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I am in the process of carrying out my thesis, I am using Webots software to measure the displacement of linear actuators and then carry out PID control for a manipulator of an ROV which is based on an excavator design, I have reconstructed the arm using shapes and transforms, as imported CAD models caused errors.

Once the parts were reconstructed then I connected everything with hinge joints with appropriate anchor coordinates, then by applying physics with actual mass of each component acquired from data sheets I tested the functionality, every time I added and connected each part.

Upon adding the arm actuator, connecting it with appropriate anchor coordinates which I double checked, then applying physics the actuator started to act unnatural, instead of falling down with the arm, as there is no force applied to the actuators yet it moves in the other direction. I have also noticed that the other actuators and their hinges are not at the correct anchor points.

I have tried to fix it but nothing seems to work apart from changing the gravity at which point the hinge joints stay 'semi-fixed', once I move the actuators it seems like the joints are fixed on a string or a spring and the actuators lift above the boom part and then the boom follows.

This is my first time using a robotics simulation software so it might just be lack of experience and knowledge, or might it be some parameter and or the large weight?

Screenshot

I have recorded a quick video as well showing the issue.

Glorfindel
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kuba
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1 Answers1

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Your model needs some calibration because it uses heavy masses. By default Webots can handle lightweight models properly, but if you are using an heavyweight model, you should adapt some parameters in particular:

  1. The WorldInfo.CFM value should be reduced and the WorldInfo.ERP should be adapted, see explanations here.
  2. If you use ContactProperties, you should also adapt the softCFM/softERP there.
  3. You may also need to adjust some parameters with joints, such as the spring and damper properties and the stopERP and stopCFM of the HingeJointParameters.

Disclaimer: I am a Webots developer working at Cyberbotics.

Olivier Michel
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  • This answer fixed the discussed issue, but upon further development - adding more actuators, they do not function properly, I will add a video to help visualise my problem. The two boom actuators function properly, the arm actuator seems like it is 'inverted' so when it extentds the arm goes down as it should but the actuator 'disattaches' and moves up. The boom actuator does not work properly at all. I have used the same method to create all these actuators. I checked all of the anchor points and they are correct. Too complicated hinge joints,or wrong geometry? https://youtu.be/zg8bPrNQG5A – kuba Mar 03 '23 at 16:13