Smart Sensor Systems by Gary W. Hunter, Joseph R. Stetter, Peter J. Hesketh, Chung-Chiun Liu ensors and sensor systems are control and conditioning stages that environment in a given application or vital to our awareness of our will provide both excitation and signal that can be modified to meet the needs Ssurroundings and provide safety, data logging and conditioning. The of a wide range of different applications. security, and surveillance, as well as data acquisition layer will convert the A second major implication of smart enable monitoring of our health and signal from analog to digital and acquire sensors is the development of a new environment. A transformative advance additional parameters of interest to generation of smart sensors that can be in the field of sensor technology provide compensation when needed for networked through the communication has been the development of smart thermal drift, long term drift, etc. The interface to have the capability of sensor systems. The definition of a embedded intelligence will continuously individual network self-identification smart sensor may vary, but typically monitor the discrete sensor elements, and communication allowing at a minimum a smart sensor is the validate the engineering data being reprogramming of the smart sensor combination of a sensing element with provided, and periodically verify sensor system as necessary. Further, the output processing capabilities provided by a calibration and health. The processed from a number of sensors within a given microprocessor. That is, smart sensors are data becomes information and can then region can be correlated not only to basic sensing elements with embedded be transmitted to external users. The verify the data from individual sensors, intelligence. The sensor signal is fed to user can choose the complexity of the but also to provide a better situational the microprocessor, which processes the data transmitted: from a single reading awareness. Such communication data and provides an informative output to a complete download of the sensor can be between a single smart sensor to an external user. A more expansive system’s parameters. and communication hub or between view of a smart sensor system, which is One major implication of smart individual smart sensors themselves. used in this article, is illustrated in Fig. 1: sensor systems is that important data can These types of capabilities will provide a complete self-contained sensor system be provided to the user with increased for a more reliable and robust system that includes the capabilities for logging, reliability and integrity. Intelligent because they are capable of networking processing with a model of sensor features can be included at the sensor among themselves to provide the end response and other data, self-contained level including but not limited to: self- user with coordinated data that is power, and an ability to transmit or calibration, self-health assessment, based on redundant sensory inputs. display informative data to an outside self-healing, and compensated Further, information can be shared in user. The fundamental idea of a smart measurements (auto zero, calibration, a more rapid, reliable, and efficient sensor is that the integration of silicon temperature, pressure, relative humidity manner with on-board communications microprocessors with sensor technology correction). The capability of the smart capability in place. can not only provide interpretive sensor to perform internal processing A driving goal in the development power and customized outputs, but also allows the system not only to provide the of smart sensor systems is the significantly improve sensor system user processed data, but also the ability of implementation of systems in a performance and capabilities. the sensor to be self-aware and to assess nonintrusive manner so that the The smart sensor possesses several its own health or status and assess even information is provided to the user functional layers: signal detection the validity of the processed data. The wherever and whenever needed, as well from discrete sensing elements, smart sensor system can optimize the as in whatever form is needed for the signal processing, data validation and performance of the individual sensors application. In effect, the objective of interpretation, and signal transmission and lead to a better understanding smart sensor research is the development and display. Multiple sensors can of the data, the measurement, and of sensor systems to tell the user what be included in a single smart sensor ultimately, the environment in which they need to know in order to make system whose operating properties, the measurement is made. Overall, the sound decisions. While this article is not such as bias voltage or temperature, presence of the microprocessor-sensor a complete survey of all the activities can be set by the microprocessor. The combination allows the design of a core in the development of smart sensor sensor elements interface to signal system that is adaptable to a changing systems, it is a brief sampling of some of the enabling smart sensor technologies, two examples of smart sensor systems, and a discussion of potential Smart Sensor System Processed ramifications of this technology. Sensor Information Smart Sensor us Power to User System Components ul l s im ca The components of a smart sensor on St on l l ti ti system as depicted in Fig. 1 include S ca Opti ca sensors, power, communication, and l/ ni ni mica Analog-Digital-Analog OR signal processing typically provided by he Signal Processing rica a microprocessor. The description of NS /C mmu mmu User advances in microprocessor technology al SE Co Co Elect is beyond the scope of this article, but ic Commands ys for Sensor recent advances are enabling sensor systems to function remotely on very Ph Operation little power. There are many examples of technology advancements in sensors, power, and communications that can Fig. 1. A smart sensor system as presented herein. The core of a stand-alone smart sensor system enable future smart sensor systems. The includes sensors, power, communication, and signal processing. ideal goal is to have a self contained The Electrochemical Society Interface • Winter 2010 29 Hunter, et al. 1 µm wide bridge, a sensitivity of 2.05 will be important for our further (continued from previous page) mohms/ppm for helium and 0.71ohms/ advancement of the smart sensor ppm for methane at 3.6 V operation has system. Specifically, it is suggested that smart sensor system that is cost-effective, been demonstrated. The microfabricated the Li-ion, Li-polymer, and metal-air reliable, self-monitoring, reconfigurable, sensor elements in Fig. 2 have extremely rechargeable batteries can be appropriate and can operate indefinitely. Simply low power consumption, on the order energy sources for smart sensor systems. put, just as microfabrication approaches of 4 mW continuous and, <4 µW when For example, Li-ion and Li-polymer are enabling the revolution in operated on a duty cycle to read every rechargeable batteries have an open microprocessor technology and MEMS millisecond. In principle, this would circuit potential of approximately 3.6 sensor elements,1 microfabrication and allow the operation of this sensor for V, and an energy density of 160 and nanotechnology will play a notable months to years using a single small 130-200 Wh/kg (watt-hour/kilogram), role in the development of smart sensor battery.2 The microfabrication processing respectively, which will be sufficient systems.2 Below are examples of several is compatible with CMOS processes for the needs of many smart sensor potentially enabling technologies for and therefore makes integration of systems.4 smart sensor systems. the electronic interface for the sensor Energy harvesting is a process by feasible on a single substrate. which energy can be derived from Many sensors will require multichip an external source, captured, and Low Powered Sensor Elements solutions, however, in order to achieve stored. Piezoelectric crystals or fibers, optimal sensing and processing. While thermoelectric generators, solar cells, Microfabrication methods make approaches may vary for other sensor electrostatic, and magnetic energy it possible to build very small and types, sensor elements that provide data capture devices can be considered5 low power sensors. One example of with minimal power consumption can for local power needs. A piezoelectric microfabricated sensors that could be enable long lived smart sensor systems. energy system will produce a small integrated into a smart sensor system voltage when it is physically deformed. is a sensor based on a microhotplate. Power: Battery or This deformation can be caused by Microfabricated hotplates offer a lower mechanical vibration that may be power platform for high temperature Energy Harvesting generated by the proper mounting and metal oxide conductometric sensors. the placement of the smart sensor system Femtomolar isothermal desorption has A smart sensor system will require 3 in an appropriate (e.g., mechanically been carried out by Shirke et al. with energy to support and operate all 6 6 o vibrating) operating environment. heating rates up to 10 C/s and minimal components including the sensors Thermoelectric generators consisting power consumption due to the small themselves. If the sensor elements and of the junctions of two dissimilar thermal mass of the microhotplates. communication system both have low materials produce a small voltage in An ultra-low power bridge built with power designs and are compatible, the presence of a thermal gradient. polysilicon surface micromachining is the total energy for the system is 2 Typical performance of 100-200 uV/°C shown in Fig. 2. This sensor responds correspondingly low. This can enable per junction is achievable. Depending to ambient gas changes in nanoseconds lower installation costs and more on the location of the smart sensor and having a measured transient response convenient deployment options. Small its power requirement, small energy time-constant of 12 µs in helium.