Wheel lift sensors used during dynamic testing of light vehicles

Transcription

1 SUBJECT: Wheel lift sensors used during dynamic testing of light vehicles TO: D.O.T. Docket No. NHTSA FROM: Patrick Boyd, NHTSA NVS-123 DATE: March 31, 2003 The attached description briefly describes NHTSA s setup procedures for sensors used to measure wheel lift during dynamic testing. Two methods are given. Diagrams that help to illustrate these procedures, and the equations used to determine the wheel lift, are also provided. This report is being docketed in response to questions at NHTSA s public workshop on dynamic rollover testing held at our Vehicle Research and Test Center in East Liberty, Ohio on December 3,

2 US Department of Transportation National Highway Traffic Safety Administration NHTSA s Setup Procedures For Wheel Lift Sensors A Brief Overview March

3 SETUP PROCEDURES The National Highway Traffic Safety Administration s (NHTSA) Vehicle Research and Test Center (VRTC) uses two methods to configure the infrared displacement sensors used to measure wheel lift during dynamic rollover resistance testing. Both methods assume the sensors have been properly calibrated before attaching them to the vehicle (i.e., the precise gain of each sensor is known). To determine the method used to setup the wheel lift sensors the camber angle for each wheel/tire assembly should be measured with the test vehicle at rest on level ground. While Method #1 can be used for all types of suspensions, NHTSA recommends this method be used if the magnitude of the camber angle exceeds ± 0.3. Method #2 can be used for camber angles within a range of ± 0.3. The 0.3 camber angle corresponds to a vertical offset of less than 1/16 of an inch across a 12-inch wide tire and is therefore considered insignificant. Although Method #1 is universally applicable, it requires more calculations than used in Method #2. Figure 1 presents two sample scenarios. Each requires a different configuration. The diagram to the left shows the tire/wheel package at a 5 camber angle (requires Method #1). The diagram to the right in Figure 1 shows a zero camber situation (Method #1 or Method #2 can be used). VRTC has found Method #2 to be applicable for many solid axle type suspensions, but can be used on independent suspensions that have zero camber at rest. -5 Camber Angle 0 Camber Angle Figure 1: Shows two different scenarios the diagram to the left it 0 camber and the diagram to the right is a situation in which the tire has 5 of camber. Method #1 Method #1 can be used for all camber angles and suspension types. The left-side illustration of Figure 2 shows a wheel/tire assembly with a camber angle of 5. The assembly is under a load and has static tire compression. Configuration of the wheel lift sensors used on this assembly requires Method #1 for the proper setup. The right-side illustration of Figure 2 shows the same assembly but is unloaded with the tire just barely touching the floor. This represents a zero wheel lift condition. The orientation of the associated with the right-side assembly (i.e., the 1

4 position of the tire with respect to the ground) is used to determine the sensor outputs that correlate with the wheel/tire assembly s camber angle and zero wheel lift condition. Loaded Tire Unloaded Tire Figure 2: On the left is a diagram showing the loaded tire with a 5 camber angle. To the right is the same tire unloaded at zero wheel lift. The latter is used to determine the sensor outputs. Figure 3 presents applicable equations along with a diagram of the wheel in a zero lift situation showing how the variables and constants are defined. Knowing the camber angle, Dist1, and Dist2, the offset for each sensor (OffsetDist1 and OffsetDist2) is determined. With the wheel/tire assembly at zero wheel lift, the equations used to determine the offset for each sensor are: OffsetDist1 = Dist2 x Tan (Camber Angle) OffsetDist2 = (Dist2-Dist1) x Tan (Camber Angle) Dist1 and Dist 2 are measured with a tape measure with the wheel/tire assembly unloaded. The camber angle is measured with a digital level with the wheel/tire assembly in the zero wheel lift state. While the vehicle is at zero wheel lift, OffsetDist1 and OffsetDist2 are computed. The data acquisition system is then set accordingly so that each corresponding sensor s output match the offset computed for that sensor. 2

5 Figure 3: Wheel lift diagram above shows how sensor output is determined with the tire/wheel assembly in the zero wheel lift position. 3

6 Method #2 Method #2 is used for camber angles within a range of ± 0.3. Method #2 is a simplified version of Method #1. Method #1 uses equations that take a constant and multiply it by the tangent of camber angle to determine the initial offset of the sensor output. Taking the tangent of a camber angle that is approximately zero is zero, so no offset is needed to determine the sensor outputs with regards to camber angle for Method #2. To begin the setup, the wheel/tire assembly should be raised to the zero wheel lift state. To raise the wheel/tire assemblies installed on a solid axle type suspension from the ground, a single floor jack is placed in the center of the axle and raised until the load on the wheel/tire of interest is nearly zero. For vehicles with independent suspensions, two floor jacks may be required. The first is used to raise the vehicle near the wheel/tire of interest (i.e., to unload that corner of the vehicle). The second jack can then be used to carefully raise the control arm of the wheel/tire of interest in a manner that preserves a camber angle within a range of ± 0.3. With either method, the wheel/tire assembly of interest is raised until the tire is just touching the ground (zero wheel lift). At this point the wheel/tire assembly should rotate with only a slight surface drag. At some location on the wheel lift sensor assembly, a reference mark shall be made. Using a tape measure, the vertical distance from the reference mark to the ground is recorded (D1). The vehicle is then lowered and the floor jack(s) removed. Using the previously defined reference mark, the tape measure is used to measure the new vertical distance to the ground (D2). The difference between these two measurements (D1-D2) is the static compression of the loaded tire, as indicated in Figure 4. With the vehicle at rest on level ground, the zero offset of the wheel lift sensors is adjusted such that the magnitude of the sensor outputs (i.e., the signals transmitted to the data acquisition system) are equal to the magnitude of the static compression of the loaded tire. Since the camber angle of the wheel/tire is approximately zero, the values of both sensors at the assembly of interest shall be set to the same value. This value will be negative. VRTC has found the compression of most tires to range from 1.5 to 0.5 inches. 4

7 Figure 4: Both wheel/tire assemblies demonstrate zero camber situations. The diagram on the left is with the vehicle at rest on level ground. The figure to the right is with the tire just touching the ground. This is defined as zero wheel lift. Shown are the reference points and how static tire compression is determined. Comments Both methods make use of the assumption that the inside edge of the tire is the closest part of the tire to the ground when wheel lift occurs. However, wheel lift can be determined for any part of the tread along the bottom of the tire by varying Dist2. Also of note, the sensors used by VRTC are sensitive to surface emissivity. For this reason, they should be calibrated on a surface that is similar to the surface on which the tests will be performed. VRTC uses a piece of matte black paper to simulate a uniform dry asphalt test surface for both configuration methods. Use of a shiny painted shop floor during the calibration or configuration of the sensors should be avoided. USE OF SENSOR DATA TO DETERMINE WHEEL LIFT Once calibrated and configured, the wheel lift sensors may be placed into service. As illustrated in Figure 5, the term wheel lift describes the distance from the inside edge of an inboard tire to the ground. The equations used to derive wheel lift from sensor data recorded in the field are provided in Figure 5. Once wheel lift is resolved, the result is filtered with a 200 ms, one-pass, digital running average filter to reduce noise and remove instantaneous peaks in the data. NHTSA defines wheel lift as at least two inches of simultaneous wheel lift of the vehicle s two inboard tires from the ground. In the case of a right-steer J-Turn, the inboard tires would be the located on the right side of the vehicle, as perceived by the driver. 5

8 Figure 5: Method used to determine wheel lift. 6

9 CONTACT INFORMATION: Devin Elsasser Research Engineer Transportation Research Center Inc. Phone: (937) Garrick Forkenbrock Project Engineer NHTSA Vehicle Research and Test Center Phone: (937)