Poka-yoke

Poka-yoke (ポカヨケ - pronounced "POH-kah YOH-keh" means "fail-safing" or "mistake-proofing" &mdash; avoiding (yokeru) inadvertent errors (poka)) is a behavior-shaping constraint, or a method of preventing errors by putting limits on how an operation can be performed in order to force the correct completion of the operation. The concept was originated by Shigeo Shingo as part of the Toyota Production System. Originally described as Baka-yoke, but as this means "fool-proofing" (or "idiot proofing") the name was changed to the milder Poka-yoke.

An example of this in general experience is the inability to remove a car key from the ignition switch of an automobile if the automatic transmission is not first put in the "Park" position, so that the driver cannot leave the car in an unsafe parking condition where the wheels are not locked against movement. In the IT world another example can be found in a normal 3.5" floppy disk: the top-right corner is shaped in a certain way so that the disk cannot be inserted upside-down. In the manufacturing world an example might be that the jig for holding pieces for processing only allows pieces to be held in one orientation, or has switches on the jig to detect whether a hole has been previously cut or not, or it might count the number of spot welds created to ensure that, say, four have been executed by the operator.

Implementation
Shigeo Shingo recognises three types of Poka-Yoke :


 * 1) The contact method identifies defects by whether or not contact is established between the device and the product. Colour detection and other product property techniques are considered extensions of this.
 * 2) The fixed-value method determines whether a given number of movements have been made.
 * 3) The motion-step method determines whether the prescribed steps or motions of the process have been followed.

Poka-yoke either give warnings or can prevent, or control, the wrong action. It is suggested that the choice between these two should be made based on the behaviours in the process, occasional errors may warrant warnings whereas frequent errors, or those impossible to correct, may warrant a control poka-yoke.