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?