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Stability methods

 

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Introduction

Every simulation of any system quickly encounters places where numerical stability becomes an issue. Integrating discrete steps always will have this problem.

Racer internally runs at 1000Hz, but still, some physics processes tend to cause overshoots when integration. There's a multitude of stability issues, which will be touched on, sometimes briefly, sometimes with a better explanation.

The clutch

The clutch can be a high-friction device. When just looking at the engine, a 450Nm clutch gives high frequencies when synchronising the clutch rotational velocity with that of the engine.

Racer adds a 'locked' state to the clutch whenever velocity reversal (a term that is applicable to many parts of the simulation) takes place; when integrating (apply the clutch torque over 1 timestep) makes the clutch crossover the engine velocity, the clutch is set into locked state. Only when the engine torque tops the clutch torque will the unlocked state be entered.

Note that the clutch pedal influences the clutch torque. Effectively, the clutch torque is clutchPedal*clutchTorque. So when depressing the clutch pedal, the engine is more likely to go into unlocked state (slipping clutch).

Suspension damper forces

Some cars, especially race cars, have high-rate suspension & tire characteristics. Some forces can be limited by predicting their influence over the next timestep. Damper forces are one such force; they can never increase the amplitude of the wheel. Racer v0.6.3-v0.8.4 used the following method:

v0.8.5 tried to improve on this:

Anti-rollbar forces

The anti-rollbar can also go quite stiff. However, we can avoid it overshooting, although not entirely physically correct perhaps.

Wheel rotational velocity & braking

The braking force is a problematic area.


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(last updated November 13, 2012 )