Rockwell Automation 1771-QB Linear Pos. - Manual (Page 20)

Positioning Concepts
Chapter 2
2Ć4
In Figure 2.4:
desired velocity is the desired speed of axis motion from one position to
another
position command equals the integration of velocity over time
actual position value (transducer feedback) is the actual position of the
axis as measured by the LDT
following error equals position command minus actual position
velocity command is generated by amplifying the following error and
converting the result into an analog output
D/A (Digital to Analog convertor) generates the analog output controlling the
servo valve
K
P
(proportional gain) is the component that causes an output signal to
change as a direct ratio of the error signal variation
Proportional Gain
The following error is a function of the velocity command divided by the
proportional gain (K
P
). To generate the velocity command, multiply the
following error by the proportional gain. Proportional gain can be expressed in
ips/mil (where 1 mil = 0.001 inches) or mmps/mil (where 1 mil = 0.001 mm).
For example, with a velocity of 12 ips and a gain of 1 ips/mil, the following
error is:
Following Error = Velocity/Gain
= 12 ips/(1 ips/mil)
= 12 mil
When you increase the gain, you decrease the following error and decrease the
cycle time of the system. However, the capabilities of the system limit the gain.
Too large a gain causes instability.