A "Safe approach" tracking controller for short headway AGT operation

This paper presents a vehicle following control strategy for short headway AGT systems; it is also applicable to longer headway AGT systems. The strategy handles the vehicle overtake situation with the same basic controller as is used in steady-state string operations. A tight control loop is tied around each vehicle in the string. This control loop is isolated from large transients, such as occur during overtake situations, by constraining the vehicle to follow a well-behaved target trajectory which closes on the vehicle immediately ahead in accordance with a "safe approach" operating policy. By this means, the controller operates on small signals at all times. The safe approach target trajectory is determined by the states of the leading and trailing vehicles, the states of a vehicle which may become a new lead vehicle during merges, and the jerk, acceleration, and velocity constraints of the AGT system. The paper is organized into five sections. Section II presents a proposed "safe approach" operating policy which specifies velocity/spacing relationships for dynamic string operation. This policy is compared with steady-state policies such as constant time headway, constant separation and constant K-factor. It is shown that only the "safe approach" operating policy is consistent with collisionless service maneuvers. Section III then describes how such a safe approach policy can be used to implement vehicle longitudinal control systems. An example control system is discussed which uses a basic velocity control loop for all overtaking and headway control operations. Simulation results are included to demonstrate the performance of this design. Section IV discusses ways in which the same safe approach policy may be further extended to handle emergencies and merge/diverge operations. Finally, Section V presents concluding comments.