This research provides a capacitance-based method for monitoring the integrity of tires and other polymeric products during manufacturing and throughout the useful product life. Tires are complex composite structures composed of layers of formulated cross-linked rubber, textiles, and steel reinforcement layers. Tire production requires precise manufacturing through chemical and mechanical methods to achieve secure attachment of all layers. Tires are subjected to a variety of harsh environments, experience heavy loads, intense wear, heat, and in many cases, lack of maintenance. These conditions make tires extremely susceptible to damage.

Figure 1. The Parallel Plate Capacitor design.
The sensor presented here utilizes the existing dielectric rubber and conductive steel belts within the tire to create a parallel plate capacitor, which can detect the formation of gaps between layers due to a loss of cross-linking or environmental effects, cuts within a layer due to punctures or slicing, and decreased layer thickness due to overall wear. This system can alert the driver to tire problems before accidents occur, increasing driver safety, improving quality control from the manufacturing site, and decreasing costs due to rim and frame damage and accidents caused by tire malfunctions. Overall, this research provides a mechanism for continuously monitoring tire stability and uses current tire materials for device construction. Similar technology on hoses, seals, and polymers has also been developed.

Capacitance describes the ability of a material to store energy in the form of an electric field, which is a measurable quantity. Using capacitance produces a very sensitive instrument, as capacitance can be measured in the picoFarad range. Capacitors can also be constructed from a variety of materials. This provided a means for the application of capacitance sensing to a variety of technologies using materials already found in manufacturing. The parallel plate design is shown in Figure 1.

Several sensor designs were evaluated based on manufacturability, sensitivity, number of modes of damage detectable, and influence on tire performance. The designs used here provided the best solutions based on the criterion. The first design utilized conductive rubber for the conductive plates of the parallel plate capacitor. A second rubber with low carbon black content was mixed and milled as the dielectric layer. Each prototype contained four conductive layers that were all 81" long and 0.04" thick. Two different widths were tested: 0.8" wide and 1.2" wide. The layers were applied as seen in Figure 2. Ten tires at the 1.2" width were constructed, and five tires at the 0.8" width were constructed. Leads were placed throughout on the conductive layers. The tire size was P225/70R16.

A second tire design was constructed using wire mesh in place of conductive rubber. The tires were assembled in layers as in the multi-layer rubber design. Leads were soldered onto the wire mesh. These tires were size P235/65R17. Five tires of this prototype were created.

For all prototypes, capacitance and resistance measurements were taken throughout with a digital multimeter. After layering, the tires were cured for 15 minutes at a temperature just below 400 °F. Then, the tires were allowed to cool. Capacitance and resistance were monitored during the cooldown process. Static capacitance and resistance testing was also done using an LCR meter.

Shearography was also used to image the surface strain on the prototype tires. Shearography is a non-destructive test in which a photograph is taken when illuminating the tire, and this photograph is compared to a second photograph in which the tire is subjected to stress by placing it in a vacuum. In a shearography image, white lines represent strain. Shearography provides a method of imaging to ensure solid cross-linking in tires. Therefore, at least 18 hours after curing, shearograms were taken of the prototype tires.

Dynamic tests were conducted using a rolling resistance test stand. The prototype tires were placed on a rim and inflated to 30 or 40 psi, depending on the test setup. The test stand simulated normal forces felt by the tire at various speeds. The tire was mounted on a test axle and a metal wheel component spins at designated speeds, interacting with the tire and providing the appropriate normal force to the tire.

Figure 2. The multi-layer rubber Capacitance Circuit Prototype tire construction.
Testing results on the multi-layer rubber designs showed irregular and unexpected capacitance signals. Therefore, the tires were tested to ensure they were properly constructed. Shearography results and tire cross-sections indicated the tires had not cured, causing an air gap that resulted in unexpected capacitance measurements. The reason for the curing problem was discovered: the conductive rubber could not cure under normal curing temperature and conditions due to antioxidants in the sulfur-based cure. A peroxide-curing environment would be required for curing the conductive layers of this material configuration. Further testing was done on the wire mesh tire design to eliminate the material compatibility issue. Despite the unexpected results from this experiment, the test had significant implications. Capacitance sensing can provide a method of quality control to test tires before they leave a manufacturing facility.

The tire was tested to determine if the sensor could detect damage to the layers due to a cut formed in the tire. A drill was used to create a small hole between the belt layers at one point on the tire and the output voltage was measured. This cut was labeled as Stage 1 damage. Then, a second cut was applied to the same location of the tire. The cut was approximately 3" circumferentially around the tire, creating a small gap between the capacitive layers. The second cut was labeled Stage 2 damage. The results show that after the Stage 1 cut, a drop in voltage was seen. This drop became larger after the second cut was applied.

The results show that a change in signal due to damage as applied during testing results in a voltage change of between 14.5 and 48.9%, depending on load. Overall, a successful capacitance sensor and circuit were developed that can accurately detect changes in tire materials and connections and provide an alert to the operator when changes occur.

This work was done by Brittany Ann Newell, Gary Krutz, Keith Harmeyer, and Michael Holland of Purdue University. The full technical paper on this technology is available for purchase through SAE International at .