Motor sanity check

[krich]

Now that we've completed our first robotics project, I'm doing research on another robotics project that my daughter came up with.  She wants to build a robotic rat for Halloween.  That gives me 9 months of build time.  Something that will scurry around under foot as trick or treaters walk up the driveway.  My first reaction was "no way I'm putting a several hundred dollar robot out there with a bunch of kids".  Well, the challenge of building such a thing has gotten the best of me and I've agreed to "look into it".  Plus, the challenges I would be forced to solve will be useful in other areas.

The way I figure, is this thing has got to be FAST, or I have no doubt that it WILL get stomped on by the first teen who walks up to the door.  If I can't make it fast, nothing else matters.   I know I'll have to smooth the motor acceleration to limit wheel slippage and gear strippage and sensor speed will be an issue, but why worry about that when I haven't even found the motors yet?

So, motors first.  I'm new to all this, so I read up on the Statics and Dynamics tutorials.  Looking at the RMF calculator, I've begun to realize that this is much harder than I thought.  I need this thing to go at least 3ft/s (~2mph), preferably in the 6-8ft/s range and with at least acceleration to full speed in 4 seconds, pref 2.  I'm running into a problem where I need a bigger motor, creating more battery and motor weight, requiring a bigger motor, etc, etc.  The small 6V motors I want to use are deficient in the RMF calculations by a factor of 10.  Not even close.  The 7.2V motors look alright, but I'm starting to worry about peak Amps, battery life and weight.

Here's a sample of what I am getting:

Mass:  ~2lb, Velocity: 6ft/s, Acceleration:  3ft/s/s, Efficiency: 75%, Incline: 5deg, Wheel D: 3 in, Wheels: 2

That gives me an RMF of 7.37 @ 458RPMs.  I'd rather just use gearhead motors since the gear reduction is already done.  So, I'm looking for something about 450RPMs.  I found this motor at Trossen 36:1, 24mm Planetary Gearmotor, RS-370.  That gives me a RMF of 7.92 @ 444RPMs.  Does this sound about right?

That thing has a stall current of almost 10A.  That rules out all of the H-Bridge ICs I was thinking of using (L293, L298, SN754410, LMD18201).  Any suggestions?

Ken.

[SmAsH]

well you would want it to go fast enough so the children cant catch it...why dont you put it on two motors with two little wheels underneath it then make a shell..i think moving feet would be very hard to make it go fast...
 RAT

[krich]

Yep, 2 powered wheels, 3" diameter (or so).  No way I'm building a walker that goes 8 ft/s.  I wish I was that good, but I'm not.

[SmAsH]

lol ive done things like this before i just put sensors in the eye holes and used the differential drive system.... worked quite well (scared the crap out of mum and friends) i tested it at my mothers dinner party lol

[airman00]

To make it fast just use larger wheel diameters

to control the motors use high powered MOSFETS w/ heatsinks or maybe buy a commercially made H bridge ( not IC)

question: What you want is a fast R/C car basically , in a rat disguise ,  rite?  So just get a really fast R/C car , maybe a gas one???  And cover it with wood on the electronics or metal on the electronics ( so it shouldn't break if you step on it) , then put a rat disguise over the whole thing

[krich]

I just found this:

Sabertooth 2x5 Dual Regenerative Motor Controller

Some things it does that are perfect for what I need to do:

• Regenerative - while braking it uses the back current from the motor to charge the battery, more battery life
• Analog, R/C, Serial, Serial Packet interfaces - spit an analog value out of your MCU analog port to control speed/direction, no PWM hassles
• Exponential Response - response to inputs are exponential, use with 'bots that have high acceleration or top speeds.
• Ultrasonic switching - you can't hear it whine.
• Over-current protection - when a punk stomps on it and jams a wheel to stall point, it'll simply shut down rather than explode.  (bonus!)
  
• Cheap - $60 plus $2.50 shipping.  I can't beat that with my soldering skills.
  

.

I can't see anything wrong with it.  At all.

Ken.

[SmAsH]

lol that looks cool but wont it be too big for a rat?

[krich]

To large?  Its less than 2 inches square and weighs less than an ounce.  Either way, I'm not going for a normal sized rat, I'm going for an R.U.S.

[cooldog]

for the motors use the motors from old cd players there small and go fast

[maverick monk]

but they have no torque, id go with a 380 or 540 sized motor, probobly380 or youl draw too much power. then get a gear reduction unit. my current one is 10.84:1 and with a 30000 rpm motor (basic 540 size) I am getting up to 25mph

[krich]

25MPH?   

Now that's what I'm talking about!

[airman00]

krich what you are looking for is a gas r/c car , they go really fricken fast!

[SmAsH]

naa go with nitro..now that id have!!


~smash

[ed1380]

nitro would eb harder to use.

get a profesional grade electric RC