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counts / user unit parameter and the scale factor defining volts
per user unit match up. This is because, even when position
feedback comes from the analog input, velocity loop closure is
still based on the incremental encoder feedback.
EXAMPLE 1:
•
An actuator / motor system has 10 inches of total stroke with a 2-
pitch ballscrew and a 1000 line encoder.
•
This system would have an encoder resolution of 1000 * 4 *2 =
8000 encoder counts per inch. This parameter would be entered
on the main sequential programming screen.
•
Now assume that linear, absolute analog feedback is available
that provides 9 volts of output range over the 10 inch stroke.
•
Also assume that voltage increases with travel in the same
direction that produces positive encoder count accumulation.
The correct scale factor for analog position feedback would then
be 9 / 10 = .9 volts per inch.
CAUTION!!!
It is important that the sign of these parameters
provides for the same polarity, i.e. if the analog feedback polarity
does not match the equivalent position feedback from the
incremental encoder, the system will be unstable and a run-
away condition would result.
ANALOG POSITION OFFSET
Once the scale factor is properly set, the "Analog Position Offset"
parameter can be set to provide a position reference point. This
parameter defines the absolute position that corresponds to zero
volts analog input. Some systems may not ever produce a zero
volt analog signal, but this parameter is useful and valid
nonetheless.
EXAMPLE 2: Using the system outlined in Example 1:
•
Assume the zero position to be in the center of the 10 inch travel
range, i.e. at 5 inches.
•
With a defined a scale factor of .9 volts per inch and stated that
the analog feedback voltage ranges from 0 to 9 volts over the 10
inches of system travel, then 5 inches of physical position would
correspond to 5 * 0.9 = 4.5 volts at 5 inches.
•
What is desired is an absolute position of zero corresponding to
4.5 volts. Thus, at zero volts, the position would be (0-4.5) *
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