• 86.2 Actual position
￭
Actual position scaling
When position is not measured directly from the load, it must be scaled to match
the actual load position. Actual position can also be scaled to correspond to the
scale and range of desired load units.
The feedback for position control is scaled by the load gear (86.14/86.15) and feed
constant (86.16/86.17). If no scaling (load gear × feed constant = 1) is applied, then
86.2 is identical to revolutions of the position feedback source. The position
controller output is in units/s, so it will be additionally converted by load encoder
scaling parameters 86.21 and 86.22 to corresponding motor shaft speed in rpm,
which is in turn used as a reference for the speed controller.
Instead of the maximum range, a modulo range (86.18/86.19) can be defined for
a repeated, application-specific position range. See section Modulo
operation (page 44).
￭
Actual position tracking
You can use parameter 86.25 to disable or enable actual position tracking over a
power-down.
The parameter should generally be set to
Disabled
with linear SSI encoders to
avoid the storing and restoring of false offsets, and with a rotary encoder if it
stays within its range. The parameter should be set to
Enabled
with a rotary encoder
that is likely to wrap around its total absolute range.
By default, the following functions are disabled:
• Restoring of actual position encoder absolute position wrap counter
(90.13/90.23) during power-up.
• Wrap count power-up correction if the position has changed more than half
of the range.
￭
Terms and definitions
86.2 Actual position = 86.8 Actual position raw/Resolution (increments/rev) × Load gear ×
Feed constant × Modulo range
Load gear = Load revs/Load encoder revs
Feed constant = Load units/Load revs
Modulo range = factor to define load units range for modulo operation
Load gear × Feed constant = Load units/Load encoder revs
Load encoder scale = Motor revs/Load encoder revs
Load encoder revs = encoder revs of position feedback source
Gear ratio = Motor revs/Load revs
Load revs = load-side revolutions after gear ratio
40 Position control program features
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