Sensorless Control of a Brushless DC motor using an Extended Kalman estimator

Within the appliance motor control industry there is an increasing trend to move towards using brushless dc motors. This trend is in response to recent legislation mandating improved efficiency standards in appliance applications. Brushless dc motors do however require rotor position information in order to select the appropriate commutation angle. In cost sensitive applications such as appliances a sensorless commutation scheme is often desirable. This paper describes the development of such an algorithm based on an Extended Kalman estimator. In its simplest form this algorithm provides estimates for position and velocity based on the back-EMF segments from a trapezoidal driven motor. Unlike previous sensorless algorithms, based on the Extended Kalman techniques, this algorithm doesn’t require stator current measurements. The algorithm simply requires four resistor divider networks to scale the three back-EMF and dc link voltages and a low cost embedded DSP motor controller. INTRODUCTION Recent legislation imposing efficiency standards in appliances has forced appliance manufacturers to migrate to brushless dc motors in their applications. The introduction of brushless dc motors has however increased the level of complexity in controlling these applications. Moreover, brushless dc motors typically require some form of position transducer in order to select the correct commutation sequence. However in such cost sensitive applications the additional cost associated with adding position transducers is not acceptable or even practical. The traditional sensors were Hall sensors mounted in the machine. This requires more complex machine construction and can reduce the reliability of the overall system. This paper presents a sensorless control algorithm for a brushless dc motor, which requires only four resistor divider networks from which estimates of position, velocity, and disturbance torque information can be derived. Furthermore this algorithm provides instantaneous position and velocity information unlike the classical zero crossing approach. This algorithm was developed for and implemented on the Analog Devices ADMC330 DSP motor controller [Murray 96]. The algorithm requires approximately 500 DSP instructions and executes in approximately 13 s. BACK EMF CHARACTERISTICS OF BRUSHLESS DC MOTORS Consider the brushless dc motor as shown in fig [1-2]. When driven as a trapezoidal motor only two of the windings are active at any one time. In this sensorless control application the inactive winding is used to measure the back EMF from which estimates of position and