Operation & Software Manual
210
Direct Drives & Systems
Chapter C: System functions
ETEL Doc. - Operation & Software Manual # DSC2P 903 / Ver. F / 3/6/05
13.13 Encoder scaling and mapping
This paragraph describes how to correct the position given by the encoder along an axis.
The linear and rotary encoders have a measurement error with regard to the actual position of the axis. This
error comes in the form of a linear error according to the position (a
µ
m of the encoder is not necessarily a
standard
µ
m) on which is added a random error. The following drawing gives a good idea of the error:
In the application requiring a very high absolute precision on the movement, it may be necessary to rectify the
wanted position according to the known errors of the scale. These corrections will cancel the errors of the
measuring system and will position the moving load with a better absolute precision.
To sum up, we will use the term of
encoder
mapping
to talk about the compensation of the random error along
the scale and the term of
encoder scaling
to talk about the linear correction of the scale. The second function
allows the user to proportionately influence the movement references.
Whether it is the trajectory generator or the DSMAX which gives the position set point, the data firstly goes
through the jerk filter defined by parameter K213 and then through the encoder scaling and mapping correction.
Remark:
Both corrections are given with regards to the absolute position on the scale and then, they will be
activated only after the homing!
The encoder scaling is not available in the rotary modes (MMD=17 or 19 or 24 or 26)
. If the
user wants to use this correction with a rotary motor, the mode MMD = 1 must be used but in that
case the movement is limited to -2*10
9
to +2*10
9
[udi].
13.13.1 Encoder scaling
Parameter
K168
determines the point where the correction is null. The principle is to add a correction in dpi on
the reference position. The correction is calculated as follows:
Correction = (position - K168) * K169 / 100000
Available on
DSC2P
DSC2V
DSCDP
DSCDL
DSCDM
Position measured
with a perfect scale
Position measured
with a real scale
Perfect actual position
Position given by the scale
K168
0 in dpi after
the homing
Correction in dpi
to be done on
the reference
0
Scale, absolute position in dpi
Scale, absolute position in dpi
Gradient: K169 / 100000
