The Information Model for Poka Yoke:

USB2

In today’s post, I will be looking at poka yoke or error proofing using an information model. My inspirations for this post is Takahiro Fujimoto, who wrote the wonderful book “The Evolution of a Manufacturing System at Toyota” (1999) and a discussion I had with my brother last weekend.

I will start with an interesting question – “where do you see information at your gemba, your production floor?” A common answer to this might be the procedures or the work instructions, or you might answer it as the visual aids readily available on the floor. Yet another answer might be the production boards where the running total along with reject information is recorded. All of this is correct. A general definition of information is something that carries content, which is related to data. I am not going into Claude Shannon’s work with information in this post. Fujimoto’s brilliant view of information is that every artifact on the production floor, and in fact every materialistic thing carries information. Fujimoto defines an information asset as the basic unit of an information system. Information cannot exist without the materials or energy in which it is embodied – its medium.

info asset

This information model indicates that the manufactured product carries information. The information it carries came from the design of the product. Information is transferred and transformed from the fixtures/dies/prints etc onto the physical product. Any loss of information during this process results in a defective product. To take this concept further, even if the loss of information is low, the end-user interaction with the product brings in a different dimension. The end-user gains information when he interacts with the product. If this information matches his expectations, he is satisfied. Even if there is minimal loss of information from design to manufacturing, if the end product information does not match the user’s expectations, the user gets dissatisfied.

Lets look at a simple example of a door.  A door with a handle is a poor design since the information of whether to push or pull is not clearly transferred to the user. The user might expect to pull on the handle instead of pushing on it. The information carried by the door handle is to “open the door using handle”. It does not convey whether to push or pull to open the door.

handle

Perhaps, one can add a note on the door that says, “Push”. A better solution to avoid the confusion is to eliminate the handle altogether so that the only option is to push. The removal of the handle with a note indicating “push” conveys the information that to open the door, one has to push. The information gets conveyed to the user and there is no dissatisfaction.

This example brings up an important point – a defect is created only when an operator or machine interacts with imperfect information. The imperfect information could be in the form of a worn-out die or an imperfect work instruction that aids loss of original information being transferred to the product. When you are trying to the solve a problem on the production floor, you are updating the information available on the medium so that the user’s interaction is modified to achieve the optimum result. This brings us to poka yoke or error-proofing.

If you think about it, you could say that the root cause for any problem is that the current process allows that problem to occur due to imperfect information.  This is what poka yoke tries to address. Toyota utilizes Jidoka and poka yoke to ensure product quality. Jidoka or autonomation is the idea that when a defect is identified, the process is stopped either by the machine in an automated process, or by the operator in an assembly line. The line is stopped so that the quality problem can be addressed. In the case of Jidoka, the problem has already occurred. In contrast, poka yoke eliminates the problem by preventing the problem from occurring in the first place. Poka yoke is the brainchild of probably one of the best Industrial Engineers ever, Shigeo Shingo. The best error-proofing is one where the operator cannot create a specific defect, knowingly or unknowingly. In this type of error-proofing, the information is embedded in the medium such that it conveys the proper method to the operator and if that method is not followed, the action cannot be completed. This information of only one proper way is physically embedded onto the medium.

Information in the form of work instructions may not always be effective because of limited interaction with the user. Information in the form of visual aids can be effective since it interacts with the user and provides useful information. However, the user can ignore this or get used to it. Information in the form of alarms can also be useful. This too may get ignored by the user and may not prevent the error from occurring. However, the user cannot ignore the information in the form of contact poka yoke since he has to interact with it. The proper assembly information is physically embedded in the material. A good example is a USB cable where it can be entered in only one way. The USB icon on top indicates that it is the top. Apple took this approach further by eliminating the need of orientation altogether with its lightning cables. The socket on the Apple product prevents any other cable from being inserted due to its unique shape.

Final Words:

The concept of physical artifacts carrying information is enlightening for me as a Quality Engineer. You can update the process information by updating a fixture to have a contact feature so that a part can be inserted in only one way. This information of proper orientation is embedded onto the fixture. This is much better that updating the work instruction to properly orient the part. The physical interaction ensures that the proper information is transferred to the operator to properly orient the part.

As I was researching for this post, I came across James Gleick who wrote the book, “The Information: A History, a Theory, a Flood”. I will finish off with a story I heard from James Gleick regarding information: When Gleick started working at the New York Times, a wise old head editor told him that the reader is not paying for all the news that they put in to be printed. What the reader is paying them was for all the news that they left out.

Always keep on learning…

In case you missed it, my last post was Divine Wisdom and Paradigm Shifts:

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