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[DPRG] Re: Pittman Motors at Tanner's

Subject: [DPRG] Re: Pittman Motors at Tanner's
From: David P. Anderson dpa at io.isem.smu.edu
Date: Mon Aug 13 23:19:42 CDT 2001

Hi all,

Rodent writes:

> My big robot weighs about 10 lbs,  has 12-volt 44 rpm motors and 4" diameter
> wheels -- it has plenty of power. It was severely underpowered when I had
> the 100 rpm motors. I would be affraid the Pittman motors may be turning a
> bit fast to have any power, especially with a 5" wheel.

Good comments.

With a 2:1 final gear reduction between the Pittman gearhead and wheel
axle that would give around 35 rpm on 12 volts.  That's about the top
speed of a couple of my bots weighing in around 7 or 8 lbs.  This might
be the optimal way to mount them as well, with spur and pinion gears
driven from motors mounted fore and aft of a central axle.  

It's hard to know without actually using the motors.  On SR04 I played
around with a final ouput of 1.5:1 to 1.8:1 to find the best performance,
using much smaller 12 volt motors with a 40:1 gearhead, 5 inch wheels.

It really does make a difference, especially at different speeds.  The
Pittman motors with a final 2:1 reduction would effectively yield 120:1,
which should be beaucoups of torque.  Folks wanting the higher speeds
could still go to an 18 volt motor supply.  These motors are advertised
as "19.1 volt" motors.

So your optimal final wheel rpm would be around 45 for a 4 inch wheel?

I had arrived at 35 rpm for a 5 inch wheel, looking for more ground
clearance.  That's still in the same ball park.  But I might want to 
go faster for a "contest" robot.  

What kind of motors are you using?

onward,
dpa


> 
> ----- Original Message -----
> 
> > These little Pittman motors seem ideal.  They can be driven at 70 rpm
> > with a 12 volt battery.  With a 5" wheel that's
> >
> >                  2 pi R =
> >     2 * 3.14159 * (5/2) = 15.707950 inches per revolution.
> >
> >         15.708 * 70 rpm = 1099.556 inches per minute
> >
> >       ~1100 inches per minute
> >        91.6 feet per minute
> >        5500 feet per hour
> >       ~1 mph.
> >
> 
> Which seems just about ideal.
>
> That is about twice as fast as any of my existing robots.  Faster is not
> necessarily good.  It gets much harder to control because the robot has
> less time to sample his sensors and make corrections.  These motors
> could be mounted transfering power to the wheel with a final output
> gear for another 2:1 reduction or whatever.  This has some advantages
> for motor placement.  Or a smaller wheel could be used...



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