Dynamic Wire: Crimp Force Monitoring

"As customers demand more for their money, manufacturers must find ways of ensuring that electrical equipment is reliable. Therefore testing the crimp force on the terminals is more important than ever."

Rich Griffith, Sensotec Inc, USA, and John Perkins, Sciemetric Instruments Inc, Canada

The process of manufacturing wire and termination assemblies used in automobile wiring harnesses involves crimping terminals on the ends of individual wires of various lengths. The quality of this connection determines whether or not the electrical signal transmitted across each wire reaches the destination reliably over time. Unfortunately, the quality of the crimp cannot always be determined by visual inspection.

Manual pull tests or electrical checks, while effective, are slow and costly. Electronically monitoring the mechanical force as it is applied during each terminal crimping operation has been shown to provide an excellent indication of the integrity of this electrical connection.

The automotive electrical systems of today have undergone a transition from essentially a few circuits to complete systems that play an increasingly important part in the performance and safe operation of the vehicle.

Power windows, power door locks, sophisticated audio systems, which all used to be considered luxuries, are becoming standard on more and more models. Onboard sensors monitor passenger compartment temperature, windshield washer fluid levels, and many other critical and non-critical parameters. Several onboard computers are used to monitor and control engine and transmission performance, anti-lock brake and supplemental restraint systems, and to perform detailed diagnostic checks.

With all of the latest sophisticated automotive electronics, the quality of onboard wiring and connections has become a critical part of the wire harness assembly operation. Prefabricated wiring harnesses are necessary to simplify the interconnection of all the electrical systems, and the individual crimped wire/terminal must be 100 per cent inspected to ensure the performance of these completed wiring harnesses.

Careful and precise application of the force applied during the wire/terminal crimping operation is critical if one is to achieve a reliable electrical connection- insufficient force can result in a loose terminal, while excessive force can damage the terminal and/or wire.

Most of the wire crimping presses in use today are mechanically driven and adjusted, and the crimping force cannot easily be controlled. Measuring the actual force applied during the crimping cycle is not a trivial task either, due to the high speed operation and high shock encountered.

For this application, the force measurements were made with a dynamic loadce1l mounted under the die tooling for the crimp-on electrical terminal.

The dynamic load ce1l contains a crystal that generates a charge when compressed. This phenomenon is known as the piezoelectric effect and is a sensing technology commonly used in accelerometers for high-frequency acceleration and vibration monitoring. This charge is directly proportional to the-force applied and can easily be converted to a voltage output signal through the use of a charge amplifier. The dynamic load cell offers infinite resolution, micro-inch deflection for high frequency response and the ruggedness of billions of compression cycles. The low profile (to 0.19in) and small diameter (less than 1 in) allow them to be incorporated into existing machinery easily, without major changes.

Simply monitoring the force applied during each crimp cycle is not sufficient to identify a bad crimp versus a good one. The way that the force is applied and removed over time (i.e. the force waveform) is the key to achieving a good crimp. An electronic signature analysis system can monitor the electrical terminal crimp force and compare the force waveform of each crimp with previously learned acceptable force profiles stored in the memory.

Through systematic analysis of the force waveform, (signature) abnormalities in the crimping operation can be identified with high accuracy and excellent reliability.

Typical signature analysis criteria include global or local maxima and minima, envelope conformance, area under the curve, average, and curve fitting. In addition to identifying machine problems that cause insufficient or excessive force to be applied, the signature analysis technique can also identify other problems that can occur during the crimping operation. The crimp force waveform will exhibit distinct changes for any of the following reasons:

• Missing wire or Terminal;
• Missing wire strands;
• Scrap caught in terminal;
• Missing ears on terminal;
• Wire improperly placed in grip;
• Wrong wire gage or wire not stripped;
• Double crimp (two terminals).

By developing a library of acceptable waveforms rather than just ensuring that the crimp force applied is within a certain range, more defects can readily be identified; and better quality control is assured. These acceptable waveforms can be used as acceptance or rejection criteria, thereby providing 100 per cent inspection of each wire and terminal crimped assembly without time-consuming manual or visual inspection, A crimp cycle may be triggered with a proximity switch or when the crimp force exceeds a certain preset force level. The dynamic force curve is compared against the learned curve, and depending on the defined tolerances and limits, a pass or fail condition will be issued on a relay that can, generate an alarm or stop the crimping machine. The built-in signature learning capability is important, with crimp force monitors to account for cyclic shifts in the nominal signature due to variations with time, materials and temperature (Figure 1).

Fewer wires with multiplexed signals in the more complex automotive electrical systems dictate higher reliability of the onboard wiring and terminal components in automobiles today. Individual automotive wire/teminal dynamic crimp force monitoring and signature analysis is a quality improvement that results in fewer rejections of finished wiring harnesses. The use of compression-type, piezoelectric load cells under the terminal crimp machine die plate provides individual crimp force data that may be used with signature analysis systems to ensure that only properly crimped wire/terminal assemblies are sent on to the wire harness assembly operations.