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Stepper Motor Feedback

Stepper motors are simple to drive and control.  However there is a risk of losing step when current is kept low to save on energy use.  Sensorless feedback can help detect a stall condition, but it requires careful tuning and can not provide the same reassurance as checking the physical shaft angle with a position sensor.  Optical encoders are traditional position sensors, but they can stop working with even small amounts of environmental contamination including dust, and require careful mechanical alignment to perform well.  CambridgeIC's miniature through hole rotary sensors offer a robust and precise alternative that is not affected by dirt and dust. 

By providing position feedback, the host can detect step loss and recover from it.  This allows designers to aggressively reduce motor size and hence cost, and reduce operating current and hence energy use.

In a typical application, a CambridgeIC through-hole rotary sensor is mounted to the rear of the stepper motor, and a matching target to the rotating shaft.  In the examples below these are 21mm miniature through hole parts, and the stepper motors are NEMA8 size configured for linear motion.  In the left hand image the motor is a non-captive type and the target is mounted to a brass extension of the motor's armature.  In the right hand image the motor turns the ballscrew, and the target is mounted to that ballscrew with a brass fitting.

NEMA8 non-captive linear stepper motor with 21mm throguh hole rotary screened sensor and targetMoons NEMA8 stepper motor with 21mm rotary through-hole encoder

The sensor is connected to a CambridgeIC CTU chip for processing.  The 21mm through-hole rotary sensor can be connected to Type 4 CAM204 circuitry allowing two axes to be measured with the same CTU chip.  Alternatively, if 3 or 4 axes are needed, Type 1 circuitry may be used.

The CTU chip connects to the host system with an SPI interface.

The host controls the stepper motor with traditional techniques, reads the angle reported by the position sensor system and checks that the expected and measured angles match within expected tolerances.  If the host detects a loss of synchronisation, data from the sensor can be used to recover absolute position and resume operation, most likely with a higher motor current in order to avoid a stall condition.

CambridgeIC linear sensors can also be used with stepper motors.  They are typically mounted at the point of load, rather than on the rotating motor shaft.  The CAM204 processes multiple axes, so that two or more linear and/or rotary sensors can connect to the same processing chip.  This helps reduce component count and complexity.