Our last posting introduced the idea of four quadrant drives. Four quadrant drives allow the motor to generate torque in the same direction or in the opposite direction that the shaft is spinning. If the motor is operating in the first or third quadrant the rotor will lag the stator’s magnetic field. If the motor is operating in the second or fourth quadrant, then the rotor is leading the stator’s magnetic field. In either case, the maximum torque that a stepper can generate is when it’s lagging or leading the stator’s commanded position by one full step.
In our previous discussions of our new “Hybrid” drive we always talked about the rotor lagging the stator. We introduced the “variable current mode” which increases the motor’s current in a linear fashion as a function of the rotor’s lag angle. We also introduced a new concept of “position make up” where the drive didn’t deliver all the commanded steps because the rotor was lagging by 1.1 full steps. It then reinserted the steps that it didn’t take so the correct number of steps taken was still accurate, and equally, or maybe even more important, the stepper didn’t stall.
So now let’s look at our “Hybrid” drive with the motor’s operation in the second or fourth quadrant. The rotor is leading the stator’s commanded position. The stator’s magnetic filed is trying to pull back or retard the rotor’s motion. We can do the same thing we did with the “variable current mode.” That is, if the rotor is leading by ½ of a full step, then let the winding current is 50% of its maximum rating. If the rotor is leading by ¾ of a full step then the current is 75% of the maximum rating. Again it’s a linear relationship. The bigger the lead (or lag) the higher the current.
But what happens when the rotor leads the stator’s commanded position by 1.1 full steps?
If we remove steps, like we did when it was lagging, we would just exacerbate the situation by increasing the lead. So what do you think we should do?
I love it when everyone agrees.
We should add steps so the leading rotor doesn’t get any further than 1.1 full steps ahead.
The control keeps track of the number of extra steps that were taken to keep the motor from stalling and removes them from the “step stream” at an appropriate place and time, so that the rotor moves the correct number of steps. Again, equally as important the motor didn’t stall.
Now for you purest, if the maximum torque takes place at one full step what is the torque value at 1.1 full steps? Hopefully you remember from our previous postings that the stepper motor’s torque follows a sine wave with the maximum torque taking place at the sine 90 degrees. This is when the rotor is displaced by one full step from the commanded position.
So the torque at a 1.1 full step lead or lag is found by using the following equation:
100% (sine (1.1*90)) or 100% (sine 99 degrees) = 98.78% of the maximum torque. Thus, in all practicality the motor is generating 99 % of it maximum torque between 81 and 99 degrees whether the rotor is leading or lagging and 100% at exactly 90 degrees. That’s a broad area of high torque operation for our “Hybrid” drive to work its magic.
Now our “Hybrid” drive has a “variable current mode” and “position make up” that work as a stall prevention mechanism when the rotor is leading or lagging
We’ll have more nifty ideas next time.