Alrighty, next series of major purchases has been made;
Galil DMC-2183 8-axis motion controller
Galil SDM-20640 4-axis stepper motor driver daughter board
So while the wallet charges back up, I am now looking at the next link in the chain; stepper motors!
My design constraints are at least well-defined since I have a driver circuit to work within, which is kind of backwards from how you're supposed to approach this. The Galil driver is certain to be 'good enough' for my rather modest means, so it's down to milking the most I can out of it for the money --same as everything else so far.
1) Driver current is 3A maximum:
This effectively puts a cap on the amount of torque I can get from my motors. Every motor winding design has a different torque:current ratio, but peak torque is always attained at peak current (and at slow speeds). According to materials on Gecko's site about sizing servo power supplies, the bipolar-parallel winding configuration my steppers will use can only draw at most 2/3 of the rated current in practice (I think the rated value is for a *stalled* motor condition, which a good machine design won't operate them in). Unless I'm mistaken
--please tell me if I'm mistaken-- that means my 3A driver circuits can run 4A steppers at most, and I'd need a +8A power supply to feed them.
2) Driver voltage is 50V maximum:
Voltage limits put a cap on how fast you can go, and also what types of motor windings can be used. Current is held to a fixed level during the peak/constant torque portion of the motor's performance so the windings don't fry, and since torque & speed correspond to power, a higher operating voltage capability means higher speed before torque falls off (i.e. current drops due to impedance effects at high speed). To get the most from my drivers, I need power supply that can operate at around 48V (50V isn't as common, is all)
48V isn't super energetic, but it is high enough that it can damage certain stepper motors (it can drive current too hard into a coil that isn't build with enough inductance to resist it) or at least cause them to run poorly, and hot (more likely with only 3A to play with). Going by the voltage-rating formula out there in many places (Vmax = 32*sqrt(inductance)), it appears that if I wish to use the full 50V capacity of my driver for power output, I must use motors with impedance near 2.44mH, or slightly higher. If much lower, the motors will cook at 50V, if impedance is much higher the stepper driver's voltage won't drive the motor aggressively enough to operate at higher speeds (torque falls off badly)
3) Motor wiring is bipolar-parallel:
There's three ways the two sets of coils inside the motor can be hooked up to power and driven; bipolar series, unipolar, and bipolar parallel (in rough order of increasing torque rating). From what I gather, unipolar is an archaic configuration that older drivers were designed around, before more modern switching circuits became available; they can still be wired as series, or as parallel (but only using half their coil turns), but we generally can't drive stuff in unipolar wiring anymore. That leaves series and parallel, and is basically a function of how much speed you need; series can be driven at low current, but has very high inductance so lots of voltage is needed to spin the motor quickly (more than any driver can supply). The trade off is not equal, so unless you don't have much current *and* don't need speed, bi-polar is more efficient all around. For simplicity, this is the only motor configuration I'm looking to use.
I've already found a reasonably-priced 48VDC power supply that can deliver up to 9A.
So, I'm looking for bipolar-parallel motors in the 3-4A range with impedance around 2.5mH. *Not* looking at torque figures except for comparison (like I said, I'm having to do this a little backwards
). If any of you know of some vendors that have a good selection, I'd like to hear about them; checking out the OMC/StepperOnline catalog, there are a plethora of NEMA23-size stepper motors in all manner of power levels, but I'm not seeing that 'perfect' combination of my design requirements that will let me get the most out of my machine. If I have to compromise on anything, I think it should be high impedance since that mostly affects high speed operation, and as I've mentioned, lathes don't gain as much from rapid traverse moves as do mills and routers. My target rapid speed of 420rpm is less than half the 1000rpm that the inductance figures are measured at, but I don't know how much lower they would be.
Here's my list of hopefuls;
(I'm also listing inertia to get an idea of the 'agility' of these motors)
24HS34-4004D; 35$, 4A, 3.5mH, 325.7oz-in, 840g/cm^2 **NEMA24** my understanding is these still roughly fit the NEMA23 form-factor, which is fine)
23HS45-3004D; 75$, 3A, 9.0mH, 354.0oz-in, 810g/cm^2 (inductance is rather high, by my 'rapid speed' rpm is only 420rpm, so maybe OK...price is silly)
23HS45-4208D; 75$, 4.2A, 2.3mH, 276.1oz-in, 810g/cm^2 (would have to run at slightly-lower torque at all speeds, but curve is flatter...price is silly)
23HS33-4008D; 25$, 4A, 1.8mH, 283.2oz-in, 530g/cm^2 (I'm limited to around 42V and less power, but torque/inertia ratio is much higher)
*this last one is the front-runner, unless my assumptions are bad or there's a better option
There's lots of others, but this 4A/3mH region seems to be a bit sparse. Tons of options one amp higher, or if I had 80V to drive a higher-inductance motor with, but these are the only options I have unless I want to step down to <250oz-in torque ouput (probably still okay, but why not get more if it's available?)
TCB
