Unexpected output when using the MOTION function

Marcos Salgado
Marcos Salgado Altair Community Member
edited January 19 in Community Q&A

Hello everyone,

I am currently working on a model where a gas spring needs to be implemented. To better understand the definition of this spring I've prepared a simple model to assess some features of cylindrical joints. I established a time-dependant displacement motion on the joint to simulate the piston stroke, and also defined an output request to obtain the force needed to create that motion. However I did not get a proper result by using the MOTION function.

The model is very simple, it consists only of a slider (the piston) and a cylinder connected by a cylindrical joint (varname 'j_cylindrical_joint_1'). The translational degree of freedom is the displacement in the X-axis.

image

image

The motion assigned to the joint is a linear displacement with respect to the time, in the form of 'displacement=A*TIME', where 'A' is a positive integer (varname 'mot_slider_1'). To measure the force that creates this motion I used the MOTION function in the following form:

`MOTION({mot_slider_1.idstring},{0},{2},{j_cylindrical_joint_1.i.idstring})`

When plotting the output I obtained a graph of null force throughout the simulation time.

image

I would like to know which is the mistake that is preventing me from obtaining a proper force output.

Thank you in advance.

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Answers

  • Praful
    Praful
    Altair Employee
    edited January 19

    Hi Marcos - The motion you have provided is of constant velocity (effectively). There is no acceleration. And there are no other resisting forces such as springs or damper that opposes the motion. Hence your force measured by MOTION will be 0.0.

    Regards

    Praful

     

  • GTT Adam
    GTT Adam
    Altair Employee
    edited January 19
    Praful Prabhu_20784 said:

    Hi Marcos - The motion you have provided is of constant velocity (effectively). There is no acceleration. And there are no other resisting forces such as springs or damper that opposes the motion. Hence your force measured by MOTION will be 0.0.

    Regards

    Praful

     

    Hi Marcos,

    Praful is correct, in that if there is no resistance then no force is required to move the object.

    However the syntax for the MOTION expression is also incorrect. Since you are using the i marker for that joint as the reference marker, then you need to measure FZ, as the Z axis aligns the cylindrical joint marker. 

    Simply change the expression to `MOTION({mot_slider_1.idstring},{0},{4},{j_cylindrical_joint_1.i.idstring})` and put either friction in the joint or a spring attaching the slider to ground in order to confirm this syntax works.

     

    Hope this helps!

    Adam Reid

  • Praful
    Praful
    Altair Employee
    edited January 19
    Adam Reid_21142 said:

    Hi Marcos,

    Praful is correct, in that if there is no resistance then no force is required to move the object.

    However the syntax for the MOTION expression is also incorrect. Since you are using the i marker for that joint as the reference marker, then you need to measure FZ, as the Z axis aligns the cylindrical joint marker. 

    Simply change the expression to `MOTION({mot_slider_1.idstring},{0},{4},{j_cylindrical_joint_1.i.idstring})` and put either friction in the joint or a spring attaching the slider to ground in order to confirm this syntax works.

     

    Hope this helps!

    Adam Reid

    Good catch ! @Adam Reid . I did not pay close attention ;)

     

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