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RPM of servos.... — Parallax Forums

RPM of servos....

TEJATEJA Posts: 8
edited 2007-10-11 17:13 in Robotics
HI....

can anyone plzzzz tell me...regarding the speed of CONTINIOUS SERVOS.....is there any other
way i can make them run any faster then 60-RPM...

when i participated in LINE-FOLLOWER competetion,,,,my BOE-BOT screwd it up...the only reason being i was the slowest 2 trace the line..


can i find any upgrades for existing SERVOS ??

any faster servos available in markets???/

or should i use any STEPPER motors 4 higher speeds..??

but...are STEPPER motors as precise and sharp as SERVOS.....???




please someone answer all these...troubles ..

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2007-10-07 04:48
    If you want speed, particularly controllable speed, a geared DC motor is probably the best way to go. If you use Parallax's HB-25 motor controller, the DC motor will still look like a servo to the Stamp and you could use your existing programs with only minor changes.

    Stepper motors could be used, but the controllers needed would work differently with your programs ... not a big problem, but the HB-25 and a DC motor is the easiest way to go.
  • TEJATEJA Posts: 8
    edited 2007-10-07 06:18
    thanx a lot mike.....but i have seen in description that,,a single..HB-25 motor controller may control 2-DC motors.......is it right..???

    do we have 2 spend $50 on this mcro-controller (hehehe,,,, now my financial position is...rather bugged up freaked.gif )
  • eyyYoeyyYo Posts: 14
    edited 2007-10-07 09:46
    Im borrowing the thread quickly ;> Can u use any dc motor you want, with HB-25? So i can like use the dc motor from my old fan (12V), etc.
  • D FaustD Faust Posts: 608
    edited 2007-10-07 12:36
    You should e ble to use any DC MOTOR that you want with the HB-25 as long it doesn't draw to much current and is withing the voltage ranges.

    The HB-25 can control two DC motors IF they are moving in the same direction. This won't work if you want to steer. It you want direction and speed control of 2 motors you need two HB-25s.

    My plan for controlling two dc motors is to use the PWMPal to make pwm for a L293NE transistorized H-bridge.

    This might help: http://forums.parallax.com/showthread.php?p=525689

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  • eyyYoeyyYo Posts: 14
    edited 2007-10-07 12:53
    Okey.. Because im doing the steering for my robot. I think im going to go with modified servos on that one. But then i will use a much bigger motor for the cutting (building a robot that cuts grass). Can I use a relay (5V for the On/Off on the relay) to controll it, since its just going to go in one speed? Then i connect the relay to a pin on the stamp, and HIGH 4 turns the motor on, with 12V. Or am i thinking wrong?

    Thanks. (Sorry for the crappy english, btw)
  • D FaustD Faust Posts: 608
    edited 2007-10-07 13:26
    I'm not sure if CR servos will be powerful enough to push and pull your bot in grass.· A stamp can't supply enough curretn to turn on a relay.· You should probably connect a stamp pin to the base of a transistor and run 12V through the transistor to power a 12V relay.· The power for the motor would go through the relay.· I think you need a back-EMF protection diode, but I'm not sure how that works, maybey somebody else can explain that.· Hope this helps.

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  • TEJATEJA Posts: 8
    edited 2007-10-07 13:36
    D faust,
    the quick link u have sent was useful thnx...but..can we try controlling DC motors directly using BS2 controller.....like...can we think of using HIGH command...to turn it on or off.....and for reversing directions of Dc motors,,,,can we think of something like ON-OFF switch that can connect I/O pins of BS2 with either the GND or VDD....is this possible....so by changing the way DC motor is connected to the +ve and -Ve terminals...can we chabge the way it rotates na??
  • Mike GreenMike Green Posts: 23,101
    edited 2007-10-07 14:17
    No. The Stamp can only directly switch about 20-25ma which is not even enough for most relays. You have to have some kind of high current driver, whether it's a power transistor or logic level MOSFET or a specialized IC like the H-bridge drivers.

    You theoretically need the reverse diodes for switching any inductive load like a motor or relay. Some MOSFETs and some Darlington transistors have this built-in. There's a good description on the Wikipedia ... search for "wiki snubber". Basically, as the magnetic field collapses in the relay coil or the motor coils, it induces a reverse voltage in the coil. If this is allowed to rise unlimited, it can damage the attached circuitry. The reverse diode dissipates this energy (mostly in the resistance of the motor windings) preventing the voltage spike.
  • Paul Sr.Paul Sr. Posts: 435
    edited 2007-10-07 14:22
    TEJA said...
    D faust,
    the quick link u have sent was useful thnx...but..can we try controlling DC motors directly using BS2 controller.....like...can we think of using HIGH command...to turn it on or off.....and for reversing directions of Dc motors,,,,can we think of something like ON-OFF switch that can connect I/O pins of BS2 with either the GND or VDD....is this possible....so by changing the way DC motor is connected to the +ve and -Ve terminals...can we chabge the way it rotates na??

    1. You can turn a DC motor ON or OFF directly [noparse][[/noparse]using a transistor or relay] with a BS2 pin - but no direction or speed control.

    2. If you add another relay OR replace the relays with an H-Bridge, you can then add DIRECTION control.

    3. If you add PWM [noparse][[/noparse]external components - the BS2 will not natively provide continuous PWM for motor speed control], you can then control SPEED.

    4. A combination of 3 and 4 will get you SPEED and DIRECTION control.

    HB-25 combines of all of the above allowing control using only 1 BS2 pin.

    Edit: Mike got in before I finished! As usual with a great response. I believe this is still a good simple guide!

    Post Edited (Paul Sr.) : 10/7/2007 2:27:36 PM GMT
  • D FaustD Faust Posts: 608
    edited 2007-10-07 15:53
    It looks to me like you would want the Pololu dual serial motor controller.· it's 20$ form parallax I think.· It would require a little code adjustment, but should work if your motors don't draw too much current.


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  • k4shfk4shf Posts: 21
    edited 2007-10-07 17:01
    What voltage were the other robots running their servos at?

    My $.03 (inflation)

    Tim T.
    ·
  • D FaustD Faust Posts: 608
    edited 2007-10-07 17:18
    [noparse]:)[/noparse] Ha Ha

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  • eyyYoeyyYo Posts: 14
    edited 2007-10-07 21:33
    Mike Green: Thank you, i will look into it!
  • TEJATEJA Posts: 8
    edited 2007-10-08 01:33
    This pololu dual motor controller appears good( atleast it is afordable 4 me hheheh.....)

    thank u guys..
  • D FaustD Faust Posts: 608
    edited 2007-10-08 01:54
    Make sure you test you motors for current usage. The pololu controller supplies 1 amp max (I think, check the datasheet)

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  • TEJATEJA Posts: 8
    edited 2007-10-08 03:26
    yeah.gif·
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2007-10-08 09:50
    You could add bigger wheels to the servo to go faster with the same RPM, but that would be a mere mechanical solution.

    If you are really brave, you can build your own motor controller. Google 'H-bridge' to research.· Beam robotics even has some very tiny 6 transistor 'free form - no circuit board' H-bridges that are great for use with hobby motors.· Each will cost you $1-2 USD.

    Of course, it is a trade off. The HB-25 is a very durible design and likely to outlast anything you build yourself. It is intended to prove some hefty pushing power and can drive an all-terrain outdoor robot from solid 12v motors.

    But, I'm as much interested in the knowledge gained as the result. And face it, one can only go so far with servos. After all, they have hollow plastic drive shafts and plastic gears.

    When shopping for RRM, I don't think you need to go over 300 RPM tops.· With a 5 inch diameter wheel, that will keep you busy. Of course, you can handle higher RPM with 1/2 wheels.

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    ···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

    Post Edited (Kramer) : 10/8/2007 9:57:14 AM GMT
  • MightorMightor Posts: 338
    edited 2007-10-08 11:30
    I used a simple circuit with a L293D, that's a little IC with 4 half H-bridges in it that you can hook up together to make 2 full H-bridges. They cost almost nothing and may be adequate for your needs. If used together with 2 simple solarbotics motor, you're probably looking at about $15-$20 in total including wheels and the IC. The nice thing about the L293D is that you won't need any extra external fly-back diodes as those are built-in. Apparently the SN754410 is a drop in replacement for the L293D but can handle a bit more power and has a lower overhead.

    Gr,
    Mightor

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  • D FaustD Faust Posts: 608
    edited 2007-10-08 11:50
    What do you use for PWM?· or do you just have have the motor be full on or off?

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  • MightorMightor Posts: 338
    edited 2007-10-08 11:55
    I use PWM, yeah. I wrote a bunch of SPIN/ASM code to do that for me. I can post it later if you want, I'm on my way out the door now.

    Gr,
    Mightor

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  • D FaustD Faust Posts: 608
    edited 2007-10-08 11:57
    Okay, I'm trying to do it with a BASIC stamp, I wanted to know if you were doing it with hardware. Thanks anyway. No need for code postage.

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  • number5number5 Posts: 6
    edited 2007-10-11 17:13
    I recently built a shoe-box size DC motor robot.
    It uses an OEM Basic Stamp as controller, 2X12 VDC gearhead Motor, 2X5 in monster truck tires and 7.2Ah lead acid battery.
    In demo you will see a simple TTL driven DC motor driver (discrete H-bridge) and PnP Opto Sensor that I designed for it.

    Object Avoiding Rover Robot Demo:


    Testing DC Gearhead Motor and H-Bridge Driver:
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