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Motor Control

Demands for smoothness, efficiency and cost effectiveness drive motor control design.  Technologies including optical or magnetic encoders, resolvers or sensorless control are traditional solutions, but none really nails all three demands well on its own.  CambridgeIC's CAM502 IC used with Type 6 Precision Through Hole Rotary Sensors offers an attractive alternative. 

CambridgeIC technology delivers precise angle feedback with a minimum of Group Delay and the high sample rates needed for high quality control across a wide speed range.  They are designed to be deeply embedded with the motor and its control system, eliminating additional housings, bearings, mountings, calibration and adjustment traditionally associated with add-on encoders.

The block diagram below illustrates how CambridgeIC technology may be applied to servo motor control.  Motor feedback is provided by a Type 6 Precision Through-Hole sensor, with its rotating target typically mounted to the rotating motor shaft which passes through both sensor and target.  Sensor processing is performed with the CAM502 IC, typically mounted to an electronics PCB together with other drive and control circuitry.

A microcontroller typically performs motor control.  It reads CAM502 measurements over an SPI interface, typically every 200µs.  The CAM502 also provides a Sample Indicator signal which enables the microcontroller to time stamp measurement results, enabling high quality velocity estimation across a wide dynamic range.

A Field Oriented Control algorithm is usually used for efficient and smooth control.  Its control inputs are instantaneous rotor phase angle and a current control signal.  Rotor phase angle input can be taken directly from the CAM502.  Alternatively, for higher speed applications it can be beneficial to compensate for the Group Delay inherent in the measurement process (140µs) plus the time to transfer information to the microcontroller over SPI (16µs using Burst SPI at 10Mbit/s).  This is performed by adding a compensating lag angle to angle measurements, where that lag angle is the velocity estimate times the known delay.

An inner control loop controlling acceleration is usually implemented as a current control loop, with feedback taken from current measured in the motor drive circuitry.  This is controlled by the output of a velocity control loop, whose feedback is the velocity estimate derived from CAM502 data.

Where motor position control is also required, the demanded position is compared with the CAM502 position measurement to derive an error signal input to a position control loop.

An additional CAM502 chip and rotary or linear sensor can be used to provide position feedback at the point of load.  This enables the final position to be precisely controlled without errors due to backlash and compliance in the transmission.  In this case the second CAM502's position data is fed straight into the position control loop, while the first CAM502 is used only for velocity estimation and commutation.

Attractive features of the solution include...

  • Smooth and efficient control at low speed due to...
    • High resolution sensing, and
    • Accurate angle measurement
    • Absolute angle output, so motor angle is known at start-up
  • Simple mechanical design and installation due to...
    • Immunity to misalignment of sensor parts in all directions
    • Through-hole sensor and targets
    • Low profile
    • No calibration
  • Robust installation due to...
    • Immunity of inductive sensing technology to dirt and dust
    • Immunity to vibration
  • Broad application to a wide range of motor types and pole counts, due to...
    • Different sensor sizes available
    • Absolute angle output, supporting any number of motor pole pairs

Applications include...

  • Industrial motor control across a wide range of motor sizes and types
  • Traction motors for electric vehicles
  • Heating, Ventilation and Control (HVAC)
  • Lab automation
  • Pan and Tilt motion platforms