Steve Blackmore wrote: > > It's not only home made encoders, expensive commercial encoders have the > same problem. > > > The problem is, without using external hardware, anything above 150 ppr > or so limits the spindle speed severely. 240 rpm is very slow for a CNC > machine threading. Most of my threading is done at 700-1000 rpm, and > that is well within the capabilities of my Z axis. Compared to > commercial machine even that is slow. > > Here, with my hardware and PC, EMC can't read pulses reliably over about > 200 rpm with a 1024 encoder, which is totally impractical. > Well, there's the problem, the sampling rate is just too low. What is the huge resistance to using any external hardware, when it is clearly the entire stumbling block in this situation? I can understand with an experimental machine like the CNC'd Etch-a-Sketch, but if you are retrofitting an actual machine tool, the cost of a hardware interface module just isn't all that high. >> Even if some folks have problems with encoder noise, I think the >> problems more likely stem from the fact that homebrew encoders tend to >> have a low PPR value - that might explain some of the observations. >> > > But we are stuck with low PPR, it would appear, due to latency. > No (if I may be so blunt), due to a stubborn insistence on using software encoder counting where they are inappropriate. (In the form of "full disclosure" I sell boards that contain hardware encoder counters, and the only appropriate use I have found for the HAL encoder counter is for a manual pulse generator (jog dial).) >> I dont fully understand the trajectory planner, but my gut feeling >> it's a control loop oscillation which gets excited with the encoder >> quantization/noise spectrum gets too close to the loop frequency. >> I agree entirely with this analysis, although the quantization noise doesn't need to approach the loop frequency, but possibly a response peak in its transfer function.
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