GB1601471A

GB1601471A - Dispensing apparatus

Info

Publication number: GB1601471A
Application number: GB948977A
Authority: GB
Inventors: Denis Rothwell
Original assignee: NAT RES DEV; National Research Development Corp UK
Current assignee: NAT RES DEV; National Research Development Corp UK
Priority date: 1975-12-10
Filing date: 1977-03-07
Publication date: 1981-10-28

Description

(54) DISPENSING APPARATUS (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION. a British Corporation established by Statute.

of Kingsgate House, 66-74 Victoria Street, London, S.W.1., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to dispense ing apparatus, and is concerned in particular, but not exclusively. with apparatus for medical use for dispensing a drug from a syringe at a very slow controlled rate. The present invention is an improvement in or modification of the invention described in the specification accompanying British patent application No. 50657/75 (Patent No.

1,528,385), hereinafter referred to as the parent specification.

It is sometimes a requirement in the medical field to provide a slow infusion of a drug into a patient from a portable dispensing apparatus so that the patient can remain mobile while receiving the infusion. Such an infusion may be required to take place over several hours, during which time a very small quantity of drug. for example two millilitres, must be infused at an accurately controlled rate.

In known dispensing apparatus for this purpose, the plunger of a syringe is compressed by a lead screw which is driven by a clockwork or electric motor. Between the motor and the lead screw is a high-reduction gear box.

It is one object of the present invention to provide an improved dispensing apparatus for dispensing medical material at a very slow controlled rate.

There is claimed in the said parent specification apparatus hereinafter referred to as "apparatus as hereinbefore defined" for dispensing material comprising a container for material, a piston for displacing material from the container, a D.C. rotary electric motor, mechanical coupling means connecting the rotor of the electric motor to the piston, the mechanical coupling means being adapted to move the piston in linear motion upon rotation of the rotor of the electric motor, means for supplying electric current to energise the electric motor, bistable switching means switchable between an ON state in which the supply means is connected for supplying current to the electric motor and an OFF state in which the supply means is not connected for supplying current to the electric motor, timing means for switching the switching means to the ON state at periodic time intervals, and distance moniitoring means for switching the switching means to the OFF state each time the total cumulative displacement of the piston from a reference position increases by a predetermined increment.

By a D.C. rotary electric motor is meant a rotary motor adapted to be energised when in normal continuous operation by a source of direct current.

The apparatus described in the parent specification has particular advantage in medical applications where the said container is constituted by the barrel of a standard medical syringe, and the piston is constituted by the plunger of the syringe.

Thus there is also claimed in the said parent specification apparatus for dispensing material comprising a frame for supporting a medical syringe, a lead screw, carrier means coupled to the lead screw and arranged to move linearly upon rotation of the lead screw, the carrier means being adapted to co-operate with the piston of a medical syringe to effect depression of the piston upon rotation of the lead screw, a rotary electric motor having its rotor mechanically coupled to the lead screw to effect rotation of the lead screw upon rotation of the rotor of the electric motor, means for supplying electric current to energise the electric motor, bistabe switching means switchable between an ON state in which the supply means is connected for supplying current to the electric motor and an OFF state in which the supply means is not connected for supplying current to the electric motor, timing means for switching the switching means to the ON state at periodic time intervals, and distance monitoring means for switching the switching means to the OFF state each time the total cumulative distance of the carrier nut means from a reference position increases by a predetermined amount.

In the said parent specification, it is explained that in normal operating conditions one form of bistable circuit which can be used is a circuit known as a latch circuit which has two control inputs. Each pulse fed to a first of these control inputs switches the circuit to the ON state if the circuit is in the OFF state, but has no effect if the circuit is in the ON state already. Each pulse fed to a second of the control inputs switches the circuit to the OFF state if the circuit is in the ON state, but has no effect if the circuit is in the OFF state already. In such an arrangement, if under abnormal operating conditions the distance monitoring means becomes temporarily inoperative, for example due to contacts sticking, the electric motor will continue to run until the monitoring means returns to normal operation or until the whole apparatus is switched off.

In order to reduce any risks which might arise from this form of abnormal operation, there is described in the said parent specification an alternative arrangement which gives to a certain extent fail-safe operation if the monitoring means becomes temporarily inoperative. Thus there is also claimed in the said parent specification an arrangement in which the timing means is arranged to feed timing signals to the bistable switching means and the monitoring means is arranged to feed to the bistable switching means in normal operation an OFF signal each time the total cumulative displacement of the piston from the said reference point increases by a predetermined amount, the bistable switching means being arranged to be switched on in normal operation by a timing signal from the timing means and to be switched off in normal operation by an OFF signal from the monitoring means, but being arranged in such a manner that in the absence of an OFF signal from the monitoring means due to malfunction, the bistable switching means is switched off by a timing signal from the timing means.

Embodiments described in the said parent specification may be arranged in which in the absence of OFF signals from the monitoring means due to malfunction of the monitoring means over several cycles of operation of the timing means, the bistable switching means is arranged to be switched on and off by a series of periodic timing signals from the timing means.

In accordance with the present invention there is provided an improvement in or modification of the apparatus claimed in the last mentioned claim of the said parent specification, the improvement or modification comprising an arrangement of the bistable switching means such that in the absence of an OFF signal from the monitoring means over several cycles of operation of the timing means, the bistable switching means is arranged to occupy its OFF state without repeated action by timing signals which in normal operation would switch to its ON state.

In accordance with the present invention in one aspect there is provided an improvement in or modification of the apparatus claimed in last mentioned claim of the said parent specification, the improvement or modification comprising an arrangement of the bistable switching means such that in the absence of an OFF signal from the monitoring means due to malfunction, the bistable switching means is switched off by a timing signal from the timing means and is maintained in the OFF state, despite further timing signals from the timing means, until reset after correction of the malfunction.

In accordance with the present invention in another aspect, there is provided an improvement in or modification of the apparatus claimed in the said last mentioned claim of the said parent specification, the timing means being arranged, in the improvement or modification, to provide a first series of periodic ON signals for switching the bistable switching means to the ON state, and a second series of periodic signals of the same frequency as the first series but delayed relative to the first series by an amount such that in normal operation an OFF signal from the monitoring means occurs between an ON signal from the timing means and a delayed signal from the timing means, the bistable switching means being arranged to be such that in the absence of an OFF signal from the monitoring means after an ON signal from the timing means, the bistable switching means is switched to the OFF state by the next delayed signal from the timing means.

In some arrangements the bistable switching means may comprise a logic circuit composed of logic elements, including for example two flip flop circuits, arranged to provide various other fail-safe modes of operation selected according to the required functions of the apparatus.

In one arrangement the bistable switching means may be arranged to be responsive to receipt of two or more consecutive clock signals from the timing means without receipt of an intervening pulse from the distance monitoring means, and in response to such an occurrence may be arranged to indicate (for example visually or audibly) that a fault has developed and/or to modify or prevent further energising of the motor until the fault has cleared or until the apparatus is restarted by an operator.

In another arrangement the bistable switching means may be arranged to be responsive to receipt of two or more consecutive OFF signals from the distance monitoring means without receipt of an intervening clock pulse from the timing means (which would occur if for some reason the motor was running too fast), and in response to such an occurrence may be arranged to indicate (for example visually or audibly) that a fault has developed and/or to modify or prevent further energising of the motor until the fault has cleared or until the apparatus is restarted by an operator. This may be done for example by inhibiting the action of the clock pulse immediately following detection of the fault, delaying thereby energisation of the motor by one time period and reducing thereby the average motor speed.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings. in which: Figure 1 is a plan view showing apparatus for dispensing material for medical purposes by actuation of a standard medical syringe, embodying the invention of the said parent specification: Figure 2 is an end view of the apparatus of Figure 1; Figures 3 and 4 are plan and side views respectively of an index switch suitable for use in the apparatus of Figures 1 and 2; Figure 5 is a block circuit diagram of an electronic control circuit embodying the invention of the said parent specification for providing electric current for driving the apparatus shown in Figure 1; Figure 6 shows graphs of voltage and displacement of the syringe against time and illustrate the operation of the control circuit of Figure 5; Figure 7 is a circuit diagram of an improvement in or a modification of the circuit shown in Figure 5, and shows an embodiment of the present invention; Figure 8 is a truth table for two of the circuit elements used in the circuit of Figure 7; Figure 9 shows four timing sequences explanatory of the operation of the circuit of Figure 7; Figure 10 is a circuit diagram of a further improvement in or modification of the circuit shown in Figure 5 and shows a further embodiment of the present invention; and Figure 11 shows five timing sequences explanatory of the operation of the circuit of Figure 10.

Referring to Figures 1 and 2, there is shown medical dispensing apparatus embodying the invention of the said parent specification for slow infusion of a drug from a standard disposable syringe 11 having a capacity of for example 2.5 ml. The syringe comprises a cylindrical barrel 12 from which the drug is dispensed through an outlet 13 by longitudinal movement of a piston or plunger 14. The cylinder 12 is held in a housing 17 by a clip 82.

The housing 17 also contains a battery 22, an electronic control circuit 23 and a miniature D.C. motor (with integral gear box) indicated at 24. The output shaft 25 of the motor 24 is coupled by reduction gearing 26 to a lead screw 28, which is parallel to the plunger 14 of the syringe 11. The battery 22 is retained by a spring 35.

Mounted on the lead screw 28, is a carrier 83 which operates to propel or carry the plunger 14 into the cylinder 12. The carrier 83 comprises a loose fitting sleeve 84, and a half nut 85 which is urged into engagement with the lead screw 28 by an end extension 86 of a spring 87 coiled around the sleeve 84.

The carrier 83 has an upstanding flange 88 which co-operates with the end of the plunger 14. The half nut 85 can be disengaged from the lead screw 28 by pressing the sleeve 84 sideways towards the plunger 14 against the extension 86 of the spring 87.

In Figures 3 and 4 there are shown plan and side views respectively of one example of the index switch 64 indicated generally in Figure 1. Although in the description to follow with reference to the circuit diagrams in Figures 5 to 11, the index switch will be described as providing one OFF pulse per revolution of the shaft on which the index is mounted, more than one index element can be mounted on the shaft of the index switch and such an arrangement is shown in Figures 3 and 4. The switch 64 has three terminals labelled 93, 94 and 95 in Figure 3, of which terminals 94 and 95 only can be seen in Figure 4. Terminal 95 is common to both switching arrangements but terminal 93 or 94 is selected according to which of two delivery rates is required. As shown in Figure 4, terminals 94 and 95 are connected to two switch contacts 96 and 97 respectively which are positioned just above a cam wheel 98 mounted on an output shaft of the motor 24. The cam wheel 98 has three cam projections 99 so that the switch contacts 96 and 97 are closed once for each one third of a revolution of the wheel 98. The terminals 93 and 95 are connected to switch contacts (of which the upper control 100 is shown in Figure 3) just above a second cam wheel (not shown) similar to the wheel 98 but have only one cam projection. By selecting different terminals 93 or 94, a change of 3 to 1 in the delivery rate can be effected. Conveniently, flow rates can be arranged so that a syringe is emptied in 8 hours or 24 hours.

In Figure 5 there is shown one arrangement of the electronic control circuit 23 of Figure 1. Referring to Figure 5, a crystal oscillator 57 feeds pulses to a divider or countdown circuit 58 (which may be variable or fixed at a pre-selected ratio) which reduces the frequency of the pulses and feeds timing or clock pulses to a bistable circuit 59 which may be a circuit known as a latch switch. The bistable circuit 59 has one input 60 connected to the output of the divider circuit 58, and another input 61 the connection of which will be described below. The switch 59 is switchable to a first state, which will be referred to as the ON state, on receipt of a pulse from the divider circuit 58, and to a second state, which wil be referred to as the OFF state, on receipt of a pulse at the input 61. An output 65 of the bistable circuit 59 is connected to a switching amplifier 62 which is adapted to connect the battery 22 to the D.C. motor 24 in response to the output of the bistable circuit 59 when the latter is in the ON state.

In Figure 5, there is shown in diagrammatic form the index switch 64 which is operated by the reference index on a suitable shaft in the drive assembly between the motor 24 and the lead screw 28, and which constitutes a distance monitoring means for switching off the motor 24. Conveniently the reference index may be mounted on the output shaft 25 shown in Figure 1. In the following description it will be assumed that the contact terminal 100 in Figure 3 in used so that a single cam projection on the shaft 25 constitutes the reference index.

The input 61 of the bistable circuit 59 is fed from the index switch 64. Thus, in a simple example where only a single reference index is provided, an OFF pulse is generated by the index switch 64 once each cycle of the revolution of the reference index, and is fed to the input 61 of the bistable circuit 59.

The operation of the control circuit of Figure 5 will be described with reference to Figure 6 which shows graphs during each cycle of the clock pulses at the input 60, the graphs showing angular deflection of the reference index from a fixed reference mark, and the voltage applied to the electric motor, both against time. Considering the first cycle when the oscillator 57 is switched on, and assuming that the reference index is initially just past the fixed reference mark, the voltage pulse starts at time to and continues until the motor drives the reference index through one complete revolution. At this time, t1, the voltage to the motor 24 is disconnected but the inertia of the system carries the reference index on to say a further 90" of rotation. There then follows (in the example shown) a relatively long interval of the order of 1 second until at t2 a further clock pulse from the divider circuit 58 switches the output of the bistable circuit 59 to the ON state and causes the battery 22 to be connected again to the motor 24. In this cycle the voltage pulse ends at t3 and the time t2 to t3 iS shorter than that from to to tl. The reason for this is that the reference index starts from a position of 90" displacement from the fixed reference mark, and the OFF signal is generated by the index switch 64 earlier than in the previous cycle. Thus the advantage of deriving the OFF signals for the switch 59 from the index switch 64 (rather than switching off after a fixed time interval), is that varying amounts of over-run in each cycle are compensated for by the next cycle. Any errors on over-run are not cumulative.

In the arrangement described with reference to Figure 5, the operation has been described for normal operating conditions.

However, in applications of the invention for medical use it is important to consider how apparatus will operate should abnormal conditions arise temporarily. For example, if the index switch 64 consists of a mechanical switch it may occur that the contacts of the switch stick for some reason and the index switch 64 does not produce an OFF signal upon the first rotation of the shaft on which the index is mounted. The motor 24 may then continue to drive the lead screw 28 and may carry the index of the switch 64 through further cycles of rotation.

If however the index switch 64 were to continue to malfunction (where the bistable circuit 59 is a simple latch switch as described so far), then the motor 24 would continue to drive the lead screw 28 until the whole apparatus were switched off. To reduce the chances of such continuous actuation of the motor 24, there may be provided a modification of the circuit Figure 5 which is shown in Figure 7.

Thus there will now be described an arrangement for providing fail safe operation in a modification of the circuit shown in Figure 5, the modification being shown in Figure 7 with explanatory diagrams in Figure 8 and 9. The circuit of Figure 7 is described in broad outline by the block diagram of Figure 5, and like elements are indicated by like reference numerals. In the case of the bistable circuit corresponding to the circuit 59 of Figure 5, this is referred to in Figure 7 as bistable circuit 59' having inputs 60' and 61', and outputs 65', to indicate the modification relative to Figure 5. The fail safe modifications consist in modifications to the logic within the bistable logic circuit 59 of Figure 5. There are also shown in more detail one arrangement of the switching amplifier 62 of Figure 5 and one arrangement of circuitry of the index switch 64 of Figure 5.

Referring to Figure 7 the crystal oscillator 57 and divider (or countdown) circuit 58 feed a series of pulses through a capacitor 154 and resistor 155 network to the input 60' of the bistable logic circuit 59'. The index switch 64 (for example having a spring contact 100 as described with reference to Figure 3) is arranged to short out to earth a positive voltage (for example 4V) fed through a resistor 163 and capacitor 170 across a resistor 171 coupled to earth. When the switch contact 100 is opened, a pulse representing a logic 1 is fed to the input 61' of the logic circuit 59'.

Considering now the output of the logic circuit 59', this appears at the output 65' and is fed to the switching amplifier 62 which comprises a resistor 172 leading to a pair of transistors 173 and 174. The emitter of the transistor 173 is coupled to the base of the transistor 174 the emitter of which is connected to earth. The collector of the transistor 173 is coupled to a terminal 175 of the motor 24, which terminal is also connected through a light emitting diode 176 to the collector of the transistor 174. The other terminal 177 of the motor 24 is connected to a four volt unidirectional voltage supply.

The effect of the switch 64 is that when the contact 68 is opened by rotation of the motor 24 a positive pulse, representing a logic 1, is applied to the input 61', of the logic circuit 59'. At other times, the input 61' is at earth, representing a logic 0. The operation of the switching amplifier 62 is that when a positive voltage representing a logic 1 is applied to the resistor 172, the transistor 174 and the light emitting diode 176 are rendered conducting and the motor 24 is energised. The circuits are arranged so that approximately two volts is taken up across the light emitting diode 176 so that during normal running two volts is applied to the motor 24. The light emission from the diode 176 affords an indication of correct running of the motor 24 by regular intermittent light emission. Continuous light emission would for example indicate a fault, as would no light emission.

Considering now the detailed circuitry of logic circuit 59', this circuit is composed of two dual J-K master slave flip flop circuits 178 and 179. Each of the flip flop circuits has three input terminals, which are labelled CLOCK J and K, a SET input and a RESET input, and two outputs Q and Q1. The relationship between these inputs and outputs is determined by the truth table shown in Figure 8. These elements are available as prefabricated standard components, and it is not considered necessary to describe the internal workings of the flip flop circuits.

The input 60' of the circuit 59' is connected to the SET 1 input of the flip flop 178, and to the CLOCK input of the flip flop 179. The input 61' is connected to the RESET inputs of both the flip flops 178 and 179. The CLOCK, J1 and K1 inputs of the flip flop 178 are all connected to earth, and the output Q1 of the flip flop 178 is connected to the J2 input of the flip flop 179.

The K2 input of the flip flop 179 is connected to a small positive voltage representing a logic 1, for example 4 volts. The SET 2 input of the flip flop 179 is connected to earth.

The arrangement of connectioms shown of the circuit 59' has the effect on the working of the apparatus which is illustrated in Figure 9. In normal working, each CLOCK pulse (as shown in the timingsequence (a) in Figure 9) drives the output Q1 to zero and drives the output Q2 to a logic 1. The arrival of each switching pulse (as shown in timing sequence (b) of Figure 9) drives the output Q1 back to a logic 1 and in normal operation resets the output Q2 to a logic 0. However, where a fault arises, for example where the contact of the switch 64 sticks in the open position so that no OFF pulse is provided after flock pulse, the effect is that the output Q1 remains at logic 0 (as shown in timing sequence (c) in Figure 9). This allows the motor 24 to remain energised by the output Q2 for the whole of a period of the clock pulses (as shown in timing sequence (d) of Figure 9), but upon the next clock pulse arriving at the CLOCK 2 input of flip flop 179, the output Q2 is returned to zero because the appropriate OFF signal has not been applied to the RESET 2 input of the flip flop 179. The motor 24 then remains off until the contacts of the switch 64 are mechanically closed for example by manual operation or by self clearing to restore normal operation. Conveniently a manual button is provided on the apparatus for such resetting.

There will now be described with reference to Figures 10 and 11 a further modification of the circuit of Figure 5. Although the circuit shown in Figure 7 detects a fault when the contacts of the index switch 64 are inoperative, the end result is that the motor steps running entirely until reset. In a further modification for failsafe purposes shown in Figure 10, the bistable switching circuit (here labelled 59") is arranged so that when a fault is detected the motor is not switched off entirely, but is arranged to run at a rate approximately equal to the re quired metered rate. A warning of a fault is indicated and the circuit will normally be reset or corrected as soon as possible, but in the meantime the circuit is arranged to run at a timed rate (as opposed to a rate dependent upon distance travelled by the carrier on the lead screw) until repair can be effected.

Referring to Figure 10 the crystal oscillator 57 and divider circuits 58 are arranged to have two outputs 180 and 181 which provide two series of timing signals of the same frequency but delayed by half a period relative to each other. The first series of timing pulses is referred to as Clock 1 pulses and the trailing series of pulses is referred to as Clock 2 pulses as shown in Figure 11. The Clock 1 pulses are arranged to constitute ON pulses for the bistable switching means sty'.

The Clock I pulses and Clock 2 pulses are sharp negative going pulses and are fed through capacitors 182 ansd 183 respectively to the bistable switching circuit 59". A circuit 184 is provided indicated in a broken line box at 84 for the purpose of arranging the input levels of the clock pulses at suitable magnitude to be received by the input of the bistable circuit 59".

The bistable circuit 59" comprises a pair of three input NAND gates 185 and 186 connected to function as a bistable circuit.

The NAND gates are referred to as gates A and B having inputs Al, A2 and A3 and B1, B2 and B3, and outputs A4 and B4 respectively. The output A4 is connected to the input B1 and the output B4 is connected to the input A3. The clock 1 pulses from the output 180 are connected to both the Al and A2 inputs, and the Clock 2 pulses from 181 are connected to the B2 input. The output from the A4 output is connected via a resistor 187 to a modified Darlington pair comprising elements 173 and 174 connected in the same manner as described with reference to Figure 7 except that the collector of the transistor 173 is connected through a resistor 188 to the +4 volt line voltage and the collector of the transistor 174 is connected directly to the terminal 175 of the motor 24. A further modification of the motor connections compared with this circuit of Figure 7 is that the light emitting diode 176 is, in Figure 10 connected between the +4 volt line by way of a resistor 189 to earth by way of the switch 64. The switch 64 is arranged to be normally open but to be closed by rotation of the index cam of the distance monitoring means. The OFF signals from the switch 64 are taken from a point between the switch 64 and the LED 1 76 and are fed by way of a capacitor 191 to the input B3 of the NAND gate 186.

Referring to Figure 11, the first two cycles illustrated in the timing sequences show the circuit of Figure 10 in normal operation.

Each Clock 1 pulse switches the output A4 from a logic 0 to a logic 1 and the output remains in this condition, energising the motor 24, until an OFF pulse fed to the input B3 of the NAND gate 186 switches the output A4 back to a logic 0.

In the last three cycles shown in Figure 11, the effect is illustrated of an absence of OFF signals from the switch 64. In such a case the output A4 remains at a logic 1 until a Clock 2 pulse switches the output of the NAND gate 186 B4 from 0 to 1. Upon this occurring, the output B4 switches the output A4 of the NAND gate 185 back from a logic 1 to a logic 0 thus switching off the motor. In this fault condition, the circuit 59" continues to act as a simple bistable switching from the ON condition to the OFF condition alternately under the control of the Clock 1 and Clock 2 pulses until either the OFf pulses are resumed or until the machne is switched off and repaired.

Thus it will be seen that the time for which the motor is energised (represented by the graph labelled A40UT) is approximately the same in the fault condition as in the normal running. The difference however is that both the ON instant and OFF instant of the energising pulses A4 is determined by timing pulses in the fault condition, whereas in the normal running the ON instant of each energising pulse is determined by Clock 1 pulses but the OFF instant of A4 is determined

Claims

**WARNING** start of CLMS field may overlap end of DESC **. quired metered rate. A warning of a fault is indicated and the circuit will normally be reset or corrected as soon as possible, but in the meantime the circuit is arranged to run at a timed rate (as opposed to a rate dependent upon distance travelled by the carrier on the lead screw) until repair can be effected. Referring to Figure 10 the crystal oscillator 57 and divider circuits 58 are arranged to have two outputs 180 and 181 which provide two series of timing signals of the same frequency but delayed by half a period relative to each other. The first series of timing pulses is referred to as Clock 1 pulses and the trailing series of pulses is referred to as Clock 2 pulses as shown in Figure 11. The Clock 1 pulses are arranged to constitute ON pulses for the bistable switching means sty'. The Clock I pulses and Clock 2 pulses are sharp negative going pulses and are fed through capacitors 182 ansd 183 respectively to the bistable switching circuit 59". A circuit 184 is provided indicated in a broken line box at 84 for the purpose of arranging the input levels of the clock pulses at suitable magnitude to be received by the input of the bistable circuit 59". The bistable circuit 59" comprises a pair of three input NAND gates 185 and 186 connected to function as a bistable circuit. The NAND gates are referred to as gates A and B having inputs Al, A2 and A3 and B1, B2 and B3, and outputs A4 and B4 respectively. The output A4 is connected to the input B1 and the output B4 is connected to the input A3. The clock 1 pulses from the output 180 are connected to both the Al and A2 inputs, and the Clock 2 pulses from 181 are connected to the B2 input. The output from the A4 output is connected via a resistor 187 to a modified Darlington pair comprising elements 173 and 174 connected in the same manner as described with reference to Figure 7 except that the collector of the transistor 173 is connected through a resistor 188 to the +4 volt line voltage and the collector of the transistor 174 is connected directly to the terminal 175 of the motor 24. A further modification of the motor connections compared with this circuit of Figure 7 is that the light emitting diode 176 is, in Figure 10 connected between the +4 volt line by way of a resistor 189 to earth by way of the switch 64. The switch 64 is arranged to be normally open but to be closed by rotation of the index cam of the distance monitoring means. The OFF signals from the switch 64 are taken from a point between the switch 64 and the LED 1 76 and are fed by way of a capacitor 191 to the input B3 of the NAND gate 186. Referring to Figure 11, the first two cycles illustrated in the timing sequences show the circuit of Figure 10 in normal operation. Each Clock 1 pulse switches the output A4 from a logic 0 to a logic 1 and the output remains in this condition, energising the motor 24, until an OFF pulse fed to the input B3 of the NAND gate 186 switches the output A4 back to a logic 0. In the last three cycles shown in Figure 11, the effect is illustrated of an absence of OFF signals from the switch 64. In such a case the output A4 remains at a logic 1 until a Clock 2 pulse switches the output of the NAND gate 186 B4 from 0 to 1. Upon this occurring, the output B4 switches the output A4 of the NAND gate 185 back from a logic 1 to a logic 0 thus switching off the motor. In this fault condition, the circuit 59" continues to act as a simple bistable switching from the ON condition to the OFF condition alternately under the control of the Clock 1 and Clock 2 pulses until either the OFf pulses are resumed or until the machne is switched off and repaired. Thus it will be seen that the time for which the motor is energised (represented by the graph labelled A40UT) is approximately the same in the fault condition as in the normal running. The difference however is that both the ON instant and OFF instant of the energising pulses A4 is determined by timing pulses in the fault condition, whereas in the normal running the ON instant of each energising pulse is determined by Clock 1 pulses but the OFF instant of A4 is determined by the distant monitoring means constituted by the index switch 64. Thus under the abnormal conditions the amount of material dispensed is not accurately metered since the start and stop of the motor is switched by Clock pulses and no account is taken of-variations in over-run of the motor, for example due to friction changes. However the running in the fault condition approximates to normal running and is unlikely to cause harm for a short period until the machine is repaired. It will be appreciated that an indication of fault is made by the LED 176 which will remain either continuously ON or continuously OFF depending on the nature of the fault of the contacts of the switch 64. WHAT WE CLAIM IS:

1. Apparatus as hereinbefore defined for dispensing material, in which the said bistable switching means is arranged to be switched on in normal operation by a timing signal from the timing means and to be switched off in normal operation by an OFF signal from the monitoring means, but in which the bistable switching means is arranged in such a manner that in the absence of an OFF signal from the monitoring means due to malfunction, the bistable switching means is switched off by a timing signal from the timing means, and, in the

absence of OFF signals from the monitoring means over several cycles of operation of the timing means, the bistable switching means is arranged to occupy its OFF state without repeated action by timing signals which in normal operation would switch it to its ON state.

2. Apparatus as hereinbefore defined for dispensing material in which the said bistable switching means is arranged to be switched on in normal operation by a timing signal from the timing means and to be switched off in normal operation by an OFF signal from the monitoring means, but in which the bistable switching means is arranged in such a manner that in the absence of an OFF signal from the monitoring means due to malfunction, the bistable switching means is switched off by a timing signal from the timing means and is maintained in the OFF state, despite further timing signals from the timing means, until reset after correction of the malfunction.

3. Apparatus as hereinbefore defined for dispensing material in which the said bistable means is arranged to be switched on in normal operation by a timing signal from the timing means and to be switched off in normal operation by a OFF signal from the monitoring means, and in which the timing means is arranged to provide a first series of periodic ON signals for switching the bistable switching means to the ON state, and a second series of periodic signals of the same frequency as the first series but delayed relative to the first series by an amount such that in normal operation an OFF signal from the monitoring means occurs between an ON signal from the timing means and a delayed signal from the timing means, the bistable switching means being arranged to be such that in the absence of an OFF signal from the monitoring means after an ON signal from the timing means, the bistable switching means is switched to the OFF state by the next delayed signal from the timing means.

4. Apparatus according to any preceding claim adapted for medical use in which the said container is constituted by the barrel of a standard medical syringe, and the piston is constituted by the plunger of the syringe.

5. Apparatus according to any preceding claim in which the bistable switching means are arranged to be responsive to receipt of two or more consecutive clock signals from the timing means without receipt of an intervening pulse from the distance monitoring means, and in response to such an occurrence are arranged to indicate that a fault has developed and/or to modify or prevent further energising af the motor until the fault has cleared or until the apparatus is restarted by an operator.

6. Apparatus according to any preceding claim in which the bistable switching means are arranged to be responsive to receipt of two or more consecutive OFF signals from the distance monitoring means without receipt of an intervening clock pulse from the timing means and in response to such an occurrence are arranged to indicate that a fault has developed and/or to modify or prevent further energising of the motor until the fault has cleared or until the apparatus is restarted by an operator.

7. Apparatus according to Claim 6 including means for inhibiting the action of the clock pulse immediately following detection of a fault, delaying thereby energisation of the motor by one time period and reducing thereby the average motor speed.

8. Apparatus according to any preceding claim in which there is provided means for controlling the rate at which the material is dispensed comprising means for varying the frequency of the timing signals produced by the timing means.

9. Apparatus according to any preceding claim in which the timing means comprises a crystal oscillator, and the output of the oscillator is divided by a variable ratio divider circuit for controlling the frequency of the timing signals produced by the timing means.

10. Apparatus according to any preceding claim in which the distance monitoring means comprises means responsive to movement of-at least one reference index on a rotary element past a fixed reference position.

11. Apparatus according to Claim 10 in which the coupling means connecting the electric motor to the piston includes a lead screw rotated by the motor, and the monitoring means is arranged to generate at least one OFF signal for the switching means upon each rotation of a rotary element coupled to the lead screw.

12. Apparatus according to any preceding claim in which the switching means for supplying electric current to the motor comprises a solid state electronic circuit for supplying unidirectional electric current from a battery.

13. Apparatus for dispensing material substantially as hereinbefore described with reference to Figures 1 to 9 or with reference to Figures 1 to 11 of the accompanying drawings.