Guided operator solutions remove assembly errors for manufacturer.

Assembly cells making several different products in one work-space have a high potential for operator error. Mitsubishi Electric’s Pick-to-Light guided operator system has removed errors by controlling the picking sequence using lights, doors, operator terminals and touch sensors. A test cell recorded a huge improvement in the quality, picking accuracy, assembly speed and consistency, that has led to a roll-out programme.

Based in Livingston, Scotland, Mitsubishi Electric Air Conditioning Europe wanted to enhance quality control processes by reducing the opportunity for picking mistakes in its production cells where several different products are assembled. In addition, there was a desire to increase productivity by helping workers to quickly adapt to new assembly processes.

The answer came from Mitsubishi Electric, Factory Automation with its Guided Operator Solutions, a versatile system that uses a PLC to control parts picking sequences using lights, doors, terminals and touch sensors. This is coupled with high-definition HMI screens that guide operators through live assembly tasks.HMI screens have been used to replace a system of complex product cards that were previously used for work instructions. As part of the integrated Guided Operator Solutions, the screens allow for dynamic instructions and prompts to be added to a pick sequence that ensures the correct part is selected.

Production section manager, Mitsubishi Electric Air Conditioning Plant, Audrey Smith explains: “We operate semi-automated assembly cells, each of which offers the facility to assemble several different products. Since we make several products in one work space there is a high potential for operator error. To combat this, operators have to have a high level of training to work on the assembly line.” The high level of training required in the past also added to the overall cost of production, and so was an obvious target for improvement.

Greg Hookings, Senior Manager Strategic Business Development EMEA for Factory Automation at Mitsubishi Electric Europe adds: “The complexity of the products was clearly a challenge for the operators and was resulting in picking mistakes. This often resulted in re-work of products, which significantly reduced the efficiency of the assembly process. Furthermore, the process to identify which components were required in different air conditioning systems was also time consuming.”

Evaluation phase

For the evaluation phase, a pick-to-light system with visual indicators on the parts bins was combined with light curtains or touch switches for pick confirmation. Mechanised doors that opened in sequence were also used to create a physical barrier to prevent wrong part selection. The effectiveness of each combination was compared and the results combined with user preferences to gauge overall effectiveness for different component bins and process sequences.

The variety of operator guidance devices ensured that a wide range of picking scenarios are accommodated and tested. Each assembly station was then equipped with the most effective option, improving quality, while also making the operator more productive and the production line more efficient. The installation built for the air conditioning plant was commissioned around Mitsubishi Electric MELSEC Q series modular PLCs and GOT2000 HMI operator terminals, all connected over a dedicated network.

Smith explains: “From an operator’s point of view, rather than complex paper-based assembly instructions, the HMI now presents a set of clearly defined work instructions. It gives the operator a sequential visual indication of the parts to be picked for building each assembly, and their quantities. Although the work instructions for the assembly of these products are complex and detailed, the installation has had an immediate impact on the assembly cells. Picking has not only become inherently more reliable and error-free, but also much faster. Quality and overall productivity have both shown a marked improvement.”

Importantly, the application has also addressed the operator training challenge at the plant. The terminals now guide operators to the correct parts and the GOT HMI shows clear images of the product to be assembled. This has reduced the resource required for training, whilst improving the time taken to learn each process.

Roll-out programme

Smith concludes: “The success of the trial on the first two stages of the assembly process has been dramatic. Compared to the original process, we have seen a huge enhancement in the quality control process, picking accuracy, assembly speed and consistency. We are now looking at implementing similar systems on all six stages of the assembly line, plus the parts loading process. We are passionate about making improvements and the benefits of this technology are now really showing their worth.

Plant wide integration

Another key benefit for the air conditioning plant was the excellent integration of the hardware. The PLC and the HMI seamlessly integrated with the other components in the system, while providing the potential to link to wider automation systems at the plant. This will enable new product specifications to be sent directly to the production cells and an inventory report to be sent back for logistical planning. Further, connectivity to other guided assembly cells throughout the production facility in the future could also allow for workflow information to be passed downstream.

See a workstation in operation here