To ensure that appropriate actions are taken when a failure occurs in an RFID system, feedback devices are used to signal these events to responsible personnel or to a back-end system, or they can be programmed to take action. Feedback devices are also used at verification points, where they signal the validity or invalidity of a tag's function, encoding, and/or placement on the product. The most frequently used feedback devices are light stacks, sound devices, and diverters.
Although you might think of a flashy light show at an old Styx concert, a light stack is a little simpler than the name might imply. It is usually a single tube with red, green, and yellow lights in it. Light stacks are often used to indicate whether the tag applied to the product or packaging functions (green for a read, red for no-read). Light stacks are used with an interrogation zone; the tag's function is verified, and the appropriate light indicates the result. There are several ways to make this work.
One method is to connect the light stack to an external I/O port through a relay circuit or a programmable logic controller (PLC) that can be programmed to turn on and off different lights indicating various conditions, such as reads, no-reads, read of a specific tag, and so on. PLCs also can be connected through a network and receive commands from the middleware.
Sometimes the light stack can be connected to the I/O port on the reader directly, because some readers can manage low-voltage devices without relays or PLCs and are protected from voltage spikes and ground loops.
Besides a tag read (green light) and no-read (red light or no light), light stacks can also provide other information. A red light often indicates an interrogator error, and a yellow light usually follows the booting process of the interrogator. The colors of a light stack can have different meanings, depending on the way the lights were programmed and what meaning was assigned to each color when you planned the business process change for the system.
By sound, I mean noise. Beepers, buzzers, horns, or other devices producing various sound effects can be useful in environments where the interrogation zone and the light stack are out of the line of sight or as a supplement to other devices. Sound devices are not suitable for high-noise environments or places where a quiet environment is a priority. A great application for using a sound device is for asset management so that expensive items such as laptops or secure servers can be tracked through points of egress. A buzzer can be an added security device and is often coupled with a surveillance camera to track when a high-value item leaves the premises.
Diverters, or divert gates, are used to divert a product on a conveyor or a manufacturing line based on the data read from the tag. They are also used for diverting products with nonfunctioning tags for exception processing. Divert gates have been used for a long time with barcode readers, and they work on the same principle when used with RFID systems.
A diverter is placed behind the read point and automatically routes all boxes to exception processing until a valid tag is read. Then the diverter opens and lets the valid box continue on its route. This way, all unread boxes go to exception processing, where they can be retagged, verified, and sent back on the line.
A similar principle is used for routing boxes into their appropriate dock doors. The diverter routes the boxes based on the data on the tag read by the read point. In this case, the diverter is usually managed through PLC and middleware, which indicates whether the box belongs to the particular location. This decision is made at every intersection, and boxes are routed accordingly.
Diverters work well only when the product being diverted has the same or similar RF transparency as other products on the line. Otherwise, for example, a case of water in front of a case of toilet paper on a conveyor might be read in reverse order, and the diverter would divert the wrong item. This problem can be solved by spacing the products on the conveyor and verifying that the right product is being loaded at the dock door.