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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
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).
