Should We Switch to Servos?

In this installment of The Answer, a reader learns that to achieve accurate positioning using steppers in machine centers they now need to operate more slowly, but are there advantages in changing to AC servos?

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We’ve used steppers in our machining centers for a long time, and micro-stepping has helped us achieve better resolution along the way. Our problem is that we now have to operate more slowly to achieve accurate positioning. Are there advantages in changing to AC servos that are worth the extra cost? Any tips to pass along?

—from May ’07 Control Design

ANSWERS

Accuracy, Volume Warrant Servos

Stepper motors have extremely high torque, but only when deflected from their target position by nearly a full detent position. Micro-stepping can give greater resolution, but under load won’t deliver the accuracy often advertised or the accuracy of a closed-loop servo.  

A servomotor can deliver full torque with little or no deflection by using the Integral Compensation in all servo control loops. It’s best to use a servo with reasonably high encoder resolution, but nothing approaching micro-stepping resolution. This is because with friction or loading going on, you won’t get the benefit of the resolution advertised for the micro-stepped step motor.

Servomotors vary in effectiveness in machine-tool applications. Typical machine tools use large ball screws, reflecting high inertia to the servo motor, so be sure to choose a high-inertia servomotor. This can be counter-intuitive because motor manufacturers advertise low-inertia servos as being more advanced or better. In many applications they are better, but not in machine tools.

If you mount handles on the rear shafts of the motors, it’s better to make them as small as possible when using servos. The added inertia of the handle will work against you for controllability. 

Bear in mind that it’s a common misconception that high-inertia motors are better because they have a lot of inertia. In fact, they’re better because they have a large diameter rotor that gives the magnetics better mechanical advantage.

High inertia is a by-product of this construction, and shows the motor can perform against large or high-inertial loads. You wouldn’t want to add inertia to the back of the motor, thinking that you’re making a higher-inertia or better motor.

Because servos typically are more expensive than steppers, deciding whether to switch must consider two factors. First, the servo will offer greater accuracy under heavy loading. If greater accuracy is required, then you need a servo. Second, the servo is faster, and will make the machine more productive by quadrupling traverse rates. It’s not difficult to find out how soon a servo will pay for itself. If the machine is seldom used, servos won’t be worth it. If the machine is used all the time, servos will be a great investment.

Robert Bigler, CEO,
Animatics, www.animatics.com

AC Servo Has Smaller Footprint

Yes, there are advantages of using an AC servo versus a stepper, but don’t just look at cost. Look at the savings associated with the smaller footprint of an AC servo. This smaller footprint also will give you lower weight, which can be advantageous depending on the application. It might also allow you to make the overall machine smaller.

For performance, an AC servo is the right solution. It will cost more, but the throughput gained might be worthwhile. You mentioned that you need to run at a lower speed to achieve positioning. The advent of high-resolution feedback devices makes low and high-speed operation more feasible. A common feedback resolution used to be 1,024 lines/rev (4,096 counts post quad). Now, a typical absolute feedback device has 17 bits of resolution or 131,072 counts per revolution.

This higher resolution allows a tighter velocity loop, increasing system bandwidth and letting you settle into position sooner. Your circle, diamond, and square test will never look better.

An AC servo’s drawback is its lower rotor inertia. But, since you can use a higher resolution feedback device with increased bandwidth, you can have a higher load/rotor inertia mismatch.

Pat Berkner, product manager, rotary servo motors,
Parker Hannifin, www.parker.com

Feedback Can Be the Difference

The choice is between a stepper with a micro-stepping drive and a brushless servomotor with sinusoidal commutation. Both are AC synchronous motors, but one is controlled without feedback.

When is the extra cost and complexity of a feedback sensor with its associated circuitry worth it?

First, the feedback sensor permits the motor to always run at a torque angle of 90°, yielding the maximum torque per unit current. With an open-loop stepper, the general rule of thumb is not to exceed a 30° torque angle in normal operation, so that you always have a 2:1 torque margin for unexpected events. This means you’re always planning to get less than half the maximum torque per unit current. So, an open-loop stepper will run much hotter for a comparable size, or probably would have to be sized considerably larger than a brushless servomotor.

Second, you have to be more conservative with an open-loop stepper because a momentary problem can lead to an unrecoverable error that the controller doesn’t know about. Using feedback gives you greater confidence when pushing the system to its limits because you know it can detect and recover from a momentary disturbance.

In one sense, an open-loop stepping system really isn’t open-loop because there’s a feedback loop in the motor’s electromagnetics. But this feedback loop really applies only to proportional gain between the commanded step/microstep setting and the physical position of the rotor.

There’s no integral action to overcome steady-state errors, and there’s virtually no derivative action to provide damping. Continuous high currents are required to minimize steady-state errors  without integral action, and very small micro steps are required to minimize ringing effects. In a servo system, it’s easy to add integral and derivative action to optimize feedback loop performance.

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