DC Motor Guidelines?
January 4, 2013
I am building an autonomous vaccum cleaner robot. I am using an existing cordless vaccuum as the base (why reinvent the wheel?) and putting powered wheels and a brain on it.
I am looking for guidelines as to how to spec motors for this (and other) projects. One wouldn't think it should not bring about so much thought, but here it is. Although there is a distinct possibility I am overthinking this.
I am using 6V cordless screwdriver motors connected directly to the drive wheels with a L293 H Bridge. With PWM and no load, these draw 0.4 amps per side (0.8 amps total) which seems a lot, although as you would expect, the screwdrivers do have a lot of torque. I was thinking that it might be good to replace these with smaller geared motors, but I am not really sure what I should use as criteria. For instance, I saw Sparkfun.com had a geared motor rated at 6V - 60 ma, 140 rpm, 26 oz in. Now if I remember my physics that means it has enough power to pick up 26 oz and move it 1" in a second (x2 since will be two motors.) The cordless vaccum weighs about 5# with the cordless screwdrivers, acrylic, Arduino etc but let us say it has a rolling resistance around 0.5 pounds of pressure. So in one second, these motors will have 52 oz/in (3.25 pound/in/second) or should be able to move my robot 6.5"/second. (6.5*3600)/12 = 1950'/hour or at the VERY best 0.36 mi/hr. Those aren't going to cut the mustard unless I misunderstand what I am reading.
If I want my robot to move at 1 mile/hr (about right for the brush to pick up stuff on carpet), I need to have (1/0.36) * 52 oz/in = 144.04 oz/in/second or 9 pounds/in/sec. Each of my motors then has to be 72.02 oz/in/sec which seems really, really high for very slow performance. The robot will have inertia to overcome and then to keep it moving will require less power so maybe this is an issue, non issue. But if the load is a continuous 0.5 pounds these numbers seem correct if I use two motors.
My other concerns were what if it runs into a slight incline going from the wood to tile in my kitchen? What if it gets stuck on a pencil that my kids left on the floor? If folks who have been there, done it could share with me their suggestions or guidelines, I would greatly appreciate it.