Page 35 - EE Times Europe Magazine | March 2020

EE|Times EUROPE 35

MOTOR CONTROL DESIGN
Microstepping for High-Performance

Motion Control

By Maurizio Di Paolo Emilio

tepper motors are brushless synchro-
nous electric motors powered by direct
current that rotate in steps, keeping
Sthe rotor stationary under a specific
power supply. Pulse-width–modulated (PWM)
pulses, sent in a well-defined sequence via
electronic drivers, turn the stepping motor in
one direction.
Stepper motors exhibit a special feature:
the ability to rotate the rotor shaft by a few
degrees very precisely and without the need
for sensors to detect the shaft’s angular posi-
tion. In short, a complete revolution of the
motor can be divided into a certain number of
steps. The stepper motor’s width determines
the number of steps.
Inside a stepper motor, there are several
windings/coils arranged circularly on the
stator that operate like an electromagnet. The
stator converts the magnetic field to electric
current. The number of steps declared by the
manufacturer corresponds to the number of
groups of coils (called phases) electrically Figure 1: Electrical diagram of a stepper motor and main components
connected to each other. Within each group, (Image: Maxim Integrated)
there are several pairs of coils in parallel.
Each coil pair is arranged and electrically
connected around the stator. Each phase is
activated in sequence to allow the motor to
rotate, one step at a time.
The stepper motor operates with accurate
positioning and speed control by synchro-
nizing with the pulse signal output from
the microcontroller to the motor driver. The
motor driver reacts to the microcontroller’s
signals by delivering power pulses to the
stepper motor at the output.
The stepper motor is operated by electri-
cal impulses that feed a part of the stator
in sequence. The sequence with which the
stators are excited determines the direction of Figure 2: Generic layout of a bipolar and unipolar motor (Image: Maxim Integrated)
rotation. Through the technique of micro-
stepping, it is possible to obtain a more fluid
and linear movement and better positioning. There are different micro- The main applications for stepper motors are robots, office automa-
stepping modes, with lengths from 1⁄3-full-step to 1⁄32-full-step. tion (OA) equipment, and medical/nursing equipment, which require
motors with superior precision motor control and better resistance to
DRIVERS FOR STEPPER MOTORS environmental influences. Motor control solutions such as resolvers
The stepper motor forms an incremental motion drive. With every and encoders are used to optimize the movement and guarantee the
command pulse that reaches the system, the motor completes a finite correct position of the motors.
rotation. The rotation is achieved by powering the phases in a given Stepper motors with a resolver enable high-precision motor control
sequence and with a given current direction (Figures 1 and 2). even in harsh environments with heat, dust, or vibration and maximize
There are different stepper motor configurations and even more the available torque. The resolver is a type of inductive displacement
ways to drive them. The most common stator configuration is two coils. transducer — an electromechanical device for the measurement of
These are arranged around the stator’s circumference in such a way angular displacement. The device detects the variation of magnetic
that when driven with square-wave signals, the motor turns. To turn induction flux in the solenoid.
the motor in the opposite direction, simply reverse the ratio between As an example, Renesas Electronics Corp. has developed a resolv-
phases A and B of the signals. er-to-digital converter (RDC) that supports MinebeaMitsumi’s new

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