Rockwell Automation Publication 1756-AT017A-EN-P - January 2013
8 Position-based Output Control with the MAOC Instruction
Constant versus Changing Velocity
The MAOC instruction calculates schedules without acceleration compensation.
Between each CUP, the instruction assumes a linear velocity. However, physical
systems behave in a non-linear fashion due to mechanics and physics. This can
lead to slight deviations between the scheduled position and physical position of
an event.
During times of high acceleration or deceleration, outputs can change state
sooner or later than planned. For example, in
Figure 4, the intended result is to
turn the output On where position A intersects the physical position graph at
time D. The velocity compensation estimates that this will happen where
position A crosses the schedule position graph at time C. Because the physical
position lags behind the schedule position, the output turns On where position B
crosses the physical position graph. This demonstrates an output changing state
earlier than expected during acceleration.
This example is graphical only, and numerical difference is dependent on the
acceleration profile, the mechanical system, the available dynamic response, and
the duration of the coarse update period.
Figure 4 - Example of Constant versus Changing Velocity
osition
Time
C
D
Schedule Position
Physical Position
CUP 1
CUP 2
CUP 3
CUP 4
A
B
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