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Choosing A Step Motor: Real Example (1)
- Jul 28, 2017 -

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.

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