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

An integrated controller/driver solution simplifies this architecture. A fully integrated solution takes care of everything done in the traditional way using a single device. An example of such a device, the AMIS-30624, is shown in Fig. 2. This device contains the controller, speed, position, current, diagnostics and power stage. This solution offers simple construction and is well suited to applications where the required current in the coils is close to the maximum operating current provided by the single-chip controller/driver.

There are basically two reasons for departing from the single-chip solution: the need for stepper-motor drive-current control and the need for controller flexibility. In a fully integrated solution, the controller is embedded by the chip manufacturer and can be a microcontroller or a programmable state machine. However, some users have developed a high level of expertise in these systems, developing motion-control algorithms designed for use on standard microcontrollers or DSPs. Even in such cases, an intelligent, integrated motor-driver chip, combined with the standard microcontroller, can bring a strong simplification of the architecture.

The type of chip requires, as an input, a next-step command and does all that is required to deliver the PWM drive signal at the coils of the motor (Fig. 3). One microcontroller can control more than one motor, because the PWM generation is done in the intelligent, integrated motor-driver device.

Fig. 4 shows a controller/driver reference design incorporating two integrated driver chips. This design provides a highly integrated solution for driving two stepper motors using a single microcontroller, which is programmed via a CAN bus. The motor driver internally generates all required information on motor speed, position, coil current and diagnostics on open or shorted coils or overheating.

Benefits of Integration

The AMIS-30522 is a microstepping stepper-motor driver for bipolar-stepper motors. The IC can be interfaced via I/O pins and the SPI bus to an external microcontroller or a DSP. This external controller can be very simple, as the AMIS-30522 contains a current translation table and advances the rotor a single microstep on every rising (or falling) edge on the NXT input pin.

On this chip, the DIR register, or input pin, defines the direction of rotation. This implies that the PWM signal is generated in the driver chip and uses a proprietary PWM algorithm for reliable current control. The NXT step can be a full step or a microstep down to 1/32 of a full step. Microstepping operation overcomes the design trade-offs between minimum speed, audible noise and step losses from resonance phenomena. It also increases torque at low velocities.

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© 2007 Penton Media, Inc.