Velocity sensing using actuator coil back-emf voltage

An apparatus for measuring the radial velocity of a head in a disc drive having a disc and an actuator controllably positionable with respect to the disc, the actuator including the head and a coil of a voice coil motor, the apparatus comprising:

• a current sense resistor electrically connected in series with the coil;
• control circuitry for controllably applying, current to the coil and the current sense resistor;
• and
• velocity sense circuitry responsive to the current sense resistor and the coil, the velocity sense circuitry measuring the motion-induced back emf in the coil as the coil is moved relative to the disc, wherein the velocity sense circuitry comprises:
• a first differential amplifier, responsive to the current sense resistor and the coil, for generating a total voltage drop signal, the total voltage drop signal indicative of the total voltage drop across the current sense resistor and the coil;
• a second differential amplifier, responsive to the current sense resistor, for generating a dc current signal, the dc current signal indicative of the magnitude of the current applied by the control circuitry to the coil and the current sense resistor;
• a subtractor, responsive to the first and second differential amplifiers, for generating a coil voltage drop signal, the coil voltage drop signal indicative of the dc voltage drop across the coil;
• a differentiator, responsive to the second differential amplifier, for differentiating the dc current signal to generate a differentiated dc current signal;
• and
• a summing junction, responsive to the subtractor and the differentiator, for summing the coil voltage drop signal and the dc current signal to generate a velocity signal indicative of the radial velocity of the head.

An apparatus for measuring the radial velocity of a head in a disc drive having a disc and an actuator controllably positionable with respect to the disc, the actuator including the head and a coil of a voice coil motor, the apparatus comprising:

• a current sense resistor electrically connected in series with the coil;
• a control circuitry for controllably applying current to the coil and the current sense resistor;
• and
• velocity sense circuitry responsive to the current sense resistor and the coil, the velocity sense circuitry measuring the motion-induced back emf in the coil as the coil is moved relative to the disc, wherein the velocity sense circuitry comprises a servo processor, the servo processor measuring the radial velocity of the head by:
• generating a total voltage drop signal, the total voltage drop signal indicative of the total voltage drop across the current sense resistor and the coil;
• generating a dc current signal, the dc current signal indicative of the magnitude of the current applied by the control circuitry to the coil and the current sense resistor;
• generating a coil voltage drop signal, the coil voltage drop signal indicative of the dc voltage drop across the coil;
• differentiating the dc current signal to generate a differentiated dc current signal;
• and
• summing the coil voltage drop signal and the dc current signal to generate a velocity signal indicative of the radial velocity of the head;
• and
• wherein the servo processor uses the velocity signal to control the position of the head.

A disc drive, comprising:

• a rotatable disc;
• an actuator assembly adjacent the disc which positions a read/write head over tracks defined on the disc in relation to current passed through a coil of the actuator assembly;
• a current sense resistor of selected resistance connected in series with the coil;
• a control circuit operably connected to the coil and the current sense resistor which selectively applies the current in a first direction so that the current passes through the coil and then through the current sense resistor, and a second direction so that the current passes through the current sense resistor and then through the coil, the first and second direction of current selected in relation to a desired radial movement of the read/write head;
• and
• a velocity sense circuit operably connected across the coil and the current sense resistor which generates a velocity signal indicative of a radial velocity of the read/write head relative to the disc in relation to a measured voltage drop across the coil and the current sense resistor and a measured resistance of the coil.