Wheel unbalance measurement system and method

• US 4,285,240 A
• Filed: 01/11/1980
• Issued: 08/25/1981
• Est. Priority Date: 01/11/1980
• Status: Expired due to Term

• Current Assignee: Snap-On Incorporated
• Original Assignee: FMC Corporation
• Inventors: Gold, Kenneth S.
• Primary Examiner(s): Gill, James J.
• Application Number:
  US06/111,159
• Time in Patent Office: 592 Days
• Field of Search: 73/462-466, 364/463, 364/576
• US Class Current: 73/462
• CPC Class Codes: G01M 1/225 for vehicle wheels in situ G01M1/28

Abstract

An off-the-car wheel unbalance measuring system has a rotationally driven wheel mounting shaft supported in a pedestal together with a pair of force transducers mounted in the pedestal adjacent to and spaced along the shaft. The force transducers are coupled mechanically to the shaft and provide periodic electrical output signals indicative of unbalance forces transmitted through the shaft when the shaft is driven rotationally. The angular position of the shaft is monitored with respect to an angular reference position at a predetermined number of angular increments during each full revolution of the shaft. The transducer output signals are converted in electrical circuitry within the system to digital form, and a multiplication operation is performed on the digitized signals at each angular increment using sine and cosine representative factors corresponding to that particular angular increment. The sine and cosine factors are stored in memory and are called up from storage in accordance with the monitored angular position of the shaft. The system circuitry operates to sum the digitized signals including sine and cosine factors thereby providing operating data from which unbalance force magnitude and angular position is calculated. Operation of the system while a known unbalance is mounted on the shaft and also while the shaft is running free provides data from which calibration constants for the system are calculated and thereafter applied to the operating data to correct unbalance measurements for shaft unbalance and transducer idiosyncrasies.

Claims

1. A method of measuring unbalance in a rotating body mounted on a rotating shaft coupled to a force transducer providing an unbalance signal indicative of the instantaneous unbalance magnitude component relative to a known angular reference, comprising the steps of digitizing the unbalance signal, sampling the digitized unbalance signal at each of a predetermined number of angular increments of shaft rotation, storing a plurality of digital sine and cosine representative quantities corresponding to predetermined ones of the angular increments, combining the sampled digitized unbalance signal with the corresponding sine quantity and with the corresponding cosine quantity, the step of combining being limited to include no more than one addition step so that the combinations are obtained during the time the shaft transits each angular increment, whereby separate combined digital quantities are obtained containing sine factors and cosine factors, summing the quantities containing the sine factors over a predetermined number of angular increments, summing the quantities containing the cosine factors over the predetermined number of angular increments, whereby data is obtained indicative of unbalance magnitude and angular position relative to the known angular reference.

2. A method as in claim 1 wherein the step of storing comprises the steps of storing a series of digital numbers each having only two information bits of primary interest.

3. A method as in claim 1 wherein the step of storing comprises the steps of storing a series of digital numbers having only one information bit of primary interest, and wherein the step of combining comprises only shifting the digitized unbalance signal in accordance with the number of digital places in the stored digital number, whereby the execution time for obtaining the combined digital quantities is reduced compared to the execution time for conventional digital multiplication.

4. A method as in claim 1 together with the steps of storing the data, introducing parameters descriptive of the rotating body, and defining the unbalance magnitude and angular position corresponding to the introduced parameters.

5. A method as in claim 1 together with the steps of mounting a known weight at a known position on the shaft, rotating the shaft, thereby obtaining calibration sums containing known force transducer error, removing the known weight and rotating the shaft unloaded, thereby obtaining unloaded shaft unbalance sums, combining the calibration sums and the shaft unbalance sums so that calibration constants are obtained, and applying the calibration constants to the combined digital quantities containing sine factors and cosine factors thereby providing data corrected for transducer errors and shaft unbalance.

6. A method as in claim 5 together with the step of storing the calibration constants in nonvolatile memory, whereby they are retained during power-off conditions and further calibration is necessary only after mechanical changes occur which are related to the force transducers or the rotating shaft.

7. In combination with apparatus for measuring unbalance in a rotatable body wherein at least one force transducer is disposed to provide an electrical signal indicative of periodic unbalance forces coupled thereto through a shaft adapted to support the rotatable body for rotation thereon, and wherein an analog-to-digital converter is coupled to receive the electrical signal and to provide a digital word output corresponding thereto, the improvement comprising means for controlling said analog-to-digital converter to provide said digital words at a predetermined number of angular increments during each revolution of the shaft, a memory containing a plurality of stored digital sine and cosine representative values, ones of said digital sine and cosine representative values corresponding to predetermined ones of said predetermined number of angular increments, means for modifying each of said digital words in accordance with the corresponding stored digital sine and cosine representative values and for limiting each modification to include only one addition function so that digital sine and digital cosine representative quantities are provided during the time the shaft is traversing each of said angular increments, and means for summing the digital sine quantities and the digital cosine quantities for each of said angular increments, whereby digital data is provided for determining unbalance force magnitude and direction in the rotatable body.

8. The combination of claim 7 including a processor operating to convert the data to signals indicative of unbalance weights which when counterbalanced provide a substantially balanced rotatable body.

9. A method of calibrating apparatus for measuring unbalance in a body mounted on a spin shaft having an angular reference position relative to a supporting framework therefor and being coupled to a force transducer which provides an unbalance signal having an instantaneous magnitude indicative of a component of the unbalance force which results when the shaft is spun, comprising the steps of applying a known unbalance weight to the spin shaft at a known angular position relative to the angular reference position, spinning the shaft with the known unbalance weight applied thereto, digitizing the unbalance signal indicative of the known unbalance weight, storing a plurality of digital sine and cosine representative quantities corresponding to predetermined angular increments of spin shaft rotation, modifying the digitized unbalance signal in accordance with the corresponding digital sine representative quantity at a plurality of the angular shaft increments while the shaft is being spun, modifying the digitized unbalance signal in accordance with the corresponding digital cosine representative quantity at a plurality of the angular shaft increments while the shaft is being spun, summing the modified signals containing unbalance signal and sine representative factors, summing the modified signals containing unbalance signal and cosine representative factors, whereby uncorrected calibration sums are obtained, spinning the shaft with no load applied, digitizing the unbalance signal indicative of no load shaft unbalance, modifying the digitized no load unbalance signal in accordance with the corresponding digital sine representative quantity at a plurality of the angular shaft increments while the shaft is being spun to obtain combined no load signals containing sine representative factors, modifying the digitized no load unbalance signal in accordance with the corresponding digital cosine representative quantity at a plurality of the angular shaft increments while the shaft is being spun to obtain combined no load signals containing cosine representative factors, summing the combined no load signals containing sine representative factors, summing the combined no load signals containing cosine representative factors, whereby shaft unbalance sums containing sine and cosine representative factors are obtained, applying the shaft unbalance sums containing sine and cosine representative factors to the uncorrected calibration sums containing sine and cosine representative factors respectively, thereby obtaining calibration sums corrected for shaft unbalance and for transducer error in the apparatus.

10. A method for digitally acquiring calibration constants used in calculation of unbalance data obtained from a wheel balancing apparatus having a spin shaft with a removable known unbalance weight mounted thereon, the spin shaft being mounted on a framework which provides a spin shaft angular reference, and a force transducer mounted in the framework coupled to the spin shaft providing an unbalance signal indicative of a component of the unbalance force relative to the angular reference when the spin shaft is rotated, said method comprising the steps of digitizing the unbalance signal, storing a plurality of digital sine and cosine representative quantities corresponding to predetermined angular increments of spin shaft rotation, processing the digitized unbalance signal in accordance with corresponding ones of the digital sine representative quantities at each angular increment, processing the digitized unbalance signal in accordance with corresponding ones of the digital cosine representative quantities at each angular increment, summing the processed digitized signals containing sine representative factors, summing the processed digitized signals containing cosine representative factors, whereby calibration sums containing force transducer errors are obtained, repeating the foregoing digitizing, processing and summing steps with the known unbalance weight removed from the spin shaft, whereby calibration sums containing shaft unbalance errors are obtained, and digitally correcting the calibration sums containing force transducer errors with the calibration sums containing shaft unbalance errors, whereby digital calibration sums are provided from which apparatus calibration constants are calculated, whereby unbalance data is provided which is substantially free of force transducer and shaft unbalance errors.

11. The method of claim 10 wherein the steps of processing include addition operations, together with the step of limiting such operations to one during each processing step.

12. Apparatus providing calibrated digital measurement of unbalance in a rotating body, comprising a framework, a spin shaft having an angular reference position relative to said framework, means for mounting said spin shaft for rotation in said framework and for rotating said spin shaft, means mounted in said framework and coupled to said spin shaft for providing an unbalance signal indicative of a component of unbalance force resulting from rotation of said spin shaft, means for detecting angular location of said spin shaft relative to said reference position at a plurality of angular increments during each rotation of said shaft, an analog-to-digital converter for digitizing said unbalance signal, a memory operating to store a plurality of digitial sine representative quantities and a plurality of digital cosine representative quantities corresponding to predetermined ones of said angular increments, a known calibration weight configured for attachment to said spin shaft at a predetermined angular position thereon relative to said reference, means for combining said digitized unbalance signal at each of said angular increments with corresponding ones of said digital sine representative quantities providing digital sine quantities and with corresponding ones of said digital cosine representative quantities providing digital cosine quantities, means for summing said digital sine quantities and for summing said digital cosine quantities so that when said shaft is spun with said calibration weight attached and with said shaft free of any extraneous weight uncorrected calibration sums and unloaded shaft sums are obtained, and means for modifying said uncorrected calibration sums with said unloaded shaft sums to obtain calibration data corrected for shaft unbalance and for calculating and storing digital calibration constants in accordance with said calibration data, whereby error data of both mechanical and electrical origin are detected and stored to provide for removal of such errors from subsequent unbalance data.

13. Apparatus as in claim 12 wherein said means for combining includes means for providing a single digital addition operation to obtain each digital sine and digital cosine product.

14. A method of calibrating an apparatus for measuring mass unbalance in a rotating body wherein the body is mounted in a known position on a rotating shaft supported in a frame and the shaft is coupled to a sensor which provides an electrical signal indicative of the mass unbalance, comprising the steps of

spinning the shaft while it is unloaded and monitoring the substantially periodic electrical signal,

recovering the fundamental sine and cosine coefficients from the unloaded periodic electrical signal,

storing the fundamental coefficients from the unloaded signal as shaft unbalance values,

spinning the shaft while it is loaded with a known unbalance mass and monitoring the substantially periodic electrical signal,

recovering the fundamental sine and cosine coefficients from the known loaded periodic electrical signal, and

storing the fundamental coefficients from the known loaded signal as calibration values,

so that when electrical signals are obtained when the body having unknown unbalance is rotated on the shaft and such signals are processed by applying the shaft unbalance and calibration values, corrected mass unbalance location in the body is obtained.

15. A method as in claim 14 together with the step of preserving the stored shaft unbalance and calibration values when the apparatus is shut down.

16. A method as in claim 14 together with the step of spinning the shaft with the body having unknown mass unbalance mounted thereon, obtaining a substantially periodic unknown unbalance electrical signal, and selecting between a low range and a high range for processing the unknown unbalance signals corresponding to relatively small and relatively high unbalance forces respectively.

17. A method as in claim 14 wherein the body having unknown unbalance is located in a plane displaced from the plane in which the sensor is located together with the step of translating the unbalance detected by the sensor to the plane of the body.

18. A method as in claim 17 wherein the step of translating comprises the steps of separating the plane of the body into two planes intersecting the shaft axis at predetermined locations and translating the detected unbalance to the two planes so that compensation for the unbalance applied in the two planes will substantially eliminate any couple introduced by the body about the diameter of the rotating shaft.

19. Apparatus for measuring mass unbalance in a rotating body wherein the body is mounted in a known position on a rotating shaft supported in a frame wherein the shaft is coupled to a sensor which provides an electrical signal indicative of the unbalance of the rotating mass, the improvement comprising

means for receiving the electrical signal when the shaft is rotated unloaded and for providing a fundamental sine and cosine coefficient from the unloaded electrical signal,

a memory for storing the fundamental coefficients from the unloaded signal,

said means for receiving being arranged to also receive the electrical signal when the shaft is rotated with a known mass unbalance for providing a fundamental sine and cosine coefficient from the known loaded signal,

said memory being arranged to also store the fundamental coefficients from the known loaded signal, and

means for processing the electrical signal provided when a body having an unknown mass unbalance is rotated on the shaft, said last named means being connected with said memory to use the stored fundamental coefficients from the unloaded and known loaded signals whereby data indicative of the unbalance mass location in the body is provided which is corrected for both sensor errors and shaft unbalance errors.

20. Apparatus as in claim 19 wherein said memory is non-volatile, whereby stored quantities are preserved when the apparatus is shut down.

21. Apparatus as in claim 19 together with means sensitive to the magnitude of the electrical signal for selecting between a low range and a high range so that said means for processing the electrical signal accommodates signals indicative of relatively small and relatively high unbalance forces respectively.

22. Apparatus as in claim 19 wherein the body is mounted in a plane remote from a plane in which the sensor is mounted, together with means for translating the data indicative of unbalance mass location to the plane of the body.

23. A method of measuring unbalance in a rotating body mounted on a rotating shaft coupled to force transducers which provide unbalance signals indicative of the instantaneous unbalance magnitude component relative to a known angular reference, comprising the steps of

spinning the shaft with no load mounted thereon, whereby the force transducers provide a no load unbalance signal,

computing no load unbalance signal numerical quantities representative of the phase and magnitude of the unbalance relative to the known angular reference,

spinning the shaft with a known calibration weight at a known position mounted thereon, whereby the force transducers provide a calibration unbalance signal,

computing calibration unbalance signal numerical quantities representative of the phase and magnitude of the unbalance relative to the known angular reference,

subtracting the no load unbalance signal numerical quantities from the calibration unbalance signal numerical quantities to provide corrected calibration numerical quantities,

computing calibration constants from the corrected calibration numerical quantities thereby relating force transducer output magnitude and phase to known unbalance forces,

spinning the shaft with an unknown unbalance weight mounted thereon, whereby the force transducers provide an unbalance signal,

computing the unbalance signal numerical quantities representative of the phase and magnitude of the unbalance relative to the known angular reference,

subtracting the no load numerical quantities from the unbalance numerical quantities to provide modified unbalance numerical quantities,

and applying the calibration constants to the modified unbalance numerical quantities thereby obtaining corrected unbalance data.

24. The method of claim 24 together with the steps of storing the no load numerical quantities and the calibration constants, whereby the steps of spinning the shaft with no load and with a known calibration weigth mounted thereon may be performed once for a plurality of spins with unknown unbalance weights mounted thereon.

25. The method of claim 24 together with the step of translating the corrected unbalance data to the plane in which the unbalance weight center of mass is located.

26. The method of claim 24 wherein the unknown unbalance weight comprises the unbalanced weight in a wheel having a rim with a tire mounted thereon, together with the step of translating the corrected unbalance data to the two planes of the wheel rim, whereby dynamic unbalance in the wheel is measured.