[ros-users] Flexible & remote transmission

[Jeroen Willems]

To problem can be simplified to a motor which is connected through a ~20cm
axis with a joint. Both, the motor and joint, are having position encoders.
We assume that the main dynamics of the axis (the transmission) can be
modeled as a linear spring ( T_spring = k * (theta_joint - theta_motor) ).

This problem is probably also due the fact I'm have problems understanding
the mechnism_model. I'm used to that the controller is getting a joint
error, it outputs a control signal to the robot (torque applied to the motor
for instance). So the mechanism_model is somewhat confusing to me because
you put the effort directly on the joints and then via a transmission the
effort on the actuator is determined. If I get it correctly.

I'm using an hexapod so imagine that if you eventually want to jump, you
want to use the kinetic energy of the springs.  When landing you also want
to use the
springs to reduce the impact on the robot by obtaining a part of the energy.

Jeroen


On Tue, Dec 14, 2010 at 23:39, Stuart Glaser <sglaser at willowgarage.com>wrote:

> Hi Jeroen,
>
> Could you describe what you're trying to model in more detail?  In
> particular, where are the encoders, motors, springs, gears, and joints
> in relation to each other?
>
> -Stu
>
> On Tue, Dec 14, 2010 at 4:37 AM, Jeroen Willems <jjpa.willems at gmail.com>
> wrote:
> > Okay I'm looking into the flexibility between the actuator and the joint
> by
> > adding a custom transmission but I'm having some difficulties.
> >
> > I've looked into the existing transmissions and those are pretty
> > straightforward (except for the pr2_gripper).
> > I want to add the (linear, rotational) flexibility, I will neglect the
> > damping for simplicity.
> > I'm used to use the equations of motion:
> > Load:  Jl theta_L_ddot + k ( theta_L - theta_M ) = 0
> > Motor: Jm theta_M_ddot - k ( theta_L - theta_M ) = u
> >
> > I don't get it how to implement this in the transmission. I was thinking
> of
> > a little bit of rewriting as in the attachment but it did not influence
> > anything at all.
> > Would be great if someone has an idea, solution or suggestion on how
> > implementing this flexibility.
> >
> > Thanks in advance
> >
> > Jeroen
> >
> > On Sat, Nov 20, 2010 at 21:07, Stuart Glaser <sglaser at willowgarage.com>
> > wrote:
> >>
> >> Hi Joeren,
> >>
> >> For both of these, the solution is to write a transmission.  Both
> >> issues require a specific translation between actuators and joints,
> >> and the best way to encode that translation is in a transmission.
> >>
> >> -Stu
> >>
> >> On Fri, Nov 19, 2010 at 1:36 AM, Jeroen Willems <jjpa.willems at gmail.com
> >
> >> wrote:
> >> > Hi All,
> >> >
> >> > I'm currently modeling a hexapod in Gazebo but I'm currently having
> some
> >> > difficulties.
> >> >
> >> > I have basically 2 questions, I hope that this is not a problem for
> one
> >> > post
> >> > but both have to do with the transmission:
> >> >
> >> > 1) Between the actuators and the joints are torsion springs. I haven't
> >> > found
> >> > something that incorporates flexibility between actuators and joints.
> >> > Is there some solution or workaround to obtain this or should I try an
> >> > alternative solution like writing a custom transmission?
> >> >
> >> > 2) As you can imagine I have 6 legs consisting of three links,
> basically
> >> > an
> >> > elbow manipulator.
> >> > The last two links can be regarded as in a 2D-plane. The third link is
> >> > remotely driven.
> >> > This means that when rotating the second link the third link does not
> >> > change
> >> > the rotation with respect to the body.
> >> > So in a 'standard' situation the third link rotates with the second
> link
> >> > so
> >> > the angle between those two does not change.
> >> > I hope this is a bit clear.
> >> > So again is there some solution or workaround?
> >> >
> >> > Thanks!
> >> >
> >> > Jeroen
> >> >
> >> > _______________________________________________
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> >> > https://code.ros.org/mailman/listinfo/ros-users
> >> >
> >> >
> >>
> >>
> >>
> >> --
> >> Stuart Glaser
> >> sglaser -at- willowgarage -dot- com
> >> www.willowgarage.com
> >> _______________________________________________
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> >> https://code.ros.org/mailman/listinfo/ros-users
> >
> >
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> >
>
>
>
> --
> Stuart Glaser
> sglaser -at- willowgarage -dot- com
> www.willowgarage.com
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