# How create a model of real joint with parasitic movements in SimMechanics

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Jaroslav on 16 Jan 2012
Answered: Steve Miller on 25 Jun 2021
Hello,
I will create a model of real planar joint defined by stiffness and damping matrices. The joint movements are in x and y axes and rotation around z-axis. For model of this, I have used block “Custom Joint” with same primitive joints (P1 and P2 for planar moving in x and y, and one Revolute for rotating around z). As actuator was used block “Joint Spring & Damper”. When stiffness matrix is diagonal, all is correct. Problem is when stiffness matrix is in the form (real planar joint):
K=[Kx 0 0; 0 KyF –KyM; 0 –KthetaF KthetaM]; the stiffnesses KyM and KthetaM are equal.
How can create model of this joint in SimMechanics because I can define only one stiffness for one primitive joint, but for one primitive, for instance y-direction, I have two stiffness, one main KyF, and one parasitic –KthetaF.
Thx.

Steve Miller on 25 Jun 2021
It sounds like you need the movement along one axis to affect the stiffness and damping along another axis. If that is the case, then you can measure the distance and velocity measurements for each axis, perform the math in Simscape (or Simulink, or MATLAB) to calculate the forces and torques, and then apply the forces/torques to the respective axes. The key thing to watch out for is that you measure and apply the forces/torques in the correct reference frame.
--Steve