Poka Yoke is a Japanese term that refers to an idea of quality assurance that utilizes good design or uses engineering devices to prevent or detect failure modes and causes of the failure modes. “poka” is the operation and “yoke” is the prevention of mistakes. In English, poka yoke means mistake-proofing or error proofing.
Above picture: A poka yoke device detects the bottle without a red cap and the robot hand will take it out.
1. Poka Yoke concepts
A Poka Yoke device is a production device that prevents defective products from being produced or detects defective products produced. Base on the kind of control, there are two categories of poka yoke and poka yoke device.
• Prevention: The Poka Yoke prevents the defects that are about to occur.
• Detection: The Poka Yoke detects the errors that have occurred.
Base on the error/defect control level, there are three types of poka yoke:
• Warning: Poka yoke device can detect the failure mode or cause of failure mode then warns the operator about the failure or cause of failure. The operator then decides on how to react to the notification. This level is the lowest level of control since the operator can choose to ignore the warning, then defects will be produced or transferred to the next process.
• Stop: At this level, the device detects the abnormalities or the causes of the problem and stop the machine. The operator needs to stop the current action. Otherwise, he cannot proceed further. This kind of control is safer than Notification Device since the operator cannot continuously continue without resolve the problem first.
• Error/Defect Control: At this level, the device can detect the failure or cause of failure and auto fix the error or auto guide the defective part to quarantine area (like an NG box) without any interruption of the process.
2. Prevention Poka Yoke
2.1 Error Warning Device
This kind of poka yoke recognizes errors that may lead to the defective product and use lamps/buzz to warn the operator. If the operator ignores the signal, the process results in a defective product. The operator must stop the operation to resolve the error before restart production to secure the product quality.
Example: In a drilling process. An operator must set the part in the correct position, then the operator press the button to start drilling. Sometimes, the operator does not set the part correctly, and drilling turns out a defective product.
A guide fixture used to guide the part, and a limit switches used to determine if the position of the part is correct. If the operator does not set the part in the right place, the limit switch is not active. The lamp is turned on to notify the operator about the problem when the operator presses the Start button. The downside of this kind of device is the operator can ignore the notification and keep the operation.
2.2 Error Locking Device
This kind of poka yoke can also detect specific errors or abnormalities that can lead to product defects, however, instead of notifying the operator, it stops the machine’s operation or locks the part to avoid the operator proceed further. The operator has to resolve without resolve the error.
Example: with the same process as above, instead of setting a lamp to notify the operator. If the operator does not set the part correctly, the limit switch will not activate, and drilling will not work.
2.3 Error Free Device
This kind of poka yoke prevents the cause of failure mode by jig/fixture or product design precaution. The operator cannot stray from standard procedures and make the defective part.
Example: The operator must set the jig at with correct position before milling. However, the operator can set the part on the mill jig upside down or in reverse inattentively. This mistake causes the milling defects. Improvement: by changing the design of mill jig that can completely prevent the upside down or in reverse.
3. Detection Poka Yoke
3.1 Alarm Poka Yoke
This device detects defective product and uses lamps and (or) buzzers to notify the operator. Then the operator decides to remove the defective product out of the current station.
However, the operator can make a mistake if he ignores the alarm. The defective product will be transferred to the next process.
3.2 Locking Poka Yoke
Stop for defects. This kind of device can detect when the machine has produced a defective product and immediately stops the machine’s current operation or function so that it prevents defective product transferred to next process. However, the defective product remains in the station, and the operator must remove it before restarting the machine.
Example: If a wrong component is provided for an assembling process, the operator cannot set it in the assembling fixtures. The operator has to stop the operation to remove the incorrect component before resuming regular production.
• A stop device is a low level of poka yoke because it requires production stop before resetting the regular production. Therefore, with short cycle time production, the team may use auto Poka yoke.
• With stop device, it possible that the operator removes the defective part out of the process. However, the operator forgets to put it in the quarantine box. That is a risk of mixing defective part with good part.
3.3 Quarantine Poka Yoke
This type of poka yoke detects and removes defective products out of the station and resume regular production without interruption.
Example: In a bottling process, the cap must be assembled in the bottle. Proximity Switches is used to confirm present/absent of metal cap. This switch connects to a divider switch. Whenever a defective bottle detected, divider closes and let this bottle come back to cap assembling process.
4. What kind of poka yoke should be used?
Ok, so how to select the right poka yoke type for your process? To answer this question, we should start with the requirement of the process. You can use the process PFMEA to find the need of control.
Example: Severity, Detection and RPN (RPN = S x O x D) are high and assume that you need to lower the RPN than 120. You cannot change the Severity rank. Therefore you need to reduce the Detection rank by 4. In the list below of Poka Yoke devices’ Detection rank, you have two option
Detection ranking of Poka yoke device (The ranking system may be different between companies, this table is an example of detection ranking)
Defects alarm poka yoke -> 6
Defects Locking Poka yoke -> 5
Quarantine Poka Yoke-> 4
Error Free Poka Yoke give us 1 in Detection ranking
The second thing they also need to consider is which poka yoke device you can add. Not all poka yoke device can be used practically with any process. The team has to consider engineering issue, timing and the budget. Example, a Quarantine Poka Yoke is much more expensive and need more time to implement than other devices.