ExpressionBasedBushingForce definition

[Ryan Byrne]

Hi everyone,

I was hoping someone could shed some light on this. I am trying to implement an ExpressionBasedBushingForce. The API says that the expressions should be in terms of the bushing deflections theta_x, theta_y, etc. The model successfully loads into OpenSim, but when running static optimization on the model I receive an error stating that there is "no value specified for theta_y". I thought that OpenSim would automatically recognize these variables. Do I need to define these variables somewhere in the .osim file? I'm having a hard time finding information on this in the API.

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<ExpressionBasedBushingForce name = "trialbushing">
					<isDisabled>false</isDisabled>
					<!--One of the two bodies connected by the bushing.-->
					<body_1>sacrum</body_1>
					<!--The other of the two bodies connected by the bushing.-->
					<body_2>lumbar5</body_2>
					<!--Location of bushing frame on body 1.-->
					<location_body_1>0 0.009 0</location_body_1>
					<!--Orientation of bushing frame in body 1 as x-y-z, body fixed Euler rotations.-->
					<orientation_body_1>0 0 0</orientation_body_1>
					<!--Location of bushing frame on body 2.-->
					<location_body_2>0 -0.018 0</location_body_2>
					<!--Orientation of bushing frame in body 2 as x-y-z, body fixed Euler rotations.-->
					<orientation_body_2>0 0 0</orientation_body_2>
					<Mx_expression>"1.5*theta_x+2.0*theta_y"</Mx_expression>
					<My_expression>"1.5*theta_x+2.0*theta_y"</My_expression>
					<Mz_expression>"1.5*theta_x+2.0*theta_y"</Mz_expression>
					<Fx_expression>"1.5*theta_x+2.0*theta_y"</Fx_expression>
					<Fy_expression>"1.5*theta_x+2.0*theta_y"</Fy_expression>
					<Fz_expression>"1.5*theta_x+2.0*theta_y"</Fz_expression>
				</ExpressionBasedBushingForce>

[Thomas Uchida]

There is a test (testForces.cpp) that generates model files containing ExpressionBasedBushingForce components. I think you just need to remove the quotation marks from the expressions. Here are the relevant lines from the test file (note that this file was generated in OpenSim 4.0):

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<ExpressionBasedBushingForce name="translational_linear_bushing">
    <socket_frame1_connectee_name>base_body_offset</socket_frame1_connectee_name>
    <socket_frame2_connectee_name>ball_offset</socket_frame2_connectee_name>
    <frames>
        <PhysicalOffsetFrame name="base_body_offset">
            <socket_parent_connectee_name>base_body</socket_parent_connectee_name>
            <translation>0 0 0</translation>
            <orientation>0 0 0</orientation>
        </PhysicalOffsetFrame>
        <PhysicalOffsetFrame name="ball_offset">
            <socket_parent_connectee_name>ball</socket_parent_connectee_name>
            <translation>0 0 0</translation>
            <orientation>0 0 0</orientation>
        </PhysicalOffsetFrame>
    </frames>
    <Mx_expression>0.000000*theta_x</Mx_expression>
    <My_expression>0.000000*theta_y</My_expression>
    <Mz_expression>0.000000*theta_z</Mz_expression>
    <Fx_expression>10.000000*delta_x</Fx_expression>
    <Fy_expression>10.000000*delta_y</Fy_expression>
    <Fz_expression>10.000000*delta_z</Fz_expression>
    <rotational_damping>0 0 0</rotational_damping>
    <translational_damping>0 0 0</translational_damping>
</ExpressionBasedBushingForce>