BEMF detection relies on the fact that, just as a current-carrying conductor in a magnetic field will be subjected to a force (causing it to move), a conductor moving in a magnetic field will create an opposing EMF. The amplitude of this EMF is a linear function of the speed of the motor: most important for stall detection implementations, it is zero when the motor is blocked.

Like most features, the exact implementation depends on the selected driver architecture. An intelligent driver like the AMIS- 30522 makes the BEMF voltage available on an external pin, allowing feedback to a microcontroller.

A more highly-integrated device such as the AMIS-30624 has detection circuitry embedded, with threshold levels set via simple I2C commands. BEMF can also be used to implement dynamic torque conditioning, allowing potential reductions in motor size and cost, and improvements in energy efficiency.

BEMF is a time-varying function of the velocity of motion of the motor. The phase difference between the BEMF voltage and the current in the coils is influenced by the mechanical load on the motor axis: as mechanical load increases, the phase difference also increases. The sampled BEMF level will therefore decrease with increasing mechanical load if the BEMF is always sampled at the same time - a phenomenon known as load angle.

Load angle can be observed via an external pin on the AMIS- 30522 device. Increased mechanical Loading - indicated by a voltage Drop - can be compensated by selecting a higher current, and hence increasing the torque of the motor. Such a dynamic torque conditioning strategy means that the designer no longer needs to initially dimension the system for the "worst-case scenario" of the peak expected load. Instead, a smaller, less expensive motor can be used.

Integrated motor control ASSPs ease the task of designing stepper motor subsystems - reinforcing the existing trend towards use of such motors in increasingly diverse applications. They reduce BOM, simplify design and cut time-to-market. Moreover, they allow design engineers to focus on adding value rather than implementing low-level control strategies, and simultaneously enable more sophisticated control strategies and richer feature sets.

Guido Remmerie is director and Peter Cox is Engineering Product/ Account Manager of Industrial Products at AMI Semiconductor Inc.