IXMS150PSI datasheet(5/10 Pages) IXYS

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Application Information

Introduction

The advantages of step motors are well

known. They may be operated in an

open loop fashion, the accuracy of

which is mostly dependent on the

mechanical accuracy of the motor. They

move in quantized increments (steps)

which lends them easily to digitally

controlled motion systems. In addition,

their drive signals are square wave in

nature and are therefore easily gene-

rated with relatively high efficiency due

to their ON/OFF characteristics.

But step motors are not free of prob-

lems. Their large pulse drive wave-

forms create mechanical forces which

excite and aggravate the mechanical

resonances in the system. These are

load dependent and difficult to control

since step motors have very little

damping of their own. At resonance a

step motor system is likely to lose

synchronization and therefore skip or

gain a step. Being an open loop system,

this would imply loss of position infor-

mation and would be unacceptable. A

common method of solving this problem

is to avoid the band of resonance

frequencies altogether, but this might

put severe limitations on system

performance. Steppers have 200 steps

per revolution or 1.8 degrees per step.

The highest resolution commercially

available steppers have 400 steps per

revolution or 0.9 degrees per step.

Microstepping Mode

One way to circumvent the problems

associated with step motors while still

retaining their open loop advantages is

to use them in the microstepping mode.

In this mode each of the steps is subdi-

vided into smaller steps or microsteps".

Applying currents to both phases of the

motor creates a torque phaser which is

proportional to the vector sum of both

currents. When the phasor completes

one turn (360 electrical degrees), the

motor moves exactly four full steps or

one torque cycle. Similarly, when that

phasor moves 22.5 electrical degrees

the motor will move (22.5/90) 100 =

25 % of a full step. Thus the position of

the motor is determined by the angle of

the torque phasor. When used with an

appropriate motor a positioning accu-

racy of 2 % of a full step can be achie-

ved, equaling 0.036 degrees for a 200

full steps per revolution motor. In this

manner the motor can be positioned to

any arbitrary angle. A common way to

control the angle of the torque phasor is

by applying to the motors phases two

periodic waveforms shifted by 90

electrical degrees.

Let the phase current equations be:

A = IO cos θe

(1)

B = IO sin θe

(2)

Note that θe is the electrical position.

The resulting torque generated by the

corresponding phases would then be:

A = K0 iA = K0 I0 cos θe

(3)

B = K0 iB = K0 I0 sin θe

(4)

where K

0 is the torque constant of the

motor. Substituting Eqs. (1), (2) into (3),

(4) and doing vector summation the

resulting total generated torque mea-

sured on the motor shaft is given by:

g = K0 I0

(5)

Note that in this case we have zero

torque ripple.

Using this technique one can theore-

tically achieve infinite resolution with

any step motor. Since the drive current

waveforms are sinusoidal instead of

square, the step to step oscillations are

eliminated and the associated velocity

ripple. This greatly improves perfor-

mance at low rotational speeds and

helps avoid resonance problems. In an

actual application, the extent to which

these things are true depends on how

the two sinusoidal reference waveforms

are generated.

Seemingly we have lost the quantized

motion feature of a stepper when used

in this mode. This can be regained by

defining the term microsteps per step.

Each full step is subdivided into micro-

steps by applying to the motors phases

those intermediate current levels for

which their vector sum tracks the circle

of Fig. 2 and divides the full step (90

electrical degrees) into the require

number of microsteps. An example of

the required phase currents for full step

and four microstep per step operation

are shown in Fig. 1 and 2 respectively.

Phase Current Matching

Requirements

Assuming microstepping is being used

for resolution improvement and not as a

resonance avoidance technique, a step

motor can be selected knowing the

torque needed, its specified step

IXMS 150

Fig. 1 Full Step Drive Waveforms

accuracy, and the required resolution or

the number of microsteps per step.

Next, one must determine the accuracy

required of the phase currents to main-

tain the accuracy of the complete

system. Equations (1) - (4) clearly

indicate that errors in the absolute

value or phase of the phase currents

will impact positioning accuracy.

Another observation is that by keeping

the ratio of the phase currents i

A/iB

constant, errors in their value will result

Fig. 2 Four Microstep per Step Drive

Waveforms

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