Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

42 Chapter 1 Motor and Motion Control Systems
Thomson Saginaw also offers its linear actuators with either Halleffect
or potentiometer sensors for applications where it is necessary or
desirable to control actuator positioning. With Hall-effect sensing, six
pulses are generated with each turn of the output shaft during which the
stroke travels approximately 1 ⁄32 in. (0.033 in. or 0.84 mm). These pulses
can be counted by a separate control unit and added or subtracted from
the stored pulse count in the unit’s memory. The actuator can be stopped
at any 0.033-in. increment of travel along the stroke selected by programming.
A limit switch can be used together with this sensor.
If a 10-turn, 10,000-ohm potentiometer is used as a sensor, it can be
driven by the output shaft through a spur gear. The gear ratio is established
to change the resistance from 0 to 10,000 ohms over the length of
the actuator stroke. A separate control unit measures the resistance (or
voltage) across the potentiometer, which varies continuously and linearly
with stroke travel. The actuator can be stopped at any position
along its stroke.
Stepper-Motor Based Linear Actuators
Linear actuators are available with axial integral threaded shafts and bolt
nuts that convert rotary motion to linear motion. Powered by fractional
horsepower permanent-magnet stepper motors, these linear actuators are
capable of positioning light loads. Digital pulses fed to the actuator
cause the threaded shaft to rotate, advancing or retracting it so that a load
coupled to the shaft can be moved backward or forward. The bidirectional
digital linear actuator shown in Figure 1-33 can provide linear res-
Figure 1-33 This light-duty linear
actuator based on a permanent-magnet
stepping motor has
a shaft that advances or retracts.