Abstract-Cooperative adaptive cruise control (CACC) employs wireless intervehicle communication, in addition to onboard sensors, to obtain string-stable vehicle-following behavior at small intervehicle distances. As a consequence, however, CACC is vulnerable to communication impairments such as latency and packet loss. In the latter case, it would effectively degrade to conventional adaptive cruise control (ACC), thereby increasing the minimal intervehicle distance needed for string-stable behavior. To partially maintain the favorable string stability properties of CACC, a control strategy for graceful degradation of one-vehicle look-ahead CACC is proposed, based on estimating the preceding vehicle's acceleration using onboard sensors, such that the CACC can switch to this strategy in case of persistent packet loss. In addition, a switching criterion is proposed in the case that the wireless link exhibits increased latency but does not (yet) suffer from persistent packet loss. It is shown through simulations and experiments that the proposed strategy results in a noticeable improvement of string stability characteristics, when compared with the ACC fallback scenario.

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In this paper, performance analysis of a team of unmanned vehicles (agents) that are subject to actuator faults is investigated. The team goal is to accomplish a cohesive motion in a modified leader-follower architecture by using a semi-decentralized optimal control strategy. The controller, which is recently proposed by the authors, is designed based on minimization of individual cost functions by using the available information from the neighboring sets. It is shown that a loss of effectiveness (LOE) fault in an actuator does not deteriorate the stability nor the consensus seeking goal of the team. This fault would only result in a different transient behavior, e.g., a change in the agent's convergence rate, without a change in the consensus value. On the other hand, if the fault in one or more of the agents is of the float type, either in the leader or the followers, the team could not maintain its consensus any longer, however the stability of the team can still be guaranteed. Moreover, the leader and the healthy followers adapt themselves to the follower's change when a float fault occurs in one of the agents. Finally, the behavior of the team in presence of the lock-in-place (LIP) actuator fault is also investigated. Simulation results are provided to demonstrate the performance of the team subject to the above three actuator fault scenarios.