Quality assurance in the production of wire harnesses

January 2018 — Wire harnesses are a central component of modern vehicles. Highest possible quality here is essential. For quality assurance in a complex and largely manual manufacturing process, the use of a specialised Manufacturing Execution System is indispensable.

Digitization has long been an integral aspect of vehicle technology. With each generation of models, the implementation of IT continues to rise. Vehicles are crammed full of it from engine control through assistance systems to media centres with digital systems. These developments are by no means nearing their conclusion as work is already underway on the next steps with the keywords of Connected Car and the emergence of autonomous driving. A core element of vehicle IT is the wire harness. It acts as a backbone connecting the digital components to each other or to other components such as the engine or chassis and also supplies the various devices with power.

This wire harness is now crucial for vehicle efficiency and safety. Malfunctions or even failures of a stereo system are annoying and with assistance systems they can still be offset by an attentive driver, but at the latest in autonomous vehicles faults will no longer be acceptable – either in individual components or in the wire harnesses. Therefore also in this regard quality requirements will increase dramatically. Faulty wiring connections, a wire break or a loose connection in a wire harness system may not only be crucial to the success of individual model series but also to human life. Therefore not only the important systems, but also the wire harness system must be designed with redundancy.

In this regard, wire harnesses have also differentiated themselves through the wide range of electronic systems which can be selected in the diverse configurations available. Whereas, for example, Asian automotive manufactures limit themselves in one model to specific different variants of wire harnesses, it has become commonplace in the German premium segment to provide any combination of configurations with a special wire harness. The reason for this high degree of effort is, on the one hand, the restricted space in the vehicle, which is why laying unnecessary wires must be avoided as far as possible and, on the other hand, the weight of the wires themselves: Wire harnesses are now not only very long, they also have a considerable weight with 50 kg to 60 kg not uncommon. Since manufacturers have to take advantage of all opportunities to save weight to reduce fuel consumption and harmful emissions, wire harnesses are of course also subject to this criterion. The consequence is: Everything becomes more bespoke and miniaturized.

The major international manufacturers of wire harnesses, such as Leoni or Delphi must not only provide the highest quality, but they must also be extremely flexible; for the reasons stated, wire harnesses must at times be produced in very small batch sizes, sometimes in a batch size of one, without this incurring any loss of quality. Basically the production of wire harnesses is very labour-intensive – the manual proportion accounts for up to 95% – which is why manufacturers are always looking for (new) sites in countries with low labour costs. To achieve this, new employees in large numbers have to be trained which in turn also has an impact on efficiency and quality. Therefore, fault prevention is a key factor in manufacturing as is total traceability in order to isolate faults in the event of recalls. Quality assurance can start at various points in the production process.

From design to fabrication

The process starts with design and development before production itself. The vehicle manufacturer provides a so-called 150%-drawing of the design of his wire harness to the manufacturer. This includes all possible variants of a wire harness, for example all possible designs of a media centre and climate control system. The engineers of the manufacturer use this drawing to create a production drawing, a work plan and corresponding parts lists. However, the manufacturer regularly issues schedule changes and at very short intervals – even daily – and the planning must be adjusted to each of these updates.

This activity is carried out mainly manually by relatively large teams. As always when manual activities are concerned, and particularly if these have to be carried out under time pressure, the error rate is high. Furthermore, the manual implementation is also contingent on individual working methods and depending on which engineer was working, a production plan may potentially look different. Also, the frequent changes to the drawings are not always systematically managed and documented and the technical level of a vehicle is not always unambiguously and continuously documented.

As all these aspects tend to be to the detriment of quality, it makes sense to use a software-controlled planning system. This approach is very innovative and with PPG (Production Plan Generation) a first solution has been proposed by DIIT. In such a system the manufacturer can use a scripting language to specify rules which define how a production plan should be structured, detailing for example which steps are to be carried out mechanically, which depends on the respective machinery, and what is to be carried out manually or semi-automatically. These rules must be flexible so they can be adjusted. For instance, they could look like this:

If single wire and
0.35 < cross-section < 6 mm²
Then standard machine.

Using such rules as a basis, the system reads in the drawing of the automotive manufacturer, analyses it and then generates production data, therefore the work plans and parts lists. In addition to a considerable saving in work time, the quality advantages are in relation to quality through the elimination of manual fault sources and also in the standardization of implementation procedure. Such software can also calculate the production documents on the basis of a specific just-in-time delivery schedule. The appropriate wire harness is then produced and delivered at the right time for this special vehicle in a batch size of one.

The production of wire harnesses

Below the ERP level the task of the Manufacturing Execution System (MES) is the control and documentation of the production of a wire harness. However, as wire harness production has many specific aspects, “general” MES systems could not be used. They fail for example in the technical IT description of a wire harness. Therefore, different wire harness manufacturers have developed their own control systems. However, these refer to “custom-made orders” which no longer fit when production processes or ERP systems change. Another issue is that know-how is often undocumented and linked to specific employees. Therefore, leading wire harness manufacturers, such as Delphi, Yazaki, Sumitomo, Leoni and Nexans rely on systems which have been developed specifically for the wire harness industry and in this regard the solutions from DIIT have now been established as a quasi-standard.

Wire harnesses are produced in three phases of cutting area, pre-assembly and final assembly: Wire cutting in the cutting area is highly automated. CST machines cut the wires into specific lengths (cut – C), strip off the sheathing (strip – S) and apply seals and terminals (terminate –T). More complex machines can also perform additional process steps and for example, twist the wire. Wires pieces produced in this way are grouped into bundles and made available to the next production step.

A special MES such as Cutting and Assembly Optimization “CAO” ensures the optimum distribution of the orders across the various machines in order to increase productivity. But quality and traceability are also important here. In a CST machine the clocking is usually very high and the wires are sometime extremely thin. Of course it is primarily the machine itself that ensures precision but the MES also makes an important contribution by transferring control data, such as crimp data, directly to the connected machines, by enforcing quality assurance process steps such as scanning, and by monitoring tool maintenance cycles. To set up the traceability, CAO assigns a unique ID to each bundle and manages it continuously throughout its history.

The next production phase of a wire harness is highly manual. In the first pre-assembly phase the wire bundles produced in the cutting area are assembled into simple kits using procedures such as crimping, welding and twisting. The history is of course updated. When producing a kit a new ID is generated and all components included in the kit are saved so that the history is complete. Since manufacturers, as already mentioned, often relocate their production for cost reasons complete plants at new sites rise up sometimes recruiting thousands of new employees. However, a lack of experience and qualifications signify high risks for product quality. The Software therefore places great emphasis on instructions to employees directly at the workplace.

Setting up the traceability

In final assembly the wire harnesses are assembled on so-called assembly boards. Generally the assembly boards run clocked across many workstations. At each workstation the employee who works there must perform the correct work steps. Given that wire harnesses are generally produced to be customer-specific, these steps differ from wire harness to wire harness. Therefore, every employee must potentially do very different things in every rhythm – an additional challenge for quality assurance. The Production and Logistics System “PLS” from DIIT controls this complex process with screens which provide employees with detailed displays of the relevant work steps to be performed. Employees can also be prompted by the system to confirm certain steps, perform test steps or input QA relevant data. The software can thereby provide a high level of quality in these highly manual processes.

The history is of course updated in the final assembly. The finished wire harness now consists of thousands of components and the system retains a complete history for each component. The system knows on which machine, by which employee and with which tools the individual components were produced. This means, for example, that it can be subsequently determined which vehicles have wire harnesses incorporated with wire from a specific batch. This also enables any repairs or recalls to be isolated and any product liability issues can be quickly clarified.

Software-controlled quality control is now essential for wire harnesses in an increasingly digitized vehicle technology. Otherwise the necessary quality for the demanding tasks cannot be guaranteed – and against the backdrop of an ever broader product liability “guarantee” is to be interpreted literally here. However, a “normal” MES is not designed for these very special requirements and here only relevant special solutions will help further.

Bernd Jost, DIIT AG

DIIT AG
Justus-von-Liebig-Ring 11a
82152 Krailling/Germany
Contact person is Mrs. Patricia Lange
Tel.: +49 89 893250-0
patricia.lange@diit.de
www.diit.de

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