Bipolar Stepper Tests

The bipolar stepper has a limit as to how fast it may be stepped. This speed limit will vary across the many types of bipolar steppers manufactured. There is also a “load” limit for steppers. How much force is the stepper capable of delivering? This force is usually measured as the torque the motor is capable of generating. We’re going to look at some stepping-speed issues, take a look at running a bipolar stepper directly from the pins of a PIC16F84A without any diode protection, and see if we can come up with a method of measuring torque.

Stepping Speed

You can drive the bipolar stepper with the proper sequencing on the lead wires, but there is no way to tell if the motor actually made that step. Two things can prevent the expected stepping (all other things being equal). First, you can change drive signal too quickly for the motor to “keep-up”, and second, you can have too great of a load on the motor. The motor has a physical limit on it’s start up speed. It must overcome the inertia of the rotor and any attached load and accelerate it to its next stator pole position. The greater the load, the greater the inertia making the start-up even slower. Additionally, inertial forces work to keep the motor in motion when stepping-pulses stop.

You can step faster with lighter load, and you can only drive a specific load at some maximum rate. These limits of speed and load will be different for each type of bipolar stepper. It’s pretty obvious that higher voltage, higher current, and higher torque specification motors will be able to drive a heavier load. It’s not so obvious what maximum speed is allowed. Many manufacturers will supply a speed/torque chart. That can be used as a guide as to the safe limits of operation for your motor.

So the video demonstrates what can happen if the motor is driven too quickly. Tests are run with a load on the servo arm that I have mounted on the motor shaft. The load is just a screw and a short hex standoff. A comparison is made to these tests with the “load” removed and shows an improvement in max motor speed. As expected. There is no real data generated, but you get the idea that driving speed and load are factors that must be considered. The video demonstrate what can happen if you accelerate the motor too quickly and what can happen if you try to decelerate too quiclky. From the tests that are shown, it’s easy to see that there are fail points for operating outside of these boundaries.

Driving Naked

I once saw a video that demonstrated running a small bipolar stepper straight from a small microcontroller. I have since lost that URL and have not been able to find the video. But that video fascinated me in that there was no diode protection. So what’s going on? Well, my video demonstrates that this is probably not possible over an extended period of time.

Naked stepper scope shot

When I connected the motor the the PIC for the first time, the output waveform looked completely normal. After a short period, the signals degraded to what you see above. However, the motor itself continued to run normally. This situation lasted for a few hours. Eventually the motor started acting erratically. It began stepping clockwise and counter-clockwise seemingly randomly. It became apparent that the output drivers were slowly being destroyed by the motor “kick-back”. It is graphically shown why you need protection diodes. The faster the diode the better. Schottky diodes are preferred.

Measuring Torque

Unfortunately, this part is being delayed indefinitely due to mechanical, programmer and weather issues. I hope to have the torque measurement added in the near future.

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