Abstract:
The invention provides a system and method for measuring tire forces wherein the force contributions of the test machine may be separated from the force contributions of the tire. The invention achieves the separation of the forces by equipping the test station with a plurality of accelerometers that allow the forces and the moments of the components of the measurement station to be calculated and accounted for in the overall force measurement. In one embodiment, the test station uses a slip ring disposed between the rotating rotor to which the tire is mounted and the stationary bearing housing to allow data to be easily gathered from the accelerometers mounted on the rotor. The slip ring also allows tires to be easily mounted and dismounted.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to systems used to measure forces in pneumatic tires. More particularly, the present invention relates to a system for measuring tire force generation and tire force transmission as a loaded tire rolls over a road surface. Specifically, the present invention relates to a system and method for measuring tire forces wherein the force contributions of the test machine are separately measured from and added into the resulting force measurement. 
     2. Background Information 
     Tire manufacturers desire to compare the forces generated and transmitted by different tire constructions to determine a preferred tire construction for a particular application. The subject forces are created when a loaded tire rolls over a road surface. A tire rolling over a road surface creates vibration forces that will create tire noise. Minimizing or controlling tire noise is one of the objects of tire design. Force testing provides the tire designer a method for comparing alternative tire constructions. 
     Tire forces have been measured in the past by mounting the subject tire on a rim that is then mounted to a rotor. The rotor is rotatably mounted on a bearing housing that is connected to a load cell or force measurement device. The load cell or force measurement device is configured to measure three axial forces and the moments about the reference axes. One type of load cell is known in the art as a Kistler load cell and is available from Kistler of Switzerland. The tire is brought into contact with a rotating simulated road surface that causes the tire and rotor to rotate. The load cell measures the forces and delivers the measurements to a recording device such as a computer that records the load cell forces. The readings may be displayed for use by investigators in a variety of different ways. One method of displaying the data is to perform a Fourier transformation on the data to display it in the frequency domain. 
     In past systems, the inertial forces experienced by the rotor, the bearing housing and the measurement device would be measured by the load cell in combination with the tire forces. The inertial forces do not influence force measurements at low frequencies because the inertial forces are generally higher frequency forces. At the higher frequencies, the machine forces combine with the tire forces to yield useless results. For instance, those skilled in the art know that tires have certain measurable resonances such as the resonances caused by the tire cavity and the body cords. Another known resonance occurs in the wheel to which the tire is mounted. Although these resonances are known to exist, the existing tire force measurement systems do not accurately reveal these resonances because the forces of the test machine hide these resonances. The persons testing the tires desire a system that accounts for the machine forces when it measures the tire forces. 
     In addition to the force accounting, the users desire a system that is as easy to use as prior art systems. When multiple tire constructions must be measured and compared, each tire must be loaded onto the test machine and tested. The test tires are currently loaded onto machines by using standard lug nuts to hold the wheel to the rotor. The entire loading and test process takes less than 5 minutes. Any improved system should be as quick and easy to use as these systems so that they will be readily accepted by those who perform the tests. 
     SUMMARY OF THE INVENTION 
     The invention provides a system and method for measuring tire forces wherein the force contributions of the test machine are separately measured and accounted for in the overall measurement of the tire force. The invention accounts for the machine forces by equipping the test station with a plurality of accelerometers that allow the forces and the moments of the components of the test station to be independently calculated from the overall measured forces. 
     The system of the invention allows wheels to be quickly mounted to the test station so that they may be tested as quickly as in known test stations. In one embodiment of the invention, a slip ring is disposed between the bearing housing and rotor so that the data from the rotor accelerometers may be gathered by the data gathering device of the test station. The slip ring allows the tire-wheel assemblies to be mounted and dismounted from the rotor without disconnecting the accelerometers from the data gathering device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of the system for measuring tire forces according to the present invention. 
     FIG. 2 is a side view of the system of FIG.  1 . 
     FIG. 3 is a view similar to FIG. 2 with the rotor and bearing housing depicted in section. 
     FIG. 4 is a graph showing the results of the prior art system compared to the invention results. 
     FIG. 5 is model of the system showing how the force equations are developed. 
     FIG. 6 is model of the system showing how the moment equations are developed. 
    
    
     Similar numbers refer to similar parts throughout the specification. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A test station according to the concepts of the present invention is indicated generally by the numeral  10  in the accompanying drawings. Test station  10  is used in conjunction with a moving road surface simulator  12  such as the rotating wheel depicted in FIG.  1 . The exterior surface  14  of simulator  12  simulates the roughness and frictional characteristics of a typical driving surface. For instance, surface  14  may be configured to simulate a coarse road surface. Most of simulator  12  may be disposed below a floor  16  such that only a small portion  18  is exposed though an opening  20  defined by floor  16 . The tire  22  to be tested at station  10  contacts portion  18 . Rotation of simulator  12  will cause tire  2  to rotate to simulate a moving vehicle tire. 
     Station  10  includes a computer  24  that is configured to record the operating conditions of station  10  and forces generated and transmitted by station  10 . Computer  24  includes the necessary memory and processing units to store and execute the instructions that make the calculations of the present invention. In one embodiment of the invention, computer  24  may be a personal computer. Station  10  further includes a base  30  that is mounted to floor  16  and supports the other elements of station  10 . A force measurement device or load cell  32  is supported by base  30 . Base  30  includes a adjustment device that is adapted to move force measurement device  32  and tire  22  with respect to simulator  12  to create a load on tire  22  that simulates the weight of a vehicle. Force measurement device  32  is adapted to measure forces along the X, Y, and Z axes as well as the moments about these axes. One type of force measurement device  32  is known in the art as a Kistler load cell. 
     A bearing housing  34  is carried by force measurement device  32 . Housing  34  rotatably supports a rotor  36  on bearings  38 . The structure and operation of bearing housing  34  and rotor  36  are known to those skilled in the art. Tire  22  is mounted to a rim  40  that may be connected to rotor  36  by conventional connectors such as the lug bolts  42  depicted in the drawings. 
     In the past, tire  22  would be rotated by simulator  12  and force measurement device  32  would record the forces and moments over time. A typical result of this test is depicted by line  44  on FIG.  4 . The vertical axis of this graph represents the magnitude of the measured force while the horizontal axis of the graph represents frequency. Line  44  includes all of the forces measured by force measurement device  32 , bearing housing  34 , and rotor  36 . These vibration forces generally occur at higher frequencies and will mask the higher frequency resonances of tire  22  that need to be measured to compare different tire constructions. 
     In the present system, station  10  is configured to measure the forces of measurement device  32 , bearing housing  34 , and rotor  36  and account for these forces during the force calculations. This system thus yields only the forces created and transmitted by tire  22  during the test procedure. These forces include the high frequency resonances that are known to occur in tire  22  that were hidden by vibration forces in the past. 
     Station  10  includes a force measurement system design to measure the inertial forces generated by the motion of and transmitted by measurement device  32 , bearing housing  34 , and rotor  36 . In the embodiment of the invention depicted in the drawings, four accelerometers  50   K, B, R  are connected to each of measurement device  32 , bearing housing  34 , and rotor  36  to measure the accelerations of four points disposed about each element. These measurements are combined with the masses of the elements to calculate the forces and moments at their centers of gravity. These forces and moments are combined with the overall measured forces and moments so that the test station forces are accounted for during the test procedure. FIG. 4 depicts line  52  that represents the forces of tire  22  measured by the method of the present invention. One should note that the vertical scale is logarithmic and at some points of line  44  differ from line  52  by a factor of 10. In FIG. 4, the measured tire had known acoustic cavity resonances between 200 and 300 Hz. These resonances are measured by station  10  and identified by the numeral  54 . 
     The forces and moments of each component are calculated by measuring the acceleration of the center of gravity of each component. The acceleration is then multiplied by the mass of the component to calculate the force components at the center of gravity. The force components are then used to calculate the moments of each component. The accelerations of the centers of gravity are measured by mounting accelerometers on each component at known locations with respect to the center of gravity of the component. 
     In one embodiment of the invention, accelerometers  50  are disposed at equally-spaced locations about each element of station  10 . Accelerometers  50  may be spaced apart by 90 degrees and are located on the X and Z axes of station  10  as depicted in the model shown in FIG.  5 . Accelerometers  50   K1, K2, K3, and K4  may be disposed on the exterior surface of force measurement device  32 . Each accelerometer  50   K  is in communication with computer  24  by a suitable connectors. Accelerometers  50   B1, B2, B3, and B4  may be disposed on the exterior surface of bearing housing  34  and in communication with computer  24  by suitable connectors. Accelerometers  50   R1, R2, R3, and R4  may be disposed on an interior surface of rotor  36  and in communication with computer  24  by suitable connectors. The location of each accelerometer  50  with respect to the center of gravity of its test station component is known so that the acceleration components recorded by the accelerometers may be used to calculate force components at the centers of gravity. 
     A slip ring  60  may be positioned between rotor  36  and bearing housing  34  to allow accelerometers  50   R  to rotate with rotor without tangling the connections between computer  24  and accelerometers  50   R . Slip ring  60  may be a slip ring provided by Michigan Scientific of Charlevoix, Mich. Slip ring  60  allows tire  22  to be quickly mounted onto rotor  36  and dismounted from rotor  36  so that the test procedure of the invention does not require any more time than the test procedure of the prior art. In another embodiment of the invention, slip ring  60  may be positioned outside of rotor  36 . In this embodiment, the slip ring must partially disassembled so that tire  22  may be mounted and dismounted. 
     Each accelerometer  50  is positioned a known vector  52   K, B, R  from the center of gravity  54   K, B, R  such that the acceleration components at the center of gravity  54   K, B, R  can be calculated by summing the acceleration components at all of accelerometers  50   K, B, R . In one embodiment of the invention, accelerometers  50   K, B, R  are sampled at 6250 Hz during the test procedure. The acceleration components A x, y, z  are then multiplied by the mass of the component to yield the force components F x, y, z  acting on the center of gravities  54   K, B, R . These force measurements are transformed to the frequency domain by performing a Fourier transformation on the time-domain data. 
     These force measurements are also used to determine the moments created by the test station components. In one embodiment of the invention, the “y” moment component is considered to be negligible and is not considered in the final calculation. The moment in the “y” direction will only yield the moment created by the bearings which has been found to be negligible. The total moment measurement includes the summation of: (1) the moment for the force measurement device  32 ; (2) the cross product of the moment producing forces in device  32  with their distance to the point of moment summation; (3) the inertial moments due to angular acceleration; and (4) the inertial moments due to translational acceleration of the supporting machine masses. The gyroscopic moment terms are also negligible and are, therefore, ignored. The sum of these moments are added together and used instead of the moments measured by the force measurement device  32 . 
     The measured forces and moments of the test station components are added to the total force measurement in order to yield the forces and moments of the tire. Addition of the forces of the test station components yields the tire forces because of the phase or sign of the test station forces. The phase or sign of these forces causes them to be eliminated from the overall measurements yielding the desired test results. 
     In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. 
     Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described.