Applied Motion Products investigated how closed loop step motor systems outperform traditional open loop step motors in terms of torque. To demonstrate that, Applied Motion Products used two equally sized step motors. On the one hand, researchers used a conventional motor that they’ll drive open loop with a basic microstepping drive. On the other hand, researchers used a StepSERVO closed loop integrated motor.

As Applied Motion Products compared these two motors side-by-side, researchers can see that they both have a NEMA 23 frame size and the stators, which are the torque-generating portions of the motors, are the same length. They have the same windings, so they will essentially perform the same except that this motor is being driven open loop and the other motor is being driven closed loop.

The way this test works is that each motor will be commanded to make identical point-to-point moves. In these point-to-point moves, each motor will accelerate to a fixed velocity, run at that velocity for a brief period of time and then decelerate to a stop. Throughout these tests, the target velocity will be 10 rev/sec or 600 rpm.

With a moderate acceleration rate of 100 rev/sec2, researchers saw that both motors perform this move just fine.

Now they increased the acceleration rate to 500 rev/sec2 and observe what happens. The open loop motor started to lose steps. This is happening because the open loop motor does not have enough torque to accurately accelerate and decelerate the motor at this higher rate.

However, researchers also noted that the closed loop StepSERVO motor continued to accurately position with this more aggressive move profile. In fact, they saw an increase in the acceleration rate all the way to 2500 revs/sec2, and the StepSERVO motor continued to position accurately. Meanwhile the open loop motor had nowhere near enough torque to complete this move.

The reason the StepSERVO motor could do this is because of the closed loop servo control, which allows the motor to generate peak torques as much as 50% higher than the open loop motor.

Another thing to notice in this test is that with the increased acceleration rate the move time of the StepSERVO motor is much shorter. Depending on the target velocity, the closed loop motor will decrease move time by as much as 40% or more. In simple terms, a shorter move means more cycles per minute and greater throughput for a machine or process.

Applied Motion Products
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