# Simscape Fluids Fixed-Displacement Motor Issues

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Trevor Howard on 16 Feb 2020
Answered: Yifeng Tang on 18 Aug 2020
Hello,
I'm using Simscape Fluids on MATLAB 2019a, and my model is not matching my team member's hand calculations at all. Currently we are designing a hydraulic bicycle for use in a competiton. I understand losses play a factor in our design, but I am getting 8.168 mph out of the model when it's expected to be around ~20 mph.
All of the hand calculations are accurate up to the motor subsystem, which makes me think somehting is wrong with the motor model. Also, in the motor subsystem, the inertia was calculated by finding the effective intertia on the motor shaft by summing the kinetic energies in the system. The damping coefficient for the rotational damper was found by taking the ideal motor torque divided by the ideal motor angular velocity. I can post calculations for these if necessary.
I was hoping that someone could take a look at my model (particularly the motor subsystem), and see if they see anything incorrect with the system.
If it's useful, we're using a Eaton 26702-DAB motor (0.54 in^3/rev), and a Parker MGG20010-BC1B3 pump (0.218 in^3/rev).
Thank you for the help.

Yifeng Tang on 18 Aug 2020
Hey Trevor,
I know this may be too late for your competition. But what I noticed in your model is that the fixed-displacement pump and fixed-displacement motor parameters may not correspond to the actual device. The "Nominal shaft angular velocity" and "Nominal pressure drop/gain" are likely the most important parameters besides the displacement. It may not be straightforward to figure out these parameters but if you have datasheet from the manufacture, it may be helpful.
Without setting these parameters, even if we lower the motor damping to practically zero, the wheel speed is still < 12mph.
Thanks,
Yifeng

R2019a

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