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Beating Torque Ripple in Brushless Servos

Cogging torque compensation is a technique intended for applications where more accurate torque control is required.

GraphicPermanent magnet brushless AC servomotors are tending to replace the conventional brushed DC servo motors in many robotic and machine tool applications. The reasons include: Superior mechanical, thermal and dynamic performance, as well as the elimination of problems associated with brushes and commutators of conventional DC motors.

However, for applications where accurate torque control is required (precise axis force control) and when very low speed ripple across the whole speed range is required (contouring and machining), AC drives cannot directly replace brushed DC drives because of their higher torque ripples.

The influence of torque ripples in a traditional speed-regulating loop is clear. The frequency on these ripples is proportional to the motor shaft speed. At high speed, high frequency torque ripples are naturally filtered by the shaft inertia. At low speed, high frequency torque ripples are well filtered by the PI or PI2 feedback controller. But when torque ripple frequency is close to the speed loop bandwidth (critical speed), torque ripples can cause important speed variations.

One source of torque disturbances comes form the mismatch of feeding currents and motor EMFs, leading to electromagnetic torque fluctuation. Two different current control systems are available in brushless servo drive technology, "six steps" current control and "sinusoidal" current control. In low torque ripple applications, the sinusoidal current control system is preferred, because of smoother current and torque change in the motor.

The three phases are fed by sinusoidal currents of which the phase and frequency depends on the rotor position. The motor shaft is equipped with a suitable high resolution resolver or precision encoder to generate the waveform Torque Ripple Graphicof the current references. Two or three current control loops are used to force the stator currents to follow sinusoidal current references. Torque amplitude is controlled by current amplitude. This control scheme is generally associated with sinusoidal EMF AC servomotors. The total instantaneous torque is the sum of the torques produced by the three phases.

Cogging torque is also an important source of torque ripples in brushless servo motors. This originates form the interaction between stator slots and rotor permanent magnets. This imperfection is greatly dependent on the motor design. Cogging torque amplitude can be substantially reduced by the appropriate choice of the magnet width relative to the slot pitch. Further amplitude reduction may be obtained by skewing the stator teeth relative to the rotor magnets or skewing the rotor magnets relative to the stator teeth, but only at the expense of added complexity in motor construction and some loss of output torque.

An alternative approach is to minimize the torque ripple amplitude in the servo drive design and reduce it using a compensation term in the control scheme.

Reducing Torque Ripple

Infranor has developed a compensation method called Cogging Torque Compensation, which reduces the torque ripple due to the motor imperfections in the sinusoidal current control scheme. The method is based on torque disturbance observation and eliminating harmonics. Cogging torque is observed and compensated for by a feed-forward term. The observation procedure is necessary in order to neglect torque ripples proportional to current amplitude and other torque disturbances and must be executed before the motor is coupled with the load. Cogging torque is then estimated as a function of the shaft position when the motor is moving one revolution under servo control.

Torque ripples due to motor EMF harmonics are also reduced by current and EMF matching. Motor EMF harmonics are first identified by spectral analysis of the motor terminal voltages during a test operation. The optional feed current waveform is then calculated. The number of current harmonics to be used depends on the order of torque ripples to be minimized and their amplitude is limited in order to limit additional motor losses. During the standard operation of the controller, currents are forced to follow these optimum references in order to match motor EMFs.

This control scheme is available on the SMT-BD1 amplifier and allows cogging torque compensation on any brushless permanent magnet motor. Experimental results show that the method is effective in reducing torque ripples over the whole torque range for both sinusoidal and trapezoidal EMF motors.