Health monitoring of structures with cable members under tension




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University of Connecticut





HEALTH MONITORING OF STRUCTURES WITH CABLE MEMBERS UNDER TENSION

 

 

Senior Design Team

 

Christopher Von Kohorn – Mechanical Engineering

Lead on cable vibration

Jeff Urban – Electrical & Computer Engineering

Lead on digital design

Eric Snapper - Electrical & Computer Engineering

Lead on signal processing

 

Sponsor

Dr. Jonathan Russell - Civil Engineering, Coast Guard Academy

 

Advisors

Dr. Sung Yeul Park – Electrical Engineering

Dr. Kevin Murphy – Mechanical Engineering

Dr. John Bennett – Mechanical Engineering

Dr. Rich Dino – Business / Management



December 2009 ECE 4901


Table of Contents


  1. Abstract………………………………………………………………………………………………………3

  2. Introduction……………………………………………………………………………………………….4

    1. Problem Statement…………………………………………………………………………..4

    2. Background……………………………………………………………………………………….5

  3. Theory………………………………………………………………………………………………………..7

  4. Approach…………………………………………………………………………………………………….9

  5. Design Concept…………………………………………………………………………………………10

    1. Power Supply…………………………………………………………………………………..12

    2. Accelerometer…………………………………………………………………………………14

    3. Band-Pass Filter………………………………………………………………………………16

    4. PC Oscilloscope……………………………………………………………………………….17

    5. Attachment and Installation Tool…………………………………………………..18

  6. Concerns…………………………………………………………………………………………………..19

  7. Data Processing……………………………………………………………………………………….21

    1. Peak Fitting……………………………………………………………………………………..25

  8. Castleman Lab Setup.……………………………………………………………………………..26

    1. Lab Setup Calculations……………………………………………………………………30

  9. Broadcasting Tower Testing…………………………………………………………………….32

    1. Waterford Tower Testing…………………………………………………………………32

    2. WHUS University of Connecticut Radio Tower Testing………………….33

  10. Timeline……………………………………………………………………………………………………34

  11. Budget……………………………………………………………………………………………………..35

  12. References……………………………………………………………………………………………….36

  13. Appendix A – Code for Determining Theoretical Frequencies……………….37

  14. Appendix B – Dr. Russell’s Patent Information………………………………………51

  15. Appendix C – Key Terms & Phrases…………………………………………………..…..52

  1. Abstract


The current methods for measuring the tension of guy cables attached to broadcasting towers are slow and expensive. This project involves constructing a device based in part on a technique developed by Dr. Jonathan Russell of the Coast Guard Academy for measuring the tension by comparing the observed frequency spectrum of the guy cable to its theoretical frequency spectra under various tension conditions.


A first-stage prototype device composed of a power supply, accelerometer, filter, PC oscilloscope, and a laptop computer has been assembled to perform initial testing. This prototype will allow further experimentation to determine necessary specifications for a later stage system that will be self-contained. The PC scope and laptop will be replaced with a digital circuit based on an FPGA or microcontroller.


Building on experience gained during field testing of the system, the team is developing a universal attachment for the accelerometer to the guy cable under test, as well as an installation tool to allow safe installation and removal of the sensor from ground level.


The data from the accelerometer is converted to the frequency spectrum via FFT. The geometric conditions of the cable to be examined are entered into MatLab to determine multiple theoretical tensions and their associated natural frequencies.


Currently, this design has not yet been fully tested. The team is still in the process of constructing the peak extraction code as well as modifying the Matlab code given to the team by the team’s sponsor, Dr. Russell.



  1. Introduction


Problem Statement:


Instruments are to be designed that measure the cable tension in guy wires supporting broadcasting towers through the application of an algorithm developed by Dr. Jonathan Russell of the Coast Guard Academy. The algorithm will be enhanced by the University of Connecticut Senior Design Team and implemented in a portable laptop based system as well as a self-contained permanently installed system. The technique determines the tension of a cable by identifying the observed resonant frequencies and determining the best match within a matrix of predicted natural frequencies for the guy wire predicted at a given tension and temperature. Dr. Russell’s has provided code to determine the natural frequencies for a cable given its geometry and material properties. The Senior Design Team is enhancing the algorithm by automating the peak extraction and comparison processes, and building electronic systems to identify the tension automatically.


Background:


Cable tension structures are those that incorporate metallic cables under tension as structurally significant elements. Such structures include cable stay bridges, broadcasting towers, and high-voltage power lines. The cable tension of guy wires attached to antenna towers (Analog & Digital: AM / FM / TV / Loran) must be monitored periodically to ensure that cables maintain their tensile load. While likelihood of failure is low, costs of failure can be high. Failure can result in loosening of the cable, bending or twisting of the central tower or cable breakage. Such failures can damage the tower, causing loss of operation, possible human injury, and requiring replacement of the tower.


Towers are typically 1000 ft tall, ranging up to 2000ft. The central truss is formed as a triangle shaped 10ft on a side, with columns 3-6” wide of 0.5” wall tubing. Cables are strung between points on the tower and the ground in groups of three at each of several heights on the tower. These cables control the movement of the tower, rather than hold it fixed in place, and often hold on the order of 40,000 lbs tension. Full size towers incorporate heavy steel cables, inches thick, which typically sag 50 – 70ft from vertical. The fundamental (base) frequency of each cable ranges from 2Hz down to 0.2Hz (0.5 second to 5 seconds).


The current method of performing accurate cable tension measurements uses a slow, labor, personnel and equipment intensive process of attaching a coupling to each cable and physically measuring the cable tension using a hydraulic cylinder to take the load. The method generates results with 5 – 10% accuracy. This method must be repeated for each cable. Less accurate methods are used for small towers and for initial construction of larger towers. On method involves line-sighting using a scope installed parallel to the cable, near the base. Another involves exciting a pulse in the cable and counting the cycle time for the pulse to move between the base and tower.


With any method, the center tower will require its own visual inspection to check straightness, non-twistedness, and to check bolted connections and bracket attachments. When the tower is first constructed, with the cable tensions being adjusted for the first time, this process can take weeks to accomplish. This is because the tensions of the cables must be adjusted simultaneously until the equilibrium position of the tower is attained.


Vibration techniques to determine tension in bridges are mature technologies that have been widely studied and applied. These techniques begin to fail on broadcast tower guy cables as they grow larger and incorporate significant sag and significant lumped masses at points along their length. Tower guy cables present a different problem than bridge risers, which are comparatively massive, stiff and taut. Bridge analysis techniques are accurate only for the 1st, 2nd and 3rd frequencies, not afterwards. On guy cables, these lower frequencies may have much lower amplitudes than higher frequencies, which can be used up to approximately the 20th harmonic. Furthermore, the bridge techniques do not allow for the inclusion of lumped masses along the length of the cable. The presence of significant point masses on a cable has been shown to cause banding of the natural frequencies, a significant problem to overcome when attempting vibration analysis of a system.


Dr. Russell approached the Tech-Knowledge Portal Program (TKP, an EDA University Center within the Office of Technology Commercialization) with a product / business concept that should solve these problems, allowing vibrational measurement techniques to be applied to large guy cables on towers. The technique uses frequency spectrum data up to the 20th harmonic of the fundamental frequency to accurately predict natural frequencies, despite the presence of lumped masses, significant sag, and inclined cable spans. After discussion with Dr. Russell, he has expressed interest in funding this as an Entrepreneurial Senior Design project. He would be interested in participating in a commercial startup business should the project be successful.




Figure 1: Tension tool for small cables



Figure 2: Line-sighting using a scope


  1. Theory

 

This section of the report details the basic concepts behind the thesis work Dr. Russell performed on determining the tension of guy cables from their natural frequencies. Due to its complex nature, the mathematics are not detailed, but can be found in his thesis, included in the list of references near the end of this report.


Other existing vibration measurement techniques are not applicable to broadcasting tower cables, due to their unique properties. In these cables, the lower frequencies are often obscured due to effects from cable sag. Damping is insignificant in the cables and can be ignored. Vibrations due to excitement from ambient conditions including gentle breezes should be sufficient to produce meaningful measurements.


The vibration behavior of a guy cable can be compared to an elastic catenary system with negligible damping. In such a system, the cable is modeled as a stiff spring that forms a catenary shape. However, in the case of broadcasting towers, additional geometric conditions such as point loads from anti-galloping cables and point masses from non-conducting elements inserted in the cable must be considered.

 

Dr. Russell’s core technology is an accurate method that can determine expected frequency modes, usually up to the 20th natural frequency, on cables with significant slack and installed lumped point masses. This method is used to determine the expected natural frequencies (base frequency and harmonics) at incremental levels of tension on either side of the design (specified) tension. After a cable is measured to determine its ambient frequency response, it is compared to a matrix of expected frequencies to find the best match tension condition.(Figure 3: Flow chart of Dr. Russell's method, which uses a manual process of matching observed and expected natural frequencies


Dr. Russell’s algorithm determines these theoretical natural frequencies by employing finite element analysis on the system. By doing this, constraints such as the point masses can be considered on a single element as opposed to attempting to average out the mass along the cable. This allows Dr. Russell’s method to predict the banding due to these point masses that occur.


By employing this finite element technique, a global stiffness matrix is generated. Dr. Russell’s algorithm then uses the Newton Raphson method to simultaneously root solve for the vertical component of tension at the base of the guy cable, the horizontal component of tension at the base of the cable, and the unstrained length of the cable (if the unstrained length is not known).


Using these determined tension and unstrained length values, the algorithm then solves for the any number of natural frequencies required by the end user.




Figure 3: Flow chart of Dr. Russell's method, which uses a manual process of matching observed and expected natural frequencies

Approach

 

By implementing Dr. Russell’s cable tension measuring algorithm, a smaller and more efficient portable device will be designed to monitor the cable tension. The completed device should be far more cost effective than the current process, meeting a need within the niche business of cable installation and inspection. The permanently installed version will offer continuous monitoring, not presently available in the marketplace, and may serve as a platform for additional monitoring applications.


The portable type may be temporarily attached to the cable to take a quick reading, and the collected data compared to baseline data from the original installation. This would be a fast and reliable method, and could conceivably eliminate a majority of the work associated with the currently used procedures, while delivering significant cost savings. The device would be usable by a minimal service team, ideally a single technician.


The permanent type could be permanently attached to a cable, performing measurements on a periodic or semi-continuous basis. Data communication could be wireless or hard-wired, and by a schedule or on-demand. The device would need to be durable under weather conditions, wear and corrosive effects, and would require a power source, which may be hard-wired, or harvest vibrational or solar energy.


The upgrade from a portable to a permanent system will allow the team to reduce costs, allow as-needed maintenance inspections, and may offer a platform for additional applications. The team will pursue both versions, beginning with the portable type to verify and refine our technique. This will be used as a test fixture to better understand the problem, refine minimum specifications and justify the design of the permanent, self-contained system. After creating an initial, working portable system, the team will swap in prototype components and build additional functionality into the working system to transition to the permanent self-contained system.


  1. Design Concept


The team’s initial design goals are to create a handheld device that can gather and store data to be easily accessed for analyzing. To fulfill these needs, the team is initially using a digital PC oscilloscope that allows the team to gather data from our accelerometer and easily transfer the results to MATLAB. This setup is convenient because the data can be stored for repeated experimentation and interpretation as the analysis software is developed.


The digital oscilloscope package that the team has obtained is equipped with an automatic FFT function, to convert our vibrational responses to the frequency spectrum. The team has opted to collect samples in the time domain, using the FFT generated frequency spectrum only for secondary verification – implementing an FFT through code to allow for an easier transition to further design stages. The digital oscilloscope package is intended for initial testing to verify the technique and determine minimum specifications for the digital circuit. Switching to either a microcontroller of FPGA meeting the minimum specifications will reduce the overall cost and size of the device.


From here, it can either begin processing the data internally to output a concluded tension, or it can transmit the data to an offsite location where the analysis can be performed. This microcontroller/FPGA approach, however, is merely conceptual at this point in time, as the team would like to improve the coding aspect itself before moving forward with making our device more practical and marketable.



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