Design World – August 2019

106 August 2019 http://www.designworldonline.com DESIGN WORLD

spring). The ringing is the motor’s natural

resonant frequency, which is based on the

motor construction. The natural resonant

frequency is amplified when the frequency

of motor steps approaches or equals it.

This noise is most pronounced when the

step motor is driven in full step sequence

(the lowest resolution available; equal to

the motor’s step angle) and at low speeds,

typically in the range of 1 to 5 revolutions

per second.

The question of noise most often arises

when a user tests a step motor for the

first time with the motor unmounted and

uncoupled to any load. In this scenario, the

motor is free to resonate as much as it likes

without anything to damp the resonance.

Fortunately, a few easy steps can

mitigate the resonance:

Add mechanical damping to the system

by mounting the motor and coupling the

motor shaft to a load. Coupling the shaft

to a load adds some amount of inertia or

friction to the system ... and that in turn

alters or damps the motor’s natural resonant

frequency.

Reduce the step angle with

microstepping. When microstepping, the

step angle is much smaller with each step

and the natural resonant frequency is

excited less.

If neither of these steps works, consider

using a stepper drive with an anti-resonance

algorithm built into its current control logic.

Motion Control

Applied Motion Products step motors come in a wide range of

frame sizes — from NEMA 8 to NEMA 42 and beyond.

Shown here are step motors from Applied

Motion Products with various stack lengths.

6

“I need an encoder to

run a step motor, right?”

No, an encoder is not required

to run a step motor in open-

loop control. Step motors are the only type

of brushless dc motor that accurately and

repeatedly position a load using open-loop

control. Other motors need some type of

position feedback. Open-loop control works

well when:

• Motion tasks are the same over time.


• The load doesn’t change.


• The required speeds are relatively low.


• Failure to complete the motion task does
  not result in critical or dangerous
  machine failure.

If the application doesn’t meet the stated

criteria, consider introducing feedback

into the system to permit some level of

closed-loop control. Adding an encoder to

a step motor system offers benefits ranging

from basic functions that are essentially

open-loop control but with subtle, effective

improvements, to fully closed-loop control

where the step motor operates as part of

a servo control system. Contact your step

motor and drive supplier for information

on the range of feedback and closed-loop

control options they offer. DW

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Motion Control 8-19_Vs3 NEW.LE.indd 106 8/5/19 1:56 PM