Hingejoint not responding on script

Question

I have a hingejoint setup. When I press a key I can see the number at the "target position" in the inspector change but the mesh it is attached to does not move. But when I type in a random number at "target position" the mesh does move.

The script attached is very basic:

 public float defaultPosition = 0.0f;
     public float targetPosition = 50.0f;
     public float springStrength = 100.0f;
     public float springDamper = 1.0f;

     private JointSpring spring;
     
     void Awake ()
     {
         GetComponent<HingeJoint>().useSpring = true;
         spring = new JointSpring();
         spring.spring = springStrength;
         spring.damper = springDamper;
     }
     
     void Update ()
     {            
         if (Input.GetKeyDown (KeyCode.D)) 
         {
             Debug.Log ("D pressed");
             spring.targetPosition = targetPosition;
         }
         else
             spring.targetPosition = defaultPosition;
         
         GetComponent<HingeJoint>().spring = spring;
     }

-edit- The problem is stranger then I thought. I removed the script component, added it again end it worked. But once I change a public variable in the inspector it breaks. Besides that if I re-attach the script component it does not always work, sometimes I have to re-attach it several times.

Answer 1

Answer by HarshadK · Apr 07, 2015 at 11:03 AM

Instead of creating a new Spring instance here, you need to get the current spring from the HingeJoint. Something like:

  void Awake ()
  {
      GetComponent<HingeJoint>().useSpring = true;
      // Get the spring from the HingeJoing
      spring = GetComponent<HingeJoint>().spring;
      spring.spring = springStrength;
      spring.damper = springDamper;
  }

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MadMenyo · Apr 07, 2015 at 12:51 PM 0

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Nope still the same result: nothing when I use the "D" key even with get$$anonymous$$ey ins$$anonymous$$d of get$$anonymous$$eyDown. And the damn thing is moving when I change the value manually in the inspector while running.

Answer 2

Answer by stephancom · Apr 09, 2015 at 04:31 AM

This appears to be a bug, I've created a github repository that illustrates the issue

https://github.com/stephancom/unity-hinge-bug

A very simple scene, when you walk into the collider, the spring should open the door, and it does not - the values change in the project, but there is no movement. Manually changing the values while it is running works.

How do I file a bug report, and do they pay bounties?

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Answer 3

Answer by kinggryan · May 12, 2015 at 09:48 AM

I've also encountered this and devised a workaround. I'm not sure, but it seems like changing the constraints of a joint do not cause the physics engine to calculate a solution for the scene. By "prodding" the object whose joint's constraints have been changed, you can force a solution. For me, after changing the constraints of a joint, I called AddForce(Vector3.zero) on the object's rigidbody. This caused the new constraints to be enforced.

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Answer 4

Answer by MarshCZA · Nov 06, 2018 at 02:00 PM

I've had the same problem and fixed it by calling

GetComponent().WakeUp();

After you change the spring values.

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Hingejoint not responding on script

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