Our last posting we introduced the idea of “position make up.”
The idea is that we’re using the 1.1 full step rotor-stator lag information and making a choice as to whether or not the drive will send all the steps it has received to the stator.
If it doesn’t send some and sends others, because the rotor is still in motion, it can maintain the stator-rotor lag relationship of 1.1 full steps. It just counts the steps we didn’t send and puts them in our “back pocket.” And latter, when the move profile begins to slow down and the lag angle decreases, we’ll pull the same number that was put in our “back pocket” and put them back into the “step stream” at an appropriate place and time so we end up not missing any of the steps, and equally, or maybe even more important, the motor didn’t stall.
The previous discussion has been about the stepper motor, motoring. By that I mean that the motor’s shaft is rotating in the same direction that the torque is being generated in. Basically it’s moving a friction load. Thus, the rotor lags the stator position. For example let’s say the motor shaft is turning counter clockwise (CCW), and the torque its producing is in the CCW direction also. For ease of discussion, let’s call this type of motion as operating in the first quadrant as shown in Figure 1 below.
Turning the shaft in a clockwise (CW) direction with a friction load attached is represented in the third quadrant. Again the motor is generating torque in the same direction as its spinning.
So how do you get a motor to operate in the second and fourth quadrants? I’m glad you asked.
There are several ways to do that. Picture a “pure” inertial load with no friction.
Spin it up to speed in the CW direction. What quadrant is the motor operating in while it’s accelerating? If you said three, you’d be correct.
What quadrant would you be operating in once it gets up to speed?
The third is correct again. Since this is a “pure” inertial load the only friction the motor has to over come is the motor’s own bearing friction which is very, very low. There may be some “windage” (hey, no giggling in the back there!) losses too depending on the shape of the inertial load. Thus, the motor is operating in the third quadrant, but very close to the origin. In fact it might scoot into the second quadrant for a moment or two as the acceleration ramp transitions from the acceleration portion of the move profile to the steady state slew speed.
What quadrant is the motor operating in when the motor decels?
Aha, it’s in the second quadrant this time.
The load and the motor’s own rotor inertia is trying to keep the system spinning in the CW direction, so the motor has to generate a torque in the opposite direction that it is spinning to slow the inertial load down.
Other examples of a motors operating in the second or fourth quadrants are when they lower a vertical load, or moving away from a spring load. Intermittent loads that come and go while the motor is moving can cause the motor to shift into and out of the second or fourth quadrant too.
The second and forth quadrants are areas of operation where some motor drive types regenerate the stored kinetic energy and actually pump it back into the power supply or the AC line. These four quadrant drives are typically more expensive than a single quadrant drive because they have a higher component count and are more complex.
More about second and forth quadrant operation next time.