Motion System Design
Taking the right step  with stepmotors

Taking the right step with stepmotors

Servomotors and stepmotors are candidates for incremental control applications. Engineers should consider both types and determine which gives the best competitive edge for a given machine. Often, the choice comes down to accuracy.

Stepmotors and servomotors are both used successfully for positioning in equipment worldwide. However, designers sometimes overlook their distinct characteristics. The two motor types offer different advantages that should be considered during motion design.

Stepping motors are widely used in applications requiring less than 1.5 hp. Simplicity through open-loop operation makes for cost-effective design; tuning adjustments (required by servos) are unnecessary. If the distance to be moved is relatively short and the load unchanging, a stepping motor provides sufficiently rapid moves.

In contrast, servomotors are widely used in myriad applications; high peak torque and speed capability are their strengths. Feedback allows monitoring and immediate correction for any errors; corrective controls also heighten accuracies. Whether the distance to be moved is long or short, a servomotor provides sufficiently fast speeds and rapid positioning.

Stepper technology

Stepmotors move or index in fixed angular increments. These motors are used with a command signal that appears as a series of digital pulses. One pulse causes the motor to increment or move through one angle of motion — so one pulse moves the stepping motor through 2.8°, for example. This specific angular movement is repeated for every pulse.

There are various stepmotor designs — those that step through 0.8°, 1.8°, and 2.8°, for example — and various types (such as variable reluctance, synchronous induction, hybrid, and permanent magnet) with the latter being most common.

Most stepping motors are used in an open-loop configuration: A command for movement is issued from the drive, to the motor, and then the motor moves. Here, the command is simply sent; no signal feedback is returned to inform the drive that any operation has, or has not, occurred. In other words, the open-loop control assumes that the movement or operation has taken place.

Stepping motors have two unique speed-torque characteristics. The first is that a stepmotor's peak torque is the same as its continuous torque capability — so there is no extra torque available. As a result, the stepper can stall, or lose steps, if the load becomes too high or inconsistent.

If this occurs, the machine is homed to a known reference position. To prevent this, it is usually recommended that steppers be oversized by applying a safety factor of at least two times the application torque.

The second unique stepmotor characteristic is the slope of the speed-torque curve. Consider the S-T plot on page 19: At 500 rpm, the available torque drops to about 30 to 40% of the stall torque value; as speed increases, available torque drops further.

This characteristic makes stepping motors better suited for applications with lower-speed operation.

Servomotor technology

Prime features that differentiate servomotors from others are:

  1. Motor diameter
  2. Feedback device
  3. Closed-loop operation, and
  4. Product design and manufacturing.

Servomotors are specifically designed to have small diameters, yet maintain a given output horsepower or torque. Consider typical one-horsepower motors: An induction motor with such power is 5.6 to 7.6 in. in diameter; a PMDC servomotor is 4.0 in.; and a brushless servomotor is 3.5-in. square. How is the servomotor made smaller? Its permanent magnets eliminate large bulky windings (as found in the stator of an ac induction motor) while maintaining the magnet field strength.

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A smaller diameter aids in reducing rotor size, which in turn lowers the rotor's mass (or inertia) to allow for faster acceleration and positioning. How does this affect equipment output? If a machine using a vector motor (also called an induction motor) produces one part, the same machine with a dc servomotor can produce four parts; with a brushless servomotor, it can produce eight parts — because a servomotor reaches target speeds faster.

What is more, a servomotor uses a feedback device in the form of a tachometer, resolver, or encoder. A tachometer provides speed information; both resolvers and encoders provide speed and position information. Coupled with a feedback device, the motor provides improved accuracy and precision. (The most suitable feedback depends on specific application requirements.)

Whatever the form, feedback on servomotors provides closed-loop control in which there are command and feedback signals. Feedback signals provide the control with information to monitor the process, measure the accuracy of the task being accomplished, and issue corrections to fix any errors.

One could say that a servomotor is part of a team — a team of components used to control the position, direction, and speed of a load. The other devices include the control, power supply, and positioner. All work together to accurately move the load.

Servomotors are different in one final way: During their design and manufacture, material selections and winding production are given extra consideration. Materials are selected to allow operation over wider temperatures — most often to 155°C. In contrast, ordinary motors are designed to operate at 90° to 125 °C.

During manufacture, the amount of wire inserted between the slots of the laminations (termed slot fill) is also increased. For example, typical induction motors have 65% fill, while typical slot fill for a servomotor is 75 to 80%. Higher slot fill presents production challenges, but improves efficiency and allows the servomotor to deliver extra torque.

Performance comparison

Consider two motors of the same wattage — 100 W — of approximately the same diameter. Let us review their acceleration capabilities to determine just how fast these two motors can reach speed and position a load.

Two methods of analysis exist. The first considers only the amount of continuous torque available from the motor, whereas the second considers the peak torque available.

Using the first approach (based on continuous torque figures) plotted speed-torque curves reveal that a stepper's speed-torque characteristics at 300 rpm allow 8.76 1b-in. of continuous torque; the servo has 2.8 lb-in. Let us assume that the motors accelerate a load of 0.00035 lb-in.-sec2 — equal to the stepping motor's inertia. The stepper reaches 300 rpm in 2.8 msec, and the servo in 4.3 msec.

Then at 1,800 rpm, the stepper's available torque is 3 lb-in. while that of the servo is 2.8 lb-in. Full speeds are reached in 47 msec and 26 msec respectively.

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Now let us analyze performance with the second approach — by considering peak torque capability. Again, plotted speed-torque curves reveal that at 300 rpm, the stepper has 8.76 lb-in. of torque available; meanwhile, the servomotor has 8.49 lb-in. peak torque available. To summarize, the stepper reaches 300 rpm in 2.8 msec (as in the last analysis) and the servo in 1.4 msec.

At 1,800 rpm, the stepper can output 3 lb-in. whereas the servo can output 8.49 lb-in. Therefore, the time it takes these motors to get to speed is 47 msec and 8.7 msec respectively — meaning that the servo reaches a higher speed faster.

When comparing torque-speed curves for different models, note that a stepper motor's peak torque is the same as its continuous capability; in contrast, a servomotor's peak torque is typically three to four times its continuous torque capacity. The amount of available torque is important in applications, as this is used for acceleration that allows for fast positioning.

The use of feedback devices in a closed loop results in higher positional accuracy. An application with a ballscrew mechanism (for which typical pitch is 5 revolutions/in.) exhibits accuracy of roughly 0.004 to 0.008 in. with a stepper … and 0.001 to 0.0001 in. with a servomotor. These figures account for no mechanical backlash — a phenomenon that decreases accuracy, no matter what kind of motor powers the machine.

Note that when using a stepping motor in an application, if it is useful to obtain and report position data, it is always possible to add feedback.

Manufacturer performance curves reveal that a stepping motor's output power peaks at 900 rpm; in contrast, a comparable servomotor outputs peak power at 3,000 rpm. Why? In short, steppers are designed to deliver optimized power at lower speeds, while servomotors perform best at higher speeds. Therefore, if an application requires continuous running (especially at a low rpm) a stepping motor will perform well. On the other hand, if an application requires low speed and high slew speeds, a servomotor may be the better choice.

The most important caveat here is that the wattage or power rating of a given motor can be a misleading factor if not fully defined and understood.

Instead, when selecting the motor technology for an application, it's better to ask: How much accuracy really is necessary? As accuracy increases, so does cost. A servomotor and control can cost 2.5 times as much (or more) as a comparable stepping motor and control. Stepping motors are the lowest-cost velocity and motion control solution in applications for which their performance is appropriate.

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