Wireless technologies take safety to a new level

02 December 2008

The basic methods of mustering using manual paper, T card or passive card/tag reader based systems have a major disadvantage.
They require the person to remember to sign or swipe in and out of plant areas and it is time consuming to tally that all the personnel that have assembled at the different muster points equates to the number of people on the plant.

In order to overcome the problem of remembering to sign or swipe in and out of plant and to speed up the whole process active RFID technology has been adopted whereby the tag responds to a low frequency RF field (LFR - usually around 100 KHz to 125 KHz) by sending a transmission to the reader to confirm its presence. Ranges of several metres are possible with field strengths that are within the 2W limit allowed for an IIC hazardous area. Using two readers or antenna it is possible to determine the direction of movement of the person wearing the tag, so it is possible to count the numbers of people in certain areas or zones of the plant.

The final hurdle to overcome is where does one search for the missing personnel after a muster procedure has identified they are missing? The desire by plant operators to be able to track their work force and know their exact location to within a few metres accuracy is becoming more compelling due to ever tightening legislation that is designed to protect the welfare of workers in industry.

There are several real time location tracking technologies that can be applied, each with their own key benefits and disadvantages. Be it proprietary hardware, will only work outdoors or indoors, accuracy may be affected by multi-path RF reflections or that the cost of the infrastructure is prohibitive, as is often the case, no one technology can resolve the problem in its entirety.

One location technology we are all familiar with in our everyday lives is GPS which in outdoor areas will provide location accuracy to sub 10 metres by receiving signals from at least 3 satellites and using Time Difference Of Arrival (TDOA) algorithms to calculate the position of the receiver. This technology does not work indoors as the RF signal is not powerful enough to penetrate though buildings and needs a wireless network of some sort to update the location tracking software with the last location due to the fact the GPS satellites only transmit and have no receive mode. Certain commercial products are now becoming available such as a mobile telephone with a GPS receiver, dead man sensor and emergency call button.

Another familiar technology is the WLAN or WiFi network that most of us have at home or in the office. If there are a number of access points providing blanket coverage in cells across a site it is possible to use the Received Signal Strength Indication (RSSI) to calculate the position of the tag, again by using a triangulation calculation algorithm much like that used by GPS except based on signal strength. This principle of location works indoors with an accuracy of sub 5 metres as well as outdoors but with much less accuracy. The reason for this being the fact that the relationship between WiFi signal strength and distance becomes exponential after about 50 metres which means the cost of installing a high density matrix of access points would become prohibitive. It is much more than is needed for most mobile computing applications, where an access point every 100 to 150 metres will more often than not provide adequate outdoor connectivity and throughput. Some vendors have also developed their own receiver that has a TDOA function as well and will work outdoors up to 200 metres with sub 5 metre accuracy. The only downside with this is the proprietary infrastructure, which unlike a WiFi network cannot be used for any other purpose meaning the return on investment takes much longer than a WiFi based location system.

Some companies are demanding location accuracy of sub 5 metres for use in a very dense metal environments such as a processing unit on a refinery where it is impossible to visually spot someone from more than a few metres away. In a 50 x 50 x 50 metre unit consisting of metal pipes, vessels, walkways and stairs the only technology that can provide a solution to date is ultra wide band (UWB). This location technology -- similar in principle to radar -- can perform to less than 30 cm accuracy, even better is some cases. The other advantage of this technology is that it is possible to calculate location in 3D with suitably placed location receivers. However the density of the infrastructure required makes this technology prohibitive to use over a complete site.

Providing an economical solution to mustering is now possible and practical to implement but what solution can provide economical location and tracking of people across a complete facility? It is hard to imagine industry investing huge sums of money to locate personnel down to cm accuracy when there is no need and due to the high cost of the infrastructure. It is a common convention now that WiFi is a good investment for industry and can improve working practices, overall efficiency and safety. The Process Industries are now starting to install blanket WiFi coverage in their plants for data and voice applications.

Using the WiFi network as the primary means of RTLS makes sense as it has already been installed for other reasons so the ROI is greatly improved. New tags are being developed that combine GPS and UWB technologies with WiFi and LFR to provide an ultimate solution for plant wide location tracking. Where the level of location accuracy is not adequate by using WiFi alone, such as in large open outdoor areas, the GPS mode would be used. By installing UWB receivers in the places where a sub 1 metre accuracy is required and using the UWB transmitter in the Tag in these areas will keep the costs of the complete RTLS infrastructure to a minimum.

At Extronics taking this type of personnel protection system to the next level is a key objective along with our technology partners. We are starting to look at ways to offer lone workers the safety and security of even having their “life signs” such as body temperature and pulse rate monitored along with the traditional motion and tilt. With an ability to communicate directly back to the control room - the lone worker is no longer alone!

The Author
John Hartley, technical director of Extronics provided an insight into RFID technology and the way in which Extronics will be using this technology to provide enhanced lone worker protection.