MXPA99001003A - Method of enhancing the measurement accuracy of a tire uniformity machine - Google Patents

Abstract

A method for enhancing the measurement accuracy of a tire uniformity machine by combining signals generated by supplementary load cells with signals generated by corresponding measuring load cells to vibration signals generatedby the tire uniformity machine.

Description

METHOD TO IMPROVE THE ACCURACY OF THE MEASUREMENT OF A MACHINE MEASURING THE UNIFORMITY OF THE RIMS FIELD OF THE INVENTION This invention relates to the field of improvement in the accuracy of the measurement of a machine, and more particularly, to a method for improving the accuracy of the measurement of a machine that measures the uniformity of the tires by combining differential signals. generated by complementary piezoelectric batteries with signals generated by measuring the piezoelectric batteries to cancel the signals generated by the vibration of the machine that measures the uniformity of the tires.

BACKGROUND OF THE INVENTION In the manufacturing technique of pneumatic tires, the flow of rubber in the tire mold or minor differences in the dimensions of the tires. belts, heels, coatings, running surfaces, reinforcing fabrics of rubberized ropes, etc., sometimes cause deformities in the final rim. Deformities of sufficient amplitude will cause variations of force on a surface, such as a road, against which the wheels roll, which produces vibrational and acoustic alterations in the vehicle on which the tires are mounted. Regardless of the cause of the variations of the force, when these variations exceed a minimum acceptable level, the race of a vehicle that uses these tires will be adversely affected. In the past methods for correcting excessive force variations have been developed by removing the rubber from the rims and / or the central region of the rim rolling surface by means of grinding. These correction methods are commonly performed with a machine that measures the uniformity of the rim, which includes a unit for rotating a test rim against the surface of a freely rotating load wheel. In this test arrangement, the load wheel moves in a manner dependent on the forces exerted by the rotating tire and the forces are measured by properly placed measuring devices, for example, piezoelectric batteries. When a tire that is being tested produces less than acceptable results, the rim and center rib grinders are used to remove a small amount of the tread from the rim at the precise location of the deformities detected by the measuring devices. . As the tire rotates, it is measured and rectified at the same time. In a machine that measures modern tire uniformity (MULL), as a Model No. D70LT available from Akron Standard Co. of Akron Ohio, force measurements are interpreted by a computer and the rubber is removed from the surface of the tire. rolling of the tire using computer controlled rectifiers. Examples of machines that measure tire uniformity using these methods are described in U.S. Patent Nos. 3,739,533, 3,946,527, 4,914,869, and 5,263,284. Any vibration generated by the rim is detected by the machine that measures the uniformity by means of its measuring elements of the variation of the force, that is, the piezoelectric batteries. Small amounts of vibration caused by extraneous forces are acceptable because electronic filters are used to eliminate this strange noise. When the motor bearings are worn out or the grinding wheels are defective or improperly installed, or noise and vibration from machines external to the machine that measure the uniformity of the rim are present, excessive vibration will occur. The detection of this excessive vibration by the force measuring elements, ie the piezoelectric batteries, can cause inaccurate measurement of the tire force variations that are being measured in the machine that measures the uniformity of the rims. This in turn can result in the tread surface of the rim being ground in wrong locations to eliminate excessive variations in tire strength., resulting in longer rectification times, fewer processed tires and more discarded tires. At present, the prior art method for detecting excessive vibration is to use an external vibration analyzer with a mobile accelerometer, which a technician manually places on different points of the machine that measures the uniformity of the rim. The problems with this technique are that the equipment is expensive, several hours are taken to complete the analysis of the vibration and the dead time resulting from the machine that measures the uniformity of the tire is costly, also, given that the defects in the machine They are usually discovered on infrequent occasions, they are often undiscovered problems before they result in more costly damage. As already described, efforts have been made, and further improvements are made to the accuracy with which the uniformity of the plants is corrected. However, nothing in the prior art suggests constant detection and correction for external vibrations in the machine that measures the uniformity of the tires before and while the variations of force in the rim that is being corrected are measured and reduced by rectification of the tires.

SUMMARY OF THE INVENTION According to the invention, an apparatus is provided for improving the accuracy of the measurement of a machine that measures the uniformity of the tires. The apparatus includes primary piezoelectric batteries that support a load wheel shaft with a freely rotating load wheel mounted on the shaft. Complementary piezoelectric batteries are each mounted to the machine that measures the uniformity of the tires in close proximity to one of the corresponding primary piezoelectric cells. Each of the complementary piezoelectric batteries has a fixed mass attached thereto. An electrical signal conditioner includes a plurality of signal conversion sections that convert the voltage measurement signals from the force that are generated in the primary piezoelectric cells into force measurement signals that can be input to a computer. A plurality of differential input sections convert the differential voltage signals of the complementary piezoelectric cells into differential signals that can be input into the computer. A summing section of the signals is provided to sum the force measurement signals and the differential signals and to produce the difference between the signals of the measurement and the differential signals for the computer.

Also according to the invention, a method for improving the accuracy of the measurement of a machine that measures the uniformity of the tires consists of the following steps. The forces that are generated by the vibration of the machine that measures the uniformity of the tires are monitored with piezoelectric primary batteries that support a wheel axle of the machine that measures the uniformity of the tires, with a wheel freely loaded rotary mounted in it and the force measurement voltage signals are generated in response to it. The forces generated by the vibration of the machine that measures the uniformity of the tires are monitored with complementary piezoelectric batteries mounted to the machine that measures the uniformity of the tires and the signals of the differential voltage are generated in response to this. The voltage signals of the force measurement that are generated in the primary piezoelectric batteries are converted into analog voltage measurement signals. The voltage signals of the differential output are inverted and converted into analog differential signals. The analog voltage measurement signals and the analog differential voltage signals are summed and the analog voltage signals summed equal to their differences are sent to the computer to substantially cancel the effect of the vibration forces that are generated in the measuring machine the uniformity of the tires. An object of the present invention is to provide a method for measuring the amount of foreign vibration in a machine that measures the uniformity of the tires. Another objective is to avoid the problems and limitations of prior art methods. Other objects of this invention will be apparent from the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS The structure, operation and advantages of the presently preferred embodiment of the invention will be more evident from the consideration of the following description taken together with the accompanying drawings, wherein: Figure 1 is a schematic illustration of a a plan view of a machine that measures the uniformity of the rims, according to the invention, with a rim mounted thereon; Fig. 2 is a schematic illustration of a side view through line 2-2 of Fig. 1, showing a load wheel mounted between two primary piezoelectric cells generating output signals in response to force vibrations of a rim, two complementary piezoelectric batteries mounted to the structure of the machine that measures the uniformity of the rims, and electrical circuits to detect and cancel strange vibrations detected by the machine that measures the uniformity of the rim; Figure 3 is a schematic illustration of electrical circuits interconnecting one of the piezoelectric cells. primary to a corresponding complementary piezoelectric cell; and Figures 4a and 4b, collectively Figure 4, is a flow diagram of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION With reference to Figures 1 and 2, there is illustrated a machine that measures the uniformity of the rims (MULL) 10 according to the invention adapted for mounting a rim 12. The rim 12 is usually a pneumatic tire having a tread surface of the circumferential rim with upper and lower flange regions and a central region between the upper and lower rim regions. The rim 12 can be mounted on a crown or rim 14 secured to a rim shaft 16 and inflated to a desired pressure. A variable speed motor 17, shown in dashed lines, rotates the axle of the rim 16 and the rim 14. The rim 12 can be placed under load by a load wheel 18, which is supported rotatably on an axis fixed 20 which extends through the load wheel and is suspended from the primary piezoelectric batteries 22, 24 at its end. The primary piezoelectric batteries 22, 24, in turn, are mounted to the supports of the structure 26, 28, respectively, of the machine that measures the uniformity of the tires. Each of the primary piezoelectric batteries 22, 24 includes a lateral piezoelectric stack section conventionally used to measure the lateral force exerted by the rim 12 against the load wheel 18 in a direction parallel to the axis of rotation, extending around which it rotates the loading wheel. The primary piezoelectric batteries 22, 24 also include a section of radial piezoelectric piles conventionally used to measure the radial force at the point of intersection of the rim 12 and the load wheel 18, exerted by the rim 12 against the load wheel 18. and through the axis 20 around which the load wheel rotates. An important aspect of the present invention relates to the arrangement of complementary piezoelectric cells 30, 32 mounted in close proximity to the primary piezoelectric cells 22, 24, as it may be on the supports of the structure 26, 28 respectively, as shown. Each complementary piezoelectric battery 30, 32 has a fixed mass, such as a weight 34, 36, respectively, attached thereto for simulating the fixed mass attached to each of the primary piezoelectric cells 22, 24, respectively. Although complementary piezoelectric stacks 30, 32 are shown mounted to the supports of the structure 26, 28 in one location, it is also within the terms of the invention to mount them in other locations on the MULL 10, as long as they are very close to each other. close and oriented in the same direction as piezoelectric batteries 22, 24. During the testing process of a rim 12 for non-uniformities, the load wheel 18 is compressed against the wheel to load the inflated rim with a specific force (for example, 600 to 1900 Ib) to simulate road conditions. Shaft 20 is mounted to bearing blocks (not shown) and moved by conventional means, such as with an electric motor (not shown) operating through a ball and screw connection, to move the load wheel 18 in and out of the coupling with the rim 12. A ridge grinding unit 38 is located substantially 180 ° with respect to the rim 12 from the load wheel 18. The grinding unit of the rim 38 includes upper rim rectifiers and bottom substantially identical 40a and 40b (only the upper flange rectifier 40a is illustrated and described), which includes grinding wheels that are powered by motors and move independently in and out of engagement with the rim flange regions 12. As shown, the upper flange rectifier 40a has a grinding wheel 42a fed by a motor 44a and can be moved in and out afue of the coupling with the rim portions of the rim 12 by any conventional means, as it can be a servo hydraulic device (not shown). A central grinding unit 46 is located adjacent the wheel 12 approximately 90 ° counterclockwise around the rim 12 from the load wheel 18. The central grinding unit 46 has a grinding wheel 48 which is powered by a motor 50 and it moves towards and away from the coupling with the central region of the running surface of the rim 12 by conventional means, such as with a servo hydraulic device (not shown). The voltage signals, proportional to the amplitude of the radial and / or lateral forces, are generated by the primary piezoelectric cells 22, 24 and are introduced through lines 52 and 54, respectively, into an electric signal conditioner. The electric signal conditioner 56 includes substantially identical signal conversion sections 58 and 60 which convert the force measurement voltage signals generated by the primary piezoelectric cells 22, 24 respectively, into signals that are conditioned so that they can be be entered and stored in a computer 62. The signal conversion sections 58 and 60, as shown in Figure 3, each include at least one amplifier 64 connected by lines 52, 54 to the primary piezoelectric batteries 22, 24 , respectively. The amplified output signal from the amplifier 64 is routed through the line 66 to an anti-aliasing filter 68 for cutting the high frequency outputs, ie, greater than about 45 Hertz, from the batteries primary piezoelectric 22, 24 so that the high frequency content in the amplified piezoelectric battery signal does not cause distortion (aliasing) in the analog-to-digital conversion. The electric signal conditioner 60 also includes a low pass filter 70 connected in series to the anti-distortion filter 68 via line 72. The low pass filter 70 attenuates frequencies greater than 16 Hertz, from the amplified output signal of the amplifier 60 of so that the bandwidth of the signal is limited to the frequencies that are generated by the tire and the load wheel. The output signal from the low pass filter 62 is directed through the lines 74, 76 to the signal summing sections 78, 80, respectively, as described below. The present invention relates to the measurement and elimination of deviations from the prescribed specifications of the MULL 10, that is, operating under conditions without foreign vibrations, caused mainly by extraneous vibrations generated by or induced in the machine 10. The foreign vibrations are caused by rotating components such as the shaft of the engine-driven rim 16, the tire grinding units 38 and the central grinding unit 46, the wear of the motor bearings, the defective or inadequately installed grinding wheels and / or noise and vibration from machines external to the machine that measures the uniformity of the tires 10. The primary detectors for many of the necessary measurements that are used in the operation of the MULL 10 are primary piezoelectric batteries 22, 24 that are normally in differential mode electric. This fact, combined with the inherent low impedance of the primary piezoelectric batteries 22, 24, usually provides a signal output of adequate quality for the MULL 10. The piezoelectric cells 22, 24, however, can not discriminate between the components of the signal generated by the measurement of the strength of the rim and a vibration or force component that is generated by a strange vibration generated or detected by the MULL 10. To solve the problem of the readings of the primary piezoelectric batteries 22 , 24 corrupted by strange vibrations, complementary piezoelectric batteries 30, 32, with features substantially identical to the primary piezoelectric batteries 22, 24 are mounted to the MULL 10 to be oriented in the same direction and in close proximity to the primary piezoelectric measuring piec 22, 24, respectively. The complementary piezoelectric cells 30, 32 have a fixed mass, such as weight 34, 36, respectively, attached thereto. The output voltage signals of the complementary piezoelectric cells 30, 32 are directed towards the pairs of electrical lines 80 and 81 in substantially identical differential input sections 84, 86, respectively. Since the differential input sections 84, 86 are substantially identical, only the differential input sections 86 are shown and described. Referring to Figure 3, the pair of power lines 82 are inverted in the differential input section 86 and are input to a new gain adjusting amplifier 88. The differential voltage signal exiting the gain adjustment amplifier 88 then it is directed to a line 90 and a phase adjustment amplifier 92. The voltage output signal from the phase adjustment amplifier 92 is then sent through the line 94 to the signal adding section 78. The signals of The voltage across the electrical lines 76 and 94 is directed through the resistors 96 and 98, respectively, and then combined in the line 100 and input to a summing amplifier 102. The summing amplifier 102 subtracts the voltage signals from the lines 94 and 76 and the amplified signal resulting from the summing amplifier 102 and then send them from the signal adding section 80 through the line 102 to the computer 62. At the same time, the The resulting amplified signal leaving the signal adding section 78 a) through the line 106 is directed to the computer 62. Each complementary piezoelectric cell 30, 32 and the piezoelectric primary measurement battery, corresponding 22, 24, measure almost the same components of noise and vibration when the MULL 10 is running without load on the load wheel 18. Since both the primary piezoelectric batteries 22, 24 and their corresponding complementary piezoelectric batteries 30, 32 are operating in the differential mode, the pair of signal 81, 82 of the complementary piezoelectric batteries can be reversed, as shown in Figure 3, without adverse electrical effects. By using this configuration, a positive vibration in the MULL 10 will generate positive signal output from the primary piezoelectric gauges 22, 24 and a negative signal output from the complementary piezoelectric cells 30, 32. Assuming that the primary and complementary piezoelectric batteries 22 and corresponding 30, 24 and 32 and the masses attached to each of them are identical, the two output voltage signals that are sent through lines 74 and 76 from the signal conversion sections 58 ,. 60 and the two differential output voltage signals that are sent through the lines 95, 94 from the differential input sections 84, 86 will theoretically cancel each other in the summing sections of signals 78, 80. However, an error Very small residual j could still be present due to the phase shift induced by the slightly different locations of the corresponding primary and complementary piezoelectric cells. The gain of the amplifier 88 can be adjusted to take into account the difference between the mass 34, 36 of the complementary piezoelectric cells 30, 32 and the mass associated with each of the primary piezoelectric cells 22 and 24. The amplifier 92 provides an adjustment of phase angle that can be adjusted to correct the difference in the location of the complementary piezoelectric batteries 30, 32 with respect to the corresponding primary piezoelectric batteries 22 and 24. That is, the machine 10 can be calibrated with the rotating portions, as motors and rectifiers can be turned off. First, an external vibration is induced in the machine 10 by a conventional vibration generating device, such as a power amplifier model number 2706 and a vibrator model number 4809 of Beuel and Kjaer of Denmark. The gain and phase of the voltage output signals from the complementary piezoelectric cells are adjusted with amplifiers 88 and 92 in sections 84 and 86 so that there is a minimum difference between the output voltage signals that are sent by the summing sections 78 and 80 as calculated by the computer 62. This calibration effectively cancels the vibration signals induced in the MULL 10 that are detected by the primary and complementary piezoelectric batteries. Referring to Figure 4, a flow diagram of the present invention is illustrated. When a MULL 10, which incorporates the electrical signal conditioner 56 of the present invention is operating without force applied by the load wheel 18, the voltage signals input through the lines 104 and 106 to the computer 62 will be close to 0 As a load is applied by the load wheel 18 against the rim 12, the piezoelectric measuring batteries 22, 24 will respond to the additional load plus the vibration components produced by the rotating components and any extraneous vibration. The complementary piezoelectric cells 30, 32 will only detect the vibration components produced by the rotating components and any extraneous vibration. When the voltage signals of the corresponding primary and complementary piezoelectric batteries 22, 30 and 24, 32 are subtracted from the summing sections 78, 80, the vibrational components measured by the primary piezoelectric cells will be canceled. The remaining signal components, generated by the primary piezoelectric batteries 22, 24, for the most part, will result from the loading forces of the rim against the load wheel 18. That is, the two analog signals that are sent from the summing sections of signals 78, 80 correspond to the radial and / or lateral forces monitored with very little background noise generated by the rim 12 loaded against the load wheel 18 for a predetermined period of time. The computer 62 independently samples the analog signals to be input from the summing sections 78, 80 for a predetermined time and converts the analog signals into digital signals. Next, the computer 62 converts the digital signals into a frequency domain signal representation using a Conventional Fast Fourier Transform (FFT, fourier, fast, conventional transforms) program for each of the signals of the smaller primary piezoelectric batteries. than the complementary piezoelectric batteries that are monitored. When calculating an FFT for independent primary piezoelectric stack signal acquisition, the phase shift for each piezoelectric stack can be calculated and it is possible to generate a correction table for each frequency present in the spectrum. Also, by storing the results of the FFTs for each primary piezoelectric battery 22, 24, the signals can be acquired periodically while operating without a load on the rim and the new FFT can be calculated and compared with the original FFR. The comparison of the frequency spectrum and the amplitudes of each rotary component of the MULL 10 will allow an operator to discover the malfunction of some part of the rotating components. The malfunction can generate an alarm signal. The computer 62 then operates on the representation of signals in the frequency domain to calculate a power spectrum, as described in the previously described US patent application entitled "Method of Vibration Analysis of a Machine for the Machine Measuring Uniformity of The tires, discrete frequency components in hertz compared to the amplitude or magnitude of the discrete frequency components in pounds. The selected frequency components are then compared to the selected frequency groupings representing critical frequencies of different moving parts of the machine that measure the uniformity of the tires. 10. An acceptable amplitude for the selected frequency groups, representing the critical frequencies of the frequencies. moving parts that operate as designated, is entered into the computer. If the amplitude of the different frequency groups generated from the voltage signals sent by the summing sections 78 and 80 is greater than the acceptable amplitudes for the selected frequency groups corresponding to the different moving parts of the machine which measures the uniformity of the tire, an alarm signal is issued by the computer. The alarm signal indicates that a rotating portion of the machine measuring the uniformity of the tires 10 is defective. The alarm signal can be entered into a screen monitor and / or used to activate an alarm device such as a light or an audible alarm, that is, a bell or buzzer to alert an operator of the machine that the machine which measures the uniformity of the tire 10 is vibrating at a level beyond an acceptable limit. The mobile component of the machine that measures the uniformity of the rim 10 or the external vibration source that is causing the unwanted vibration, can be isolated as generally described in the application entitled Vibration Analysis Method of a Machine for the Machine That Measures The Uniformity Of The Tires. The computer 62 is conventionally programmed to determine taper, lateral force values, radial finish and values of the radial force of rim 12, and to control the corrective grinding action, as described in the application of the United States Patent. Series No. 08 / 534,809, entitled METHOD OF CORRECTION OF THE CONICIDAD, THE RADIAL FINISH AND THE VARIATIONS OF FORCE IN A PNEUMATIC RIM, assigned to The Goodyear Tire &; Rubber Co., the assignee of the present invention, is connected to the grinding unit of the flange 24 and to the central grinding unit 26 to place these grinding units as required. Also, within the terms of the present invention is to scale the capacity of the complementary piezoelectric piers and the mass of the reference weight attached to the piezoelectric piers, and by this means reduce the mass and the pcal size of the weights referred to 34 and 36 to facilitate the installation. Although the invention is described in conjunction with a machine that measures the uniformity of the tires, it is within the scope of the invention to eliminate the foreign vibrations induced in other types of machines incorporating piezoelectric batteries. It is evident that, according to this invention, the apparatus and method for improving the accuracy of the measurement of a machine that measures tire uniformity by subtracting the differential signals generated by the complementary piezoelectric batteries with signals generated by the batteries has been provided. Piezoelectric primary gauges to cancel the portion of the signals from the primary piezoelectric batteries that correspond to the vibration of the machine that measures the uniformity of the tires so that the objectives, means and advantages established in the foregoing are satisfied. Although the invention has been described in combination with embodiments thereof, it is evident that many alternatives such as modifications and variations will be apparent to those skilled in the art in light of the above teachings. Accordingly, the invention is intended to cover all of these alternatives, modifications and variations that come within the scope of the appended claims.

Claims (15)

1. CLAIMS 1. An apparatus for improving the measurement accuracy of a tire uniformity measuring machine consists of: a plurality of primary piezoelectric batteries supporting a load wheel shaft with a freely rotating load wheel mounted on the shaft; a plurality of complementary piezoelectric cells, each mounted to the machine that measures the uniformity of the tires in close proximity to a corresponding one of the plurality of primary piezoelectric cells, each of the plurality of complementary piezoelectric cells having a fixed mass attached to the same; an electrical signal conditioner including: a plurality of signal conversion sections that converts the voltage signals for force measurement generated by the plurality of primary piezoelectric cells into force measurement signals that can be input to a computer; a plurality of differential input sections for converting the differential voltage signals of the plurality of complementary piezoelectric cells into differential signals that can be input to a computer; and a summing section of signals to sum the force and differential signal measurement signals and send the difference between the signals of the measurement and the differential signals to the computer. The apparatus of claim 1, wherein the plurality of differential input sections each is connected to one of the plurality of complementary piezoelectric cells by a pair of lines that are inverted and connected to an amplifier, each having the plurality of differential input sections a phase adjustment amplifier and a gain adjustment amplifier connected in series with each other. The apparatus of claim 2, wherein the plurality of signal conversion sections each includes at least one amplifier connected by lines to one of the plurality of primary piezoelectric cells. The apparatus of claim 3, wherein the plurality of signal conversion sections each includes an anti-distortion (antialiasing) filter connected in series to the output of at least one amplifier. The apparatus of claim 4, wherein the plurality of signal conversion sections each includes a low pass filter connected in series with the anti-distortion filter. The apparatus of claim 4, wherein the fixed mass attached to each of the complementary piezoelectric cells is a weight that corresponds to the fixed mass attached to the corresponding mass of the plurality of primary piezoelectric cells. The apparatus of claim 1, wherein the tire uniformity measuring machine has a plurality of rotating components including the freely rotating load wheel, a motor driven shaft, and a plurality of motor-driven rotary rectifiers. 8. The method for improving the accuracy of a machine that measures the uniformity of the tires, includes the steps of: monitoring forces generated by the vibration of the machine that measures the uniformity of the tires with piezoelectric primary batteries that support an axis wheel of the machine that measures the uniformity of the rims, with a freely rotating load wheel mounted on it and that generates voltage signals from the measurement of the force in response to it; monitor the forces generated by the vibration of the machine that measures the uniformity of the tires with a plurality of complementary piezoelectric batteries mounted to the machine that measures the uniformity of the tires and that generate differential voltage signals in response to this; converting the force measurement voltage signals generated by the plurality of primary piezoelectric cells into analog voltage measurement signals; invert and convert signals, from differential output voltage to analog differential signals; add the signals of the analog voltage measurement and the analog differential voltage signals and send the analog voltage signals summed equal to their difference to the computer to substantially cancel the effect of the vibrational forces generated by the machine that measures the uniformity of the tires The method of claim 8 includes the step of: mounting complementary piezoelectric piers in close proximity to a corresponding one of the plurality of primary piezoelectric piers. The method of claim 9 includes the step of: attaching a fixed mass to each of the plurality of complementary piezoelectric cells to simulate a fixed mass attached to the corresponding mass of the plurality of primary piezoelectric cells. The method of claim 9, wherein the step of converting the voltage signals from the force measurement generated by the plurality of primary piezoelectric cells into analog signals from the voltage measurement includes the step of amplifying the analog signals of the measurement of tension. 12. The method of claim 11, wherein the step of inverting and converting the voltage signals of the differential output includes the step of amplifying the analog differential voltage signals. 13. The method of claim 12 further includes the steps of: converting the summed analog voltage signals into digital signals; convert digital signals into a frequency domain representation; convert the representation of the frequency domain into a power spectrum that represents the amplitudes of the different frequencies; and compare the amplitudes of different frequencies with acceptable amplitudes for the selected frequency groups. 14. The method of claim 13 includes the step of: converting the digital signals into a frequency domain representation by operating mathematically on the digital signals with a Fast Fourier Transform equation (Fourier transform). 15. The method of claim 14 includes the step of sending an alarm signal from the computer when at least one of the different frequency amplitudes is greater than at least one of the acceptable amplitudes for the selected frequency groups.