Denford Novamill Mach3 conversion

[EddyCurrent]

Duff drawing removed, please ignore

[Miller]

Great stuff EddyCurrent! I also need a power supply and that's pretty much the hardware done with. The motors are 1.5A for X and Y, and respectively 3A for Z axis. They are all rated at 30VDC, which I take it it's the max voltage?

Anyway, what is the optimum voltage to feed the stepper drivers? I can get a a nice deal on a quality 24-28V 10A power supply.

[JAZZCNC]

No I think Eddy's wrong.

You want to connect to the PWM+10V(pin3) and 0V(gnd) to (Pin2) the 10K resistor is just showing the POT resistance. You are controllong the 0 to 10V signal from PWM on the BOB so don't need it.
But Like John said best check if the Spindle controller 0 is floating or at mains potential.? Putting to Gnd on the BOB could fry it.?

Edit: correction to pins

[irving2008]

I think you're being a bit categorical here Irving! You can't rely on a limited slew rate to do the smoothing for you. And your comment on ground referencing isn't helpful because by definition "signal ground" is what the control inputs are referenced to! The question is, can the 0v on the controller be safely connected to the signal reference on the vfd?

John, not categorical but maybe I'll accept I was being too generic. I hadn't read whole thread, but was responding to specific posts. I can rely on VFD not needing external capacitor cos it works for me and many others. Most VFD have internal low pass filters anyway.
And the diagram posted by the OP shows a VFD.

However, it now transpires that's all irrelevant to the OPs requirements.

I agree with Eddy, to show COM as 'chassis' but have it at some unspecified high voltage is wrong in my book.

[JohnHaine]

OK, I've sort of been inspired by this thread to work out a simple circuit to interface the PWM from a BoB to a non-mains-isolated speed controller such as the Sprint or KE type thyristor units. This circuit does not need an auxiliary power supply, it runs from the 10v available from the controller itself. It uses an ST Micro dual "rail-to-rail" op amp to hopefully get the full 10V control range. It isn't tested yet but offered just as an example, but I think it should work.

I've just noticed an error in my labelling - on the input side the pin labelled "0V BoB" should go to the output you have selected for the PWM speed control, not the 0V rail!

On the controller side, it looks like for the Sprint box that Miller has, +10 V comes from terminal 1; the control input ("0-10V to controller") goes to terminal 2; and 0V is on terminal 4.

This could easily be made on a small piece of strip board, but take care with the IC2 connections as there will be mains voltages on pins 5 and 4 so make sure they are well isolated from 1 and 2.

I suggest configuring Mach for about 25 Hz PWM base frequency. One problem you may find is that the low speed end where the machine may not start until the speed is turned up a bit from zero; or may run very slowly even though set to zero. It's quite complicated to replicate the function of the "min speed" pot in the controller.

I do need to get round to adding speed control from Mach to my Novamill, so this should get tested sometime! But any questions or feedback welcome.

John.

[Miller]

No worries Eddy, and thanks everybody especially John. The circuit looks easy enough, I should be able to test it sometimes next week.

Alternatively, I have found an off the shelf converter from Axiomatic:

http://www.axiomatic.com/TDAX13020X.pdf

But I have a feeling it's going to be much more expensive than the diy version John proposed, so not really an option.

On another note, can somebody confirm a 24-28v 10A power supply would do, and what voltage do you feed to stepper drivers?

[JohnHaine]

Hello Miller,

Let me know if it works and any questions.

On the PSU, the one I use in my Novamill conversion is similar to the 36V one here: but I got mine direct from China via ebay for rather less money IIRC. Look for topengineer1989 as the seller. I use the common 2M542 drivers. 36 volts is fine to drive the steppers assuming the drivers are rated up to 40 or 50 volts. I think I have set the X and Y currents to just less than 2A and the Z probably 3 but could check those.

John.

[EddyCurrent]

I had to get up at 4:30 am this morning to get rid of my duff drawing, couldn't sleep knowing it was there

On the controller side, it looks like for the Sprint box that Miller has, +10 V comes from terminal 1; the control input ("0-10V to controller") goes to terminal 2; and 0V is on terminal 4.

John, it looks to me like the input to the drive is terminal 3, terminal 2 goes to a pot for 'minimum speed' which creates a potential divider along with the external pot to give an offset from zero.

[JohnHaine]

Oops! Eddycurrent, you're quite right, terminal 3. Terminal 2 is not connected.

Miller, the eletrolytic you selected is a surface mount type, is that what you wanted? The other one is leaded. Maplin is a good place for miscellaneous components. Well, the 36 V works fine for me, lower voltages may work but the drives are fine with 36. That's the package I bought too of the BoB and drivers.

John.

[JohnHaine]

STOP!!!

The key phrase is written in the thick black box round the speed controller schematic. It says:

WARNING. The electronics is at mains potential do not earth!

This controller, like mine, is a thyristor type with all the control circuitry run direct from the mains. It also needs a dc input from 0 to 10 v on pin 3 relative to the 0 V on the controller to vary the speed. There is no sign of a low pass smoothing filter to convert PWM to the dc control voltage.

To control the speed from the BOB you will need an opto isolator driven from the PWM output, which will safely transfer the PWM to the high voltage environment of the speed controller. Then you need a simple low pass filter to smooth the PWM, to generate the control voltage. The was a circuit on the Mach 3 boards a little while back, I'll try to track it down.

To reiterate, if you connect the 0 V on the BOB to the controller as shown in EddyCurrent's picture, on switching on the mains will be applied through a rectifier to ground. If you're lucky the fuse will blow, but rectifiers blow up faster than fuses! Guess how I know?