How to model a tendon contracting
How to model a tendon contracting
(OP)
Currently I am working with a unique actuator which under the simplest explanation is like a small rope which we can control the contraction of. This actuator can easily be cast in flexible polymers and then used to control the shape/motion of the cast structure. In the past the FE models have simply used forces applied at each attachment point of the actuator. This is no longer really a valid solution due to the longer lengths used and the large deflections needed.
The model can basically be thought of as cylinder approximately the size of a finger with a line element running top and bottom along the axis at the surface. These line elements will represent the contracting actuators. As they contract(strain) the cylinder would bend similar to a finger curling.
I would like to be able to control the strain of the actuators directly in the model. One method I have considered is to model the line elements as truss elements with a material with a thermal expansion constant. Change the temperature of the model and that would control the actuation. Then I would later be able to relate the Abaqus temperature to the fluid pressure that we actually use to control the actuators. The issue with this approach is the actual resulting strain of the actuator is also dependent on its modulus of elasticity. If the modulus were infinite the change in temperature would directly control strain, but of course the whole infinite modulus is an issue. I will also need to know the overall force the actuator needs to maintain that strain level. Note the actuators have no appreciable bending stiffness, so modeling them with 2 force members like trusses should be sufficient.
I realize the problem is a little unorthodox. Is there another element type I could couple with a state variable like temperature/charge/etc. to get this kind of control of a line element?
Thanks,
EBD
The model can basically be thought of as cylinder approximately the size of a finger with a line element running top and bottom along the axis at the surface. These line elements will represent the contracting actuators. As they contract(strain) the cylinder would bend similar to a finger curling.
I would like to be able to control the strain of the actuators directly in the model. One method I have considered is to model the line elements as truss elements with a material with a thermal expansion constant. Change the temperature of the model and that would control the actuation. Then I would later be able to relate the Abaqus temperature to the fluid pressure that we actually use to control the actuators. The issue with this approach is the actual resulting strain of the actuator is also dependent on its modulus of elasticity. If the modulus were infinite the change in temperature would directly control strain, but of course the whole infinite modulus is an issue. I will also need to know the overall force the actuator needs to maintain that strain level. Note the actuators have no appreciable bending stiffness, so modeling them with 2 force members like trusses should be sufficient.
I realize the problem is a little unorthodox. Is there another element type I could couple with a state variable like temperature/charge/etc. to get this kind of control of a line element?
Thanks,
EBD





RE: How to model a tendon contracting
You could then control the position of the nodes using displacement boundary conditions.
RE: How to model a tendon contracting
I guess I could setup a series of constraint eqs from one node to the next which basically takes their initial distance apart then constrains it to the new smaller value. I think this would work but that would get to be a whole lot of constraint equations. I was hoping for something a little less tedious.
RE: How to model a tendon contracting
RE: How to model a tendon contracting
RE: How to model a tendon contracting
RE: How to model a tendon contracting
If I could find an element or material property that already exists which couples strain in a line element to some variable I can control that would really be a powerful method for solving this problem. Like thermal expansion couples strain to temperature (but without the previously mentioned issue). I know there is also a piezoelectric coupling but I think I would have a similar issue to the temperature coupling. I was really hoping that someone knew of another similar way to couple this.
RE: How to model a tendon contracting
Any ideas on how to enforce a strain on a line element like this? Basically directly say that each line element must contract X amount of strain?
Thanks
RE: How to model a tendon contracting
That is use a script to set up your model and not CAE. The equations for nodal displacements would be part of the Python script that sets up the model & not in the model as such.
RE: How to model a tendon contracting
In the attached figure I have labeled some nodes along the actuator. Concerning appling BC directly to those nodes, if we look at the third element from N3 to N4. All I know is the initial distance between N3 and N4 and that there new distance between them should be less (whatever the strain I tell the actuator). I don't know the new position of N3 or N4, just the new distance between them.
[img
RE: How to model a tendon contracting
RE: How to model a tendon contracting
Really this all just boils down to finding a way to directly-control/prescribe/define the axial strain in a line element even through the line element may bend with the structure (Note this would be a truss element so there is zero bending stiffness).
RE: How to model a tendon contracting