Probably the smallest 3 axis controller???

[666]

My first real post...

I made this design that i use myself...

Runs unipolar motors of allmost any size (I use JVL 342 Nema 34 for my machine)

Read the howto.txt... It will pretty much explain itself...

Regards... 666

[zwdeal]

666, I am attaching my schematics here to continue discussion on it.



Yes, I think without those resistors, it will switch faster, and as you pointed out, we really don't need that kind of speed. My PIC is 12F675 (could have used 12F629) and using its internal oscillator at ~4Mhz.

I had one accident that a piece of aluminum chip got stuck between two terminals of one MOSFET, for some reason that killed both the FET and the PIC. So I decided to add those resistors, just in case when PIC's pin (connected to the gate) is low and then it is shorted to V supply by accident. I know I need to enclose the circuit, but I was too happy to see it work and chips flying . . .

[666]

Okay... Sounds reasonable, but normally i don´t polish my FET´s with aluminium chips during a gcode run... LOL... no offense...

I´m just a big fan of simplicity i guess...

And it was my opinion as a designer, not a user since the resistors won´t affect the use of your circuit in a negative way.

You could as well change the resistor values to 1K or 1K2.

The fets have a very high input impedance, so i think it might even work with 100K as well...

Conclusiuon: Less drilling & soldering is in my opinion better...

And by the way... You can actually óverclock´the PIC´s you have chosen...

I bet they will run perfect at 8MHz too.

I once had a 16F84 /04 running at 12MHz... NO sweat...

I got better input (step/dir) timing in TurboCNC. Made my motors run more steps/second before ´cranking out´... Nice feature in jog mode if your axis has a long way to travel...

Let the chips fly... And keep me updated... Nice work.

[zwdeal]

This is my second shot at stepper driver. The first one was ULN2803 but that limited me to max of 1A. But it worked for its purpose.

I just happen to have these MOSFETs from my last hobby project -- PWM'ing high powered Luxeon LED's for my ring photography light. I basically use it as a flash, only lights up at full power (8 3W Luxeon LEDs) when camera's shutter closes.

Then when I ran out of power for my steppers with ULN2803, I scrambled up this, and it worked the very first TIME, so I used it to chip some aluminum block and then the accident. I only converted my mill into CNC for fun, not even planning to use it seriously, so I never bothered enclosing it.

My software actually has a timer, after 5 second of "idling", ie, no stepping signals, I shut down all MOSFET's. I find it easier to use, particulary when I adjust Z zero, I can use the knob to lower the end mill to workpiece and then set Z to zero on Mach 3. I think I should cut it down to 0.5 seconds. Is that a good idea?

Simplicity is key to me, too. I follow KISS (keep it simple stupid) rule all the time (because I am very lazy).

[666]

Sounds reasonable to cut down the idle time as long as you keep in mind that you might need holding torque even if the axis´s are´nt moving. For instance at tool change or manual workpiece movement/turning. I allways keep my steppers on holding torque even if the machine has finished its cycle of gcode.

I don´t even use enable signals at my machines.

If i blow my steppers... I must have made a design mistake...

They´re allways ÓN´, but i have the option of adding a solid state relay that cuts the supply to the steppers. E.G. on safety or limit switch activation.

SSR´s are fast and reliable...

I have even thougt of building a bipolar stepper controller just by the use of SSR´s!!! Until now i have made a H-Bridge just by the use of SSR´s and som simple control logic (74ls74/74ls86).

Imagine a biploar stepper controller crankin´out 25 amps per phase without breaking a sweat... And built-in opto isolation is included in the package!!!

I used Crydom SSR´s for the experiment. I get them for free at work...

One day they might drive my steppers...

[zwdeal]

Yeah, determining the "right" idling time is impossible, but I think it got be done because motors heat up fast. It seems my mill (Sieg X1) has enough friction to hold on its own, so I think I will cut it down to 1 second.

Wow SSR based H bridge, how many SSR's per winding?

I am thinking about unipolar chopping design using PIC's built-in voltage comparator. When I was playing with Luxeon LED's, I had to build a constant current driver using inductor. Motor winding can be treated as inductor, so it ought to be possible. The PIC I am thinking is 16F684 which I just happen to have a bunch. This would eliminate those bulky power resistors.

Well, depends if I have time to do this, busy at work lately.

[666]

The PIC as a current limiter.... Sounds smooth...

Looking forward to hear how you do that... You could for instance use a a/d converter and measure voltage drop over a shunt resistor. Could be interresting to see if it can be done. Think it would be a perfect and fast solution. You could then use DIP switches to set the drive current or make a routine for the PIC that measures the DC resistance in the windings...

A single H-bridge is made by using 4 SSR´s for each winding. 8 for each stepper.

A 3 axis controller should have 24 SSR´s and the control logic ic´s. They can be stacked on both sides of a heatsink or attached to the cabinet wall, so it would´nt be a problem. As i get the time i´ll pull off an experiment. I allready got all the SSR´s.

[zwdeal]

8 SSR's per motor sound a little to much to construct, then again I am the lazy type :-) Good idea, but what are the current ratings on those SSR? Last time I used a SSR, it has about 140mA rating, not even enough for the tiny motor I am moding of a coolant spray system.

A/D conversion seems too slow, then again, the built-in voltage comparators seem slow, too, but definitely faster and more reliable than A/D conversion.

My scheme is this, use PIC's internal programmable Vref, its internal voltage comparators (PIC16F684 has two) to monitor voltage across sense resistors (shunt resistors as you put it) and turning on and off the MOSFET's. I am also thinking that I need some resettable fuse (PTC?) to protect motor in case of PIC failure (imagine 50V acrosss a motor winding of 1 ohm DC resistance). The software part does not seem that difficult.

So, if I use the MOSFET that I have, FQP13N10L (rated 100V, 12.5A), I probably can push the driver to handle 70V, 9A (consider massive heatsink and fans blowing). This could be a heck of driver.

But I just have to learn why people use the so called "fixed off" time scheme for chopping. Also, I need to learn those mid-band harmonic stuff. Or maybe I don't have to handle the mid-band thing because it is intended for SIMPLE driver.

The auto measure of motor DC resistance idea is an EXCELLENT one.

[666]

Hi again...

I´m not an expert in current chopping.

I suggest you ask Alan Garfield (www.fromorbit.com).

He´s really good on that subject.

About the SSR´s... They trig on 3-32VDC input at very few mAmps.

The Crydom versions i have in stock can pull 25 Amps at 250VAC or DC on the output side.

They have built in triac´s instead of SCR´s and they don´t care if it´s AC or DC thet must handle as output...

[zwdeal]

I thought TRIAC can only be used for zero crossing AC -- to turn a TRIAC off once on, no current should flow throw the device for certain time in order for it to turn off.

But I guess the one you have has special features to avoid this problem.

[666]

A Triac is just 2 Thyristior´s with the gates connected... One will trig on negative and one on positive signals...

You don´t have any negative signals i the DC (0-X volts plus...) Only one of the SCR´s will trig... So AC or DC... Dowsn´t matter...

As it goes for bridge rectifiers you can supply DC to them as well. Only 2 of the diodes wil conduct curent, but the +- on the output side is allways the same.

I made a foam cutting PSU for hotwire. Ajustable from 2 to 14 volts AC using a triac circuit. Is delivers 40 amps on the output side. Not a single heatsink used... Just switching on the primary (230V) side of the transformer.

It´s a 14.5V E-Core transformer from an APC UPS. Powerful enough to make a 2mm massive copper wire glow. I wonder if it can be adapted to deliver High current DC for my big JVL steppers, but the triac puts out a great deal of noise, so i have to smoothen the DC with some very large CAP´s 22000uF or larger.

Cons: Noise on the DC line to be dealt with. Can be fixed.

Pros: Only apx. 4 amps applied to the triac. No heatsink needed. Simple and dirt cheap. It´s just a modified light dimmer circuit...

[666]

Oh... Forgot to tell that to turn the triac off you need a cap+diode mounted as a clamping circuit. Will produce a negative short spike as the signal hits o volts. If the other built-in SCR goes on... Doesen´t matter since you only have a positive supply applies to the triac.

This is NOT needet in Crydom SSR´s. They have built in compensation for that...

Regards...