Rockwell Automation 1771-QB Linear Pos. - Manual (Page 18)

Positioning Concepts
Chapter 2
2Ć2
Closed-loop positioning is a precise means of moving an object from one
position to another. In a typical application, a positioning device activates a
servo valve controlling the movement of fluid in a hydraulic system. The
movement of fluid translates into the linear motion of a hydraulic cylinder. A
transducer monitors this motion and feeds it back to the positioning device. The
positioning device, in turn, calculates a positioning correction and feeds it back
to the servo valve.
Important: Throughout this manual we refer to servo valves, but you can also
use the analog outputs to control proportional valves or an electric servo.
Linear Displacement Transducer
A linear displacement transducer (see Figure 2.2) is a device that senses the
position of an external magnet to measure displacements.
Figure 2.2
Linear Displacement Transducer
50034
Magnet
Transducer
Head
Magnet mounted to
the piston of actuator
The transducer sends a signal through the transducer wave guide where a
permanent magnet generates the return pulse. You can use the time interval
between the transducer’s signal and the return pulse to measure axis
displacement.
Circulations
Some linear displacement transducers provide circulations or recirculation to
improve resolution. (See Figure 2.3.) This technique stretches the pulse by a
factor of two or more and results in finer resolution in the circuitry monitoring
the pulse width.
ClosedĆLoop Positioning