stepper motors glitching

[routercnc]

Hi 2e0poz,

It's a good suggestion, but the monitor is not on at the moment (see previous post). Neither is the PC. To track the problem down I'm running the driver board from a function generator which is providing square wave signals into the parallel socket on the driver board. Therefore the problem must be with the 33v power supply to the board, the board itself (which is a new replacement), or the stepper motor (of which I have 3 and they all glitch in the same way).

But I think you are right in that there must be some kind of interference from something, or bad earth somewhere.

Hoping to have another go at this after work tonight . . .

Barry

[Swarfing]

Sorry Barry thats me all over skimming instead of reading.......I suffer a lot of issues with switch mode power supplies near my radio gear, they cover a lot of frequencies across the different circuits that make up the supply unit. It may be worth probing with a frequency counter and see if any clash with the driver. What can sometimes be seen as a spiky output could in fact be a number of signals overlapping.
Paul

[routercnc]

Cracked it !!

Well, I've found the source of the problem anyway. The driver board can take 24v - 33v, so I set up 2x12v 7Ah lead acid batteries to power it by DC, eliminating the 330Watt mains power supply. Now there are no glitches at any speed, from barely moving, right up to 12KHz. I ran it for several minutes like this, changing the frequency on the function generator and it was smooth all the way.

It is remotely possible that the glitching is removed by dropping the voltage (although this doesn't feel like the answer), but I have no easy way to check that since the mains supply only goes down to 33v. Could I use a high wattage resistor to drop the voltage from the mains power supply to 24v to check? What value and resistor type to use if I wanted to try this?

So, I have a square wave function generator, into the stepper parallel port of the driver board, and one stepper motor connected. The board is running from 24v DC and does not glitch at all. However, this is not a long term solution since the batteries are quite small and old and I think they won't take to running the CNC machine.

Now, what is going on here? The power supply was from Roy at DIY CNC and was recommended. Obviously I will email him for advice on this since something about this power supply, or some interaction with the driver board, is causing the glitching, but I'm not electronics expert so need a bit of help.

Can I put ferrite rings around the DC cables?
My VFD came with a mains filter, would this sort of thing help on the main side of the power supply?
What about these choke type devices on the mains side of VFDs?
Could I safely scope the DC voltage to the board provided by the mains power supply to see if there is noise on the supply when the steppers run?

Out of time to try anything more today, but starting the see a way forward!

Thanks
Barry

[John S]

I hate switched mode power supplies. Steppers need to get rid of excessive power as they decelerate, it's known as back EMF.
Normal power supplies like a transformer, cap and rectifier can absorb this back EMF, switched mode can't.

Not a perfectly technical explanation but it gives the gist.

Barry can you try a convention power supply?

.

[irving2008]

Well somethnig to try is providing some additional hi-frequency bypass capacitors on the power input to the driver board. An 0.1uF/100v in parallel with an 0.01uF/100v should do, this will eliminate any switching spikes from the motors going up into the power supply and upsetting its regulation. Also to absorb back emf a large electrolytic capacitor might be useful, something like a few thousand uF at 100V.

I agree with John tho, if you can try a linear power supply then do so...

[routercnc]

Hi John, Irving2008

Thanks for your suggestions. This does all make sense now, and probably explains why the stepper ran smooth at the higher rpm, since the pulses would be closer together and the deceleration of the motor between them would be less, creating less back EMF.

I tried both the 0.1uF capacitor, and the 0.01uF + 0.1uF, and both combinations reduced the kick of the glitch, and also brought the frequency down from >8KHz = no glitches to >6KHz = no glitches. A small improvement, but not solved. I tried a 1000uF 35v capacitor which I had, and this brought the frequency down to about >5.5KHz = no glitch. Roy at DIY CNC said that he had sold alot of these supplies and had no problems, so it could be faulty. He suggested I try a 4500uF - 10000uF 35v capacitor across the power supply in the first instance, so I have ordered a 10000uF 35v unit from ebay. If this does not work, then I'm still open to try a replacement.

Otherwise, I did have a look around the internet for an equivalent linear supply of about 30 - 33v, up to 10A, and didn't find much about. I could probably live with a lower current rating since there are 3 axis of up to 2A each, but some overhead is nice. Alot of people had made their own supply from a transformer, diode rectifier circuit, and capacitor(s), but I don't trust my electronics enough for mains. Where are you buying them from?

Thanks
Barry

[irving2008]

I'd be wary of putting a 35v rating capacitor on there permanently, the back emf could easily get to 45+ volts and lead to premature capacitor failure.

Makig a linear power supply is easy. Its hard to find ready-made linear supplies as there's little market for them outside the CNC world. I'd be happy to take you through it, almost no soldering required!

[John S]

Right 3 axis at 2 amps each is 6 amps but as they are never running at the same time, and believe it or not they don't draw 2 amps then the rule of thumb is 70 % of total. However to give a bit of over head we'll say 6 amps at 30 -33 volts.

So we select a transformer in this case from RS because I'm idle and they deliver for free.
6 Amp at 25 volts #223-8207 £ 27.82

Next we need a capacitor 10,000uF 63 volt working #255-0232 £10.44

Then a bridge rectifier 15 amp 50 volt working #515-5559 £1.50

Now because it is smoothed by the capacitor and converted into DC that 25 volts now becomes 25 x 1.414 = 35.5 volts.

All these components above are either screw fixing or you crimp on spade terminals, no soldering needed.

[irving2008]

I'd go higher on the bridge rectifier... 15A 100v, part # 687-5642 @£1.75
This will need to be mounted on a 5degC/W heatsink or better (or the aluminium case you mount it all in if its at least 200mm x 150mm)

Also its 25 * 1.414 - 1.1 (forward drop of diode) = 34v, which might be a tad hi for a 30V driver.... 8% regulation its going to be nearer 37v off load... just need to check max driver voltage.

10000uF capacitor will give 2% ripple @ 6A which is more than adequate, for 3% ripple you could go as low as 6,800uF (381-9093) @ £8.59

Dont forget fuse holder (384-011, £1.23) and slo-blo fuse @ 1.5A, crimp terminals for the fuse holder and bridge rectifier, ring terminals for the capacitor, 10A-rated 'chocolate blocks' for input and output

[z3t4]

Genuine question here 'cos this is most definitely not my home territory. That transformer is 300VA (2x25V, each at 6A). Do you need 300VA, or are you just putting in a nice healthy margin?
I'm interested in this, having seen the telecoms PSU's on the 'bay* at 13 quid for 50V @ 9A. Was already to unlock the safe and trade in some beloved Jaffa cakes, but it's a SMPSU so after this thread I'm not so sure.

*Item number 380071301709

Cheers,

John