Do not forget that if you appy a very good gear then you can hold any power you want. It is nothing about the torque of the motor. It is the same as you have a lift fork and you can lift off 2 tonns while with your hand you can make only 100Kg. Positioning is the same: the motor rotates lets' say 1 whole cycle to make 0.0001" movement or something like that and then it measures the position and if not statisfied can be rotate one more this or that way... it is everything about gears and measuring the position, pretty much less depends on the motor. Tamas On 16/08/06, Denny Esterline wrote: > > > > > For controlling position I believe you are thinking of what's called > a > > > > "stepper" motor. It is also "brushless", but has many more poles > > > > allowing for relatively precise positioning. > > > > > > Servos in HDD are also "brushless", position quite precise under PID. > > > > Very true, but I wouldn't consider them "motors" per se, which is what > > the thread was about. > > > > A brushless motor isn't very good for positioning since it doesn't have > > a great deal of "holding power" at stall. I'm sure you could devise some > > PID system to turn a brushless motor into a pseudo servo, but frankly a > > stepper is just so much easier I don't really see the point. > > > > TTYL > > Sorry, I can't see your logic here. Most (all?) modern CNC machining > centers use brushless motors to drive the ballscrews. Holding a 10,000 pound > part against a 5k rpm cutting tool driven by a 30hp motor and maintaining > 0.0002" accuracy is certainly one of the more demanding positioning > applications I can think of. > http://www.haascnc.com/details_HMC_HS.asp?ID=37#HMCTreeModel > > Of course they cost ~$0.5Million. > > -Denny > > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- unPIC -- The PIC Disassembler http://unpic.sourceforge.net -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist