Download Application Note: Position Feedback Device Comparison

Application Note: Position Feedback Device Comparison
Limit Switches - a simple switch which is set or reset mechanically. Limit switches are
the least expensive and least accurate form of feedback.
Resolver - a transformer with a rotating secondary. A fundamental sine wave (usually
400 or 2500 hz) is placed on the primary. As the secondary is turned, a sine wave
appears with a phase shift from the fundamental which is proportional and digitalized by
means of a resolver to digital (R to D) converter. Resolvers are rugged, noise and
temperature immune. Cost is moderate, implementation is moderate, and accuracy is
god. Resolvers are often mounted on the motor. However, increased accuracy is
obtainable with the use of a precision instrument rack.
Incremental Encoders - optical devices which usually consist of a glass disc with an
"A" channel, a "B" channel and a once around "marker". Encoders interface easily to
computers but are not as rugged or noise immune as resolvers. Their cost is moderate
and accuracy is good.
Absolute Encoders - similar to incremental encoders except they have many channels
which provide absolute positional information within 360°. Beyond 360°, continuously
powered types with memory allow absolute position over many rotations. They are
usually more expensive than incremental encoders.
Inductosyn - a conductor of many hairpin turns (called a stator) is placed on a substrate
and excited by a D.C. voltage. A similar conductor with 2 separate conducting paths
(called a rotor) is passed over the stator, resulting in 2 series of pulses with a 90° offset
being sent from the rotor. Alternately, the stator may be excited by equal-amplitude
sinusoidal signals in time quadrature. The rotor output will then be a constant amplitude
sine wave with a phase shift proportional to the position of the rotor. This is very much
like a "linear" resolver. These devices are very accurate and more expensive than
resolvers or encoders.
Glass Scale - similar to an encoder except it is a linear device. A glass rod is marked
with a series of transparent and opaque areas. A light is shown through the windows and
a sensor on the other side counts the number of times the beam is broken. These devices
can be accurate, but also very expensive.
Laser Interferometer - counts the wavelengths of light to determine position. It is the
most accurate, most expensive and most-difficult-to-implement form of position
feedback.