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Trying rto figure out H-bridge controllers and encoders but its hard going — Parallax Forums

Trying rto figure out H-bridge controllers and encoders but its hard going

whiteoxewhiteoxe Posts: 794
edited 2014-05-02 06:25 in General Discussion
You can call me rigidigital or Whiteoxe, I have no idea why I called myself whiteoxe

I am struggling to find out how to use a H-bridge motor controller and an encoder. The idea is to drive two contiuos servo motors on a home made chassis. Ill have the wheels and chassis done in nless than two weeks. Then im gonna be back to motorcontrollers and encoders. I really dont know if the micro controller is going to be sending signals to the H-bridge or encoder or some other majic.

This is the latest on the subject i have found http://itp.nyu.edu/physcomp/Labs/DCMotorControl

Its not servo motors but im gonna build it anyway. The H-bridge there dosent look anything like the H-bridges ive been looking at all over the place. If you bother to follow the link, the H-bridge looks like a little IC but its is apparently a H-bridge motor controller. it looks like the encoders Im about to buy from sparkfun ?

and im posting this elsewhere in the hope of some quick replies

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-04-30 21:54
    If your "servo" motor is a continuous-rotation RC-type servo, then the H-bridge will not help you. These motors have their own built-in drive capability and respond only to logic pulses (1000us to 2000us) from the microcontroller. OTOH, if the motors are simple DC motors with built-in encoders, then an H-bridge is the appropriate controller.

    Which are they?

    -Phil
  • xanaduxanadu Posts: 3,347
    edited 2014-04-30 22:05
    H-Bridge > ADC > servo pulses > servo/encoder PID?
  • whiteoxewhiteoxe Posts: 794
    edited 2014-05-01 01:48
    Phil Pilgrim (PhiPi) wrote: »
    If your "servo" motor is a continuous-rotation RC-type servo, then the H-bridge will not help you. These motors have their own built-in drive capability and respond only to logic pulses (1000us to 2000us) from the microcontroller. OTOH, if the motors are simple DC motors with built-in encoders, then an H-bridge is the appropriate controller.

    Which are they?



    -Phil
    I have first looked art the ones sold at Parallax, they are possibly ewhat I'll get but just now I looked at these on ebay, very cheap and no postage

    http://www.ebay.com.au/itm/4-2-6-0V-Blue-Tower-Pro-SG-90-9g-Micro-Servo-w-Arms-for-RC-Helicopter-/390832532594?pt=AU_Toys_Hobbies_Radio_Controlled_Vehicles&hash=item5aff6f2c72#ht_2390wt_1105


    are encoders used to keep track of revolutions ?
  • T ChapT Chap Posts: 4,223
    edited 2014-05-01 03:26
    An encoder is one method to keep track of revolutions, but there are other methods as well. Keeping track of revolutions is one thing, keeping track of exact position is another. An encoder can allow you to have 200, 400, 800, 1600 steps of precision per revolution depending on which type you use. A lot of encoders have an index output as well so that you can track revolutions. There are methods of using a magnet attached to a shaft with a hall sensor that can check revolutions. The question really boils down to what are you trying to achieve. Then, after you decide what the goal is, then you think in terms of the mechanical attachment of the device. If you want an encoder so you can have precise positioning of the motor within a revolution, then you can use a US Digital type encoder with quadrature output, there is a rotary object in the examples that come with the Prop Tool, this requires 2 wires and will count the number of lines on the encoder. Then you can decide if the goal with an encoder is to have a closed loop so that a PID loop is keeping the motor at a precise position.

    Like I stated, first decide what is the goal.
  • AGCBAGCB Posts: 327
    edited 2014-05-01 05:54
    The parallax Standard servo is near double the price but also double the power FWIW
  • whiteoxewhiteoxe Posts: 794
    edited 2014-05-01 15:08
    Thanks everyone. I learned maybe most of what i needed to understand.

    mike.
    Ps. What do you think of these servos,will cost very lille for two where if i buy the ones on this site the cost will be $38, (thats $9.50 postage included) and they are for the lightweight robot that Duane gave directions for.

    These contiuous servos --> http://www.ebay.com.au/itm/4-2-6-0V-Blue-Tower-Pro-SG-90-9g-Micro-Servo-w-Arms-for-RC-Helicopter-/390832532594?pt=AU_Toys_Hobbies_Radio_Controlled_Vehicles&hash=item5aff6f2c72#ht_2390wt_1105 $6.94 each + Free postage
  • ercoerco Posts: 20,256
    edited 2014-05-01 17:15
    Those are standard 180-degree position servos, not continuous rotation.

    Continuous rotation servos are somewhat rare. Some but not all 9-gram standard servos can be modified for continuous rotation. Not sure about that particular Tower servo. Some cheap Ebay China servos only have a sector (half gear) on the output shaft, so they cannot be modified. The only one I have tried is the Hobby King HXT900, which is medium complexity to modify, but works very well for a robot drive servo. NOT for heavy robots by any means. The heavier the bot, the sooner it will wear out.
  • ellipserellipser Posts: 43
    edited 2014-05-02 06:25
    Do you want continuous rotation or only partial rotation?

    Encoders are typically used for continuous rotation. Those servo motors you linked to are typically for sector rotation. So when you say you'd like to use an encoder on a servo motor, it's not typically done that way, because you get position control already from the servo motor.
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