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Need help to drive slow

Discussion in 'Newbie Q & A' started by Ken Kay, Jan 9, 2017.

  1. Ken Kay
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    Ken Kay New Member

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    Our high school team is currently building a competition robot with a 2 wheel direct drive system. We are currently using 2 castle mamba monster escs with 2200kv motors directly attached to each wheel. We can drive very fast which we don't need, driving slow causes the motors to stutter and driving is erratic. We need slow and precise movement, maybe 10-20mph max. Any help would be greatly appreciated as we are all new to this.

    :dizzy:
     
  2. BenFink
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    BenFink Studio Forums Elite
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    When you drive slow with the motor out of the car does it still stutter?
     
  3. Ken Kay
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    Ken Kay New Member

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    No, it only happens when we set it down with the weight of the bot. Currently it is maybe 5lbs, it's max weight will be 14-15lbs when finished
     
  4. Darkhorse_Steele
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    Darkhorse_Steele Well-Known Member

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    I would look for a gear reduction kit.

    Sent from my LGLS665 using Tapatalk
     
  5. redford
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    redford Well-Known Member

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    Does the combo have a sensor cable and have you connected this?
    Can you use a motor with less KV?
    That would be my ideas.
     
  6. GrablinGold
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    GrablinGold Well-Known Member

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    I agree, Gear reduction would be the way to go. directly connecting the the wheel to the motor isn't going to work all that well.
     
  7. GrablinGold
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    GrablinGold Well-Known Member

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    also you should post up a pic, id love to see the bot!
     
  8. drzoo2
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    drzoo2 Studio Forums Elite
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    Motor KV x Input Voltage = Rotor RPM
    Rotor RPM / Gear Reduction = Axle RPM
    Tire diameter x Pi (3.14) = Tire circumference (that is the distance traveled in one revolution)
    Tire circumference x Axle RPM = Wheel speed (if tire diameter is measured in inches, that would be inches per minute. You could convert that to MPH)

    So, some assumptions
    2200KV
    12.6volts input
    4 inch tire diameter
    10:1 gear reduction

    2200KV x 12.6V = 27,720 Motor RPM
    27,720 RPM / 10 Reduction = 2,772 axle RPM
    4" x 3.14 = 12.56" traveled per tire revoltution
    12.56" x 2,772 = 34816.32 inches/min.

    Converted to MPH
    34816.32 inches per min / 12inches per foot = 2,901.36ft/min
    2,901.36ft per min / 5280 ft per mile = 0.5495 Miles per minute
    0.5495 Miles per minute x 60 sec per hour = 33 MPH

    So, with all of that above, you need gear reduction. My example included 10:1 and still calculated 33MPH. Rather large tires and high input voltage but still. What input voltage and tire size are you using? Over gearing will cause a brushless motor to cog. Worse being sensorless. Since your direct drive, your way, overgeared and may overheat the motor and/or ESC.
     
    #8 drzoo2, Jan 9, 2017
    Last edited: Jan 9, 2017
  9. BenFink
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    BenFink Studio Forums Elite
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    I think that is forgetting the weight and resistance of the vehicle, I did the math and calculated that my traxxas slash has a max speed of 196.62 mph. I probably got something wrong.

    2200kv Sidewinder 8th on 25.2 Volts (fully charged 6 cell lipo)
    4 inch tires
    54 tooth spur, 17 tooth pinion

    would the fact that it is 4 wheel drive impact it also? More tires so more resistance.
     
  10. drzoo2
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    drzoo2 Studio Forums Elite
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    True. It's theoretical. There are losses but it isn't that much. At most 10% worse case

    Take the total gear reduction. Transmission and axles. These two multiply.

    A quick calc using what I could find on gearing for the slash makes it close to 70ish


    Sent from my XT1575
     
    #10 drzoo2, Jan 9, 2017
    Last edited: Jan 9, 2017
  11. BenFink
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    BenFink Studio Forums Elite
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    The slash goes motor - pinion - spur - drive line - differentials - wheels. The spur to axle is 1:1 speed ratio because it is hooked up directly. I need to look at my diffs really quick to see what ratio they are, I believe they should be 1:1

    there is a diff reduction, I need to figure out what it is.
     
    #11 BenFink, Jan 9, 2017
    Last edited: Jan 9, 2017
  12. drzoo2
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    drzoo2 Studio Forums Elite
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    Without taking over this thread, your saying you have no transmission?

    NV....center diff... I looked at the 2wd. Answer is still 70ish. I used my EXO calc to figure it. It used a center diff also so my mistake yielded the correct answer.
     
  13. redford
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    redford Well-Known Member

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    Good morning! :)

    He said:

    "... driving slow causes the motors to stutter and driving is erratic. ..." I think that's the cogging without the sensor wire.
     
  14. GrablinGold
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    GrablinGold Well-Known Member

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    If you have no gear reduction between the motor and the wheel i'm not sure a sensor wire will solve all your woes. Also if you intend on increasing the weight of the vehicle without gear reduction you risk burning up the motors/ esc. It would be just too much resistance on the motor system. What you need is low end torque, which you cant get without gear reduction.
     
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  15. redford
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    redford Well-Known Member

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    That's why I had also addressed the engine with less KV.
    Here would probably be a few pictures very helpful.
     
  16. RustyWrench
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    RustyWrench ...I Always Edit...
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    What batteries are you using on the Bot? 2s LiPo, 6s LiPo, NiMh, or something else? It is important for understanding the complete system... Are you using 2 ECS's?

    I would look at changing out the Brushless motors and replacing them with a couple of Brushed Drill Motors this is the easiest and most effective way to eliminate the cogging and gaining low speed control.

    Figure out your tire size, the aprox weight of the bot so you can calculate a gear ratio. Don't forget to take in account the power system when you calculate.



    Dean
     
  17. HalNutt
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    HalNutt Studio Forums Elite
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    Without changing the bot brushed motors will give you low speed control. Brushless motors are for high speed revolving granted everyone like rosie whipping around the jetsons but not in real life. 15-20 lbs your gonna need some old black and decker hole hawg brushed motors or equivalent. The 540 motor is probably gonna overheat with that load without a gear reduction to transfer torque. If its a buget build tag sales and corded sawzalls hole hawgs skil sawz on those engines running Direct Current thru an esc will give you nice smooth control without gearing
     
  18. 00whynot
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    00whynot Moderator
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    Yup you need a gear reduction somehow. those motors want to spin at thousands of RPM. You can try mounting the tire to a shaft on a bearing and have the shaft of the motor press against it to turn it. but chances of that working are slim. they want to turn fast. best bet would be to try some 55~70t brushed crawler motors. the esc's you have can be set up for brushed motors.
    But honestly a couple crawler ESC and motor combos would be the best way to go.
     
  19. RustyWrench
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    RustyWrench ...I Always Edit...
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    You may be right... Hard for me to say for sure unless I was building it myself.

    But. You could also go the Pawn Shop or Second Hand Store and pick up a couple of Matching Cordless Drills for cheap and harvest the motors. lol... Sorry I like the Drill Motor concept.



    Dean
     
  20. HalNutt
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    HalNutt Studio Forums Elite
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    I'm not sure that the drills will have enough torque to maintain slow speeds unless they go with a hammer drill ir larger wind. Its only a school project so prob diesnt have to have mych longevity I guess
     

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