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Choosing A Step Motor: Real Example(2)

Jul 31, 2017

Tilt operation:

Speed range, 5 deg/sec

Torque, 12 oz-in.

Gear ratio, 1:1

Drive voltage, 12 to 40 Vdc

Acceleration and deceleration rates were not severe for either operation.

Pan operation is the most demanding of the two axes, so the following discussion concentrates on it. Panning requires smooth motion because, when a camera with a long lens mounts on the unit, any shaking will be seen as blurring on the video monitor.

The first choice for a test motor was a 1.8-deg/step, size 17 motor with the customer’s 12-Vdc driver. This motor had a rated holding torque of 22.2 oz-in., far above the pan operation’s torque requirement. However, it delivered only marginal performance at the top speed of 666 pulses/sec (200 deg/sec). The motor moved the load at desired speed sometimes, but lost synchronism and missed steps at others.

We then decided to test a similar motor, but one that could produce more torque. A 0.9 deg/step, size 23 motor with a 12-Vdc driver was selected. Even though this motor had a rated holding torque of 80 oz-in., it too could perform only marginally at top speeds. The inadequacy of the size 23 motor here was a clue that there was a torque-at-speed problem — a problem not necessarily caused by the motor, but by the driver serving the motor.

A step motor’s windings consist of a series circuit having both resistance and inductance. The inductance causes motor current (thus, torque) to build over a finite time, even when the winding is hit with a step change in voltage (a voltage pulse). If the pulses come very fast (higher speeds), current may not have time to build to rated value before the next pulse hits; the motor may not reach rated torque.

Some drivers in effect add resistance in the motor winding. (Remember: resistive and inductive currents are 90 deg out of phase.) The result is a softening of the effect of inductance on current buildup time with increasing resistance, thus letting the motor come closer to rated torque at higher pulse rates.

However, the method raises the resistive power loss in the motor. And at higher speeds, it may still not be enough to let the motor reach full rated torque. That seemed to be the case here. We then tested the same two motors, but with a unipolar, constant-current, pulse-width-modulated (PWM) “chopped-voltage” driver. This driver could drive the motors with up to 24 Vdc. A PWM driver tends to let its step motor reach higher speed, because the higher voltage applied to the motor windings lets motor current rise toward rated value faster; the motor can produce more torque at higher speeds. In tests with this drive, both the size 17 and size 23 step motors reached desired performance.

With the torque hurdle cleared, we had to look at smoothness of motion throughout the speed range. At low speed (10 pps), both the 1.8 and 0.9-deg/step motors shook too much. When accelerated, they showed typical resonance effects. The motors shook violently and lost synchronism before reaching the desired top speed. Low-speed shaking also made the video camera picture jittery and unacceptable. However, we saw that the 0.9- deg/step motor motion was much smoother than that of the 1.8-deg/step motor.

Because as step size decreases, smoothness increases, we decided to try a high-resolution, 5-phase, 0.36-deg/step motor and 24-Vdc driver combination. In testing, we could move the camera very smoothly at high speed (300 deg/sec) and low speed (1.5 deg/sec). Also, there was no noticeable jitter on the video monitor at any speed in the range.

In the next test, a full-size telephoto lens was loaded onto the camera. The motor performed well. Then, to more accurately simulate the entire load of the system, we added the real mechanical components required for the tilt axis. Everything worked well. Moreover, we were able to run the tilt operation up to 200 deg/sec — well above requirements.

Driving voltage still needed consideration. All testing had been done at 24 Vdc. But could a 0.36-deg/step motor reach the 3,333-pps maximum speed at only 12 Vdc? When the motor was tested with a 12-Vdc driver it performed as expected — it had difficulty reaching the higher speeds. Because the customer wanted to keep power consumption as low as possible, we tested the motor at increasingly higher voltages until it ran satisfactorily at the higher speeds. Through trial and error, we determined that the motor ran well at as little as 17-Vdc applied voltage. In general, you should drive a step motor with as low an applied voltage as possible. As motor drive voltage increases, the motor tends to run hotter and create more electromagnetic or “winding” noise. Also, it can increase the amplitude of any transmitted mechanical noise the motor produces. Because the original motor drive specification was 12 to 40 Vdc, we were well within the camera mechanism’s heat-dissipating capability.

With motor and driver types determined, it was time to find out if the system could meet the customer’s cost restraints. Unfortunately, it was not cost-effective for the customer to purchase the motor and an off-the-shelf driver in the quantities needed. We decided to help the customer build his own driver. By supplying the logic and power chips, along with the technical support to help integrate them into the final design, the customer was able to build a complete motion system under budget.

What you have just seen is a thought process to use when selecting a step motor for an application:

• Define the task well and be sure you understand it.

• Understand the mechanical system as well.

• Determine torques needed to move, accelerate, and decelerate the system.

• Estimate the performance you can expect from a given step motor and driver combination. You can find such information in manufacturers’ data sheets.

• When you have all that information compiled, you — the designer — should assemble and thoroughly test a complete system. If you don’t have the facilities for testing, work with an independent test lab or with the motor and driver supplier. But stay close, monitor the tests, and know what is going on. Don’t be afraid to test several configurations. Trial and error is still the best way to see whether you have selected the best motion system for your application.

What makes a step motor tick?

The rotor of a typical hybrid step motor has two soft iron cups that have teeth (usually 48 or 50) on their surfaces. They guide the flux from a permanent magnet to the rotor-stator air gap. Each cup is axially magnetized with polarity opposing the other. In most cases, the teeth of one cup is offset from the teeth of the other by one-half tooth pitch for a 2-phase step motor.

The stator also has many teeth, but generally one or two fewer than the rotor. When a tooth on the stator is energized with a North polarity, a corresponding tooth on the rotor with a South polarity aligns with it. Likewise and at the same time, another tooth on the stator of the same phase is energized with a South polarity, which attracts a tooth on the North pole cup to align with it. By energizing neighboring stator teeth one after the other in a rotating sequence, the rotor begins to turn correspondingly as its teeth try to align themselves with the stator teeth.

Shortening steps

A regular 2-phase hybrid step motor moves in an oscillatory manner. The effects of these irregular rotations and corresponding resonance can range from high audible noise to missed steps. The best way to minimize the effects of resonance is to make the step size smaller. There are two popular methods to reduce step size of a common hybrid step motor: “microstep” a regular 2-phase, 1.8-deg step motor, or use a 5-phase step motor.

The microstepping method reduces step size electronically. This is done by proportional control of the current in each phase to create an intermediate step between the motor’s “cardinal” steps. With this method, a 1.8-deg/step motor can be driven at up to 25,000 steps/rev.

The 5-phase method adds a pole pair to the stator of a regular 2-phase hybrid step motor design. This changes the rotor-stator offset of a 2- phase step motor from one quarter to one tenth the rotor pitch. The resulting full-step size in a motor with 50 rotor teeth is 0.72 deg, or 500 steps/rev. The 5-phase step motor has much lower irregular rotations compared with a full-step, 2-phase motor, and has practically no resonance effects. The torque ripple of a 5-phase step motor vs. a regular 2-phase step motor is also greatly reduced. The difference in lowest to highest torque value of a regular 2-phase step motor is 29%. The same value for a 5-phase step motor is only 5%. This reduced torque ripple is one reason for the 5-phase step motor’s smoothness.