Quality Scan: Ensuring Wireless Data Integrity
More and more often, wireless sensors and metrology instruments are being deployed in production areas. Wireless data transmission is critically needed in many operations, but is vulnerable to interference.
A variety of issues, including electromagnetic interference (EMI)—whether environmental or user-related—can cause data errors in wireless systems. Network configurations and software exist to minimize these interruptions.
Data in a wireless data acquisition system is carried by radio frequency (RF) transmission. RF is highly susceptible to corruption and alteration by a variety of EMI, and the end result can be degradation of the reported performance of a piece of equipment, lost or corrupted data, or a wide range of other failure mechanisms.
Some typical EMI types in production-shop environments include DC fields, quasi-AC fields, and magnetic, AC, and RF and transient electromagnetic fields. In addition, EMI problems are most prevalent in the RF spectrum—good RF handling is necessary to keep the data intact. Regardless of the source of EMI, system designers must develop robust wireless data collection networks and sensors that perform well in the presence of EMI.
A proven method that has been employed by our company for several years is to establish a mesh network. A mesh network has a single and central "gateway" function where all system-wide commands and network management can occur. Data from the network also return to this gateway. Numerous routers are present, and can be added to enable multiple paths for Over the Air (OTA) transmissions.
This mesh network is inherently robust to interference by the very nature of the system configuration. In other words, it has no single point of failure where it counts, on the RF-hostile shop floor.
One of the key advantages of wireless data collection is freedom from the bench, from host PCs, from wires, and from the limitations of bringing a part to the tool for inspection.
In turn, when using a wireless system, data being recorded may not be immediately visible. For example, when an operator presses a button on the tool or data cable to record data, the data are displayed at a PC/gateway that may be located out of the operator's view. A viable wireless data system will alert the operator, at the location where the operator is standing, that data actually made it to the correct destination.
Advanced systems for wireless data collection have indicators/enunciators on the data-transmission device to show the operator the status of the system, and confirm the successful transmission of data. If the end node has the ability to show the user that data transmission has failed, the operator can immediately recognize a problem with the network and do what is necessary to correct it.
When system status is available to the user, data can have more integrity. If the user ignores or misses the feedback, however, data can still be corrupted. As an additional safeguard to prevent losing data, the end-node radio can incorporate a means to store data recorded when the main system is down.
Modern wireless data-collection system designs incorporate a storage feature that collects any data "taken" at the end node, and holds the data until the system becomes available again. Once the system is back on line, the end node will restore its data to the gateway/PC host system.
During the transmission of data, many types of interference can occur, and modern wireless systems include mechanisms that are able to identify corrupted data and mark the data as such. By using cyclic redundancy check(ing) (CRC) and parity-error detection methods, a wireless datacollection system can tag or discard corrupted data.
Wireless data collection systems should communicate between end node and gateway, and seek to ensure that uncorrupted data arrives at the gateway. If the system tags the data sent with a CRC or parity error, the gateway informs the end note to resend the data again, and ensures that good data ultimately get to the data set.
Also, consider a wireless collection system that offers end nodes that provide a degree of resistance to common contaminants on the shop floor, such as coolant, cutting oils, and dirt.
This article was first published in the March 2009 edition of Manufacturing Engineering magazine.