Simscape use of inertia and mass blocks

I am trying to build a simplified model of a piston compressor in Simulink. I started with an ideal velocity source with 265 rpm, added the intertia of the crankshaft to it, converted the rotation to a linear movement with a wheel and axle block, added the mass of the piston and connecting rod to it. The translation mechanical converter block compresses the air. The goal is a suction volume of 2500 l/min. After running the simulation the suction volume is at 7e-6 l/min and I get a warning regarding the inertia and mass blocks. I struggling to find out where exactly the problem is with these blocks and how to solve it. Have I used the blocks correctly and set the references properly?

[VBBV]

Specify a tolerance for suction volume in the simscape model relative to rotational speed of crankshaft

[VBBV]

Your initial conditions also need to set according to model equations you coded in simulink

[Kai]

Many thanks for your answers. Can you give me a little hint, how to set the initial conditions? I don't think you mean the beginning values of the inertia and mass blocks.

I am also not sure what you mean with a tolerance for the suction volume. I haven't seen something similar in the Matlab examples for the simscape toolbox, could you please explan that?

[Kai]

ok, a rotational friction block between the ideal angular velocity source and the wheel and axle block seem so solve the warnings. I don't want to consider any friction losses in my simulation is there another alternative than reducing the friction to very small values, because this still affects the simulation result?

[Yifeng Tang]

Hi Kai,

I suspect there is something else going on in your model. It's currently set up with wheel and axle as the conversion mechanism between rotational and translational motion. There isn't any reciprocal mechanism that exists in a piston compressor. As the constant angular velocity is driving the wheel, the cylinder will be extended or compressed. Because it's an ideal velocity source, the hardstop you have in the model won't stop the motion. Instead, the velocity source will apply huge torque to overcome the very stiff spring from the hardstop.

Looks like you need to consider using the Slider-Crank block instead: https://www.mathworks.com/help/physmod/simscape/ref/slidercrank.html

Answer 1

[Yifeng Tang]

I suspect there is something else going on in your model. It's currently set up with wheel and axle as the conversion mechanism between rotational and translational motion. There isn't any reciprocal mechanism that exists in a piston compressor. As the constant angular velocity is driving the wheel, the cylinder will be extended or compressed. Because it's an ideal velocity source, the hardstop you have in the model won't stop the motion. Instead, the velocity source will apply huge torque to overcome the very stiff spring from the hardstop.

Looks like you need to consider using the Slider-Crank block instead: https://www.mathworks.com/help/physmod/simscape/ref/slidercrank.html