I (Steer-by-Wire Grad. Project)

[Know_seeker]

Hello there,

i am doing a graduation project for my bachelor on Steer-by-Wire, i have to make a modeling and simulation on simulink. i reached the idea that the torque output from the motor that actuates the steering axles to rotate the wheel around the z - axis (Yaw) changes with the velocity of the car (not proportianal ) i am trying to find this relation to be set in the controller in order to control the torque output , i hope that someone can help me where i can find this relation

thank you :)

 

[Mech_Engineer]

I don't think I could quantify it for you, but it's nonlinear with speed not only due to friction but also due to gyroscopic forces on the tires at higher speeds. It's relatively simple to calculate the required force at zero velocity, and from there required force goes down. So if your system can put out enough force to turn the tire while stationary, it can put out enough force at any speed. The rest is just a response curve based on speed.

My wife's car has an electric power steering system which varies assist force based on speed- it puts out maximum force at zero to about 10mph, and reduces as speed increases. By the time you're doing 65 mph, assist force has dropped significantly.

 

[Know_seeker]

Thank you very much for the replay, that was very helpful info, and saves a lot of time and search.

any more advice would be welcome

 

[Mech_Engineer]

Thinking about it, since your system is steer-by-wire sterring "feel" will basically be simulated or non-existent (are you going to provide force feedback at the steering wheel?). Because of this, the amount of force you're putting out just needs to be enough to turn the wheel, it doesn't actually have to dynamically vary. As long as your system is strong enough to turn the wheel while stationary, it should be able to turn it at any speed.

You might look into varying the steering ratio as a function of speed, this is a high-end option in some cars these days and wouldn't be difficult to implement in a full steer-by-wire application.

 

[Know_seeker]

ok now for stationary,
what i want to do now is to select a motor based on maximum torque which is at stationary,
for my model i will make the assumption that the car is in air, so the torque equation i will use is as follows
T=equiavalent Moment of inertia*angular acceleration + Damping coefficient*angular speed
i don't have big problem here except that i don't know what am i going to use as the constant parameters Inertia and damping

thank you

 

[Mech_Engineer]

If you're calculating the torque required for a stationary vehicle,there are no dyanmic elements to the calculation. It should simply take into account the coeffieicnt of friction for the tire on the surface and the normal force on that tire.