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Making a servo out of a regular motor... — Parallax Forums

Making a servo out of a regular motor...

Jorge PJorge P Posts: 385
edited 2011-05-25 11:17 in Robotics
Hi all,

I am interested in making a servo motor from scratch by using a normal·DC motor.· I have yet to find a schematic of the internal workings of any servo.

The battery and motor I am using are automotive, A car battery and a power window motor.· Napa Part# 655-1374 as it is all metal construction and fairly low cost for its power 105Kgf.cm, abd its common application in Ford vehicles.· I have been told this is a commonly stocked part and always available at Napa Autoparts Stores.· I intend on using limit switches or reed switches to keep track of armiture position of the servo, while still having the ability for continuase rotation.

Any schematics or links or guides·would be verry helpfull with consideration the the amp and volt requirement of the motor.

Thanks in advance,
Jorge

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http://WhatsAvailable.org Software and Gadgets for Windows 7.

Comments

  • John R.John R. Posts: 1,376
    edited 2010-03-17 19:05
    Check out this thread in the Sandbox: http://forums.parallax.com/showthread.php?p=884440

    And specifically this link: http://www.fieroaddiction.com/servo.html·from the post by Agent420.

    They basically hack the controller board from a "standard" servo, and attache it to a motor controller for a bigger motor.· They then hook up the "electronic guts" of the standard servo to the mechanical mechanism to get the feedback.

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    John R.
    Click here to see my Nomad Build Log
  • kwinnkwinn Posts: 8,697
    edited 2010-03-17 19:15
    A little more detail of the application would help. Is the motor to be just on/off or is it's speed controlled, does it go in one direction only or must it be able to go in reverse, what accuracy do you need for the armature position, and do you mean the motor armature position or the output shaft position?
  • John R.John R. Posts: 1,376
    edited 2010-03-17 19:17
    There are many ways of making a "servo" motor. Basically you're talking about a motor, some type of sensor, and contorlling logic and associated hardware.

    Hobby servos use a variable resistor on the output shaft as a sensor. Higher end servos use encoders (quadrature or absolute). Some systems use a linear sensor to monitor the final output of the motor.

    The controlling logic needs to know the desired position, and make a comparison, then send the appropriate voltage (polarity and amplitude) to the motor.

    Advanced systems may also apply a brake of some type if the motor is at the correct position and should remain stationary. In your case, I don't thing this will be necessary, as I believe that motor involves a worm gear reducer, and the load will not be able to drive the motor when the motor is "idle".

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    John R.
    Click here to see my Nomad Build Log
  • Jorge PJorge P Posts: 385
    edited 2010-03-17 19:27
    kwinn said...
    A little more detail of the application would help. Is the motor to be just on/off or is it's speed controlled, does it go in one direction only or must it be able to go in reverse, what accuracy do you need for the armature position, and do you mean the motor armature position or the output shaft position?
    The motor will go forward & reverse, and speed controll will be needed.· It will be used as not only a drive motor(no speed control required as I am using bycicle sprockets and geer change mechinism (shimano)), and it will also be used for steering (pitman arm style).· The bot will be·4 wheeled bot in a 2 wheel drive setup similar to a small go cart.
    John R said...
    In your case, I don't thing this will be necessary, as I believe that motor involves a worm gear reducer, and the load will not be able to drive the motor when the motor is "idle".
    Yes, this motor is configured like that.

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    http://WhatsAvailable.org Software and Gadgets for Windows 7.

    Post Edited (Jorge P) : 3/17/2010 7:32:31 PM GMT
  • LeonLeon Posts: 7,620
    edited 2010-03-17 20:07
    You can do away with the electronics in a model control servo, apart from the pot, and do everything in software by reading the pot with an ADC. You could do something similar by connecting a suitable pot to the motor.

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    Leon Heller
    Amateur radio callsign: G1HSM
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2010-03-17 20:35
    Sounds like you are trying to do something similar to the Parallax Motor Mount and Wheel kit:

    http://www.parallax.com/Store/Robots/RoboticAccessories/tabid/145/CategoryID/22/List/0/SortField/0/Level/a/ProductID/507/Default.aspx

    That should give you an idea on one way you could do it.

    Robert
  • Jorge PJorge P Posts: 385
    edited 2011-05-21 16:42
    Time to elaborate a bit more about what I am trying to do. The motors I am using are Automotive Power Window motors due to their torque and low cost.
    • Bot Frame - The frame is purchased from a tractor supply, it is a 4 wheel, hand pull utility wagon. much like to old radio flyers, only a bit bigger. The rear axel will be modified with the guts and sprokets from a rear rim of an 18 speed Mountain Bike
    • Drive System - One motor will rotate the main crank of the bicycle sprocket set. Geering is changed in the same manner as used on bycycle.
    • Stearing System - The second motor will be for the steering system. This motor needs the position feedback I am asking for help on.
    That is a basic summery of the base. I weighed the parts at about 40Lbs total for the bot platform base. It will end up being much heavier, about 130 to 150 Lbs with all the atachments.

    What I need help with is the Motor for steering, A pot will be the feedback for position, so in a sense I am turning the motor into a servo using something like the L293D H-Bridge IC. The H-Bridge will be controlled by the Propeller... Will I be able to do this with an H-Bridge if I use a POT for feedback of the position???

    I am bumping this thread as It still needs some feedback and is still a work in progress...
  • Martin_HMartin_H Posts: 4,051
    edited 2011-05-21 17:25
    In the book "Programming the Propeller in Spin" Harprit Sandhu has a chapter where he builds a servo motor using a pot for feedback and an H bridge to control a DC motor. It might be worth a look. He also has a section on stepper motor control using the same H bridge.
  • kwinnkwinn Posts: 8,697
    edited 2011-05-21 17:29
    Jorge P wrote: »
    Time to elaborate a bit more about what I am trying to do. The motors I am using are Automotive Power Window motors due to their torque and low cost.

    What I need help with is the Motor for steering, A pot will be the feedback for position, so in a sense I am turning the motor into a servo using something like the L293D H-Bridge IC. The H-Bridge will be controlled by the Propeller... Will I be able to do this with an H-Bridge if I use a POT for feedback of the position???

    I am bumping this thread as It still needs some feedback and is still a work in progress...

    Yes, you can control the motor with an H bridge if you use a pot for steering position feedback. If the pot is connected to the regulated power supply (I'm assuming a 3.3 or 5V input to the pot, but it could be something else) the pot wiper will have a range of motion of approximately 270 degrees. That should be more than the range of the steering mechanism. Ideally the "straight ahead" voltage would be half of the input voltage. Steering in one direction results in less than half the voltage, and in the other direction greater than half the voltage.
  • Jorge PJorge P Posts: 385
    edited 2011-05-21 22:27
    Great, 270 degrees would be twice what I need, I am guessing here but the steering is roughly 120 to 130 degrees of motion from left to right. I have all the parts, now I just need to get some welding experience, but I think I will pay someone with more experience so I don't screw up my project. Once I get this finished, I will list it in completed projects and try to give the dimensions and part numbers for everything used. It's already been about over year since I started this project and it will be a bit longer than a year more since i want to fully document it and have it look professional.

    Thanks for your help with this.
  • kwinnkwinn Posts: 8,697
    edited 2011-05-22 08:39
    You're welcome, and good luck with the project.

    BTW, you have several ways of reading the position from the pot. You can use an external adc chip or a sigma/delta adc using 2 prop pins to measure the wiper voltage, or an r/c circuit and measure the time constant of the pot/capacitor circuit. Turning will be simple. Going straight will be harder.
  • Jorge PJorge P Posts: 385
    edited 2011-05-24 16:36
    I think an R/C circuit would be more accurate than ADC wouldn't it??? With this eventually steering a 100+ lbs bot, I want the most precision as possible, this bot will be functioning around humans and pets so precision is my main focal point right now. I am also considering adding IR emitters and a sensor to detect the center position of the motor to keep the wheels straight when needed.
  • kwinnkwinn Posts: 8,697
    edited 2011-05-25 11:17
    Either one would be more than adequate for the task although an adc is probably more precise. The problem is more with calculating the wheel position required to get the robot to go where you want over variable terrain. If you think about how we steer a car or other vehicle, we look ahead and turn the steering wheel to point the vehicle towards our target destination. In essence we use the feedback from our visual system to make continuous corrections to our course to keep it centered on the destination. The control program for your robot will need to use the sensors available to do something similar. That may be as simple as using IR and/or ping sensors to avoid obstacles and compensate for terrain while maintaining a specific heading, or as complex as a full visual system with target recognition, course plotting, etc.
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