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motor driver — Parallax Forums

motor driver

vla7vla7 Posts: 79
edited 2007-11-27 07:21 in BASIC Stamp
Can someone give me some advice on how to chose a motor driver and where to get one.· Do they carry them in Radio Shack?· I'm trying to convert an RC car into a robot car.· It has two motors - one for rotation of the rear wheels and one for turning the front wheels.· I just want to be able to do the basics - go forwards, backwards, and turn left and right.· It seems alot simpler to just buy a motor driver then building my own H-bridge.
Thanks.

Comments

  • FranklinFranklin Posts: 4,747
    edited 2007-11-22 02:45
    A lot of the less expensive RC cars (like you get at RS) have a circuit board you can hack and directly control the motors. Others use servos that the stamp can control. Hard to tell just what you have and how you would control it. (More info , please)

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  • vla7vla7 Posts: 79
    edited 2007-11-22 02:48
    It just has two dc motors. No servos.
  • slamerslamer Posts: 52
    edited 2007-11-22 18:39
    When choosing a speed controller it's good to know some things like. What kind of motor is it? (perminate magnet), (brushless), (stepper),... ETC.··What it the voltage of the motor? What is the Motors Stall Current Draw? Do·I need to have the motor run in Fwd and Rev? There are lots of speed controller available for small motors. Once you can answer some of the basic question about sizing the controller for the job.·Heres some that might work for your application.

    http://www.robotcombat.com/marketplace_antweights.html

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  • DJSandbergDJSandberg Posts: 56
    edited 2007-11-25 18:52
    Most RC cars use an Electronic Speed Controller or ESC.· They take·a signal from the remote and convert it into forward and reverse.· Only the more expensive units have reverse.· These are available at any hobby store or online at www.towerhobbies.com.

    Parallax has the HB-25 which I've used and recommend.· It is overkill for your RC car project, but you will be able to use it on almost anything you build in the future. I'm using two of them on a 63 pound robot and they work great.· You can't beat the value and ease of use.· It also takes a simple one wire connection to control it.

    Daryl

    ·
  • Steve JoblinSteve Joblin Posts: 784
    edited 2007-11-25 22:13
    You are correct vla7... there are many good motor controllers available so you don't need to build your own h-bridge... selecting the right one is important as the features and capabilities are all unique. Lucklily, Parallax carries a nice selection that should suit your needs... they are all listed here: http://www.parallax.com/Store/Accessories/MotorServoControllers/tabid/160/List/0/CategoryID/35/Level/a/Default.aspx?SortField=ProductName,ProductName

    Parallax used to have a nice little summary chart to help you select, but now that the website has changed, I have no idea where it is.

    My personal favorite for small DC motors... Pololu's Dual Serial Motor Controller... Parallax sells one of them, but check out Pololu's website for their other versions as well:·http://www.pololu.com/products/elec.html#motocon
  • J.T.J.T. Posts: 31
    edited 2007-11-26 21:19
    I'm in the same boat.· My project is pending because I don't know which motor speed controller to purchase.· I think my biggest factor is that the battery that came with the vehicle is a

    1320mAh 11.1V 3 Cell Li Poly 3s



    I don't know if this was being regulated before it got to the motors, but based on the voltage, can anyone recommend which speed controller would likely be best?· I'm leaning toward the pololu, but not sure about the voltage.· I would switch to a 9.6 volt battery if that is what is needed, but I already have the lithium polymer battery.



    thanks!
  • Steve JoblinSteve Joblin Posts: 784
    edited 2007-11-26 22:14
    Volts are important, but I think the most important factor is how many amps the motors draw... you need to make sure that the motor controller is delivering as many amps as the motor will possibly need (which usually is the stall current of the motor).
  • J.T.J.T. Posts: 31
    edited 2007-11-27 04:04
    Thanks,

    So how do I measure the amps that the motor is pulling?· Do I just run the motor and put a multi-meter on the two leads of the motor?

    Also, the vehicle has 4 motors, so the battery will be driving all 4 at the same time.· I'm guessing that when running all 4, the results of amperage are different?· Would it be less, or more?
  • Mike GreenMike Green Posts: 23,101
    edited 2007-11-27 04:23
    Yes, you either get the information from some kind of datasheet or you measure it yourself. The current when the motor is under load will be higher than when it's unloaded or lightly loaded.

    The amount of current drawn is additive. 4 identical motors under identical mechanical load will draw 4 times the current of one motor.
  • J.T.J.T. Posts: 31
    edited 2007-11-27 06:16
    Thanks Mike,

    I haven't run the amp test yet, but it looks like I may be needing Pololu's dual serial motor control vs. the micro version of this simply for the·fact that the volt range for the micro is·too small (up to 9V).· I did see another post where someone confirmed with Pololu that they can·run up to 9.6 volts, but my battery is 11.1 volts.·

    Odd, though that the smaller micro version can handle up to 2 amps per motor, where as the regular one is 1 amp continuous per motor.··So·the micro can handle less volts, but more amps. (assuming you run only 1 motor, it can handle up to 2 amps).

    As far as·volts go, it seems from the description that the motor controller also regulates the voltage to the motors?· Is this true?· Can I use the 11.1 volt battery and somehow in the code tell the motor controller to only allow 9.6 volts to the motor,·or whatever?

    (FYI, I plan to ask this in the Pololu forum, but am having trouble registering)
  • J.T.J.T. Posts: 31
    edited 2007-11-27 06:32
    I think I found a data sheet.· Not sure if this is good news...

    Load speed at 7.2V is 16200 RPM 0.50A. At maximum efficiency, RPM=14060 and current draw is 3.29A.

    At stall it says 21.6A on the current....


    ·
  • Mike GreenMike Green Posts: 23,101
    edited 2007-11-27 06:44
    The motor controller does not regulate the voltage to the motors. It regulates the motor speed by turning on the full voltage only part of the time so the average amount of power to the motor is some percent of full power.

    That stall current is pretty hefty and might quickly burn out the motor (and fry your motor controller unless it's an HB-25).
  • slamerslamer Posts: 52
    edited 2007-11-27 07:21
    It would be a bad idea to run a speed controller at a higher voltage than it's designed for. If it's rated for 9 VDC that is the maximum voltage you should ever feed it. If you motor is made to run on 11.1 VDC then you could possibly run the motor with a lower voltage say 9.6 VDC.·But you will sacrifice the motors top speed (RPM)·and it's power output will be a lot less.

    You should always try to get the stall current ratings of motors though, before sizing your speed controller. It varies greatly depending on the internal resistence of the motor. Carlo talks about it here. http://www.robotbooks.com/robot-motors.htm

    Of course these are huge motors by comparison·but the theory is the same. Also you may find that you can look up the motor(s) you have by searching for the RC car Model·you are using for parts. It's not widely known but Radio Shack often has listings for replacement parts for the cars that they sell.


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    undefinedundefinedundefinedundefinedSteven Kirk Nelson (slamer)

    Team K.I.S.S
    Build Safe, Build Mean, Build Strong!
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