Error-proofing, or poka yoke, was developed by Shigeo Shingo in Japan in 1961. The idea behind error- proofing is to design processes in a way that mistakes, or errors, are impossible to make or at least are easy to detect and correct. Error-proofing devices fall into two major categories: prevention and detection.
A prevention device affects the process in such a way that it is impossible to make a mistake. The concept of prevention follows five principles: elimination, replacement, facilitation, detection, and mitigation. Elimination refers to the need to design work and processes that eliminate the potential for error. Replacement means replacing a faulty process with another, more reliable process, that has less of a potential for error. Facilitation makes it easier for the operator to perform without error. Detection encourages the use of methods that enable an error to be easily spotted, either at the original workstation, or at the very next operation. Mitigation, refers to minimizing the effect of the error if it does occur.
A detection device signals the user that a mistake has been made, so that the user can quickly correct the problem. This can be accomplished through the design and use of fail-safe devices, counts, redundancy, magnifying the senses, or special checking and control devices. Fail-safe devices fool-proof an action by preventing the work from being done any other way. Mechanisms or work holding devices signal the operator when the work has been done correctly. Limiting mechanisms on tools may be used to prevent a tool form exceeding a certain position or amount of force. Counts can help an operator keep track of where they are in a process. Redundancy can be used when identifying parts, for instance, labeling a part both with color and a bar code. Double checks can be used to determine if the work has been completed correctly. By magnifying the senses, tools can be used that provide feedback in a variety of ways that alert an operator to whether or not their process is operating the right way. Lastly, special checking and control devices of varying levels of complexity may be designed to help an operator detect whether or not the work they are performing is correct.
Some examples of error-proofing methods:
Fail-safe devices
- Alarms: Used to signal depletion of material supply and abnormalities.
- Foolproof fixtures: Fixtures used to check the quality of work from preceding operations.
- Limiting tools: Use of a regulated torque wrench or air tool to prevent over-tightening.
Magnification of senses
- Locating indexes and fixtures to aid in part positioning.
- Optical magnification to improve visibility.
- Use of pictures in place of numbers.
- Multiple signals to improve recognition and response.
Redundancy
- Multiple-identity codes: These are intended to prevent product mixups, e.g., color codes or other recognition schemes.
- Redundant actions and approvals: Two operators working independently of each other to perform a task.
- Multiple test stations: Checking height, weight, etc. of parts.
Special checking and control devices
- Automatic dispensing devices: Liquid filling and other portion control devices.
- Software to detect incorrect information: “Spell check” in word processing and software to detect errors in data.
- Handheld devices to check or perform calculations.
- Automatic recording of information: Use of bar codes.