Driver ICs Elevate Design of Stepper-Motor Control

Aug 1, 2007 12:00 PM
By Guido Remmerie, Director of Industrial ASSPs, and Peter Cox, Product Manager for Industrial ASSPs

Applications

Dynamic positioning implies that an object is moved from one position to another at a speed that is designed specifically for that motion. In most cases, this motion is high speed. Acceleration and deceleration are adapted to the specific design of the motor and its specified load to avoid oscillations and resonances in the system (including the motor itself).

This demanding dynamic positioning is being used in dynamic headlight positioning and dynamic flap positioning for automotive climate control. In industrial applications, there is also a need for smooth dynamic motion in surveillance-camera positioning, air-flap and water-valve positioning in climate-control systems, and dose pumps in process control. Very high-speed dynamic motion is occurring in manufacturing equipment like weaving equipment, pick-and-place equipment, industrial robots, X-Y-Z tables and in the dynamic stage lighting of the entertainment business.

Control Architectures

The traditional architecture for stepper-motor-driver electronics is shown in Fig. 1. The core of the circuit is a microcontroller, usually with the program code embedded in Flash memory. For some applications, a DSP is more suitable. The microcontroller executes the motion-control algorithm based on feedback from the motion of the system, as well as the motor itself.

The first feedback is provided by a Hall sensor, which basically provides information on the rotor position. It can be used to keep track of the rotor position, or to monitor the motion and detect a possible stall condition or blocked rotor. In simple cases, an end-of-loop position switch is sufficient. Additional options include optical-position coding or a resistive potentiometer mounted on the motor shaft. All of these options increase system complexity, pc-board area and cost.

A resistor in series with the motor driver collects information on the motor current, and an ADC converts it to digital data, which is then fed to the controller. Other diagnostics are possible at the expense of additional analog circuitry. The microcontroller or DSP delivers a PWM signal to drive the motor coils. This signal is amplified by analog circuitry that drives the power stage, which in turn drives the coils of the motor.

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© 2007 Penton Media, Inc.