What is “Poka Yoke”?

The term “Poka Yoke” usually refers to any behavior or procedure constraint that prevents incorrect patterns or operations. A simple example of a “Poka Yoke” is, within manual gearbox cars, that you have to press on the clutch pedal in order to start the engine, or else the car ignition won’t even activate. By pressing the pedal (a process step, therefore a procedure constraint), the interlock prevents any unintended movement by the car. Then, time after time, the process becomes a behavior pattern that the driver automatically performs as a force of habit (usually, at least).

The term “Poka Yoke” comes from the Japanese words poka = mistake and yoke = advert or avoid. The term was originally applied by Shigeo Shingo in the 1960’s. Shingo was an industrial engineer at Toyota who redesign an assembly process in which workers usually forgot to include a spring under a switch. By performing a two-step process that included a special placeholder for the spring, Shingo ensured that the worker wouldn’t be able to forget to place the spring first. By eliminating the mistake factor at its source, it can be ensured that a defect won’t reach the customer. Within “Poka Yoke” Shingo identified 3 types of approaches for detecting errors:

•    Contact: identifying defects by testing physical attributes (color, size, shape, etc.) through the use of jigs
•    Constant or Fixed Value: where certain amount of processes or movements need to be made.
•    Motion or Sequence Steps: where a certain prescribed steps need to be achieved in order to advance in the process.

The goal on the implementation of any of these steps is for the operator to be alerted or prevented to continue the process in order to detect the error being made and avoiding defects to go further down the production line or reaching the customer, thus reducing mistakes costs within a company.

Screw assembly processes common issues

Within mass production processes, operators working on repetitive tasks are prone to make mistakes (even the most experienced ones) because of fatigue or repetitive distraction, where a force-of-habit repetition makes the operator less attentive to the task at hand. One such of these operations is screw assembly, where an operator’s task requires to tighten a set number of screws. With few screws (4 or less) the probability of the operator missing one is really low (yet, not zero), but with 5 or more it quickly increases. For example, a flat TV board assembly easily uses some 10 to 15 same sized screws of small size; just one missed screw compromises the sturdiness of the assembled device.

HIOS provides several options to solve these two “Poka Yoke” approaches with tools that verify both the amount of screws being placed (Fixed Value) and the proper tightening conditions –torque, installation depth, through cycle time- (Sequence Steps). The HIOS Controlled Torque electric screwdrivers with screw counter capabilities provide a perfect solution for the missed screws problem by addressing these two of the “Poka Yoke” approaches in relation to screw assembly: Fixed Value -the amount of screws- and Sequence Steps –the tightening parameters-.

HIOS BLG-BC2 Brushless Screwdriver Expert System (JUKUREN)

We present the HIOS BLG-BC2 Brushless Screwdriver Expert System (JUKUREN) with built-in Screw Counter and Pulse Counter.

The BLG-BC2 allows you to:
•    Detect screw fastening errors in real time by counting the pulses of the motor
•    Detect misplaced, recessed, floating, missing washers, seized parts, slipped or different size screws
•    Direct teaching of max. and min. parameters for simple setting (only pulse values need to be set)
•    Remotely control and record pulse and tightening values for traceability (serial and parallel – PC or PLC*)
•    View the immediate result of the operation on the built-in panel
•    Ensure a complete tightening process with audible alarm.
* I/O external cable w/sample app and the parallel signal (PLC) converter sold separately.

With the BLG systems you have a simple setting tool powerful enough to detect screw assembly errors and provide operation traceability to improve process performance, such as evaluation screw quality by rejecting operations that failed due to screw issues such as mix screws, different pitches, different lengths, cam-out grooves, slippage, etc.-
For more details and optional accessories, visit our HIOS product webpage and download the HIOS line-up catalog.