Sensor fusion by using a sliding observer for an underwater breathing system Salvatore Carotenuto, Luigi Iannelli, Sabato Manfredi, Stefania Santini Abstract— This paper addresses the sensor fusion problem applied to an underwater breathing apparatus. In such systems hardware redundancy is used for safety reasons: in order to overcome any dangerous situations eventually coming from sensor faults, the measurement is taken by using three sensors that measure the same variable (oxygen partial pressure). Thus a sensor fusion scheme is needed in order to feed-back the measurement for controlling purposes. The work presents a fusion algorithm that uses a sliding mode observer for taking into account model information, and combine the different measurements by looking at how they behave with respect to the observer prediction. A forgetting factor approach is used for making more reactive the algorithm. Extensive simulations show the effectiveness of the proposed solution comparing results with a more traditional approach like the voting logic. I. I NTRODUCTION AND PROBLEM MOTIVATION Closed-circuit underwater breathing apparatus, commonly known as ‘rebreather’, is a device that permits completely autonomous diver operations at a very deep depth without an umbilical. The term closed-circuit rebreather (CCR) refers to the recirculation of the breathable mixture. In order to recirculate gasses, all rebreather concepts include a mouthpiece, through which the diver breathes, connected with a collapsible bag that inflates when he exhales, and deflates when he inhales. This bag is usually called counterlung. The macroscopic chemical effects on breathed gasses are (partial) oxygen subtraction for metabolic use with a con- sequent carbon dioxide increase. All other gasses different from oxygen are inert with respect to the respiration process and flow through the lungs without being chemically trans- formed. This means that exhaled gasses can be recycled (or better re-breathed), provided that oxygen content is restored and carbon dioxide is removed. To reestablish a breathable mixture, rebreathers must be equipped with a device (usually a chemical scrubber) for CO 2 removal and a supply valve for O 2 injection into the breathing loop. CCR has a feed-back electronic controller that, based on the measure of the oxygen level in the counterlung, injects pure oxygen by operating a solenoid supply valve, so as to regulate oxygen partial pressure to a given set-point value during the entire dive. Luigi Iannelli is with the Department of Engineering, Universit` a del Sannio in Benevento, P.zza Roma, 21 - 82100 Benevento, Italy. Salvatore Carotenuto, Sabato Manfredi and Stefania Santini are with the Department of Computer and Systems Engineering, Universit` a di Napoli Federico II, via Claudio, 21 - 80125 Napoli, Italy. e-mail: luigi.iannelli@unisannio.it, {salcarot, smanfred, stsantin}@unina.it Controlling the oxygen partial pressure during all phases of a dive is a crucial task for the correct and safe use of a CCR. The actual oxygen content in the breathed mixture must be within specific limits by considering that (i) the oxygen partial pressure inside the breathing loop should never fall below 0.16 [atm] to avoid hypoxia (it can rapidly bring the diver to unconsciousness [1]); (ii) breathing oxygen at high partial pressure (greater than 0.5 [atm]) can be toxic [2] (Central Nervous System (CNS) oxygen toxicity is related to both oxygen pressure level and duration of the exposure [3]). However O 2 partial pressure in the breathing loop is subject to variations due to disturbances such as the diver individual metabolism, which depends on the workload and the internal pressure of the counterlung which changes with the dive profile. While variations of O 2 partial pressure due to the depth can be easily predicted, metabolic oxygen consumption rate can vary from person to person by a factor of 6 (or more) in normal conditions, and as much as 10-fold in extreme conditions, depending on the activity level. In order to keep the oxygen partial pressure at a desired value, it is important to design a controller that is robust to such disturbances. This is crucial since the oxygen set point is chosen as the maximum safe value throughout the dive, thus the non-oxygen portion of the breathing gas (the part that determines decompression obligations) is kept at a minimum. This allows the diver to stay longer at depth without incurring a decompression obligation [4], and also to speed up the decompression process whenever an obligation is incurred. When working in critical conditions or environments, such as under the sea, the control-loop has to be fault-tolerant. In other words the control strategy of the rebreather system has to be ‘robust’ against sensor failures. An error on the actual oxygen level measurement, especially in the set point proximity, can cause a disease to the diver and may have fatal consequences. Thus, in the counterlung control-loop some hardware redundancy is desired: usually the system output (oxygen partial pressure) is measured through three sensors so that eventual sensor faults can be detected and isolated for the diver safety. Of course, by introducing this redundancy, an algorithm that decides which measurement value to give to the controller is needed. The problem of designing this algorithm is called ‘sensor fusion’ problem. In this paper a sensor fusion algorithm that uses sensors information as well as model information, is proposed. The scheme is based on a sliding mode observer [5]. The Proceedings of the 44th IEEE Conference on Decision and Control, and the European Control Conference 2005 Seville, Spain, December 12-15, 2005 ThIB20.3 0-7803-9568-9/05/$20.00 ©2005 IEEE 7662