A niche area of motion control is found in voice coils. Voice coils are single phase linear motors with a limited travel of less than one electrical cycle. They are useful for their response speed, precision, and accuracy.
Voice Coil Control
Voice coils work using the Lorentz force, which is generated by an electric current moving through a coil of wire in a fixed magnetic field. The direction of the force is determined by the right-hand-rule using the current and magnetic field directions. Therefore, it’s very easy to create a simple back and forth motion by simply reversing the current direction.
In that sense, using a servo drive to control a voice coil is analogous to brushed DC servo motor control, the main difference being motion is translational instead of rotational. In the case of a servo motor control, current is proportional to torque, whereas in a voice coil control, current is proportional to force.
Voice Coil Applications
An audio speaker is one of the most common examples of a specialized voice coil. The back and forth motion coupled with the short range of movement and precision allow speaker voice coils to create the vibrations that make audible noise from electrical signals. Microphones work with the same technology, but in the opposite way, generating an electric current from the coil’s motion through the magnetic field when vibrations cause it to move.
The other most common use of voice coils is in the read-write arms of spinning hard drives (HDDs). The voice coils in a hard drive’s read-write arm bring it down to be in contact with the spinning disc at the precise moment to read or change a bit of data. With billions of bits on a drive, this needs to be dead-on.
ADVANCED Motion Controls' Voice Coil Capabilities
- All off-the-shelf ADVANCED Motion Controls servo drives (both analog and digital) can operate single phase motors, including voice coils.
- Most servo drives can give excellent performance when used with voice coils, providing a fast motion response and high bandwidth within their range of travel.