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Detect stepper motor stall with back EMF technique (Part 2)

Posted: 21 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Back EMF? back electro-motive force? stepper motor?

Here's a look at a stepper motor driven in full step mode to see the effects of torque on back EMF (BEMF). Figure 1 illustrates an unloaded motor being driven in full step mode. The red is the current while the purple is the voltage on the phase. The thin black line is a feeble attempt at estimating the back EMF.

In an unloaded motor (figure 1), the back EMF leads the phase current. What are seen here is a skewed BEMF peak and a prolonged low (near zero) period. This is the torque first speeding up the rotor then slowing it down. Just spinning the motor would provide a very symmetrical BEMF waveform.

Figure 1: Unloaded motor driven in full step mode.

Looking at figure 2 for a loaded motor, we can see the loading is more in line with the current it is being fed. The back EMF is more symmetrical to the driving currents. The zero crossing point is more in the middle, or between the two driving current regions. If we were to load this motor much further, it would stall.

Figure 2: A loaded motor driven in full step mode.

You can see why systems that use stepper motors severely overdrive their motors to ensure that they never, under all normal operating conditions, approach stall.

Now, if we were to compare these waveforms with what appears during stall, we can see a dramatic difference.

Figure 3: Motor in hard stall.

In figure 3 we see that there is virtually no back EMF during the non-driven intervals. This would be nice if there wasn't some play in the mechanical aspect of the system. Typically, a stalled rotor will actually vibrate as it tries to move. As we know, any rotational movement will translate to BEMF.

Looking at a stalled but vibrating rotor (figure 4) we can see that there is somewhat of an issue with BEMF detection.

Figure 4: An example where the motor is in stall but allowed to "vibrate."

Comparing these waveforms with the previous running waveforms we can see that there is some overlap. Of course, this figure is showing the behavior of a full step mode driven motor. This is a bit different than a motor driven in micro step mode. In micro step mode we are only looking during that short moment when the current is zero. It is like reading a document through a straw. You can only see a small portion at a time. It may seem limiting but it is enough.

To get some idea what the BEMF looks like on average for a given motor we built a simple system that will check BEMF synchronously with the stepper motor phasing.

Figure 5: Simplified block diagram of Back EMF detection circuit.

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