Effect of Stiffness and Mass on Gazebo Simulation stability

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Hi All,

I've been developing a model that has hydraulic actuators that are as a result very stiff in that a small change to either the load on the actuator or on the valve spool will result in a relatively instant and relatively large change in the states of the actuator. These motors are represented by a class hydmotor;

//Hydraulic motor models
    private:
        hydmotor motor_joint1, motor_joint2, motor_joint3, motor_joint4, motor_joint5, motor_joint6, motor_joint7, motor_gripper;

during the update of the plugin the motors are run through a simulation step of their own based on their own states for pressureload using Gazebo provided dt (from get sim time) and GetVelocity(), returning the result (the force on the joint) via SetForce(axis, value):

    motor_joint1.dt=dt;
    //motor_joint1.u = 0.5;
    motor_joint1.thetaspeed =joint1->GetVelocity(0)*motor_joint1.gearratio;
    //Calculate the Motor Dynamics
    motor_joint1.updateinput();     


    joint1->SetForce(0, (motor_joint1.getTorque()*motor_joint1.gearratio));

dt is calculated from sim_Time. the method:

motor_joint1.updateinput();

Is basically where the next state iteration is calculated for the motor (where the states are pressureload, and rotationalspeed and the input is the valve position.

These have been incorporated into a Gazebo model via a plugin so that it can talk to ROS. To get it to work required a dt value of around 0.1 ms in the solver. Anything larger and the model ( a serial robot made of a number of links) would end up as a jumbled heap. I tried increasing the iter value about 100 times and can see that the model works for a short time before becomming unstable and flying apart.

My question is why would stability be affected if I only add an additional variable of type hydmotor?

I would expect it to run slower sure, but not to affect the overall stability.

The stability issue can be corrected by making the solver timestep even slower. at the moment it sits at about dt = 0.000044 with iters = 95.

Cheers