Poka-yokes are mechanisms used to mistake-proof an entire process. Ideally, poka-yokes ensure that proper conditions exist before actually executing a process step, preventing defects from occurring in the first place. Where this is not possible, poka-yokes perform a detective function, eliminating defects in the process as early as possible.

Many people think of poka-yokes as limit switches, optical inspection systems, guide pins, or automatic shutoffs that should be implemented by the engineering department. This is a very narrow view of poka-yoke. These mechanisms can be electrical, mechanical, procedural, visual, human, or any other form that prevents incorrect execution of a process step. Poka-yokes can also be implemented in areas other than production such as sales, order entry, purchasing, or product development where the cost of mistakes is much higher than on the shop floor.

Prevention-based Poka-Yokes
Prevention-based mechanisms sense an abnormality that is about to happen, and then signal the occurrence or halt processing, depending on the severity, frequency or downstream consequences. There are two approaches for prevention-based poka-yokes:

Control Method:
This method senses a problem and stops a line or process, so that corrective action can take place immediately, thus avoiding serial defect generation.

Warning Method:
This method signals the occurrence of a deviation or trend of deviations through an escalating series of buzzers, lights or other warning devices. However, unlike the control method, the warning method does not shut down the process on every occurrence.

Detection-Based Poka-Yokes
In many situations, it is not possible or economically feasible to prevent defects, particularly where the capital cost of the poka-yoke mechanism, far exceeds the cost of prevention. The three categories of detection-based poka-yokes are as follows:

Contact Method:
This method detects any deviation in shape, dimensional characteristics or other specific defects, through mechanisms that are kept in direct contact with the part. A subset of this category is the non-contact method, which performs the same function through devices such as photoelectric cells.
Fixed Value Method:

This method is used in operations, in which a set of steps is sequentially performed. Sometimes this is referred to as odd part out method, in which parts left over after assembly signal a defect. Fixed value also includes critical condition detection (pressure, temperature, current, etc.) through electronic monitoring devices.

Motion Step Method:
This method ensures that a process or operator does not mistakenly perform a step that is not part of the normal process. An example of this is colour coding of electronic components on drawings and totes to prevent using mixed or incorrect parts.

Building a Strong Foundation
The poka-yoke philosophy requires a strong foundation in total quality management. First, organizations must learn to be customer focused. Second, organizations must promote quality ownership at the source, and they need to ensure proper investment in their people, which enables them to be truly empowered. Third, a clear distinction needs to be made between good versus bad quality. These are the building blocks of an effective poka-yoke effort.