1. Field of the Invention
The present invention is directed generally to a method and apparatus for sensing the depth of a tire tread, and more particularly, to a noninvasive method and apparatus for measuring a tread depth remaining on a tire while minimizing structural damage to the tire itself.
2. Description of Related Art
New tires are formed with a tread that includes channels through which water, snow and other environmental debris commonly found on roads can pass as the vehicle travels along a public roadway. By allowing the water to enter the channel, the amount of water passing between the roadway surface and an outermost tire surface is minimized, thereby reducing the tendency of the water to lift the vehicle from the roadway surface. Such channels are mandated on all tires to be installed on vehicles that are licensed to travel public roadways, and depending on the type of tire, come in a variety of depths, widths and patterns.
Tread depth is a vertical measurement between the outermost surface of the tire that will actually contact the roadway surface and the bottom surface of the tread's channels that is approximately parallel to the outermost surface of the tire. In the United States, tread depth is measured in increments of 1/32 of an inch to determine whether the tire is safe for use with a vehicle licensed to navigate public roadways. Traditionally, this measurement has required the partial insertion of a conventional ruler into the channel until a terminal end of the ruler contacts the bottom surface of that channel. The tread depth is then read by an onlooker viewing the ruler's scale extending out of the channel from the plane of the tire's outermost surface.
Measuring the tread depth of a tire with a ruler is difficult and inaccurate. Often the tire with the tread depth being measured is installed on a vehicle that is resting on the ground. Wheel wells, quarter panels and other body parts of the vehicle on which the tire is installed can obstruct the onlooker's view of the ruler, and can often interfere with the insertion of a lengthy ruler into the channel at a location from where it can be easily viewed by the onlooker making the measurement. Further, the fine 1/32 inch increments of the tread depth measurement and the inconsistent viewing angle of the onlooker onto the ruler from tire to tire often skew the subjective reading of the ruler by the onlooker. Such inaccuracies can result in the erroneous conclusion that a questionable tire is safe to travel along public roadways when, in reality, it is not.
Other devices for measuring tread depth rely less on the subjective reading of a ruler's scale than the use of a ruler does. For example, a tire tread depth gauge includes a single demarcation distinguishing an acceptable tread depth from an unacceptable tread depth. The gauge can be smaller than a ruler, thereby facilitating use of the gauge in cramped spaces where a ruler may not otherwise be useable. Further, the single marking on the gauge simplifies measurements and provides an objective determination as to whether the remaining tread depth of a tire is acceptable for continued use on a vehicle licensed to travel public roadways. One form of such a gauge is a U.S. Lincoln Penny. As a rule of thumb, when the Lincoln Penny is inserted into the channel head first and Lincoln's entire head is visible when observed from a plane of the tire's outermost surface, the tire's tread has been worn to approximately 2/32 of an inch, and is considered legally worn out in most States.
Although the use of a commercial gauge or simply a Lincoln Penny provides a more objective measurement tool than a ruler, it is still prone to inconsistencies that can lead to erroneous measurements. For example, the onlooker making the measurement may view the extent to which Lincoln's head is visible from a plane other than that which is in the plane of the tire's outermost surface. Thus, the same measurement can vary each time the measurement is performed. And regardless of the objectivity of the measurements made with a gauge or a ruler, measurement of a tire's tread depth with a gauge requires use of the vehicle on which the tire is installed to be discontinued for considerable periods of time for the inspection. Further, keeping records documenting tread wear is time consuming when the tread depth is manually measured with rulers and gauges. Tread wear records for each individual tire in such cases must be manually created and maintained, which also requires each tire to first be manually identified and distinguished from other tires that are to be inspected.
Attempts have been made to automate the measurement of a tire's tread depth. Such devices utilize many laser light sources and sensors positioned in an array to illuminate the outermost surface of the tire's tread, along with side wall profiles. The measurements performed by the array of lasers and sensors encompassed the entire circumference of the tire, and accordingly, required the tire to be removed from the vehicle and rotated by a dedicated stand positioned adjacent to the array of lasers and sensors. Such an arrangement is cumbersome, and makes inspection of tires provided to a fleet of vehicles impractical.
Accordingly, there is a need in the art for an apparatus for accurately and consistently measuring the tread depth of a tire. The apparatus can measure the remaining tread depth of the tire without physically contacting the tire with a measuring device, and can optionally identify and differentiate a plurality of tires from each other in addition to performing the tread depth measurement. Further, the apparatus can optionally measure the depth of a plurality of channels across a width of the tire's tread, and can optionally generate, maintain and update tire wear records automatically, without human intervention.