WO2002043898A2

WO2002043898A2 - Device and process for acoustic inspection of the quality of cold reforming joint connection

Info

Publication number: WO2002043898A2
Application number: PCT/US2001/042952
Authority: WO
Inventors: Dieter Mauer
Original assignee: Newfrey Llc
Priority date: 2000-11-16
Filing date: 2001-11-16
Publication date: 2002-06-06
Also published as: DE10056859A1; AU2002241471A1; WO2002043898A9; WO2002043898A3

Abstract

The present invention relates to a process for checking the suitability of a connection between at least two plate-like components consisting of the following stages: registering a number of acoustic signals during a cold reforming joining process for connecting at least two plate-like components; processing the acoustic signals; evaluating the acoustic signals by means of suitable software and hardware and emitting a corresponding signal.

Description

DEVICE AND PROCESS FOR ACOUSTIC INSPECTION OF THE QUALITY OF COLD REFORMING JOINT CONNECTION

FIELD OF THE INVENTION

[0001] The present invention provides a device and a process for the acoustic inspection of the quality of cold reforming joint connections and for monitoring the integrity of a punch machine. In particular, a device and process which monitors acoustic transmissions produced during the formation of punch riveted and clinch connections.

BACKGROUND OF THE INVENTION [0002] It is well known to join two or more sheets of metal with a rivet. Jt is also known to use self-piercing rivets that do not require a prepunched hole. Such self lercing or punch rivet connections can be made using a solid rivet or a hollow rivet.

[0003] A punch rivet connection is conventionally formed with a solid rivet by placing the parts to be joined on a die, The parts to be joined are clamped between a hollow clamp and the die. A punch punches the rivet through the workpieces such that the rivet punches a hole in the parts thereby rendering pre-punching unnecessary. Once the rivet has penetrated the parts to be joined, the clamp presses the parts against the die, which includes a ferrule. The force of the clamp and the geometry of the die result in plastic deformation of the die-side part to be joined thereby causing the deformed part to partially flow into an annular groove in the punch rivet. This solid rivet is not deformed.

[0004] Traditionally, joining devices are used to form such punch rivet connections. More specifically, the punch is actuated by a hydraulic cylinder unit. The cost of producing such joining devices is relatively high and process controls for achieving high quality punch rivet connections has been found to be problematic. In particular, joining devices are subject to variations in the force exerted by the punch owing to changes caused by tooling wear or variations in material properties of the material being fastened. Furthermore, changes in the quality of the joint produced are substantially dependent on the life of the toolipg and the quality of the rivets being used.

[0005] When forming a punch connection or joint with a hollow rivet, as well as a semi-hollow rivet, the punch cause the hollow rivet to penetrate the punch-side part to be joined and partially penetrate into the die-side part to be joined. The die is designed to cause the die-side part and rivet to be deformed into a closing head. An example of such a joined device for forming a punch rivet connection with a hollow rivet is disclosed in DE 44 19 065 A1. Hydraulically operating joining devices are also used for producing a punch rivet connection with a hollow rivet.

[0006] It is also known to employ a computer system for monitoring various characteristics of a blind rivet setting system. For example, reference should be made to U.S. Patent No. 6,276,050 B1 entitled "Riveting System and Process for Forming a Riveted Joint" which issued to Mauer et al. on August 21, 2001, U.S. Patent No. 5,661,887 entitled "Blind Rivet Set Verification System and Method" which issued to Byrne et al. on September 2, 1997, and U.S. Patent No. 5,666,710 entitled "Blind Rivet Setting System and Method for Setting a Blind Rivet Then Verifying the Correctness of the Set" which issued to Weber et al. on September 16, 1997. These U.S. patents are incorporated by reference herein.

[0007] Known processes for checking the quality of joint connections, in particular clinch and punch riveted connections, destroy the riveted connection and only provide information on individual samples from a series of joint connections. In the automotive assembly sector for example, which is an important field of application for punch riveted connections, several thousand punch rivets per car are automatically processed. To determine the quality of the joint connections, a test piece from a punch riveted connection is produced at regular intervals by the rivet setting machine, for example every 10,000 riveted connections, and is cut along the punch rivet axis and the cut edge is ground so the quality of the connection can be assessed by optical analysis. This optical analysis is undertaken by experts who feed this test piece to the rivet setting machine manually. As the rivet setting process is not interrupted in an automated assembly procedure during the non-automated time-consuming measuring and testing process, a period of time can pass between the appearance of deficiencies in quality in the rivet setting process and stopping of the rivet setting process which can potentially lead to expensive finishing or even to removal of the partially assembled product. A further disadvantage of this known measuring and testing process lies in its insufficient ability to provide information regarding the quality and suitability of a riveted connection. By destroying the riveted connection by means of sawing open, grinding and polishing along the central axis of the rivet it is only possible to assess the quality and suitability in this cutting plane. The radial expansion of the rivet shank in particular is a crucial criterion for assessing the quality and suitability of a riveted connection. As only the radial expansion along a diameter can be assessed with a section, however, the ability of this measuring and testing process to provide information is not sufficient for assessment of the radial expansion of the entire rivet shank.

[0008] |n order to obtain a riveted connection of high quality and suitability, as described for example in DE 19701 780, it is necessary to make both the geometric and material-conditioned parameters of the ιplate-)ike components to be connected and of the rivet as consistent with one another as ppssible. The hardness, or combined hardness, of the punch rivet and the plate-like components to be connected and the rivet dimensions, for example shank length, diameter, geometry and homogeneity of the cutting edge, the thickness of the rivet shank wall and the surface quality, are therefore crucial. In addition to these workpiece-conditioned parameters and variables, the condition of the die, in particular its surface quality and iπtactness, as well as the surface quality of the punch, is crucial for the quality and suitability of a riveted connection, The quality and suitability of a riveted connection is also decisively influenced by the dimensional changes to the rivet and the plate- like components which occur during the rivet setting process. The radial expansion of the rivet shank inside the lower plate-like component and upsetting of the rivet shank merit particular attention here.

[0009] It has proved to be particularly problematiclal when connecting aluminum sheets with steel rivets that, owing to the cold reforming method of the aluminum in the die, traces of fines from the lower aluminum sheet line the steel die during a prolonged period of rivet setting cycles and therefore reduce the reforming volume of the die pocket. This change in the georhetry of the die can lead to stress points and cracks in the surface regions of the die pocket and can ultimately lead to a forced rupture of the die.

SUMMARY OF THE INVENTION [0010] In accordance with the present invention, a system se'nses an acoustic signal created when two or more workpieces are joined. In another aspect of the present invention, a self-piercing rivet is employed to join ,the workpieces. A further aspect of the present invention uses a self- piercing rivet which does not fully penetrate the die-side workpiece in an acceptable joint. Still another aspect of the present invention employs an electronic control unit and one or more sensors to determine a riveting characteristic and/or an actuator characteristic. In still another aspect of the present invention, an acoustic sensor is used to monitor a noise signature produced during a joint formation. In yet another aspect of the present invention, an acoustic sensor is used to monitor the status of ςαmpόnents in a joint forming machine. Unique software employed to control the riveting machine is also used in another aspect of the present invention. A method of operating a riveting system is also provided.

[0011] The riveting system of the present invention is advantageous over conventional devices in that the present invention employs an acoustic receiver incorporated into the joining device. Furthermore, the present invention advantageously employs an electric motor to actuate the riveting punch thereby providing higher accuracy, lower maintenance, less energy, lower noise and less temperature induced variations as compared to traditional hydraulic drive machines. Moreover, the electronic control system and software employed with the present invention riveting system ensure essentially real time quality control and monitoring of the rivet, riveted joint, workpiece characteristics, actuator power consumption and/or actuator power output characteristics, as well as collecting and comparing historical processing trends using the sensed data.

[0012] The riveting system and self-piercing hollow rivet employed therewith, advantageously provide a high quality and repeatable riveted joint that is essentially flush with the punch-side workpiece outer surface without completely piercing through the die-side workpiece. The real- time characteristics of the rivet, joint and workpieces are used in an advantageous manner to ensure the desired quality of the final product. Furthermore, the performance characteristics may be easily varied or altered by reprogramming software set points, depending upon the specific joint or workpiece to be worked upon, without requiring mechanical alterations in the machinery. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Accordingly, the present invention provides a process and a device which facilitate destruction- free checking of cold reforming joint connections, in particular punch riveted and clinch connections. Furthermore, the present invention employs a process and devices which provide information regarding the condition of the tools used, in particular punches, tool holders and dies.

[0013] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: [0015] Figure 1 is a diagrammatic view showing the preferred embodiment of the riveting system of the present invention;

[0016] Figure 2 is a partially diagrammatic, partially elevational view showing the preferred embodiment riveting system; [0017] Figure 3 is a perspective view showing a riveting tool of the preferred embodiment riveting system;

[001 B] Figures 4 and 5 shows a device according to the invention on a punch rivet device with C-shaped frame;

[0019] Figures 6 and 7 are schematic representation showing a riveting tool having an acoustic sensor coupled to a controller of a first alternate embodiment of the riveting system;

[0020] Figures 8a-8b are graphs showing acoustic emission characteristics of another embodiment riveting system; and

[0021] Figure 9 is a flow chart describing the process used in conjunction with one embodiment of the system.

PETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0022] The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0023] Referring to Figures 1 and 2, a joining device for punch rivets, hereinafter known as a riveting system 21, includes a riveting machine or tool 23, a main electronic control unit 25, a rivet feeder 27, and the associated robotic tool movement mechanism and controls 28, if employed. Riveting tool 23 further has an electric motor actuator 29, a transmission unit, a punch 31 , a clamp 33 and a die or anvil 35. Die 35 is preferably attached to a C-shaped frame 37 or the like, The C-shaped frame 37 also couples the advancing portion of riveting tool 23 to a set of linear slides 39 which are, in turn, coupled to an articulated robot mounted to a factory floor. A linear slide control unit 41 and an electronic robot control unit 43 are electrically connected to linear slides 39 and main electronic control unit or electronic controller 25, respectively. The slides 39 are actuated by a pneumatic or hydraulic pressure source 45.

[0024] The transmission unit of riveting tool 23 optionally includes a reduction gear unit 51 and a spindle drive mechanism 53. Plunger 31 , also known as a punch assembly, includes a punch holder and punch, as will be described in further detail hereinafter. A data monitoring unit 55 may be part of the main controller 25, as shown in Figure 2, or can be a separate microprocessing unit, as shown in Figure 1, to assist in monitoring signals from the various sensors. Various types of sensors are distributed throughout the riveting system 21. These include at least one vibration or acoustic sensor 57. Additionally the riveting system 21 can optionally contain positional sensors such as LVDTs and force transmission sensors 59.

[0025] In the device according to the invention shown in Figures 3- 5, acoustic sensor 57 are arranged on or in a C-shaped frame 37, wherein further acoustic sensor 57 can be arranged on the force flow line 63 of the C- shapεd frame 37 shown by dashed lines. These further acoustic sensor 57 can be mounted on the C-shaped frame 37 according to the user's requirement profile and to verify individual results. The acoustic sensor 57 is operatively connected to data monitoring unit 55 to evaluate thp signals received by it, in which hardware the acoustic signals are processed and evaluated and then imparted to the user optically, acoustically or in another way on a display which is not shown. It is also possible for the results to flow directly into the process control 25, so process controller 25 is influenced on the basis of the results determined. A punch 31 is also shown in Figure 4 which drives a rivet 65 into two plate-like components. An acoustic receiver 67 is located immediately next to the punch 31 of the setting device 67 which is indirectly connected via the supply device 69 to a magazine which is not shown, and is therefore constantly supplied with punch rivets. The die 70 which receives the plate-like components for shaping during the plastic deformation of the same is shown below the plate-like components to be connected. [0026] Figure 4 shows a further preferred embodiment of the present invention, wherein the acoustic sensor 57 is arranged immediately above the punch 31 inside the setting device 67. This punch 31 is also connected via an electrical line to the data monitoring unit 55 in which the signal is evaluated. This data monitoring unit 55 is, as already described above, operated by software (further described below) which is, and can optionally be connected to two different output devices or can directly influence process control on the basis of the results determined.

[0027] Figure 6 shows a simplified electrical diagram of the preferred embodiment riveting system is shown in Figures 1-4. Main electronic control unit 25, such as a high speed industrial microprocessor computer, having a cycle time of about 0.02 milliseconds purchased from Siemons Co., has been found to be satisfactory. A separate microprocessor controller 61 is connected to main electronic control unit 25 by way of an analogic input output line and an Encoder2 input which measures the output of the acoustic sensor 57 through a digital signal. Controller 61 further receives an electric motor signal and a resolver signal. The load cell force signal as well as the acoustic sensor is sent directly from the tool connection 81 to the main electronic control unit 25 while the proximity switch signals (from the feeder, feed tube and spindle home position sensors) are sent from the too) connection 71 through an input output delivery microprocessor module 91 and then to main electronic control unit 25. Input output delivery microprocessor module 91 actuates error message indication lamps 93, receives a riveting start signal from an operator activatable switch 95 and relays control signals to feeder 27 from main electronic control unit 25. An IBS/CAN gateway transmits data from main electronic control unit 25 to a host system which displays and records trends in data such as joint quality, workpiece thickness and the like. Controller 61 is also connected to a main power supply via fuse. [0028] Figure 7 diagrams the analysis and signal conditioning of the signals from the acoustic sensor 57. As can be seen, the acoustic sensor 57 produces a signal 71 which is passed through a first filter 73, This filter can be a high pass, low pass, notch or other filter known in the art. Next, the signal 71 Is passed through an amplifier 75 and an optional second filter 77. The signal 71 from the acoustic sensor 57 is then passed to the control unit 25 which performs various analysis on the signal. This analysis included but is not limited to Fourier-analysis, root-mean square analysis, amplitude and time duration, integration, differentiation.

[0029] Noises at a frequency which cannot be detected by humans are produced during setting of a connector, such as a blind rivet, punch rivet or, during the connecting of sheet parts, by means of clinches, owing to the mechanical load of the crystal lattice in the structure of the punch rivet 65 and/or of the plate-like components to be connected. These noises or acoustic signals are characteristic and can provide information regarding the instantaneous status of the cold reforming joining process as characteristic mechanical loads act on the connecting means (punch rivet, blind rivet or similar) at each instant of this joining process which can be clearly and unambiguously assigned with the aid of empirical establishing of ideal processing stages.

[0030] These noises emanating from the lattice structure of the fastener and/or of the plate-like components to be joined are measured or registered according to the invention by means of the acoustic sensor 57, optionally converted into a signal, subsequently passed to suitable software and hardware in order to be processed and evaluated there in order, finally, to emit a signal which permits rapid assessment of the quality and/or suitability of the jointed connection or the condition of the tools, such as the punch or the die or a tool holder (for example wear or breakage of a C-shaped frame).

[0031] After investigation by empirical force/path and noise courses during a certain number of ideal punch rivet setting processes, a reference noise course Is determined using statistical methods, which course can be used for automatically assessing and evaluating further rivet setting processes with the aid of the process according to the invention. This evaluation takes place, for example, by means of suitable software which calculates and compares the actual values of the measured acoustic signals with set-point values, based, for example, on the above-described empirically ^• determined ideal curves, and therefore decides on the basis of an algorithm whether when certain acoustic signals are produced, a riveted connection or a rivet setting process or clinch process lies within certain tolerance limits and can therefore be considered suitable or does not meet the quality requirements.

[0032] If it does not meet such quality requirements, the rivet setting process, for example, can be stopped immediately and the punch rivet or blind rivet can be removed from the started jointed connection. In this way, it can be particularly advantageously determined that punch riyets which' have not yet penetrated completely into the uppermost plate-like component and therefore have a certain peripheral gap between rivet head and uppermost plate-like component can be easily removed again from the uppermost plate- like component by means of a tong-like tool. [0033] In addition to some clear signals, a plurality of acoustic noise values are produced during processing of the acoustic signals' with the process according to the invention, which noise values can be filtered qut by means of suitable filters, for example on acoustic or electronic paths, in order to thereby produce a signal pattern which provides information for further processing and evaluation in appropriate software and hardware,

[0034] A further advantage of the process according to the invention is in the adaptability of the processing and evaluation software as an appropriate reference signal curve can be determined empirically for each punch rivet 65 and type of material of the plate-like components to be joined in various combinations in each case, and these can be recorded in a menu, so when the tools or punch rivets are changed or replaced a corresponding adjustment is made automatically by means of the software in order to make a reproducible and reliable checking process of various punch rivets or clinch connections possible. As a further advantage of the process according to the invention, it is possible to automatically teach-in such a reference curve by means of a certain number of rivet setting processes, so with new rivet types for example, the first 10 or 200 rivet processes are recorded in a memory and processing region of the device according to the invention suitable for this purpose and are converted by means of special software into a corresponding reference curve.

[0035] Figure 8a is an amplitude/time graph showing a sequence of a single riveting operation or cycle. The amplitude is indicative of the force and displacement of punch 31 and its interface with the rivet. A predetermined alert limit 83 is set, which is indicative of a problem with either the rivet joint or a malfunction In the riveting system 21. This malfunction can represent a fracture of the punch 31 or die 35. [0036] Measurement of the sheet metal/workpiece material properties can also be accomplished during the processing of the joint. For example, it is possible to determine the volume fraction or location of a fiber reinforcement in a composite material at each joint location by analyzing the aooustic signature of the rivet insertion. Noises are generated by the fracturing of, for example, carbon fibers. By listening to the amplitude and the signature of the noise generated by the fracture of these fibers at the different points of rivet insert (measured by the punch stroke location or as a function of time), a reliable determination of the volume fraction or depth of the material can be made. [0037] Figure 8b represents an analysis which can be run on the signal 71. By running a root-mean square of the signal, then subsequently integrating, an energy value can be determined. An evaluation of the overall amount of energy or by evaluating the processed signal intensity, a determination of the quality of the joint can be accomplished, For example, the hardness of the rivet can be determined by the maximum value. Further, by analysis of the length of time of the acoustic signal, further assessment of the quality of the joint can be made. Predetermined error can be set which would indicate a change of the process control limits,

[0038] By monitoring the trends of the acoustic emission signature over time, as well as by monitoring the force of the punch (or alternatively, the clamp or motor torque), a machine failure forecast determination can be made. This would indicate that the parameters which control the rivet gun can be changed or that the tooling needs to be changed. Furthermore, by watching a decrease in the acoustic emission signature as well as a decrease in the force, an unexplained material difference of the workpiece can be detected. Although each joint will have an individualized signature, statistics will help to analyze the processes over a long time.

[0039] The joining force to be applied by the punch is conventionally measured during a setting process by means of sensors which determine the ratio of force applied and path of the punch traversed and thereby control the joining process. As the process data "force" and "path" move in a certain range of tolerance, the power: path ratio is determined during the rivet setting process in a tolerance band and when the tolerance limits are exceeded a signal is produced or the rivet setting process is optionally stopped' instantly. Despite this tolerance band, it can happen that radial expansion of the punch rivet 65 is either insufficient or unsymmetrical (for example owing to cracking in the rivet shank) and therefore the quality of the riveted connection is deficient although the force: path ratio lies within the predetermined .tolerance band.

[0040] Owing to the high mechanical load of the tools to produce punch riveted and clinch connections (between approximately 20 and 150 kN), these can change during the automated rivet setting or clinch process, for example erode and wear, and therefore the quality of the jointed connection is influenced substantially. While total failure of the tool can be determined with conventional testing processes, it cannot be determined, however, which of the previously set punch rivets are still of suitable quality, Therefore it is difficult inter alia to determine which punch rivet 65 has been correctly set and when the tools used are starting to become unsuitable.

[0041] The noises emitted owing to the movement of the lattice planes in the metal crystals are a direct function of the technical parameters of the workpieces used. It should therefore be taken into account that the noises from steel rivets which penetrate into plate-like components made of aluminum produce a certain acoustic signal sequence of a certain frequency which is different to the signal generation of a steel punch rivet 65 which penetrates into steel plate-like components. For example, the; acoustic signals which are produced during the punching of the upper plate-like component, occur earlier and have a lower frequency than the, acoustic signals which are produced during reforming of the radial spreading of the cutting shank end of the punch rivet, and, owing to the harder material of the punch rivet, emit a higher frequency at a later instant.

[0042] To assess the quality of joint connections, in particular of punch riveted connections, it is necessary, as described at the start, to set ideal-typical criteria which make a statement regarding the quality of the punch riveted or clinc connection achieved possible. To assess the suitability it has proved expedient, according to the invention, to measure the penetration depth of the punch rivet 65 into the plate-like components in a plurality of suitable punch rivet 65 connections, furthermore to determine the undercut of the spread punch rivet 65 within the plate-like components and to determine the extent of expansion and upsetting. It has also proved to be advantageous to measure and register the signal and noise pattern which accompanies the ideal-typical reshapings at the same time as these measurements in order to construct a reference curve.

[0043] After fixing the tolerance variables it is possible with the process according to the invention and using the device according to the invention, to precisely determine the quality and/or suitability of the riveted connection while taking into account the different types of rivet 65 or clinch shape. For example, cracks, which are produced in the rivet 65 during the punch rivet process, punch breakage and die breakage or tendencies of the above-mentioned towards fatigue damage or force damage can be clearly detected from the signal course,

[0044] Shown in Figure 9 is a flow chart of a process according to the invention for checking the quality of a join between at least two plate-like components. The following steps are carried out: - registering a number of acoustic signals during a cold reforming joining process for connecting at least two plate-like components, - processing the acoustic signals, - evaluating the acoustic signals by means of suitable software and hardware,

- emitting a corresponding signal.

[0045] The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.

Claims

CLAIMS What is claimed is:

1. A method for checking the quality of a connection between at least two workpieces comprising:

(a) cold reform joining the workpieces together;

(b) sensing at least one acoustic signal during the cold reforming joining process; and

(c) comparing the sensing signal to a prior stored signal value.

2. The met od according to Claim 1 , wherein at least one acoustic receiver designed as a sensor receives acoustic signals.

3. The method according to Claim 1 , wherein the at least one acoustic signal is filtered and subsequently converted into electrical and digital signals.

4. The method according to Claim 1 , wherein registration and conversion of the acoustic signals are simultaneous.

5. The method according to Claim 1, wherein the various acoustic signals are analyzed simultaneously or sequentially.

6. The method according to Claim 1, wherein the signal to be emitted flows back into a process control of the reforming joining process for evaluation.

7. The method according to Claim 1 further comprising automatically varying a joining characteristic based on the sensed acoustic signal.

8. A device for checking the suitability of a connection between at least two plate-like components, the device comprising: at least one acoustic receiver operably receiving acoustic signals; an electronic control unit operably receiving the acoustic signal from the acoustic receiver, the electronic control unit operably processing the acoustic signal and comparing the acoustic signal to a prior value.

9. The device according to Claim 8, wherein the at least one acoustic receiver is arranged either one of directly on a punch rivet device or directly on a tool holder.

10. The device according to Claim 8, further comprising a punch, at least one punch holder and at least one die, the sensor being, located adjacent at least one of the punch, the punch holder and the die.

11. The device according to Claim 8, wherein the tool holder is a C- shaped frame and at least one sensor is coupled to the C-shaped frame.

12. The device according to Claim 8, wherein the acoustic receivers designed as sensors have piezoelectric elements.

13. The device according to Claim 8, wherein the acoustic receivers designed as sensors are microphones.

14. The device according to Claim 6 further comprising a joint forming characteristic being automatically varied in response to the acoustic signal.

5, A riveting system comprising:

(a) a riveting tool including: a drive system; a punch linearly movable in response to actuation by the 5 drive system; and a die aligned with the punch and spaced away from the punch when the punch is in a retracted position;

(b) an acoustic sensor coupled operable to receive acoustic signals produced by the actuation of the riveting tool and produce electronic o signals thereof;

(c) at least one electronic control unit electrically coupled to the riveting tool, the electronic control unit being operable to receive signals from the acoustic sensor; and

(d) a self-piercing rivet operably moved toward the die by 5 advancement of the punch,

16, The system of Ctajm 15 wherein the electronic signals are indicative of a riveting characteristic.

0 17. The system of Claim 16 wherein the acoustic sensor is a piezoelectric sensor.

18. The system of Claim 17 wherein the sensor is positioned adjacent the punch. 5

19. The system of Claim 16 further comprising: at least two workpieces desirably joined together by the rivet; wherein the riveting characteristic indicates the quality of 0 the riveted joint.

20. The system of Claim 19 wherein the riveting characteristic indicates the length of the rivet.

21. The system of Claim 19 wherein the riveting characteristic 5 indicates at least one material property of one of the workpieces.

22. The system of Claim 19 wherein the sensor indicates a component failure in the riveting tool.

o 23. The system of Claim 19 further comprising: a clamp linearly advancing at least partially with the punch; wherein the acoustic sensor sends a signal responsive to a force applied by the punch, 5

24. The system of Claim 19 further comprising a load cell capable of measuring forces generated by the punch.

25. The system of Claim 19 wherein the electronic control unit 0 deenergizes the drive system and sends an error signal if the, electronic control unit determines that the riveting characteristic is undesirable.

26. The system of Claim 19 wherein the electronic control unit allows the riveting tool to operate in a subsequent riveting cycle if the 5 electronic control unit determines that the riveting characteristic is acceptable.

27. The system of Claim 19 wherein the electronic control unit modifies at least one characteristic of the rivet tool in response to the electronic signal. 0

28. The system of Claim 15 further comprising a robpt operable to move the riveting tool relative to the electronic control unit.

29. A riveting system comprising: a riveting tool including:

(a) an electric motor; (b) a transmission operable to convert rotary motion caused by energization of the electric motor to linear motion;

(c) a rivet advancing member linearly movable in response to actuation by the transmission;

(d) at least one vibration sensor operable to sense the activation of the riveting tool and producing a signal indication thereof; at least one electronic control unit electrically connected to the riveting tool, the electronic control unit being operable to control energization and deenergization of the electric motor; a rivet operably driven by the punch; and a second sensor located in the riveting tool and electrically connected to the electronic control unit, the sensor operably sending a signal to the electronic control unit indicating a riveting characteristic.

30. The system of Claim 29 wherein the acoustic sensor indicates the quantity of force applied by the rivet advancing member,

31. The system of Claim 29 wherein the second sensor is a load cell operable to sense force applied by the rivet advancing member on the rivet.

32. The system of Claim 29 wherein the control unit operable to calculate the root-mean square of the signal.

33. The system of Claim 29 wherein the control unit is operable to calculate an integration of the signal.

34. The system of Claim 32 wherein the control unit is operable to adjust the energization of the electric motor in response to the root-mean square calculation.

35. The system of Claim 33 wherein the control unit is operable to adjust the energization of the electric motor in response to the result of the integration calculation.

36. The system of Claim 32 wherein the control unit is operable to disengage the motor if the signal is above a predefined limit.