Poka-Yoke (Error Proofing)

Poka-yoke devices are classified by their function and timing. By function: Prevention poka-yoke makes it physically impossible to make the error — an asymmetric connector that only fits in the correct orientation, a fixture that only accepts the right part, or a software form that will not submit with missing required fields. Detection poka-yoke identifies when an error has occurred and alerts the operator before the defective unit proceeds further — a sensor that detects a missing component, a weight check that flags under- or over-weight assemblies, or a vision system that identifies incorrect label placement. By timing: Contact methods use physical shape, size, or other attributes to detect errors through direct contact with the product. Fixed-value methods verify that the correct number of operations has been performed (e.g., counting the number of screws installed). Motion-step methods verify that required process steps are performed in the correct sequence. Prevention poka-yoke is preferred because it eliminates the defect entirely, but detection poka-yoke is valuable when prevention is not feasible.

Effective poka-yoke implementation starts with defect data analysis — which defects occur most frequently and have the highest impact? Pareto analysis of quality data identifies the vital few defect types that warrant poka-yoke investment. For each target defect, the team analyzes the failure mode: exactly how does the error occur? What physical action or omission leads to the defect? This understanding guides the poka-yoke design. Simple, low-cost poka-yoke devices are preferred: guide pins, custom fixtures, limit switches, light curtains, color coding, and checklists. Complex, expensive solutions should be reserved for high-severity defects where simpler methods are insufficient. Involve the operators in the design process — they understand the work intimately and often have ideas for mistake-proofing that engineers would not think of. Test the poka-yoke thoroughly before full deployment, including testing the failure mode (does it actually prevent/detect the error under realistic conditions?). Document the poka-yoke as part of standard work so it is maintained and not removed.

While poka-yoke is primarily a quality tool, it has significant benefits for production scheduling reliability. Defects disrupt schedules: when a defective batch is discovered, the scheduler must fit rework into an already-full schedule, or worse, schedule an emergency production run to replace scrapped units. By preventing defects at the source, poka-yoke eliminates these disruptions and makes the production schedule more predictable. The quality factor in OEE improves (more good parts from the same production time), effectively increasing schedulable capacity. Changeover-related defects — startup rejects produced while the machine is settling into the new product — are a common scheduling headache that poka-yoke can address through setup verification devices that confirm correct tooling, material, and settings before production starts. For schedulers using LinePlanner, fewer quality-related disruptions mean fewer emergency schedule changes, better on-time delivery, and more confidence in the production plan.