Axis correction

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Axis correction

Deviation of actual position value

The actual position value delivered by the encoder system and the real actual position value on the axis may vary for a number of reasons. Such non-linear inaccuracies can be compensated by axis error correction (using position- and direction-dependent correction values). For this, a correction value table is filled with values for each of the two directions. The respective correction value is produced from the current axis position and the direction of movement by means of cubic, jerk-stabilized interpolation. The actual position value is adapted on the basis of the corrected table. Both tables contain 250 interpolation points.
The correction range is within the value range delimited by parameters P 0591 ENC_ACOR_PosStart "Start position" and P 0592 ENC_ACOR_PosEnd "End position correction". The start position is preset on the user side; the end position is determined on the drive side.

Possible cause of deviations

Inaccuracy of the measuring system
Slack in mechanical elements such as the gearing, coupling, feed spindle etc.
Thermal expansion of machine components.

Bild: Slack between two mechanical components

Slack in the mechanism

Parameters

P. no.	Parameter name Setting	Function
P 0530	ENC_Encoder1Se	Channel selection for the 1st encoder used
P 0531	ENC_Encoder2Sel	Channel selection for the 2nd encoder used
P 0590	ENC_ACOR_Sel
(0)	OFF	No encoder selected
(1)	1. encoder	1st encoder selected
(2)	2. encoder	2nd encoder selected
P 0591	ENC_ACOR_PosStart	Definition of correction range: The range is defined by parameters P 0591 ENC_ACOR_PosStart Start position and P 0592 ENC_ACOR_PosEnd end position. The start position is user-specified; the end position is determined on the device side from the maximum value of correction table interpolation points used and the interpolation point pitch
P 0592	ENC_ACOR_PosEnd
P 0593	ENC_ACOR_PosDelta	Interpolation point pitch: The positions at which the correction interpolation points are plotted are defined via parameters P 0593 ENC_ACOR_PosDelta Interpolation point pitch and P 0591 ENC_ACOR_PosStart Start position. Between the correction interpolation points, the correction values are calculated by cubic spline interpolation.
P 0594	ENC_ACOR_Val	Actual position
P 0595	ENC_ACOR_VnegTab	Values of the correction table for negative direction of rotation in user units.
P 0596	ENC_ACOR_VposTab	Values of the correction table for positive direction of rotation in user units.

Vorgehensweise: Axis correction

With P 0530 ENC_Encoder1Sel channel selection for SERCOS: 1st encoder
With P 0531 ENC_Encoder2Sel channel selection for SERCOS: 2nd encoder
Selection of the encoder whose actual position value is to be changed, with
P 0590 ENC_ACOR_Sel
Enter interpolation point pitch in P 0593 ENC_ACOR_PosDelta
The correction values are determined using a reference measurement system (e.g. laser interferometer). The interpolation points for the various directions within the desired correction range are approached one after another and the corresponding position error is measured.
The interpolation point-specific correction values are entered manually in tables
P 0595 ENC_ACOR_VnegTab (neg. direction) and
P 0596 ENC_ACOR_VposTab (pos. direction).
Save values
Restart
P 0592 ENC_ACOR_PosEnd now shows the position end value of the correction range
Start control (in position control execute homing) and then move to any position.
The momentary correction value is written to P 0594 ENC_ACOR_Val. This value is subtracted from the approached position value. This applies to all positions.

End position = interpolation point pitch multiplied by number of interpolation points (table values) + start position (only if start position ≠ 0).

Determining the direction of movement

Position control

The direction of movement is produced when the time-related change in position reference (speed pre-control value) has exceeded the amount of the standstill window in the positive or negative direction.

Speed control

The direction of movement is produced when the speed reference has exceeded the amount of the standstill window in the positive or negative direction.

Bild: Corrected actual position value

NOTE:

Parameterization is carried out in the selected user unit for the position as integer values. It is advisable to use the same number of correction interpolation points for the positive and negative directions. The first and last correction values in the table must be zero in order to avoid instability (step changes) of the actual position value. Differing correction values for the positive and negative directions at the same interpolation point will lead to instability in the associated actual position value when the direction is reversed, and so possibly to a step response adjustment to the reference position.