Encoder Primer

Introduction

Encoders are the best device for tracking the position of an object. They come in 2 main types, rotary and linear. Rotary encoders track angular position while linear track position in one spatial dimension.

How they work

Encoders work by counting ticks that are spaced evenly and very close together. On higher quality encoders the ticks are closer together resulting in greater measurement accuracy.

Types of encoders

Rotary

Mechanical encoders consist of a metal disc with slots cut into it which spins beneath a series of wire brushes. When the brushes are over the slot the circuit remains open, but when the brushes contact the disc they close the circuit. These are brushes at different radii and the encoder is guaranteed to have a unique pattern of closed circuits for each set step in angular position.

Because of the physical complexity mechanical encoders have to be larger than the alternatives to get the same accuracy.

Optical encoders have a similar disc to mechanical encoders except it has a number of opaque or transparent areas. Then a light source in conjunction with a set of photo detectors perform the same function as the wire brushes in a mechanical encoder. For each angular position step there is guaranteed to be a unique set of active photo detectors.

Linear

Optical linear encoders dominate the high performance market for linear encoders. Typical incremental scale periods can get down to sub-micrometre and with interpolations accuracy can be as fine as nanometre.

Magnetic linear encoders use Hall Effect readheads to measure ticks, they typically have measurement resolutions in the order of micrometres.

Capacitive linear encoders work by sensing the capacitance between reader and scale. Commonly used in digital calipers. The downside is they are vulnerable to foreign materials such as dust or dirt. Resolution is in the order of micrometres.

Quadrature encoding

File:Quadrature encoder.png

Top: Representation of a quadrature encoder wheel. Bottom: Output from the encoder.

Quadrature encoders are common, using two output channels to dictate both a change and the direction of change. In a quadrature system, two parallel mechanical switches or optical slots are offset slightly. This way, as the slots pass by the sensor, the staggered output indicates both the number of pulses that have occurred (the change in position) as well as which output channel is leading the other (direction of change).

Choosing an encoder

The first thing to consider when choosing an encoder is what degree of accuracy you are going to require. Is the application a simple human interface (knob or something similar) or are you trying to precisely track the movement of a motor? Mechanical, magnetic, and capacitive encoders have the advantage of being extremely cheap. So even though they don't have as high accuracy as optical encoders, for applications that don't require it such as turning a knob on a control panel they are ideal. For any situation where you need modest to high accuracy, you only real choice is an optical encoder.

Products that fall under this category

• 1047 - PhidgetEncoder HighSpeed 4-Input
• 1057 - PhidgetEncoder HighSpeed
• 1052 - PhidgetEncoder
• 3530 - Optical Rotary Encoder ISC3004

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Make sure to mention that 4-pin encoders still work with either controller, as long as everything is connected up properly.