Poka-Yoke Explained

Jonathan Trout, Noria Corporation
Poka-Yoke  

What Is Poka-Yoke?

Poka-yoke (pronounced PO-ka yo-KAY) is the use of a mechanism or device that helps an equipment operator (or anyone) avoid mistakes. In Japanese, poka-yoke translates to "mistake-proofing" or "inadvertent error prevention" and was originally described as baka-yoke, which means "fool-proofing." The purpose of the poka-yoke technique is to eliminate product defects by preventing them in the first place, correcting them or drawing attention to human errors as they occur. In manufacturing, this means not accepting, creating or allowing defects to move down the line.

Self-driving cars are slowly becoming a fairly common sight on the roads. Tesla recently showed a video of its Model 3 backing out of a parking space and slowly maneuvering its way to pick up its owner, who had called it via his smartphone. In another more terrifying instance, a Model 3 driver from Florida experienced seemingly inexplicable braking from the Tesla's autopilot feature while traveling at highway speeds, only to find out a split second later that the car in front of them swerved out of the lane to reveal a stopped car. Tesla's sensor had automatically detected the hazard and acted (without human input) to avoid a crash.

Self-driving cars are a modern consumer-based example of poka-yoke. They operate by using multiple sensors and alerts like automatic braking and a variety of cameras to "see" everything around them in real time. In fact, automatic braking systems are becoming a common feature in most new cars, helping prevent accidents due to human error. Common areas where poka-yoke works well include:

  • when a step in a process is determined to leave room for human error, causing mistakes and defects to occur (this is especially true of steps that rely on the operator's attention, skill or experience);
  • in a service-related process where the customer can make a mistake that affects the output;
  • with a step in a process involving a hand-off, where output is transferred to another worker (or customer in a service process);
  • when a small error early on in a process can cause a major problem later down the line; and
  • when the consequences of a mistake are expensive and/or dangerous.

Shigeo Shingo of the Toyota Production System (TPS) adopted poka-yoke methodologies to be implemented at any step in a manufacturing process where something can go wrong or an error can be made. For example, this would include using a digital counter to track the number of spot welds on a product, ensuring the welder makes the correct number of welds before sending it down the line. The bottom line is manufacturing defective parts usually ends up being very costly. Many customers find one defect and return the entire batch of parts out of precaution. This is where poka-yoke techniques play a critical role between human error and the causes of defects.

Poka-Yoke vs. Quality Control

human errors/cause of defects

Shingo made the important distinction between mistakes and defects. He said mistakes are inevitable in most cases but can be detected and corrected immediately. Defects are mistakes that manage to make it all the way through a system and reach the customer. Poka-yoke strives to prevent mistakes from becoming defects. Shingo viewed quality control as a three-level hierarchy of effectiveness that includes poka-yoke:

  1. Judgment inspection: inspectors inspect products
  2. Information inspection: using statistical process control (SPC) to monitor conditions within the process
  3. Before the fact: using poka-yoke

In this hierarchy, the least effective form of quality control is the use of inspection, while the most effective is using autonomous procedures and poka-yoke devices. This prevents or highlights defects without the need for any type of judgment inspection or relying on an operator to do something.

 

Types of Poka-Yoke

There are three recognized types of poka-yoke that, when implemented, can greatly reduce human errors by effectively making it impossible to make a mistake in a given process.

  1. Contact method: The contact method of poka-yoke uses some type of sensing device to identify defects in a part's shape, size, color or other physical attribute. Things like notches with matching locator pins, interference pins, limit switches and proximity switches are used to make certain a part is positioned correctly before work begins.

    Contact methods are good in any circumstances that encourage mistakes. These are situations that involve rapid repetition, infrequent production or issues with the surrounding environment like poor lighting, extreme temperature, excess humidity, noise, dust or any other factor that may distract an operator. Paul Dvorak outlines four areas for potential problems that require solutions using the contact method in his Machine Design article, "Poka-Yoke Designs Make Assemblies Mistakeproof."

    1. Look at all areas a product will fail if parts are assembled incorrectly.
    2. Look for small features critical to proper assembly.
    3. Beware of relying on subtle differences to determine the top from the bottom or the front from the back, especially if parts are painted dark colors.
    4. Beware of excessively complicated designs that confuse inexperienced operators.
  2. Constant number (fixed-value) method: The constant number (fixed-value) method of poka-yoke is used in processes where the same action is repeated several times. In other words, the fixed-value method alerts operators once a certain number of movements has been made. A great example of this method is giving an operator a bin containing the exact number of parts needed to complete a task. If there is a part left over, the product would be considered defective and not allowed to move to the next stage. So, if the operator needed to install six bolts, the bin would only contain six bolts.

    Dvorak gives an example of an operator charged with tightening six bolts and using a poka-yoke technique of dipping a wrench in diluted paint. The operator then can easily see where the untightened bolts are (untightened bolts will be lacking paint).

  3. The sequence (motion-step) method: The sequence (motion-step) method of poka-yoke is employed when a process requires several different activities be completed in sequence by the same operator. The poka-yoke device for this method is created to detect whether each motion is performed and to alert the operator when a step is skipped. A good example of a sequence poka-yoke device is utilizing a simple proximity switch that opens only after all components are loaded in the correct order. Likewise, the device can detect when each component is used (or removed) from its dispenser. If one component is not removed, an alert is sent to let the operator know not to proceed.

Ideally, poka-yoke devices ensure a process is designed so mistakes are prevented before they happen. When this is not possible, poka-yoke takes on more of a detective role, eliminating defects in the process as early as possible. A prevention-based poka-yoke system acts before the defect happens; devices sense an abnormality is about to happen and issue an alert. A detection-based poka-yoke system immediately signals an operator as soon as a mistake is made. It then does not allow the process to continue, enabling the operator to quickly fix the error.

Poka-yoke devices take on one of two approaches: the control approach and the warning approach.

  • Control approach: Devices using the control approach sense a problem and stop the process, so actions can be taken to correct the issue immediately.
  • Warning approach: Devices using the warning approach detect and alert to the occurrence of a deviation (or trend of deviations) through an escalating series of warning devices (buzzers, lights, etc.). The difference between this approach and the control approach is that the warning approach does not shut down the process every time an error is detected.
 

Poka-Yoke Examples

The modern-day use of poka-yoke devices in self-driving cars, including automatic braking systems and object sensors, has already been discussed; however, there are plenty of these devices in every-day life that you probably take for granted.

  • Automotive industry: Poka-yoke devices go beyond automatic braking systems when it comes to vehicles. Radar and video sensors constantly monitor the distance and speed of surrounding cars and objects, helping prevent collisions in the event of a sudden stop. These sensors can fall under the control and warning approaches (discussed earlier), depending on whether they actually stop the vehicle or just alert the driver to impending danger.

    Lane-keeping assist uses the control approach by automatically sensing and returning the car to the correct lane when the driver is unresponsive. Likewise, electronic stability control helps reduce accidents during a skid due to over-correcting; the condition being controlled in this instance is a wet or icy road. When the car is introduced to this condition, the system automatically kicks in to prevent an unwanted result.

    Even more poka-yoke devices can be found within the automotive industry, including blind-spot sensors and adaptive headlights. Also, the automatic transmission in most cars requires the vehicle to be in the "park" or "neutral" position before it can be started.

  • Around the house: Many things used on a daily basis have built-in poka-yoke devices or sensors that make them safer without much thought being given to them. Microwaves, washing machines, dryers, dishwashers and other household appliances might not start until a door is shut or may use sensors to know when to stop drying clothes or filling with water. Electrical plugs have an earth pin that is longer than the other pins, so they can only be inserted one way, which is a classic contact method of poka-yoke. Other examples include child-proof tops of pill bottles that prevent accidental ingestion, elevator doors that have sensors causing them to open when there is an obstruction, and lawnmowers with a bar on the handle that, when released, shuts off the mower to prevent accidents.
  • Manufacturing industry: In lean manufacturing, the goal is to produce defect-free products the first time. Mistake-proofing is a built-in quality-assurance technique to ensure this happens.

    Common examples of poka-yoke devices in manufacturing include magnets in a food-processing plant to detect and remove metal pieces before packaging, interlock switches that can identify the position of a machine's guard and switch off the machine when the guard is lifted, safety mats near dangerous machinery that automatically trigger a machine shutdown when someone steps on them, personal protective equipment like gloves that are in eye-catching colors for the food industry in case they fall into the food, and standardized containers at workstations that contain exact quantities of material.

The list can be fairly extensive. These are just a few examples of how poka-yoke devices work, not only in manufacturing but also in daily life.

 

Poka-Yoke Implementation

There are a few things to consider when initially employing the poka-yoke techniques and methodology. Before anything can happen, management must be willing to devote resources and support for implementing not only the poka-yoke procedures but the devices as well. Next, you'll need to provide adequate training to operators on the new process(es) and equip them with the tools and skillset necessary to do their job.

  • Build the team: The first step in implementing poka-yoke is building a cross-functional team to review current processes. Your team should include operators, quality technicians, manufacturing and quality engineers, and production supervisors. Each team member should be trained on poka-yoke methodologies and have a working knowledge of the processes in question. It's also a good idea for them to know about any process error or product defect history.
  • Select the process: Choose a process you want to evaluate. The selection could be based on the overall impact it has on the company's key performance indicators (KPIs), the complexity of the process, frequency of errors, warranty history or its quality-control history. Try to select a process that has a significant chance for improvement.
  • Map the process: With your team, review each step in the process flow chart, looking for any areas where defects have occurred in the past or have a high potential to occur. If you don't have a process flow chart, create one.
  • Identify risk: With your team, brainstorm all the possible errors that could occur within the process in question, keeping in mind when human errors are likely to happen. Failure mode and effects analysis (FMEA) is a great tool for identifying risk.
  • Find the source: Start with each potential error and work your way back through the process flow to figure out the point of initiation. Brainstorm ways to prevent the error by examining the point of initiation.
  • Outline your options: Now that you've identified potential errors, brainstorm ways to make it impossible for those errors to occur. Options should include simplifying the step so the action is simpler to perform than the error, eliminating the step by asking if it is necessary and adds value, and implementing poka-yoke to make it impossible to create an error during the step.
  • Choose the method: If you decide to implement poka-yoke, meaning the step can't be simplified or eliminated, it's time to choose the best poka-yoke method for the process. This means collectively deciding on the contact, fixed-value or motion-step method of poka-yoke.
  • Implementation and validation: Develop and/or select the poka-yoke device or method and implement it into the existing process, ensuring operators are aware of the changes being made and how they will be affected. Over time, measure the effectiveness of the change(s) by monitoring efficiency, error reduction, rework and scrap.

Where to Implement Poka-Yoke

Once you've decided on which process could benefit from a poka-yoke device, there are a few areas to consider placing the device or method within the workflow. The location of the device within the process workflow can affect its level of effectiveness.

The best location for a poka-yoke device is before an operator or the machine starts a task. In other words, it's best to prevent any errors by not allowing the process to even begin until a certain set of conditions are met.

The second-best location for a poka-yoke device is during a particular step within a process. This way, an operator or machine receives an alert in the event of an error and stops the part or process from moving down the line.

Finally, the third-best location for a poka-yoke device is downstream after the process step is finished, typically at an inspection station. At this point, you're assuming errors or defects may have occurred and are dependent on detection controls to find the defects before sending the process down the line.

 

Benefits of Poka-Yoke

Aside from the obvious goals of eliminating human and mechanical errors and not accepting, allowing or passing along defects within a manufacturing setting, poka-yoke can yield other benefits that are not quite as evident.

  • Operators will need less training because processes containing poka-yoke devices automatically correct any deviation from what is required.
  • An increase in safety is an added benefit where workers are exposed to hazardous materials or dangerous machinery such as when working with high-inertia machines or petrochemicals.
  • Fewer quality checks through sampling and inspection are needed thanks to the mistake-proofing aspects of poka-yoke. While inspections still have their place, with poka-yoke, eliminating mistakes is built into processes through the prediction or detection of errors or defects.
  • Work becomes less repetitive for operators, as it helps them work through processes error-free the first time, which prevents them from having to go back and do it again correctly.
  • The quality of processes is improved, resulting in higher quality products. Improved processes also begin to produce effective teams that work in a coordinated effort to deliver error-free products the first time.

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