Applications Dictate Wireless Sensor Solutions

The use of wireless sensors in wellhead monitoring of oil and gas fields is a key application that shows promise. The measurements involved are conventional process measurements of pressure, level and flow using a small number of wireless measurements on each well.

Currently, these measurements are not applied at the majority of wells because wells with low production rates cannot justify the high installation cost of the sensors.

Wireless process sensors with dramatically lower installed costs can extend automated monitoring to a much higher fraction of producing wells, and returns can be high. The type of monitoring performed, however, will be different than in traditional supervisory control and data acquisition (SCADA) applications. In most cases, the application will consist of sensor readings gathered periodically, along with alarms and some logic for the detection of abnormal conditions. Readings will not be taken continuously to conserve sensor power. Periodic or end-of-day readings can be transmitted via the sensor gateway to a monitoring system. The gateway may use either wireless wide area networks or satellite communications. Transmission is periodic and infrequent, further minimizing battery power consumption.

Intelligent Light Switches

Another promising wireless sensor network application is facility lighting control. The situation is analogous to the advancement in factory automation that occurred when hard-wired automation logic was replaced with programmable logic.

Traditionally, lighting fixtures have been completely passive, and lighting is controlled by switching power to these passive fixtures. Control logic, if any, is hardwired into the facility wiring and is thus extremely inflexible. Only in a relatively small number of large and complex applications is lighting control logic separate and programmable, but actuation remains hardwired.

In a possible future design for wireless lighting systems, each lighting fixture becomes an intelligent actuator containing a wireless device that controls the lamps. Such a control system would have a very large number of actuators compared to sensors. These actuators must all operate in parallel in response to common control logic. The only sensors involved are the automatic or manual control switches. Hardwiring becomes standardized, because fixture power supply and control are now separate. The fixtures need only be wired to a continuous power supply. While power is supplied continuously to all fixtures, the lamps are switched by the embedded wireless actuators. These wireless actuators use the same power supply as the lamps, making batteries unnecessary.

One advantage of this type of system is that it can be easily reconfigured and provided with new logic as the space layout or use changes, or as additional control logic is added, ranging from an additional wall switch to energy management functionality. A second advantage is that the physical installation does not need to change to support more advanced applications and logic.

One area of commonality between the lighting and the wellhead process application is the importance of the gateway device. In lighting applications, the gateway provides execution of the logic and actuation via multicast signals to and from the many fixtures/actuators. In wellhead applications, the gateway device provides logic for detection of abnormal conditions. In both applications, gateways provide the critical functions of network communication, network management and integration with higher-level applications.

Manufacturers and developers must give precedence to fit-for-purpose requirements rather than the wireless technology itself in deployment of wireless sensor networks for industrial applications. They should also plan to use the wireless sensor gateway as a key component when specifying and deploying wireless sensor networks. Finally, when planning an application, look to examples of deployment, but evaluate the experience gained in the context of your application requirements.

Harry Forbes, hforbes@arcweb.com, is a senior analyst at ARC Advisory Group, in Dedham, Mass.

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