ASTM E556E556M-11 (Reapproved 2015)

Designation: E556/E556M − 11 (Reapproved 2015)Standard Test Method forCalibrating a Wheel Force or Torque Transducer Using aCalibration Platform (User Level)1This standard is issued under the fixed designation E556/E556M; the number immediately following the designation indicates the yearof original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the calibration of the force (ortorque) transducer and associated instrumentation of a mountedtest wheel by using a calibration platform.1.2 This test method is a static calibration, simulating thetraction force between a tire and the pavement.1.3 In the case of a force-measuring system, the instrumen-tation readout is directly proportional to the calibration forceinput.1.4 In the case of a torque-measuring system, the instru-mentation readout is a measure of the calibration force inputand the effective tire radius.1.5 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem shall be used independently of the other. Combiningvalues from the two systems may result in non-conformancewith the standard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E274 Test Method for Skid Resistance of Paved SurfacesUsing a Full-Scale TireF377 Practice for Calibration of Braking/Tractive MeasuringDevices for Testing Tires3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 calibration platform—a moving platform for applyinga force in the contact plane of a tire, and associated means formeasuring the applied force. The calibration platform consistsof a rigid plate with a high friction surface, in contact with thetire footprint, supported on a frictionless, preferably air, bear-ing. It may also be instrumented to measure vertical forces(loads).3.1.2 calibration reference signals—repeatable signals inthe range of expected wheel-force transducer system loading.These signals could either be constant voltages or preferablyproduced by a strain-gage calibration shunt resistor.3.1.3 crosstalk—the undesired effect of force readings ap-pearing on an unloaded axis of a transducer while applyingforce to another.3.1.4 horizontal traction (traction)—a force acting in ahorizontal axis through the wheel transducer; that is, lockedwheel drag force.3.1.5 hysteresis—the maximum difference between corre-sponding transducer outputs (of the wheel force transducersystem) at increasing and decreasing applied calibration force,expressed as a percentage of full load. Proven outliers areexcluded.3.1.6 nonlinearity—the maximum deviation of the trans-ducer output(s) (of the wheel force transducer system) from thebest-fit linear relation to the applied calibration force, ex-pressed as a percentage of full scale. Proven outliers areexcluded.3.1.7 test wheel—a wheel and test tire assembly mounted toa test vehicle by means of a force or torque transducer.3.1.8 vertical load (load)—force acting in a vertical axisthrough the wheel transducer; that is, weight.3.1.9 wheel force transducer system—a force-to-electricalsignal converter system including transducer(s), associatedsignal condition, zeroing, amplifying, recording, and monitor-ing instrumentation.4. Summary of Test Method4.1 The vertical static loads (forces) on the test wheel(s) andhitch are established. If the tester is of the trailer type, it must1This test method is under the jurisdiction of ASTM Committee E17 on Vehicle- Pavement Systems and is the direct responsibility of Subcommittee E17.21 onField Methods for Measuring Tire Pavement Friction.Current edition approved Sept. 1, 2015. Published December 2015. Originallyapproved in 1975. Last previous edition approved in 2011 as E556 – 11. DOI:10.1520/E0556_E0556M-11R15.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1be connected to its tow vehicle. Traction forces are applied tothe locked wheel(s) at the tire-platform interface. The calibra-tion platform consists of a bearing that is frictionless in alldirections of the horizontal plane supporting a rigid plate witha high friction surface. The force applied to this platform ismeasured by suitable electronic force transducers.4.2 Wheel transducer linearity and hysteresis are deter-mined from an electronic digital meter (x-y chart optional) andare correlated with the tow vehicle instrumentation. Techniquesare given for establishing reference signals to be used ascalibration points.4.3 With calibration platforms having force-measurementcapability in the vertical direction, test wheel load (force) isrecorded as a function of the applied tractive calibration force.4.4 With a wheel transducer having normal load (force)-measuring capability, the transducer output is compared withthe corresponding vertical output of the calibration platform.5. Significance and Use5.1 Wheel force or torque transducers are used underdynamic test conditions, and the measurements are subject tomany error sources. The static calibration recommended by thepresent standard cannot eliminate all error sources. Its signifi-cance is in providing an accurate calibration of the transducerand the associated electronics, readout, and recording equip-ment.5.2 Calibration result may be used to either make mechani-cal or electronic adjustments until the readout agrees with thecalibration input. Alternatively, calibration curves or tablesmay be prepared to be used as corrections to measured results.6. Apparatus6.1 The calibration apparatus shall consist of the basiccomponents described below, and calibrated in accordancewith 6.3 of Practice F377.6.1.1 A calibration platform for applying tractive forces atthe tire-platform interface of a test wheel, with a calibratedinternal or external reference force transducer to measure theseforces. Capability of measuring vertical forces (wheel loads) isalso desirable.6.1.2 A system for generating traction forces appliedthrough the calibration platform.6.1.3 Instrumentation for recording or indicating transduceroutputs, including digital meters and optionally an x-y recorder.6.1.4 Level, tire pressure gage and thermometer.6.1.5 Scale(s) accurate within 0.5 % of maximum expectedaxle load for the test trailer.7. Preparation of Apparatus7.1 Site—Select a sheltered, level paved area. (A hardsurface, preferably under cover, is desirable.)7.2 Calibration Platform and Associated Instrumentation—Prepare the calibration platform and associated instrumentationby following established set-up procedures and the manufac-turer’s manuals where applicable.7.3 Test Tires—Set the inflation pressure under normalvertical load (force). Inflation pressure shall be 24.0 6 0.5 psi(165.0 6 3.5 kPa) at ambient temperature (cold).7.4 Traction Tester Instrumentation—Check the test vehicleinstrumentation by following established set-up procedures forthe system with reference to the recommended procedure in themanufacturer’s manual, where applicable. Set all gains andoffsets at values normally used during friction testing. Recordthe settings.7.4.1 Force-Type Transducer—Load crosstalk into traction:if, with full normal load back on the test wheel, the tractionoutput is different from zero by more than 1 % of the appliednormal load, adjust by either rotating the transducer (ifprovisions for rotation have been made) or by changing thehitch height, that is, tilting the trailer to orient the transducerhorizontal axis parallel to the calibration plane. Then apply 500lb of traction force and note the change in load reading. If morethan 1 % crosstalk, check transducer leveling and repeat thistest.7.4.2 Torque Type Transducer—Use steps 7.4 and 7.4.1,substituting the torque output for the traction. If the load intotorque crosstalk is greater than 1 % of the applied normal load,refer to the manufacture since there are normally no useradjustments.7.5 Test Trailer— Position the trailer coupled to its towvehicle on level ground and set the trailer variables such as tirepressure, hitch height, and inflation pressure of air shockabsorbers where present to the normal operating values.Measure the hitch height from level ground to the center of thehitch. Decouple the trailer and weigh it at each point ofsupport, maintaining its relative level. Correct the weights ifneeded to comply with Test Method E274. Recouple the trailerto the towing vehicle.NOTE 1—The tow vehicle should have1⁄2 tank of water, and a full fuelload before leveling the trailer.7.6 Wheel Transducer— With the test wheel loaded, thewheel transducer traction output should be adjusted to zero.Then remove 500 lb of the load and recheck traction zero.7.7 Record all measures, weights, and dimensions, and thetrailer leveling adjustments that give zero transducer outputs.8. Frequency of Calibration8.1 It is recommended that the system be calibrated accord-ing to all sections of this specification at least once a year.8.2 For routine test to indicate normal system operationsonly, it may be sufficient to abbreviate the procedure in Section9.8.3 Calibration should be performed at least once a monthaccording to 8.2.8.4 The calibration should be performed whenever themeasured test results are inconsistent with previous values fora given test site.9. Procedure9.1 Determine that the calibration platform has been suit-ably calibrated within a reasonable period of time (see 6.1), oneyear.E556/E556M − 11 (2015)29.2 If the test vehicle is of the trailer type, hitch and align itwith the tow vehicle. The calibration may be performed withauxiliary electrical power for the instrumentation. In this case,after completing the calibration, check the system whenpowered by the vehicle electrical system with the enginerunning, to assure that the output(s) have not changed. Thepreparations described in Section 7 shall have been performed.9.3 Place the test wheel(s) on the calibration platform(s)using blocks, if needed, under the other wheels to maintain alevel plane of support for all system tires.9.4 With the test wheel raised clear of the platform, zero theoutput(s) of the calibration platform in accordance with estab-lished operating procedures. Zero the wheel transducer out-put(s). (Warning—Make sure that wheel brakes do not drag.)9.5 Align the calibration platform(s) and the associatedforce application system(s) with the test tire(s) and lower thetest wheel onto the platform.9.6 The platform shall be level within 60.25° prior to thecalibration.NOTE 2—Increased accuracy may be obtained by maintaining theaforementioned level throughout the calibration procedure.9.7 Record the ambient temperature. To meet this standard,calibration can only be done when the ambient temperature isbetween 40 and 100°F (4 and 38°C).9.8 Use digital meter(s) and optionally x-y recorder(s) toobserve the wheel-force transducer system signals at the inputto the system recorder.9.8.1 With the trailer level and with full vertical load andzero traction force applied to the test wheel(s), adjust thesystem vertical load indicator to correspond with the platformvertical load value.9.8.2 Lock the test wheel and apply a midrange tractiveforce (typically 500-lbf 2.2 kN) via the platform to the testwheel. Adjust the system traction indicator to correspond withthe platform traction value. Return the traction force to zero.9.9 Record the wheel-transducer vertical load output and theforce-place transducer vertical output, on both the systemrecorder and meter while increasing and decreasing the verticalforce applied by at least 20 % of the static wheel load value in100 lbf (445 N) increments. This can be done by adding 200 lb(shot bags) to the trailer wheel load.9.10 Establish the linearity and hysteresis characteristics forthe vertical transducer output, if applicable, using the data in9.9.9.11 Record the wheel transducer output(s) (horizontal trac-tion and vertical load) versus the platform traction and verticalload readings on both the system-recorder and the meter whileincreasing and decreasing the traction force by 100 lbf (445 N)increments through the full calibration range (typically 0 to50 % of vertical load).NOTE 3—For a two-wheeled towed test trailer, the test wheel verticalload should decrease linearly from the zero traction value as the appliedtraction force increases at a slope proportional to the H/L ratio of thetrailer3where H is the initial hitch height and L is the initial hitch-to-axlelength (see Test Method E274). The non-test wheel vertical load (force)should remain constant at the zero traction value.9.12 If the test wheel load and traction values obtained in9.9 and 9.11 agree with the platform values within 61%,apply system load and traction calibration signals and recordfor the future reference. If differences greater than 1 % arefound, the system may require maintenance or the crosstalkshould be rechecked as described in 7.6.NOTE 4—Some calibration platforms use ball or roller bearing (insteadof air bearings). Any side force on the wheel under calibration willgenerate additional friction forces in the calibration platform and causeerroneous readings and hysteresis.9.13 Perform the calibration procedure of Section 9 for thesecond wheel transducer, if applicable.10. Report10.1 The calibration report shall contain the followinginformation:10.1.1 Skid measuring system identification,10.1.2 Wheel-transducer identification,10.1.3 Date,10.1.4 Test equipment identification in accordance withcalibration report(s),10.1.5 Test tires identification,10.1.6 Static weight at wheels and hitch,10.1.7 Setting of the skid tester variables,10.1.8 Measured values H (hitch height) and L (length fromhitch to wheel center).10.1.9 Tow vehicle standardization details,10.1.10 Instrumentation check-out details,10.1.11 Orientation of the transducer-sensitive axis,10.1.12 Statement of loading conditions under which thereadouts are zeroed,10.1.13 Loading conditions under which the reference cali-bration points are applicable,10.1.14 Record of the on-board and external indicatoroutputs of each transducer output versus increasing and de-creasing traction force and vertical load (force),10.1.15 Statement of calibration values, linearity, and hys-teresis characteristics for the transducer and its associat