Electronic controls for a hospital bed

Abstract

Improved electronic controls for an adjustable hospital bed incorporating various logic devices. These devices result in the concurrent operation of several motors in response to the activation of a single switch to produce a desired bed configuration. The motors may operate simultaneously or sequentially depending upon the particular selection and the involved circuit logic. Moreover, the logic allows the position of one adjustable bed portion to prevent the functioning of the motor for another bed portion. The circuit logic may also preclude the operation of a motor absent the secure engagement of mechanical parts. Further, the logic circuitry permits switches on the selection panel to control one adjustable bed portion, but over different ranges. This becomes particularly advantageous where one switch also operates a different bed portion.

Description

BACKGROUND

Hospital beds fill a variety of functions because of the infirmed nature of many of their occupants. Furthermore, the hospital setting results in the patient spending most of his time in the actual occupancy of the bed. Consequently, these beds generally include some mechanism for altering and adjusting their configuration.

One of the bed's functions obviously includes providing a place for the patient to sleep. However, the bed must also provide a place for the patient to lounge and engage in customary daily activities, such as reading, watching television, eating, as well as some personal hygiene. Further, when the physician makes his rounds in the hospital, the hospital bed frequently provides for the examination and treatment of the patient. Recently, medical science has discovered that placing the patient at a slight angle with respect to the horizontal - the Trendelenburg or reverse Trendelenburg positions - provides some benefit for various types of infirmities. Accordingly, some beds possess the ability to place the patient in such a position.

Beds other than those used strictly in hospitals must also display similar advantages as those described above. Nursing homes, which perform many similar functions to hospitals, display a need for such flexible beds. On occasion, an infirmed person at home may also need this type of bed.

Various types of adjustable beds have attempted to accommodate these needs. Two of these in particular have found acceptance in the hospital and nursing home industries.

The first type incorporates a single electric motor which effects a single adjustment to allow the patient to sit up. It does so first by elevating the head portion of the bed which thus assumes an elevational angle with respect to the horizontal. In this position, however, gravitation causes the patient to slide towards the foot of the bed and assume an uncomfortable and perhaps unhealthy position. To preclude this gravitation, the same motor simultaneously elevates the knee portion of the bed. The raised knee portion, in effect, provides a stop mechanism to abate the sliding. Some beds of this type incorporate a separate hand-actuated mechanism for raising the overall level of the bed.

U.S. Pat. No. 3,821,821 to F. J. Burst et al. provides a drastic improvement in this general type of bed. It includes a clutching mechanism which utilizes the motor's power to raise and lower the bed, as well as to configure it. Furthermore, it permits the disengagement of the knee mechanism from the head portion and, additionally, includes a separate hand crank to independently adjust the knee. This bed also provides a hooking mechanism which, upon the elevating of the bed to its highest position, can retain either end while the other lowers to achieve one of the Trendelenburg positions.

The other type of bed permits three separate adjustments accomplished through either one or three electric motors. This bed includes a first adjustment to elevate the head, a second and independent adjustment to elevate the knee, and a third mechanism to raise the level of the bed.

The bed with a single motor also possesses complicated mechanisms through which the motor adjusts the different bed portions. U.S. Pat. Nos. 3,290,956 to W. R. Black et al., 3,602,784 to G. M. Eluer, 3,710,404 to W. J. Peterson, and 3,716,876 to A. P. Petzon et al. show a single motor and various types of electrical and mechanical devices to separately adjust the head, knee and elevation portions of the bed.

Where the bed incorporates three motors, the activation of a particular motor adjusts a single bed section. The expenditure of appreciable effort with sufficient readjustments of the various parts allows the placing of the bed in a satisfactory position for the particular activity undertaken by the patient. Goodman et al.'s U.S. Pat. No. 3,743,905 shows separate switches controlling the three motors for the movable bed portions.

Both types of beds, nonetheless, suffer unavoidable drawbacks in their general utilization by patients who must occupy them for various purposes. The first type of bed, with the coordinated head and knee motion, simply lacks versatility in the different types of positions achievable for a patient. Less knee elevation for a particular head elevation, for example, generally falls beyond the capacities of the bed. Obviously, such sophistications as the Trendelenburg or reverse Trendelenburg position present no possibility for most of these beds.

The beds having separate controls for the head, knee, and elevation portions do possess the desired versatility. They, however, do not allow for the coordinated movement of different bed portions. Each portion undergoes movement only upon the activation of the particular switch designated for that portion. Consequently, achieving the exact desired bed configuration presents a significant and deleterious burden. Moreover, to achieve certain frequently used positions, such as the Trendelenburg position or the simple, straight, flat position for sleeping often requires the manipulation of a multitude of the switches. This bed lacks the desired simplicity and ease of operation.

SUMMARY

Incorporating logic devices within the control circuitry of a hospital bed combines versatility of bed adjustments with simplicity of operation. Moreover, the logic devices allow types of control over the bed's operation not previously attempted.

Typically, a hospital bed includes at least one bed portion which moves to a number of positions and an electric motor for moving that bed portion from one position to another. Through the use of logic means, two different selection means can control the motor with the second of these two not achieving all of the positions available with the first. Consequently, the logic, in response to the first selection means, will actuate the motor to move the bed portion to positions to which the bed portions will not move in response to the second selection means.

The bed's knee mechamism reveals a number of advantages resulting from this type of dual control. The knee switch, of course, represents the first selection means and moves the knee portion through its full range of positions for whatever reason desired, such as an examination of the patient. The switch to raise the head portion should also elevate the knee portion but only enough to prevent gravitation of the patient and desirably no further. The logic in conjunction with the selection means determines the particular ranges of knee movement for the head and knee buttons.

The logic means also has utility where the bed includes two bed portions, each movable to a number of positions, and an electric motor for moving the first bed portion between its positions. Here, the logic will permit the operation of the electric motor, and thus the movement of the first bed portion, to depend upon the position occupied by the second bed portion. Consequently, when the second bed portion occupies one of a first set of positions, activation of the selection means produces the usual motion of the first portion. When the second bed portion occupies a position in the second set of positions, no such motion results, notwithstanding a selection made on the selection means.

The lowering of the head and knee portions represents an example of this latter situation. When the head portion lowers to the horizontal, the knee portion should similarly flatten. However, the head portion may, of course, begin its descent from an almost vertical position. To prevent patient gravitation, the knee portion should not lower until the head portion has gone below a predetermined level, typically 30.degree.. Depressing the switch to lower the head portion accordingly will lower the knee portion only when the head portion occupies a position below the desired level. With the head portion above that level, the knee portion will not lower in response to the lowering of the head.

The head and knee also exemplify the third type of benefits accruing from the incorporation of logic means in the bed. In this instance, the bed again includes two movable portions and an electric motor for each. However, the logic actuates both motors in response to a selection made on a single selection means. This, of course, refers to the raising or lowering of both the head and knee portions merely upon selection on the head selection switch.

Furthermore, the logic means allows for the reversing, in a single operation, of a motor when a bed portion reaches a particular point. In addition to the movable bed portion, the reversible electric motor, and the selection means, this improvement requires some means for indicating tha the bed portion had reached the position where the motor reverses.

This ability to reverse a motor's direction finds particular advantage in the Trendelenburg operation. In this operation, the elevation motor often first raises the bed to its highest position and then reverses. Acting with some means to retain one end of the bed in its raised position, the reversal lowers the other end and produces the desired inclination. The indicating means shows that the bed has reached its highest position and that the motor can reverse its direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrams the electrical components of an adjustable three-motor bed.

FIG. 2 shows the power supply components providing the various potentials of the bed's circuit.

FIG. 3 gives a detailed circuit diagram of the electrical components in the bed except the motors and the power supply.

FIG. 4 provides an alternate ground test device for the one shown in FIG. 3 .

DETAILED DESCRIPTION

Adjustable beds possess at least one portion that moves between a number of positions. Frequently, in fact, the bed has three movable portions. These generally include the head portion, the knee portion and the elevation portion, as discussed in U.S. application Ser. No. 496,212 of James S. Adams, William H. Peck and Daniel R. Tekulve entitled ELEVATING AND TRENDELENBURG MECHANISM FOR AN ADJUSTABLE BED.

The elevation portion represents the mechanism of the bed which allows the general level of the mattress to rise or lower with regards to the floor or any surface upon which the bed sits. In a three-motor bed, a separate elevation motor moves this mechanism to achieve the desired mattress height.

The head portion of the bed includes the mechanism which raises and lowers the head of the spring and mattress. Actually, the mechanism causes the head portion of the mattress to rotate about an axis which lies transverse to the longitudinal axis of the bed and parallel to the ground. This transverse axis falls at an intermediary point between the head and the foot of the bed. It comes sufficiently close to the head to also allow for a knee portion. Since the elevation of the head portion proceeds as a rotation about an axis, the angle of this rotation represents the actual amount of elevation.

A recent and remarkable development in hospital beds concerns the overall motion of the bed upon the elevation of the head. As shown in U.S. Pat. No. 3,237,212 to Hillenbrand et al., a general movement of the overall bed structure toward the head may accompany the elevation of the head. This allows the patient's head to remain close to the wall and accessory equipment for greater comfort, convenience, and safety, as well as greater maneuvering area in the hospital room. This development, of course, represents an extremely desirable facet associated with the rising of the bed's head portion.

The knee section accomplishes its elevation by rotating bed sections about two transverse axes. The first axes occurs just below the head portion of the bed. Rotation of the bed portion below this axis results in the elevation of the lower portion of the patient's body. The second axis of rotation, of course, occurs in the region of the patient's knee and permits the usual flexing of the knee. The amount of rotation about the second axes at the knee may suffice for the foot of the bed to remain at the same height it occupied prior to the elevation of the knee. Recent practice, however, prefers to elevate somewhat the foot of the bed and the feet of the patient from the position they occupied before the knee bending. Producing less rotation about the knee, of course, results in the desired elevation of the feet.

FIG. 1 diagrammatically shows the three bed motors, with the elevation motor at 11, the knee motor at 12 and the head motor at 13. The various selection means, labeled as INPUTS, ultimately control the operation of these motors. These include first of all the patient controls 14 and 15 on either side of the bed. One of the patient controls would suffice, of course, but having two provides greater convenience for the patient.

The bed position indicators at 16 provide another type of input into the control circuitry. As one of their several functions, they indicate when a bed portion has reached its limit of operation in order to turn off its motor. Further, they indicate the relative positions of various bed portions to provide coordinated operation of their motors and motions.

Lastly, most electrical adjustable beds include some sort of master control 17. The master control, when desired by the hospital or medical staff, precludes operation of any or all of the various movable portions of the bed. It may also have the switches for esoteric configurations, such as the Trendelenburg positions.

The inputs feed into the control logic 18 along the connections 19, 20, 21 and 22. The control logic 18 in turn controls the elevation motor 11, the knee motor 12, and the head motor 13 through the double connections 23, 24 and 25, respectively. The doubled representations of the connections 23, 24 and 25 result from the fact that each motor may operate in a forward and in a reverse direction; one of the connections stands for the forward direction and the other for the reverse direction. The reversibility of the motors allows for both the raising and lowering of the pertinent bed portion.

The power supply system of FIG. 2 represents one of many that can provide the various a.c. and d.c. potentials required for the operation of the bed's circuitry shown in FIG. 3. The power for the circuit appears on the leads 31 and 32 with the fuse 33 interposed for safety. The potential then appears across the primary winding 34 of transformer 35 which also connects to ground 36 for safety. Typically, the voltage supplied along the leads 31 and 32 lies within the range of 115 to 120 volts a.c. The design of the circuit, however, allows it to operate properly with the alternating current in the range of 90 to 130 volts.

Two secondary windings 40 and 50 obtain power from the transformer 35. Current from the secondary winding 40 passes to the rectifier 41. After filtering by the capacitor 42, the voltage appears as -6 volts d.c. on the connection 43 labeled V.sub.D. The connections, as shown, add -.notident.V. d.c. onto the high voltage side 32 of the basic a.c. current supply. Thus, with respect to ground, the voltage appearing at V.sub.D will have the usual 120 V. a.c. configuration superimposed on a -6V d.c. background.

The other secondary winding 50 supplies its current to the rectifier 51 followed by filtering through the capacitor 52, which supplies a d.c. potential of 12V., labeled V.sub.E, with respect to ground, along the connection 53. The 12 V. also passses through the resistor 54 and regulator 55 which, with the assistance of resistor 56, provides a regulated 5 V., V.sub.C, along the connection 56. The power supply shown in FIG. 2 suffices to provide the potentials required to operate the control circuits shown in FIG. 3.

While other designs would suffice, the unit in FIG. 2 has the advantage of isolating the V.sub.D supply of -6V. appearing on connection 43 from both the ground connection 35 and the two a.c. connections 31 and 32. This insulates the sensitive reed relay switches of FIG. 3 from the very high potential tests required by the Underwriters Laboratories. In that test, the two a.c. wires providing the operating voltage supply connect to each other and to one side of a 1,240 V. potential. The other side of the high potential connects to the ground of the system. The test, of course, requires the absence of any breakdown current leakage between the high potential connections. However, this high potential could, in fact, injure the reed relays. The separate winding 40 precludes that injury.

A particular circuit diagram incorporating logic devices and including all of the electrical components for an adjustable bed with the exclusion of the power supply and the motors appears in FIG. 3. The circuit, as shown, incorporates electronic logic components. Other classes of logic devices, including mechanical and hydraulic types, have found service in other settings. Conceivably, with the appropriate supporting structures, they too could suffice for an adjustable bed.

The diagram of FIG. 3 shows the inputs from the various types of selection means or switches at the left. To the far right lie the leads to the electric motors which, when directed by the rest of the circuit, will carry the current to operate the appropriate motors in their proper directions. In the middle appear the electronic logic components which translate the signals from the various switches and selection devices into the appropriate motor operations.

The selection means includes the various switches indicated generally at 70. These fall into the two categories of manually actuated switches and position-indicating switches.

When activated, the manual switches provide electrical signals which initiate changes in the bed configuration. Where an adjustable bed portion merely has a limited and discrete number of positions, a manual switch may have a separate setting for each of the positions the bed portion may move to. A rotary switch having the same number of positions as bed positions would suffice in this instance. Moving the switch to the appropriate setting would induce the bed portion to move to the corresponding position.

Generally, however, the adjustable bed portion possesses a continuous range of positions. A manual switching device for this situation could involve a selector moving over a circular or linear dial having a range of positions corresponding to those of the bed portion. The switch itself may involve a potentiometric device and connect to a servo system which moves the bed portion to a selected position.

More typically and preferably, the bed includes an on-off, or single-pole, single-throw switch, to induce movement of the bed portion in one direction and a second switch to produce motion in the other direction. This represents the situation for the manual switches shown at 70.

These switches include an elevation-up switch, labeled EU at 71. Depression of the EU switch 71 will generally induce the elevation motor of the bed to raise the elevation bed portion. The elevation-down ED switch 72 performs a similar function to lower the elevation portion of the bed. For the head portion, the head-up HU switch 73 and the head-down HD switch at 74 perform similar functions as do the knee-up KU switch at 75 and the knee-down KD switch at 76 for the knee portion.

The bed includes five further manual switches. The bed-flat BF switch at 77 has a three-fold function which can proceed simultaneously. First, it lowers the knee portion; second, it lowers the head portion; and third, it raises the elevation portion so that the bed assumes a high and flat configuration. The bed-flat BF switch 77 has two important purposes. The first occurs to provide a particularly desirable configuration when the physician or a nurse desires to examine the patient. Second, this switch also induces the motion to retrieve the bed from a Trendelenburg or reverse Trendelenburg position.

The Trendelenburg switch 78, of course, places the bed into the Trendelenburg or reverse Trendelenburg position. For the schematic shown in FIG. 3, the distinction between the Trendelenburg or reverse Trendelenburg positions does not derive from the electronics, but rather from mechanical couplings to the Trendelenburg switch, as shown in F. J. Burst et al.'s U.S. Pat. No. 3,821821 or J. S. Adams et al.'s U.S. application Ser. No. 996,212.

The switches discussed above normally take the form of spring-loaded on-off switches loaded in the off position. In this circuit which operates at low voltages, membrane switches present the ideal performance characteristics.

The remaining three manual switches do not possess spring-loading. These include the lock-out-elevation LOE switch at 79, the lock-out-head LOH switch at 80 and the lock-out-knee LOK switch at 81. These normally appear, together with the bed-flat BF switch 71 and the Trendelenburg T switch 78, on a separate master-control panel 17, in FIG. 1. Located away from the patient's normal reach, and frequently at the foot of the bed, the switches control bed motions which the hospital staff should undertake and not the patient.

The lock-out switches preclude operation of the indicated motors, notwithstanding the activation of other switches which otherwise would result in the movement of the bed portion. In particular, the LOE switch 79 immobilizes the elevation motor, the LOH switch 80 the head motor, and the LOK switch 81 the knee motor.

The position-indicating and dependent switches do not provide for manual activation. Rather, they indicate to the logic circuitry the positions occupied by various parts of the bed. More accurately, they indicate which of a variety of possible situations the bed occupies. For example, the limit-head-up LHU switch at 90 indicates that the head portion of the bed has reached its highest limit. The limit-head-down LHD switch at 91 similarly indicates that the head portion occupies its lowest limit. Thus, either end of the continuous range of positions produces a special indication. When the head portion assumes a position intermediate these extreme limits, no special indication results from these switches.

Intermediate the uppermost and the lowermost positions of the head portion, the limit-head-contour LHC switch 92 comes into play. The LHC switch 92 divides the range of head positions into two sets. The first set includes all positions at and above a certain point, generally 30.degree. of elevation, while the second set includes those positions below that point. As discussed below, the LHC switch 92 operates upon the depression of the head-down HD switch 74 to lower the knee portion of the bed but only when the head portion occupies its lower range, or second set, of positions mentioned above.

Similarly, the limit-knee-contour LKC switch 93 functions in the simultaneous raising of the knee portion upon the depression of the head-up HU switch 73. The LKC switch 93 stops the elevation of the knee portion by the head-up HU switch 73 when the knee portion reaches a certain height, generally 15.degree. of elevation.

As with the head portion, the knee portion also has a limit-knee-up LKU switch 94 and a limit-knee-down LKD switch 95. The bed also includes a limit-elevation-up LEU switch 96 and a limit-elevation-down LED switch 97, as well as the limit-Trendelenburg LT switch 98 for when the bed has reached its greatest angle of inclination in either the Trendelenburg or reverse Trendelenburg positions.

The hook H switch 99 represents the last position-indicating switch in the drawing. It provides an extra measure of safety in operating the Trendelenburg mechanism. The type of bed shown in U.S. Pat. No. 3,821,821 to Burst et al. or in Adams et al's application Ser. No. 496,212 rises to its highest point before inclining into the Trendelenburg position. While in its highest position, one of two hooks, depending upon which end of the bed declines, engages a catch in order to retain the other end in an elevated position. If not securely engaged, that end of the bed could possibly slip and fall when the first end lowers. Accordingly, the hook H switch 99 indicates the secure hook engagement before either end of the bed may decline.

A position-indicating switch need only translate the relative mechanical motion of the two bed parts into electrical signals. Accordingly, it may attach to one of the parts and abut against the other when the latter has reached a preselected point of travel. Alternately, it may follow a cam attached to the second part. Other common arrangements would also clearly suffice.

Most of the components for the circuit in FIG. 3 may attach to a printed circuit board with their interconnections printed upon the board itself. The switches, however, because of their locations at various points on the bed do not constitute part of the board. Accordingly, they attach to the board through the series of bus bars indicated generally at 20. The figure shows a generally convenient arrangement of the bars.

For the safety of the bed's occupant, the switches, as well as most of the circuit components, operate at voltages below about 25 V. and generally in the range of 5 to 12 V. These low voltages do not allow the current in the switches to arc over the small gap that exists immediately prior to their closing and after their opening as with higher voltages. This sparking across the gap has the desirable effect of burning off corrosion and other residues on the metallic contacts. Without such sparking, the corrosion will remain. Consequently, the low voltage switches indicated at 70 will give somewhat improved performance over an extended period of time if their contacts include a noncorrosive metal. The noble metal gold, of course, represents an ideal choice. Silver cadmium oxide represents another possibility.

However, the circuitry incorporates a further safety feature in the event that a switch does malfunction and remain open. This malfunction will result in the inoperation of a motor rather than vice versa. Thus, the inability of a switch to close properly will not cause undesired and perhaps dangerous motion of the bed.

The resistors, indicated generally at 130, translate the openings and closings of the switches 70 into electrical impulses suitable for further processing by the circuit logic components. Generally, for the logic components to operate, the signals must vary between two voltage levels, for example, between 0 V. and 5 V. Handbooks published by component producers list the voltages required for their products.

The EU switch 71, the particular resistor 131, and the connection to the voltage source V.sub.C exemplify the voltages resulting from a switch's opening and closing. While the EU switch 71 remains open, the juncture 132 connects electrically only to the resistor 131 and thence to the source of voltage V.sub.C. Inasmuch as little or no current flows through the resistor 131 in the event of the open switch 71, no voltage drop occurs across the resistor 131. Accordingly, the juncture 132 and the lead attached to it, labeled EU, remain substantially at the voltage level of V.sub.C, 5 V. in this instance. On the other hand, closing the switch 71 results in the juncture 132 connecting directly through the switch 71 to ground at 133 which, of course, lies at 0 V. Thus, closing and opening the switch results in the juncture 132 and the lead EU going between 5 V. and 0 V., respectively.

The symbolism of EU for the switch 71, and EU for the lead after the resistor 131 results from the standards adopted for a logic circuit. Generally, the voltages used in a logic circuit can exist at either one of two levels; in this case, for example, 0 V. and 5 V. represent those levels. Usually, the higher voltage level represents the positive state with the lower voltage called negative. The output of the logic components, of course, depends upon the positive or negative values of the inputs to that component.

Frequently, a component with a single input will produce the negative of that input as its output. In this case, a symbol standing by itself represents the input variable and the same symbol with a bar over its top represents the negative of the input. Alternately, the symbol with a bar may stand for the input, and the symbol itself used for the output.

In this case, for example, EU stands for the functioning of the switch 71. In particular EU is defined as positive upon an actual depression and closing of the switch 71. As discussed above, when the EU switch 71 closes and thus achieves its positive state, the juncture 132 and, thus, the lead labeled EU assumes the negative state of 0 V. Thus, the lead EU always has the negative value of the EU switch 71. Accordingly, for convenience, the lead itself has the label EU which represents its value. Similarly, other leads in the diagram will bear labels actually representing their voltage states.

Similar to the EU switch 71, most of the manually actuated switches assume their positive state when closed and their negative state when open, In addition to the EU switch 71, these include the ED switch 72, HU 73, HU 74, KU 75, KD 76, BF 77, and T 78. The lock-out switches LOE 79, LOH 80, and LOK 81 perform in the reverse fashion. To lock out a motor, the switch opens and assumes its positive state.

With the exception of the Trendelenburg hook H switch 99 and excluding the limit-head-contour LHC switch 92 and the limit-knee-contour LKC switch 93, all of the bed-position indicating switches have a positive value when open and a negative value when closed. All of the above states accord with the notion that because of corrosion possibly preventing a switch's closure, an open switch results in the inactivation of its respective motor. Thus, for example, the limit-head-up LHU switch 90, opens when the head portion reaches its upper limit. This causes inactivation of the head motor. Yet, the head portion at its upper limit represents the positive state of the corresponding switch. With the LHU switch 90 open, however, the juncture 134 reaches the higher level of 5 V. and, accordingly, assumes the positive state as does the lead LHU connecting to it.

Conversely, when the head portion does not occupy its highest position, the LHU switch 90 remains closed. This represents the negative state of the switch 90, as well as the LHU lead to the right of juncture 134.

The Trendelenburg hook H switch 99 has the opposite function of the other bed-position switches. When the hook actually engages, the switch closes to allow actuation of the motor to achieve the proper Trendelenburg tilt. This represents its positive state but H then descends to its negative state and, thus, has a converse behavior of the switch 99.

The lock-out-elevation LOE switch 79, the lock-out-head LOH switch 80, the limit-head-contour LHC switch 92, and the limit-knee-contour LKC switch 93, do not provide information to the more usual circuit logic components in the drawing. The first two disconnect the elevation and head switches from ground to inactivate them. The latter two interconnect the head-and-knee-motor leads to coordinate the movements of the head and knee sections.

As discussed above, the elevation-up EU switch 71 operates by connecting the junction 132 to ground potential. Thus, to raise the bed, the junction 132 must go from 5 V. to ground.

However, the connection between ground at 133 and the EU switch 71 first passes through the lock-out-elevation LOE switch 79. Depressing and thus opening the LOE switch 79 disconnects the EU switch 71 and the juncture 132 from their ground. Unable to connect, upon the activation of the LOE switch 79, to ground, the EU switch 71 thus becomes inoperative. For the same reason, the elevation-down ED switch 72 cannot operate upon the opening of the LOE switch 79. Accordingly, the LOE switch 79 renders both the ED switch 71 and the EU switch 72 nonfunctional and the bed will not rise or lower in response to them.

Similarly, both the head-up HU switch 73 and the head-down HD switch 74 connect to ground through the lock-out-head LOH switch 80. Activating and thus opening the LOH switch 80 prevents the raising or lowering of the head portion through the HU or HD switches 73 or 74.

The notations of EU, ED, HU and HD in FIG. 3 signifies the actual functioning of the appropriate switches. The inactivation of these switches by the LOE switch 79 or the LOH switch 80 precludes them from supplying the electrical information characteristics of the EU, ED, HU, or HD functions. Thus, this notation on the leads in FIG. 3 and in the following discussion implies that the EU switch 71, the ED switch 72, the HU switch 73, and the HD switch 74 have not undergone inactivation by the LOE switch 79 or LOH switch 80, respectively.

The LHC and LKC switches 92 and 93 also work through other switches. In particular, upon the simultaneous closing of the LKC switch 93 and the head-up HU switch 73, the ground potential of 0 V. passes through the HU switch 73 to the juncture 135, through the diode 136, and the LKC switch 93, back up to the junction 137 on the lead to the knee-up KU switch 75. Supplying approximately 0 V. to this lead, as seen from above, produces the same effect as closing the knee-up KU switch 75 to raise the knee portion of the bed. Thus, upon actuating the head-up switch 73, the knee portion of the bed also receives a signal to raise, provided the limit-knee-contour LKC switch 93 remains closed. And, the position-indicating LKC switch 93 opens only when the knee portion of the bed rises above a preselected level, conveniently 15.degree. of knee elevation.

When the knee portion elevates beyond 15.degree., the LKC switch 93 opens and precludes the transmission of the signal from the switch 73 to the knee-up KU lead at 137. This terminates the raising of the knee portion in response to the depression of the head-up HU switch 73. Nonetheless, the knee-up switch 75, of course, will permit further elevation of the knee portion if desired.

Conversely, with the head-up HU switch 73 open, the juncture 135 stays at the postive potential of 5 V. Upon closing the KU switch 75, however, the juncture 137 goes to 0 v. and remains unaffected by the 5 V. at the point 135, due to the blocking action of the diode 136.

Similarly, the head-down HD switch 74 connects to the juncture 138, the diode 139, the limit-head-contour LKC switch 92 and back up to the juncture 140 on the knee-down KD lead. As with the limit-knee-contour LKC switch 93, this connection causes the knee portion to descend upon the simultaneous closing of the head-down HD switch 74 and the limit-head-contour LHC switch 92. The LHC switch 92, however, only closes when the head portion descends below a predetermined position, typically 30.degree. of head elevation. Lowering the head portion from a position above 30.degree. will not cause the knee portion to descend until the head portion goes below 30.degree.. Again, in going in the reverse direction, the diode 139 prevents the actuation of the knee-down KD switch 76 from affecting the operation of the head portion of the bed.

The notation employed in FIG. 3 and the discussion below does not indicate the operation of the knee portion upon the depressing of the head-up or head-down switches. Accordingly, whenever the symbol HU occurs, it also connotes the operation of the knee-up motor provided the knee portion of the bed remains below 15.degree.. Similarly, the symbol HD also includes the operation of the knee-down motor provided the head of the bed has no more than 30.degree. of elevation.

The above discussion shows that proper interaction of the knee and head portions of the bed procedes through the diodes 136 and 139. Consequently, in this circuit, they function as very simple logic devices.

The circuit in FIG. 3 receives its power supply and also connects to the motors at the bus bars at the right of the drawing, indicated generally at 150. The connections at 151 and 152 in particular provide the basic source of a.c. power.

The bus bars at 50 also include two connections between the circuit and each of the three motors in the bed. One of these two connections supplies current to the winding of the motor which results in the elevation of the relevant bed portion. The other connection provides current along an alternate winding on the motor and causes the motor to operate in the reverse direction and lower the bed portion. As an example, the connection 153 provides current to the winding of the elevation motor which raises the bed. Current provided along connection 154 goes to the winding of the elevation motor that lowers the bed.

According to the typical convention, the output from connection 153 to the elevation-motor-up EMU winding is defined as positive when current actually flows through that connection to the winding. Otherwise, it remains in its negative state. This also holds true for the other connections including the elevation-motor-down EMD connection 154, and those for the head-motor-up HMU, head-motor-down HMD, knee-motor-up KMU, and knee-motor down KMD.

The current to the elevation-motor-up EMU connection 153, flows from the 120 V. a.c. connection 151, to the juncture 155, and through the electronically controlled triac switch 156. Thus, the triac 156 controls the current supply through the EMU connection 153 and consequently to the winding of the elevation motor which raises the bed.

The triac switch 156 in turn receives its control voltages from the resistor 157 and then from the reed relay device 158. The closing of the reed switch 159 within the reed relay 158 allows the potential V.sub.D to pass from the junction 160 and through the switch 159 and the resistor 157.

As discussed above, V.sub.D comes from a separate winding 40 on the transformer 35 in FIG. 2 and provides a d.c. potential of -6 V. on top of usual a.c. voltage of approximately 120 V. The V.sub.D voltage when applied to the triac 156 closes it and allows the current of 120 V. from the connection 151 to pass to the elevation-motor-up EMU connection 153. With the reed 159 open, the switch 156 also opens and no current passes to the EMU connection 153.

The reed 159, in turn, closes only when current flows in the coil portion 161 of the reed relay 158 establishing the appropriate magnetic fields. However, to have current in the coil 161, the potential EMU at the junction 162 must be in its negative state of 0 V. With EMD at the lower potential of 0 V., current from the higher potential source V.sub.C at 5 V. passes through the coil 161 and to the juncture 162, allowing the reed 159 to close. When EMU becomes positive it closely approximates the potential V.sub.C and no current flows. The diode 164 allows energy in the coil 161 to dissipate.

Thus, a negative value for EMU results in the EMU connection 153 going positive and in the operation of the elevation motor to raise the bed. Conversely, when EMU becomes positive, no current flows through the coil 161; the reed 159 opens; the EMU connection 153 becomes negative; and the elevation motor will not raise the bed. Thus EMU display opposite behavior; when one becomes negative, the other goes positive, and vice versa.

The triac 156, the resistor 157, and the reed relay 158 have greater significance than merely taking the negative of EMU to provide EMU. These components separate the high a.c. 120 V. needed to operate the elevation motor from the lower voltages generally under 25 volts which appear on the circuit logic components and, more particularly, the selection means 70 and thus near the patient.

Specifically, the reed relays physically separate the two voltages. The coil 161 operates at the lower d.c. voltage; the reed 159 operates at the high a.c. voltage; and a section of glass with other material separates and insulates the two from each other. The high voltage appearing at the other motor connections have the separation from the low voltages of the diode, reed relay, resistor and triac indicated generally at 170.

Furthermore, on the actual circuit board a printed ground line separates the high from the low voltage connections and prevents the passage of undesired current from the former to the latter. The low voltage connections of the reed relays appear on one side of the ground line with the high voltage connections on the other.

An explanation of the operation of the circuit in FIG. 3 requires a statement of the motor operation EMU, EMD, and so forth or, equivalently, of their negatives EMU, in terms of the inputs from the selection means at 70. In other words, the actual operation of the elevation, head, and knee motors depend upon the state of the various switches at 70 as indicated by their alphabetic symbols; accordingly, an analysis of the circuit relates the operation of the various motors to these inputs.

An analysis of the circuit in FIG. 3 utilizes standard symbolic logic with its techniques and notations. Since the various components shown in the logic portion of the circuit operate upon their inputs, they analogize directly to logic operator symbols.

The figure includes three basic types of components, the inverter, the NAND gate, and the NOR gate. Each of these operates only on inputs which generally assume only one of two values such as that provided by the inputs 70 in FIG. 3. Furthermore, the output of each component assumes only one of two values; the higher voltage value corresponds to the positive state, and the lower value of 0 V. to the negative state.

In this type of system where each connection assumes one of two values, the operation of each component finds expression in terms of a "truth table." This table merely relates the state of the output of a component to the state or states of the inputs.

The inverter represents one of the simpler logic devices. It merely converts a positive input into a negative output and vice versa. IN terms of the voltages used in FIG. 3, where a 5 V. potential enters the inverter, a 0 V. potential exits.

According to the operation described above, the inverter has the following truth table:

Table 1 ______________________________________ Truth Table for an Inverter Input Output ______________________________________ POS. NEG. NEG. POS. ______________________________________

The entries to the left of the vertical double line include all of the possible inputs to the inverter with the entries on the right side representing the outputs.

An example of the inverter appears at 181 in FIG. 3. It converts the value of the input, shown as HD to its negative which could carry the notation of HD. However, the negative of a negative becomes the positive and, accordingly, the output of the inverter 181 appears as HD.

The little circle attached to the right point of the triangle of the inverter symbol 181 signifies the negative functioning of the component. The same notation also appears on other items.

The NOR gate represents a further type of logic device used in FIG. 3. An example of a NOR gate appears schematically at 182. However, the NOR gate combines two other components -- an OR gate followed by an inverter; the little circle on the right of the NOR gate 182 represents the inverting function as with the inverter. Consequently, considering each step sequentially simplifies the explanation of the NOR gate.

The OR gate provides a positive output when it has at least one input in the positive state. Only when all of its inputs are negative does the output of the OR gate become negative. Accordingly, it has the following truth table:

Table 2 ______________________________________ Truth Table for an OR gate Inputs Output ______________________________________ POS. POS. POS. POS. NEG. POS. NEG. POS. POS. NEG. NEG. NEG. ______________________________________

The NOR gate has almost the same effect except that it then inverts the output of its OR-gate portion. Accordingly, the NOR gate has the following truth table which also includes the OR function.

Table 3 ______________________________________ Truth Table for a NOR gate OR gate NOR gate Inputs Output Output ______________________________________ POS. POS. POS. NEG. POS. NEG. POS. NEG. NEG. POS. POS. NEG. NEG. NEG. NEG. POS. ______________________________________

Thus, the NOR gate's output becomes positive only when both inputs occupy the negative state; with either input positive, it has a negative output.

Where an OR gate has inputs of A and B, the output has the notation A + B, because of similarities to the ordinary arithmatic function of addition. The NOR gate simply negatives the output of the OR gate and, accordingly, for the same inputs of A and B, A +to B symbolizes its output.

The NOR gate 182 exemplifies the device's functioning in the circuit. It has the value HD as one input. As seen from the figure, the other input has the value T.sub.1 + BF. From above, this results from a prior OR gate with T.sub.1 and BF as its inputs. The component T.sub.1 represents the first stage of the Trendelenburg operation in which the head and knee descend and the bed rises, as discussed below. Were gate 182 merely an OR gate, it would have the output of HD + T.sub.1 + BF; as a NOR gate, however, it has the output of HD + T.sub.1 + BF. Consequently, if HD, T.sub.1, or BF or any combination of them is positive, then the NOR gate 182 has a negative output. Where HD, T.sub.1, and BF all remain negative, then the NOR gate 182 has a positive output.

The NAND gate represents a further type of device used in FIG. 3 and appears schematically at 183. Again, the NAND gate combines the sequential application of two separate devices, the AND and the inverter. The AND gate, which appears the same as the NAND gate 183 without the little circle adjacent to its right, produces a negative output unless both of its inputs occupy the positive state. Accordingly, the AND gate has the following truth table:

Table 4 ______________________________________ Truth Table for an AND gate Inputs Output ______________________________________ POS. POS. POS. POS. NEG. NEG. NEG. POS. NEG. NEG. NEG. NEG. ______________________________________

To achieve the results of a NAND gate requires the inversion of the output of an AND gate. Accordingly, Table 5 prevents the results for a NAND gate and also includes an AND gate having the same inputs.

Table 5 ______________________________________ Truth Table for a NAND gate AND gate NAND gate Inputs Output Output ______________________________________ POS. POS. POS. NEG. POS. NEG. NEG. POS. NEG. POS. NEG. POS. NEG. NEG. NEG. POS. ______________________________________

Where the AND gate has the two inputs A and B, its output has the notation of A. B. The NAND gate inverts the AND gate output and, accordingly, has the notation A. B. This notation indicates an underlying similarity to the usual mathematical function of multiplication.

Despite their apparent dissimilarities, the NOR and NAND functions bear a relationship to each other, which receives expression in DeMorgan's Theorems. These state the following:

A + B = A . B (1)

A . B = A + B. (2)

These theorems submit to immediate proof using the usual truth tables.

DeMorgan's Theorems allow an understanding of the NAND gate 183 and the NOR gate 184 which provides one of its inputs. The NOR gate 184 receives one input from the NOR gate 182 which, from above, has the output HD + T.sub.1 + BF. LHD represents the other input to NOR gate 184. Accordingly, the NOR gate 184 has, as its output, (HD + T.sub.1 + BF) + LHD. However, applying DeMorgan's Theorems to the plus sign between the parenthesis and LHD gives (HD + T.sub.1 + BF). LHD. Lastly, since the negative of a negative becomes positive, the input of the NOR gate 184 has the following expression (HD + T.sub.1 + BF) . LHD.

The output of the NOR gate 184 has a simple interpretation according to this expression. Recalling that the dot symbolizes the AND function, the expression assumes a positive value when the terms on either side of the plus sign are also positive. The expression within the parenthesis is positive when HD, T.sub.1, or BF or any combination of them becomes positive. This occurs when depressing the HD switch 74 or the BF switch 77 or the bed undergoes the first phase of the Trendelenburg operation. For the other term, a positive LHD requires a negative LHD. Thus, the NOR gate 184 becomes positive only when the head portion has not reached the limit-head-down and the head-down, the bed-flat, or the first phase of the Trendelenburg operation is selected.

The output of the NOR gate becomes one of the inputs to the NAND gate 183. The NAND gate 183, in turn, prevents ambiguous signals to the HMD line which goes to the head-motor-down winding. By the above rules and with HU as the other input to the NAND gate 183, its output becomes

(HD + T.sub.1 + BF) .sup.. LHD .sup.. HU = HMD. (3)

Taking the negative of both sides, the expression for when the head-motor-down operates becomes

HMD = (HD + T.sub.1 + BF) .sup.. LHD .sup.. HU. (4)

Accordingly, the head motor operates to lower the head when:

1. the head-up HU switch 73 is not depressed;

2. the head portion has not reached its limit of downward motion; and

3. the head-down operation, the bed-flat operation, or phase one of the Trendelenburg operation, all of which have the effect of lowering the head portion, is selected. Thus, the logic has precluded ambiguous commands to the head motor by preventing any current to the head-motor-down winding upon the depressing of the head-up HU switch 73. Moreover, it causes the head-motor-down to cease operation upon the activation of the limit-head-down LHD switch 91. Lastly, and when the above two conditions have become satisifed, it allows the actual operation of the head-motor-down upon the activation of any of the three switches which lower the head; these include the head-down switch 74, the bed-flat switch 77, or the Trendelenburg T switch 78 while the bed remains in the first phase of the Trendelenburg operation.

By a similar process of reasoning, and from the NOR gate 185 and the NAND gate 186, the functioning of the head-motor-up HMU follows the expressions:

HMU = HU . LHU . (HD + T.sub.1 + BF). (5)

Equation (5) also has a simple interpretation in terms of the inputs to the logic circuitry. First, the operation of the head-motor-up requires, of course, the activation of the head-up HU switch 73. Further, according to the second term, the head portion must not have reached its upper limit; otherwise, the circuitry will turn off the head-motor-up.

The parenthesized expression represents selections on the inputs 70 which would normally cause the head motor to lower the head. To preclude ambiguous commands to the head motor, if any of these become activated, the head-motor-up will not operate.

Expressions (4) and (5) do not completely determine the functioning of the head motor. As discussed above, the lock-out-head LOH switch 80, when open, will prevent the operation of the head motor by either the head-up HU switch 73 or the head-down switch 74. This, of course, allows the hospital staff to preclude the operation of the head motor when required for a particular patient.

The same reasoning process, applied to the inverter 191, the NOR gates 192 through 196, and the NAND gates 197 and 198, leads to the following expressions for the operation of the knee-motor-down KMD and the knee-motor up KMU:

KMD = [(KD .sup.. LOK) + T.sub.1 + BF]. LKD .sup.. (KU .sup.. LOK) (6)

KMU = (KU .sup.. LOK) .sup.. LKU .sup.. [(KD .sup.. LOK + T.sub.1 + BF]. (7)

These equations resemble each other, and basically have the same effect. They state that the knee motor will operate in either the up or down direction provided first, that the lock-out-knee LOK switch 81 has opened; second, that the limit of motion in the desired direction has not been reached; third, that a switch selecting the direction of the knee motor opposite from that desired has not been depressed; and fourth, that a manual switch that would cause the knee motor to operate in the desired direction has, in fact, closed. With regards to these conditions, only the knee-up KU switch 75 causes the knee motor to raise the knee while either the knee-down KD switch 76, the bed-flat BF switch 77 or T.sub.1 of the Trendelenburg operation, actuated by the T switch 78, would cause the knee portion to go down. Accordingly, the latter three must not close while the knee-up KU switch 75 attempts to raise the knee. Analogous remarks also apply for the knee-motor-down KMD. Thus, the logic components prevent current flowing ambiguously through both windings of the knee motor. Moreover, the logic also incorporated the lock-out function as well as turning off the motor when the knee portion has reached its limit of motion in the desired direction.

Lastly, with regards to the knee motors, the head-up HU switch 73 and the head-down HD switch 74 activate the knee-up and the knee-down directions provided that the knee portion remains below 15.degree. and the head portion falls below 30.degree., respectively. As discussed above, depressing the head-up HU switch 73, for example, with the knee portion below 15.degree. will activate the knee-up KU leads exactly as though the knee-up KU switch 75 had closed. This satisfies the KU condition of equations (6) and (7) as fully as the actual closing of the KU switch 75.

NAND and NOR gates may further combine into various groupings with different behavior characteristics. The two NOR gates 201 and 202 represent one such grouping, called a NOR-gate flip-flop. In the NOR-gate flip-flop, the output of each NOR gate constitutes one of the inputs to the other NOR gate. The interconnections between the NOR gates 201 and 202 satisfy this criterion.

With one input thus already occupied, each NOR gate has one remaining available input. The relationships between the outputs of the NOR gates forming the flip-flop to these available inputs also finds expression in a truth table, such as the following:

Table 6 ______________________________________ Truth Table for a NOR gate flip-flop Input to Input to Output from Output from NOR gate1 NOR gate 2 NOR gate 1 NOR gate 2 ______________________________________ NEG. POS. POS. NEG. POS. NEG. NEG. POS. POS. POS. NEG. NEG.(6) NEG. NEG. (last POS. NEG. POS.) NEG. (last NEG NEG. POS. POS.) ______________________________________

Leading to the flip-flop of NOR gates 201 and 202, the inverter 203 changes ED to ED and supplies it as the input to the NOR gate 201. The input to the NOR gate 202, which comes from the NOR gate 204 and the NAND gate 205, has the value of EU + H + T.sub.1 + BF. However, the circuit only utilizes the output from the NOR gate 202, which bears the notation of MED for "maintained-elevation-down." According to Table 6 above, the output MED has the value given by the following table:

Table 7 ______________________________________ Truth Table for the value of MED Input to NOR Input to NOR MED gate 201 gate 202 ED EU + H + T.sub.1 + BF ______________________________________ NEG. POS. NEG. POS. NEG. POS. POS. POS. NEG. NEG. NEG. (last POS.) NEG. NEG. (last POS.) NEG. POS. ______________________________________

The last two lines of Table 7 merit particular attention. On both of those lines, both inputs have a negative value. Yet, the outputs on the two lines very from each other and depend not only upon the present values of the inputs, but upon their prior history. Specificallly, the value of the output in the instance where both inputs remain negative depends upon which of the two inputs last had a positive value. Thus, the flip-flop operates as a memory device, remembering which of the two inputs last had a positive value.

Lines 2 and 5 of Table 7 show that the maintained-elevation-down MED value becomes positive under two circumstances. Both of these require a negative input of EU + H + T.sub.1 + BF to the NOR gate 202. That implies that not one of these is positive. EU, T.sub.1, and BF represent functions which would raise the bed and, thus, go contrary to the elevation-down ED function. The hook H becomes positive during a Trendelenburg operation. Any of these four will disenable the maintained-elevation-down MED function.

As for the ED input to the NOR gate 201, MED becomes positive when the elevation-down Ed output of the NOR gate 202 first gores positive; the latter occurs upon the depressing of the ED switch 72. However, after the releasing of the ED switch 72, ED becomes negative but will have been the last positive. This fact allows the MED value to remain positive even though the elevation-down ED switch 72 has returned to its negative value.

Thus the flip-flop of the NOR gates 201 and 202 allow the bed to start down upon the depressing of the ED switch 72 and to continue descending even after the release of the ED switch. Pressing the elevation-up EU switch 71, the Trendelenburg T switch 78 or the bed-flat BF switch 77 will stop the downward movement of the bed.

The above description of MED then allows the expression of the elevation-motor-up EMU and the elevation-motor-down EMD in terms of the inputs 70, MED, and T.sub.3, where T.sub.3 represents the third and inclining phase of the Trendelenburg operation. Following the various inputs through the inverter 206, the NOR gates 207 through 213, and the NAND gates 214 and 215, EMU and EMD become:

EMD = [MED + (T.sub.3 .sup.. LT)].sup.. LED .sup.. [(EU .sup.. H) + T.sub.1 + BF] (8)

EMU = [(EU .sup.. + T.sub.1 + BF].sup.. LEU .sup.. ED .sup.. [MED + (T.sub.3 .sup.. LT)].sup.. LED. (9)

The capacitor 220 connects between the potential source V.sub.C and the NOR gate 201, while the resistor 221 couples between V.sub.C and the output of the NOR gate 201. Upon the application of power to the circuit, the capacitor 220 retards the rise of power to the NOR gate 201 and the resistor 221 forces its output to the positive state. The resistor 222 connects to ground from the output of the NOR gate 202, forcing it to the negative state. As a result, the NOR gate 202 initially controls the flip-flop which cannot assume a positive maintained-elevation-down MED state. Consequently, the bed will not start down on its own.

The resistor-capacitor combinations of 223-224 and 225-226, respectively, retard the intial supply of power to the separate windings of the elevation motor. This delay allows the motor tot stop momentarily before rversing its direction. Depressing a swtich which would induce the motor to raise the bed also retracts the motor from its maintained-elevation-down MED condition. The slight delay thus introduced dissipates what cold otherwise amount to signficant torque forces on the motor.

To remove the flip-flop action of the NOR gates 201 and 202, the figure also shows, in phatom, a connection from the input of the NOR gate 201 to ground at 227, instead of to the output of the NOR gate 202. Making this change on the circuit board converts the NOR gate 201 into an inverter and produces for output of the NOR gate 202 the value of ED .sup.. (EU + H + T.sub.1 + BF). As a result, the elevation-motor-down EMD operates only during the time of the actual depression of the elevation-down ED switch 72.

As alluded to above, the Trendelenburg operation has three phases, symbolized by T.sub.1, T.sub.2 and T.sub.3. The relationship between the phases derives from another type of flip-flop, composed of the two NAND gates 241 and 242 near the bottom of FIG. 3. As with the NOR-gate flip-flop, the output of the first NAND gate 241, labeled Q, constitutes one input to the second NAND gate 242. Similarly, the output of the second NAND gate 242; labeled Q*, provides an input to the first NAND gate 241. The typical NAND-gate flip-flop has the following truth table:

Table 8 ______________________________________ Truth Table for a NAND-gate flip-flop Input to Input to Output from Output from NAND gate NAND gate NAND gate NAND gate 1 2 1 2 ______________________________________ POS. NEG. NEG. POS. NEG. POS. POS. NEG. NEG. NEG. POS. POS. POS. POS. (last NEG. POS. NEG.) POS. (last POS. POS. NEG. NEG.) ______________________________________

This table compares closely to the NOR-gate flip-flop's table, Table 6. Nonetheless, unlike the NOR-gate flip-flop, with inputs to the flip-flop positive the outputs depend upon which inputs had last been negative.

The input to the NAND gate 241 derives from the usual reasoning applied to the inverters 243 and 244, the NOR gate 245, and the NAND gates 246 and 247. Accordingly, this input has the value of LHD + LKD + LEU + H. The value of the input to the NAND gate 242 has the value of BF + LEU as derived from the inverter 250 and the NAND gate 251.

The output from the flip-flop of NAND gates 241 and 242 appears in the following table which also has the negatives of the Q and Q* values.

Table 9 __________________________________________________________________________ Truth Table for the value of Q, Q, Q*, and Q* Input to Input to NAND gate 241 NAND gate 242 Q Q Q* Q* LHD + LKD + LEU + H BF + LEU __________________________________________________________________________ POS. NEG. NEG. POS. POS. NEG. NEG. POS. POS. NEG. NEG. POS. NEG. NEG. POS. NEG. POS. NEG. POS. POS (last NEG.) NEG. POS. POS. NEG. POS. (last 0) POS. POS. NEG. NEG. POS. __________________________________________________________________________

The NOR gate 260 has the output of T .sup.. Q. This defines T.sub.1 as the occurrence of the closing of the Trendelenburg T switch 78 simultaneously with the flip-flop providing a positive value for Q. The first and fourth lines of Table 9 show that Q has a positive value only upon the nonsatisfaction of one of the conditions of limit-head-down LHD, limit-knee-down LKD, limit-elevation-up LEU, or the engagement of the hook H. In fact, T.sub.1 has the purpose of allowing the head and knee portions to reach their lower limits, the bed to reach its highest level, and the engagement of the hook.

After the flip-flop, the NOR gate 261 combines T.sub.1 with bed-flat BF. The inverter 262 provides the output seen above in the diagram of T.sub.1 + BF. That, of course, has a positive value during either the first phase of the Trendelenburg operation or the bed-flat operation.

The NOR gate 263 whose output by definition provides T.sub.3, shows that the third phase occurs when Q* has a negative value and simultaneously the Trendelenburg T switch 78 remains closed. Thus, as shown in the second line of Table 9, Q* reaches a positive value to begin T.sub.3 only after the first phase accomplishes the four objectives of LHD, LKD, LEU and H. The short period of time between T.sub.1 accomplishing its objectives and Q* becoming negative constitutes T.sub.2.

The last line of Table 9 indicates that after meeting the four conditions of T.sub.1, the bed remains in T.sub.3 even though one or more no longer remain satisfied. This occurs for two reasons. First, in T.sub.3, the bed lowers and tilts precluding it from staying at its limit-elevation-up LEU position. Second, in either Trendelenburg positions, the head or knee portions may raise upon the selection of the appropriate switch.

While the hook H remains engaged, the elevation-up EU switch 71 and elevation-down ED switch 72 becomes inoperative. This readily appears from the H term in Equation (9) for EMU and in the truth table, Table 7, for MED. Thus, to raise the bed out of a Trendelenburg position requires the activation of the bed-flat BF switch 77. When the bed reaches the limit-elevation-up LEU by depressing the bed-flat switch, it no longer remains in T.sub.3.

Providing an idication of a proper connection with ground becomes especially important in the case of equipment occupied by patients. For this bed, the bulb 270 in FIG. 3 lights when the bed and its circuitry have lost contact with ground. It also shines when the common connection and the 120 V. potential have crossed in their wiring. Lastly, it indicates when the potential of the common connection, which should stay near ground, has risen unacceptably above that value.

The resistors 271 through 274 on the one hand and 275 on the other provide a potential divider to control the normal input voltage of the gate of the field-effect transistor 276. The output of that transistor, in turn, controls the state of conductivity of the more usual p-n-p transistor 277. In the absence of the proper connection to ground or the proper wiring, the voltage on the gate of the FET 276 lowers, allowing it to conduct. This lowers the potential on its drain, and thus on the base of the transistor 277 which also becomes conducting allowing the lighting of the bulb 270. The diodes 278 and 279, as well as the resistors 280 and 281, assist in the control of the transistor 277. The switch 282 tests various components of the testing circuit. By raising the grid of the field-effect transistor 276 to a value of V.sub.E = 12 V., closing the switch 282 turns on the bulb 270 provided the transistor 276 and 277 as well as the bulb 270 have not become inoperative.

An alternate ground test device appears in FIG. 4. This unit could connect to the circuit of FIG. 3 by interposing the diode 300 between potential supply V.sub.D and the juncture 160 leading to the switch portion 159 of the reed relay 158.

As discussed above, each time a motor operates, current must flow from V.sub.D through the juncture 160 and across one of the switches. As a result, in FIG. 4, any selection which operates a motor will produce current flowing across the diode 300.

The resistance of the diode 300 produces a potential drop across itself and also between the emitter and the base of the transistor 301, turning it on. At the instant the transistor 301 turns on, as well as before, the capacitor 302 bears no charge having previously discharged through the resistor 303.

Carrying no charge, the capacitor 302 remains in the negative state which, after passing through the resistor 304, provides the input to the NOR gate 305. Having both of its inputs tied together, as seen in Table 3 for the NOR gate, the NOR gate 305 acts as an inverter and provides a positive output to the NOR gate 306.

Thus, with either of its inputs positive, the NOR gate 306 provides a negative output to the NOR gate 307, which forms part of the flip-flop composed of itself and the NOR gate 308. Table 6 for the NOR-gate flip-flop shows that this negative input to the NOR gate 307 cannot affect the state of the flip-flop as initially set by the resistors 309 and 310 and the capacitors 311 and 312 to provide a negative input to the base of the transistor 313. This negative input to the transistor 313 does not allow current to pass through it or the incandescent bulb 314 which, accordingly, remains off.

When the transistor 301 turns on, the capacitor 302 begins to charge, drawing current through the coil 305 of a reed relay. The current passes through the coil 305 closing the switch portion 306 of the reed relay.

When the switch 306 closes and with a proper ground connection, a potential drop of 120 V. a. c. exists across the resistors 320 through 323. The portion of this potential appearing across the resistor 320 passes through the diode 324 to charge capacitor 325. This charging of the capacitor 325 raises the lower input to the NOR gate 306 from the negative to the positive state and, accordingly, the output of the NOR gate 306 stays in the negative state.

As the capacitor 302 charges, so do the capacitors 326 and 327. A proper choice of these capacitors, as well as the resistor 304, will cause the capacitors to charge more slowly than the capacitor 325. Upon charging, they raise the input to the NOR gate 305 from the negative to the positive causing the NOR gate 305 to provide a negative input to the NOR gate 306. However, the negative input to NOR gate 306 from the NOR gate 305 does not induce a positive output from the former; the capacitor 325 previously charged sufficiently to provide the NOR gate 306 with at least one positive input. The output of the NOR gate 307 remains positive and the transistor 313 and the bulb 314 remain off.

As the capacitor 302 reaches its limit of charge, the current diminishes in the coil 305, opening switch 306. However, the capacitor 325 stands almost isolated in the circuit at this point because of the diode 324. Accordingly, it retains its charge while the capacitors 325 and 326 discharge through the resistor 304 after the transistor 301 turns off. Accordingly, the input to the NOR gate 305 goes negative and its output becomes positive causing the output of the NOR gate 306 to remain in the negative state and bulb 314 not to light.

If the circuit has no connection to ground, then closing the switch 306, of course, does not allow the establishment of a potential drop across resistor 320. Similarly, should the wiring in the 120 V. supply line become reversed, then the upper connection 330 of the resistor 320 will no longer connect to the 120 V. line. Rather it will connect to the common line of the power supply and have a voltage near the ground. Thus, without a ground, or with reversed polarity in the a.c. supply, no potential develops across the resistor 320 and the capacitor 325 cannot charge. Consequently, the lower input to the NOR gate 306 remains negative.

Nonetheless, the capacitor 302 still charges when the transistor 301 conducts. This allows the capacitors 326 and 327 to charge and provide a positive input to the NOR gate 305 which consequently provides a negative input to the NOR gate 306. At this point, with either missing ground or reversed polarity, the NOR gate 306 has two negative inputs and provides a positive output to the flip-flop consisting of the NOR gates 307 and 308. As shown in Table 6, this resets that flip-flop so that the NOR gate 307 which previously had a positive output now provides a negative output to the base of the transistor 313. This turns on the transistor 313 and allows current to flow through it and through the bulb 314, lighting it. This lit bulb indicates that some problem exists in the wiring.

Moreover, the bulb 314 remains lit after the circuit has concluded the test. Providing a positive input to the transistor 313 represents the only way to turn off the bulb 314. This requires the flip-flop to produce a positive output from the NOR gate 307. Only a positive input to the NOR gate 308 will properly reset the flip-flop as required. However, this input remains permanently connected to the negative common line and, accordingly, cannot reset the flip-flop to turn off the bulb 314.

Thus, upon the amelioration of whatever difficulty caused the bulb to light, it can turn off only by unplugging and reconnecting the entire circuit with the power supply. When connected to the power, the resistor 309 and capacitor 311 draw the output of the NOR gate 307 to the positive state provided by the potential V.sub.D. Simultaneously, the resistor 310 and the capacitor 312 pull the output of the NOR gate 308 to the negative state. In view of the interconnections between the two NOR gates, these states properly set the flip-flop to keep the transistor 313 turned off.

After the transistor 301 becomes nonconducting, the coil 305 retains an appreciable amount of stored energy. The diode 330 dissipates this energy by allowing current to flow in the reverse direction than it has during the charging of the capacitor 302.

The Underwriters Laboratories high-potential test, discussed above, imposes 1,240 V. a.c. between ground and the 120 V. line. In FIG. 4, this places 1,240 V. across the resistor 320 and charges the capacitor 325 to levels that could damage many of the circuit components. The Zener diode 331 limits the potential across the resistor 320 to its breakdown voltage, frequently on the order 14 V.

The switch 306 remains closed only while appreciable current flows through the coil 305. Further, the capacitor 302 charges once for each operation of a bed motor and only for a small fraction of a second. Accordingly, only small intermittent pulses of electricity enter the ground system. This compares favorably with most ground-testing devices which frequently place a constant milliamp of current into the ground system. In a hospital having several hundred beds, this represents a potentially dangerous amount of electricity on a supposedly safe line.

Because of the flexibility and versatility of logic electronic components, other circuits could effectively accomplish the same or similar results as those in FIGS. 2, 3 and 4. However, the circuits in those figures have, in fact, performed the desired tests. The components used in them appear in the following Table 10.

Table 10 ______________________________________ Components Used in the Figures Component Number Identification ______________________________________ 33 1/16 A., 250 V. 34 290-12021 41, 51 FWB, 1 A., 50 V. 42, 52 330.mu.F., 25 V. 54 3.OMEGA., 1.5 W. 55 5 V., 1/2 A., TO-220 56 100.OMEGA., 1 W. 130, 131, 281 4.7 K.OMEGA. 136, 139, 164, 278, 1N4148 279, 324, 330 156 10 A., 400 V., SC146D-5. 157 82.OMEGA. 158 R 4534-1 181, 191, 203, 206, 7404 P.C. 243, 244, 250, 262 182, 184, 185, 192- 7402 P.C. 196, 201, 202, 204, 207-213, 245, 260, 261, 263 183, 186, 197, 198 7400 P. C. 208, 214, 2115, 241, 242, 246, 247, 251 220 0.1 .mu. F., 25 V. 221 1 K.OMEGA. 222 3.3 K.OMEGA. 223, 225 330.OMEGA. 224, 226, 302 100.mu.F. 271-274 2.7 M.OMEGA. 275 10 M.OMEGA. 276 2N5484 277 2N5142 280 470.OMEGA. 300 1N4001 301 2N3906 303, 309, 310 10 K.OMEGA. 304 2.2 K.OMEGA. 305-308 4001 311, 312, 325-327 .01.mu.F. 313 2N3906 314 14 V. 320 240 K.OMEGA. 321-323 1 M.OMEGA. 331 14 V. d.c., 5%, 1 W. ______________________________________

Claims

Accordingly, what is claimed is:

1. In an adjustable bed of the type having:

a. a bed portion movable to a plurality of at least three positions, and

b. an electric motor for moving said bed portion from one of said positions to another,

the improvement which comprises:

A. first selection means for selecting between at least two of said positions of said bed portion;

B. second selection means for selecting between at least two of said positions of said bed portion, at least one of said positions selectable by said first selection means not being selectable by said second selection means; and

C. logic means coupled between said electric motor and said first and second selection means for actuating said motor in response to said first or said second selection means to move said bed portion to a selected position.

2. The improvement of claim 1 wherein said bed portion has a continuous range of positions with all of the positions within said range being selectable by said first selection means and a portion of said range of positions being selectable by said second selection means.

3. The improvement of claim 2 wherein, said bed portion being a first bed portion and said electric motor being a first electric motor,

a. said bed includes a second bed portion movable over a continuous range of positions and a second electric motor for moving said second bed portion from one of said positions of said second bed portion to another; and

b. said logic means in response to said second selection means actuates said second motor to move said second bed portion.

4. The improvement of claim 3 wherein said logic means is electronic logic means.

5. The improvement of claim 3 wherein said first bed portion is a knee portion, said second bed portion is a head portion and said second selection means actuates said first electric motor to raise said knee portion to no higher than about 15.degree. and actuates said first motor to lower said knee portion when said head portion is at or below about 30.degree..

6. The improvement of claim 5 wherein said first and said second selection means each includes at least one manually activated on-off switch and a bed portion position-indicating switch.

7. The improvement of claim 3 wherein said electronic logic means includes switching means for controlling the flow of electricity through said electric motors, and wherein both said selection means and said logic means, with the exception of said switching means, only include components operating at voltages below about 25 volts.

8. The improvement of claim 7 wherein said switching means includes triacs coupled to said electric motors and reed relays coupled to said triacs.

9. The improvement of claim 7 including ground test means.

10. The improvement of claim 7 wherein said electronic logic means, with the exception of said switching means, includes components selected from the group consisting of inverters, and AND, NAND, OR, and NOR gates.

11. The improvement of claim 7 in a bed which includes a bed elevation portion, an elevation motor for moving said elevation portion, and an elevation selection means coupled to said logic means, said logic means, in response to said elevation selection means, actuating said elevation motor to move said elevation bed portion.

12. The improvement of claim 11 further including a bed-flat selection means coupled to said logic means, said logic means in response to said bed-flat selection means actuating said head motor to lower said head portion, said knee motor to lower said knee portion, and said elevation motor to raise said elevation portion.

13. The improvement of claim 3 wherein said first and second motors being reversible, said logic means includes ambiguity control means for precluding the actuation of said first or said second motor when said logic means in response to selections on said selection means would actuate said first or said second motor respectively to operate simultaneously in both its forward and reverse directions.

14. The improvement of claim 3 wherein said first and second motors being reversible, said selection means includes indicating means for developing an indication of when said first or second bed portions has reached the end of its range of positions and said logic means includes avoidance means for precluding the actuation of said first or second motors, respectively, except in a direction to move said first or second bed portions respectively, away from the end of its range of positions.

15. The improvement of claim 2 including lockout means for precluding actuation of said motor in response to the selection on either of said selection means.

16. In an adjustable bed of the type having:

a. first and second bed portions, each of said bed portions being movable to a plurality of positions; and

b. an electric motor for moving said first bed portion from one position to another;

the improvement comprising:

A. selection means for selecting between said positions of said first bed portion;

B. indicating means for developing an indication of whether the position of said second bed portion is in a first or in a second set of positions, said first and second sets of positions (a) being mutually exclusive, (b) including all the positions of said second bed portion, and (c) each containing at least one position; and

C. logic means coupled between said electric motor and said selection means and said indicating means for actuating said motor in response to said selection means to move said first bed portion to a selected position only when the position of said second bed portion is in said first set of positions.

17. The improvement of claim 16 wherein said first bed portion has a continuous range of positions.

18. The improvement of claim 17 wherein said second bed portion has a continuous range of positions.

19. The improvement of claim 18 wherein said second bed portion is the head portion of the bed and said first set of positions includes those with elevation of 30.degree. or less and said second set of positions includes all those with elevation above 30.degree..

20. The improvement of claim 19 wherein said selection means includes on-off switches and bed portion position-indicating switches and said indicating means includes position-indicating switches.

21. The improvement of claim 19 which includes lockout means for precluding actuation of said motor in response to the selection of a position on said selection means.

22. The improvement of claim 19 wherein said logic means includes switching means for controlling the flow of electricity through said electric motor and wherein said selection means and said logic means, with the exception of said switching means, includes only components operating at voltages below about 25 volts.

23. The improvement of claim 22 wherein said switching means includes triacs coupled to said motors and read realys coupled to said triacs.

24. The improvement of claim 23 wherein said logic means includes components selected from the group consisting of inverters, and AND, OR, NAND, and NOR gates.

25. The improvement of claim 19 wherein, said motor being a reversible motor, said logic means includes ambiguity control means for precluding the actuation of said motor when said logic means in response to selections on said selection means would actuate said motor to operate in both its forward and reverse directions.

26. The improvement of claim 19 including ground test means.

27. The improvement of claim 19 which, said selection means being a first selection means, includes a second selection means coupled to said logic means, said logic means actuating said motor in response to said second selection means independently of the position of said second bed portion.

28. The improvement of claim 19 in a bed which, said motor being a first motor, includes a second reversible electric motor coupled to said logic means wherein said second motor operates to move said second bed portion between its positions and wherein said logic means, in response to a selection on said first selection means, actuates said second motor to move said second bed portion.

29. The improvement of claim 28 wherein said first bed portion is the knee portion of said bed, said second bed portion is the head portion of said bed, and said bed includes an elevation portion, a reversible elevation motor coupled to said logic means, and elevation selection means coupled to said logic means, said logic means, in response to a selection on said elevation selection means, actuating said elevation motor to move said elevation bed portion over a continuous range of positions.

30. The improvement of claim 29 including a bed-flat selection means coupled to said logic means, said logic means, in response to a selection on said bed-flat selection means, actuating said head motor to lower said head portion, said knee motor to lower said knee portion, and said elevation motor to raise said elevation portion.

31. The improvement of claim 29 including

a. indicating selection means for developing an indication of when said head portion reaches an end of its range of positions, when said knee portion reaches an end of its range of positions and when said elevation portion reaches an end of its range of positions, and

b. logic avoidance means for, when said head portion is at an end ot its range of positions, precluding actuation of said head motor except in the direction to move said head portion away from said end of its range of positions; when said knee portion is at an end of its range of positions, precluding actuation of said knee motor except in the direction to move said knee portion away from said end of its range of positions; and, when said elevation portion is at an end of its range of positions, precluding actuation of said elevation motor except in the direction to move said elevation portion away from said end of its range of positions.

32. In an adjustable bed of the type having:

a. a first and a second bed portion, said first and second bed portions being movable to a plurality of positions; and

b. a first and a second electric motor for moving said first and second bed portions respectively from one position to another,

the improvement which comprises:

A. selection means for selecting between the position of said first bed portion; and

B. logic means coupled between said selection means and said first and second motors for actuating, in response to said selection means, said first and second motors to move said first and second bed portions, respectively.

33. The improvement of claim 32 wherein said logic means actuates said first and second motors sequentially.

34. The improvement of claim 32 wherein both said first and said second bed portions have a continuous range of positions; said selection means selects a direction of movement of said first bed portion; said first and second motors are reversible; and said logic means, in response to said selection means, actuates said first motor to move said first bed portion in the selected direction

35. The improvement of claim 34 which, said selection means being a first selection means, includes a second selection means coupled to said logic means, said logic means, in response to a selection on said second selection means, actuating said first but not said second electric motor.

36. The improvement of claim 35 wherein:

a. said first bed portion is the head portion of said bed and said first motor is a head motor;

b. said second bed portion is the knee portion of said bed and said second motor is a knee motor; and

c. said logic means in response to a selection on said first selection means to lower said head portion, actuates said knee motor to lower said knee portion when said head portion is elevated 30.degree. or less, and said logic means in response to a selection on said first selection means to raise said head portion actuates said knee motor to raise said knee portion to no higher than 15.degree. elevation.

37. The improvement of claim 35 wherein said logic means is electronic logic means.

38. The improvement of claim 37 wherein said first and second selection means includes on-off switches and bed portion position-indicating switches.

39. The improvement of claim 35 including first lockout means for precluding actuation of said first motor in response to said first selection means and second lockout means for precluding actuation of said second motor in response to said second selection means.

40. The improvement of claim 35 in which said logic means includes switching means for controlling the flow of electricity through said first and second electric motors and wherein said first and second selection means and said logic means, with the exception of said switching means, includes only components operating at voltages below about 25 volts.

41. The improvement of claim 40 wherein said switching means includes triacs coupled to said motors and reed relays coupled to said triacs.

42. The improvement of claim 41 wherein said electronic logic means, with the exception of said switching means, includes components selected from the group consisting of inverters and NAND, AND, NOR and OR gates.

43. The improvement of claim 35 wherein said logic means includes ambiguity control means for precluding the actuation of any particular motor when selections on said selection means would result in the actuation of said particular motor in both its forward and reverse directions.

44. The improvement of claim 35 including ground test means.

45. The improvement of claim 35 wherein said bed includes an elevation portion with a continuous range of positions, a reversible elevation motor coupled to said logic means to raise and lower said elevation portion; and elevation selection means for choosing a direction of motion of said elevation portion, said logic means in response to a selection on said elevation selection means actuating said elevation motor to move said elevation portion in the direction chosen.

46. The improvement of claim 45 including bed-flat selection means, said logic means in response to said bed-flat selection means actuating said head motor to lower said head portion said knee motor to lower said knee portion and said elevation motor to raise said elevation portion.

47. The improvement of claim 45 including

a. indicating selection means for developing an indication of when said head portion reaches an end of its range of positions, when said knee portion reaches an end of its range of positions and when said elevation portion reaches an end of its range of positions, and

b. logic avoidance means for, when said head portion is at an end of its range of positions, precluding actuation of said head motor except in the direction to move said head portion away from said end of its range of positions; when said knee portion is at an end of its range of positions, precluding actuation of said knee motor except in the direction to move said knee portion away from said end of its range of positions; and, when said elevation portion is at an end of its range of positions, precluding actuation of said elevation motor except in the direction to move said elevation portion away from said end of its range of positions.

48. In an adjustable bed of the type having:

a. a bed portion movable to a plurality of positions; and

b. a reversible electric motor for moving said bed portion from one of said positions to another,

this improvement comprising:

A. selection means for selecting between said position of said bed portion;

B. indicating means for developing an indication of when said bed portion is at a predesignated position; and

C. logic means coupled between said motor and said selection means and indicating means for, when said bed portion is not in said predetermined position and in response to said selection means:

1. actuating said motor to operate in a first direction to move said bed portion to said predetermined position, and

2. when said bed portion reaches said predetermined position, reversing said motor to operate in a second direction.

49. The improvement of claim 48 wherein, when said bed portion is in said predetermined position prior to a selection on selection means, said logic means, in response to said selection means, actuates said motor to operate in said second direction.

50. The improvement of claim 49 wherein said bed portion has a continuous range of positions, said predetermined position is at one end of said range of positions, and said indicating means is a limit switch actuated when said bed portion is in said predetermined position.

51. The improvement of claim 50 wherein the operation of said motor in said second direction in response to said selection means places the top of said bed at a non-zero angle with respect to the surface on which it sets.

52. The improvement of claim 51 wherein said motor operating in said first direction raises said bed; said improvement further includes alternate selection means coupled to said logic means; and said logic means, in response to said alternate selection means, actuates said motor to lower said bed.

53. The improvement of claim 52 in a bed which further includes a movable head portion with a continuous range of positions; a movable knee portion with a continuous range of positions; a reversible head motor coupled to said logic means and connected to said head portion for moving said head portion over its range of positions; a reversible knee motor coupled to said logic means and connected to said knee portion for moving said knee portion over its range of positions; knee selection means coupled to said logic means; and head selection means coupled to said logic means, said logic means, in response to said knee selection means actuating said knee motor to move said knee portion and in response to said head selection means, actuating said head motor to move said head portion.

54. In an adjustable bed of the type having:

a. a head portion, a knee portion, and an elevation portion, each of said portions being movable over a continuous range of positions;

b. a reversible head motor for moving said head portion, a reversible knee motor for moving said knee portion, and a reversible elevation motor for moving said elevation portion; and

c. head selection means for choosing the direction of movement of said head portion, knee selection means for choosing the direction of movement of said knee portion, and elevation selection means for choosing the direction of movement of said elevation portion,

the improvement which comprises electronic logic means coupled between said head, knee, and elevation motors and said head, knee and elevation selection means for:

A. actuating said elevation motor when a direction of movement of said elevation portion is chosen on said elevation selection means to move said elevation portion in the chosen direction;

B. actuating said knee motor when a direction of movement of said knee portion is chosen on said knee selection means to move said knee portion in the chosen direction; and

C.

1. when the raising of said head portion is chosen on said head selection means, actuating said head motor to raise said portion and, when said knee portion is in a predesignated portion of its range of movement, actuating said knee motor to raise said knee portion; and

2. when the lowering of said head portion is chosen on said head selection means, actuating said head motor to lower said head portion and, when said head portion is in a predetermined portion of its range of movement, actuating said knee motor to lower said knee portion.

55. The improvement of claim 54 which includes a bed-flat selection means wherein said logic means upon the activation of said bed-flat selection means actuates said head and knee motors to move said head and knee portions to their lowest positions and said elevation motor to move said elevation portion to its highest position.

56. The improvement of claim 55 which further includes Trendelenberg selection means wherein said logic means in response to the activation of said Trendelenberg selection means actuates said elevation motor to place the top of said bed at a non-zero angle with respect to the surface on which said bed sets.

57. The improvement of claim 56 wherein said logic means:

a. when the raising of said head portion is chosen on said head selection means,

1. actuates said knee motor to raise said knee portion when said knee portion is below 15.degree. and

2. ceases to actuate said knee motor at least when said knee portion raises to 15.degree.; and

b. when the lowering of said head portion is chosen on said head selection means, actuates said knee motor to lower said knee portion when said head portion is 30.degree. or lower.

58. The improvement of claim 57 which includes head lockout means for precluding actuation of said head motor in response to the selection of a direction on said head selection means, elevation lock-out means for precluding actuating of said elevation motor in response to a direction selected on said elevation selection means, and knee lock-out means for precluding actuation of said knee motor in response to the selection of a direction on said knee selection means.

59. The improvement of claim 58 wherein said logic means includes switching means for controlling the flow of electricity through said electric motors and wherein said selection means and said logic means, with the exception of said switching means, only includes components operating at voltages below about 25 volts.

60. The improvement of claim 59 wherein said logic means includes ambiguity control means for precluding the actuation of any particular motor if said logic means in response to a first selection made on said selection means would actuate said particular motor in a first direction and in response a second selection made on said selection means would actuate said particular motor in the reverse of said first direction.

61. The improvement of claim 60 wherein each of said selection means includes at least one manual on-off switch and one bed portion position-indicating switch.

62. The improvement of claim 61 wherein said elevation selection means includes

a. a first manual on-off switch and said logic means actuates said elevation motor to raise said elevation bed portion only during the time of a selection being made on said first switch, and

b. a second manual on-off switch and said logic means actuates said elevation motor to lower said elevation portion from the time of a selection being made on said second switch until said elevation portion reaches the lower limit of its range of positions or until the making of a selection on any manual switch in response to which said logic means would actuate said elevation motor to raise said elevation bed portion.

63. The improvement of claim 62 including:

a. indicating selection means for developing an indication of when said head portion reaches an end of its range of positions, when said knee portion reaches an end of its range of positions and when said elevation portion reaches an end of its range of positions: and

b. logic avoidance means for when said head portion is at an end of its range of positions, precluding actuation of said head motor except in the direction to move said head portion away from said end of its range of positions; when said knee portion is at an end of its range of positions, precluding actuation of said knee motor except in the direction to move said knee portion away from said end of its range of positions; and, when said elevation portion is at an end of its range of positions, precluding actuation of said elevation motor except in the direction to move said elevation portion away from said end of its range of positions.

64. The improvement of claim 63 wherein said logic means with the exception of said switching means includes components selected from the group consisting of inverters and AND, OR, NAND and NOR gates.

65. The improvement of claim 64 wherein said switching means includes reed relays coupled to said gates and triacs coupled between said reed relays and said motors.

66. The improvement of claim 65 including ground test means