Dependable Quality for Automotive Wire Harnesses
To make the connected car and autonomous driving possible, quality wire harnesses are essential. Since quality assurance is a complex and largely manual manufacturing process, the use of a specialized manufacturing execution system (MES) is paramount.
Digitization has long been an integral aspect of modern vehicle technology. Vehicles are chock full of IT, from engine control systems to driver assistance systems to media centers and the list goes on. These developments are by no means nearing their peak as work is progressing rapidly on the connected car and autonomous vehicles.
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 electrical power.
The wire harness has become critical to vehicle efficiency and safety. Malfunctions of something such as a stereo system can be annoying. Even the failure of driver assistance systems can still be offset by an attentive driver. But as autonomous vehicles become mainstream, failures will no longer be tolerable, either in the individual components or in the wire harness that connects them together. Also in this regard, quality requirements will increase dramatically. A wire break or a loose connection in a wire harness system can mean life or death. Therefore, all systems connected via the wire harness must be designed with redundancy.
Not so long ago, the wire harness for a particular vehicle model was identical for all vehicles produced of that model. Every wire harness included all of the wires for every possible option, regardless of whether the option was ordered with the vehicle. Some automotive manufacturers still build wire harnesses that way. Other automotive manufacturers design and build their wire harnesses to only include those wires that are needed for the particular options that were ordered with the vehicle. The reason for this high degree of effort is, on the one hand, the restricted space in the vehicle, which is why including unnecessary wires must be avoided as much as possible and, on the other hand, the weight of the wires themselves. Automotive wire harnesses are now not only very long, but also weigh a considerable amount, with 50 to 60 kilograms (110 to 132 lbs.) being fairly common. To save weight to reduce fuel consumption and harmful emissions, more wire harness systems are being customized for each particular vehicle.
Production of Automotive Wire Harnesses
The major international manufacturers of automotive wire harnesses, such as Delphi, LEONI, and Yazaki, provide the highest quality, extremely flexible product in very small batch sizes, without incurring any loss of quality.
The production of automotive wire harnesses is very labor-intensive — the manual labor portion accounts for up to 95% of the cost — which is why manufacturers seek manufacturing sites in countries with low labor costs. To achieve this, large numbers of new employees have to be recruited and 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.
Phase 1: From Design to Production
The process starts with design and development of the wire harness. The vehicle manufacturer provides a 150% drawing of the design of his on-board wiring system to the wire harness manufacturer. This includes all possible variants of the wire harness, based on all possible options for a particular vehicle model.
The engineers at the wire harness manufacturer use this drawing to create a production drawing, a work plan, and corresponding parts lists. However, the manufacturer regularly issues schedule changes and the planning must be adjusted to each of these updates.
As always, where 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 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 managed and documented properly in real time.
As all these aspects tend to be to the detriment of quality, it makes sense to use a software-controlled planning system. In such a system, the manufacturer can use a scripting language to specify rules that define a production plan, detailing for example, which steps are to be carried out mechanically, which depend on the respective machinery, and what production steps are 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 < 6mm2
then use standard machine
Using such rules as a basis, the system reads the drawing of the automotive manufacturer, analyzes it, and then generates production data, including the work plans and parts lists. In addition to a considerable savings in work time, the quality advantages are high due to the elimination of manual fault sources and the standardization of an 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 just in time for a specific vehicle in a batch size of one.
Phase 2: The Production of Automotive Wire Harnesses
Below the ERP level, the task of the manufacturing execution system (MES) is the control and documentation of the production of an automotive wire harness. However, as wiring harness production has many specific aspects, general MES systems could not be used. They fail for example in the technical IT description of a wiring harness.
Therefore, different wiring harness manufacturers have developed their own MES systems. However, these custom-made software solutions 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 automotive wire harness manufacturers rely on systems which have been developed specifically for the wire harness industry.
Wire harnesses are produced in three phases: cutting, pre-assembly, and final assembly.
Wire processing in the cutting area is highly automated. Cut, Strip, and Terminate (CST) machines cut the wires to specific lengths (cut – C), strip off the insulation (strip – S), and apply seals and terminals (terminate – T). More complex machines can also perform additional process steps such as twisting wires. Wires produced in this way are grouped into bundles and made available to the next production step.
A special MES 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 production rates are usually very high and the wires are sometimes very small in diameter. Of course it is primarily the CST machine itself that ensures precision and quality but the MES also makes an important contribution by transferring control data, such as crimp data, directly to the connected machines. The MES can also ensure quality assurance process steps, such as scanning the barcodes for the wire and terminals, enforcing crimp height and pull tests and by monitoring tool maintenance cycles. To set up traceability, an MES assigns a unique ID to each wire bundle produced 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, ultrasonic 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 to low cost labor countries for cost reasons, entire factories are built seemingly overnight and thousands of new employees must be recruited. However, a lack of relevant experience and technical qualifications signify high risks for product quality. The MES software places great emphasis on instructions to employees directly at the workplace.
In the final assembly area, the wire harnesses are assembled on wire harness boards. Generally, the wire harness boards are mounted on a moving assembly line where assembly personnel each mount one or more wires to the harness board. At each workstation, each employee must perform the correct work steps. Given that wiring harnesses are generally produced to be customer-specific, the work steps can differ from wire harness to wire harness. Therefore, there is a good chance that errors will be made, an additional challenge for quality assurance. An MES controls this complex process with display monitors, which provide employees with detailed information for 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 hundreds or thousands of components and the system retains a complete history for each component. The system keeps track of which machine, which employee and which tools were used to produce the individual circuits and components for each wire harness.
This means that it can be subsequently determined which vehicles have wire harnesses built 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.
Quality control managed by software is essential for producing automotive wire harnesses in increasingly digitized vehicle technology. Otherwise, the necessary quality for the demanding task cannot be guaranteed. Traceability is key for ensuring product quality and managing product recalls. Conventional MES systems are not up to the task of managing the highly specialized nature of automotive wire harness production.
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