Poka-Yoke – How to achieve zero-defect production

The Poka-Yoke principle is a simple method to avoid mistakes. Technical precautions are taken to either prevent production errors from the outset or to detect and correct them immediately. Poka-Yoke supports the goal of zero-defect production.

The inventor of the principle is the Japanese Shigeo Shingō, an engineer and co-developer of the Toyota Production System. This is a system to avoid any kind of waste in the production process. The expression Poka-Yoke is taken from Japanese and means “to avoid carelessness” (poka= errors, carelessness; yoke = to avoid).

The origin of the development of Poka-Yoke lies in statistical quality control and the analysis of error causes. However, Shigeo Shingō found this to be unsatisfactory, as errors are caused in the production process and product testing can only find errors but not avoid them. This fact is particularly important for two reasons:

1. Many causes of defects are already in the early stages of the production process. However, the costs of eliminating defects increase disproportionately the later a defect is discovered. It is therefore essential to discover errors as early as possible or to avoid them preventively.
2.  The inspection of end products is certainly always a non-value-adding activity. The inspection of products and the search for defects always causes costs, regardless of whether defects are found or not. It is therefore important to exclude or immediately eliminate errors to reduce product testing.

A further impetus for Poka-Yoke was the recognition that human error is a major source of error in production processes and technical systems. Over 70% of accidents in technical systems can be attributed to human error. An investigation in Japan has shown that approx. 30% of all human errors occur accidentally. This means that this human error is unintentional and occurs independently of the motivation and training of employees and the existence and quality of work and process standards. Especially repetitive tasks are error-prone and require special design or supervision to prevent human error.

Poka-Yoke systems are often described as foolproof or idiot-proof. However, such designations were rejected by Shigeo Shingō, who felt that they were disrespectful to working people. A more accurate description of Poka-Yoke systems is foolproof.

Poka-Yoke aims to design processes and products in such a way that errors caused by human error and carelessness can be ruled out. Poka-Yoke is used to monitor possible causes of errors and starts directly at the point of potential error occurrence. Errors are indicated directly and are not passed on to subsequent operations. This avoids expensive consequential error costs at a later stage.

To achieve this, processes and products must be designed in such a way that the errors are either impossible from the outset or are detected directly. The discovery of errors then leads to immediate action. 100% inspection and immediate reaction are core elements of the Poka-Yoke concept. The Poka-Yoke methodology is based on the firm belief that all errors are avoidable.

Three different categories of human errors are distinguished, which can be prevented with the help of Poka-Yoke: faulty memory, faulty perception, and faulty execution.

This error category contains all errors that are triggered by an incorrect reminder. This includes omitting work steps such as inserting or removing parts, pressing buttons and switches, taking measurements and recording. It also includes performing operations in the wrong order, a more incorrect number of repetitions and performing them at the wrong time. The unauthorized execution of process steps also falls into this category.

This category includes errors that are due to an erroneous perception of the performing employee. This includes the incorrect selection of types and quantities of material or parts. It includes errors in the perception of conditions, e.g. when reading procedural instructions, records and measuring instruments, as well as the incorrect assessment of hazardous situations.

This category includes all errors that can be traced back to the faulty execution of operations. This can include, for example, the incorrect holding, positioning or fixing of material and tools. The unintentional damage of workpieces, e.g. by dropping them, also falls into this category.

Poka-Yoke is often only used as a reactive measure to avoid recurring errors. The subsequent integration of Poka-Yoke measures into existing systems or processes often involves effort and costs. It is usually more cost-effective to take possible errors into account already during the planning of the systems and processes.

The implementation of Poka-Yoke measures takes place in three steps:

1. Problem analysis
2. Generation and evaluation of ideas
3. Implementation and evaluation

Before a Poka-Yoke measure can be taken, an analysis of the cause of the fault is necessary. To do this, it is necessary to trace the causal chains that cause an error. Often the causes of errors are less obvious than initially assumed. It is important to differentiate between cause and effect. The 5 Why method or the Ishikawa diagram are suitable for this.  If customer complaints have already been received due to the error, the 8D reports created for this purpose may contain important information.

In particular, if Poka-Yoke measures are to be taken for preventive error avoidance, an analysis of potential errors analogous to Failure Mode and Effects Analysis (FMEA) is useful.

Overall, it is important to note that Poka-Yoke measures are particularly suitable when human intervention in the process or the system is necessary, which can lead to inattention and errors.

In the ideal case, a Poka-Yoke measure completely eliminates the possibility of misuse from the outset. A classic example from everyday life: Cash can only be withdrawn from ATMs after the EC card. This has been introduced because EC cards are regularly forgotten in ATMs.

In many production systems, it is not possible to completely exclude all incorrect actions. Poka-Yoke measures are therefore often characterized by the fact that the fault is discovered during or immediately after the incorrect action. These measures are usually characterized by three different mechanisms, which have already been suggested by Shigeo Shingō: Detection, triggering and regulation mechanisms.

The detection mechanism describes the tools with which the production process is monitored. This usually includes sensors and sensor systems of all types:

• Sensors for the identification and localization of objects such as barcode and RFID readers,
• Sensors for shape, dimension, pressure, temperature, color, vibration, current, etc.,
• counters and instruments for time monitoring,
• Limit and proximity switches

The trigger mechanism describes the logic used to detect an error in the production process. Usually, this is a target/actual comparison. A distinction is made here between three main methods:

Depending on the sensor used, the contact method detects the impermissible deviation from the target state. This allows the identification of a faulty component or a faulty shape, weight or geometry, etc. The term contact method is misleading due to technical progress. Depending on the sensor, the inspection can also be carried out without contact.

The fixed value method is particularly useful in processes that consist of several consecutive steps. The number of substeps carried out is checked at the end of the process. If the counted number does not match the required number, an error has occurred. For example, an error can also be detected if the required number of assembly parts was not installed.

The motion step method detects errors by checking standard motion sequences. With the help of proximity sensors, it is possible, for example, to check motion sequences and determine deviations from the target sequence. The above example with the EC card also follows the step sequence method.