User's Manual

PMAC User Manual
Setting Up PMAC Commutation 99
• Plot the velocity-vs.-time graph on the screen. Calculate an acceleration value from the slope of the
curve as it leaves zero velocity.
• Decrease the Ix78 slip gain by 10% and repeat steps 3 and 4. If the acceleration from zero velocity is
greater than the first plot, continue decreasing slip gain. If the acceleration is less than the first plot,
increase Ix78 from the initial value by 10% and repeat steps 3 and 4.
• Continue modifying Ix78 slip gain until honing in on a value that provides the maximum acceleration.
When close, use progressively smaller changes to Ix78 until noticing that there is no significant
change in the motor response.
• Multiply the values of Ix77 and Ix88 together. This product is optimum for the motor (at least at its
present temperature).
• Notice the maximum velocity on the plot that provides maximum acceleration. The acceleration
stopped because of the back EMF of the motor, which is proportional to your Ix77 magnetization
current, matched the supply voltage. To get to a higher speed than this, decrease the Ix77 value in
inverse proportion to the desired increase in speed. To double the speed, decrease Ix77 to 50% of its
present value.
If able to tolerate a lower maximum speed, and a higher torque at low speed, increase the Ix77 value. If
changing the Ix77 value, change the Ix78 value in inverse proportion, so the Ix77*Ix78 product stays
constant (newIx78=IdealProduct/newIx77).
Open-Loop Microstepping Commutation
PMAC has the ability to do open-loop microstepping (direct microstepping) of standard stepper motors,
working off internally generated pseudo-feedback for both commutation and servo algorithms.
This technique is different from using PMAC with a voltage-to-frequency converter to command an
external microstepping drive; that technique does not utilize the PMAC commutation algorithms at all.
When microstepping, PMAC provides two analog outputs that are used as current commands for phases
of the motor. Typically, for a microstepping motor, the two phases are electrically independent and 90
o
out of phase with each other. In this case, the two outputs are simply bidirectional current commands for
the H-bridge amplifiers driving each phase. These amplifiers can be simple torque-mode (current-mode)
DC brush motor amplifiers.
The PMAC microstepping algorithm provides 256 microsteps per electrical cycle, which is 64
microsteps/step. On a typical 200-step/revolution motor, this amounts to 12,800 microsteps per
revolution. With the default phase update frequency of 9 kHz, PMAC can slew at over 576,000
microsteps/second (9000 full.
Figure 17 PMAC/PMAC2 Direct Microstepping System