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Applying the Jidoka Principle to a UV Coating Manufacturing Process

Posted By: Ken White on
Feb 06,2017

In his book, “The Toyota Way: 14 Management Principles from the World's Greatest Manufacturer”, Jeffrey Liker states that Alex Warren, former SVP of Manufacturing for Toyota, defines the Jidoka Principle as:

“In the case of machines, we build devices into them, which detect abnormalities and automatically stop the machine upon such an occurrence. In the case of humans, we give them the power to push buttons or pull cords— called "andon cords"— which can bring our entire assembly line to a halt. Every team member has the responsibility to stop the line every time they see something that is out of standard. That's how we put the responsibility for quality in the hands of our team members. They feel the responsibility— they feel the power. They know they count.”

When something goes wrong in a process operation, the Jidoka Principle recommends that you stop immediately and address the issue.  This ensures that faulty products are not continuously manufactured.

One of the challenges of a manufacturing company is dependence on operations to continuously monitor and validate that the product being manufactured is within specifications.  These quality checks can be numerous. If the quality checks are not performed adequately – the perfect storm can occur.

These include issues such as:

  • A device on the processing equipment has failed
  • The operator neglects to perform the intended quality check
  • Bad sub-assembly products move forward to the next process

Situation

I’d like to use an example to illustrate how that perfect storm can happen when the Jidoka Principle isn’t followed by featuring a company that used UV coating on its product.  As a part of the process, two UV power supplies were used to cure the coating as the speeds increased.  As the line speed increased, the PLC’s analog reference signal to the first UV power supply followed suit. When the line speed reached a certain point, and the first UV power supply was at maximum output, then, the second UV power supply was enabled and a second analog reference signal increased the UV output in order to increase it to maximum output. 

At the end of each run, the operator did a quality check by wiping the product to determine if the UV cured coating adhered to the product.  But, unfortunately, this only checked the product at the lower speeds and neglected to check the product that was being made at the higher speeds.

As a result, this process of only checking the product made at lower speeds led to severe quality issues. Multiple coated products were being manufactured on a single process line and were then moved to the next process.  This next process then manufactured multiple other sub-assembly products.  Several days later, one of the final products underwent a quality check, and it was discovered that the UV coating was coming off of most of the sub-assembly products.  Unfortunately, the company needed to remake many of these products due to this single sub-assembly error. It cost the company more than $125,000.

The company used its solid Manufacturing Execution System (MES) to trace the issue back to a single UV coating machine. The machine was checked and the second UV power supply was found to be adjusted incorrectly. This did not allow the UV power supply to have full power when needed.  These UV power supplies are variable and have a potentiometer which allows adjustment of the UV output. In this case, the knob was turned wrong.

Solution

The initial request was only for the operators to verify that the UV power supplies were set up correctly.  But then a team was assembled with the directive to find an alternate solution.  These UV power supplies use RS232 to communicate to the PLC.  Reference values are communicated to the UV power supply to give the desired output.  The UV power supply does not communicate back to the PLC and load values.

The team quickly learned that the maintenance department had a procedure to calibrate UV power supplies.  As part of the calibration procedures, maintenance professionals connected volt meter leads to test points on the internal High Voltage Diode Blocks and varied the output using the UV power supply potentiometer.  These diode blocks rectified the AC voltage to a high voltage DC voltage.  The test point voltage on the High Voltage Diode Blocks varied as the settings were changed and followed the curve of the high voltage that went to the magnetrons of the UV power supply.

 

Diode Block.jpg

The team chose to take a signal isolation card and connect the low voltage test points to an analog input of the PLC.  This feedback correlated to the actual output of the UV power supply and revealed when the machine was not running at 100%. The PLC program was set up to shut the machine down when the test point voltages were below the threshold.  This simple design change helped to detect future abnormalities and then automatically stopped the machine and prevented bad product from being produced.

The additional signals added to the PLC from the UV power supply gave feedback which allowed an automated “go/no go” decision concerning the output of the power supply.  If the voltage was not above the threshold, the machine alarm activated and the manufacturing process stopped.

As a part of the team that worked to find this solution, I am proud that the company is now satisfied that a quality gap has been closed and it can run coated products and have confidence that the UV power supply is providing proper voltage to the UV bulbs.  

I am now one of the experts at Avid Solutions, many of whom are plant engineers who understand the need to seek continuous improvement opportunities and apply the Jidoka Principle in order to ensure manufacturing of the highest quality product.  Our experts are experienced at finding the most valuable solution to each and every problem using EVO (Exploring Value Opportunities).  Our unique combination of plant floor experience, operations experience, and process industry knowledge can be used help solve tough control problems similar to what happened with the UV coating process manufacturing.

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