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# Four Quadrant Drives Part the Third: Torque Mode

Our last few posting introduced a number of improvements to our original stepper drive so we renamed it a “Hybrid” drive. This “Hybrid” drive operates in all four quadrants without missing any steps and without stalling. Think about that for a minute. A stepper system that doesn’t stall, when’s the last time you heard that?

Did you ever expect to see “stepper system” and “doesn’t stall” in the same sentence? Some automation engineers automatically avoided using steppers because they stalled. I strongly recommend that if you’re one of them to reconsider this innovative motion control solution. It might save you some big dollars.

The two “magic elixirs” that we incorporated in our design we called “variable current mode” and “position make up.”

The “variable current mode” controls the current flowing to the motor in a linear fashion and is a function of the lag or lead angle relationship between the rotor and the stator’s commanded position. When the rotor lags or leads by one full step the current reaches 100%. A half step lag or lead is 50%

The “position make up” algorithm is applied when the rotor lags or leads the stator’s commanded position by greater than 1.1 full steps.

When the rotor lags by 1.1 full steps we remove steps and keep track of how many we removed and then, at the appropriate time, we reinsert them into the step stream so the system takes the correct number of steps and the motor doesn’t stall.

If the rotor leads the stator’s position by more than 1.1 full steps then we add steps and keep track of how many were added and then, at the appropriate time, we remove them from the step stream so that the system takes the correct number of steps and the motor doesn’t stall. If you’re just joining this blog, I suggest you back up a number of postings and read how we developed these “magic elixirs.” You’ll find that that it’s more on the ingenious side than magic.

Sweet!

Can you think of any other “magic elixirs” we might be able to incorporate into our “Hybrid” drive? No, well keep thinking, because we can do more.

I’ll give you a hint:

We know that the maximum torque the motor produces takes place at one full step rotor-to-stator lag or lead. How can we use that knowledge in a different way from what we have been doing? What if we create a “torque mode” and modify our “Hybrid” drive yet again, so it causes the step motor to act like a torque motor.

What’s a torque motor you ask?

Take a look at figure 1 below:

Simple torque motors are AC permanent split capacitor (PSC) motors that are typically operated at reduced voltage. They are designed to operate in a stalled condition for a specified time.

For example if the PSC motor is energized with half its rated voltage the torque is fairly flat from its zero speed (stall or locked rotor) to about 25% of its rated speed. At half its rated voltage it might be able to stay in the stalled condition 100% of the time.

At 2/3 of its rated voltage you’ll get more torque, but it might only be able to stay stalled for only 20 minutes and have a duty cycle of 33%. This means that the motor has to be off for at least 40 minutes.  At its full rated voltage you’ll get even more torque, but it might be able to stay stalled for only 5 minutes.

AC PSC Torque motors are simple to control because they require only a variable AC input voltage to set the torque.

Other motor technologies can be made to act like torque motors. Some others are permanent magnet DC motors, brushless DC motors, AC induction motors and servos which of course have more sophisticated controls.

So did I whet your appetite about how we’re going to get a stepper to act like a torque-motor?

More next time.

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