Rockwell-automation 1771-QB Linear Pos. Uživatelský manuál stáhnout pdf (Strana 23)

Positioning Concepts

Chapter 2

27

Figure 2.7

Derivative

Control

50039

Linear

Displacement

Transducer

Servo Valve

Desired

Velocity

Axis

Velocity

Command

Following

Error

Position

Command

D/A

Integrator

Actual

Position

Feed

Forward

Integrator

Derivative

Deadband

Most systems have friction and play in their mechanical linkages. These

characteristics can cause a cylinder to oscillate around a programmed

endpoint–especially if you use an integral term. You can use a deadband to

reduce these oscillations.

A deadband is an area surrounding the programmed endpoint where the error is

ignored. Outside the deadband, error is reduced by one half the width of the

deadband.

If you apply a deadband to an integral term, the integral output remains constant

while the axis is within the deadband. This reduces oscillations around the

endpoint. However, if the deadband is too large, it can also reduce the

positioning accuracy of the system.

PID Band

Integral and derivative control can cause undesirable results when the axis

moves from one position to another. The integral term can cause the axis to

overshoot the programmed endpoint. The derivative term opposes changes in

error, and thereby changes in position.

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