on further thought... microstepping can help with resonance problems, so it may be worthwhile to use it at the speeds where that is a problem (not using it at higher speeds makes sense since you should have a large ratio between your chopping frequency and your step frequency). also, a trick i learned that worked very well for damping oscillation is to coat things with heat shrink tubing. in this case they were tripod legs, about 4" in diameter made from hollow heavy wall aluminum tubing on an instrument that was very vibration sensitive (enough that a "super spring", i.e. an active electro mechanical spring) sat below it on another tripod. putting heatshrink over the hollow legs made them completely dead, though it was necessary to first spray them with silicone conformal coating to keep the heatshrink in place over the long run (silicone is one of the few adhesives i've found that works well with pvc, and it was pvc heatshrink). of course for this application you want the soft, almost gooey heatshrink (and hopefully not in that large a size or it will cost as much as motors!). other things that occur to me but i have no data on that might help resonance would be applying short pulses to the opposite set of coils (though that would be tricky) to round the edges as it were (i.e. reducing the sharpness of the torque change). anything to make the mechanical system more dead would definitely help, the heatshrink tubing could be put on the shafting and there are deadening materials that can be applied other places as well, if you can find one thermally conductive it could also be put on the motor though i don't know of any such material (other than greases, which could be ok if you built a secondary enclosure around the motor and filled it with a thermally conductive grease, most of which are very dead). in any case dampening material on the other flat surfaces of the bracketing etc. should help. there are probably also solutions involving a viscous oil and vane type of affair thou those would also tend to limit maximum speed (which seems to be very important in your case). in any case, the proper lubricant on the lead screws might help resonance problems. i suspect a web search might turn up some useful ideas and a lot of not so useful ideas (i often wind up doing considerable sifting through search results, but i'm often looking for odd information). protecting the lead screw from oscillating is definitely important, i think i'd use microstepping and consider filling it's area with grease even it it slightly increased the dynamic load. in any case it's definitely something you want to keep an eye on in testing. i suppose magnetic dampening could be used, with strong rare earth magnets along the length of the lead screw or in a few well chosen places, interrupted magnets would effectively shorten the length of the free part of the lead screw and raise the critical frequency. all in all though probably best to avoid cogging near critical speed by any means necessary. your application almost sounds as if it could benefit from a multi-speed transmission, though that would complicate positioning unless used as part of a servo system. i've seen home built cnc mill modifications that used a timing belt and pulleys, which would make backlash less of a problem for a servo system. i believe in any case that with a servo design you want the encoder right on the lead screw, though backlash could still cause oscillation problems. i do like your' idea of using a linear supply to start with at least, big, ugly, and heavy but the toughest solution possible. just remember to consider the ripple current of your filter capacitors and provide them with plenty of forced air cooling! the first thing i did when i saw my friends setup was to get him another capacitor that would be able to handle the ripple current, you often have to use one much larger than what you need for filtering although this makes it harder on the transformer and rectifier since it decreases ripple voltage and increases the ripple current still more. (physically smaller capacitors are often a bad idea for power supplies because of the heat from ripple current that must be dissipated, and i have seen heatsinks on power supply caps before!). Lee McLaren wrote: > > Thanks for the advise Philip, > I expect I don't really need microstep and FETS where looking to be the best > option from my research as well, for a power supply I was just going to get > a mains 240 - 120v transformer and feed it through a rectifier if my output > stage can handle the voltage. I expect I don't want anything to go wrong > though! ---------- -- "We cannot simply suspend or restrict civil liberties until the War of Terror is over, because the War on Terror is unlikely ever to be truly over," Judge Gerald Tjoflat wrote for the three-member court. 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