Anticogging: Torque ripple suppression,modeling,and parameter selection

Smooth motion is critical to some robotic applications such as haptics or those requiring high precision force control. These systems are often direct-drive, so any torque ripple in the motor output must be minimal. Unfortunately, low-torque ripple motors are expensive. Low cost brushless direct current motors are becoming more prevalent, especially from the hobby remote control community. These motors often have the required high-torque density; however, they also have significant torque ripple. This paper presents a low cost method for anticogging, the compensation of cogging torque. While other methods exist to compensate for current-based torque ripple (mutual or reluctance torque), none have addressed cogging torque, except by adding expensive force sensors. This paper presents two methods that use a position sensor (already present for servo motors) to map cogging torque to rotor position. The map is played back according to position reported from the sensor to cancel the cogging torque. The design and testing of a low cost haptic arm using anticogging shows validation; however, the approach is much broader, and can be applied to any precision force application. A model of torque ripple sources are included as a function of pulse-width modulation frequency to help choose the optimal pulse-width modulation frequency to minimize torque ripple. Test results on eleven different motors show a removal of up to 88% of torque ripple with no added cost in robotic servo applications, and in some cases having better performance than motors that are over nine times as expensive.