In the last posting we looked at the dynamic performance of a stepper motor and we discussed the fact that the faster the motor spins the less time the drive has to get the current into the motor’s winding and the torque falls off as the speed increases. Did anyone reading it say, “Hey wait a minute. What about the counter EMF effect. That slows down the current rise in the winding too?”
Well if you did, you get two gold stars and an at-a-boy/girl. Good for you, because it’s an important point that I didn’t talk about in the last posting. It is a fact that a stepper motor generates a “back voltage” when the rotor spins. It’s called a Counter Electromotive Force, or counter EMF.
Spin a permanent magnet inside a coil and it will generate a voltage.
The polarity of the voltage that’s generated by the stepper motor is opposite or “counter” to that of the power supply. So the counter EMF is always fighting the power supply and slowing down current rise time and thus, the current flowing into the winding. If it aided the current flow, I think they would have called it “positive EMF,” but then I think we would have entered in to the realm of perpetual motion.
As we noted in the last posting, not only is the step time shorter the faster the rotor spins, but the counter EMF is slowing the current rise time too. At low speeds the power supply can keep the current at its rated value and the torque output relatively constant. Once the speed gets to a point that the current flowing in the winding begins to fall off then, the motor produces less and less torque the faster the rotor spins.
From all the previous blog postings we know:
- That small changes in the winding current can be controlled by using bipolar chopper drive
- That the torque is in a fairly linear relationship with the winding current.
- That the motor can be microstepped to get finer position resolution.
- That a stepper motor has only three moving parts.
- The rotor
- The front bearing
- The rear bearing
- Maybe a non-contact encoder disk
- That stepper motors are robust and are used in many motion control applications.
- That because of their simplistic design, they are the least expensive motion control solution for many applications.
- That stepper motors are brushless.
- The faster the step rate the lower the torque
- That the maximum torque at any given speed that the motor can produce is when the rotor leads or lags the commanded position by one full step.
- That the motor will stall if the error between the commanded position and the rotor position exceeds two full steps.
For all the positive things that we listed about stepper motors and their controls it’s the very last thing in the list, the motor can stall, that make some engineers shy away from using it in their application. It’s not a good thing if the motor stalls when it’s trying to position something very accurately.
Technological evolution added an encoder many years ago to the back of the motor to keep track of how many steps the motor took and the control technology algorithms did a “count and compare.” Basically it “counted” the number of steps that the control sent out to the motor and “counted” the encoder steps back to see if they “compared” to what was sent out. If they agreed with each other, then everything was good.
If the encoder count was less than what was sent, then the motor stalled some place during the move. An output could be set to indicate that the motor stalled and the machine operator could manually intervene. This is fine if the manufacturing process is producing inexpensive items so that the failure or stall didn’t waste a lot of product.
These controls could even calculate the difference between what was sent out and what was returned by the encoder and use that difference to try to make another move to correct the error. The problem with that is the make-up moves were usually at a slower speeds and the machine’s throughput diminished.
To get around the stall issue let’s bring all that we know about bipolar drives and stepper motors and design a new and innovative bipolar control that keeps track of the rotor’s position and the commanded position and makes “adjustments” on the fly that will help prevent stalling.
More on this innovative design next time.