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Measurement-based Feedforward Tuning:
Friction Feedforward
Friction feedforward on the XMP is designed to compensate
for the static friction in a system. It assumes that the breakaway friction
is the same as kinetic friction.
There are two simple ways to measure this friction.
Both methods are based on increasing the Output Offset in the control
algorithm (Filter Object) and looking at the way the stage moves in the
face of friction. The Output Offset is used extensively in both methods.
Below is a screenshot of where the Output Offset parameter is found in
Motion Console.
Method 1
Use the breakaway torque for determining
friction feedforward and output offset.
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- Increase the Output Offset until you get a small
amount of motion.
Let's call this offset value OffsetF.
- Decrease the Output Offset until you get a small
amount of motion.
Let's call this offset value OffsetR.
- The friction feedforward value is the average of OffsetF and
–OffsetR.
That is, Kfff = (OffsetF – OffsetR) / 2
- The output offset value is the average of OffsetF and OffsetR.
That is, Output Offset = (OffsetF + OffsetR) / 2.
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Method 2
Use the torque value that brings motion
to a rest after first exceeding breakaway torque for determining friction
feedforward and output offset.
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- Increase the Output Offset until you get a small
amount of motion.
Decrease the output offset until motion stops.
Let's call this offset value OffsetF.
- Decrease the Output Offset until you get a small
amount of motion.
Increase the output offset until motion stops.
Let's call this offset value OffsetR.
- The friction feedforward value is the average of OffsetF and
–OffsetR.
That is, Kfff = (OffsetF – OffsetR) / 2
- The output offset value is the average of OffsetF and OffsetR.
That is, Output Offset = (OffsetF + OffsetR) / 2.
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What is a small amount of motion?
A "small amount of motion" differs from system to system, but in general
you want to make a move that overcomes the mechanical hysteresis in a
system (lip seals, oil films, etc.). In the face of these forces, it is
common to put a small offset that results in only a few counts of motion.
From that point, a significant increase in output offset is required to
be able to visually see more of a continuous motion. This is because some
of the parts that contribute friction are flexible and will move a bit
with little force until they "take a set" — where they will require
more force for further motion to occur. You will likely not want to use
the value that gets the stage to move 1 or 2 encoder counts because this
amount of movement is often highly non-linear and dependent on behaviors
of individual bearings and seals. A larger movement will not be as drastically
affected by these effects. When "a small amount of motion" is referred
to above, consider this to be the amount of motion where everything has
"taken a set" and the stage is ready to continue to move very slowly as
a result of the output offset.
Friction Feedforward Calculation
Friction feedforward on the XMP is calculated as follows:
Friction feedforward contribution to DAC
= commanded
velocity sign (-1, 0, or 1) * Kfff
Acceleration and friction feedforward are equally applicable
to PID and PIV.
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