Applying torque to joined actors.

georgatos7 · 02-25-2015 06:54 PM

I have gone through the tutorials on how to use joints and on how to apply forces, torque etc on actors.

I have made a test hinge joint on two parts and i have applied torques to their actors but i haven't found a way to apply torque in relation to the joint and the two joined actors.

[Image: Joint.png]

Trying to apply torque on each actor like it is a standalone entity is going to have a very different simulation outcome than applying it based on the joint and in relation to the two joined actors.

Is this possible in the current physX implementation?

3DRaddict · 02-25-2015 10:55 PM

Not too sure of your particular setup, but here are a few pointers you might like to try, if you have not done so already.

1) If the joint structure is to be in a fixed position then make the fixed actor to be static and the hinged actor to be dynamic

2) If the joint structure is to be moveable (eg a moveable platform) then make the moveable actor to be kinematic and the hinged actor to be dynamic
Note that a kinematic actor can only be moved via 'kinematicMoveTo()' and not via applied force like a dynamic actor.

3) If both actors are dynamic then try making the one actor significantly heavier than the other.

4) Fix the one actor to the floor via a fixed joint. (use NULL for the floor actor pointer)

When one body is significantly heavier than the other, make the lighter body the second actor in the joint. Similarly, when one of the objects is static or kinematic (or the actor pointer is NULL) make the dynamic body the second actor.

**Esenthel** · 02-25-2015 11:06 PM

So what you want is a method that applies 'torque' at specified position and not the center of mass if I understand it right?
In that case I think that would be a combination of angular velocity + linear velocity.

For example if you're applying a 'force' at a specified position

then PhysX does something like this:

Code:

PX_INLINE void addForceAtPosInternal(PxRigidBody& body, const PxVec3& force, const PxVec3& pos, PxForceMode::Enum mode, bool wakeup)

{

    if(mode == PxForceMode::eACCELERATION || mode == PxForceMode::eVELOCITY_CHANGE)

    {

        //ESENTHEL Ps::getFoundation().error(PxErrorCode::eINVALID_PARAMETER, __FILE__, __LINE__, "PxRigidBodyExt::addForce methods do not support eACCELERATION or eVELOCITY_CHANGE modes");

        return;

    }

    const PxTransform globalPose = body.getGlobalPose();

    const PxVec3 centerOfMass = globalPose.transform(body.getCMassLocalPose().p);

    const PxVec3 torque = (pos - centerOfMass).cross(force);

    body.addForce(force, mode, wakeup);

    body.addTorque(torque, mode, wakeup);

}

Applies both linear force and torque.

So I think doing with torque at custom position would require applying both torque and linear force.
But at the top of my head I don't know the formula for the force/torque vectors/intensities.

**Esenthel** · 02-25-2015 11:10 PM

Or perhaps you want to apply some modifications to the joint itself, from PhysX PxRevoluteJoint.h header:

Code:

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//

// Copyright (c) 2008-2014 NVIDIA Corporation. All rights reserved.

// Copyright (c) 2004-2008 AGEIA Technologies, Inc. All rights reserved.

// Copyright (c) 2001-2004 NovodeX AG. All rights reserved.  

#ifndef PX_REVOLUTEJOINT_H

#define PX_REVOLUTEJOINT_H

/** \addtogroup extensions

  @{

*/

#include "extensions/PxJoint.h"

#include "extensions/PxJointLimit.h"

#ifndef PX_DOXYGEN

namespace physx

{

#endif

class PxRevoluteJoint;

/**

\brief Create a revolute joint.

 \param[in] physics the physics SDK

 \param[in] actor0 an actor to which the joint is attached. NULL may be used to attach the joint to a specific point in the world frame

 \param[in] localFrame0 the position and orientation of the joint relative to actor0

 \param[in] actor1 an actor to which the joint is attached. NULL may be used to attach the joint to a specific point in the world frame

 \param[in] localFrame1 the position and orientation of the joint relative to actor1 

@see PxRevoluteJoint

*/

PxRevoluteJoint*    PxRevoluteJointCreate(PxPhysics& physics, 

                                          PxRigidActor* actor0, const PxTransform& localFrame0, 

                                          PxRigidActor* actor1, const PxTransform& localFrame1);

/**

\brief Flags specific to the Revolute Joint.

@see PxRevoluteJoint

*/

struct PxRevoluteJointFlag

{

    enum Enum

    {

        eLIMIT_ENABLED            = 1<<0,        //< enable the limit

        eDRIVE_ENABLED            = 1<<1,        //< enable the drive

        eDRIVE_FREESPIN            = 1<<2        //< if the existing velocity is beyond the drive velocity, do not add force

    };

};

typedef PxFlags<PxRevoluteJointFlag::Enum, PxU16> PxRevoluteJointFlags;

PX_FLAGS_OPERATORS(PxRevoluteJointFlag::Enum, PxU16)

/**

\brief A joint which behaves in a similar way to a hinge or axle.

 A hinge joint removes all but a single rotational degree of freedom from two objects.

 The axis along which the two bodies may rotate is specified with a point and a direction

 vector.

 The position of the hinge on each body is specified by the origin of the body's joint frame.

 The axis of the hinge is specified as the direction of the x-axis in the body's joint frame.

 \image html revoluteJoint.png

 A revolute joint can be given a motor, so that it can apply a force to rotate the attached actors.

 It may also be given a limit, to restrict the revolute motion to within a certain range. In

 addition, the bodies may be projected together if the distance or angle between them exceeds

 a given threshold.

 Projection, drive and limits are activated by setting the appropriate flags on the joint.

 @see PxRevoluteJointCreate() PxJoint

*/

class PxRevoluteJoint : public PxJoint

{

public:

    /**

    \brief return the angle of the joint, in the range (-Pi, Pi]

    */

    virtual PxReal getAngle()                            const            = 0;

    /**

    \brief return the velocity of the joint

    */

    virtual PxReal getVelocity()                        const            = 0;

    /**

    \brief set the joint limit parameters. 

    The limit is activated using the flag PxRevoluteJointFlag::eLIMIT_ENABLED

    The limit angle range is (-2*PI, 2*PI) and the extent of the limit must be strictly less than 2*PI

    \param[in] limits The joint limit parameters. 

    @see PxJointAngularLimitPair getLimit()

    */

    virtual void            setLimit(const PxJointAngularLimitPair& limits)            = 0;

    /**

    \brief get the joint limit parameters.

    \return the joint limit parameters

    @see PxJointAngularLimitPair setLimit()

    */

    virtual PxJointAngularLimitPair getLimit()            const            = 0;

    /**

    \brief set the target velocity for the drive model.

    The motor will only be able to reach this velocity if the maxForce is sufficiently large.

    If the joint is spinning faster than this velocity, the motor will actually try to brake

    (see PxRevoluteJointFlag::eDRIVE_FREESPIN.)

    If you set this to infinity then the motor will keep speeding up, unless there is some sort 

    of resistance on the attached bodies. The sign of this variable determines the rotation direction,

    with positive values going the same way as positive joint angles.

    \param[in] velocity the drive target velocity

    <b>Range:</b> [0, PX_MAX_F32)<br>

    <b>Default:</b> 0.0

    @see PxRevoluteFlags::eDRIVE_FREESPIN

    */

    virtual void            setDriveVelocity(PxReal velocity)            = 0;

    /**

    \brief gets the target velocity for the drive model.

    \return the drive target velocity

    @see setDriveVelocity()

    */

    virtual PxReal            getDriveVelocity()            const            = 0;

    /**

    \brief sets the maximum torque the drive can exert.

    Setting this to a very large value if velTarget is also very large may cause unexpected results.

    The value set here may be used either as an impulse limit or a force limit, depending on the flag PxConstraintFlag::eDRIVE_LIMITS_ARE_FORCES

    <b>Range:</b> [0, PX_MAX_F32)<br>

    <b>Default:</b> PX_MAX_F32

    @see setDriveVelocity()

    */

    virtual void            setDriveForceLimit(PxReal limit)            = 0;

    /**

    \brief gets the maximum torque the drive can exert.

    \return the torque limit

    @see setDriveVelocity()

    */

    virtual PxReal            getDriveForceLimit()        const            = 0;

    /**

    \brief sets the gear ratio for the drive.

    When setting up the drive constraint, the velocity of the first actor is scaled by this value, and its response to drive torque is scaled down.

    So if the drive target velocity is zero, the second actor will be driven to the velocity of the first scaled by the gear ratio

    <b>Range:</b> [0, PX_MAX_F32)<br>

    <b>Default:</b> 1.0

    \param[in] ratio the drive gear ratio

    @see getDriveGearRatio()

    */

    virtual void            setDriveGearRatio(PxReal ratio)            = 0;

    /**

    \brief gets the gear ratio.

    \return the drive gear ratio

    @see setDriveGearRatio()

    */

    virtual PxReal            getDriveGearRatio()        const            = 0;

    /**

    \brief sets the flags specific to the Revolute Joint.

    <b>Default</b> PxRevoluteJointFlags(0)

    \param[in] flags The joint flags.

    @see PxRevoluteJointFlag setFlag() getFlags()

    */

    virtual void                    setRevoluteJointFlags(PxRevoluteJointFlags flags) = 0;

    /**

    \brief sets a single flag specific to a Revolute Joint.

    \param[in] flag The flag to set or clear.

    \param[in] value the value to which to set the flag

    @see PxRevoluteJointFlag, getFlags() setFlags()

    */

    virtual void                    setRevoluteJointFlag(PxRevoluteJointFlag::Enum flag, bool value) = 0;

    /**

    \brief gets the flags specific to the Revolute Joint.

    \return the joint flags

    @see PxRevoluteJoint::flags, PxRevoluteJointFlag setFlag() setFlags()

    */

    virtual PxRevoluteJointFlags    getRevoluteJointFlags(void)                    const    = 0;

    /**

    \brief Set the linear tolerance threshold for projection. Projection is enabled if PxConstraintFlag::ePROJECTION

    is set for the joint.

    If the joint separates by more than this distance along its locked degrees of freedom, the solver 

    will move the bodies to close the distance.

    Setting a very small tolerance may result in simulation jitter or other artifacts.

    Sometimes it is not possible to project (for example when the joints form a cycle).

    <b>Range:</b> [0, PX_MAX_F32)<br>

    <b>Default:</b> 1e10f

    \param[in] tolerance the linear tolerance threshold

    @see getProjectionLinearTolerance() PxJoint::setConstraintFlags() PxConstraintFlag::ePROJECTION

    */

    virtual void                setProjectionLinearTolerance(PxReal tolerance)                    = 0;

    /**

    \brief Get the linear tolerance threshold for projection.

    \return the linear tolerance threshold

    @see setProjectionLinearTolerance()

    */

    virtual PxReal                getProjectionLinearTolerance()            const                    = 0;

    /**

    \brief Set the angular tolerance threshold for projection. Projection is enabled if 

    PxConstraintFlag::ePROJECTION is set for the joint.

    If the joint deviates by more than this angle around its locked angular degrees of freedom, 

    the solver will move the bodies to close the angle.

    Setting a very small tolerance may result in simulation jitter or other artifacts.

    Sometimes it is not possible to project (for example when the joints form a cycle).

    <b>Range:</b> [0,Pi] <br>

    <b>Default:</b> Pi

    \param[in] tolerance the angular tolerance threshold in radians

    @see getProjectionAngularTolerance() PxJoint::setConstraintFlag() PxConstraintFlag::ePROJECTION

    */

    virtual void                setProjectionAngularTolerance(PxReal tolerance)                    = 0;

    /**

    \brief gets the angular tolerance threshold for projection.

    \return the angular tolerance threshold in radians

    @see setProjectionAngularTolerance()

    */

    virtual PxReal                getProjectionAngularTolerance()            const                    = 0;

    /**

    \brief Returns string name of PxRevoluteJoint, used for serialization

    */

    virtual    const char*            getConcreteTypeName() const { return "PxRevoluteJoint"; }

protected:

    //serialization

    /**

    \brief Constructor

    */

    PX_INLINE                    PxRevoluteJoint(PxType concreteType, PxBaseFlags baseFlags) : PxJoint(concreteType, baseFlags) {}

    /**

    \brief Deserialization constructor

    */

    PX_INLINE                    PxRevoluteJoint(PxBaseFlags baseFlags) : PxJoint(baseFlags) {}

    /**

    \brief Returns whether a given type name matches with the type of this instance

    */

    virtual    bool                isKindOf(const char* name) const { return !strcmp("PxRevoluteJoint", name) || PxJoint::isKindOf(name); }

    //~serialization

};

#ifndef PX_DOXYGEN

} // namespace physx

#endif

/** @} */

#endif

these methods are not currently exposed by the engine.

If this is what is needed then I could expose API for that, please be specific which methods would you need.

georgatos7 · 02-25-2015 11:38 PM

Yeah i will take a close look into those Esenthel and i'll let you know, thanks a lot. Also thanks for the suggestions 3dRaddict but from a first look its the drive/motor functionality that i need but ill be more specific.

3DRaddict · 02-26-2015 06:08 AM

I agree... the joint drive/motor functionality would certainly be an added benefit. Its something I've been used to in other PhysX based 3D engines, and its missing from Esenthel. (hence the workaround of adding torque to revolute jointed actors)

georgatos7 · 02-26-2015 03:35 PM

Hey Esenthel well the functionality that i was refering to is the drive/motor of the revolute joint which seems to be controlled by the methods below.

-setDriveVelocity(PxReal velocity)
-setDriveForceLimit(PxReal limit)
-setLimit(const PxJointAngularLimitPair& limits)

And probably to customize on the behavior you would need to change the "Limit", "Drive" and "Freespin" flags.
-setRevoluteJointFlag(PxRevoluteJointFlag::Enum flag, bool value)

For useful joint input.
-getAngle()
-getVelocity()

Now i don't know if this is too hard to implement at this stage.

**Esenthel** · 03-05-2015 10:19 PM

This doesn't sound that complex, so I'll try to implement this soon!

georgatos7 · 03-05-2015 10:30 PM

Awesome, thanks a lot. I bet it's gonna be a nice addition to the existing joint functionality.

**Esenthel** · 03-09-2015 12:28 AM

Just added following methods to the Joint class in the GitHub source:

Code:

// hinge joint specific (following methods are supported only in PhysX 3)

    Flt  hingeAngle          ()C; // get joint angle in range (-PI, PI]

    Flt  hingeVel            ()C; // get joint velocity

   Bool hingeDriveEnabled   ()C;   Joint& hingeDriveEnabled   (Bool on   ); // get/set if drive is enabled

   Bool hingeDriveFreeSpin  ()C;   Joint& hingeDriveFreeSpin  (Bool on   ); // get/set if the existing velocity is beyond the drive velocity, do not add force

   Flt  hingeDriveVel       ()C;   Joint& hingeDriveVel       (Flt  vel  ); // get/set drive target velocity, 0..Inf, default=0

   Flt  hingeDriveForceLimit()C;   Joint& hingeDriveForceLimit(Flt  limit); // get/set the maximum torque the drive can exert, setting this to a very large value if 'hingeDriveVel' is also very large may cause unexpected results, 0..Inf, default=Inf

   Flt  hingeDriveGearRatio ()C;   Joint& hingeDriveGearRatio (Flt  ratio); // get/set the gear ratio for the drive, when setting up the drive constraint, the velocity of the first actor is scaled by this value, and its response to drive torque is scaled down. So if the drive target velocity is zero, the second actor will be driven to the velocity of the first scaled by the gear ratio. 0..Inf, default=1

georgatos7 · 03-09-2015 01:44 AM

Awesome.

3DRaddict · 03-09-2015 06:51 AM

Wonderful!

@Greg
If you could do the same for prismatic/sliding joints that would be even more wonderful!

Applying torque to joined actors.
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