Method for determining the back electromotive force induced in a voice-coil motor driven in discontinuous mode

A method for determining a back electromotive force induced in a coil of a voice-coil motor by its motion, driven in a discontinuous mode by commanding alternating on-phases and off-phases, the method comprising:

• sensing a drive current flowing in the coil or a drive voltage on the coil at an end of an on-phase;
• estimating an amplitude of a voltage disturbance induced in the coil by mutual induction between a permanent magnet of the voice-coil motor and the moving coil as a function of the sensed drive current or drive voltage;
• sensing voltage on the coil during a following off-phase with no current flowing in the coil;
• and
• determining the back electromotive force induced in the moving coil as a difference between the sensed voltage and the estimated amplitude of the voltage disturbance.

A method for determining a back electromotive force induced in a coil of a voice-coil motor by its motion, driven in a discontinuous mode by commanding alternating on-phases and off-phases, the method comprising:

• sensing an electrical condition of the coil at an on-phase;
• estimating a voltage disturbance induced in the coil by mutual induction between a permanent magnet of the voice-coil motor and the moving coil as a function of the sensed electrical condition;
• sensing voltage on the coil during a following off-phase with no current flowing in the coil;
• and
• determining the back electromotive force induced in the moving coil as a difference between the sensed voltage and the estimated voltage disturbance.

A method for driving in a discontinuous mode a voice-coil motor by alternating on-phases, during which a drive current flows through the coil, to off-phases during which the coil is tristated, the method comprising:

• sensing voltage on the coil during off-phases with no current flowing in the coil;
• sensing the drive current flowing through the coil or the drive voltage applied to the coil at an end of an on-phase;
• determining a back electromotive force induced in the moving coil as a function of the sensed drive voltage on the coil during off-phases;
• forcing in the coil a current pulse having an amplitude and a duration determined as a function of the determined back electromotive force;
• comparing with a threshold an absolute value of a difference between the drive current flowing through the coil or the drive voltage applied to the coil at the end of the on-phase and the value at an end of a previous on-phase;
• and
• if the threshold is surpassed, then the back electromotive force is considered constant and equal to a last determined value for a number of successive off-phases.

An apparatus comprising:

• a voice-coil motor (VCM) comprising a permanent magnet, and a coil adjacent said permanent magnetic;
• a microprocessor coupled to said VCM for determining a back electromotive force induced in said coil by its motion, and for driving said VCM in a discontinuous mode by commanding alternating on-phases and off-phases;
• an operational amplifier coupled to said VCM for sensing an electrical condition of said coil at an on-phase;
• an analog-to-digital converter coupled to an output of said operational amplifier for digitizing the sensed electrical condition;
• said microprocessor coupled to an output of said analog-to-digital converter for estimating a voltage disturbance induced in said coil by mutual induction between said permanent magnet and said moving coil as a function of the digitized sensed electrical condition;
• said operational amplifier sensing voltage on said coil during a following off-phase with no current flowing in said coil;
• and
• said microprocessor determining the back electromotive force induced in said moving coil as a difference between the sensed voltage and the estimated voltage disturbance.