Choosing a step motor: Real example

It isn’t all that hard to select a step motor once you understand how one works and its major limitations. A step motor converts electrical input energy into rotational mechanical output energy. The box “What Makes a Step Motor Tick?” gives some theory. The step motor is a convenient device that, when given a predetermined sequence of input voltage pulses, moves in discrete, predictable, angular increments (step angles). Common step angles are 7.2, 3.6, 1.8, 0.9, 0.72, 0.36, and 0.18 deg. Which step angle to use depends upon your application. The pan and tilt surveillance camera of Figure 1, for example, uses motors of small step angle to help alleviate jitter and apparent blur as the camera moves and stops. We’ll see later how the motors were chosen.

The use of digital signals such as input pulses means that the step motor can be operated open loop — that is, without feedback. Thus, there is no need for an encoder or related electronics. The step-angle error, or deviation of the shaft’s true position from theoretical angular position, is small and nonaccumulative; error detection or correction is not needed. You can close the loop on a step motor if you choose, however.

Top questions

As an application engineer, some of the questions I find most useful when trying to size a step motor include:

• What torque is required to move the mechanism and at what speed must the mechanism move? Remember, total torque includes both the running torque required plus the torque required to accelerate and decelerate the system. Acceleration and deceleration torque is often overlooked. I’ve seen people try to move what amounts to a Mack truck with a Volkswagen engine, with little success.
• What is the system inertia reflected back to the motor shaft? Inertia is influential where high acceleration and deceleration are needed. The amount of acceleration and deceleration torque depends greatly on system inertia. A rule of thumb: Load inertia as seen at the motor shaft should be less than 10 times the rotor inertia of the motor selected. The higher the motor speed, the closer the ratio of inertias should be to 1:1. Most mechanics textbooks and many manufacturers’ catalogs show how to calculate inertia.
• What type of motion in needed? For example, must the machine travel a given distance in a fixed time, or must it reach a given speed after a fixed time? Knowing the required motion helps determine what type of ramping profile to use. Figure 2 shows some examples.
• What is the driven mechanism? Will it be a lead screw or something else? Many airline ticket printers use pinch rollers to feed tickets across the print head. Many rotary index tables use pulleys both to slow the rotary speed of the system and to reduce reflected load inertia to the motor. The answer is important in determining the resolution of the motor, the speeds needed to move the rotor, and how far the rotor must move.

Keep a sharp eye

The following describes how we determined the step motor needed for a specific case — a pan and tilt surveillance camera. Two step motors are required, one for the pan operation; one for tilt.

The first details we needed: speed and torque requirements for the mechanism. We also needed reflected load inertia values. If the mechanism required high acceleration or deceleration, it would be important information affecting selection of the step motor and type of motor driver to use. The performance parameters the customer gave us were:

Pan operation:
Speed range, 3 to 200 deg/sec
Torque, 9.2 oz-in.
Gear ratio, 3:1
Drive voltage, 12 to 40 Vdc

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.

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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.