Title:
PUMPS
United States Patent 3572979


Abstract:
In a blood pump a fluid pressure circuit is provided which is operative to feed a variable frequency pulsating flow of water to one side of a flexible diaphragm. The other side of the flexible diaphragm is connected into a patient's bloodstream and thus flexure of the diaphragm results in a pulsatile flow of blood on said other side of the diaphragm. The specification describes a number of devices for alternately applying suction and pressure to the water in said fluid pressure circuit and thereby to the diaphragm as well as describing one practical form of pumping head including such a diaphragm.



Inventors:
MORTON PAUL GREVILLE
Application Number:
04/809681
Publication Date:
03/30/1971
Filing Date:
03/24/1969
Assignee:
PAUL GREVILLE MORTON
Primary Class:
Other Classes:
128/DIG.3, 417/390, 417/427, 600/16, 604/151
International Classes:
A61M1/10; F04B43/113; (IPC1-7): F04B9/08; F04B17/00; F04B35/00
Field of Search:
103/152,45 417
View Patent Images:
US Patent References:
3451347VISCOUS SUSPENSION PUMPING MEANS1969-06-24Chimura
3250226Hydraulic actuated pumping system1966-05-10Voelker
3208448Artificial heart pump circulation system1965-09-28Woodward
3048121Hydraulic actuated pump1962-08-07Sheesley
2815715Surgical pump1957-12-10Tremblay
2186972Pumping apparatus1940-01-16Hollander et al.
0491116N/A1893-02-07



Primary Examiner:
Freeh, William L.
Assistant Examiner:
Vrablik, John J.
Claims:
I claim

1. An artificial heart pump for connection into a vascular system including:

2. An artificial heart pump according to claim 1 in which the spool valve is disposed within the reservoir of hydraulic fluid.

3. An artificial heart pump according to claim 1 including means for adjusting the position of the valve body relative to the two-land spool to vary the times during which the second chamber of the diaphragm pump is connected with the outlet of the first pump and the inlet of the second pump.

4. An artificial heart pump according to claim 2 including means for adjusting the position of the valve body relative to the two-land spool to vary the times during which the second chamber of the diaphragm pump is connected with the outlet of the first pump and the inlet of the second pump.

5. An artificial heart pump according to claim 1 in which the diaphragm pump comprises:

Description:
This invention relates to pumps for use in cardiac surgery.

The roller-type pump used in present-day cardiac surgery has the disadvantage that the outflow of blood from the pump is essentially of a nonpulsating nature thus being different from the natural outflow of blood from a heart, and furthermore the very localized squeezing action applied by the rollers through a flexible diaphragm to the blood itself during operation of such a roller-type pump results in damage to the constituents of the blood.

According to this invention, a pump for use in cardiac surgery includes a housing separated into two areas by a flexible diaphragm, one of the areas being adapted for connection into a patient's bloodstream for taking over the function of the patient's heart, and the other area forming part of a control fluid circuit which is operable to apply a pulsating control fluid pressure to said other area whereby to cause a pulsating flow of blood from said one area.

Preferably, two lines are provided from said one area for connection into the patient's bloodstream, each line being provided with a nonreturn valve and the valves being arranged so that one valve permits flow into said one area and the other valve permits flow from said one area.

The housing and the flexible diaphragm may be in the form of tubes, in which case the diaphragm tube is mounted coaxially within the housing tube and is sealed to the latter at its ends, the bore of said tube constituting said one area and an annular space between said tubes constituting said other area, and said nonreturn valves are carried by end fittings for the housing tube.

It will be appreciated that the frequency of pulse of said pulsating control fluid pressure may be varied so as to vary the frequency of the pulsating flow of blood.

In a preferred form of this invention the control fluid in the control fluid circuit is an incompressible fluid.

The control fluid circuit may include first conduit means providing communication between said other area and a pressure source of said control fluid, second conduit means providing communication between said other area and a suction source of said control fluid, valve means for controlling communication between said other area and said pressure source on the one hand and said suction source on the other hand, and actuating means adapted to actuate said valve means such that the control fluid pressure and the suction pressure are applied separately to said other area whereby to apply said pulsating control fluid pressure.

The valve means may comprise a spool valve unit and, where the control fluid is an incompressible liquid (for example water), the spool valve unit may be mounted within a reservoir of the control liquid so as to be completely submerged within the liquid.

The present invention conveniently enables a blood pump to be produced for use in cardiac surgery which is sterile, easily cleaned, rugged, easily operated and cheap, and which can provide a pulsating outflow of blood without subjecting the blood to a localized squeezing action, and furthermore, which may be adjusted to provide a pulsating blood flow of a desired frequency so as to simulate the output of a heart.

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

FIG. 1 is a schematic diagram illustrating the principle of operation of a blood pump in accordance with this invention,

FIG. 2 illustrates a practical embodiment of a valve and valve actuating mechanism for a blood pump according to this invention,

FIG. 3 is a diagram illustrating the relative positions and the profiles of the cams employed in the embodiment of FIG. 2,

FIG. 4 is a second practical embodiment of a valve and valve actuating mechanism for a blood pump according to this invention,

FIG. 5 is a modification of the embodiment of FIG. 4,

FIG. 6 is a third practical embodiment of a valve and valve actuating mechanism for a blood pump according to this invention,

FIG. 7 illustrates a practical embodiment of a pumping head for use in a blood pump in accordance with this invention,

FIG. 8 is a schematic circuit diagram of electrical means for operating the actuating device according to another preferred embodiment of the invention, and

FIG. 9 shows a circuit waveform of the pressure signal applied to the pipe 18 of FIG. 5.

Referring to FIG. 1, a blood pump includes a pumping head 10 which comprises a housing 11 divided internally into two areas 12 and 13 by a flexible diaphragm 14.

The area 12 on one side of the diaphragm 14 is provided with two nonreturn valves 15 and 16 arranged so that one permits flow into the area 12 and the other permits flow out from the area 12. The pumping head 10 is adapted to be connected into the patient's blood system by way of the nonreturn valves 15 and 16, the area 12 providing a bypass around the patient's heart.

The area 13 on the other side of the diaphragm 14 forms part of a control fluid circuit. The control fluid circuit is a closed circuit, that is a circuit in which no fluid losses have to be made up. The preferred fluid is water. The area 13 is connected to a device 17 by a pipe 18. The control fluid circuit also includes two centrifugal pumps 19 and 20, a reservoir 21 and pipes 22 to 26 respectively connecting the output of the pump 19 to the device 17, the input of the pump 19 to the reservoir 21, the output of the pump 20 to the reservoir 21, the input of the pump 20 to the device 17, and the reservoir 21 to the device 17. The surface of the water contained within the reservoir is open to atmosphere.

The device 17 includes valve means for connecting the pipe 22 to the pipe 18 or the pipe 26 so that the output of the pump 19 is placed in conduit communication with either the area 13 or the reservoir 21. The device 17 also includes valve means for connecting the pipe 25 to the pipe 18 or to the pipe 26 so that the input of the pump 20 is placed in conduit communication with either the area 13 or the reservoir 21.

In operation of the blood pump, the pumps 19 and 20 are driven continuously so that water is circulated around the closed control fluid circuit. The water pressure in the area 13 governs the position of the diaphragm 14 and movement of the diaphragm causes blood to be drawn into or expelled from the area 12 through the appropriate valve 15 or 16. It will be seen that when the output of the pump 19 is first connected to the area 13 through the actuating device 17, the diaphragm 14 is deflected suddenly so that blood is suddenly expelled from the area 12. It will also be seen that when the conduit communication between the output of the pump 19 and the area 13 is blocked by the diversion of the water through the device 17 to the reservoir 21, and when the input of the pump 20 is connected to the area 13 through the device 17, suction pressure is applied by the pump 20 to the diaphragm 14 drawing the diaphragm towards the device 17 and sucking blood into the area 12. It will be understood that by suitably controlling the interconnection between the pipes 22, 25 and 26 on the one side of the device 17 and pipe 18 on the other side of the device 17 the pressure in the area 13 acting on the flexible diaphragm can be varied in a pulsating manner so that the outflow of blood from the area 12 is correspondingly pulsatile. Furthermore, it will be understood that the frequency of pulse can be varied to suit requirements.

It will be appreciated that various refinements and modifications to the system illustrated in FIG. 1 may be employed without departing from the basic principle of operation. For example, it is not essential to have a closed control water circuit; where the water supply pressure is suitable the pipe 22 may be connected to a domestic water tap, thus dispensing with the pump 19. Furthermore, it is not essential to connect the pipe 25 to the pipe 26 when it is not required to apply the suction pressure to the area 13; the pipe 25 may simply be blocked by any convenient means. Moreover, it is not essential to employ a centrifugal pump 20; any other suitable means for providing the required suction pressure may be employed, for example a standard venturi device.

Experiments have been carried out and these show that, in a blood pump in accordance with this invention, the pump 19 should preferably be capable of supplying water at a rate of flow sufficiently great to eject the full stroke volume (that is up to 30 ccs. in the case of a dog and up to 100 ccs. in the case of a man) from the area 12 in less than 0.1 seconds. This needs a water pump capable of delivering, in the case of a dog, greater than 4 gallons per minute, and for a man, greater than 131/2 gallons per minute. The suction device 20 should preferably be capable of applying a vacuum of more than 20 feet of water at flow rates of about 1 gallon per minute and 4 gallons per minute for dog and man respectively.

Referring to the embodiment illustrated in FIGS. 2 and 3, there is illustrated a practical form of the device 17 of FIG. 1. Where appropriate the reference numerals of FIG. 1 have been applied to the corresponding parts. The centrifugal pump 20 is replaced by a water jet pump 30 which operates on the venturi principle. The inlet of the pump 30 is connected to the reservoir (not shown) through a pipe 31, and the pipe 25 is connected to an intermediate port 32 of the pump 30.

The three pipes 22, 25 and 26 are passed through a valve mechanism of the actuating device 17. In this arrangement the pipe 26 is a branch from the pipe 22, the connection being upstream of the valve mechanism.

The valve mechanism includes three pairs of opposed anvils, the upper anvil 33 of each pair being carried by a fixed bar 34 and the lower anvil 35 of each pair being carried by the end of a corresponding one of three levers 36 which are pivotally mounted at their other ends on a rod 37. The actuating means for the valve mechanism comprises a cam shaft 38 which extends below the three lower anvils 35 and carries three cams 39, 40, 41, which each cooperate with a corresponding one of the levers 36. The cams 39 and 41 are similarly profiled and similarly positioned relative to the cam shaft 38 so that they impart similar movements to their respective cooperating levers 36 whereby they control flow through the pipes 25 and 26 respectively. The cam 40 controls flow through the pipe 22. The relative positions and profiles of the cams 39, 40 and 41 can be seen from FIG. 3.

In operation of a blood pump incorporating the arrangement described above, the cam shaft 38 is rotated about its axis. Thus it will be seen that when the cams 39 and 41 hold their corresponding lower anvil member in their uppermost positions, these anvil members 35 cooperate with their corresponding upper anvil members 33 to close the pipes 25 and 26 which are made of rubber or any other suitable flexible material, leaving the pipe 22 fully open and supplying water under pressure to the area 13. As the cam shaft 38 rotates, the lower anvil members 35 operated by the cams 39 and 41 fall, opening the corresponding pipes 25 and 26, and thus diverting some of the water under pressure from the pipe 22 to the pipe 26 and also applying suction pressure to the area 13 through the pipe 25 so as to initially gradually reduce the pressure in the area 13 until rotation of the cam shaft 38 closes the pipe 22 by way of the cam 40. When pipe 22 is closed no water under pressure is supplied to the area 13 and full suction pressure is applied. Further rotation of the cam shaft 38 opens the pipe 22 and then closes the pipes 25 and 26, and it will be seen that as the cam shaft 38 rotates, a pulsating control water pressure is applied to the diaphragm 14.

It will be understood that the variation of pressure applied to the diaphragm 14 during one rotation of the cam shaft 38 can be varied by suitable selection of cam profiles and cam positions. Moreover, the frequency of pulse of the pulsating control pressure applied and thus of the pulsating flow of blood induced by the blood pump can be altered by varying the speed of rotation of the cam shaft 38. A tap (not shown) may be provided in pipe 22 whereby the flow of water flowing through the pipe 22 may be altered so as to correspondingly alter the amount of blood which is caused to flow by the blood pump.

Referring to FIG. 4, there is illustrated a second practical form of the device 17 of FIG. 1. Where appropriate the reference numerals of FIG. 1 have been applied to the corresponding parts. The device 17 in this embodiment comprises a spool valve unit which is immersed within the water contained within the reservoir 21. Consequently the pipe 26 connecting the actuating device 17 to the reservoir 21 is not required. The spool valve unit has a balanced two-land spool 42. The spool 42 is mounted for reciprocatory movement within a valve body 43. The pipes 18, 22 and 25 are connected to the valve body in such a way that the spool 42 connects either the pipe 22 or the pipe 25 to the pipe 18 and blocks the other. A bearing housing 44 is mounted on the valve body 43 so as to support a crank 45 for rotation. The radially outer end of the crank 45 is pivotally connected to one end of a connecting rod 46, the other end of the connecting rod being pivotally connected to an extension member 47 carried by the valve spool 42.

In operation of a blood pump incorporating the actuating device described above, the crank 45 is rotated continuously so that the valve spool 42 is reciprocated within the valve body 43 through the connecting rod 46 and the extension member 47. Reciprocation of the valve spool 42 connects the pipe 22 or the pipe 25 to the pipe 18 alternately, thus applying pressure or suction to the flexible diaphragm 14 alternately, as in the arrangement described hereinbefore with reference to FIGS. 2 and 3. The length of the connecting rod 46 may be adjusted so that the valve spool 42 dwells longer in either the pressure applying or suction positions as required. As in the previous embodiment, the frequency of the pulsating pressure applied to the diaphragm 14, and thus the frequency of the pulsating flow of blood expelled from the pump, may be altered by varying the speed of rotation of the crank 45, and the flow of the water flowing through pipe 22 may be altered so as to alter the amount of blood which is caused to flow by the blood pump.

Location of the spool valve unit below the level of water in the reservoir 21 permits said valve to be made with liberal tolerances, since water leakage and air entrainment due to a poorly fitting spool are no problem.

Referring to FIG. 5, this shows a modified form of the arrangement of FIG. 4 and like parts have been given the same reference numerals. In this modified arrangement, the spool valve 48 is carried by a screw device 67 which is arranged with the longitudinal axis of its threaded part 68 vertical and with the threaded part engaged in two longitudinally aligned tapped blocks 69, 70 carried one at each end of the valve body 43. The screw device 67 is mounted in a fixed structure 71 outside the reservoir 21 in such a way that it is prevented from moving along its longitudinal axis but may be rotated about that axis. The valve body 43 is orientated in such a way that the longitudinal axis of the spool 42 is vertical and is suitably held against lateral movement so that rotation of the threaded part 68 of the screw device 67 causes vertical movement of the valve body 43.

In this modified arrangement, the extension member 47 described with reference to FIG. 4 for connecting the spool 42 to the connecting rod 46, is replaced by a spring coupling 72 which is flexible in bending but is fairly stiff in tension. The bearing housing 44 of FIG. 4 is also dispensed with, the crank 45 being in the form of an eccentric and being supported by a shaft 73 by which it is rotatably driven.

The operation of this modified arrangement is the same as for the arrangement of FIG. 4, except that the adjustment, whereby the spool 42 dwells longer in the pressure-applying or suction position as required, is effected by rotation of the screw device 67 so that the valve body 43 is raised or lowered relative to the spool 42.

Referring to FIG. 6, there is illustrated a third practical form of the device 17 of FIG. 1 employing a spool valve in a manner similar to the embodiments of FIGS. 4 and 5. Where appropriate, the reference numerals of FIGS. 1, 4 and 5 have been applied to the corresponding parts.

In this arrangement a three-land spool valve 48A is mounted below the level of water in the reservoir 21 with the longitudinal axis of its valve spool 42A vertical. The bottom port of the spool valve 48A opens directly into the reservoir 21; the other three ports are connected to the pipes 18, 22 and 25 in a similar manner to the spool valve 48 of FIG. 5, so that the spool valve 48A controls the flow of water through the pipe 18 in much the same way. A three-land spool valve 49 is also mounted below the water level within the reservoir 21 with the longitudinal axis of its spool 50 vertical. The spaces below the lower land of each spool valve 48A and 49 are connected together by a pipe 51 which is also connected to the space below a piston 52 in a cylindrical 53. The cylinder 53 is also mounted below the water level within the reservoir 21 with its longitudinal axis vertical. The piston rod 54 of the piston 52 and the valve spools 42A and 50 all extend vertically and are loaded respectively with weights 55 to 57.

The spool valve 49 has four ports 58 to 61 respectively which are spaced apart from each other vertically, the lowermost port being the port 58. The port 58 opens directly into the reservoir 21, the port 59 is connected to the pressure pipe 22 through a branch pipe 62, the port 60 is connected to the pipe 51 through a branch pipe 63, the port 60 is connected to the pipe 51 through a branch pipe 63, and the port 61 is connected to the suction pipe 25 through a branch pipe 64. The three lands of the spool 50 are arranged so that the port 60 communicates with either the port 61 or the port 59, and thus with either suction or pressure, through the space between the upper and middle lands of the spool, and so that the port 58 always communicates with the space between the middle and lower lands of the spool 50 and is never connected to either of the ports 59 or 61. The cylinder 53 carries an adjustable stop 74 which is abutted by the topside of the piston 52 at the top of the piston's stroke.

In operation of a blood pump incorporating the actuating device described above, the spool valve 48A controls the supply of fluid pressure to the pumping head 10 through the pipe 18 in much the same way as has been described above with reference to FIGS. 4 and 5. The difference between the embodiment of FIGS. 4 and 5 and the present embodiment lies in the actuation of the valve spool 42A.

Starting from the normal rest position in which the valve spools 42A and 50 and the piston 52 are at the bottom of their respective strokes and in which the ports communicating with the respective pressure-applying pipes 22 and 62 are open, when water under pressure is supplied through the pipes 22 and 62, it is applied to the flexible diaphragm 14 of the pumping head 10 through the spool valve 48A and the pipe 18 and to the area under the piston 52 through the pipes 63 and 51. The piston 52 moves vertically upwards against the action of the weight 55, until it abuts stop 74, whereupon there is a buildup of water pressure beneath the lower lands of the spools 42A and 50 forcing the spools to move vertically upwards against the action of the respective weights 56 and 57. The inertia of this upward movement carries the spools 42A and 50 through the point at which they close the respective ports communicating with the respective pressure-applying passages 22 and 62 to the position in which they open the ports which are in communication with the respective suction-applying passages 25 and 64. It will now be appreciated that suction pressure is applied to the flexible diaphragm 14 and to the area below the piston 52 so that the piston 52 descends under the influence of its weight 55.

Once the piston 52 reaches the bottom of the cylinder 53 the suction pressure acts upon the underside of the lower lands of the valve spools 42A and 50 causing them to descend under the influence of their respective weights 56 and 57. This downward movement carries the spools 42A and 50 through the point at which they close the respective ports communicating with the respective suction-applying passages 25 and 64 to the position in which they open the ports which are in communication with the respective pressure-applying passages 22 and 62. It will be understood that once this cycle of motion of the valve spools and the piston has been initiated, it will continue as long as water pressure and suction pressure are applied through the respective pipes 22 and 62, and 25 and 64.

The magnitude of the weight 56 may be adjusted so that the valve spool 42A dwells longer in either the pressure-applying or suction-applying positions as required. The frequency of the pulsating flow of water supplied to the diaphragm 14 through the pipe 18 may be altered by adjusting throttle valves 75 provided in the pipes 51, 62 and 64, or by adjusting the position of the stop 74, or by altering the magnitude of the weights 55 to 57, and the volume of water flowing into the pipe 18 may be altered by adjusting throttle valves 76 provided in the pipes 22 and 25.

Springs may be employed in place of the weights 55 to 57. Such springs would preferably be compression springs acting on the topside of the spools 42A and 50 and of the piston 52. However, should the weight of the spools 42, 50, and the piston 52 be sufficient to ensure movement of the spools through the midposition (that is the position shown in FIG. 6 where the lands block both the pressure and the suction ports) then external loading means such as the weights 55 to 57 or the compression springs referred to above may be disposed with.

It will be appreciated that both the spool valves 48A and 49 may be replaced by two land spool valves 48 of FIG. 5. However, in this embodiment it would be necessary for the valve spool of such a two-land spool to be very good sliding fit in the valve body in order to minimize the likelihood of water under pressure from the pipes 22 or 62 leaking past the lower land into the pipe 51. Use of three-land spools as in the arrangement of FIG. 6 avoids this problem.

Referring to FIG. 7, there is illustrated a suitable form of pumping head 10 which may be employed in a blood pump in accordance with this invention. The pumping head 10 is generally tubular in form with the nonreturn valves 15 and 16 being located at each end of the head 10. Extending between the two valves 15 and 16 and surrounding the appropriate inlet or outlet of the respective valve 15, 16, is a thin tube of flexible material, such as suitable rubber or polythene, which acts as the flexible diaphragm 14 separating the areas 12 and 13. Coaxially surrounding the tubular diaphragm 14 is a thick tube 65 of a transparent plastics material, although it is to be understood that any other suitable material may be employed. The tube 65 is clamped at each end to the respective valve body of the corresponding valve 15, 16, by screwed end fittings 66. The tubular diaphragm 14 has enlarged shaped end portions 67 which fit into correspondingly shaped annular grooves 68 in the respective valve bodies; the shaped end portions 67 are clamped between the tube 65 and said respective valve bodies by the end fittings 66. The pipe 18 extends radially through the wall of the tube 65 and opens into the area 13 at a position adjacent the inlet valve 15 via an annular water feed groove 18A formed in the tube wall 65, which groove prevents the diaphragm 14 from blocking the pipe opening in operation.

In operation of the pumping head 10 illustrated in FIG. 7, it will be seen that when water under pressure is supplied through the pipe 18 it forces its way around the tubular flexible diaphragm 14 inside the thick tube 65 and presses the tubular flexible diaphragm 14 radially inwards, thus reducing the volume of the area 12 between the two valves 15, 16 and within the tubular flexible diaphragm 14 so as to force blood out of that area 12 through the nonreturn outlet valve 16. Furthermore, it will be understood that when the pressure of the water supplied through the pipe 18 is subsequently reduced and replaced by suction pressure, the flexible diaphragm 14 is sucked radially outwards until it contacts the inner surface of the tube 65, thereby increasing the volume of the area 12 and drawing blood into that area 12 through the nonreturn valve 15.

It will be appreciated that the pipe 18 opens into the space 13 adjacent the valve 15 to ensure that a maximum amount of blood may be fed, if required, through the valve 16 during the pressure applying stroke.

All the arrangements described so far have employed an essentially mechanical means for operating the device 17. It is also possible to use electrical means for this purpose, and a suitable circuit is illustrated in FIG. 8. A high-gain amplifier 100, a positive feedback resistor 101, a negative feedback network 102 and a capacitor 103 together form a multivibrator which produces an output signal of the form shown in FIG. 9. This signal is applied to the coil 109A of a reed relay, whose contact 109B in turn drives the coil 110A of another relay. The contact 110B of the latter relay energizes one or other of two solenoids 111 and 112 according as the coil 110A is energized or not. The solenoids 110 and 112 are connected to opposite ends of a spool valve 113 which may be similar to the spool valve 48 of FIG. 5. Thus the pressure signal applied to the pipe 18 will vary substantially, as the waveform of FIG. 9.

The circuit includes a pair of linked switches 104A and 104B. With these switches in the positions shown, the multivibrator is free-running; by moving the switches to their other positions, the multivibrator can be synchronized with a patient's own heartbeat by means of signals fed in on line 114 from a cardiograph.

The operation of the circuit is as follows. Assume that the switches 104A and 104B are in the free-running positions as shown, and that the output of the amplifier 100 is slightly positive. This positive output is fed back through the resistor 101 to the positive input to the amplifier, driving its output more positive, so that it immediately saturates with its output at the maximum positive level. This positive output is also applied through the network 102 to the negative input of the amplifier 100; however, the capacitor 103 is also connected to this negative input, so that the voltage at this negative input can only rise gradually as the capacitor charges through the network 102. Eventually however the voltage at the negative input of amplifier 100 will exceed the voltage at the positive input from resistor 101, and the output of the amplifier will therefore go negative. Resistor 101 will now act to hold the output at its maximum negative level, and this negative level will be fed back through the network 102 to charge the capacitor 103 in the opposite direction. Eventually, the signal at the negative input of amplifier 100 will again overpower the signal from the resistor 101 at the positive input, and the output will go positive one more. The circuit will continue to switch between positive and negative outputs in this manner indefinitely.

The network 102 consists of two separate paths. When the output of amplifier 100 is positive, diode 105 is forward-biassed and current can flow through the variable resistor 106, diode 107 being cut off; when the output of amplifier 100 is negative, current can flow through diode 107 and variable resistor 108, diode 105 being cut off. By adjusting the resistors 106 and 108 the durations of the positive and negative outputs from the amplifier 100, i.e. of the times t1 and t2 respectively (FIG. 9), can be individually adjusted.

In order to operate the circuit in the synchronized mode, the switches 104A and 104B are moved to their other positions. The signals on line 114 are amplified by an amplifier 115 and coupled over a coupling network 116. Also a variable resistor 117 is connected in series with resistor 106, thus increasing the period t1. This decreases the natural frequency of the multivibrator, so that the synchronizing signal can increase its frequency again to the required value.

The power supply to the relay coil 110A and to the solenoids 111 and 112 may be the AC mains.