Automatic Orientation System

Abstract

Disclosed is an automatic orientation system for structures. Utilized in the system is an electrical control circuit which selectively operates the proper lift device in response to predetermined signals from an orientation sensor. Provided is an override circuit permitting orientation at various heights, and a storage circuit to move the lift devices out of position when not in use.

Description

BACKGROUND OF THE INVENTION

This invention relates to equipment for the orientation of structures, and more particularly, to equipment for automatically leveling vehicles such as camping and house trailers.

Certain types of land vehicles, including for example, camping trailers and self-contained motor homes, generally must be leveled after having been moved to a location at which they are to be occupied. Leveling may be desired for a number of reasons, depending upon the use to which the vehicle will be put. For example, a complex mobile laboratory may require leveling to assure proper operation, and a house trailer or camper is leveled for proper operation of the refrigerator and plumbing, and for the comfort and convenience of the occupants. Usually, orientation operations are performed by hand. Manual leveling of any large vehicle however, is a very laborious, time consuming, trial and error process. Furthermore, camping trailers are often moved, so the leveling operation must be performed repeatedly. A fast, automatic system, therefore, is highly desirable.

Hydraulic automatic leveling system are known, but the use of hydraulic control for such apparatus has many limitations, and therefore such systems have not received wide acceptance. The size and weight of hydraulic systems make them inconvenient for use on vehicles which often must travel over poor roads or rough terrain. Also, the bulky and cumbersome pipes used to connect the control unit to leveling jacks prevent easy retraction thereof.

The object of this invention, therefore, is to provide an automatic orientation system for land vehicles which is inexpensive to build, can be put into operation quickly, and does not appreciably increase the weight of the vehicle. A further object is to provide a system as described above in which the leveling jacks may be moved out of position when not in use.

SUMMARY OF THE INVENTION

This invention is characterized by an automatic orientation system for land vehicles including a plurality of orientation jacks and a power source for their operation. All control operations are performed from one central location, and the control system couples an orientation sensor to the jacks in such a manner that the proper jack or jacks will be automatically actuated until correct orientation is achieved.

One feature of this invention is the utilization of an electric control system. Among the advantages realized by the use of an electric system are light weight and the freedom to use a small and light sensing system. A small sensor is sufficient because only switch contacts must be actuated, rather than the large, bulky valves, which are required for hydraulic systems. In addition to the important advantage of reduced weight, a light sensing unit has the further advantage of being easily supported. A light weight sensor may be mounted with an adjustable support, thereby permitting the relative orientation of the vehicle to be easily changed. Also, since an electrical sensor will operate with a relatively short lever arm, it can take any of several forms. Examples include a small, light weight vertically disposed pendulum, and a small planar disc, centrally supported and held horizontally by gravitational force.

Another feature of this invention is the use of relatively high speed electric motors coupled through gear reduction trains to the jacks. The gear trains function as brakes and separate brakes on each jack are therefore not needed.

Yet another feature of this invention is the incorporation of initiation and automatic switching circuits in the control mechanism, and thrust sensors in the jacks. Upon actuation of the initiation circuit, all jacks simultaneously lower until reaching the ground. Then, upon receipt of a proper signal from the thrust sensors, the switching circuit disconnects the initiation circuit and automatic orientation is begun.

Another feature of the invention is the use of a storage system which swings the jacks into horizontal positions when the vehicle is not in use. This is advantageous since vehicles upon which this system will be used are often required to travel off roads or on unimproved roads. Movement will therefore be facilitated if all parts of the vehicle are as high above the ground as possible.

Still another feature of the invention is the inclusion of an override circuit with which the jacks may be independently actuated and the automatic orientation circuit bypassed. Any one jack can be set at the final height desired, then the trailer will be leveled at that height. This circuit is also advantageous in that there are times when unusual orientations of the vehicle are desired for short periods of time, yet adjustments or the orientation sensing device would be inconvenient. In the event of a malfunction of the sensor or orientation circuit, proper orientation can still be attained by use of the override circuit.

DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become more apparent upon an examination of the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a perspective view of a camping trailer equipped with the disclosed automatic orientation system;

FIG. 2 is a detail and partial cross-sectional view of a jack and related coupling mechanism employed by the system;

FIG. 3 is a cross-sectional view of the orientation sensor used in a preferred embodiment of the system;

FIG. 4 is a schematic illustration of a storage mechanism used in the system; and

FIGS. 5-8 show a diagram of the control circuit employed in the system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1 there is shown a trailer 11 equipped with an automatic orientation system 12 as disclosed herein. In the interest of clarity, a body 13 of the trailer 11 has been shown in outline form only, and attention is directed to a frame 14' thereof. Mounted on the frame 14' are a pair of brackets 14 that retain four orientation jacks 15, 16, 17 and 18 supported by orientation jack mounts 19, 20, 21 and 22, respectively. Also mounted on the frame 14' is a central control housing 24 and a storage motor assembly 25, the latter being coupled to the orientation jacks 15-18 by storage cables which are described hereinafter. An orientation sensor 26 is mounted below a floor 27 of the trailer 11, a portion of which is shown. Connecting the jacks 15-18, the central control unit 24, the storage motor 25 and the orientation sensor 26 are electrical cables, also described hereinafter. Contained within the central control housing 24 is a portion of a control circuit 28, comprising an override circuit 23, an orientation circuit 29, an initiation circuit 30, a storage control circuit 31, and a switching circuit 32. The control circuit 28 is shown in FIGS. 5-8, and described below. Contained within the orientation jacks 15-18 and their mountings 19-23 are retraction sensors 33 and force sensors 34, described below.

Referring now to FIG. 2 there is shown a detail and partial section of the jack 15 and jack mount 19. It will be understood that the other orientation jacks 16, 17, 18 are constructed and mounted similarly. A motor assembly 35 is coupled to an acme screw 36 by an enclosed gear system. Meshed with the teeth of the screw 36 is a bronze nut 37 which is firmly secured to an inner ram 38, both of which are disposed inside an outer shaft 39 in such a manner that they cannot rotate therein. The lower end of the ram 38 protrudes from the lower end of the shaft 39, and is connected to a self-adjusting foot assembly 41. Such self-adjusting feet 41 are well known and are not considered part of the invention. An expanding rubber sleeve 42 extends from the lower end of the shaft 39 to the foot 41, covering and protecting the lower end of the ram 38. Jacks of this type are conventional and well known. One feature which makes this type of jack preferable in the present system is that no brake is required, and the jack will not slip.

Disposed in the jack 15 is the retraction sensor 33, constructed as follows. Retained in switch cavity 43 is a switch 44, which is connected with the storage circuit 31. A tapered portion 45 on the upper end of the nut 37 operates through an actuation pin 46 to control the switch 44 as the nut moves vertically.

Extending upwardly from the top of the jack 15 are two lugs 47 aligned one behind the other so that they appear as one in FIG. 2. Extending downwardly from a mounting block 48 are two more lugs 49, also aligned. Lugs 47 and 49 are disposed closely adjacent to each other and secured together by a pivot rod 51. A bell crank 52 is mounted on the end of the rod 51 with a pin 53. The rod 51 which may rotate with respect to lugs 49, is fixed with respect to lugs 47. Attached to the end of the bell crank 52 is a storage control table 54 which is connected to the storage motor mechanism 25.

Mounting block 48 is secured to the bracket 14 by elongated screws 55 which have one sense of freedom, thereby allowing slight vertical movement of the mounting block. Disposed between the mounting block 48 and the bracket 14 is a conically shaped disc spring 56, which resists vertical movement of the jack 15. Above the mounting block 48 is the thrust sensor 34. Extending upward from the mounting block 48 into a cavity 57 is a tapered contact actuator 58. Disposed in a switch cavity 59 in the bracket 14 is a switch 61 which is connected to the initiation circuit 30 and the switching circuit 32. Enclosed in a channel 62 between the actuator 58 and the switch 61 are four actuator spheres 63. When the biasing force exhibited by the disc spring 56 is exceeded, and the actuator 58 moves upward, the spheres 63 are forced to the left and operate the switch 61.

Referring now to FIG. 3, there is shown a section of the orientation sensor 26 mounted on the underside of the floor 27. A mounting plate 64 is secured to the floor by screws 65. Screws 66 attach a frame 67 to the mounting plate 64 and disposed therebetween and around the screws are rubber washers 68. Adjustments in the position of the sensor 26 can be made by manipulation of the screws 66, which causes compression of the rubber washers 68 and changes in angle between the frame 67 and the mounting plate 64. A socket 69 retaining a ball 71 is held in place in the frame 67 by a ring 72. A lead wire 73 is connected between the top of the conductive ring 72 and the orientation circuit 29. Connecting the ball 71 with a metal disc 74 and a weight 75 is a short lever arm 76. Retaining four annularly disposed contacts 77-77c is a cap 78. Since the ball 71 and lever arm 76 are electrically conductive, the edges 79 of the disc 74 are electrically connected to the wire 73 and serve as contacts, thereby forming four switches 81-81c. The frame 67 and cap 78 are molded of non-conductive material. Holding the weight 75 and disc 74 in position is a plunger 82 of a solenoid 83. When the solenoid 83 is energized, the plunger 82 moves in a downward direction releasing the weight 75 and disc 74, which then assume a horizontal position in response to gravitational force. If the floor 27 is substantially level, all switches 81-81c remain open; but if the floor is not level, the switch in the uppermost position will close. Protecting the entire assembly 26 is a cover 84.

Referring next to FIG. 4 there is shown a bottom view of a jack storage control mechanism mounted on the frame 14'. Mounted near the center of the frame 14' is the storage motor assembly 25 which includes a motor 85 and gear train enclosed in a housing 86. Through the gear train, the motor 85 is connected to a storage disc 87 on which are mounted two disc pulleys 88 and two switch actuators 89 and 91. Switches 92 and 93 are controlled by the actuators 89 and 91 respectively and connected to the storage circuit 31. Connecting the bellcrank 52 with a bellcrank 52a is the storage cable 54, and connecting a bellcrank 52b with another bellcrank 52c is another storage cable 94. Between bellcrank 52 and bellcrank 52a, cable 54 passes around two frame pulleys 96 and an end pulley 97. Cable 94 passes through corresponding pulleys. Connecting end pulleys 97 is a central cable 98, which is wrapped around disc pulleys 88 and two disc guide pulleys 99.

Referring next to FIGS. 5-8, there is shown a schematic diagram of the control circuit 28 contained in the housing 24. Connecting a battery 101 with a pilot light 102 and the control circuit 28 is an on-off switch 103. Completing the complement of switches in the control circuit 28 are four override switches 104, 105, 106 and 107, an initiation-orientation switch 108 and a jack storage control switch 109. Override switches 104-107 are single pole-single throw, and the initiation-orientation switch 108 and the storage control switch 109 are single pole-double throw types. One contact of each of the override switches 104-107 and the center contacts 110, 111 of switches 108, 109, respectively, are connected to a common positive buss 112. Connecting the control circuit 28 with the left rear jack assembly 15 are wires 113 thru 120. Switches 44 and 61 are of the type known as double pole-double throw and each have two common contacts 121, 122 and 123, 124, respectively, and associated contacts 125, 126 and 127, 128 on switch 44 and 129, 130 and 131, 132 on switch 61. The motor assembly 35 has three terminals including a common negative 133, a positive terminal 134 to be energized when upward motion is desired and another positive terminal 135 to be energized when downward motion is desired. Jacks 16 thru 18 are constructed and wired similarly, and corresponding contacts and wires are numbered similarly except that for jack 16 numbers are suffixed with an a, for jack 17 with a b and for jack 18 with a c.

Connecting the orientation sensor 26 and control circuit 28 are wires 73 and 136 thru 140. Switches 81-81c include, respectively, contacts 77-77c and common contact 79. Solenoid 83 comprises a positive terminal 141 and a negative terminal 142.

Storage motor assembly 25 is connected to the control circuit 28 by wires 143 thru 147. Motor 85 has three terminals including a common negative terminal 148 and two positive terminals 149 and 151 one of which must be energized when upward or downward motion, respectively, is desired.

Returning now to the control circuit 28, there are shown contacts 152 and 153 of switch 108 and contacts 154 and 155 of switch 109. A relay 156, which is connected to the switching circuit 32, is a single pole-double throw type, and has a common contact 157 and relay contacts 158 and 159. A relay coil 161 has a positive terminal 162 and a negative terminal 163.

During operation of the preferred embodiment, the vehicle, for example the trailer 11 shown in FIG. 1, is first parked in a desired location. Switch 103 is then turned to the "on" positive igniting lamp 102 and energizing positive common buss 112 and applying negative potential to wires 117a, 119-119c, 136 and 143. Negative potential is therefore applied to terminals 133-133c, 142 and 148. The jacks 15-18, which during movement of the trailer 11 were retained in an inactive position, parallel to the floor 27, must first be moved to active position. With the jacks 15-18 in the inactive position, switches 92 and 93 are as shown by the broken line in FIG. 5; that is, switch 92 is closed and switch 93 is open. Switch 109 is placed in the "down" position connecting contacts 111 and 154 thereby energizing the storage circuit 31 by applying positive potential to wire 145, which passes through switch 92 to terminal 151, energizing motor 85. Operation of motor 85 causes the storage disc 87, shown in FIG. 4, to rotate in a counterclockwise direction allowing end pulleys 97 to move away from the storage motor assembly 25. The effective lengths of cables 54 and 94 between frame pulleys 96 and bell cranks 52-52c are increased, causing rods 51 to rotate and thereby allowing jacks 15-18 to lower by gravitational force, into an active vertical position. As the jacks 15-18 assume the active positions, the storage disc 87 reaches a position which causes actuator 89 to contact and throw switch 92. Positive potential is disconnected from terminal 151 when switch 92 is opened thereby stopping motor 85. Since actuator 91 has now moved away from switch 93, the present position of switches 92 and 93 is shown by the solid lines in FIG. 5. Switch 109 is now returned to the neutral position. All jacks 15-18 are now in the active, i.e. vertical, position, but none is in contact with the ground.

Next, the initiation circuit 30 is actuated by placing switch 108 in the "down" position, thereby connecting contacts 110 and 152 and applying positive potential to contacts 157 and 162 of the relay 156. Since none of the jacks 15-18 is in contact with the ground, thrust sensors 34-34c are not actuated and switches 61-61c are as shown by the solid lines in FIG. 5. Relay contact 163 is connected to contact 123b by wire 117b. Since contact 123b is connected to dummy contact 130b, relay 156 does not change state when switch 108 is thrown. Positive potential is applied to contact 158 and thereby to wires 116-116c, which are connected with terminals 135-135c through contact pairs 124 and 132, 124a and 132a, 124b and 132b, and 124c and 132c. Motors 35-35c are therefore energized to operate all jacks 15-18. For example, in jack 15 the acme screw 36 (shown in FIG. 2) is rotated causing the bronze nut 37 and inner ram 38 to move in a downward direction. When any jack, for example 15, contacts the ground and the resistance of the disc spring 56 overcome, the mounting plate 48 and actuator 58 are driven in an upward direction moving switch 61. This action can be initiated at any desired force level by selecting a disc spring 56 having suitable characteristics. Actuation of switch 61 moves contact 124 from 132 to contact 131, stopping motor 35. Contact 123 is moved to engage contact 129 as shown by the dashed lines in FIG. 5.

When the fourth jack contacts the ground, the jack motors 35a-35c are similarly deenergized. Also the contact pairs 123 and 129, 123a and 129a, 123b and 129b, and 123c and 129c have been closed completing the switching circuit 32, comprising wire 117a, contacts 123a, 129a, wires 118a, 117, contacts 123, 129 wires 118, 118c, contacts 129c, 123c, wires 117c, 118b, contacts 129b, 123b, and wire 117b which applies negative potential to relay contact 163, thereby causing relay 156 to change state.

The hereinbefore described change of state of relay 156 energizes the orientation circuit 29 by connecting contacts 157 and 159 and applying positive potential to wire 73, which is connected to contact 79 and solenoid terminal 141. Solenoid 83 is thereby energized, and draws plunger 82 down into the position shown by the dashed lines in FIG. 3. The disc 74 is nowreleased and will seek a position which is level, and if the sensor frame 67 is not level, contact 79 will operatively engage a contact 77-77c completing a circuit. The degree of disorientation required to actuate the sensor 26 can be varied according to the construction of the sensor, but magnitudes of one degree are typical. The contact which will be connected with edge 79 is the one which is highest. Assume, for example, that the front right corner of the trailer 11 is low. The sensor 26 is in such a position that contact 77b will be nearest to the rear left corner and therefore the highest. In this situation, contact 79 will engage contact 77b applying positive potential to wire 139 which is connected with wire 115b and in turn to terminal 135b. Motor 35b is thereby energized lifting the right front corner of the trailer 11 until the orientation of sensor frame 67 causes contacts 77b and 79 to open, deenergizing motor 35b. This procedure will continue with different jacks until disc 74 is freely suspended in a horizontal position, and not touching any of contacts 77-77c. The trailer 11 is then level. Switch 108 is then returned to the neutral position, and switch 103 turned "off." Leveling at any desired elevation may be achieved by use of the override circuit 23. Beginning with switches 104-109 in open or neutral positions and switch 103 "on," the left front jack 16, for example, is actuated. Closing switch 105 applied potential to wire 115a, and therefore to terminal 135a, starting motor 35a, thereby raising the front left corner. When the front left corner has achieved the desired elevation, switch 105 is released and jack 35a shuts off. Switch 108 is then placed in the "down" position, and the trailer is leveled as hereinbefore described. When the trailer 11 is leveled at the desired elevation, switch 108 is returned to the neutral position, and switch 103 is turned off.

When the vehicle is to be moved, the jacks 15-18 are returned to their inactive positions. First, switch 103 is turned "on," and switch 108 is moved to the "up" position connecting contacts 110 and 153, thereby applying positive potential to wires 114-114c. Retraction sensors 33-33c are not at this time actuated and are as shown by the solid lines in FIG. 5. Positive potential is then applied to terminals 134-134c thru contacts sets 122 and 127, 122a and 127a, 122b and 127b and 122c and 127c. This potential causes the motors 35-35c to rotate in directions opposite that which was described above. For example, opposite rotation of motor 35 raises the bronze nut 37 and inner ram 38 shown in FIG. 2. When the ram 38 is fully withdrawn, the actuator 46 is moved putting the switch 44 in the position shown by the dotted line in FIG. 5, thereby turning off motor 35. After full retraction of all jacks 15-18, all switches 44-44c will have changed state as described above to deenergize all motors 35-35c. This operation of switches 44-44c also completes a series circuit comprising wire 113b, contacts 121b, 126b, wires 120b, 113c, contacts 121c, 126c, wires 120c, 147, switch 93, wires 146, 120, contacts 126, 121, wires 113, 120a, contacts 126a, 121a and wires 113a and 144. Having no further need for the motors 35-35c, the orientation circuit can now be deactivated by moving switch 108 to the neutral position. Switches 92 and 93 are presently in the positions shown by the solid lines in FIG. 5. Next, the storage circuit 31 is activated to withdraw the jacks 15-18 into their inactive positions. Switch 109 is then thrown into the "up" position connecting contacts 111 and 155, thereby applying a positive potential to wire 113b and to terminal 149 of motor 85. Rotation of motor 85 now causes storage disc 87, shown in FIG. 4, to rotate in a clockwise direction. End pulleys 97 are thereby drawn in toward the storage motor mechanism 25. The effective lengths of the storage cables 54 and 94 between bell cranks 52-52c and frame pulleys 96 are thereby shortened, rotating the jacks 15-18 into inactive positions. When the jacks 15-18 have reached inactive positions, disc 87 will have reached a position such that the actuator 91 will change switch 93 to the position shown by dotted lines in FIG. 5, thereby deactivating the storage circuit 31 and stopping motor 85. The jacks 15-18 are now fully withdrawn and in inactive positions, and switch 109 is placed in the neutral position and switch 103 in the "off" position. The trailer 11 is now prepared for transportation to a new destination.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, the disclosed orientation system could be used on vehicles or structures other than a trailer. Also, other forms of sensors, for example a common pendulum, could be used, but the described disc sensor is highly preferred since it is more compact. Other numbers of jacks such as two or three could be used, and wheel jacks could be added to the override circuit to facilitate changing of tires. Also, by the incorporation of suitable solenoid valves, the present control system can be used to activate other types of lift devices including, for example, hydraulic, pneumatic or mechanical lifts. It is to be understood, therefore, that the invention can be practiced otherwise than as specifically described.

Claims

What is claimed is:

1. An automatic orientation system comprising:

a. a plurality of lift devices mounted in spaced apart positions on a structure to be oriented, each of said lift devices comprising a movable lift member for engaging a support surface to produce movement of said structure relative thereto;

b. power means for actuating said lift member independently;

c. orientation sensing means for sensing the orientation of said structure; and

d. electrical control circuit means responsive to said orientation means for energizing said power means to selectively and independently actuate said lift members so as to produce a desired orientation for said structure; said control circuit comprising an orientation circuit including a plurality of contacts connected with said power means, said plurality of contacts being operatively coupled with said orientation sensing means so as to be selectively opened and closed thereby; said control circuit means further comprising an initiation circuit for simultaneously inducing actuation of all of said lift devices by said power means and an electrical relay means for alternatively activating and deactivating said initiation and orientation circuits.

2. A system according to claim 1 wherein said plurality of lift devices comprises at least three lift devices, and said lift members thereof project from two-dimensionally spaced positions of said structure.

3. A system according to claim 1 wherein said control circuit further comprises an overide circuit means operative to selectively and independently actuate any of said lift devices with said power means.

4. A system according to claim 1 wherein each of said lift devices comprise a force sensor for sensing reaction forces encountered by said lift members, and said control circuit further comprises switching circuit means for actuating said relay means to activate said orientation circuit and deactivate said initiation circuit in response to sensing of a predetermined reaction force by any of said force sensors.

5. A system according to claim 4 wherein said control circuit further comprises an overide circuit means operative to selectively and independently actuate any of said lift devices with said power means.

6. A system according to claim 4 wherein each of said lift devices comprise a retraction sensor for sensing full retracting of said lift members, and said control circuit further comprises deactivating circuit means for decoupling said power means from any of said lift devices in response to sensing by its retraction sensor of full retraction of its lift member.

7. A system according to claim 6 including storage control means for moving each of said lift devices between active and inactive positions, and wherein said control circuit means further comprises storage control circuit means for automatically actuating said storage control means to move said lift devices into said inactive positions in response to sensing by all of said retraction sensors the full retraction of its associated lift member.

8. A system according to claim 7 wherein said control circuit further comprises an overide circuit means operative to selectively and independently actuate any of said lift devices with said power means.

9. A system according to claim 2 wherein said orientation sensing means comprises a pendulum member freely suspended from said structure and adapted for gravity induced movement in response to changes in the orientation of said structure produced by each of said lift devices.

10. A system according to claim 9 wherein said control circuit comprises an orientation circuit including a plurality of contacts opened and closed by movement of said pendulum member.

11. A system according to claim 10 wherein said lift devices comprise jacks, said power means comprise a source of electrical power and a reversible electric motor operatively coupled to each of said jacks, and said plurality of contacts are connected between said electric motors and said source of electrical power.

12. A system according to claim 11 wherein said control circuit further comprises an initiation circuit for simultaneously connecting all of said motors to said source of electrical power, and an electrical relay means for alternatively connecting and disconnecting said initiation circuit and said orientation circuit from between said source of electrical power and said electric motors.

13. A system according to claim 12 wherein each of said lift devices comprise a force sensor for sensing reaction forces encountered by said lift members, and said control circuit further comprises switching circuit means for actuating said relay means to activate said orientation circuit and deactivate said initiation circuit in response to sensing of a predetermined reaction force by any of said force sensors.

14. A system according to claim 4 wherein each of said force sensors comprise electrical contacts coupled to said relay means by said switching circuit means, contact actuator means movable to actuate said contacts in response to reaction force encountered by said lift member, and biasing means for resisting actuating movement of said contact actuator means.

15. A system according to claim 14 wherein said contact actuator comprises a thrust member coupled to said lift member, and said biasing means comprises a conical spring member resistively engaging said thrust member.

16. A system according to claim 13 wherein said control circuit comprises overide circuit means operative to selectively and independently connect any of said motors to said source of electrical power.

17. A device according to claim 16 wherein said sensor elements comprise electrical contacts, and said actuator disc comprises electrical contact portions disposed to contact said sensor element contacts in response to predetermined relative orientations between said frame and said actuator disc.

18. An automatic orientation system comprising:

a. a plurality of lift devices mounted in spaced apart positions on a structure to be oriented, each of said lift devices comprising a movable lift member for engaging a support surface to produce movement of said structure relative thereto; each of said lift devices comprising a retraction sensor for sensing full retraction of said lift members;

b. power means for actuating said lift members independently;

c. storage control means for moving each of said lift devices between active and inactive positions;

d. orientation sensing means for sensing the orientation of said structure; and

e. electrical control circuit means responsive to said orientation means for energizing said power means to selectively and independently actuate said lift members so as to produce a desired orientation for said structure, said control circuit comprising deactivating circuit means for decoupling said power means from any of said lift devices in response to sensing by its retraction sensor of full retraction of its lift member and storage control circuit means for automatically actuating said storage control means to move said lift devices into said inactive positions in response to sensing by all of said retraction sensors the full retraction of its associated lift member.