Force direction in the bushing force element
Posted: Mon Nov 11, 2024 1:29 pm
Hi,
I am implementing intervertebral disc joint stiffness using ExpressionBasedBushingForce class in the spine model. I like to better understand the direction of these passive joint torques. For example, I defined a bushing element at the L2/L3 joint using the expression below where I am only considering the rotation at the sagittal plane (about the z-axis).
When I run a static analysis with the coordinate of the L2/L3 joint set to +1 degree (theta_z = 0.01745 rad), below is the force output I am reading.
Since positive 1 degree represents an extension motion at the joint, the passive force should oppose this motion and create a negative force in the opposite direction. The model calculates a negative passive torque value of -2.6 Nm at L2 segment. Is this implementation correct or do I need to flip the sign in the force expression?
I am implementing intervertebral disc joint stiffness using ExpressionBasedBushingForce class in the spine model. I like to better understand the direction of these passive joint torques. For example, I defined a bushing element at the L2/L3 joint using the expression below where I am only considering the rotation at the sagittal plane (about the z-axis).
Code: Select all
<ExpressionBasedBushingForce name="EBBE_L2-L3">
<socket_frame1>/jointset/L2_L3_IVDjnt/lumbar3_offset</socket_frame1>
<socket_frame2>/bodyset/lumbar2</socket_frame2>
<Mz_expression>step(a_z)*2.60*a_z+(1-step(a_z))*(1.80*a_z); a_z=57.2957800*theta_z</Mz_expression>
- bushing_force.PNG (11.93 KiB) Viewed 930 times
Since positive 1 degree represents an extension motion at the joint, the passive force should oppose this motion and create a negative force in the opposite direction. The model calculates a negative passive torque value of -2.6 Nm at L2 segment. Is this implementation correct or do I need to flip the sign in the force expression?