Feedback on speed

Dec. 8, 2005
A novel process offers an inexpensive way to monitor the speed of small permanentmagnet motors.
Roger Baines
Johnson Electric
Shelton, Conn

For many years control makers monitored servomotor velocity and rotor position through use of various built-in or attached devices. The last few decades have seen a mushrooming of applications for motors that incorporate velocity control. These new applications frequently need small and competitively priced motors to be practical.

This has presented a need for a small motor and velocity feedback generator at a low cost. Signal generators attached to or integrated into motors typically add size and expense.

The engineers at Johnson Electric went back to basics to come up with a configuration that did not. They used a permanent-magnet direct-current (PMDC) motor in creating their new sensor. When the armature in a PMDC motor rotates, various regular and cyclic changes take place that have a frequency and, often, amplitude that varies proportionally to motor velocity.

For example, there is an almost-sinusoidal ripple in the supply current caused by commutation. Accelerometers placed on the motor housing detect a torque ripple. And the magnetic flux density changes in the air gap between the magnets and armature because of "armature reaction" within the permanent magnets.

The air gaps between permanent magnet stators and rotating armatures must be kept small to create a magnetic circuit of minimum reluctance and to avoid flux leakage. Usually this air gap does not exceed 0.5 mm (about 0.020 in.) Johnson Electric engineers chose to look closely at this air gap with its varying flux density. A simple detector could collect this information and send a signal out to a velocity controller.

Basic physics says any conductors placed in a varying magnetic field experience an induced voltage. The amount of that voltage depends on the rate of change of the magnetic flux passing through the wire. Engineers at Johnson Electric conducted numerous experiments using a very-small wire mounted as a single-turn coil on the inner face of the permanent magnets.

The sensing coil mounted on the inner face of the PMDC magnets such that it was in the air gap between the magnets and armature. Most permanent magnets used today are ceramic and, as such, are good insulators. This made it possible to fasten coils made from fine round wires or flat thin ribbons to the nonconducting magnets. A painted or sprayed insulating coating was another possibility.

The number of turns and the pitch of the conductors of the coil are determined by the arc-length of the magnet, the number of salient poles on the armature, and the signal strength required. Connections to the sensor coils are at the ends of the permanent magnets and carry through to the end cap for termination outside. The signal produced by the coil is processed and amplified to produce a velocity feedback for motor-speed regulation.

The speed sensor coil is suitable for radio-controlled toys, cordless appliances and tools, and small robotic applications.

A small PMDC motor uses permanent magnets mounted to the inside of the motor housing, with the armature turning beneath the magnetic poles. The air gap between the armature and magnets is kept small, typically less than 0.020 in., to keep flux reluctance low and maximize the magnetic circuit. The speedsensor coils mount to the face of the permanent magnets within this air gap.

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