Test Current draw on Motors / Drivers

[skylar]

Okay, I just spent a couple hours learning about stepper motors, drivers, and all sorts of fun stuff. My recommended approach is actually to go with one of these low-current (and thus low-torque) motors:

https://www.sparkfun.com/products/9238 http://www.adafruit.com/products/324?gclid=CKe_ntyQ-7wCFYFhMgodNwwATg

And power it with these low-current boards (the big easy is also fine, but has bonuses we probably don't need and aren't applicable in the lower-power and battery-operated situation):

http://www.pololu.com/product/2131

Lower current means longer battery life, which seems key after doing the calculations on the high-current models! Current on these is rated per-coil (2 coils used in a bipolar stepper) so the actual draw on the stepper is twice that (esp when the motors are holding still). So, for a 1.7A system that's 6.8A just for the motors. These 330mA motors would only draw 1.2A in total.

We need to measure current for a couple reasons:

• over-powering the motors can damage them (e.g., too much current). It's good to power the motor then turn up the pot until you get the desired meter reading. We should measure our drivers to set them (with motors in a still, powered, motionless mode).
• microstepping actually draws a variant current on the motors. there are all sorts of interesting algorithms the motors use to vary this current, but it will be interesting to see the actual draw on the individual power leads to the coils while running the wheels to test.
• we should measure current drawn by the two drivers from the power source to test our theory on total current draw by the system (eg. ~1.2A). This (by my understanding) should be constant even though the power draw by each coil at any time might be lower.

Once we understand our current draw on these motors we should also test the holding power. Making the robot go should be a problem. The motors can be powered less than the rating (e.g.. 12V in this case) though there are two consequences higher heat build up (due to more work to charge the coils in time) and lower torque (well, because the coils are charging slower - same issue!). If the robot can stand on 5V and the system doesn't overheat, then things might be okay on serialized 6.6V lipo batteries or a 7.4V lipo. If we need more torque, we can crank the voltage up to 9+ with AA batteries in serial.

As @jonmagic mentioned, we might not need that much torque since the robot will be in constant motion. Rather than holding still, it will be micro stepping constantly to adjust itself.

[skylar]

@jonmagic I did a lot test last night and looks like my estimates were pretty close. The system actually seems to draw less power than above but mostly because the drivers (or engines?) seem to draw only 70% current by default to accommodate for peaks.

Some numbers:

• Power at 12V / 2A (via hard-drive power supply brick)
• Each engine coil draws around 220-260mA
• Voltage on the engine driver bus was just above 9V, sometimes jumping another 1V. Not sure why it's not 12V yet.
• When testing draw on the whole engine setup (4 coils) current was steady around .840mA, but drops quite a bit as engines step (they need less current when allowing movement)
• the spark core was drawing around 170-220mA

All this suggests we need about 1.2 A to sufficiently power the system (900mA for the motors, 200mA for spark) at peak periods.

[skylar]

I was not able to get the system to power on a 5V power yet. Luckily I found a piece of awesome power kit in a hard disk interface kit. Now that I've got things working at home on 12V and know what and where to measure, I might go back and try things agin on 5V.

I doubt the motors will work on less than 9V from what I've seen so far.