Other Parts Discussed in Thread: DRV8301, MOTORWARE
Hello,
We are looking at using an Instaspin solution to replace our current BLDC motor drive system, which uses trapezoidal commutation with Hall sensors. We are doing position control through a high reduction gear train (100:1 to 1000:1 depending on application). In our current system, we use an absolute magnetic encoder on the output shaft. This encoder drives a position control loop, which outputs a target velocity to a velocity/current control loop. Velocity is controlled solely using the Hall sensors. This gives us easy control over the absolute position of the output shaft, and the position control loop handles any backlash or other nonlinearities in the gear train basically automatically, without having to know anything special about the motor or drive train.
I have a few questions about the Instaspin solution that hopefully someone can answer:
1. Is it possible using the Instaspin solution to decouple position for position control / profile generation and position for motor commutation in a manner similar to our current system? All of the documentation I've seen shows these two position quantities tied together. I've been looking at some of the labs, and it looks like you could provide different positions to STPOSCONV_run and CTRL_run, but I'm not sure if that will cause any problems.
2. We frequently need to be able to hold a position under load at zero speed, as well as start under load. Even though the position control accuracy provided by hall commutation isn't ideal, the high gear ratio we use means it works acceptably well for our application. The operating environment for most of our products basically rules out optical or mechanical encoders. The design of our system would make it difficult, but not impossible, to add an additional magnetic encoder on the motor, but we'd like to avoid that if at all possible. I've seen references to using hall effect position sensing at low speed & FAST position sensing at higher speeds, and that is the option we'd probably go with. I'm looking at Lab 11e, and it looks like the strategy is to determine the motor speed using the FAST system and then, depending on the speed, select the angle from either the hall sensors or the FAST system. The angle is then fed into the FOC system, and it looks like the PWM output is overridden and the motor is operated in trapezoidal commutation mode when the Hall sensors are used for position sensing. I assume that in this mode, it should perform almost identically to a standard trapezoidal BLDC drive system, correct? I'm guessing it doesn't make sense to try to use the FOC PWM output with such low-resolution position data. What kind of performance (resolution, response time) does the FAST system have in terms of sensing velocity, especially at low velocities? If we did need to add an additional encoder, do you have any information on the required sample rate/latency/resolution? Most of the magnetic encoders we've worked with have an update rate of ~10KHz and a latency of 100-200 uS. I would assume that you would want these to be at least as fast as your motor control loop, but if you have any information on the performance implications of encoder latency, that would be appreciated.
Thanks.
