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HB-25 and robot arm. Axis drifting. — Parallax Forums

HB-25 and robot arm. Axis drifting.

I have 6 HB-25 on my Scorbot robot arm.

I'm using some simple Arduino joystick programming to control each driver with PWM.

When the drivers have no power the arm axis's are difficult to move manually. The are all brushed dc motors with harmonic gearboxes.

When the drivers are powered the axis's affected by gravity drift down. ( shoulder, elbow, wrist).

The base rotate does not move as gravity has no effect on it.

The joystick program on the Arduino works like its suppose to.

Anybody have any ideas??


Comments

  • Video of arm drifting.
    Tha axis moves normally when using the joystick.
    When I stop it’s starts to dtift down.

    https://youtube.com/shorts/NOzJKhSMeSk?feature=share

  • I'm REALLY interested in learning about this robot arm. I haven't seen anything like this.

    Is this a brushed motor and actual harmonic drive? Or is it a stepper motors? What is the encoder resolution?

    It looks like powder coated cast metal. No?

  • JonnyMacJonnyMac Posts: 8,311

    Do you have encoders on the joints?

  • jdoleckijdolecki Posts: 717
    edited 2022-11-03 01:11

    @The_Master said:
    I'm REALLY interested in learning about this robot arm. I haven't seen anything like this.

    Is this a brushed motor and actual harmonic drive? Or is it a stepper motors? What is the encoder resolution?

    It looks like powder coated cast metal. No?

    It’s Scorbot from a university

    Yes all metal dc motors with harmonic drives.

    They are on ebay but everyone thinks they are made of gold so they asking alot.

    The controller is obsolete.

    https://intelitek.com/

  • @JonnyMac said:
    Do you have encoders on the joints?

    Yes a/b with index pulse.

    Home switches are hard wired into the motors.

  • I haven't use the HB-25 specifically, but have used other cheaper motor driver board.

    There were 4 modes: R rotate, L rotate, Off (open), Off (shorted)

    The last two modes made a BIG difference in performance. Off (motor shorted) provides significant braking. Maybe that will help you here?

  • @The_Master said:
    I haven't use the HB-25 specifically, but have used other cheaper motor driver board.

    There were 4 modes: R rotate, L rotate, Off (open), Off (shorted)

    The last two modes made a BIG difference in performance. Off (motor shorted) provides significant braking. Maybe that will help you here?

    What driver boards did you use?

  • just google: L298N You can get a 4-pack of driver boards for $11-$12

    I just read the specs they can do 24V 18amps so will probably even work

    If you know of any good youtube videos showing what this thing is capable of please post them. Im interested in the top performance this servomotor arm was capable of at those mass and power levels

  • MicksterMickster Posts: 2,304

    @jdolecki

    So it drifts due to gravity but it's not actually losing track of position, right?

    IOW, when you grab control in manual jog, the joint angles are still correct?

    It sounds to me that it has torque limiting (aka: soft-servo) which enables the joints to be compliant. When not actually being commanded to move, it makes sense to allow only enough current (motor torque) to hold position. Seems like the torque limiting is set too low.
    Collaborative robots (aka: Cobots) use this technique (plus sensors) so that they don't need protective cages and can work side-by-side with humans.

    Craig

  • MicksterMickster Posts: 2,304

    @The_Master said:
    just google: L298N You can get a 4-pack of driver boards for $11-$12

    I just read the specs they can do 24V 18amps so will probably even work

    I would look at some of the really nice drivers on Aliexpress.

    Craig

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