Alarm Means For Cobalt Therapy Machine

Abstract

An alarm means on a cobalt therapy machine of the type in which the patient rests on a stretcher and a moving rotating head of a cobalt irradiation device moves relative to the stretcher. Transducers measure changes in the forces on the stretcher, and so provide an alarm upon any interference between the moving parts and the stretcher by measuring the difference between an output signal and a reference signal.

Description

The present invention relates to alarm means used to give warning when a force applied to a body changes in a manner indicating the need for the intervention of an operator.

One application of the invention is to a cobalt therapy machine of the type in which the patient is rested on a stretcher, and a moving rotating head of the cobalt irradiation device, and a counterweight associated therewith, pass in close proximity to the stretcher and to the patient. In such an arrangement, it is important that an alarm be given if the moving head or the counterweight contacts the stetcher or the patient, or if the patient makes a sudden movement which might bring him into contact with moving parts.

According to the present invention, alarm means comprise transducer means arranged to ascertain the magnitude of a force applied to a body and to provide an output signal indicative of the said magnitude, monitoring means having first and second operating states, the first operating state being such that the monitoring means repeatedly checks the said output signal and developes a reference signal indicative of the instantaneous magnitude of the force within predetermined error limits, and the second operating state being such that the monitoring means repeatedly checks the said output signal against the reference signal which was present at the time the monitoring means switched from the first operating state to the second operating state, and alarm means arranged during the second operating state to provide an alarm should the said output signal deviate by more than a predetermined amount from the magnitude indicated by said reference signal.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective drawing of a therapy unit of commercially available form, used for the irradiation of a selected body portion of a patient from a cobalt 60 source, in the treatment of cancer;

FIG. 2 is a side elevation showing the manner in which two strain gauges are mounted on a stretcher support of a therapy couch shown in FIG. 1;

FIG. 3 is a plan view of the detail shown in FIG. 2;

FIG. 4 is a diagram showing the connection of the two strain gauges of FIG. 2 in an alarm circuit; and

FIG. 5 is a timing diagram showing the operation of the circuit of FIG. 4

Referring first to FIG. 1, this illustrates a known form of irradiation thereapy unit. The patient 1 is supported on a support or table 2 and when properly positioned relative to a sourcehead 3 receives irradiation from a cobalt 60 source contained in the sourcehead. The machine includes a unit support base in the form of a structural steel framework mounted beneath the treatment room floor. This support base carries at one end a support frame and drive housing 10 which carries on one side a massive U-shaped structure 11 of which one leg carries the sourcehead 3 and the other forms a counter weight 13. The structure 11 is pivotal about a horizontal axis 15. The cobalt 60 source is arranged to produce a beam of radiation which is directed along axis 16 and intersects the axis 15 normally. When the structure is so oriented that the axis 16 is vertical, then this axis 16 is also coaxial with a pivotal axis 17 for a treatment stretcher 19. Stretcher 19 is pivotally mounted on the unit support base 4 mentioned above, and its axis of rotation is thus rigidly positioned with respect to the horizontal axis of rotation of the structure 11.

The patient is supported on the table 2 and that table is carried by the stretcher 19 in a manner permitting several modes of adjustment. During these relative movements between the patient and the sourcehead, there is always the danger that the patient or the stretcher will come into collision with other moving parts, and the present invention is directed to ensuring that any such collision will provide at least an alarm signal to enable serious con-sequences to be avoided.

Referring now to FIGS. 2 and 3, these figures show how two strain gauges 31 and 33 are fitted to the support for the stretcher 19. The arrangement is such that upon an increase in the vertical load on the stretcher, the strain gauge 31 suffers a reduction in resistance and the strain gauge 33 suffers an increase in resistance. A wide ring 34 is mounted on the base of the therapy couch, and bolted to the couch is a beam 35 terminating in a spindle 36 carrying a roller 37 which runs on the ring 34. The arrangement is such that the beam is always stressed to hold the roller firmly against the ring. The two strain gauges are cemented respectively to the upper and to the lower surfaces of beam 35, and an increase in the load on the couch causes flexing of the beam and thus a stretching of one strain gauge and a shortening of the other.

Referring now to FIGS. 4 and 5, these diagrams show the manner in which the two strain gauges 31 and 33 are incorporated in an alarm circuit. From what has been said above it will be appreciated that the two strain gauges act differentially, i.e., for a given change in load, their resistances vary in opposite senses. The two strain gauges are connected in a resistive Wheatstone Bridge 41 energized by a direct current source 43. The output from bridge 41 is applied to a comparison means or dual operational amplifier 45. This amplifier provides an output which varies from a maximum of one polarity, progressively through zero, to a maximum of the opposite polarity, as the output from the bridge 41 varies through its working range. The difference voltage developed in bridge 41 is amplified by a factor of approximately 15,000.

The output from the amplifier 45 is fed into three detecting means or "trigger" circuits 47, 49 and 51, which are designed to detect, and to react to, different states of the amplifier output. Trigger circuit 47 starts emitting pulses when the output from the amplifier 45 nears saturation in the positive direction, corresponding to increased deflection of the stretcher 19 in a downwards direction. Trigger circuit 51 starts emitting pulses when the output from amplifier 45 nears saturation in the negative direction, corresponding to increased deflection of the stretcher 19 in a upwards direction. Trigger circuit 49 starts emitting pulses when the output from amplifier 45 passes through a null. The deflection of the stretcher 19 referred to is deflection as distinct from displacement caused by its positioning means, since the deflection is detected by the strain gauges acting between the stretcher and its supports. Thus deflection is set up when the patient is installed on the stretcher, when any external force acts on the stretcher, in an appropriate direction, and even (as a result of inertial loadings) if the patient makes a violent movement once on the stretcher.

The outputs from the two trigger circuits 47 and 51 are applied as an ON pulse to a sub-circuit on switching means 54 which consists of a 3-input gate and a 2-input gate and forms a NOR gate 55 arranged to "reset" a flip-flop 56. The "O" output from flip-flop 56 is applied to a NAND gate 57 which controls the passage of clock pulses from a clock circuit 58 to an eight-bit binary counter 61. The output from the counter 61 is applied to a ladder network 63 which feeds one input of the operational amplifier 45 so that the output from the ladder netword is added to the signal from the strain gauges. The combination of clock circuit 58, counter 61 and ladder network 63 define a reference signal means and provides an output from the ladder network which commences at an upper voltage and then reduces stepwise through 256 steps (i.e. 2.sup.8 steps) to a low value, and then returns substantially instantaneously to its upper voltage and then steps back to the low value. This action is repeated as long as the clock pulses are fed into the counter 61.

Since in the amplifier 45 the signal from the strain gauges is combined with the output from the ladder network 63, during each cycle of the counter 61 the amplifier output goes through null, which causes the trigger circuit 49 to produce an OFF signal pulse which is applied to flp-flop 56 to set that device so as to remove its "O" output and to set up a "1" output which is applied to a NAND gate 65.

A terminal 67 is so connected that when the driving motor for rotating the structure 11 is energized, an existing 115 volts 60 Hz supply is removed from terminal 67. Terminal 67 is connected to NAND gate 69 having as second input a "ground" connection. The output from gate 69 is applied as second input to gate 65. It is also applied to a NAND gate 71 having ground as its second input, and providing the third input to gate 57.

The output of gate 65 is applied to a driver relay 73 which when energized activates an alarm 75 and effects shut-down through motor control 77 of the motor driving the structure 11.

It will be seen from a study of FIGS. 4 and 5 that the main safety feature is that the collision device has the collision indication as its normal condition. In order to have a "NO COLLISION" signal, several conditions must be met. Thus the gate 65 will provide a signal causing the driver relay 73 to sound the alarm and to shut down the driving motor through the control 77 unless input signals are present on both its inputs. The presence of the input derived from gate 69 indicates that the supply voltage has disappeared from input terminal 67, i.e., that the structure 11 is on the move. The presence of the second input, from flip-flop 56 set to the "1" condition, indicates that the "O" trigger circuit 49 has operated to show that counter 61 has been properly set to suit the loading on the couch at the instant immediately before the structure 11 began to move.

Considering now the normal use of the apparatus, the initial condition is with the structure 11 stationary and the patient being positioned on the stretcher 19 and the apparatus being set up ready for therapy. Since the structure 11 is stationary, the supply voltage appears on terminal 67, so that gate 69 provides no signal to gate 65 which in turn supplies no signal to energize the driver relay 73. The alarm 75 therefore does not sound. The nurse or radiographer will be with the patient, and can supervise the operation of the apparatus during setting up to ensure that nothing untowards happens. During this period, the load on the stretcher will be chainging, so that the bridge 41 will change its output from time to time as the resistances of the two strain gauges 31 and 33 vary. At each different loading on the couch, initially one of the two trigger circuits 47 and 51 will have operated, thereby (through gate 55) resetting the flip-flop 56 to the "O" condition. Considering gate 57, this has three inputs; that from he clock 58 is intermittent, in the form of a constant series of pulses; that from flip-flip 56 is present when the flip-flop is reset to the "O" condition; and as long as there is the supply voltage on terminal 67 (indicating that the structure 11 is not moving), through the two negation gates 69 and 71 a voltage is applied to the third input of gate 57. Thus a series of pulses will be applied by gate 57 to the counter 61, the counter on which will continue to increase and so increase the voltage applied by ladder network 63 to the input of the amplifier 45. Once the output from amplifier 45 reaches "O," this will be detected by "O" trigger circuit 49, which will set the flip-flop 56 to the "1" condition. Since this removes the "O" output from the flip-flop 56 applied to the gate 57, that gate no longer passes clock pulses to counter 61, which remains at its existing setting. Of course, any appreciable change in the load on the couch will initiate another cycle of resetting of the counter 61.

Once the patient is left alone, quiescent on the stretcher, counter 61 will be at rest. When structure 11 is moved, removing the voltage from terminal 67, the alarm driver relay 73 is rendered effective. There being signals on both inputs of gate 65, no output will be applied to the driver relay 73. However, if for any reasons the output from the two strain gauges 31 and 33 changes by more than a predetermined amount, the output from the amplifier 45 will change from zero and will trip one of the trigger circuits 47 and 51. This through gate 55 will reset the flip-flop 56 to the "O" condition, removing the "1" output of the flip-flop from the gate 65. In the absence of one of its inputs, gate 65 will supply an output to alarm driver relay 73 to sound the alarm 75 and to operate control 77. Once the structure 11 comes to rest, the counter 61 will immediately start rebalancing the inputs to the amplifier 45. However, this will not permit the structure 11 to start moving until it is again manually started by the radiographer.

It will be seen that should any dangerous situation arise, the movements of the parts is stopped automatically. The operator can resume movements of the parts by manually starting the driving motor, taking appropriate precautions to ensure that the patient is in no danger.

To ensure adequate fail-safe measures, the apparatus is so arranged that a condition of collision is indicated unless a number of safety conditions are satisfied simulataneously.

1. POwer (115 VAC, 50-60 Hz) must be supplied both to the collision device and to the collision relay. Power failure in either device results in a collision indication.

2. DC power must be present to energize the bridge and the amplifier (.+-. 8 VDC); as well as to feed the different logic circuits (+ 4.7 VDC).

3. The amplfier must be out of saturation, in linear operation. This is to compensate for the possible failure of one of the trigger circuits.

Claims

I claim:

1. A patient irradiation machine comprising;

a. a table on which said patient is supported;

b. a base on which said table is supported;

c. transducer means arranged to ascertain the magnitude of a supporting force exerted between said table and said base;

d. monitoring means having an input coupled to said transducer means and first and second outputs, said monitoring means further having first and second operating states;

e. reference signal means for generating a reference signal of varying magnitude when said monitoring means is in its first operating state, said reference signal means being coupled between the first output and the input of said monitoring means, said monitoring means switching from its first to its second operating state when the magnitude of said reference signal corresponds to the output of said transducer means; and

f. alarm means coupled to the second output of said monitoring means, said alarm means being energized when said monitoring means is in its second operating state and the difference between the magnitude of said reference signal at the time said monitoring means switched from its first to its second operating state and the output of said transducer means exceeds a predetermined amount.

2. A patient irradiating machine as defined by claim 1 wherein said moinitoring means comprises

a. comparison means having its input coupled to the outputs of said transducer means and said reference signal means;

b. detecting means having its input coupled to the output of said comparison means, said detecting means providing a first output when the output of said comparision means has a first predetermined value indicative of balance between the outputs of said transducer and reference signal means and a second output when the output of said comparison means has a second predetermined value indicative of a predetermined unbalance between the outputs of said transducer and reference signal means; and

c. Switching means having first and second inputs coupled respectively to the first and second outputs of said detecting means, a first output coupled to said reference signal means and a second output coupled to said alarm means, the first or second output of said switching means being energized when said monitoring means is in its first or second operating stage respectively.

3. A patient irradiating machine as defined by claim 2 wherein the reference signal generated by said reference signal means varies cyclically between a higher magnitude and a lower magnitude and wherein said detecting means, when said monitoring means is in the first operating state, stops the cyclic variation in the reference signal when the output of said comparison means reaches said first predetermined value, said detecting means further, when said monitoring means are in the second operating state, initiating the cyclic variation in the reference signal when the output of said comparison means reaches said second predetermined value.

4. A patient irradiating machine as defined by claim 3 wherein said reference signal means comprises a counter, pulses being applied to said counter until said detecting means stops the cyclic variation in said reference signal and said monitoring means is switched from its first to its second operating state.

5. A patient irradiating machine as defined by claim 4 which further comprises a movable member; a motor coupled to said movable member to move said member relative to said table, energization of said motor changing said monitoring means from the first operating state to the second operating state.

6. A patient irradiating machine as defined by claim 5 which further comprises means for coupling a voltage to said alarm means and to said counter, said voltage deactivating said alarm means and permitting activation of said counter when said motor is deenergized.

7. A patient irradiating machine as defined by claim 4 wherein said reference signal means further comprises a ladder network coupled between the output of said counter and the input of said comparison means, said reference signal changing in a step-like manner in one direction between the higher magnitude and the lower magnitude.