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Rotary encoder with absolute readout offers high resolution and low cost

-January 28, 2013

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Rotary encoders are typically used in positioning systems with servo feedback in which the cost of the encoder usually is of minor importance. Encoders, however, are also used in user interfaces to encode the positions of knobs—the volume knob on an audio system, for example. For those knobs, you have the choice between either a potentiometer boasting low cost, high resolution, and absolute readout but only limited travel—typically less than 340°—or a mechanical-optical rotary encoder, which has endless travel but a higher cost, low resolution, and only relative readout. The Design Idea presented here attempts to combine the advantages of the potentiometer with the endless operation of the mechanical-optical rotary encoder.

The encoder uses standard potentiometer construction techniques and is thus easily produced. It basically is a dual-wiper quadrature endless pot. It consists of a full ring of resistive material, which is powered from opposite sides and on which two electrically independent wipers move. The wipers are mechanically connected to each other at an angle of 90° (Figure 1).


Figure 1 The encoder is a dual-wiper quadrature endless potentiometer that consists of a full ring of resistive material, which is powered from opposite sides and on which two electrically independent wipers move.

An ADC on a microcontroller reads out the two signals; firmware uses both signals to determine in which quadrant the axis is located. Once the quadrant is known, the signal of both wipers can be used to calculate the position of the axis. When a wiper reaches the top or bottom power connections, its signal should be ignored because of nonlinear response (Figure 2). Both wipers cannot be in this nonlinear position at the same time because of the 90° angle between the wipers. Today, even the most basic microcontrollers offer a 10-bit ADC, so the combined signals give an 11-bit resolution, or better than 0.2°. The microcontroller can ignore the absolute readout if the application does not require it or when a software reset is useful.


Figure 2 When a wiper reaches the top or bottom power connections, its signal should be ignored because of nonlinear response.

This quadrature endless pot provides a user experience similar to the old tuning knob of a classical analog radio. It offers new possibilities in human-interface design and can give a quality feel in consumer products at low cost.

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