Poka-yoke systems swiftly detect errors in processing machinery, enabling 100% inspection and prompt corrective feedback in Zero Quality Control (ZQC), ensuring minimal defects and enhanced efficiency while integrating seamlessly into production for real-time or informative post-process inspections.
Poka-Yoke Systems in Source Inspections
A true Zero Quality Control (ZQC) implementation ensures that poka-yoke systems are integrated into source inspections. This ensures errors are caught before production, preventing the creation of defective products. By identifying issues upfront, companies can maintain high-quality standards and minimize wastage downstream in the production process.
Poka-Yoke Systems in Informative Inspections
Informative inspections using poka-yoke systems play a crucial role in defect prevention. Whether conducted as self-checks during the process or successive checks at the next stage, they prevent defect propagation down the line. While not eliminating all defects, they’re more effective than statistical sampling or lacking feedback altogether, significantly reducing defects in production processes.
Poka-yoke systems regulate the production process and prevent defects by using one of two approaches:
- A control system stops the equipment when an irregularity happens or locks a clamp on the workpiece to keep it from moving on when it is not completely processed.
- A warning system signals the operator to stop the machine or address the problem.
Methods for Using Poka-Yoke Systems
There are three main methods for using poka-yoke systems:
- Contact methods: These methods capitalize on product design features, like asymmetry, to promptly identify errors during processing, enabling continuous improvement in both product and equipment design for enhanced mistake-proofing. Contact methods employ physical or energy-based sensors to detect product interaction:
- Physical contact methods, like limiting switches activated by screws, ensure proper assembly.
- Energy sensing devices, such as photoelectric beams, detect positional deviations without direct contact.
- Passive devices like guide pins prevent incorrect positioning in jigs.
- Fixed-value methods: Fixed-value methods are employed when a specific number of parts or operations is required. This approach relies on devices such as limit switches or counters to track movements or part quantities. Once the predetermined count is reached, the product is released. Alternatively, parts can be counted beforehand; any surplus indicates missing components. These methods ensure accurate assembly or operation by enforcing the completion of the specified tasks or parts.
- Motion-step methods: The motion-step method in poka-yoke systems senses process actions within expected time frames or sequences to prevent errors:
- Sensors, like photoelectric switches connected to timers, monitor process movements.
- Lack of expected movement triggers equipment stoppage or warnings. For instance, a label-dispensing machine halts if labels aren’t removed within cycle time.
- Sequencing aids assemblers in selecting correct parts for product models, often facilitated by code recognition systems.
Types of Sensing Devices
The sensing devices used in poka-yoke systems can be divided into three broad categories:
- Physical contact sensing devices: Physical contact sensing devices, like limit switches and microswitches, activate electrical signals upon direct touch, crucial in automated processes. They confirm object presence and position, detecting improper part positioning, processing issues, or broken tools. Microswitches, compact and affordable, fit well in tight spaces. Touch switches offer high sensitivity to detect object presence, position, breakage, or dimensional accuracy. Trimetrons ensure quality control by sounding alarms or halting equipment when conditions deviate.
- Energy sensing devices: Energy sensing devices detect errors without physical contact, offering versatility in various applications:
- Photoelectric switches utilize light beams to inspect transparent objects, assess welds, and verify size or colour of items, object passage on conveyors, or proper part supply and feeding.
- Beam sensors employ electron beams to gauge liquid levels in containers or object passage on conveyors.
- Proximity switches respond to distance changes from objects or magnetic force, suitable for magnetically sensitive materials. For instance, they can halt machines and signal alarms when detecting depletion of magnetically sensitive parts.
Other types of energy sensing devices and typical uses include:
- Fiber sensors: detect motor revolutions, color coding marks, or marks on translucent objects or printed matter.
- Area sensors: detect random breaks in a fixed area, such as hands placed in hazard areas or parts dropping from a conveyor.
- Position sensors: control cylinder strokes or determine screw heights.
- Dimension sensors: ensure correct product dimensions.
- Vibration sensors: detect product ejection errors, width distortion, seam position, or start of processing.
- Displacement sensors: detect and measure warping, thickness, and fluid level heights.
- Tap sensors: detect incomplete tap screw machining.
- Metal passage sensors: detect metal in motion.
- Color-mark sensors: detect colored marks or differences in color.
- Double-feed sensors: detect two products fed at the same time.
- Weld position sensors: select joints such as weld lines in coil stock, scams in pipes and cans, or splices in wires.
- Sensors that detect changes in physical conditions: Condition change sensing devices, the third type of poka-yoke sensors, detect alterations in physical conditions through three main categories:
- Pressure: Pressure gauges and pressure-sensitive switches identify pressure variations, crucial for detecting issues like oil supply interruptions.
- Temperature: Heat-activated devices like thermometers, thermostats, and thermistors monitor surface temperatures of dies, electronic parts, and motors, aiding in machine maintenance and industrial temperature control.
- Electrical current: Metal delays and current eyes, like nugget testers, detect changes in electrical currents, regulating defect causes and monitoring weld strength.