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Arm Movement Backwards
12-15-2019, 09:20 PM (This post was last modified: 12-15-2019 09:21 PM by stefanscott.)
Post: #1
Arm Movement Backwards
Hi.

I'm a high school teacher and our Robot club has been working on a competition project that we are hoping to include the PyArm into our solution. The students have assembled the arm (we're using 50cm rails and lead screw), and have been able do some basic operation with the pyBot app as well as the Blockly blocks. However, we've noticed an unanticipated behavior; all the stepper motion seem to be mirrored.

For instance, when using the app, we've oriented the arm to face away from us. If we click on the upper-left area of the app (let's say 45 degrees to the left of vertical, so the arm would be straight), the robot arm movement is opposite (ends up 45 to the right of vertical).

Similarly, when changing the z-values, a higher positive value brings the robot arm down (even though the illustration in the app goes up). To reach the top of the platform, we needed to use the Blockly editor and give a value of around -300mm I believe.

The stepper cables seem to match the orientation of the illustrations (blue wires toward the side with the ESP chip). Any idea what we're missing?

Also - can the gripper be controller through the app? I haven't been able to spot where that control would be.
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12-16-2019, 12:41 PM
Post: #2
RE: Arm Movement Backwards
Are you using A4988 stepper motor drivers? Using DRV or similar will change the spinning direction of the motors. Check if the MOTOR1 belt is rotating the right GEAR and the same for the MOTOR2.
If you do not find the source of the problem, just change the polarity of the motor cable, connect it backwards: just rotate the cable connector 180º and re-connect it. That will change the spinning direction of the motor. Simple but practical.

The gripper: yes, it can be controlled. In REAL TIME MODE: If you press and hold the mouse´s wheel button and shift the mouse to the right-left the gripper will rotate accordingly. The left button will open/close the gripper.
TRAJECTORY MODE: use the left and right keyboard buttons to rotate the gripper. Left mouse button to open/close the clamp.

Let me know if that worked :-)
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12-17-2019, 10:51 PM
Post: #3
RE: Arm Movement Backwards
Thanks! Reversing each stepper cable at the devia board did indeed fix that issue. We still have some issues with getting the servos to work correctly. When the board is powered on the gripper opens and closes at the end of the calibration sequence, but when operating in trajectory or real time mode we aren't getting any motion. Not from the blockly interface either.

The servos were also backwards at some point, and I know those 9g servos can burn out. Since the artist doesn't move during calibration, I think might be its problem. We'll try testing it on its own tomorrow.

I should say we are seeing the gripper illustration move in the app, just not on the arm itself. Am I right in assuming the motion should happen immediately after the left click?
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12-20-2019, 06:41 PM
Post: #4
RE: Arm Movement Backwards
(12-17-2019 10:51 PM)stefanscott Wrote:  Thanks! Reversing each stepper cable at the devia board did indeed fix that issue. We still have some issues with getting the servos to work correctly. When the board is powered on the gripper opens and closes at the end of the calibration sequence, but when operating in trajectory or real time mode we aren't getting any motion. Not from the blockly interface either.

The servos were also backwards at some point, and I know those 9g servos can burn out. Since the artist doesn't move during calibration, I think might be its problem. We'll try testing it on its own tomorrow.

I should say we are seeing the gripper illustration move in the app, just not on the arm itself. Am I right in assuming the motion should happen immediately after the left click?
It should move the same moment you rotate or open gripper when you are in "real time", that is a good way to test it. But during the calibration, both servos should move too. Check if the plastic part (gripper base) is not touching the forearm not leaving it to rotate freely.
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