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Poka-yoke (ポカヨケ) [poka yoke] is a Japanese term that means
"mistake-proofing". A poka-yoke is any mechanism in a lean manufacturing process that helps an equipment operator avoid (yokeru)
mistakes (poka). Its purpose is to eliminate product defects by
preventing, correcting, or drawing attention to human errors as they occur. The
concept was formalised, and the term adopted, by Shigeo Shingo as part of the Toyota
Production System. It was originally described as baka-yoke,
but as this means "fool-proofing" (or "idiot-proofing") the name was changed to the milder poka-yoke.
More broadly, the term can
refer to any behavior-shaping constraint designed into a process to prevent incorrect operation by the
user. Similarly, a constraint
that is part of the product (or service) design is considered Design for Manufacturability or Design for X. A
modern Poka-Yoke application is when a driver must press on the brake pedal (a
process step, therefore a poka-yoke)) prior to starting an automobile. The
interlock serves to prevent unintended movement of the car. An additional
poka-yoke would be the switch in the car's gearshift that requires the car to
be in Park or Neutral before the car can be started. These serve as
behaviour-shaping constraints as the sequence of "car in park (or
neutral)" and/or "Foot on brake" must be performed before the
car is allowed to start. Over time, the driver's behaviour is conformed to the
requirements by repetition and habit.
The term poka-yoke was
applied by Shigeo Shingo in the 1960s to industrial processes
designed to prevent human errors. Shingo redesigned a process in
which factory workers, while assembling a small switch, would often forget to
insert the required spring under one of the switch buttons. In the redesigned
process, the worker would perform the task in two steps, first preparing the
two required springs and placing them in a placeholder, then inserting the
springs from the placeholder into the switch. When a spring remained in the
placeholder, the workers knew that they had forgotten to insert it and could
correct the mistake effortlessly.
Shingo distinguished
between the concepts of inevitable human mistakes and defects in
the production. Defects occur when the mistakes are allowed to reach the
customer. The aim of poka-yoke is to design the process so that mistakes can be
detected and corrected immediately, eliminating defects at the source.
Poka-yoke can be
implemented at any step of a manufacturing process where something can go wrong
or an error can be made. For
example, a jig that
holds pieces for processing might be modified to only allow pieces to be held
in the correct orientation, or a
digital counter might track the number of spot-welds on each piece to ensure
that the worker executes the correct number of welds.
Shigeo Shingo
recognized three types of poka-yoke for detecting and preventing errors in a mass production system:
1. The contact method identifies product defects by
testing the product's shape, size, color, or other physical attributes.
2. The fixed-value (or constant
number) method alerts the operator if a certain number of movements are not
made.
3. The motion-step (or sequence)
method determines whether the prescribed steps of the process have been
followed.
Either the operator is
alerted when a mistake is about to be made, or the poka-yoke device actually
prevents the mistake from being made. In Shingo's lexicon, the former
implementation would be called a warning poka-yoke, while the latter would be
referred to as a control poka-yoke.
Shingo argued that
errors are inevitable in any manufacturing process, but that if appropriate
poka-yokes are implemented, then mistakes can be caught quickly and prevented
from resulting in defects. By eliminating defects at the source, the cost of
mistakes within a company is reduced