Smith Predictor Compensating for Vehicle Actuator Delays in Cooperative ACC Systems

Xing, Haitao; Ploeg, Jeroen; Nijmeijer, Henk · 2018 · OpenAlex-citations

DOI: 10.1109/tvt.2018.2886467

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Summary

This paper addresses the challenge of maintaining string stability in Cooperative Adaptive Cruise Control (CACC) systems while minimizing intervehicle distances to maximize road throughput. CACC utilizes wireless vehicle-to-vehicle communication to allow for shorter spacing than traditional Adaptive Cruise Control (ACC). However, inherent time delays, particularly vehicle actuator delays, significantly compromise string stability and force the use of larger, less efficient time gaps. The authors propose applying a Smith predictor to compensate specifically for vehicle actuator delays, aiming to decouple individual vehicle stability from these delays and enable smaller, string-stable time gaps. The study models a homogeneous CACC string using a simplified longitudinal vehicle dynamics model derived via feedback linearization, which accounts for actuator delay and response time constants. The proposed control strategy integrates a Smith predictor into a standard proportional-derivative (PD) controller framework with a constant time gap spacing policy. The Smith predictor eliminates the actuator delay from the closed-loop characteristic equation by utilizing a delay-free model of the vehicle dynamics. The authors analyze individual vehicle stability using the Routh-Hurwitz criterion and evaluate string stability through frequency-domain analysis of the transfer functions relating vehicle signals. The theoretical analysis is supported by experimental validation using a platoon of two passenger vehicles. The results demonstrate that the Smith predictor significantly improves stability margins. For individual vehicle stability, the allowable range for PD controller gains is expanded, making the stability conditions less strict and independent of the actuator delay. Regarding string stability, the minimum string-stable time gap is drastically reduced. Without the compensator, the minimum time gap exceeds 0.3 seconds; with the Smith predictor, it can be reduced to as low as 0.02 seconds while maintaining stability. The authors also identify a "tracking latency" inherent to the Smith predictor, where the actual steady-state distance is larger than the virtual regulated distance due to the delay compensation. They provide a method to account for this latency to ensure safety and throughput goals are met. Experimental results with two vehicles confirm the practical feasibility of the approach, showing effective disturbance attenuation and stable platoon behavior. The significance of this work lies in its ability to enhance the performance of CACC systems by mitigating the negative impact of actuator delays. By enabling smaller intervehicle distances, the proposed method can potentially double highway capacity compared to manually driven or standard ACC vehicles. The Smith predictor offers a computationally efficient alternative to complex methods like Model Predictive Control, providing a straightforward way to synthesize string-stable behavior. This approach allows for the full exploitation of CACC benefits, improving both traffic flow efficiency and safety through robust, delay-compensated control.

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