Issue 103, February 1999
Truck Speed Limiter Control

CONTROL REQUIREMENTS

When the truck approaches the maximum velocity, the pneumatic valve reduces the throttle-opening angle of the fuel-pump arm so the maximum velocity (VS) is not surpassed. Even if the driver continues to push the accelerator, the speed limiter has to ensure a smooth ride at the maximum velocity.

However, because of the dead time and nonlinearities involved with this control action, an actual overshoot and hunting occur when using a proportional or on/off controller. Adding a differential and integral part yields a PID controller model.

A PID controller generates the command value as a linear combination of the error (P), the derivative of the error with respect to time (D), and the integral of the error with respect to time (I). To tune a PID controller, the combined weights of these three components must be chosen so they approximate the nonlinear behavior of the process under control at its operating point.

Although this technique works with processes that are at only one operating point, it fails when the operating point moves. With the truck-speed limiter, the operating point moves because of different load situations such as driving uphill or downhill, as well as driving empty or with a full load. Additionally, the characteristics of the pneumatic valve and the fuel injection are highly nonlinear and vary from one truck to another.

If a PID control algorithm is used in a truck-speed limiter, it can only be tuned accurately for one operation point and one type of truck. For other operation points and different truck types, overshoot and hunting occur.

To compensate for this, European legislation permits speed limiters to operate within a certain tolerance, as shown in Figure 1. Once the maximum speed is reached, an initial overshoot of 5 km/h is tolerated. Afterward, the speed is kept constant within an interval of ±1.5 km/h. But, the overshoot and hunting tolerated by the legislation result in annoying speed fluctuations.