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« on: November 25, 2017, 02:46:57 PM »
Hi Azalin,
I think we often spend too much time worrying about spindle speed. Quite a few of the jobs I do on my mill are engraving type jobs, PCB routing particularly.
All I require is that the spindle starts, go to max RPM and stay that way until the job is done. I don't need/want it to go backwards, I don't care what the true
RPM is, I mean I can tell at a glance whether the spindle is going OK.
May I suggest rather than attempting to set your machine up so that you have fine speed control and true speed feedback with all the complication and confusion
of software setup and hardware compatibility that you settle for a simpler solution. When your comfortable with that then start extending your control and then
only to the extent required to do the machining you require. Do you actually require precise indexing and speed control for rigid tapping UNLESS you are doing
rigid tapping?
The video you linked is a very simple and common set up. It defines two outputs: one for ON/OFF control and the other as a PWM signal that results in a 0-10V
analogue signal to your VFD/spindle driver. It may be, like my engraving job, that you don't need the PWM speed control, you might have a potentiometer and
knob that you can lean over and tweak to do the job at hand. If you need to be able to go in reverse you will need more outputs.
You could use as few as two: one for FWD and the other as REV and with neither output active STOP. Or you could have three outputs: one for ON/OFF,
and the other two for FWD and REV. Use whatever seems simplest to adapt to the input circuit of your VFD/driver.
Likewise most VFD/drivers accept an analogue voltage and most BoBs output an analogue voltage usually from a nominated pin so connect one to the other.
At this stage who cares about spindle feedback? You will know immediately if the spindle is doing what its supposed to or close to it. Even if you do have an index
pulse setup and true RPM displayed without some sort of closed control loop you can't adjust the spindle speed anyway. And before you ask Mach is not a closed loop
controller. You will no doubt have seen posts about it and using some clever coding and/or hardware you can induce Mach to close a control loop but its performance
is pretty lethargic. There are external motion controllers that have closed loop control capacity, often called PID, but they cost extra, a lot extra as a rule and it
begs the question 'how precise/detailed/accurate/dynamic do I require my spindle speed control to do my machining?'
If you find that you need really precise indexing and speed control for rigid tapping say the buy yourself a decent AC servo and drive. You can shag around with
feedback controllers and encoders until 'the cows come home' and still not match the control offered by a matched AC servo and drive. All that is required with such
a matched pair is a Step/Direction signal pair....no encoder feedback, no PID tuning, no headaches!
Keep your spindle speed control solution as simple and cheap as you can...save your time and dollars for other things!
Craig
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