Chapter7 Longitudinal Control for Vehicle Platoons(33)

VEHICLE CONTROL ON AUTOMATED HIGHWAY SYSTEMS
A popular control architecture proposed for an automated highway system (Varaiya, 1993) is hierarchical and has the 4 layers shown in Figure 7- 1. The 4 layers are a) The network layer b) The link layer c) The coordination layer d) The regulation layer The network layer controls entering traffic over the entire network and assigns a route to each vehicle as it enters the system. The link layer is a roadside layer for a section of a highway. It broadcasts target values for speed and platoon size for that road section. These target values are based on information about the aggregate traffic state (density, speed, flow). The link layer estimates proportion of vehicles destined for various exits and advises vehicles where to change lanes in order to reach exits. It receives information about incidents or congestion downstream and accordingly reassigns vehicle paths. It determines which The coordination layer resides on each vehicle. maneuver to initiate at any time so as to conform to the assigned path; it also coordinates that maneuver with neighboring vehicles so that the maneuver can be executed safely. It commands the regulation layer to execute the feedback law that executes the maneuver. The regulation layer executes maneuvers. These maneuvers include a) Steady state maneuvers of lane keeping and either speed control or vehicle following. b) Transient maneuvers of lane change, highway exit, highway entry, longitudinal split from a platoon and join a platoon.
Chapter 7
regular highways (driven manually). Thus an AHS vehicle is a personal vehicle that provides point to point service from any origin to any destination. The driver can drive on regular highways from home until he or she arrives at an AHS, travel under automated control on AHS, take an exit, and then travel again under manual control on a regular highway or local road to get to the final destination point. Thus, unlike a railway or other public transit system, an AHS provides point to point travel by leveraging the existing infrastructure of regular highways and roads.
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Figure 7-1. Control Architecture
The longitudinal control task discussed in this chapter is the responsibility of the regulation layer. So also is the lateral control task, discussed in chapter 3.
7. Longitudinal Control For Vehicle Platoons
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Chapter 7
LONGITUDINAL CONTROL FOR VEHICLE PLATOONS
AUTOMATED HIGHWAY SYSTEMS
Automated highway systems are the subject of intense research and development by several research groups, most notably by the California PATH program at the University of California, Berkeley. In an automated highway system (AHS), the objective is to dramatically improve the traffic flow capacity on a highway by enabling vehicles to travel together in tightly spaced platoons. The system requires that only adequately instrumented fully automated vehicles be allowed on this special highway. Manually driven vehicles cannot be allowed to operate on such a highway. Figure 5-2 in chapter 5 shows a photograph of eight fully automated cars traveling together in a tightly spaced platoon during a demonstration conducted by California PATH in August 1997. More details on this experimental demonstration are described in section 7.9 of this chapter. While the primary motivation for the development of AHS is to obtain significant improvements in highway traffic flow capacity, an AHS will also provide significant safety benefits. This is because over 90% of accidents on today's highways are caused by human error (US Department of Transportation, 1992). Since automated systems reduce driver burden and replace driver operation with automated operation, it is expected that an AHS will provide significantly improved safety compared to driving on today's highways. An important feature to be noted is that an AHS is a dual mode form of transportation. A vehicle instrumented to travel on AHS can also tr Contr
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Figure 7-2. Structure of longitudinal control system
7.3
LONGITUDINAL CONTROL ARCHITECTURE
In the regulation layer, the longitudinal control system is responsible for executing steady state and transient longitudinal maneuvers. This longitudinal control system will also be designed to be hierarchical, with an upper level controller and a lower level controller as shown in Figure 7-2. The upper level controller determines the desired longitudinal acceleration for each vehicle. The lower level controller determines the throttle andlor brake commands required to track the desired acceleration. Vehicle dynamic models, engine maps and nonlinear control synthesis techniques (Choi and Devlin, 1995a and 1995b, Hedrick et al, 1991, Hedrick, et. al., 1993) are used by the lower controller in calculating the real-time brake and throttle inputs required to track the desired acceleration.