Measuring pressure

1. A method of determining a negative pressure generated by a suction pump of a topical negative pressure, TNP, system, the method comprising:

disconnecting a drive voltage from the suction pump;

determining an electromotive force, EMF, generated by a free-wheeling element of the pump;

determining an actual pump speed responsive to the determined EMF;

comparing the actual pump speed with a predetermined desired speed;

adjusting the drive voltage responsive to a difference between the actual pump speed and the desired speed; and

reconnecting the adjusted drive voltage to the pump.

2. The method as claimed in claim 1 , wherein adjusting the drive voltage comprises adjusting the drive voltage responsive to a scaled difference between the actual pump speed and the desired pump speed.

3. The method as claimed in claim 2 , further comprising:

averaging a plurality of preceding scaled values with a current scaled value, and adjusting the drive voltage responsive to an average of the scaled values.

4. The method as claimed in claim 2 , further comprising:

prior to use, determining scaling constants associated with the TNP system.

5. The method as claimed in claim 1 , wherein determining the EMF comprises:

measuring a terminal voltage across the pump with zero supplied current.

6. The method as claimed in claim 1 , further comprising:

disconnecting the drive voltage by determining a control signal provided to a pulse width modulation, PWM, generator, an output generated by the PWM generator being arranged to drive the pump.

7. A method as claimed in claim 1 , wherein determining the EMF comprises:

allowing a predetermined period of time to elapse subsequent to disconnection of the drive voltage; and

subsequent to expiry of the predetermined period of time, sampling a steady state value of EMF generated by rotation of a pump shaft.

8. The method as claimed in claim 1 , further comprising:

periodically repeating disconnecting the drive voltage, adjusting the drive voltage, and reconnecting the adjusted drive voltage to the pump.

9. The method as claimed in claim 8 , further comprising:

selecting an EMF sampling rate for determining the adjusted drive voltage for the pump responsive to a pump speed determined for the pump.

10. The method as claimed in claim 9 , further comprising:

reducing the EMF sampling rate as the actual pump speed increases.

11. Apparatus that determines a negative pressure generated by a topical negative pressure, TNP, system, comprising:

a pump configured to provide a negative pressure responsive to a pump speed;

a pulse width modulation, PWM, generator configured to provide an output signal which provides a drive voltage for the pump; and

a processor configured to calculate an electromotive force, EMF, generated by a free-wheeling element of the pump, compare an actual pump speed associated with the calculated EMF with a desired speed to determine a difference in the speeds, and determine a new drive voltage by increasing or decreasing the drive voltage based on the difference,

wherein the PWM generator is configured to receive a control signal that disconnects the pump from a drive voltage and reconnects the new drive voltage to the pump.

12. The apparatus as claimed in claim 11 , further comprising a sensor for determining the EMF generated by the free-wheeling element of the pump.

13. The apparatus as claimed in claim 11 , further comprising a controller for generating the control signal that disconnects the pump from the drive voltage and reconnects the new drive voltage to the pump.