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Robot travel speed — Parallax Forums

Robot travel speed

Spiral_72Spiral_72 Posts: 791
edited 2011-01-06 13:49 in Robotics
This is kind of an odd-ball question but:

What travel speed would you recommend for a bot base? I plan to use it for obstacle avoidance, probably with the PING and/or IR, some decision making, maybe line following etc. Specs that might help:

I would like to keep the base at or under 16" x 16" and the entire bot under 10lbs.
I'd be using my BS2 for the time being.
Terrain is indoors, tile/carpet
I hope to use about a 3.5" wheel DIA for ground clearance on carpet (not that it matters)
I plan to use my four cell NiMh pack (hopefully)

I can work with inches per minute, inches per second, furlongs per hour, motor speed with a wheel diameter or whatever you have.

I'm using Parallax continuous rotation servos now with a ~2" wheel. It's underpowered and slow, won't run on carpet and is generally pretty pathetic. Gearhead motors are expensive so I'd like to purchase only one pair. From the speed, I'll determine torque requirements, etc.

Comments

  • Mike GMike G Posts: 2,702
    edited 2011-01-06 08:17
    Velocity will be some function of your sensor(s), processing time, and project requirements. It's tough to speculate with the info you provided.

    I'd be more worried about torque than velocity. Get the highest torque for the money.... no less than 6rpm, 20rpm, 50rpm?
  • ercoerco Posts: 20,256
    edited 2011-01-06 08:38
    Slower is more controllable. Speed literally gets you into trouble faster. Have a look at my old thread at http://forums.parallax.com/showthread.php?118861 Same size platform you're talking about, with great $10 gearmotors that come with wheels.
  • Spiral_72Spiral_72 Posts: 791
    edited 2011-01-06 09:16
    YEA! I remember you posting that now :) I found you link to the gearmotors last night.... they're still pretty cheap, $9.79 I think which is a very good price. I was hoping to get by with a low voltage battery, but it seems 12V & 24V is pretty standard. The 6V units I've been looking at are overpriced for some reason.

    I'm glad you replied, I value your opinion. You seem to be experienced in the builds I described. I was looking in the 60rpm category. I suppose that WOULD be pretty fast once I consider it.

    With a 4" wheel and:
    6RPM = 6.3ft/min
    20RPM = 20.9ft/min
    50RPM = 52.4ft/min

    10RPM = 10.5ft/min. I think I'm going to shoot for the 10-15ft/min area unless anyone wants to disagree. That should keep current relatively low and as a wild guess, give me time for processing.... I think. If I can find the right motor and torque, I may go as high as 20-25ft/min just to have the speed option. Brushed DC motors in my experience develop poor torque at less than 50% rated speed. I saw (on here) where someone claimed a PWM control using two 555 timers give them 100% torque WAY down low. Maybe I need to experiment first.
  • Spiral_72Spiral_72 Posts: 791
    edited 2011-01-06 09:17
    OH! HA! That was you Erco:

    http://forums.parallax.com/showthread.php?126435-Motor-speed-control-with-PWMPAL&highlight=gearmotor

    ...and come to think of it, I'm pretty sure I have that Radio Shack book! I'll have to look.
  • ercoerco Posts: 20,256
    edited 2011-01-06 09:44
    Those motors are rated at 24V, and they are very efficient. I bought a dozen of them for future projects. With low loads they work quite well at 18V, even 12V and are proportionally slower. Obviously at higher voltage they have more speed and torque. Amazingly, their stall current is just 1 amp at 24V. Main downside is the batteries (SLAs work fine) and many popular speed controllers can't handle 24V. But you can build your own cheap PWM speed controller easily for this motor. At 1 amp stall, you can use cheap transistors, you won't miss their 0.7 volt drop out of 24V total.
  • Spiral_72Spiral_72 Posts: 791
    edited 2011-01-06 10:00
    These look promising at 5V: http://www.bgmicro.com/robotmotor.aspx

    and 46ozin seems like it would be enough to run slower. It's still 62RPM though a 297:1 gear. That baby must be humming! 18K rpm motor. Wow.


    This could be the ticket though: http://www.bgmicro.com/MOT1011.aspx
    edit: Maybe not. The motor looks like it pulls 800mA no load. That's a lot man.
  • ercoerco Posts: 20,256
    edited 2011-01-06 13:49
    First motor looks like what Pololu's 3Pi robot uses.

    Second motor looks like mine, BUT:

    1) It has the GOOD strong wheels that you can't find anymore; new ones are black & thin plastic
    2) these are used
    3) motor must be different; lower voltage and higher current
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