Nonlinear observer and parameter estimation for electropneumatic clutch actuator Hege Langjord * Glenn-Ole Kaasa ** Tor Arne Johansen * * Department of Engineering Cybernetics, NTNU, Trondheim, Norway, e-mail: {hegesan,torj}@itk.ntnu.no ** Statoil Research Centre, Porsgrunn, Norway Abstract: This paper proposes a nonlinear observer for an electropneumatic clutch actuator. Parameter estimation of the clutch load characteristic and friction are also treated through adaptation. Estimates of piston velocity, chamber pressures and dynamic friction state are made based on piston position measurement only. Persistence of excitation conditions for convergence of the estimation error are derived. The performance of the observer and the parameter estimation is evaluated and compared to experimental measurements. 1. INTRODUCTION In this paper we propose a nonlinear observer for an electropneumatic clutch actuator intended for heavy duty trucks. Pneumatic actuators are common as industrial actuators, Smaoui et al. [2005], because of their desired properties; clean operations, low cost, high force-to-mass ratio and easy maintenance, Aziz and Bone [1998] and Ahn and Yokota [2005]. The main drawback, compared to hydraulic actuators, is their nonlinear behavior. This arises from the compressibility of air, stiction and high friction forces. Pneumatics are still preferred in our system, as pressurized air is already present in trucks. The clutch system considered is a system well suited as an Automated Manual Transmission (AMT) system, which is consisting of automated clutch motion during gear shifts and direct transmission through the clutch disc. As in any automotive system, cost is an important factor, and it is desired to have only a position sensor present in the system and avoid sensors for velocity, pressure and other variables. But for control purposes, as considered in earlier work Sande et al. [2007], Langjord et al. [2008a], [2009] and [2010], as much real-time information of the system as possible is wanted, which makes estimation important. The main goal of the work presented in this paper is to design a nonlinear observer for estimation of the other states together with adaptation of clutch load characteristics. Off-line estimation of this characteristic was treated in Langjord et al. [2008b], but as it tend to drift with wear and temperature changes, adaptation of the load characteristic is treated in addition to adaptation of the viscous friction coefficient. General designs for nonlinear observer design are devel- oped for particular classes of nonlinear system. Wu et al. [2004] consider nonlinear observability analysis for a pneu- matic actuator system, and concludes that in general it is not feasible to guarantee pressure estimate only from ⋆ This work has been sponsored by the Norwegian Research Council and Kongsberg Automotive ASA. measurements on position. Therefore, observers presented for pneumatic actuator systems are designed ispecially for the treated system. Bigras and Khayati [2002] presents an approach to design an observer for a pneumatic cylin- der system for which the connection port comprises a non-negligible restriction, Pandian et al. [2002] propose a continuous gain observer and a sliding mode observer to estimate pressure in a cylinder actuator, and Gulati and Barth [2005] present two Lyapunov-based, one energy- based and one force-error based, pressure observers for a pneumatic actuator system. The theses of Kaasa [2006] and Vallevik [2006] consider observer designs for the same clutch actuator system as we do, but these do not provide sufficient conditions for convergence of estimate errors. They also consider a three-way proportional valve as con- trol valve while we consider on/off-solenoid valves. The presented observer in this paper is a deterministic observer with linear output-injections and adaptation laws for load characteristics and viscous friction coefficient, where the state errors converge to zero under persistence of exitation (PE) conditions. 2. SYSTEM AND MODEL Fig. 1. A schematic of the system In Figure 1, a schematic of the considered clutch actua- tor system is shown. The electronic control unit (ECU)