Design World – August 2019

104 August 2019 http://www.designworldonline.com DESIGN WORLD
DESIGN WORLD August 2019 105

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Micrometer Positioning

Stages

Micrometer Driven Ball &

Crossed Roller Stages

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• Standard selection of stepper and servo motor
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voltage specified in the speed-torque curve is

essential information. If ignored, say by using

a 12-V supply when the published curve uses a

48-V supply, the motor won’t reach the expected

torque. See Figure 2 (two speed-torque curves

of the same step motor and drive combination)

for more in this.

4

“Can’t I run this step motor

with a couple of PLC

outputs? Why do I need a

drive?”

Two-phase stepper drives use a set of eight

transistors connected to form an H-bridge.

Creating an equivalent H-bridge from PLC

outputs would require eight outputs. Some

two-phase step motors with six lead wires are

driven with as few as four transistors. For these,

you could use four PLC outputs to rotate a

step motor forward and backward. However,

a stepper drive does much more than simply

sequence the transistors in the H-bridge.

Stepper drives regulate the current in each

phase of the motor using PWM switching of the

bus voltage. As noted in the previous section on

voltage, the supply voltage must be high enough

to overcome BEMF and produce torque at speed.

Stepper drives with microstepping capabilities

further refine the PWM switching logic to

ratio the current in each phase according to a

sine wave, getting finer positioning than a step

motor’s basic step angle.

Moving beyond the most basic stepper

drives, those that have trajectory generators on

board can automatically ramp the motor speed

up and down according to preset acceleration

and deceleration rates.

Using PLC outputs to drive a step motor

could be a neat project for someone interested

in dissecting how step motors work. For any

serious motion control project, you’ll want a

proper drive.

5

“The step motor is so noisy

... there must be something

wrong with it.”

Every time a step motor takes a step, it generates

a little bit of ringing noise as the rotor settles

into position (think of the classic mass on a

Motion Control 8-19_Vs3 NEW.LE.indd 105 8/5/19 1:52 PM