(Automotive applications for stepper motors may include headlight leveling, adaptive headlamps (that is where the headlamps turn right or left with the steering wheel), EGR (exhaust gas recirculation) valves, and adjustable mirrors. Non-automotive apps for the method described in this series would be any stepper motor application where the current is around 1A.)
Part 1 of this series covered stepper motor basics.
To more easily see the effects of torque on back EMF (BEMF) the following is a look at a stepper motor driven in full step mode. Figure 9 (below) 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.
Figure 9, Unloaded motor driven in full step mode
In an unloaded motor (Figure 9), 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.
Looking at Figure 10 (below) 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 10, 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 11, Motor in hard stall
In Figure 11 (above) 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 12, below) we can see that there is somewhat of an issue with BEMF detection.
Figure 12, 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 13, Simplified block diagram of Back EMF detection circuit
With our microprocessor A to D sampling we were able to obtain several thousand BEMF readings in a short period of time and generate a histogram of the values. This provided an understanding as to what to expect.