PWM/speed control question

[ngs-audiotechniek]

Hi there,



At this moment I am converting an old Emco Turn 120 to Masso. Everything is working out great but I have 1 question.



Because almost all the conversions of this machine that I came across on the great web the original Siemens spindle is replaced by an simple 3phs motor and a VFD. I wondered why. Because the original Siemens motor is a very nice motor to use. You need quite some electrical knowledge to get this working but I figured it out! Almost...



The problem that I have is that the speed for the spindle is controlled via PWM.

but....

The frequency of that signal determines the speed. Not the duty cycle...

I tried it using an lab signal generator and it worked fine. The speed of the signal is determined by frequency. It works from 0/10Khz at around 5v. No matter what the duty cycle is speed is controlled via the frequency.

Is there somehow an solution that I can create an frequency variable signal via Masso of via an external pcb?



Thanks in advance



Is there somehow an solution t

 

[zombieengineer]

@ngs-audiotechniek - What is the model of the spindle VFD driver?

I would be very surprised if it was not possible to change this from frequency control to PWM using a software parameter change.

Found several listings on eBay of voltage to frequency convertors.

 

[ngs-audiotechniek]

@zombieengineer thanks for the quick response.



I used the original driver of this machine. See the first picture.

I modified it and get rid of all the components that I don t need. It turned out as the second/third picture. (It is still an test setup)

The pcb that you are talking about does exact what I don t need. I already tried them. It converts the o-10v signal to an frequency signal. But... it is an set frequency and a variable duty cycle that is controlled via the level of the 0-10v signal.

Using this setup the motor will do the following. The Max rpm is set to 2000 rpm.
Using the MDI and say S500 M3 there will happen nothing.

I stepped it up by S100 per time. If I cross the S1000 command the motor starts turning at its max RPM

CW and CCW is working fine by the way.



What I need to provide for the motor to get speed control is

0-10v to 0-10Khz at +/- 5v dc

The duty cycle of the signal does not really matter.

 

[zombieengineer]

@ngs-audiotechniek

Did a quick search on the internet for Emco Turn 120 and it turns out the original machine has a number of quirks that makes it kind of unique (5 phase stepper motors as opposed to 2 phase stepper motors that are normally used). Modern VFD drives are extremely configurable (read: too configurable).

Do a search on eBay for "voltage to frequency convertor 10kHz" and you should find several listings for around USD$10 of a board you can wire in.

If the output voltage from the V2F board is more than 5V (strongly suspect it is - the one I am looking at has a 12V regulator on board) then you can add a resistor to the output (suggest 680?) and use a 4.7V zener diode (1N750 or 1N4732) to act as a voltage clamp (a zener diode only starts conducting above a specified voltage - poor mans voltage regulator).

 

[ngs-audiotechniek]

@zombieengineer

Yes its kind of a special machine but I have everything working (took me a while) except the speed control.

I have some of these boards to convert voltage to frequency but these boards adjust the duty cycle of the converted signal. Not the frequency.

Tried different boards an manufacturers. They all adjust the duty cycle of the converted signal at an set frequency.



I will do some measurements tonight an take some pictures so you can see what I mean.



Thanks for the help so far!

 

[zombieengineer]

@ngs-audiotechniek

The voltage to frequency boards could being doing one of two things:

• Constant on duration but variable off duration (variable frequency with variable duty cycle)
  
• Constant duty cycle but variable frequency (on & off duration are adjusted simultaneously)
  

I suspect that most of the prebuilt circuit boards are the "constant on" duration.

How sensitive is the speed to duty cycle & frequency?

 

[ngs-audiotechniek]

@zombieengineer



Your correct. Today I received the same PCBs (according to the sellers website) but these PCBs that I received today are 0-10V > 0-10Khz (frequency) converters. These pcbs do the job! The other pcbs are 0-10V > PWM. I think the seller (read China) does not really know what he is actually selling.

It took me a while, with the spindle connected together with an oscilloscope to get the correct signal but it works 99% perfect.

The only thing is that if the speed in Masso is set to S0 and the CW or CCW signal is still hi there is some noise that will effect the spindle driver so the spindle stutters a little bit at approximately 9 rpm (according to the Masso speed encoder) I think that the noise is generated by the converter.

I now temporarily fixed this by making a simple relay circuit that will pull the 0-10Khz also to the ground. I will make an simple final correct circuit if all the other stuff for the machine is finished.

Not really a problem because the G-code files always end with an M5 command.

Thanks for you help so far!

I will post some pictures one of these days.