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96 • Chapter 6 Programming Motion DMC-3425
Stepper motor operation is specified by the command MT. The argument for MT is as follows:
2 specifies a stepper motor with active low step output pulses
-2 specifies a stepper motor with active high step output pulses
2.5 specifies a stepper motor with active low step output pulses and reversed direction
-2.5 specifies a stepper motor with active high step output pulse and reversed direction
Stepper Motor Smoothing
The command, KS, provides stepper motor smoothing. The effect of the smoothing can be thought of
as a simple Resistor-Capacitor (single pole) filter. The filter occurs after the motion profiler and has
the effect of smoothing out the spacing of pulses for a more smooth operation of the stepper motor.
Use of KS is most applicable when operating in full step or half step operation. KS will cause the step
pulses to be delayed in accordance with the time constant specified.
When operating with stepper motors, you will always have some amount of stepper motor smoothing,
KS. Since this filtering effect occurs after the profiler, the profiler may be ready for additional moves
before all of the step pulses have gone through the filter. It is important to consider this effect since
steps may be lost if the controller is commanded to generate an additional move before the previous
move has been completed. See the discussion below,
Monitoring Generated Pulses vs. Commanded
Pulses.
The general motion smoothing command, IT, can also be used. The purpose of the command, IT, is to
smooth out the motion profile and decrease 'jerk' due to acceleration.
Monitoring Generated Pulses vs. Commanded Pulses
For proper controller operation, it is necessary to make sure that the controller has completed
generating all step pulses before making additional moves. This is most particularly important if you
are moving back and forth. For example, when operating with servo motors, the trippoint AM (After
Motion) is used to determine when the motion profiler is complete and is prepared to execute a new
motion command. However when operating in stepper mode, the controller may still be generating
step pulses when the motion profiler is complete. This is caused by the stepper motor smoothing filter,
KS. To understand this, consider the steps the controller executes to generate step pulses:
First, the controller generates a motion profile in accordance with the motion commands.
Second, the profiler generates pulses as prescribed by the motion profile. The pulses that are generated
by the motion profiler can be monitored by the command, RP (Reference Position). RP gives the
absolute value of the position as determined by the motion profiler. The command, DP, can be used to
set the value of the reference position. For example, DP 0, defines the reference position of the A axis
to be zero.
Third, the output of the motion profiler is filtered by the stepper smoothing filter. This filter adds a
delay in the output of the stepper motor pulses. The amount of delay depends on the parameter that is
specified by the command, KS. As mentioned earlier, there will always be some amount of stepper
motor smoothing. The default value for KS is 2, which corresponds to a time constant of 6 sample
periods.
Fourth, the output of the stepper smoothing filter is buffered and is available for input to the stepper
motor driver. The pulses that are generated by the smoothing filter can be monitored by the command,
TD (Tell Dual). TD gives the absolute value of the position as determined by actual output of the
buffer. The command, DP sets the value of the step count register as well as the value of the reference
position. For example, DP 0, defines the reference position of the A axis to be zero.