Karl Hedrick

Real-time slip-based estimation of maximum tire-road friction coefficient

This paper presents a real-time maximum tire-road friction coefficient estimation method and field test results. The estimator is based on the relationship between the wheel slip ratio and the friction coefficient. An effective tire radius observer and a tire normal force observer have been designed for the computation of the slip ratio from wheel speed and vehicle speed measurements. The effective tire radius observer has been used so that the proposed method works for all driving situations. A tractive force estimator, a brake gain estimator, and a normal force observer have been used for the estimation of the friction coefficient. The proposed estimation method for the maximum tire-road friction coefficient has been implemented using a fifth wheel and typical vehicle sensors such as engine speed, carrier speed, throttle position, and brake pressure sensors.

Obstacle Detection for Small Autonomous Aircraft Using Sky Segmentation

A vision-based obstacle detection system for small unmanned aerial vehicles (UAVs) is presented. Obstacles are detected by segmenting the image into sky and non-sky regions and treating the non-sky regions as obstacles. The feasibility of this approach is demonstrated by using the vision output to steer a small unmanned aircraft to fly towards an obstacle. The experiment was first verified in a hardware in the loop (HIL) simulation and then successfully implemented on a small modified remote control plane using a large inflatable balloon as the obstacle.

Effects of vehicle-vehicle/roadside-vehicle communication on adaptive cruise controlled highway systems

We study the effect of vehicle vehicle/vehicle-roadside communication on the performance of adaptive cruise control (ACC) systems. Two simulation works are presented One is a single ACC vehicle simulation using MATLAB/SIMULINK. A cut-in scenario and a braking scenario are tested Communication greatly saves control effort in the former scenario, while has little effect in the latter. The other work simulates ACC controlled highway merging with SHIFT language. The results show beneficial effects of communication in terms of braking effort, waiting-to-merge queue length, and main lane traffic shock wave caused by merging.

Active suspension using preview information and model predictive control

The objective of a suspension system is to maximize the passenger ride comfort and vehicle road holding quality. Passive systems present a trade-off between these objectives and the required suspension travel. An appropriate active suspension control overcomes this tradeoff and provides maximum ride comfort and road holding quality within the available suspension travel. In this paper, we show model predictive control (MPC) to be a design of choice for control of active suspension systems utilizing previewed road information. MPC design explicitly incorporates all hard constraints on state, control and output variables. It generalizes the approaches based on feedback linearization and dynamic inversion from single step control to multiple step control over a receding prediction horizon. MPC is shown to provide excellent improvements in the ride and road handling qualities of the vehicle over realistic terrain profiles. MPC works well even in the presence of noise in the previewed information. Implementation of MPC on the UCB active suspension test rig also shows the feasibility of the MPC algorithm in real-time application.

Observer-based identification of nonlinear system parameters

This paper deals with an observer-based nonlinear system parameter identification method utilizing repetitive excitation. Although methods for physical parameter identification of both linear and nonlinear systems are already available, they are not attractive from a practical point of view since the methods assume that all the system, x, and the system input are available. The proposed method is based on a sliding observer and a least-square method. A sufficient condition for the convergence of the parameter estimates is provided in the case of Lipschitz nonlinear second-order systems. The observer is used to estimate signals which are difficult or expensive to measure. Using the estimated states of the system with repetitive excitation, the parameter estimates are obtained. The observer based identification method has been tested on a half car simulation and used to identify the parameters ofa halfcar suspension test rig. The estimates of nonlinear damping coefficients of a vehicle suspension, suspension stiffness, pitch moment inertia, equivalent sprung mass, and unsprung mass are obtained by the proposed method. Simulation and experimental results show that the identifier estimates the vehicle parameters accurately. The proposed identifier will be useful for parameter identification of actual vehicles since vehicle parameters can be identified only using vehicle excitation tests rather than component testing.

Dynamic Tire Force Control by Semiactive Suspensions

This paper presents a semi-active suspension control algorithm to reduce dynamic tire forces including the development and application of observers for bilinear systems with unknown disturbances. The peak dynamic tire forces, which are greatly in excess of static tire forces, are highly dependent on the dynamic characteristics of vehicle suspensions. One way to reduce dynamic tire forces is to use advanced suspension systems such as semi-active suspensions. Semi-active control laws to reduce dynamic tire forces are investigated and a bilinear observer structure for bilinear systems with unknown disturbances is formulated such that the estimation error is independent of the unknown external disturbances and the error dynamics are stable for bounded inputs. The motivation for the development of a disturbance decoupled bilinear observer comes from the state estimation problem in semi-active suspensions. An experimental study on the performance of a semi-active suspension to reduce the dynamic tire forces is made via a laboratory vehicle test rig. The semi-active suspension has been implemented by the use of a modulable damper, accelerometers and a personal computer. Experimental studies using the laboratory test rig show that the performance of the semi-active suspension is close to that of the best passive suspension for all frequency ranges in the sense of minimizing the dynamic tire forces and that the dynamic tire force can be replaced by the estimated one. The dynamic tire forces for both passive and semi-active control test cases are compared to show the potential of a semi-active suspension to reduce the dynamic tire forces.

Networked Control System Design over a Wireless LAN

We analyze and implement a networked control system where the communication between the sensors, the controller, and the actuator takes place over a wireless LAN (802.11b) ad hoc system. The wireless LAN system introduces random delays and packet loss in the feedback loop. We present an extended version of the separation principle of Linear Quadratic Gaussian (LQG) control with both random delays and packet loss in the feedback loop. This paper is motivated by many of the implementation constraints encountered during the setup of an experimental NCS. We see from our experimental data that the optimal LQG controller that takes the delays and loss into account indeed performs better than traditional control methods.

On-board road condition monitoring system using slip-based tyre-road friction estimation and wheel speed signal analysis

Abstract This paper presents an on-board road condition monitoring system. The road condition is continuously evaluated in terms of slipperiness and coarseness and is classified into four grades, normal (μmax ≤ 0.5), slippery (0.3 ≤ μmax< 0.5), very slippery (μmax< 0.3), and rough surface (gravel). A non-linear curve fitting technique is adopted to estimate the maximum tyre-road friction coefficient using the so-called ‘magic formula’. The characteristic of the relationship between friction coefficient and slip, i.e. the value of maximum friction coefficient μmax varies significantly with different surfaces, but its corresponding slip value λmax does not vary much, is exploited in the road condition classification algorithm. For surface coarseness analysis, a separate classifier based on the variance of filtered wheel speed signal is implemented. Experimental results demonstrate the feasibility of the road condition monitoring system for detecting slippery and rough road surfaces in close to real-time. In addition, the proposed slip-based friction estimation algorithm has the merits of robustness to vehicle-tyre variance and easy calibration as opposed to past slip-based friction estimation approaches in the literature.

Tyre-road friction coefficient estimation based on tyre sensors and lateral tyre deflection: modelling, simulations and experiments

The estimation of the tyre–road friction coefficient is fundamental for vehicle control systems. Tyre sensors enable the friction coefficient estimation based on signals extracted directly from tyres. This paper presents a tyre–road friction coefficient estimation algorithm based on tyre lateral deflection obtained from lateral acceleration. The lateral acceleration is measured by wireless three-dimensional accelerometers embedded inside the tyres. The proposed algorithm first determines the contact patch using a radial acceleration profile. Then, the portion of the lateral acceleration profile, only inside the tyre–road contact patch, is used to estimate the friction coefficient through a tyre brush model and a simple tyre model. The proposed strategy accounts for orientation-variation of accelerometer body frame during tyre rotation. The effectiveness and performance of the algorithm are demonstrated through finite element model simulations and experimental tests with small tyre slip angles on different road surface conditions.

Analysis of bird formations

Birds in V formations are frequently observed and two main hypotheses have emerged in the biology/ornithology literature to explain this particular geometry: (i) it offers aerodynamic advantages and (ii) it is used to improve visual communication. Both explanations require a bird to track its predecessor. Observations of flocks suggest that this task is difficult for birds in large formations. We explain this phenomenon using a simple bird model and systems theory. This result has implications for the coordinated control of unmanned aerial vehicles. In particular, predecessor-following is an inherently poor strategy for formation flight.