WO2018146467A1

WO2018146467A1 - Control of a pump driven by shape memory alloy wire

Info

Publication number: WO2018146467A1
Application number: PCT/GB2018/050341
Authority: WO
Inventors: James Howarth; David Charles William Richards; Andreas FLOURIS
Original assignee: Cambridge Mechatronics Limited
Priority date: 2017-02-08
Filing date: 2018-02-07
Publication date: 2018-08-16
Also published as: GB201702038D0; GB201913010D0; GB2574535B; GB2574535A

Abstract

A pump includes a pumping arrangement comprising a movable element arranged to pump a fluid and shape memory alloy wires coupled to the movable element for moving the movable element. The resistance of at least one of said one or more shape memory alloy wires is measured. A control system supplies drive signals to the shape memory alloy wires and monitors the measured resistance for verifying the operation of the pumping arrangement. The control system detects abnormal operation of the pumping arrangement on the basis of the measured resistance and detects abnormal resistance to movement of the movable element.

Description

Control of A Pump Driven By Shape Memory Alloy Wire The present invention relates to a pump driven by shape memory alloy (SMA) wire. The pump has particular application for the pumping of relatively small amounts of fluid, for example a therapeutic product that may be delivered to a human body.

By way of example, infusion pumps are medical devices used to administer a predetermined amount of therapeutic product (e.g. insulin) to the human body (e.g. in the subcutaneous tissue) in a controlled manner. In the case of insulin infusion pumps, the delivered volume of insulin at a basal rate (background) and at a bolus rate (an increased dose for a mealtime) should be of sufficient accuracy to ensure that the glucose

concentration in the bloodstream is always maintained within the desired levels.

Current products maintain accuracy by controlling and measuring the volume of insulin they deliver. One such method involves the use of pressure sensors inside the insulin flowpath. The insulin flowrate can be calculated by comparing the pressure measurements against known flowrate data. A method that can be used in pumps with a leadscrew-driven piston is to measure the rotations of the shaft that drives the piston. The shaft rotation is related to the piston displacement and therefore can be used to calculate the volume of delivered insulin. A method for delivering a bolus dose involves the use of a mechanical system that releases a fixed volume of insulin when a button is pressed fully and releases no insulin when the button is pressed partially.

A particular issue is that the insulin flowpath can become blocked, for example when a kink is formed or when debris builds up inside the tubing. Such obstructions are called occlusions. Occlusions stop the insulin flow and no accurate dosing is possible until the occlusion is detected and corrected. Therefore, occlusions should be detected as quickly as possible so that an alarm can be raised to notify the user. Occlusions cause the insulin pressure inside the pumping unit to increase progressively as the infusion device tries to pump the insulin. A common method to detect occlusions is to detect the progressive increase in insulin pressure using pressure sensors or by detecting the progressive increase in force required to actuate the pumping unit. The pressure and force increase progressively because of the elastic response of the system.

For verifying proper operation, for example dose accuracy and occlusion detection, such current products are limited by the requirement to use accurate pressure, rotation or force sensors which can be expensive or by using complex mechanical systems which can be bulky.

According to a first aspect of the present invention, there is provided a pump comprising: a pumping arrangement comprising a movable element arranged to pump a fluid; one or more shape memory alloy wires coupled to the movable element for moving the movable element; a resistance measurement circuit arranged to measure the resistance of at least one of said one or more shape memory alloy wires; and a control system arranged to supply drive signals to the one or more shape memory alloy wires, and arranged to monitor the measured resistance for verifying the operation of the pumping arrangement.

Thus, the pump uses one or more SMA wires to move a movable element of a pumping arrangement that is arranged to pump a fluid, for example a therapeutic product such as insulin. Use of SMA wire as an actuator for the pump has numerous advantages compared to other types of actuator, particularly for miniature devices. Such advantages include provision of high forces in compact arrangements. In order to verify the operation, a control system that supplies drive signals to the one or more SMA wires also monitors the electrical resistance of at least one of the one or more shape memory alloy wires that is measured by a resistance measurement system. As the electrical resistance provides an accurate measure of the length of the SMA wire, this allows verification of the proper operation in a convenient and reliable manner, without requiring any additional sensor elements.

For example, the control system may calculate the length of the at least one of the one or more shape memory alloy wires from the measured resistance. The control system may control the drive signals in response to the measured resistance so as to pump a desired amount of fluid.

In one type of pump, the movable element has a reciprocating stroke and is arranged to deliver a fixed volume on each stroke. In that case, the control system may monitor the measured resistance and verify that the movable element moves the full extent of the stroke.

The control system may detect abnormal operation of the pumping arrangement on the basis of the measured resistance and may output a warning signal in response to detecting abnormal operation of the pumping arrangement.

In addition, the control system is arranged to detect abnormal resistance to movement of the movable element, for example as may occur in the case of an occlusion. The control system may output a warning signal in response to detecting abnormal resistance to movement of the movable element.

Various techniques for detecting abnormal resistance to movement of the movable element may be applied. Some non-limitative examples of such techniques, which may applied in any combination, are as follows.

In a first example, the one or more shape memory alloy wires comprise opposed shape memory alloy wires coupled to move the movable element in opposite directions, one of the opposed shape memory alloy wires being coupled to the movable element in series with a spring. In this case, the resistance measurement circuit is arranged to measure the resistance of both of the opposed shape memory alloy wires, and the control system is arranged to calculate a measure of the tension in the spring from the measured resistances, and to detect abnormal resistance to movement of the movable element on the basis of the calculated measure of the tension in the spring.

In a second example, the control system is arranged to monitor the power of the drive signals and to detect abnormal resistance to movement of the movable element on the basis of the power of the drive signals being abnormally high.

In a third example, the pump comprises a temperature sensing arrangement arranged to sense the temperature of the one or more shape memory alloy wires. In this case, the control system is arranged to monitor the sensed temperatures and to detect abnormal resistance to movement of the movable element on the basis the sensed temperature being abnormally high.

In a fourth example, the control system is arranged to detect abnormal resistance to movement of the movable element on the basis of the measured resistance changing abnormally slowly.

Further according to a first aspect of the present invention, there is provided a method of controlling a pump in a corresponding manner.

According to a second aspect of the present invention, there is provided a pump comprising: a pumping arrangement comprising a movable element arranged to pump a fluid; opposed shape memory alloy wires coupled to the movable element for moving the movable element in opposite directions, one of the opposed shape memory alloy wires being coupled to the movable element in series with a spring; a control system arranged to supply drive signals to the one or more shape memory alloy wires, the control system further being arranged to monitor the length of the spring and to detect abnormal resistance to movement of the movable element on the basis of the monitored length.

Thus, the pump uses opposed SMA wires to move a movable element of a pumping arrangement that is arranged to pump a fluid, for example a therapeutic product such as insulin. Use of SMA wire as an actuator for the pump has numerous advantages compared to other types of actuator, particularly for miniature devices. Such advantages include provision of high forces in compact arrangements. In order to detect abnormal resistance to movement of the movable element, one of the opposed shape memory alloy wires is coupled to the movable element in series with a spring. As the spring is coupled in series, the length of the spring is indicative of the force applied by the SMA wire. Accordingly, a control system that supplies drive signals to the one or more SMA wires also monitors the length of the spring and detects abnormal resistance to movement of the movable element on the basis of the monitored length.

Further according to the second aspect of the present invention, there is provided a method of controlling a pump in a corresponding manner.

To allow better understanding, an embodiment of the present invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which:

Figs. 1 and 2 are schematic views of a first pump;

Figs. 3 and 4 are schematic views of a second pump;

Fig. 5 is a diagram of a control system; and

Fig. 6 is a graph of power dissipated by an SMA wire of the second pump in some different circumstances.

Figs. 1 and 2 illustrate a first pump 1 and Figs 3 and 4 illustrate a second pump 2. The first and second pumps 1 and 2 are piston pumps that are arranged as follows.

The first and second pumps 1 and 2 pump a fluid. The fluid may be a liquid. The fluid may be a therapeutic product. Such a therapeutic product may in general be of any type and have any therapeutic effect. By way of example, the therapeutic product may be insulin.

The first and second pumps 1 and 2 each comprise a pumping arrangement 3 arranged as follows. The pumping arrangement comprises a cylinder 4 in which a piston 5 is movable by sliding. The piston 5 therefore forms a movable element and is connected to a piston rod 6.

The cylinder 4 has a one-way inlet valve 7 and a one-way outlet valve 8, which allow flow of fluid in the direction of the arrows only. The piston 5 has a reciprocating stroke between the position shown in Fig. 1 where the cylinder 4 has a maximum volume and the position shown in Fig. 2 where the cylinder 4 has a minimum volume. Thus, when the piston 5 undergoes reciprocating movement, the piston 5 pumps a fluid by expelling fluid from the cylinder 4 through the outlet valve 8 during the compression stroke (from Fig. 1 to Fig. 2 or from Fig. 3 to Fig. 4) and drawing fluid into the cylinder 4 through the inlet valve 7 during the induction stroke (from Fig. 2 to Fig. 1 or from Fig. 4 to Fig. 3). The piston 5 therefore pumps a fixed volume of fluid on each stroke.

In addition, the first and second pumps 1 and 2 each comprise an SMA actuator arrangement 10 arranged as follows.

In each of the first and second pumps 1 and 2, the SMA actuator arrangement 10 comprises a first SMA wire 11 and a second SMA wires 12 arranged as follows. The first and second SMA wires 11 and 12 are each coupled between a fixed body 13 and the piston rod 6. The fixed body 13 is fixed with respect the pumping arrangement 3.

The first and second SMA wires 11 and 12 are opposed to each other and on contraction drive movement of the piston 5 in opposite directions, as shown by the arrow A. Thus, contraction of the first SMA wire 11 drives the induction stroke of the piston 5 and contraction of the second SMA wire 12 drives the compression stroke of the piston 5. In the state shown in Fig. 1 , the temperature of the first SMA wire 11 is increased compared to the temperature of the second SMA wire 12 by supplying it with a drive current of larger electrical power. This increase in temperature has caused the length of the first SMA wire 11 to decrease, and the piston rod 6 is pulled to the left. Conversely, in the state shown in Fig. 2, the second SMA wire 12 is heated and contracts, causing the length of the first SMA wire 11 to increase, and the piston rod 6 is pulled to the right.

In the first pump 1 shown in Figs. 1 and 2, the second SMA wire 12 is coupled between the fixed body 13 and the piston rod 6 in series with a mechanical spring 14. Thus, as the first SMA wire 11 is coupled directly between the fixed body 13 and the piston rod 6, the spring 14 effectively couples the first and second SMA wires 11 and 12 together.

In the example shown in Figs. 1 and 2, the spring 14 is arranged between the second SMA wire 12 and the piston rod 6. As an alternative the spring 14 could be arranged between the second SMA wire 12 and the fixed body 13.

In the second pump 2 shown in Figs. 3 and 4, the spring 14 is omitted and so the second SMA wire 12 is coupled directly between the fixed body 13 and the piston rod 6.

The first and second pumps 1 and 2 are controlled by a control system 20 shown in Fig. 5 and configured as follows.

As is conventional, the length of the first and second SMA wires 11 and 12 is varied by varying the temperature of the first and second SMA wires 11 and 12 by regulating the power of the drive signals that pass through them. The first and second SMA wires 11 and 12 reset to their original length at ambient temperature. The control system 20 is connected to the first and second SMA wires 1 1 and 12 and supplies drive signals thereto. The control system 20 may be implemented in any suitable manner, for example in an integrated circuit chip. The control system 20 includes a drive circuit 21 arranged to generate the drive signals, and a control unit 22 that is arranged to control the drive circuit 21. The drive circuit 21 may be implemented by suitable electronic components. The control unit 22 may be implemented by a processor executing an appropriate program.

The control unit 22 controls the power of the drive signals supplied by the circuit 21. For example, the drive signals may be pulse-width modulated signals whose pulse- width is controlled by the control unit 22 to vary the power of the drive signals and thereby control the first and second SMA wires 1 1 and 12.

The control system 20 further includes a resistance measurement circuit 23 that is connected to the first and second SMA wires 1 1 and 12 and measures the resistances Rl and R2 thereof. A measure of the resistance of the first and second SMA wires 1 1 and 12 output from the resistance measurement circuit 23 is supplied to control unit 22 which uses it as a feedback signal to control the power of the drive signals under closed loop control.

The control unit 22 of the control system 20 monitors the measured resistance and verifies the operation of the pumping arrangement 3 as follows. The control unit 22 detects abnormal operation of the pumping arrangement on the basis of the measured resistance, and outputs a warning signal in response to detecting abnormal operation of the pumping arrangement. The warning signal may be of any suitable type, for example an electrical signal, a visible signal or an audible signal.

The resistance of the first and second SMA wires 1 1 and 12 is related to their length, and so the control unit 22 calculates the length of the first and second SMA wires 1 1 and 12 from the measured resistances, and hence the position of the piston 5.

In the first pump 1 , the length of the first SMA wire 1 1 is directly related to the position of piston rod 6 and hence the piston 5. Therefore, the measured resistance of the first SMA wire 11 is used to calculate its length which is used to detect the position of piston rod 6 to verify that the full piston stroke has been achieved.

In the second pump 2, the length of both the first and second SMA wires 1 1 and 12 are directly related to the position of piston rod 6 and hence the piston 5. Therefore, the measured resistances of both the first and second SMA wires 1 1 and 12 are used to calculate their lengths which are used to detect the position of piston rod 6 to verify that the piston 5 has moved the full extent of its stroke. For actuation, the control unit 22 may target a predetermined value for each of the resistances Rl and R2 at different instances that respectively correspond to the ends of the stroke of the piston 5. The drive signal flowing through the first and second SMA wires 11 and 12 can be varied until the predetermined resistance values have been achieved so that the pump is fully actuated and a fixed volume of fluid is pumped on each stroke of the piston 5. A number of repeated pump actuations can be performed to pump a desired amount of fluid.

The desired amount may be chosen depending on the nature and use of the fluid. For example, in the case that the fluid is a therapeutic product, then the desired amount may be chosen to provide a desired therapeutic effect. For the example of insulin, the desired amount may be in a range from a lower limit at or below 0.05 U/hour to an upper limit at or above 20 U/hour, corresponding to a lower limit at or below 1.735 μg/hour to an upper limit at or above 714 μg /hour. Also, for the example of insulin, the flow range may cover both basal and bolus insulin delivery, which for a typical population of patients may be of the order of three orders of magnitude. For example, the flow rate range may be 0.025 to 25 U/hour (U represents an international unit of insulin which is the biological equivalent of 34.7 μg of human insulin), in increments of 0.025 U/hour.

The control unit 22 also detects abnormal resistance to movement of the piston 5, and outputs a warning signal in response to detecting such resistance to movement. The warning signal may be of any suitable type, for example an electrical signal, a visible signal or an audible signal. Several different techniques for detecting abnormal resistance to movement may be applied, some examples of which are as follows.

A first technique for detecting abnormal resistance to movement of the piston 5 that is applied in the first pump 1 is as follows.

During normal operation, in an instance where the first and second SMA wires 11 and 12 have stabilised at a desired values of Rl and R2, the first and second SMA wires 11 and 12 and the spring 14 have an equal tension that is also equal to the initial tension in the system. The initial tension can be determined during initiation.

The piston 5 is moved across the extent of the compression stroke by the second SMA wire 12. During the compression stroke, the tension in the first and second SMA wires 11 and 12 and the spring 14 are monitored by the control unit 22 monitoring the measured electrical resistances Rl and R2 of the first and second SMA wires 11 and 12. In particular, as the resistances Rl and R2 are indicative of the length of the first and second SMA wires 11 and 12 and the total combined length of the first and second SMA wires 11 and 12 and the mechanical spring 14 is constant, so the control unit 22 calculates the length of the mechanical spring 14 which is a measure of the tension in the spring 14, the length and tension of the spring 14 being proportional. The tension in the second SMA wire 12 is equal to the tension in the spring 14.

This tension is monitored during the compression stroke and can be compared against known data stored in the control unit 22 to detect abnormal resistance to movement of the piston. For example, the presence of an occlusion will cause a progressive increase in the pressure, above the pressure when no occlusion is present, increasing the resistance to movement of the piston 5. This will be observed as a progressive increase in the tension in the spring 14 and the second SMA wire 12 during the compression stroke.

Additionally, the control unit 22 may also monitor the measured electrical resistance R2 and may detect abnormal physical resistance to movement when the target value for the resistance R2 is not achieved. This will occur in the case that the force required to drive movement of the piston 5 is larger than the maximum actuation force provided by the second SMA wire 12.

A similar method in reverse may be implemented also in the induction stroke. The first technique for detecting abnormal resistance to movement of the piston 5 cannot be applied in the second pump 2 because of the absence of the spring 14. Instead, any of the following techniques may be applied in the second pump 2 (or in principle in the first pump 1 also).

A second technique for detecting abnormal resistance to movement of the piston 5 that is applied in the second pump 2 is for the control unit 22 to monitor the power of the drive signals and to detect abnormal resistance to movement of the piston 5 on the basis of the power of the drive signals being abnormally high. This is because an abnormally high resistance to movement, for example due to an occlusion, increases the power that that is dissipated in the first and second SMA wires 11 and 12.

By way of example, Fig. 6 illustrates the power P2 dissipated by the second SMA wire 12 of the second pump 2 over time during the compression stroke. The solid line 30 in Fig. 6 represents the power P2 dissipated in normal operation. The power rises to the peak 31 and settles to the plateau 32. The presence of an occlusion will cause the power P2 to follow one of the dotted lines 33, which each have a similar peak 31 but settle to respective plateaus 34 at a higher power level than during normal operation. Typically, in the case of an occlusion, the power of the plateau increases with each successive actuation because of the larger force requirement. Similarly, when the actuation force produced by the second SMA wire 12 is lower than the force required to move the piston 5, the power will remain at the level of the peak and will not plateau as shown by the dashed-dotted line 35. This is because the length of the second SMA wire 12 will remain constant and actuation will not be possible. Such profiles of the shape of the power over time are detected by the control unit 22 as indicating abnormal resistance to movement of the piston 5. The same methodology can be used to detect occlusions in the inlet fiowpath before the pumping arrangement 3 by monitoring the power PI of the first SMA wire 11.

A third technique for detecting abnormal resistance to movement of the piston 5 that is applied in the second pump 2 is for the control system 20 to include temperature sensors 24 which act as a temperature sensing arrangement arranged to sense the temperature of the first and second SMA wires. In this case, the control unit 22 monitors the sensed temperatures and detect abnormal resistance to movement of the piston 5 on the basis the sensed temperature being abnormally high. An abnormally high sensed temperature is indicative of an abnormally high resistance to movement, for example due to an occlusion, for the same reasons as discussed above with respect to an increase in power. The temperature of the first and second SMA wires 11 and 12 above the ambient temperature is indicative of the electrical power dissipated in the first and second SMA wires 11 and 12. The temperature measurements can be compared against the electrical current measurements as a validation check or temperature parameters Tl and T2 can be used instead of the power values PI and P2 for occlusion detection as described above.

A fourth technique for detecting abnormal resistance to movement of the piston 5 that is applied in the second pump 2 is for the control unit 22 to detect abnormal resistance to movement of the piston 5 on the basis of the measured resistance changing abnormally slowly. An abnormally slow change of electrical resistance is indicative of an abnormally high resistance to movement, for example due to an occlusion, for the same reasons as discussed above with respect to an increase in power. In this manner, the detection may be performed on the basis solely of the resistance measured by the resistance measurement circuit 23.

The various techniques for detecting abnormal resistance to movement of the piston 5 may be used in combination for increased accuracy of detection.

Various modifications to the first and second pumps 1 and 2 are possible. For example, the opposed first and second SMA wires 11 and 12 may be replaced by a single SMA wire biased by resilient biasing element such as a biasing spring.

In addition the form of the first and second pumps 1 and 2 as piston pumps is not limitative. In general, the techniques described herein may be applied to any type of positive displacement pump (e.g. diaphragm pump) in which one or more SMA wires drive movement of a movable element that pumps a fluid. For example the cylinder 4 and piston 5 may be replaced by any chamber having a volume that is varied by a movable member, for example formed by a flexible wall.

Claims

1. A pump comprising:

a pumping arrangement comprising a movable element arranged to pump a fluid; one or more shape memory alloy wires coupled to the movable element for moving the movable element;

a resistance measurement circuit arranged to measure the resistance of at least one of said one or more shape memory alloy wires; and

a control system arranged to supply drive signals to the one or more shape memory alloy wires, and arranged to monitor the measured resistance for verifying the operation of the pumping arrangement.

2. A pump according to claim 1, wherein the control system is arranged to calculate a measure of the length of the at least one of said one or more shape memory alloy wires from the measured resistance.

3. A pump according to claim 1 or 2, wherein the movable element has a reciprocating stroke and is arranged to deliver a fixed volume on each stroke, and the control system is arranged to monitor the measured resistance and to verify that the movable element moves the full extent of the stroke.

4. A pump according to any one of the preceding claims, wherein the control system is arranged to control the drive signals in response to the measured resistance so as to pump a desired amount of fluid.

5. A pump according to any one of the preceding claims, wherein the control system is arranged to detect abnormal operation of the pumping arrangement on the basis of the measured resistance.

6. A pump according to claim 5, wherein the control system is arranged to output a warning signal in response to detecting abnormal operation of the pumping arrangement.

7. A pump according to any one of the preceding claims, wherein the control system is arranged to detect abnormal resistance to movement of the movable element.

8. A pump according to claim 7, wherein

the one or more shape memory alloy wires comprise opposed shape memory alloy wires coupled to move the movable element in opposite directions, one of the opposed shape memory alloy wires being coupled to the movable element in series with a spring, the resistance measurement circuit is arranged to measure the resistance of both of the opposed shape memory alloy wires, and

the control system is arranged to calculate a measure of the tension in the spring from the measured resistances, and to detect abnormal resistance to movement of the movable element on the basis of the calculated measure of the tension in the spring.

9. A pump according to claim 7, wherein the control system is arranged to monitor the power of the drive signals and to detect abnormal resistance to movement of the movable element on the basis of the power of the drive signals being abnormally high.

10. A pump according to claim 7 or 9, further comprising a temperature sensing arrangement arranged to sense the temperature of the one or more shape memory alloy wires, and wherein the control system is arranged to monitor the sensed temperatures and to detect abnormal resistance to movement of the movable element on the basis the sensed temperature being abnormally high.

11. A pump according to any one of claims 7, 9 or 10, wherein the control system is arranged to detect abnormal resistance to movement of the movable element on the basis of the measured resistance changing abnormally slowly.

12. A pump according to any one of claims 7 to 11, wherein the control system is arranged to output a warning signal in response to detecting abnormal resistance to movement of the movable element.

13. A method of controlling a pump that comprises a pumping arrangement comprising a movable element arranged to pump a fluid and one or more shape memory alloy wires coupled to the movable element for moving the movable element, the method comprising: supplying drive signals to the one or more shape memory alloy wires;

measuring the resistance of at least one of said one or more shape memory alloy wires; and

monitoring the measured resistance for verifying the operation of the pumping arrangement.

14. A method according to claim 13, further comprising calculating the length of the at least one of said one or more shape memory alloy wires from the measured resistance.

15. A method according to claim 14, wherein the movable element has a reciprocating stroke and is arranged to deliver a fixed volume on each stroke, and method comprises monitoring the measured resistance and verifying that the movable element moves the full extent of the stroke.

16. A method according to any one of claims 13 to 15, wherein the supplied drive signals are controlled in response to the measured resistance so as to pump a desired amount of fluid.

17. A method according to any one of claims 13 to 16, further comprising detecting abnormal operation of the pumping arrangement on the basis of the measured resistance.

18. A method according to claim 17, further comprising outputting a warning signal in response to detecting incorrect operation of the pumping arrangement.

19. A method according to any one of claims 13 to 18, further comprising detecting abnormal resistance to movement of the movable element.

20. A method according to claim 19, wherein

the one or more shape memory alloy wires comprise opposed shape memory alloy wires coupled to move the movable element in opposite directions, one of the opposed shape memory alloy wires being coupled to the movable element in series with a spring, the resistance of both of the opposed shape memory alloy wires are measured, and the method further comprises calculating a measure of the tension in the spring from the measured resistances, and detecting abnormal resistance to movement of the movable element on the basis of the calculated measure of the tension in the spring.

21. A method according to claim 20, further comprising monitoring the power of the drive signals and detecting abnormal resistance to movement of the movable element on the basis of the power of the drive signals being abnormally high.

22. A pump according to claim 19 or 21 , further comprising sensing the temperature of the one or more shape memory alloy wires, and detecting abnormal resistance to movement of the movable element on the basis the sensed temperature being abnormally high

23. A method according to any one of claims 19, 21 or 22, further comprising detecting abnormal resistance to movement of the movable element on the basis of the measured resistance changing abnormally slowly.

24. A pump according to any one of claims 19 to 23, further comprising outputting a warning signal in response to detecting abnormal resistance to movement of the movable element.

25. A pump comprising:

a pumping arrangement comprising a movable element arranged to pump a fluid; opposed shape memory alloy wires coupled to the movable element for moving the movable element in opposite directions, one of the opposed shape memory alloy wires being coupled to the movable element in series with a spring; and

a control system arranged to supply drive signals to the one or more shape memory alloy wires, the control system further being arranged to monitor the length of the spring and to detect abnormal resistance to movement of the movable element on the basis of the monitored length.

26. A method of controlling a pump that comprises a pumping arrangement comprising a movable element arranged to pump a fluid and opposed shape memory alloy wires coupled to the movable element for moving the movable element in opposite directions, one of the opposed shape memory alloy wires being coupled to the movable element in series with a spring,

the method comprising:

supplying drive signals to the one or more shape memory alloy wires;

monitoring the length of the spring; and detecting abnormal resistance to movement of the movable element on the basis of the monitored length.