# Voice coil motor force control using slow decay mode

1. Aug 6, 2011

### lagfish

Hi,
I am trying to control a voice coil motor's force, which is directly proportional to the current through its coil. The direction of the force needs to be reversed, so I thought an H-bridge would be a good way to do it. I have read a lot about H-bridges but I am still confused about a few points. I am using a "chop drive" current control. Please tell me if the following makes sense:
A chop drive switches the coil voltage between supply and "off", the duty cycle of which determines the average current. However, a inductive load cannot be turned off conventionally, as the collapsing magnetic field will induce a massive potential that will destroy all switches. There are two modes of turning the coil "off": slow decay and fast decay.

During slow decay, the inductor is shorted to itself, and the inductor discharges through its own coil resistance the on resistance of the two switches. The induced magnetic field, although decaying, is still present and continues to provide a force proportional to the current and in the same direction as previous. If the motor has a velocity, the movement of the coil through the permanent magnet's field will induce a second current in the coil opposite in direction, as per Faraday and Lenz's Laws. This induced current will increase the rate of current decay. When the voltage in the inductor becomes less than the back EMF, which is equivalent to saying the decaying current becomes less than the velocity induced current, the current will switch directions and the induced magnetic field will oppose the permanent magnet's field, providing braking.

During fast decay, the inductor sees negative supply voltage, and the rate of current decay should be equal to the curreng rising rate. If the motor is in motion, the induced current will again help increase the rate of decay. Since the back EMF is the same sign as the supply, the velocity will provide generation.

In either cases, the force produced by the coil is equal to its instantaneous current, which is the decaying current minus the velocity induced current.
Am I getting this right?
Thanks in advance.

2. Aug 10, 2011

### lagfish

anyone?

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