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Thread: Suggest a Microcontroller for Delta Robot Project

  1. Post Suggest a Microcontroller for Delta Robot Project

    Hi every one!

    I have just started working on my delta robot project as part of my Mechatronics Engineering.

    As every one knows that delta robot is a parallel robot. Which means that three arms of delta robot must be controlled in parallel for the dynamics of robot. So for parallel motion of three servo motors of delta robot, there should a microcontroller capable of controlling the servos in parallel.

    First thing that came to my mind is "Arduino", but wait, can it control servos in parallel? While Arduino is not a parallel microcontroller. Although it can work as parallel using multi-tasking with occam-pi or some other embedded OS but that is very difficult to learn and still not able to drive servo motors in parallel (according to my thinking).

    I have found some delta robots built using Arduino, using "writeMicrseconds" technique but that is not a solution for parallel control of servo motors.

    As I have searched over the internet I have found two parallel microcontrollers:
    1. Parallax Propeller P8X32A-Q44 (32-bit, 8 core Parallel microcontroller) for only $7.
    2. xCore Multicore Microcontroller (ARM Based) from XMOS with pretty good price tags.

    But Arduino can also serve the purpose as there are delta robots built using Arduino.

    Now, I am confused which microcontroller should I choose. I have worked with Arduino in some projects. I have studied about Parallax Propeller and it seems to be very easy to learn (Propeller SPIN).

    Actually, what I want to do is to calculate the robot dynamics on MATLAB and then serially send the angles to the microcontroller which in turn should drive the three servos of delta robot.

    Please some one guide me!

  2. #2
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    Re: Suggest a Microcontroller for Delta Robot Project

    What type of servo?

    The ATmega microcontrollers in various Arduino boards have been used to simultaneously control 32 PWM servos (lynxmotion SSC-32), 20+ Dynamixel servos (Vanaduim/Trossen arbotix, Arduino Mega, Robotis CM-5/510/700, etc.), and control 4+ stepper motors (assorted RepRap boards with Pololu stepper motor drivers).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  3. Re: Suggest a Microcontroller for Delta Robot Project

    Three Digital servo motors each with 210 degrees angle limit. And all the three with different position values but running in parallel.

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    Re: Suggest a Microcontroller for Delta Robot Project

    'Digital servo motors' is not a very useful description. Are they controlled by the 'hobby R/C' PWM scheme, a serial command protocol, a parallel data interface? The only real question is how many pins are required to control the three servos, because the operating frequencies of even the cheap/old 8-bit ATmegas are far higher than the control loop update rate used in most non-industrial servos. Basically, the microcontroller will be able to change all the pin outputs far faster than the servos will bother to check their inputs.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  5. #5

    Re: Suggest a Microcontroller for Delta Robot Project

    Note that "parallel" really means "parallel enough."
    Compared to the 20 ms typical update cycle of an RC servo the 3 ms slew between first and last PWM command may be indistinguishable.

    Alternatives include a custom servo driver code which turns off interrupts and writes the digital pins in parallel as PORTC or whatever, and uses a microsecond timer to figure out when to turn each pin on to make them all turn off at the same time.
    Or use a servo kind which supports synchronous serial control, like the AX12 or MX28. You can queue commands and broadcast a "do it now" command. Arbotix and OpenCM work well with those servos.

  6. Re: Suggest a Microcontroller for Delta Robot Project

    I am thinking to buy this servo:

    http://www.makeblock.cc/meds15-servo-motor/

    It has limit angle of 210 degrees and dead band of 5usec.

    So for 1500 - 2000 usec, Control will be as follows:

    (2000-1500)/5 = 100usec

    (210/2)/100 = 1.05 degrees

    so basically I will get control of 1.05 degree based on dead band of 5usec!

    Am I right?

  7. #7

    Re: Suggest a Microcontroller for Delta Robot Project

    1.05 degrees is very poor control, especially for a delta robot.
    Do the math: sin 1.05 degrees times 30 cm (or whatever each spoke is) is a farily long quantum!

    A more precise (and generally better) servo, for not much more money, would be one of these:
    http://www.trossenrobotics.com/p/ax-...or-6-pack.aspx
    http://www.robotshop.com/en/herkulex...bot-servo.html

  8. Re: Suggest a Microcontroller for Delta Robot Project

    Quote Originally Posted by jwatte View Post
    1.05 degrees is very poor control, especially for a delta robot.
    Do the math: sin 1.05 degrees times 30 cm (or whatever each spoke is) is a farily long quantum!l
    I could not understand the math. sin(1.05)x45cm = 0.82 degree.cm (45 link length for my robot)
    It's not good? I mean Control resolution of 0.82 degree.cm is low? Then how much is suitable?

  9. Re: Suggest a Microcontroller for Delta Robot Project

    Quote Originally Posted by jwatte View Post
    A more precise (and generally better) servo, for not much more money, would be one of these:
    http://www.trossenrobotics.com/p/ax-...or-6-pack.aspx
    http://www.robotshop.com/en/herkulex...bot-servo.html
    I think dynamixel is not a good option for me. I am new to servos, and a straight jump to dynamixel can be quite hard for me.

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    Re: Suggest a Microcontroller for Delta Robot Project

    8.2mm steps in position control, at best, is good? Granted, even the AX-12 will have only ~2.3mm steps with a 45cm arm. What do you think any hobby servos will actually be able to accomplish with control resolution that poor and usable torque levels that low (16[kg-cm]*(1/5) / 45[cm]=0.071[kg])?
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

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