Although smart wireless sensor nodes and networks are coming into maturity, the cost per measurement point is still high and functionality somewhat limited, which means that there is still opportunity to improve on the economics of wireless sensor networks. As a result, most of the wireless sensing systems being installed today are for applications that are difficult due to extreme cost, scheduling issues or lack of physical access (i.e. using wired sensors with rotating components is difficult since slip rings are inherently noisy and unreliable and only a limited number of sensors can be accommodated).
In many applications, there is a critical need to monitor moving parts, thus requiring cable-free, wireless operation. For example, wireless sensors are highly effective at observing and controlling moving objects in high temperature zone furnaces or rotating rotors and blades at high accelerations.
Meeting the need for developing associated low cost, low power, wireless sensors to achieve the required degree of surveillance in other environments and applications will require new sensing technology. Passive wireless sensors are just one of the options in the “tool box” for reducing dependency on wired connectivity and adding functionality without wires or cables.
Two passive wireless sensor technology options are viable alternatives: active RFID sensor, wherein a reader broadcasts induced power to the sensor/transmitter system, which acquires the sensor data and sends that data back to the reader; or a system that is powered via a custom thin film thermal electric generator and can be used to measure parameters such as vibration, pressure, temperature and others. The peak power dissipation for the whole chip, including memory, logic and transceiver stage, is 2uW. The low power makes it possible for the chip to run on many sources of ambient energy at extremely low cost and small size.
Passive wireless sensing technology measures physical properties using unpowered surface acoustic wave (SAW) devices and radio frequency (RF) interrogation techniques. Very few sensor systems exist that can provide reliable operation at temperatures in the 500 degrees Celsius to 1000 degrees Celsius range, pressures up to 750 psi, and in highly reactive, explosive and/or toxic gas environments. SAW sensors can already be used up to 400 degrees Celsius (752 degrees Fahrenheit) for measuring temperature, pressure, force or strain. In the near future these sensors will be able to stand temperatures up to 600 degrees Celsius (1112 degrees Fahrenheit) and comply with 433MHz standards.
Commercially available systems today are:
• Passive, battery-free, lightweight, harsh environment wireless microwave acoustic sensors and sensor arrays; these have a small footprint and around a gram in weight depending on the application;
• Stable, medium and long term (>1000 hrs);
• Operate under severe thermal shock conditions (room temperature to >700 degrees Celsius within a few seconds);
• Survive g-forces in excess of 50,000g on rotating parts;
• Use sensor arrays to operate simultaneously in groups of eight or more while retaining the ability to individually identify and read each sensor for pressure, strain and vibration measurements;
• Contain stored data that can be retrieved and changed with remote/non-contact means;
• Provide location and range information (passive tags can be located with 5-8 centimeters of accuracy within a range of 100 metres), and;
• Operate at either low power short range or high power long range.
Turbine engines, power plants and oil/gas extraction machinery are examples of harsh environment applications that require and will significantly profit from high-performance wireless sensor systems. A class of wireless sensors have been developed that address the operational environment using passive, RFID-like sensors that can be deposited on turbine blades or other surfaces and offer long term, reliable operation. This RF circuitry is “printed” on the surface similar to the processes used to make integrated circuits, and their mass and size are sufficiently small so that they do not alter the vibrational modes of the blade on which they are mounted. They are totally passive, requiring no external power to operate. They operate by receiving an interrogating signal, modulating it with the parameter value being monitored, and re-transmitting the modulated return signal to a receiver/signal processor for measured parameter estimation.
As you can imagine, there are a number of RF issues that arise from measuring strain inside an operating jet engine fan or compressor ring. In an operating turbine, multiple reflections (“multi-path signal propagation”) and Doppler shifting of the RF signals occurs along the many propagation paths, so that the RF signals to/from the sensor are corrupted and/or modulated. This situation presents a challenge for the passive wireless sensor system design that has now been overcome with the latest generation systems.
Passive sensor technology is not only being used in what is considered cutting-edge industries, but also in the more mundane ground-based transportation infrastructure. The U.S. Federal Highway Administration is investigating the use of wireless sensor system technologies to monitor structure responses at various extreme events (earthquakes, wind, scour, or other impacts). These programs are used to plan or practise emergency planning on post-extreme events, for seismic retrofitting projects and prioritizing retrofitting needs. As wireless sensor systems become inexpensive they can easily be installed on existing structures or new structures for monitoring structure during actual incidents such as an earthquake and thus provide data on how to make them safer in the future.
This article originally appeared in the September 2012 issue of Manufacturing AUTOMATION.