Choosing the right driver/controller combination for your stepper motor design
This "Product How-To" article focuses how to use a certain product in an embedded system and is written by a company representative.
Stepper motors have become the dynamic positioning solution of choice in applications ranging from vehicle climate control to industrial dose pumps and theatrical stage lighting. Better performance, decreasing size and lower cost have all contributed to their move into mainstream use.
Just as the motors and their applications have evolved, so have the drive electronics needed to control them. In particular, automotive designers have implemented distributed processing strategies that inject increasing levels of intelligence close to the motor.
This kind of control approach has permeated into more general-purpose application areas, making the typical motor/driver combination more akin to a mechatronic subsystem than a simple actuator.
Designers have three fundamental choices when settling on an architecture for stepper motor drive electronics. The traditional approaches are to use a general- purpose microcontroller or a DSP, in combination with analog driver electronics and a sensor-based feedback loop.
Increasingly, however, cost, time-to-market and complexity constraints are leading engineers to use ASSPs dedicated to motor control. Such solutions themselves fall into two types: single-chip or two-chip implementations.
Design considerations
Although the traditional microcontroller- based approach is gradually being superseded by the use of ASSPs, it is informative to first consider such a basic architecture to understand the typical design constraints and requirements of such a design.
In this case, the core controller with program code typically in flash memory delivers a PWM signal to drive the motor coils. Analog circuitry amplifies this signal and drives the power stage, which in its turn drives the coils of the motor.
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| Figure 1: Controller/driver solutions combine the controller, speed, position current, diagnostics and power stage in one chip. |
The microcontroller needs to obtain a range of information from the outside world to calculate the correct PWM outputs. In particular, it needs feedback on the rotor position.
This function is usually fulfilled by a Hall sensor, which not only provides positional information, but can also sense a stall or blocked rotor. In very simple cases, it may be possible to replace the Hall sensor with a simple end-of-loop position switch.
Other options include optical position coding or even a resistive potentiometer mounted on the motor shaft. In addition to positional data, the controller needs information on the motor current. This is sensed via a resistor in series with the motor driver and presented via an ADC as a digital input to the controller itself.
ASSP solutions integrate most of these functions into one or two devices, in the process implementing sensorless control strategies. The highest level of component integration is provided by controller/driver solutions, such as the AMIS-30624 (Figure 1 above), which combine the controller, speed, position current, diagnostics and power stage all in one chip.
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