Method and apparatus for accurately measuring a back-emf voltage on an actuator coil

An apparatus for controlling an actuator which has a moveable member and which has a coil that influences movement of the member, comprising:

• an actuator driver circuit having an input and having two output terminals;
• a first electronic switching circuit having first and second states and having first and second switches which are both open in said first state of said first electronic switching circuit and both closed in said second state of said first electronic switching circuit, said first and second switches each being coupled between a respective said output terminal of said actuator driver circuit and a respective end of the coil;
• a voltage holding circuit which includes a capacitor;
• a second electronic switching circuit having first and second states in which said second electronic switching circuit respectively effects and interrupts an electrical coupling between the coil and said capacitor, wherein when said second electronic switching circuit is in the second state said capacitor has applied thereto a voltage which represents a back-emf of the coil, the back-emf of the coil representing an actual velocity of the member, said capacitor thereafter holding the voltage while said second electronic switching circuit is in the first state thereof;
• feedback circuitry responsive to the voltage held by the voltage holding circuit and responsive to an input representing a desired velocity of the member, said feedback circuitry being operative to output a feedback signal which is a function of a difference between the desired velocity of the member and the actual velocity of the member as represented by the voltage from said capacitor;
• a circuit which includes a processor and is operative to output a control signal;
• a third electronic switching circuit operative to supply to said input of said actuator driver circuit a selected one of the feedback signal and the control signal, wherein when said third electronic switching circuit is supplying the feedback signal to said input of said actuator driver circuit the feedback signal controls said driver circuit in a manner causing the member to be urged to move at the desired velocity;
• and
• a switching control circuit which, when said third electronic switching circuit is supplying the feedback signal to said driver circuit, is operative to periodically change said first electronic switching circuit from the second state thereof to the first state thereof and subsequently from the first state thereof back to the second state thereof, and operative during the first state of said first electronic switching circuit for changing said second electronic switching circuit from the first state thereof to the second state thereof and then from the second state thereof back to the first state thereof, said second electronic switching circuit being changed from the first state thereof to the second state thereof a predetermined time interval after said first electronic switching circuit has been changed from the first state thereof to the second state thereof, said predetermined time interval being sufficient to allow a current through the coil to decrease substantially to zero.