In the back-phasing damping method, a well-timed (CCW) pulse creates an opposing torque on the rotor, causing it to accelerate from the equilibrium point of the first phase toward that of the second phase.
Select figure to enlarge.

Getting step motors up to speed has never been a problem. Getting them to stop is a different matter.

Step motors (because of their magnetodynamics) are naturally springy, so they tend to vibrate at the end of each move cycle after the final pulse. In applications requiring high throughput, the relatively long settling times take a big chunk out of the motion cycle bandwidth.

The obvious solution to this problem is to make step motors less springy; that is, find a way to reduce the vibrations at the end of the move cycle. Several techniques have been developed, from mechanical dampers to electronic switching schemes such as back-phasing damping and delayed-last-step damping. Control methods such as S-curve acceleration and deceleration are also used, especially when it comes to generating ramp up/down motion profiles.

While back-phasing damping is not the only method available to reduce settling time, it has the advantage of not causing large overshoot. As a result, vibrations and motor noise are stifled more quickly due to faster motor stopping.

In the back-phasing method, a delayed pulse opposes the rotor, causing it to accelerate from the equilibrium point of the first phase to that of the second phase. To avoid overshoot, a second delayed pulse is applied (this time in the first phase) when the rotor enters the vicinity of the second phase equilibrium point. In this method, “n+2” pulse commands are required when controlling a motor with “n-step” motion.

Learn by example

The following sections outline an example demonstration in which back-phasing damping significantly improves settling times and reduces the amount of variance between times. Because the results obtained in these demonstrations differ from usage conditions in actual motion environments (such as driver, motor, frequency, and load), back-phasing damping in other instances will vary substantially. However, under specific circumstances, namely iterative, uniform motion with homogenous load conditions, back-phasing damping can reduce settling times and therefore improve system speed and productivity.

In this demo, experimental testing was done using the back-phasing damping method to see how much settling time could be reduced compared with normal operation (no damping). Motor behavior was measured using a system that samples encoder output (the same pulses driving the motor) every 0.1 msec. For example, if the motor were run at 500 pps, the theoretical number of encoder output pulses would be.

During back-phasing, the motor driver produces a series of pulses, the transitions of which generate a damping force that stabilizes rotation dynamics.
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This number of pulses represents the sampling resolution for each sampling period.

When the motor is run at 500 pps and 50 pulses are fed, the movement completion time (output finishing time) will be.

To determine the method of analysis, the normal operating (no damping) response was compared to the response obtained when back-phasing damping was performed. The method of analysis was then determined based on the degree to which settling time had been reduced. The settling time was defined as the time required for the motor to stop all vibration, beginning from the end of the last output pulse (100 msec), until the vibration had been damped to less than ±5% of the motor’s step angle (0.72°) specification.

In this test, the pulse width used for motor movement was set at 200 μsec. The back-phasing procedure:
1. Last CW pulse
2. “Timer 1” actuation
3. Output of one CCW pulse (50 μsec)
4. “Timer 2” actuation
5. Output of one CW pulse (50 μsec)
6. Output actuation has been completed

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