[ODE] External vs. internal joint forces

Arndt Twickel ode at twickel.ws
Wed Jun 13 10:04:27 MST 2007


Basically I'm asking a question that has already been posed on this mailing
list several times (see e.g. [1][2][3][4]), but to which I couldn't find a
satisfactory answer:

I would like to know if it is possible to get information about internal vs.
external forces/torques in joints. By internal forces I mean forces applied by a
motor of the same joint, by external forces I mean all other forces.

When you use a motor you specify a desired velocity and a maximum force that may
be applied by the motor to reach the desired velocity but in the end you do not
know how much force was actually applied by the motor.

With the dJointGetFeedback() function you get the accumulated force/torque at a
joint but you cannot tell who (motor, gravity, other motors, environment, ...)
contributed how much of the force.

The only possibility I can think of at the moment (and which I would like to
avoid if possible, see e.g. [5]) is to use functions like e.g.
dJointAddHingeTorque() (where forces/torques are explicitly added to the joints)
instead of the joint motors (see also [6]).

Why do I need to differentiate between internal and external forces in joints?
I would like to:
1. implement an energy sensor that tells me how much work has been done by the
2. implement a spring coupling where I need to know the energy taken up / put
    back into the system by the spring to determine its state.

Any help is very much appreciated!


[1] http://ode.org/pipermail/ode/2006-September/020180.html
[2] http://ode.org/pipermail/ode/2006-August/019935.html
[3] http://ode.org/pipermail/ode/2007-April/thread.html#21896
[4] http://ode.org/pipermail/ode/2002-October/006096.html
[5] http://ode.org/pipermail/ode/2001-December/004757.html
[6] http://ode.org/pipermail/ode/2007-May/021917.html

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