Introduction

Tires integrated with sensors to monitor air pressure [1], strain [2], force [3], and acceleration [4] are crucial for enhancing tire reliability [5]. Normal and tangential force measurement sensor plays an important role in advancing both vehicle safety and performance. The dynamic behavior of a vehicle largely depends on the tire–road interaction. The vehicle’s movement and maneuverability are controlled by the forces generated at the contact points between the tires and the road surface [6]. There are three basic forces applied on the rolling tire: tractive force, lateral force, and normal force [7]. Therefore, anomalies in these forces might reflect issues with tires. Hence, there is a need for monitoring forces in tires. Understanding of the forces in real time can result in improved predictive maintenance strategies, optimized performance, enhanced safety, and enhanced driving experiences.

Because the tire itself is made of a flexible and stretchable material, having a flexible and stretchable sensor will provide better compatibility with the tire. Ionic liquids (ILs) have opened new opportunities for flexible and stretchable sensors [8]. Mixing an IL with a prepolymer matrix enables a way to manufacture a pressure-sensitive layer [9,10]. This layer changes its resistance under mechanical strain [11,12]. Another benefit of using ILs in a pressure-sensitive layer is that the parameters of ILs (concentration, sensor geometry, degree of polymerization, etc.) can be modified according to the application [13]. In addition, to maintain the flexibility of sensors, a multi-walled carbon nanotube (MWCNT)–polymer matrix has been used as electrodes in previous studies [13,14].

In this article, we propose a novel approach to measuring normal and tangential force using a flexible sensor. There sensors were manufacturing through a molding and screen-printing process. The fabricated sensors are capable of quantifying force change due to the deformation applied on the tire and speed. In addition, the proposed sensor can measure the revolutions per minute (rpm) of the wheel.

Materials and Methods

Sensor Design

The proposed sensors are made of three distinct layers. As shown in Fig. 1, the pressure-sensitive layer is sandwiched between two insulating layers. The designs are adapted from previous studies [8,10]. For normal force sensing, the electrodes are aligned vertically on separate layers, with pressure-sensitive material acting as a separator (Fig. 1a,b). Conversely, for tangential force sensing, the electrodes are arranged face-to-face within a single layer separated by pressure-sensitive material (Fig. 1b,c). The pressure-sensitive regions between the electrodes are referred to as taxels. Here, three types of sensor have been employed: a normal force sensor (Fig. 1a), a tangential force sensor (Fig. 1b) and a combined normal and tangential force sensor (Fig. 1c). The sensors were previously characterized in [10,11] for static testing.

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Sensor Fabrication

The sensors are manufactured using a combination of molding, screen printing, and curing techniques. Each layer of insulating or pressure-sensitive layers is first poured into a mold (Fig. 2a), and then cured (Fig. 2c) using UV light (OmniCure® S2000, Excelitas Technologies Co., Wheeling, Illinois) to form solid layers. The electrodes are subsequently screen printed onto the solidified layers (Fig. 2b).

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Tire and Wheel Design

To characterize and evaluate the performance of the proposed sensors, a miniature tire and wheel was designed (Fig. 3a,b) and additive manufactured (Fig. 3c,d). The tire was 3D printed (PolyJet™, Stratasys) using Agilus85A (Stratasys) material. Similarly, the wheel was manufactured (fused filament fabrication) using polyethylene terephthalate glycol.

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Experimental Setup and Design

The sensor was initially affixed to the wheel using double-sided tape (Fig. 4a), after which the wheel was wrapped with the tire. The sensors were then connected to a signal conditioning and data acquisition device (Fig. 4b), which recorded the data during the experiments. The tire, along with the wheel, was mounted on a motorized test stand (ESM 303, Mark-10, Copiague, New York) using a customized fixture (Fig. 4c). The tire was subsequently rolled using a commercial treadmill roller.

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This experimental setup allows precise control of the force applied to the sensor by adjusting the deformation on the motorized test stand. In this experiment, the point at which the tire just contacts the treadmill roller was designated as 0 mm deformation. Deformations of 1, 2, and 3 mm were then applied at a constant speed towards the roller to assess the effect of force on the sensor. Additionally, the speed of the treadmill was varied so that the tire and wheel rotated at 42, 68, and 96 rpm under constant deformation conditions.

Results and Discussion

First, the normal force sensor (Fig. 1a) was integrated into the tire setup. The sensor was then tested (Fig. 5a) under three different deformations (1, 2, and 3 mm) at a constant speed of 42 rpm. The results show that whenever the sensor makes contact with the treadmill roller, a voltage peak is observed in the output. Additionally, the magnitude of the voltage peak increases with greater deformation, indicating that the sensor is responsive to changes in force. Therefore, it can be concluded that the sensor can be effectively characterized for force measurement, as the voltage output correlates with the applied force on the tire.

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Subsequently, the sensor was tested (Fig. 5b) at variable speeds (42, 68, and 96 rpm) while maintaining a constant deformation of 2 mm. The graph illustrates that the number of cycles within a specific time range varies with the speed. Counting each voltage peak as one cycle and calculating the rpm for each speed, the results were 45, 72, and 96 rpm, respectively. These calculated values are close to the actual rpm. However, slight inaccuracies were observed because the data were collected over a limited time range. If the entire experiment duration were used to count the peaks, the measurement error would be reduced, leading to more accurate rpm calculations.

Next, the tangential force sensor (Fig. 1b) was integrated into the setup and subjected to the same testing parameters. Figure 6a illustrates the effect of deformation on the voltage output. As in the previous experiment, the tangential force sensor shows an increase in the magnitude of the voltage peak with greater force. However, in this case, two voltage peaks are observed per cycle. This phenomenon occurs because the tire is non-pneumatic, and the deformation in the contact patch generates an inward force on the upper portion of the tire, leading to the secondary voltage peak [15]. In Fig. 6b, the number of voltage peaks recorded by the sensor within a given time frame is consistent with the rpm count, confirming the sensor’s reliability in detecting tangential forces.

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Finally, the normal and tangential force sensor (Fig. 1c) was integrated into the tire setup. The signal characteristics are similar to those in the previous experiments. This sensor provides the normal and tangential force data from the same location (Fig. 7). Therefore, for both curves, the peak is in the same location. Similarly, in this experiment the voltage peak magnitude increases with the deformation and number of voltage peaks in a given time align with the rpm of the wheel.

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The results are promising and demonstrate significant potential for integration into tire health monitoring systems. The data obtained from the sensors can be utilized for both prognosis and diagnosis of tire-related issues, offering a proactive approach to maintenance. Additionally, the elastomer material exhibits strong adhesion to the tire, enabling precise measurement of normal and tangential forces at specific contact points. The insights gained from these measurements can accurately predict the extent of the tire’s surface contact, as well as assess the available traction and grip. Armed with this information, vehicle safety can be substantially enhanced, as drivers and maintenance systems can make informed decisions to prevent tire failures and optimize performance.

There are some opportunities for improvements too. The sensor needs to be characterized with real-life-size tires. To do that, elastomer elastic layers can be replaced with rubber. As both the IL [11] and rubber [16,17] can be printed using a direct ink write process (material extrusion 3D printing [18]), a 3D-printed rubber sensor can be manufactured. The sensor used is subjected to bending stress. For this reason, there are some issues with repeatability. However, a conformal 3D-printed sensor can be used to print over complex structures such as the inner liner of a full-scale tire to reduce bending stress [12,19].

Conclusion

This study successfully demonstrates the integration and characterization of normal and tangential force sensors in the tire for dynamic loading. The primary focus was on the development of the sensor and evaluating how its output varies under different normal and tangential force conditions. The results indicate that the sensor reliably detects these forces, with the voltage response showing a clear correlation with the applied forces. Additionally, the sensor can predict rpm of the rolling wheel. The ability to precisely measure normal and tangential force offers insights into tire behaviors, including surface contact, traction, and grip, which are critical parameters for ensuring vehicle safety and performance. Future work will focus on refining the sensor design, improving the test setups, and exploring the application in various driving conditions to further validate its effectiveness.