Friday, September 21, 2007

Driver ICs Elevate Design of Stepper-Motor Control

Stepper motors are now being used more frequently in industrial environments. Increased performance and reduced size make them increasingly attractive, and their application is no longer limited to accurate positioning. Applications such as dose pumps, valve control and even dynamic positioning (and other movements previously reserved for linear positioners) are becoming popular.
The drive electronics for such stepper motors are evolving accordingly. In the traditional way, the architecture typically includes a standard microcontroller or DSP, custom logic to provide the decoder inputs, several analog-to-digital and digital-to-analog converters (ADCs and DACs), and an H-bridge to drive the currents in the stator windings of the stepper motor.

The development of drivers and controllers spawned by automotive applications has yielded interesting products and building blocks. These functions move local intelligence close to the motor, creating a class of products known as mechatronics. Application-specific standard products (ASSPs) have become available to drive stepper motors directly from high-level position commands on a bus. Single-chip solutions integrate driver transistors, on-chip current regulation, a translation table between the exact rotor position and corresponding coil currents, a rotor positioner, speed and acceleration, and the physical layer and data-link protocol of a communication bus. The use of microstepping converts low-resolution motors into low-cost high-resolution actuators.
These single-chip motor-driver ASSPs are convenient for multi-axis positioning applications. However, system builders who want more flexibility on the control of the movement can choose dual-chip solutions. These solutions allow system designers to embed their knowledge in standard microcores or DSPs, and develop algorithms that produce the “next-step” signals to the stepper motor. The ASSP takes care of the rest and replaces the bulk, if not all, of the traditional motor-drive circuitry. This allows designers to concentrate on the programming of the motion, rather than concentrating on driving a current through a coil using complex PWM algorithms. The development time, and therefore the time-to-market, of new products reduces accordingly.
The ASSPs come with new features like embedded diagnostics and information on torque and motion. The designer can use these signals in programming the controller to detect rotor blockage and rotor position, and to perform automated adaptation of torque without the use of external sensors.

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