US 3665742 A
A strain gage bridge is fixed to the setting anvil of a rivet head-forming press and measures the force being applied on the rivets. The strain gage bridge when unbalanced by a calibrating resistor, provides a potential which is compared by a zero crossing detector to a potential representing the formation of a proper rivet. When the potential due to the force on the rivets exceeds the potential provided by the calibrating resistor, a switching circuit is energized to avoid a press lockout. Otherwise, the switching circuit locks the punch press to prohibit a new cycle from being initiated until the switching circuit is reset.
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Description (OCR text may contain errors)
United States Patent v 5] 3,665,742 Felt et al. 4 1 7 May 30,1972.
 DETECTING MISSING OR DEFECTIVE CONNECTORS Primary Examiner-Charles W. Lanham Assistant Examiner-Gene P. Crosby  Inventors. Adolph E. Felt, Joseph R. Maklary, both A Immey An drug, Sce ales, Starke & S awa" of Milwaukee, Wis.
 Assignee: A. O. Smith Corporation, Milwaukee, Wis.  ABSTRACT  Filed: Feb. 19, 1970 A strain gage bridge is fixed to the setting anvil of a rivet headforming press and measures the force being applied on the 21 A LN .1 12582 1 pp 0 rivets. The strain gage bridge when unbalanced by a calibrating resistor, provides a potential which is compared by a zero  U-S. Cl ..72/4, 72/31 ro ing detector to a otential representing the formation of  Ill. Cl. 15/28 a proper rivet. wh th t i l d t th force on th  Field Of Search ..72/3l, 25, 26, 3,4, 10 rivets exceeds the potential provided by the calibrating sistor, a switching circuit is energized to avoid a press lockout.  Rderences Cited Otherwise, the switching circuit locks the punch press to UNn-ED STATES PATENTS prohibit a new cycle from being initiated until the switching circuit is reset. 3,444,390 5/1969 Breidenbach ..72/4 2,920,783 1/1960 Kipp ..72/4 7 Claims, 2 Drawing Figures 9 mg g Z Detector i l ii i i i: 20 :t J ii I 1 i I 7 1 H i l l I i 1 H Press IJ I E[ Contgol 4 g I J 1 l i i Interlock Network 27 l Patented May 30, 1972 v 3,665,742
INVENTORS ADOLPH E. FELT JOSEPH R. MAKLARY Attorneys DETECTING MISSING OR DEFECTIVE CONNECTORS BACKGROUND OF THE INVENTION This invention relates to riveting or similar punching, stamping and connecting of workpieces. More particularly, this invention relates to detection of defects in such riveting or the like.
Assembly of workpieces, such as automobile suspension parts, is commonly done by riveting the parts together with a punch press. The rivet stock and assembly parts may be assembled at the press and riveted together by operation of a punch which compresses the rivets against setting anvils-to form heads. Forces in the order of 20 tons will be applied to the rivets in the press.
Some parts, such as an automobile front suspension assembly, require a plurality of rivets for one assembly. It may occur that a rivet could be defective, in which case the assembly would have an outward appearance of being completed; but in actual fact, it would be dangerously defective. The rivet may be short, in which case the rivet head would not be properly formed, or it may be missing. In either case, it is highly undesirable to permit the press operator to continue with the next assembly without checking the defective assembly and determining the problem. Other press assembly processes can have similar difficulties.
SUMMARY OF THE INVENTION The invention is to automatically detect missing or defective connectors for parts assembled in a press type machine. The invention has further advantages which will be apparent from the description and drawings.
In general, a defective connector is detected by measuring the force applied to the rivets or the like by the press. A normal connector will experience a predetermined amount of force when the press engages it. Where the connector is missing or is defective, the applied force is below a predetermined minimum value. When this change is detected, a signal is generated and may be used to provide an alarm indication, or lock the press out so that a new cycle cannot occur, or mechanically reject the part, or a combination of these.
To do this according to one embodiment, a strain gage bridge is fixed to a setting anvil of the punch press. The strain gages are in a bridge circuit which is unbalanced by the change in resistance when the gage is compressed by operation of the press. The resultant potential is compared to a predetermined potential corresponding to the desired predetermined minimum force which is known to indicate that the connector is defective. If the unknown potential compares favorably, a signal is provided to a switching circuit, which in this embodiment, closes an interlock switch in the press control to permit a new cycle. Where the unknown potential is insufficient, the interlock switch is not closed, and the press is locked out.
This invention may be used in an arrangement requiring a third party, such as a foreman, to insert a key to unlock the press. Quality control is thus enhanced as the problem is pinpointed immediately and further checking of parts can be eliminated.
DRAWINGS The drawings illustrate the best embodiment presently contemplated for the practice of the invention by the inventors.
In the drawings:
FIG. 1 is a perspective view of a rivet press having the apparatus of the invention;
FIG. 2 is an enlarged closeup front view of a part of FIG. I; and
FIG. 3 is a schematic circuit diagram of part of the apparatus of the invention.
DESCRIPTION Shown in the drawings is a riveting press 1 which may be any standard machine. In the machine, and disposed over a press table 2, is a ram 3 which operates to force a pair of upper anvils 4 against a pair of lower anvils 5 on press table 2, as in the conventional operation. Anvils 5 are set in appropriate wells in table 2' and extend toward upper anvils 4 through a spring loaded platform 6 which holds a workpiece, not shown, for riveting operation. 7 I 1 Other conventional equipment for the press includes a control switch 7 which energizes a generally standard press control 8. For purposes of controlling the press in accordance with the invention, control 8 is interlocked with a defectiveconnector detector 9 so that by operation of the detector, the press control will be locked out from a new cycle if a rivet is missing or otherwise defective.-
Detector 9 may be connected to power through the usual circuits in control 8 and operates to measure and compare to a known standard, the force being exerted on the rivetsin the press. To do this, detector 9 has a strain gage bridge 10 fixed to each lower anvil 5. A bridge 10 may be a bonded wire strain gage bridge, a foil gage or any other resistance gage device in common use. Each bridge 10 has four conductors 11 attached around the outside on the upper portion of their respective anvil 5 at intervals, with two active conductors, alternating with two dummy conductors. This is a standard configuration for a bridge having temperature compensation features. The strain gage conductors change resistance in a well .known manner during compression as the press forces on the lower anvils are communicated from the rivets and thus form a variable electrical impedance with the impedance proportional to the amplitude of the load impressed thereon.
The electrical connections between conductors 11 and detector circuit 9 are provided by leads 12 which extend through grooves 13 in table 2 so as to be protected from the action of the press. Also, to protect the bridges 10 from wear, cylindrical collars l4 fit over the upper portions of anvils 5 and enclose the conductors.
In a press of this size, a normal rivet will transfer about 20 tons of force to each lower anvil 5. Strain gage bridges l0 and detector 9 will, in that case, close switches (described below) in control 8 to permit another cycle. In accordance with the invention, it is found that a short rivet or otherwise defective rivet, will transfer measurably less force to the lower anvil, while a missing rivet will cause no force to be applied to anvils 5 and strain gage bridges 10. In these situations, detector 9 locks out control 8 so that the press may not initiate a new cycle. A measurement of less than l0 tons, for example, could be used as an indication of a missing rivet. Where it is desired to detect less drastic defects, such as poorly formed rivet heads, a measurement in the range of 15-18 tons may be used in this example.
When the press is locked out, someone such as a foreman places a key, not shown, in a key reset switch 15 to close the aforementioned switch in control 9 and to thus permit the press to operate again. By this, the foreman is made aware of the problem with the product. He can then check the problem as well as discard or reassemble the defective workpiece assembly.
The invention also provides an advantageous means of detecting and locking out the press when defective connectors are detected. Two identical circuits in detector 9 act in response to their respective bridges 10, and a schematic diagram of one circuit and strain gage bridge 10 isillustrated in FIG. 3. Shown therein are the separate conductors 11 in the bridge 10 with a calibration resistor 16, an amplifier I7 and a zero crossing detector 18, which compares the calibrating potential on a capacitor 19 with the amplified potential across the bridge during press operation, and then selectively switches an SCR device 20 on, in accordance with whether or not the unknown potential compares favorably.
Bridge has the usual arrangement of strain gage resistors 11, with two terminals of the bridge connected across the DC. power leads, and the other two terminals connected to appropriate terminals of amplifier 17. The operation of such bridges is well known and need not be further described here.
Calibration resistor 16 has a known resistance predetermined to unbalance the bridge so as to provide a potential, which is amplified to charge capacitor 19 an amount simulating a predetermined minimum threshold force such as 10 tons. For this function, resistor 16 is connected across one arm of the strain gage bridge 10 and has a normally closed relay switch 21 for closing the circuit.
When switch 21 is closed, a second relay switch 22 normally connects the output terminal of amplifier 17 to capacitor 19 and then to ground. In this condition, the capacitor will be charged the predetermined amount, since resistor 16 unbalances the bridge so as to simulate the predetermined minimum force. Switches 21 and 22 are operated by a relay coil 23 in the power circuit for press control 8, coil 23 being energized when control switch 7 is closed to start the press cycle. .The voltage across the coil commonly employed to operate a clutch for ram 3 may be utilized for this purpose. When the cycle is initiated, relay coil opens switch 21 to disconnect resistor 16, and switch 22 is switched to one input terminal of zero crossing detector 18, while the other input is the potential on capacitor 19. Press control 8 causes ram 3 to operate, and a force is exerted on the rivets. Thus, the strain gage bridge becomes unbalanced, and amplifier 17 applies the unknown potential through switch 22 to the zero crossing detector 18.
Detector 18 is a well known device which emits a signal when the unknown potential from amplifier 17 is greater than the potential in capacitor 19 by a sufficient amount, in the area of 500 microvolts. Therefore, when the detector passes a signal, it is known that the force exerted on the setting anvil is over the predetermined minimum, indicating that the rivet has been properly formed. Any drift of the amplifier gain will not affect this comparison, since both the calibration potential and the unknown potential would experience the difference in potential. As long as the amplifier remains stable for the I2 second period of the press cycle, the comparison will be accurate.
In this embodiment of the invention, it is desired only to lock out the press from asubsequent cycle when the predetermined minimum force has not been surpassed. As stated earlier, the signal could be used otherwise. Detector 18 is, for this purpose, connected to turn on SCR with the signal, the connection being accomplished through resistor 24. When SCR 20 is on, it provides a conducting path through a relay coil 25 that controls a normally open switch 26 in a standard interlock network 27 which interlocks with the power circuit of control 8. With the SCR on, relay coil 25 closes switch 26 to permit the press to operate.
To reset detector 10 for each press cycle, a normally closed limit switch 28 on the press opens momentarily at the beginning of the press cycle and interrupts the SCR anode circuit. This switch may be operated by a cam, not shown, driven by the motor which drives the press. If the detector turns the SCR back on, switch 26 is closed and network 27 permits a subsequent cycle to occur. If the detector does not turn SCR 20 on, switch 26 remains open and the press is locked out.
The press control may be rendered operable again by tuming key switch 15. This applies power through resistors 29, serving as a voltage divider to the SCR gate, turning it on and closing switch 26 to deactivate interlock 27 and permit control 8 to initiate another press cycle.
OVERALL OPERATION vWhen the press is initially turned on, at the beginning of a work shift, the identical circuits of detector 9 are switched to their calibration conditions, and each capacitor 19 is charged. The workman assembles a workpiece with rivet stocks on table 2; he then closes switch 7 to bring upper anvils 4 into forceable engagement with the rivets.
At the start of the press cycle, switch 21 has been opened b coil 23, and switch 22 has been similarly switched to its terminal of detector 18, to remain so for the rest of the press cycle. Also, SCR 20 has been turned off by operation of limit switch 28. If the rivets are properly formed, the corresponding force measured by strain gage bridge 10 will exceed the voltage on capacitor 19 and cause detector 18 to turn the SCR on, so that switch 26 will be closed. Interlock network 27 therefore permits the press to recycle. However, where the force measured by the strain gage bridge indicates that a rivet is missing, short or otherwise defective, detector 18 will not turn the SCR back on. Switch 26 is thus open and causes network 27 to lock out control 8 such that the operator must apply the key to reset switch 15 for further operation of the press.
In this way, the invention provides detection of defective rivet connectors. Other press machines or stamping devices for other types of connectors could also find advantageous employment of the invention in accordance with the description herein. As well, the manner of using the signal from the detector may be modified to operate an indicator or a device which rejects the part in addition to or as substitutes for the interlock means described, although the described embodiment is most advantageous and fail-safe.
Various modes of carrying out the invention are contemplated as being within the scope of the following claims, which particularly point out and distinctly claim the subject matter which is regarded as the invention.
1. A device for detecting defective or missing connectors in articles assembled by a press engaging said connectors, the device comprising:
an electrical power source, I
a force detector on said press at a location which experiences the force applied by said press to said connectors, said force detector establishing a variable electrical impedance with the impedance proportional to the amplitude of the load impressed thereon; and
comparison means including means connecting the power source to said detector and responsive to a selected change in said impedance for comparing the force experienced by said detector with a predetermined force and thereby indicating a defective or missing connector when the predetermined minimum force is not surpassed.
2. The device of claim 1, and including:
lockout means connected to said comparison means and interlocked with said press for locking out said press from a new cycle when said unknown force does not surpass said predetermined minimum force.
3. The device of claim 2, and including:
a key reset switch connected with said lockout means, said reset switch being operable to unlock said press for a new cycle.
4. The device of claim 1, wherein:
said force detector is a strain gage bridge device; and
said comparison means includes:
a calibration resistor having a predetermined resistance and connected to selectively unbalance said bridge device;
a comparison capacitor connected to be selectively charged by said bridge when unbalanced by said calibration resistor;
a comparison detector selectively connected with said bridge and connected with said capacitor; and
a switching system alternately connecting said bridge to said resirtor and capacitor, and alternately connecting said brioge and capacitor to terminals of said comparison detector whensaid press is operated to engage said connectOl'S.
5. The device of claim 4, and including:
electronic switching means connected to said comparison detector for turning on in response to said comparison detector; and
is fixed to said anvil.
7. The device of claim 6, wherein the strain gage bridge has conductors disposed about the outside of said anvil and including a protective collar fitting over said anvil and enclosing said conductors.