Updated
8/19/02

CURRENT CONTROL IN STEPPER MOTORS
by
Suded
Emmanuel

There are many ways to control current in stepper motors. One of them is to
use pulse width modulation (PWM), this means that current passing through the
motor's coils is always monitored and controlled independent of motor speed
(i.e. if the motor is at standstill or moving). In maintaining constant current,
motor torque remains constant within the operating limits of the stepper motor.

If we consider the diagram of (figure 1 below ), PWM control IC is used for
this task. The IC used (SG3524) is available from (LINEAR TECHNOLOGY) or (SGS).
The current passing through the motor is measured by (R shunt), because the
current passes through two coils at a time (unipolar operation). R shunt
measures the current in two coils. If we use 2A per coil motor the current in R
shunt should be 4A, then we have a voltage drop of 0.4 V across R shunt. This is
amplified 10 times by LM 324 (see fig 1.) so the O/P of the amp. Is 4V. This
goes to pin 1 of the PWM IC (SG 3524), (we are going to explain the operation of
3524 later). Pin 1. of 3524 is the (- input), pin 2. is the (+ input) this is
taken from a pot to make it variable (we can set the desired value) these two
values are compared by the IC and an error signal is produced which is the pulse
width modulation signal from pins (12,13) of 3524. This signal is used to drive
the PNP darlington transistor the. o/p goes to the ferrite core which converts
the pulsed energy to a DC level this is varied to keep the motor current
constant which in turn produces constant or stable torque.

Now we will consider the operation of SG 3524 PWM IC. This is a switching
regulator control circuit (see figure 2.). Its output is taken from two NPN
transistors connected as one (single ended). The transistors are switched to
ground in PWM form. The IC has an error amplifier see (figure 2.) which has
positive and negative inputs (pins 1,2). In our case, pin 1 is the measured
current 4V. (for 2A. per coil stepper motor) pin 2 is the set point set at 4V.
by potentiometer (VR 1 in figure 1.). The error signal from the amplifier drives
the PWM control circuit inside the IC. This circuit supplies pulses to the
transistors in figure 2. These are used to drive the large (PNP) darlington
transistor in figure 1. The pulses are at a frequency set by timing Rt and Ct in
figure 2. The frequency is about 20KHz this is also called the chopping
frequency (in fact PWM means that the duty cycle of that frequency is varied) of
course you know what duty cycle means. SG 3524 has other inputs like shutdown
pin 10. If this is left open it could be used to switch off the regulator by
supplying 5V. to it. The compensation input at pin 9 is used for stability.
Stick to the values in figure 2. The IC is supplied from 8 to 40V at pin 15, in
our case 24V. A 5V output is generated internally and supplied at pin 16. This
is used for the potentiometer (VR 1) in figure 1. We used a 24V supply in figure
1. That doesn't mean the motor is driven at 24V. When it is at standstill the
function of the regulator IC is to regulate the current at 2A per coil at all
times so the motor voltage is varied accordingly.

You can contact Suded at [email-masked]

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