poka-yoke

What is a poka-yoke?

A poka-yoke is a mechanism that is put in place to prevent human error. The purpose of a poka-yoke is to inhibit, correct or highlight an error as it occurs. Poka-yoke roughly means "avoid unexpected surprises" or "avoid blunders" in Japanese. In English, a poka-yoke is sometimes referred to as "mistake-proof" or "foolproof."

Essentially, a poka-yoke is a safeguard that prevents a process from proceeding to the next step until the proper conditions have been met. Poka-yokes can be either warning mechanisms or control mechanisms. Warnings provide an alert that is designed to prevent additional errors or defects from happening. Control mechanisms stop the next step of a process from occurring.

Automobiles often have a number of poka-yokes to help drivers avoid making mistakes. Should a driver exit the vehicle but fail to remove the ignition key, for example, many cars are designed with a poka-yoke that will warn the driver with an auditory alert that the key is still inside the car.

To prevent drivers from accidentally locking themselves out, some cars also have a control poka-yoke that prevents the vehicle door from locking when the key is still in the ignition in the off position.

Poka-yokes have their roots in lean manufacturing and are closely aligned with Six Sigma methodologies, continuous improvement (Kaizen) and the Toyota Way Production System. The concept of designing process steps to be foolproof was developed by Dr. Shigeo Shingo, an industrial engineer who was a Toyota consultant and author of Zero Quality Control: Source Inspection and the Poka-Yoke System.

The concept was originally called baka-yoke, but Dr. Shingo changed the name to poka-yoke after realizing that the label foolproof was humiliating to workers. Like kanban and many other lean production concepts, the concept of poka-yoke has been adopted by many other industries, including software development and healthcare.

Poka-yokes in manufacturing

In manufacturing, there are three main types of poka-yoke for quality assurance: contact methods, fixed-value methods and motion-step methods. Each can be a control method or a warning method. Contact methods rely on sensing devices that ascertain whether a product makes contact with a device. This can be physical, as in a pin that must be placed correctly, or energetic, wherein photoelectric beams sense something is not positioned correctly.

Fixed-value methods are used when a process must be done a certain number of times or when a certain number of parts are associated with the completion of a task -- for example, bolts that need tightening a certain number of times or the parts required in a package. In fixed-value, a signal is given or present when the number is reached or the product is released to the next stage upon completion.

Motion-step methods monitor whether a motion or step has occurred within a certain time frame or sequence. An example would be an indicator light that is turned on if a step in a machine cycle is not done in the proper sequence or time frame.

Motion-step methods typically rely on sensors and devices to detect that the appropriate actions have occurred. In the indicator light example, the steps of the machine cycle are all wired to the indicator board and a timer. The light is triggered if a step has not been completed in time and in sequence.