sensors Article Tire-Pressure Identification Using Intelligent Tire with Three-Axis Accelerometer Bing Zhu , Jiayi Han and Jian Zhao * State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130025, China; [email protected] (B.Z.); [email protected] (J.H.) * Correspondence: [email protected] Received: 27 April 2019; Accepted: 4 June 2019; Published: 5 June 2019 Abstract: An intelligent tire uses sensors to dynamically acquire or monitor its state. It plays a critical role in safety and maneuverability. Tire pressure is one of the most important status parameters of a tire; it influences vehicle performance in several important ways. In this paper, we propose a tire-pressure identification scheme using an intelligent tire with 3-axis accelerometers. As the primary sensing system, the accelerometers can continuously and accurately detect tire pressure with less electronic equipment mounted in the tire. To identify tire pressure in real time during routine driving, we first developed a prototype for the intelligent tire with three 3-axis accelerometers, and carried out data-acquisition tests under different tire pressures. Then we filtered the data and concentrated on the vibration acceleration of the rim in the circumferential direction. After analysis, we established the relationship between tire pressure and characteristic frequency of the rim. Finally, we verified our identification scheme with actual vehicle data at different tire pressures. The results confirm that the identified tire pressure is very close to the actual value. Keywords: intelligent tires; tire pressure; accelerometer; wheel vibration 1. Introduction Interactions between a tire and the road exert forces on a vehicle and alter its motion state. The ultimate aim of most vehicle active safety systems, such as the anti-lock braking system (ABS) or the electronic stability program (ESP), is to make the tires controllable [1–3]. The status information of the tire is especially important for all kinds of active safety systems and especially for self-driving vehicles [4,5]. At present, tire information, such as tire force and coefficient of road adhesion, is estimated by modules and filters [6–10]. During estimation, there is much simplification and linearization in the models of the vehicle, wheel, and tires, and the kinematic state of the vehicle is indeterminate. Therefore, application of estimated tire information is always restricted. To acquire more accurate and appropriate tire information, the concept of an intelligent tire system is being researched extensively [11,12]. The intelligent tire, which is equipped with sensors and other electronic equipment, is meant to measure the tire pressure, deformation, force, and other information. Its most important part is the sensing system [13,14]. Intelligent tires can be classified into five major types, based on their sensing methods, which are accelerometers, optical sensors, strain sensors, polyvinylidene fluoride (PVDF) sensors, and other sensors. Yi Xiong et al. used an optical tire sensor-based laser to measure rolling tire deformation [15]. Matsuzaki et al. calculated the in-plane strain and out-of-plane displacement with a single CCD camera fixed on the wheel rim [16]. Optical sensors can achieve noncontact measurement, hence they have no adverse effect on the tire rubber, but these sensors are expensive and consume too much power, and wheel vibration can influence their accuracy. Sensors 2019, 19, 2560; doi:10.3390/s19112560 www.mdpi.com/journal/sensors Sensors 2019, 19, 2560 2 of 13 Strain sensors and PVDF sensors have gained more attention in tire sensing [17–21]. Zhang et al. embedded a pressure-sensitive, electric conductive rubber sensor inside the tire rubber layer to extract the 3-D forces on the contact patch [17]. Armstrong et al. applied three piezoelectric film sensors, which are a kind of strain sensor, on the inner surface of a tire to detect normal pressure, deflection, and longitudinal strain [18]. Erdogan et al. proposed a method to estimate the tire-road friction coefficient using a sensor consisting of PVDF film attached to the surface of a cantilever beam [19]. However, these sensors occupy a large area of the inner surface of the tire, which will alter the elastic property of the rubber and cause abnormal measurements. In addition, the huge temperature variation of a tire has a strong impact on strain sensors and PVDF sensors. Accelerometers are now used widely for intelligent tires [22–25]. Savaresi et al. presented a direct approach to estimate the instantaneous vertical and longitudinal forces by in-tire acceleration measurements and standard wheel-speed sensors [22]. Matsuzaki et al. proposed an identification scheme for the friction coefficient involving the slip angle, and applied force estimation through the use of a 3-axis accelerometer glued on the inner surface of a tire [23]. Niskanen et al. studied the distorted contact area of a tire in wet conditions using three accelerometers fixed on its inner liner [24]. Accelerometers are compact, small, and weigh just a few grams. They hardly influence a tire when glued to its inner surface. Accelerometers also consume less energy than optical or other sensors. This makes it possible to achieve a passive intelligent tire. In addition, their output signals are easy to read and contain more information compared to PVDF or strain sensors. With all of these attractive features, accelerometers are the most suitable sensors for intelligent tires. Intelligent tires equipped with accelerometers, as described above, are mostly researched to identify and estimate tire deformation, slip angle, vertical or longitudinal force, and tire-road friction coefficients. While this information is vital, there has been little research on tire pressure using intelligent tires equipped with accelerometers. Tire pressure influences several important aspects of vehicle performance [26]. Rolling resistance increases when a tire loses pressure, which reduces fuel economy. Pressure that is too low results in stationary wear, which can cause a tire to burst. When the tire pressure is high, the tire and road contact patch will be non-uniform, which reduces adhesive force. In addition, the tire will harden, which leads to an uncomfortable ride for passengers. The tire pressure also makes sense to vehicle active safety or control applications. The cornering stiffness is influenced by the tire pressure greatly. The cornering stiffness of the front and rear tires determines the steering behavior directly [27,28]. In this consideration, the tire pressure has something to do with the active safety systems, such as ESP. The tire pressure monitoring in real time becomes more and more important for the present and the future. The vehicle control algorithm is more and more complexity, and more variables are taken into account during tire modelling. The influence of the tire pressure can be and must be taken into consideration. For example, Janulevicius et.al. investigated how driving wheels of a front-loaded vehicle interact with the terrain depending on tire pressure [29]. Toma et al. studied the influence of tire pressure on the results of diagnosing brakes and suspension [30]. With the real time tire pressure, the control strategy under different tire pressure can be optimized and enhances the vehicle performance [31]. Therefore, Tire-pressure monitoring should be the fundamental function of an intelligent tire. Tire-pressure monitor systems (TPMSs) are widely used all over the world [32,33]. There are two types of TPMS. One is called indirect TPMS and the other is direct TPMS. The indirect TPMS can work with no need for installing additional sensors. It uses the signal of the wheel speed sensors or other existing sensors [34]. This kind of TPMS is only able to judge whether the tire is lacking of pressure. It cannot provide the precise tire pressure in real time. Therefore, the use of the indirect TPMS is limited. It can only help to acquaint the drivers with the tire situation and it can do nothing with the advanced active safety systems [35]. The direct TPMS is based on a pressure sensor mounted at the tire nozzle. It is practical and durable for the present, but it is a separate system. More electronic circuits or equipment are acquired to combine this kind of TPMS with intelligent tire or advanced active safety systems. This will reduce the reliability and add complexity. If we can estimate the tire Sensors 2019, 19, x 3 of 14 function is developed by equipping accelerometer or other vibration sensors, the tire pressure identificationSensors 2019 method, 19, 2560 proposed in this paper will help to take the place of the present TPMS.3 of 13 The major motivation to install accelerometers in the tire may be to estimate tire deformation, slip angle or frictionalpressure force. precisely In this in real condition, time using if thewe sensors can id installedentify the in thetire intelligent pressure tire, by a the seamless accelerometers tire pressure which is alreadymonitoring installed function in the tire, will bethen achieved. pressure In sensors the future in whenthe tire the will intelligent be unnecessary. tire with multi-function The less electronic equipmentis developed we mount by equipping in the tire, accelerometer the more orpractical other vibration and reliable sensors, an the intelligent tire pressure tire identification system we can achievemethod [5,36]. proposed in this paper will help to take the place of the present TPMS. 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