convert resistance to motion

Discussion in 'Embedded Systems and Microcontrollers' started by dhanurbhagat, Jul 2, 2010.

  1. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    I need help in my major project.
    I need to measure multiple variable resistances simultaneously and convert their corresponding resistances into movement by servo motors and actuators.

    Which microcontroller should I use and can it be done using only one microcontroller or would I have to use multiple microcontrollers?
     
  2. hgmjr

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    Jan 28, 2005
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    How many servos do you need to control with this microcontroller?

    The Arduino Duemilanove board would be a easy microcontroller to use if your need is to get up and running in a hurry.

    Do you have any programming experience or are you going to need to scale that learning curve?

    hgmjr
     
  3. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    I'm pretty much a n00b, so am learning programming of microcontrollers.
    I have to use 9 servos at LEAST, according to my current design. The number could go up to 19 unless I find a way to use either actuators, then it would still be 9 servos and 10 actuators.
    I will be needing to take input from an equal number of variable resistances (19 at least).

    Considering these numbers, should I still go for The Arduino Duemilanove board or would you recommend something else?
     
  4. hgmjr

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    Nope, an Arduino Duemilanove does not have enough IO lines to accommodate that many servos together with the potentiometer inputs.

    By actuator, are you refering to a linear actuator?

    You should consider using the Arduino Mega for your application. The Arduino Mega is endowed with more than enough IO lines. It is limited to 16 A-to-D channels which leaves you 3 channels short of the 19 you are looking for to support your 19 potentiometer inputs. As for supporting 19 servos (if that is one of your requirements) then you would need to introduce some multiplexing (If you use the Arduino open source servo.h software application) since this software only supports up to 8 servos. Of course if you are ambitious you could write your own servo driver. However, since you have stated that you are a newbie then you need to keep it simple.

    The Arduino Mega at $65 USD is slighty more than twice the price of the Arduino Duemilanove.

    hgmjr
     
    Last edited: Nov 20, 2010
    dhanurbhagat likes this.
  5. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    Yes, a linear actuator is exactly what I meant. They can be interfaced just like servos, right?

    The Arduino Mega seems just what the doctor ordered! :)

    Another thing I would like to ask is that is there any servo mechanism that could imitate the "knuckle joint" similar to the one in our hands?
     
  6. hgmjr

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    Controlling the linear actuators with which I am familiar is distinctly different from controlling servos. A servo uses the general 1 to 2 millisecond pulse that is repeated at 20 milliseconds intervals. Whereas controlling a linear actuator involves driving a dc motor with an H-bridge. The positional feedback from the linear actuator typically includes two limit switches that are used to indicate when the actuator has reached either of its two extremes of travel as well as the output from a potentiometer that is used to indicate the position of the linear actuator within its full range of motion.

    hgmjr
     
  7. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    It seems mine using actuators isn't going to be a cakewalk.
    Does the involvement of actuators put a limit to the number of output devices (servos/actuators) I can attach to the microcontroller?
     
  8. hgmjr

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    Jan 28, 2005
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    I would say yes to your question. I say yes because each linear actuator requires you to provide 2 inputs to monitor the two limit switches, 1 a-to-d input for monitoring the position potentiometer and 2 control lines for the H-bridge (one for direction and one for motor-enable). A servo only requires one control output.

    If you need to use a linear actuator then it is what it is. If you have the option to use a servo then you can use them and perhaps get rid of the need for the potentiometer inputs altogether.

    hgmjr
     
  9. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    Considering the limitations, would you recommend using more than one microcontroller for the task? Or would it increase the complexity of the system?
    I really need to use at least 15 servos (a conservative estimate) even if I keep the design of the project to the simplest.
     
  10. hgmjr

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    Jan 28, 2005
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    If you use more than one microcontroller you will most likely need to provide a means of connecting the two together so that each knows what state the other is in. I would suggest for simplicity that you avoid multiprocessing since you are still in the learning mode.

    It sounds like you are building some sort of robot like a hexapod. I am in the early stages of a hexapod robot so I am familiarizing myself with the various leg manipulation schemes that are needed.

    I am starting with an Arduino Duemilanove but I may need to graduate to an Arduino Mega before the project is completed.

    hgmjr
     
  11. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    Well, I'm actually planning to build a robotic hand, that would imitate and replicate the movements in my hand. That is precisely the reason I need 19 potentiometers for the 19 major joints in the human hand, and an equal number of servos/actuators to control it.
     
  12. hgmjr

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    Jan 28, 2005
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    That sounds very interesting. The hexapod I am designing has 6 legs and each leg has 3 servos.

    The smallest and cheapest servo that I have found for my use is an HXT900. Have you looked at any servos yet?

    hgmjr
     
  13. dhanurbhagat

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    Jul 2, 2010
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    I'd really like to thank you for mentioning that you're making a hexapod yourself!
    I've been having sleepless nights for days now thinking of how to make the knuckle joint, and your hexapod gives me the solution for the joint!
    Thank you so much hgmjr! :)
     
  14. hgmjr

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    There are some very good examples of the efforts of others to design a hexapod robot on youtube. Well worth taking a look to give you some good ideas.

    hgmjr
     
  15. dhanurbhagat

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    Jul 2, 2010
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    Would your hexapod be moving independently at a pre-defined pace or would it be manually controlled or would it automatically detect its environment and navigate accordingly?
     
  16. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    Is the HXT900 servo the smallest available out there?
    Are there any smaller servos out there (even at the cost of their power)?
    I need to keep the size of servos as small as possible.

    I'll definitely look up the videos on youtube!
     
  17. hgmjr

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    Jan 28, 2005
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    It is early yet in the evolution of my design. My current thinking is to make it autonomous. I don't want to have it dragging wires around nor do I want to get involved in wireless at this point. I will most likely be looking at ultrasonic sensing to monitor its environment for collision avoidance.

    hgmjr
     
  18. dhanurbhagat

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    Jul 2, 2010
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  19. hgmjr

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    Jan 28, 2005
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    Here is a link to the specs on the HXT900. It is only very slightly larger than the one you are looking. The good thing is that it has almost 3 times the torque at 4.8V. You may not need torque if all you do is move the hand in unison with your own hand.

    I bought 3 of them from www.solarbotics.com and they seem to work just fine with my Arduino Duemilanove.

    hgmjr
     
  20. dhanurbhagat

    Thread Starter New Member

    Jul 2, 2010
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    Why is there such a big difference between the prices at both the websites?
    The HobbyKing one says $2.49 each and Solarbotics is selling it for $5.85 a piece! Thats almost double!
     
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