Robotic Arm Project with stepper motors

Discussion in 'The Projects Forum' started by ekw, Dec 3, 2009.

  1. ekw

    Thread Starter New Member

    Dec 3, 2009
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    Hello first time poster,

    I am a student building a robotic arm but I am having difficulties with these motors my partner bought.

    http://www.kelinginc.net/KL23H256-21-8B.pdf

    I told him to get something that can handle around 2-3 lbs load, and he went off to buy two of these on impulse. They are quite heavy (1.5 lbs each) and required a redesign of the original arm.

    I'm new to stepper motors and I have no idea how to meet the current requirements to even operate the motor. I was thinking of using a power transistor and have my PIC16F887 to control it, but then again there lies a problem with how to find a battery to output that many amps? Is there a better motor for this application out there?

    I just want to make sure I'm heading in the right direction before we waste more money on things we would need more money to use! Any help/advice will be greatly appreciated.
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    The easiest way is to connect as a unipolar motor, use low side switches, perhaps logic level mosfets to drive the motor, the PIC should be able to drive them directly. I would probably use a 5 volt drive. The current needs to be limited to 2.1 amp, by a series resistor or perhaps pwm. This is just a broad outline, and there are other approaches.

    There are also driver circuits for motors.

    In any case read the spec. sheet to make sure 5 volt drive ( assuming your are using a 5 v uC ) is enough and that the current rating is not exceeded.

    Find out what snubber diodes are and make sure they are in place.
     
  3. ekw

    Thread Starter New Member

    Dec 3, 2009
    8
    0
    Thankyou for your response.

    It caused a chain of thinking that lead me to a better understanding!

    I think i know what to do now,

    thanks again
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Keep in mind that for maximum torque, you will need to wire them as bipolar.

    If wired as unipolar steppers, they will only have 127 oz-in holding torque.
    127/16 = 7.9375 lbs-inches, or about .66 foot-lb of holding torque.
    If wired as bipolar steppers, they will have up to 185 oz-in holding torque.
    185/16 = 11.5625 lbs-inches, or about .96 foot-lb of holding torque.

    If you need 1-1/2 foot-lb of torque, you will need larger motors, or use counterbalances or countersprings on the arms to help reduce the required torque.
     
  5. ekw

    Thread Starter New Member

    Dec 3, 2009
    8
    0
    I've been trying to get the stepper motor to spin via full step and unipolar configuration.

    I can feel it pulsing, but the shaft isn't turning. I thought it just lacked current, so I pushed what mosfets i had to the limit (200mA, was running at 80-100mA) and ended up burning them out.

    Before I go buy some power mosfets that can handle greather than 200mA, I am just wondering if current is the issue and not the motor.

    Thanks
     
  6. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
  7. ekw

    Thread Starter New Member

    Dec 3, 2009
    8
    0
    Yeah the leads are correct. I just confirmed the ordering by manually placing power on the pins (wave drive) and it turned!

    Thanks for the diagram! Unfortunately I have to stick with discrete transistors.

    Just some info so that some newbies like me won't make the same mistake,
    I decided to spike the low power rated mosfet (2n7000) past its rated ID, Got the motor to turn. But as i kept testing, I must have spiked it too high or to long; the heat damaged the oxide layer and caused the gate rupture. I didn't have a resistor between the microcontroller and the gate of the mosfet. Microcontroller got hot, but no damages happened.

    Lesson learned!
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You need logic level power MOSFETs, or Darlington transistors.
    Your motors require at least 1.5A, or as much as 3.0A, depending on how you connect them. See the datasheet.

    Choose MOSFETS or Darlingtons that are rated for at least twice the current you require.
    The 2N7000 MOSFETs you used were woefully inadequate.

    You must not exceed the rated current of the MOSFETS, transistors, or stepper motors, or you will burn them up.

    You can use N-channel logic level power MOSFETs or NPN Darlington transistors.
    You will need heat sinks on Darlingtons. If you select MOSFETs rated for 10A, you might get away without using heat sinks.

    You must limit the current from the PIC uC I/O pins to 20mA maximum using suitable resistors.
     
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