tailieunhanh - Industrial Circuits Application Note Microstepping

This application note discusses microstepping and the increased system performance that it offers. Some of the most important factors that limit microstepping performance, as well as methods of overcoming these limitations, are discussed. It is assumed that the reader is somewhat familiar with stepper motor driving and the torque generation principles of a stepper motor. If not, chapter 1 and 2 of this book can be read to get the background information necessary. | Industrial Circuits Application Note Microstepping This application note discusses microstepping and the increased system performance that it offers. Some of the most important factors that limit microstepping performance as well as methods of overcoming these limitations are discussed. It is assumed that the reader is somewhat familiar with stepper motor driving and the torque generation principles of a stepper motor. If not chapter 1 and 2 of this book can be read to get the background information necessary. What is microstepping Microstepping is a way of moving the stator flux of a stepper more smoothly than in full- or half-step drive modes. This results in less vibration and makes noiseless stepping possible down to 0 Hz. It also makes smaller step angles and better positioning possible. There are a lot of different microstepping modes with step lengths from 1 3-full-step down to -fullstep or even less. Theoretically it is possible to use non-integer fractions of a full-step but this is often impractical. A stepper motor is a synchronous electrical motor. This means that the rotor s stable stop position is in synchronization with the stator flux. The rotor is made to rotate by rotating the stator flux thus making the rotor move towards the new stable stop position. The torque T developed by the motor is a function of the holding torque TH and the distance between the stator flux fs and the rotor position fr . T Th X sin f - f where fs and fr are given in electrical degrees. The relationship between electrical and mechanical angles is given by the formula fel n -4 X fmech where n is the number of full-steps per revolution. When a stepper is driven in fullstep and half-step modes the stator flux is rotated 90 and 45 electrical degrees respectively every step of the motor. From the formula above we see that a pulsing torque is developed by the motor see figure 1a which also shows the speed ripple caused by the torque ripple . The reason for this is that f - f is