Shape-based Feedforward Tuning:

Offset (Friction) Pattern — Adjusting Kfff

Notice that in the Positive Direction plot, the entire position error plot is shifted up by about 10 counts. It does not vary with velocity. This type of shift will extend during the entire move and is roughly symetrical around 0 position error. Note that the shift occurs in positive and negative directions.

Positive Direction

Negative Direction

If we make Kfff = 500, the resulting profile almost looks right. Notice that the position error is split between the zero position error line. The split comes from some error in the Kaff term that we will clean up in the next section.

	10,000 count move Trapezoidal move:       5e5 accel, decel, vel	Green - Commanded Velocity              Yellow - Commanded Acceleration                    White - Position Error
	Kfff = 500 Kvff = 0 Kaff = 100,000 Kp = 100 Ki = 0 Kd = 2,000 Peak Position Error = 8 counts

Since Kfff is constantly affecting the entire move, you will only be able to see the problems at the beginning and end of the move. Note that the position error appears to "snap in" at the end of the move, which comes from an excessive Kfff being removed at the end of the move.

NOTE: Kfff is 3x the ideal value in the following example.

	10,000 count move Trapezoidal move:       5e5 accel, decel, vel	Green - Commanded Velocity              Yellow - Commanded Acceleration                    White - Position Error
	Kfff = 1,500 Kvff = 0 Kaff = 100,000 Kp = 100 Ki = 0 Kd = 2,000 Peak Position Error = 17 counts

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