Danaher Motion
06/2005
Project
M-SS-005-03 Rev
E
67
3. 8
P
OSITION AND
V
ELOCITY
Position and velocity are the key command and feedback signals for each
axis. These properties are updated by the MC every SERCOS cycle and
may be read at any time. Their properties are double-precision floating point
numbers. Velocity Units are discussed above. The four properties which hold
these values are:
<axis>.POSITIONCOMMAND
or
<axis>.PCMD
<axis>.POSITIONFEEDBACK
or
<axis>.PFB
<axis>.VELOCITYCOMMAND
or
<axis>.VCMD
<axis>.VELOCITYFEEDBACK
or
<axis>.VFB
You can read these properties anytime the SERCOS ring is up.
Position error is defined as the difference between position commanded and
position feedback. When an axis is at rest, (i.e.,
ISSETTLED
), this simple
definition of position error is valid. However, when the axis is in motion, the
true position error does not equal the instaneous commanded position minus
the instantaneous feedback position because there is a time delay of two
SERCOS cycle times (2 ms) from when
MOVE
is issued until the feedback
position is received.
When the MC calculates position error, it automatically accounts for the
communication delay. In most circumstances, the two-cycle time delay is
correct. However, if microinterpolation is enabled in the drive, the feedback
position is delayed an additional one or more cycle times.
To manage position error problems, the MC provides a number of axis and
group properties relating to position error.
<axis>.POSITIONERRORDELAY
(new in firmware version 3.0), allows you to specify the number of SERCOS
cycle times to apply when calculating position error. The default is two.
The MC allows you to set up any axis in your system as a rotary axis. Rotary
axes are often used for mechanical mechanisms that repeat after a fixed
amount of rotation.
