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>