Closed loop motor control: Start with position sensing

Whether you realize it or not, there could be as many as a hundred electric motors in your house. Anything that moves needs a motor, sometimes more than one. Look around and count. Know that motor control is all about knowing when to start a motor, how much to speed it up and when (and where) to stop it. Almost always, these operations are possible because of “closed loop” control.

Feedback from the output is fundamental to a closed loop control system. When precise position control is required of a motor, two types of sensors are commonly used for feedback: the resolver, an analog device, or an optical encoder, which can produce a digital output.

Precise, closed-loop control over rotational position is what enables cars to self-park, satellites to lock on to signals and machines to “pick and place” objects on the factory floor.

The venerable resolver — refined over 50 years — is robust and highly accurate. Basically, it is a rotary transformer where the magnitude of the energy through the resolver windings varies sinusoidally as the shaft rotates. The resolver shines in humid, dusty, oily, or mechanically demanding environments.

An optical encoder is a disk with a specific pattern of slits that is mounted to the motor shaft. Light shone through the slits on to a photo-detector creates an output signal that is correlated to the motor’s rotary position. There are several types of encoders, some of which can also produce analog outputs that require analog signal chains. Encoders may not be as robust as resolvers, but can be much lighter and offer considerably less rotational inertia.

Read “Closed-loop motor control: An introduction to rotary resolvers and encoders,” for a detailed discussion of rotational position sensors and the electronic circuits required to ensure signal integrity and optimum performance.

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