Hello Guest it is March 01, 2021, 12:11:01 AM

Author Topic: PWM spindle - Circuit advice  (Read 22569 times)

0 Members and 1 Guest are viewing this topic.

Re: PWM spindle - Circuit advice
« Reply #10 on: August 21, 2012, 10:01:31 AM »
C1 to 47uf, or R5 to 47K
Re: PWM spindle - Circuit advice
« Reply #11 on: August 21, 2012, 03:02:36 PM »
Did some testing on my PWM to 0-10V board today.

(Just a small note as I did not mentioned in the last post. I've got no Lynx.
I connect Mach3 directly to my Chinese Huanyang HY01D523b VFD, but I've
been experiencing trouble using the same 'rc' circuit as tread starter.)

Mach base frequency is set to 1500Hz at the moment.

I discovered that you can't fix the problem by raising the circut-voltage.
Check enclosed file. This file show Mach3 speed curve vs VFD speed curve at different working voltages.

10V is optimal for 1:1 scaling between mach speed setting and VFD output, but at 10V you will not get max speed.

Maybe an opamp (rail to rail) as a voltage follower instead of the transistor is the way to go?

Andreas
Re: PWM spindle - Circuit advice
« Reply #12 on: August 22, 2012, 05:34:12 AM »
When you say base frequency, is this for PWM or Mach kernel?  I assume the former, it isn't clear from the manual that you can set it that high - they recommend 5 - 10 Hz.  But of course setting it to 1500 Hz would solve the problem with the smoothing circuit time constant, though reducing the number of speeds steps available.

As you pointed out in a previous post, an emitter follower is not an ideal buffer since it gives a -0.6 volt offset between input and output, which means that when the speed setting in Mach gives a smoothed voltage of 0.6v (6% of maximum, 180 rpm?) the VFD will see zero volts and give zero rpm; and it will also cut 180 rpm off the maximum speed.  An op-amp voltage follower would be better - but then the problem is that opamps often can't swing rail to rail anyway.  The CA3140 can swing to the negative rail if supplied from a single rail but only to about 2v below the positive rail.  So you would need both the opamp connected as a voltage follower and an auxiliary power supply giving say +12v.

Overall, if the VFD input impedance is very high, you may be better off not having any sort of buffer at all but just connecting the smoothing cap straight to the VFD control input.
Re: PWM spindle - Circuit advice
« Reply #13 on: August 22, 2012, 06:27:00 AM »
Thanks John!  :)

The PWM frequency is set to 1500Hz, I did notice the bad resolution of speed steps, but I was not aware that the PWM frequency was the problem.
I'll try to set PWM to 10Hz and increase the time constant of the rc circuit and to some more recearch.

Yes a voltage follower (opamp) with higher voltage supply should do the trix. Will test and tell :)
Re: PWM spindle - Circuit advice
« Reply #14 on: August 22, 2012, 06:55:37 AM »
Something like this..
Re: PWM spindle - Circuit advice
« Reply #15 on: August 22, 2012, 07:01:37 AM »
That should do it!
Re: PWM spindle - Circuit advice
« Reply #16 on: August 23, 2012, 07:07:30 AM »
Now if we can get mikeboothroyd or someone to start making them.
Re: PWM spindle - Circuit advice
« Reply #17 on: August 23, 2012, 02:41:23 PM »
I've buildt and tested the circuit on enclosed schematics.
Accuracy looks good except at low PWM setting.

My spindle is set to a min. speed of 7200 rpm (VFD setting), but when I set speed to 1000rpm in Mach3 the circuit give 11,5V out!  :o
(Which make the spindle speed up to max.) 

In normal operation this will not be any trouble, as I never will enter a speed lower than 7200 rpm.

I can live with this, but to be honest I'd prefer to get the circuit working in the lower region as well..
Re: PWM spindle - Circuit advice
« Reply #18 on: August 24, 2012, 07:00:22 AM »
I think your problem may be the TL082.  Looking at the data sheet its common-mode input range doesn't include the negative rail, in fact it doesn't go to less than a volt or 2 of the negative rail.  CA3140 is a much better bet as it is designed for both inputs and output to swing to the negative rail.  I suspect basically when one input goes too low the op-amp just gives up!
Re: PWM spindle - Circuit advice
« Reply #19 on: August 24, 2012, 01:26:14 PM »
You're probably right. Thanks.
It was the only opamp I had left so I gave it a try.
Will order some new and test again.

Thanks for constructive feedback!