Dne 5.11.2011 18:30, piše Kirk Wallace: > On Sat, 2011-11-05 at 14:09 +0200, Viesturs Lācis wrote: >> 2011/11/5 Slavko Kocjancic<[email protected]>: >>> Hello... >>> >>> I think the 1'st thing is to check what is wrong. Not to do something to >>> eliminate problem but not know what problem it is. >> Yes, that is exactly my question - is there anything I can do to >> diagnose the cause of problem? > My experience seems to indicate: > > _Always_ have a filter on VFD power inputs. They are not that expensive. > Proximity of a VFD to sensitive parts doesn't really indicate much. If > the hardware is not configured properly (and what is?), the VFD > interference can travel through metal frames, conduit, unrelated wires, > shielding, etcetera, and come out on the far side of the machine. > Sometimes, beads on the output wires can help. Most stepper and servo > drives are very similar to VFD's, so they may need power input filters, > or output beads too. > > Most break-out-board inputs (and others) have very high impedance, > therefore are very susceptible to induced fields on the input wiring, so > even minor interference can show up at the input pin. Think of hitting > something hard like a bell. It doesn't absorb or convert the energy very > well so it rings until the energy gets converted to sound. Hitting dry > sand converts the all of the energy instantly, so it is hard to drive a > signal into it,let alone induce noise. I've found that plain buffer > inputs have very high impedance and often need some some sort of filter > (lowers impedance, adds sand) that matches the type of signal being > read. Opto-isolated inputs seem to have more impedance, so are not as > much of a problem. Switches and relays with real contacts go from very > high impedance to very low, and bounce, so most likely need filtering. > Most real machines (my opinion) use 12 Volts for control signals to help > push the noise into the OFF voltage region. > > Others have more experience with this, but I have found that connecting > a short piece of wire on my oscilloscope probe picks up interference. I > can wave it around my machine and find the noisy spots. It seems some > amount noise is inevitable and normal, so expect to need to deal with > it, rather than eliminate it. I've heard an AM radio is also good for > scanning for interference. I recently set up HALscope to trigger on a > suspicious signal. After forcing a trigger on the scope to clear the > traces, I sat and watched HALscope until it triggered, then preesed stop > to keep the traces form getting written over. This indicated that this > signal was on the edge of ON and OFF. A resistor in the line (current > limit) and capacitor to ground (low impedance, sand, RC filter) cleared > it up. > I agree that filtering is good thing to do. But in this case (runnung near 1 hour without problem) the EMI isnt the source of trouble. But high impedance on input pins can be source of trouble here. Or better expresed to high resistance of wires/connectors...
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