Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 1 Motor and Motion Control Systems 57
but the capacitance between the separate receiving plates varies with
rotor position.
A null point is obtained when the rotor is positioned under equal areas
of the receiving stator plates. In that position, the capacitance between
the transmitting stator plate and the receiving stator plates will be equal,
and there will be no output voltage. However, as the rotor moves clockwise
or counterclockwise, the capacitance between the transmitting plate
and one of the receiving plates will be greater than it is between the other
receiving plate. As a result, after demodulation, the differential output
DC voltage will be proportional to the angular distance the rotor moved
from the null point.
Inductosyns
The Inductosyn is a proprietary AC sensor that generates position feedback
signals that are similar to those from a resolver. There are rotary
and linear Inductosyns. Much smaller than a resolver, a rotary
Inductosyn is an assembly of a scale and slider on insulating substrates
in a loop. When the scale is energized with AC, the voltage couples into
the two slider windings and induces voltages proportional to the sine and
cosine of the slider spacing within a cyclic pitch.
An Inductosyn-to-digital (I/D) converter, similar to a resolver-todigital
(R/D) converter, is needed to convert these signals into a digital
format. A typical rotary Inductosyn with 360 cyclic pitches per rotation
can resolve a total of 1,474,560 sectors for each resolution. This corresponds
to an angular rotation of less than 0.9 arc-s. This angular information
in a digital format is sent to the motion controller.
Laser Interferometers
Laser interferometers provide the most accurate position feedback for
servosystems. They offer very high resolution (to 1.24 nm), noncontact
measurement, a high update rate, and intrinsic accuracies of up to 0.02
ppm. They can be used in servosystems either as passive position readouts
or as active feedback sensors in a position servo loop. The laser
beam path can be precisely aligned to coincide with the load or a specific
point being measured, eliminating or greatly reducing Abbe error.
A single-axis system based on the Michaelson interferometer is illustrated
in Figure 1-48. It consists of a helium–neon laser, a polarizing
beam splitter with a stationary retroreflector, a moving retroreflector that