Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 1 Motor and Motion Control Systems 43
olution as fine as 0.001 in. per pulse. Travel per step is determined by the
pitch of the leadscrew and step angle of the motor. The maximum linear
force for the model shown is 75 oz.
SERVOSYSTEM FEEDBACK SENSORS
A servosystem feedback sensor in a motion control system transforms a
physical variable into an electrical signal for use by the motion controller.
Common feedback sensors are encoders, resolvers, and linear
variable differential transformers (LVDTs) for motion and position feedback,
and tachometers for velocity feedback. Less common but also in
use as feedback devices are potentiometers, linear velocity transducers
(LVTs), angular displacement transducers (ADTs), laser interferometers,
and potentiometers. Generally speaking, the closer the feedback sensor
is to the variable being controlled, the more accurate it will be in assisting
the system to correct velocity and position errors.
For example, direct measurement of the linear position of the carriage
carrying the load or tool on a single-axis linear guide will provide more
accurate feedback than an indirect measurement determined from the
angular position of the guide’s leadscrew and knowledge of the drivetrain
geometry between the sensor and the carriage. Thus, direct position
measurement avoids drivetrain errors caused by backlash, hysteresis, and
leadscrew wear that can adversely affect indirect measurement.
Rotary Encoders
Rotary encoders, also called rotary shaft encoders or rotary shaft-angle
encoders, are electromechanical transducers that convert shaft rotation
into output pulses, which can be counted to measure shaft revolutions or
shaft angle. They provide rate and positioning information in servo feedback
loops. A rotary encoder can sense a number of discrete positions
per revolution. The number is called points per revolution and is analogous
to the steps per revolution of a stepper motor. The speed of an
encoder is in units of counts per second. Rotary encoders can measure
the motor-shaft or leadscrew angle to report position indirectly, but they
can also measure the response of rotating machines directly.
The most popular rotary encoders are incremental optical shaft-angle
encoders and the absolute optical shaft-angle encoders. There are also
direct contact or brush-type and magnetic rotary encoders, but they are
not as widely used in motion control systems.