In our last posting we introduced torque motors and posed the question about how we might be able to modify our new “Hybrid” drive to act like a torque motor.
Have you thought of a way to do it?
Great! You’re getting stepper theory down pat.
Let’s see if your idea matches mine.
We know that the maximum torque is generated when the rotor lags or leads by one full step. In this case we’re only concerned with a lagging rotor. We also know that the torque generated by a stepper motor follows a linear relationship with the phase current.
So all we have to do is set the maximum current to generate the torque that is needed and create a control algorithm that keeps the rotor lagging behind the stator’s commanded position by one full step to generate a constant torque.
Let’s see how all this works in an application. Let’s assume we’re using our new “Hybrid” drive in the “torque mode” to put tension on web. When the web is stationary the rotor is stalled or in a locked rotor condition and the stator command position is one full step ahead of the rotor. Now assume the web processing starts and the web begins to move. Our “Hybrid” drive’s “purpose in life” in the “torque mode” is to always stay one full step ahead of the rotor’s position, so the “Hybrid” drive automatically begins to generate steps to keep the stator’s command position one full step ahead of the rotor. The faster the web moves the higher the pulse rate. If the web speed slows down, so does the step rate.
How does the “Hybrid” drive know what speed to operate at you ask? It’s looking at the rotor’s encoder information. If it begins to move, then the control generates an appropriate number of steps at the appropriate frequency to keep pace with the web and keep the rotor one full step ahead of the stator. There are no additional tachometers or encoders monitoring the web speed, and that relates to a nice cost savings as well as a reduced component count and increased reliability. Plus a stepper motor is happy operating in a full-current-no-motion condition. Do you remember from our last posting that the simple torque motors had a locked-rotor duty cycle that was dependent on the applied AC voltage? The same de-rating would be necessary for some of the other motor technologies too, but not for our “Hybrid” drive system.
A word of caution:
We talked about this before, but I want to say it again for those that missed it. There can be motor-to-motor variations in the output torque. These variations are a function of the motor’s manufacturing tolerance. If you don’t build in some torque safety margin when you specify your motor and you purchase hundreds of motors, you may see these variations. I like to choose a motor with at least a 20% torque safety margin.
So design the system with that margin in mind and you’ll be fine.
More on “torque mode” next time.