Rockwell Automation 1771-QA Stepper Positioning Assembly User Manual - Manual (Page 76)

3–54 Programming and Operation
Publication 1771-UM002A–EN–P – May 2000
Stepper Motor Acceleration Considerations
The stepper motor specifications will contain an acceleration
limitation. It will state some maximum acceptable acceleration
under load (change in pulse rate for a given duration, i.e. Hz/second).
This motor acceleration constraint must be satisfied and should be
considered especially when programming rapid accelerations to high
final rates. The following equation can be used for a move that
ramps from a 0Hz rate to a final rate.
HZ > Final rate
second
motor
RAMP time
motor
For example, a typical acceleration limit for a stepper motor could be
10,000 pulses per second per second for a given load. Programming
a ramp time of 1 second for a change in final rate from 0 to 10,000
pulses per second would be pushing the motor to its maximum limit
of acceleration since the slope of the ramp is 10k Hz per second.
This is the fastest acceleration allowed for the motor.
The same applies if ramping from one final rate to another such as
when an override is blended with a move in process. In this case, the
change in final rates must be considered.
HZ > Change in Final rate
second
motor
RAMP time
motor
In summary, if a programmed ramp is too quick for the
motor/machine dynamics for accelerations and decelerations, the
motor will not be able to “keep up” with the pulses being sent to it.
If this occurs, some pulses may not be executed by the stepper motor
and the indicated position value will be inaccurate. The status that is
block transferred to the PC processor would not longer state the
actual position of the motor axis.
Important: The position values represent the number of pulses sent
to the stepper translator whether or not they are
executed by the stepper motor.
Resonant Frequency
The stepper motor and load can have a resonant frequency within the
operating range of 0 to 20,000 pulses per second. When operated at
or near the resonant frequency in a steady state condition or when
accelerating or decelerating through this frequency, an increase in
noise and/or vibration can occur. In extreme cases it is possible for
the motor to oscillate and lose pulses. The resonant frequency can
vary widely depending on the characteristics of the stepper motor
and load.