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issue: April 2005 APPLIANCE Magazine

Engineering Sensors
Motor Sensor Design with Velocity Feedback

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by Roger Baines, engineer, Johnson Electric

A new sensor design provides velocity feedback in small permanent magnet d.c. motors for cordless, battery-powered appliance applications.

Figure 1. Construction of a small PMDC motor.

For many years it has been common to find various devices either built into or attached to servo motors that provide signals relating to the motor’s velocity and/or rotor position. These feedback signals allow users of those motors to control the velocity and/or the position of the rotor. During the last couple of decades, many more applications for motors that require control of the motor’s velocity have been developed, and many of these new applications require relatively small and competitively priced motors for their purpose.

This has presented a challenge to the designers of these motors who are required to design a small motor with a velocity feedback generator at a low cost. Whereas signal generators attached to or integrated into many motors add size and high cost, it was now required that, for small and low cost motors, the size and cost of any signal generator should not be significant.

Figures 2-5. Possible configurations of the coils placed on the inside surface of a permanent magnet.

New Sensor Design

When the armature in a permanent magnet direct current (PMDC) motor is rotating, there are various regular and cyclic changes taking place that have a frequency and, often, an amplitude that varies proportionally to the motor velocity.

For example, there is an almost-sinusoidal ripple in the supply current due to commutation; there is a torque ripple detectable by accelerometers placed on the motor housing; and there is a change in the magnetic flux density in the air gap between the magnets and the armature due to armature reaction within the permanent magnets.

The air gaps between permanent magnet stators and rotating armatures have to 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 20 thousandths of an inch).

Johnson Electric engineers chose to look closely at this air gap with its varying flux density to see if a simple detector could collect this information and send a signal out to a velocity controller.

It was known that any conductor placed in a varying magnetic field would experience a current made to flow within the conductor having a value that was dependent upon the rate of change of magnetic flux passing through it. The engineers conducted a number of experiments using a very small wire mounted as a single turn coil on the inner face of the permanent magnets.

Figures 1-5 show some of these experiments. Figure 1 illustrates the overall construction of a small PMDC motor. It can be seen that, on the inside of the motor housing, there are magnets and a coil mounted on the inner face of those magnets so that the coil will be in the air gap between the magnets and armature when the motor is assembled.

Figures 2, 3, 4, and 5 illustrate a few of the possible configurations of the coils placed on the inside surface of a permanent magnet. Most permanent magnets used today are ceramic and, as such, are good insulators. The coils can be fabricated from fine round wires or flat thin ribbons, and stuck permanently to the magnets. Alternatively, the coils can be painted or sprayed.

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.

About the Author - Roger Baines retired from motor manufacturer Johnson Electric in 2002 after 25 years with the company. He held numerous executive positions in engineering and operations and continues to consult with the company. If you would like more information on this paper or would like to contact Mr. Baines, please e-mail editor@appliance.com.


By mounting very thin coils or strips of wire to form a coil on the inside surface of the permanent magnets so that it lies within the small air gap without fear of becoming detached and interfering with the motion of the armature, these coils or conductors will react to changes in the flux density due to reaction with the rotating armature fields. This is done by generating small, sinusoidal voltages that have a frequency proportional to the velocity of the motor.

In the newly patented design, connections to the sensor coils can be made at the ends of the permanent magnets and carried through to the end cap for exterior termination. The small signal can then be processed and amplified to produce a velocity feedback for motor speed regulation at very low cost and without increasing the size of the motor.

Suppliers mentioned in this article:
Johnson Electric Group

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