FEM3- Sensor and Data Acquisition Technology

1. FEM3- Sensor and Data Acquisition Technology Fundamentals Education Subunit- Structural Health Monitoring Education Unit

2. Purpose • To acquaint you with the sensors and data acquisition systems used for Structural Health Monitoring (SHM). • For a given sensor category (e.g., displacement), to acquaint you with the variety of types of a specific sensor. • To help you gain a fundamental understanding of the essential components of a SHM system. • Attempt to connect what you have learned and experienced in your some of your laboratory courses with the technology and equipment used in SHM. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

3. Expected Learning Outcomes • To be able to identify and explain sensor technology. • To be able to describe typical data acquisition components and systems used for SHM. • To be able to identify factors that influence the choice of sensors. • To be able to choose a specific device for a specific monitoring or measurement application. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

4. Your Assignment • After you have read and reviewed the content provided, you will be required to: • Take an readiness exam to determine if you have achieved a satisfactory understanding of the content of FEM3 to engage in a discussion. • Submit a response to a discussion question related to the content of this module and list any questions you have concerning anything you might not understand about the material. • Student responses to the discussion question will be discussed in an interactive manner in a classroom setting on the date indicated on the Master Schedule. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

5. A Look Back • In FEM1, you were introduced to the various components of a Structural Health Monitoring System (SHM) which included Sensors and Data Acquisition Systems. • Before continuing with this FEM, it is suggested that you review the content contained in Slides 32-45 of FEM1. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

6. Drawing From Your Previous Experience with Sensors • Whether you realize it or not, you have already been introduced to the use of sensors in most if not all of your laboratory courses such as: • Geotechnical Engineering • Fluid Mechanics • Engineering Materials • Mechanics of Materials Fundamentals Education Subunit- Structural Health Monitoring Education Unit

7. Drawing From Your Previous Experience with Sensors: Geotechnical Engineering Dial Gages • Used to measure linear displacements in consolidation and unconfined compression tests • Are classified as a sensor that provides a direct measure of displacement • The data acquisition system is an individual reading and manual recording of the dial (displacement) readings Fundamentals Education Subunit- Structural Health Monitoring Education Unit

8. Drawing From Your Previous Experience with Sensors: Fluid Mechanics Piezometer • Used to measure the pressure head • Would be classified as a sensor that provides a direct measure of pressure head • The data acquisition system is the reading and manual recording of the elevation of the surface of the liquid 2 Fundamentals Education Subunit- Structural Health Monitoring Education Unit

9. Drawing From Your Previous Experience with Sensors: Mechanics of Materials Electrical Resistance Strain Gage • Classified as a sensor that provides an analog measure which, using a calibration constant or calibration curve, can be converted to strain- the gage is “stuck” to the specimen or structural member to be monitored • A monitoring unit (normally a Wheatstone bridge) measures the change in the electrical resistance of the gage. The change in electrical resistance is proportional to change in strain. • The signal from the monitoring unit can be recorded via software or strip recorder (printer). 3 Fundamentals Education Subunit- Structural Health Monitoring Education Unit

10. Drawing From Your Previous Experience with Sensors: Engineering Materials/Mechanics of Materials Hydraulic Compression Testing • Used to measure the strength and stress-strain (load-deformation) behavior of a test specimen or structural element • Classified as an analog sensor that automatically converts an analog signal (the internal hydraulic pressure acting on a piston of known diameter) to load Fundamentals Education Subunit- Structural Health Monitoring Education Unit

11. Drawing From Your Previous Experience with Sensors: Engineering Materials/Mechanics of Materials Load Cells (1) • Load cells: metal cylinders to which Electrical Resistance or Semi-Conductor Strain Gages have been mounted • Loading in tension or compression will cause the cylinder and thus the strain gages to deform (i.e., change length) Fundamentals Education Subunit- Structural Health Monitoring Education Unit

12. Drawing From Previous Experience with Sensors: Load Cells (2) • The strain gage will extend or compress causing the electrical resistance of the gage to change. • The electrical resistance is measured and converted to strain using a calibration constant or chart. • The average strain and the Young’s Modulus of the material are used to determine the stress in the cylinder. • The stress and cross-sectional area of the cylinder are used to determine the load carried by the load cell. • In reality, the last two steps can be eliminated by using the calibration constant for the load cell which allows direct calculation of load. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

13. Drawing From Your Previous Experience with Sensors: Review of Pressure Transducers (1) • Pressure Transducer: used to measure pore water pressure (excess hydrostatic pressure) in a soil test specimen. • They consist of a thin circular metal diaphragm attached to a cylindrical case. An electrical resistance strain gage is mounted on the back face of the diaphragm. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

14. Drawing From Previous Experience with Sensors: Review of Pressure Transducers (2) • Pressure acting on the front face causes the diaphragm to deflect and the strain gage to extend. • This changes the resistance of the strain gage and produces a signal that can be measured very accurately. • A calibration chart or curve allows conversion of the strain gage signal directly into pressure. • The previous step can be performed automatically using data acquisition software and computer technology. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

15. Summarizing… • Only a limited subset of sensors have been described in the previous slides. • Not all of the sensors described are necessarily used for Structural Health Monitoring applications. • Sensors for field applications such as SHM must be more robust than their laboratory counterparts. • Due to the remote and continuous collection of sensor data, the use of computer technology and software data acquisition systems are imperative. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

16. Required Fundamental Characteristics of Sensors (1) • There are two major components of a comprehensive Data Acquisition System (DAS) in a SHM application. • The sensor network • The Data Acquisition Unit (DAU) Fundamentals Education Subunit- Structural Health Monitoring Education Unit

17. Required Fundamental Characteristics of Sensors (2) • The sensor network poses some significant problems that have been addressed in modern systems to allow for the effective and accurate monitoring of the behavior and performance of bridges and their climatic and loading environment. • The most fundamental characteristics of sensors for this application will now be discussed. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

18. Required Fundamental Characteristics of Sensors (3) • The most fundamental characteristic that a SHM sensor must possess is the ability to generate a measureable and consistently reliable signal that can be detected remotely. • Any device that requires a person to observe and record the data would not satisfy this requirement. • Generally, the sensor is physically connected to a data acquisition unit by a wire conductor. • The installation and maintenance of the wire conductors have motivated the development of sensors that do not rely on wired connectors. • Over the past 20-30 years, wireless sensors have been developed whose signals can be detected using Wi-Fi or laser technology. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

19. Required Fundamental Characteristics of Sensors (4) • Another important fundamental characteristic for SHM application is “robustness”. • “Robustness” is the ability to function reliably and accurately over a long period of time in the harsh and variable climatic environment in which the bridge exists. • Ease of installation is also an important fundamental characteristic in terms of: • Cost • Time required for installation of the sensor- also a cost factor for installation personnel Fundamentals Education Subunit- Structural Health Monitoring Education Unit

20. Required Fundamental Characteristics of Sensors (5) • The characteristics previously listed are judged to be fundamentally important for a sensor, but they are not the only characteristics that need to be considered in the selection of sensors. • Actually, it’s not simply the sensor or sensors that matter but the entire sensor and data acquisition system. Temperature Data Acquisition System Fundamentals Education Subunit- Structural Health Monitoring Education Unit

21. Types and Functions of Sensors (1) • Sensors can be classified in a number of ways, but for our purposes we will classify them on the basis of the specific behavior they are designed to measure. • The table in the following slide is by no means complete but does include a representative set of sensors that might be used for SHM applications. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

22. Types and Functions of Sensors (2) Fundamentals Education Subunit- Structural Health Monitoring Education Unit

23. Types and Functions of Sensors (3) • It should be noted that at this point, we have simply identified what might be termed generic sensors that provide a means to determine a given parameter. • In reality, a sensor to “measure” any given parameter is actually a suite of sensors of various designs and capabilities. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

24. Types and Functions of Sensors (4) • Sensor design examples: functionality, means of signal transmission, and excitation requirements • Sensor capabilities examples: range of measurement, precision of measurement, sensitivity to temperature, and cost • Specific sensors selected for a SHM project will depend on a variety of factors. • One type of frequently used sensors is termed “smart sensors.” The following slide describes the characteristics of these sensors. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

25. Smart Sensors • Defined as “a device that takes input from the physical environment and uses built-in computer resources to perform predefined functions upon detection of specific input and then process data before passing it on.”10 • A smart sensor combines many components all-in-one housing including: 1) a sensing element; 2) an analog interface circuit; 3) an analog-to-digital convertor (ADC); and 4) a bus interface. • Further details concerning smart sensors are included in Appendix A. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

26. Classification of Sensors by Means of Signal Transmission (1) • Sensors can be classified according to the means that the sensor signal or output is transmitted to the Data Acquisition Unit (DAU). • A lead cable or wire is the most common and least expensive means to transmit sensor outputs to the DAU. These are called “wired sensors”. Wired Sensors Fundamentals Education Subunit- Structural Health Monitoring Education Unit

27. Classification of Sensors by Means of Signal Transmission (2) • There is a high degree of confidence that the wired sensor system will perform as designed, but there are some disadvantages to using this system: • Time and cost required for the installation of the coaxial wire of the system • High cost of a single node of a tethered network limits the number of nodes in the network to less than that considered optimal. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

28. Classification of Sensors by Means of Signal Transmission (3) • Signal transmission can also be achieved by “wireless sensors”. • As compared to wired sensors, wireless sensors reduce installation time and cost. • There is a decreased cost per sensor node, so more nodes can be incorporated into the SHM network. • More nodes means: • better coverage of localized regions of distress or potential damage • more extensive coverage globally Wireless Sensor Fundamentals Education Subunit- Structural Health Monitoring Education Unit

29. Classification of Sensors by Means of Signal Transmission (4) • Things to be concerned with include: • the hardware architecture and the embedded software that operates each sensor in the field application • the amount of battery power consumed by the transmittal of data via the wireless communication channel • While wireless SHM networks are not without limitations, at the present time they seem to be more advantageous and cost effective than wired SHM networks. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

30. A Detailed Look at the Principal Sensors Used in SHM Applications • Even though the detailed hardware architecture of wired and wireless sensors differ, they are still functionally similar. • So without distinguishing between wired and wireless sensors, let’s take a look at the hardware architecture and operational mechanisms of the following sensors. • Be aware that the following discussion only addresses a representative number of types of sensors and not all available types of sensors. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

31. A Detailed Look at the Principal Sensors Used in SHM Applications: Displacement Sensors (1) • Measurement of position/displacement can be achieved by several types of sensors including: • Proximity probes (detects the presence of nearby objects without any physical contact) • Linear variable differential transformers (LVDTs) • Rotary variable differential transformers (RVDTs) • Others: encoders, string potentiometers Fundamentals Education Subunit- Structural Health Monitoring Education Unit

32. A Detailed Look at the Principal Sensors Used in SHM Applications: Displacement Sensors (2) Proximity Probes17: • Eddy current proximity probes are sensors that measure relative proximity. • They use changes in voltage to measure shaft surfaces that rotate or reciprocate. • Because they are non-contacting transducers, proximity probes are mounted on a reasonably stationary mechanical structure, such as a bearing housing. • From the mounting point, they measure the static and dynamic displacement behavior of a structural element. Eddy Current Proximity Probe Fundamentals Education Subunit- Structural Health Monitoring Education Unit

33. A Detailed Look at the Principal Sensors Used in SHM Applications: Displacement Sensors (4) LVDT/RVDT17: • LVDTs operate on the principle of a transformer and consist of a stationary coil assembly and a moveable core. • LVDTs measure displacement by associating a specific signal value for any given position of the core. • The main advantage of the LVDT transducer over other types of displacement transducers is the high degree of power. • A manufacturer’s specifications for a selection of LVDTs that provide a range of maximum and minimum displacements are listed in Appendix B. Take a quick look but don’t spend a lot of time reviewing each slide. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

34. A Detailed Look at the Principal Sensors Used in SHM Applications: Displacement Sensors (5) Laser Displacement Sensors (LDSs): • In addition to the ability to conduct long-range measurements, the LDS is rarely influenced by aspects of the external environment such as wind and temperature (operating temperature from -10 degrees C to ~50 degrees C) and it consumes little power. • Accuracies of 0.2 mm have been reported with a maximum range of displacement well in excess of 1 meter. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

35. A Detailed Look at the Principal Sensors Used in SHM Applications: Tilt Sensors An inclinometer, or tilt sensor, is an instrument used for measuring slope, tilt, or inclination by using gravity. An inclinometer creates an artificial plane defined at zero degrees, and measures the change in angle with respect to this plane. Tiltmeters must be rigidly attached to the structure at the measurement location. See https://video.search.yahoo.com/yhs/search?fr=yhs-adk-adk_sbnt&hsimp=yhs-adk_sbnt&hspart=adk&p=tiltmeters+for+structural+health+monitoring+of+bridges+youtube#id=2&vid=4e7288be1ae46450cd7e671e838ed87c&action=click Fundamentals Education Subunit- Structural Health Monitoring Education Unit

36. A Detailed Look at the Principal Sensors Used in SHM Applications: Strain Sensors (1) Carlson Strain Meter • The Carlson Strain Meter is a device, which can be embedded in concrete to reveal internal deformations and indirectly determine stress. • It responds to any change in dimension of the concrete, due to stress, creep, temperature change, moisture change or chemical growth. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

37. A Detailed Look at the Principal Sensors Used in SHM Applications: Strain Sensors (2) Foil Strain Gauge: • Foil strain gauges take advantage of physical properties of electrical conductance and its dependence on the conductor’s geometry. • When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end-to-end. • From the measured electrical resistance of the strain gauge, the amount of induced stress may be inferred. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

38. A Detailed Look at the Principal Sensors Used in SHM Applications: Strain Sensors (3) Vibrating Wire Sensor: • This sensor is particularly adaptable to measuring strain in structural steel bridge members. • The principal means of attachment is by conventional arc welding. • Strain is determined by plucking a wire tensioned between the two blocks welded to the structural member and measuring the resonant frequency of the wire. • The tension in the wire can thus be determined and the strain deduced on the basis of the amount of the tension. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

39. A Detailed Look at the Principal Sensors Used in SHM Applications: Strain Sensors (4) Semiconductor Sensor: • The semiconductor strain gage may be though of as a strain sensitive resistor. • Generally when bonded to a stressed member, its resistance changes as a function of deformation. • This characteristic makes it useful in the fields of stress analysis, physical measurements, testing, transducers and instrumentation manufacture. • When compared to conventional metallic wire and foil gauges, semiconductor gages offer some significant advantages26 Fundamentals Education Subunit- Structural Health Monitoring Education Unit

40. A Detailed Look at the Principal Sensors Used in SHM Applications: Acceleration Sensors (1) • As we know, F=ma; thus acceleration is the amount of force required to move each unit of mass. • Given that, acceleration is determined by measurement of changes of force required to move a known mass. • Various means can be used to measure force, mechanical as well as by generating electrical or magnetic signals. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

41. A Detailed Look at the Principal Sensors Used in SHM Applications: Acceleration Sensors (2) Capacitive: • Capacitors are used in accelerometers to measure force: if a moving mass alters the distance between two metal plates, measuring the change in their capacitance gives a measurement of the force that’s acting. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

42. A Detailed Look at the Principal Sensors Used in SHM Applications: Acceleration Sensors (3) Piezoelectric/ Piezoresistance: • In some accelerometers, piezoelectric crystals such as quartz do the clever work. • You have a crystal attached to a mass, so when the accelerometer moves, the mass squeezes the crystal and generates a tiny electric voltage. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

43. A Detailed Look at the Principal Sensors Used in SHM Applications: Acceleration Sensors (4) Optical Fiber (OF): • Whereas capacitive and piezoelectric/ piezoresistance accelerometers are used extensively in SHM applications, the negative influence of the electromagnetic interference (EMI) can be a real problem when electrical signals are used to detect and transmit physical parameters in noisy environments (high EMI). • Optical fiber sensors are increasingly used because of the nonelectrical nature of signals. 28 Fundamentals Education Subunit- Structural Health Monitoring Education Unit

44. Sensor Selection Factors (1) • It’s fundamental that the effectiveness of the SHM System is dependent, among other factors, to the selection of sensors that “will provide the information required for monitoring and analysis” (ISIS Educational Module 5). • The need for the individual sensors and the array of sensors to measure the desired response and climatic parameters are crucial issues in the sensor selection process. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

45. Sensor Selection Factors (2) • Other important factors to be considered in the selection criteria include but are not necessarily limited to: • Accuracy • Precision • Sensitivity • Reliability • Installation requirements What’s the difference between accuracy and precision? • While the cost of the individual sensors is important, it’s ultimately the cost of the entire system- sensor, cables/wiring (if relevant), signal-conditioning equipment and data acquisition system- that’s important. • Power requirements • Signal transmission limitations • Robustness • Durability • Cost Fundamentals Education Subunit- Structural Health Monitoring Education Unit

46. Data Acquisition System (DAS) (1) • As described in FEM1, the sensors combined with the Data Acquisition Unit (DAU) constitute the Data Acquisition System (DAS). • In FEM1, you were provided a basic explanation of the purpose and the functions of the DAU. • However, a more detailed explanation of the functions of the DAU is needed to better understand its role and importance in SHM. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

47. Data Acquisition System (DAS) (2) • Fundamentally, data acquisition is the process of sampling signals that measure real world physical conditions and converting the resulting signals into digital numeric values that can be manipulated by a computer. • Data Acquisition Units typically convert analog waveforms into digital values for processing. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

48. Data Acquisition System (DAS) (3) • As previously described, the components of a data acquisition system include: • Sensors, to convert physical parameters to electrical signals. • Signal conditioning circuitry, to convert sensor signals into a form that can be converted to digital values • Analog-to-digital converters, to convert conditioned sensor signals to digital values • The latter two functions are performed by the Data Acquisition Unit (DAU). Fundamentals Education Subunit- Structural Health Monitoring Education Unit

49. Data Acquisition System (DAS) (4) • Data Acquisition applications are usually controlled by software programs developed using various general purpose programming languages. • Stand-alone data acquisition systems are often called data loggers. Fundamentals Education Subunit- Structural Health Monitoring Education Unit

50. Data Acquisition System (DAS) (5) • The block diagram shown in the following slide depicts the functions of a DAS, but not necessarily completely for all installations and the two transmission modes (wired or wireless). • Additional functions of a DAS, specifically the DAU, may include: • Temporary data storage • Control of sampling events • Excitation of the sensors • Providing power to the sensors • The latter two functions generally apply to wired systems only. Fundamentals Education Subunit- Structural Health Monitoring Education Unit