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

Sensorless Stall Detection

Stepper motors are mostly used in open-loop systems. This has the advantage of being a simple and, by definition, stable concept. The major disadvantage, however, is the absence of direct position feedback. If the motor is blocked, the driver/positioner continues driving the coils as if the motor is still moving. This creates noise, and more importantly, the link between real and actual position stored in the positioner is lost. However, the AMIS-30624 is able to detect when the motor is blocked using the stall detection function.

Every motor is based on the basic principle that a current in a conductor in the presence of a magnetic field creates a torque (or force). This causes this conductor to move. Conversely, a moving conductor in a magnetic field creates an EMF in the opposite direction given by

where Φ is the magnetic flux of the field. If the motion is circular with an angular speed ω(as it is in a motor), the back EMF is given by: e=EMFMOTOR=EMcos(ωt), where EM=-NωΦM.

The amplitude EM is a linear function of the speed. As a result, the back EMF is zero when the motor is blocked.

Some motor drivers are able to measure a motor's back EMF. For instance, the AMIS-30522 makes this voltage available on the SLA pin, which can be fed into an ADC inside an external microcontroller. It is then possible to detect a stalled motor with a simple algorithm in the microcontroller.

The AMIS-30624 has its detection circuit embedded. A simple I2C command sets the different threshold levels. An application explains the different steps to parameterize these levels.[2]

Dynamic Torque Control

As previously discussed, the back EMF is a function of the rotor velocity. The phase between this voltage and the current in the coils is influenced by the mechanical load on the motor axis. If this mechanical load increases, the phase difference increases as well. As a result, the sampled voltage level will decrease with increasing mechanical load if the back EMF is always sampled at the same time relative to a fixed time in the winding current cycle (Fig. 7). This phenomenon is called the load angle and can be used to compensate for overload situations.

Usually system designers parameterize the run current to cope with potential load variations. The torque delivered should be higher than the expected peak load. This leads to overdriving (and oversizing) the motor.

The load angle can be observed on the SLA pin of the AMIS-30522. If for a given velocity the voltage on this pin starts to drop, it indicates the mechanical load is increasing. This can be compensated by selecting a higher current to increase the delivered torque of the motor. With this dynamic torque generation, it is no longer necessary to dimension the system for the expected peak loads. As a result, the stepper motor can be smaller and thus less expensive.[3]

References

1. Jones, Douglas W. “Stepping Motor Physics,” www.cs.uiowa.edu/~jones/step/

2. Application Note AN_AMIS-3062x_04. www.amis.com/products/motor_controllers

3. Application Note AN_AMIS-3052x_01 www.amis.com/products/motor_controllers

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