Friday, March 11, 2011

Zone Triggering and Style Triggering of PLC





Zone triggering refers to triggering the guns based on the part profile. A typical application uses photo sensors to scan the product as it enters the powder-coating booth. The photo sensor information is stored in the PLC and creates a digital “image” of the product. This information is stored in a logical shift register and moved through the PLC memory in sync with the conveyor travel. An encoder mounted to the conveyor is used to supply conveyor movement information. As the product approaches the powder coating guns, the data in the PLC shift register is analyzed.

Each gun can be individually triggered based on its proximity to the product. The gun is triggered on when the product reaches a preset distance to the gun. This distance can be positive or negative and is in relation to the leading edge of the product. The gun is triggered off when the product trailing edge reaches another preset distance to the gun. Again, this distance can be positive or negative. By modifying these “Lead” and “Lag” distances, the application can be optimized to improve transfer efficiency and powder deposition.

The photo sensors typically represent the “Zones”. If a booth has five photo sensors, aligned vertically, it is said to have five zones. By assigning active zones to each gun, a gun will only trigger when the product has passed through the respective zone. This allows only the guns needed for a given product to be triggered, which again improves transfer efficiency.

Style triggering is most effective for product that requires guns to be triggered at precise locations on a product or multiple times across a product. Rather than scanning the product to determine the leading and trailing edges only, sensors are used to identify the specific product style. When the product reaches the correct proximity to the gun, a triggering recipe is called up within the PLC and the gun is triggered accordingly. Style triggering does not rely on the part profile for triggering and is generally synchronized with the product carrier. Multiple style recipes are stored in the PLC and are entered through the operator interface. It is not unusual for a system to have over 100 styles available.



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