When should I make the move from proximity sensors to industrial RFID?
It's all about the right tool for the right job. Sometimes proximity sensors are the best fit for your application. Other times, using industrial RFID is the way to go. While both proximity sensors and industrial RFID systems can accomplish the same task, it pays to know when to upgrade.
In most applications, RFID systems are used to store product and process data on or retrieve it from a tag. In terms of manufacturing space and looking at it on a more fundamental level, an RFID system is a very smart proximity sensor.
Whereas a proximity sensor detects the presence of a piece of metal, and knows that the pallet has arrived (or is it just a bolt that fell off the assembly robot and triggered the sensor?), an RFID system detects a data tag, reads its memory containing associated product information, and knows the identity of the part in the process station. However, this does not suggest using RFID systems instead of proximity sensors for all possible applications.
The good news is that modern RFID systems have gotten much better, easier to use, and with tag prices of just a few dollars, their usage has gone up considerably. This means that applications that previously made use of a collection of proximity sensors reading a code block—basically a piece of metal with a hole pattern—should be seriously reconsidered for an RFID upgrade. Not only does the RFID solution provide flexibility in case a system must be expanded, it is also much more secure.
Slightly misalign a single sensor and the “detected identity” is off. While this problem can be addressed by extra check signals derived from extra sensors reading larger and larger code blocks, RFID solutions have powerful error detection built in. The installation remains simple no matter how complex the data needs become. Another advantage of RFID systems is the much greater reading distance compared to the sensing range of a proximity sensor of comparable size. Where an 18 mm extended range sensor has a nominal sensing distance of 12 mm, an 18 mm RFID read/write head can read tags at 40 mm or greater. It’s easy to place the RFID read head safely out of the way of a moving pallet, and distance directly translates into process reliability.
(Image caption) A small RFID tag is mounted to pallet that allows the PLC to read data identifying the product on the pallet. The RFID system not only carries product data, it also detects the presence of the pallet. The information exchanged between the tag and the RFID reader is protected by powerful checksum methods, making data exchange secure. Misalignment never leads to wrong data.
(Image caption) A 3 x 3 code block with nine inductive sensors is used to identify the arriving pallet. To provide a marginal amount of data security, the corner location is used as a parity bit. Its “value” is selected such that there is always an even number of holes. More secure methods are possible by increasing the number of sensors and making the plate bigger.