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Old Mar 6th 2010, 05:56 AM   #1
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Join Date: Mar 2010
Posts: 1
the problem of modeling vehicle brakes

Hi there.
I have a problem with modeling brakes. How can something so seemingly simple as brakes rubbing on a wheel be so difficult to simulate (for me, at any rate!)? Call me thick if you like but this has been bugging me for years.

I am trying to write software (in Visual Basic) to simulate the velocity of a vehicle (e.g. a train).
It is a 'real-time' simulation so I am using a software 'timer' to iterate and update the calculation of the vehicle's momentum (from which I derive the velocity, v) every second, and display the value of v on a gauge. All works well until I try to simulate the brakes.

Before I run the iterative timer code, I start by assigning values to the basic variables:
m = mass of train (pounds)
P = max propulsive force acting on the driving wheels (down-force in poundals on driving axles x coeff of friction)
t = time period of iteration (seconds) (in this case 1 second, the timer period)
R = max brake force (down-force in poundals on braking axles x coeff of friction) (initially = 0)

So with brake force, R initially set to zero, and P set to a suitable value I start my train. On each 1-second time increment I re-calculate the velocity, v (feet/sec) of the train using the following equations:

[start timer loop]
mv = mv + (P - R)t
hence v = mv / m
[increment t and loop ...]

This works fine. I see the velocity rise nicely. Next, once I've got some speed up, I reduce (in real-time) the value of P to zero (power off). The velocity remains constant which is fine, too (I'm ignoring wind and frictional running resistance at this stage). Then I increase braking force, R and watch the velocity fall, and I leave the brakes 'on'. Very nice.

BUT the velocity falls through zero and now the brakes (R) are driving my train backwards as mv increases negatively. So, this equation cannot be simulating the way brakes work at all. Am I not right in thinking that brakes can only reduce existing momentum or KE, or prevent momentum or KE from being increased? They cannot create it themselves! (No matter how hard I press the brakes in my car it goes nowhere!)

How, then, can I represent brakes in a simulation such as this? What equation can I use that reflects the action of brakes?

stuart
UK

Last edited by stuart; Mar 6th 2010 at 06:00 AM.
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