MONITORING OF ROAD STRUCTURES REAL TIME ACQUISITION AND

CONTROL OF DATA

MONITORING OF ROAD STRUCTURES REAL TIME ACQUISITION AND

CONTROL OF DATA

João Pedro Santos 1, Paulo Silveira1 Luís Oliveira Santos1 Luís Calado2

1 LNEC – National Laboratory for Civil Engineering, Structures Department, Lisbon2 IST, Technical Univ. of Lisbon, Department of Civil Engineering and Architecture, Lisbon

Outline

1. Introduction

2. Structural Health Monitoring System

2.1 Overview

2.2 Data Acquisition

2.3 Real Time Processing

2.4 Data Storage

2.5 Automatic Data Transfer

2.6 Alarm Triggering

3. Application to Existing Structures

4. Concluding Remarks

LNEC is responsible for the structural health monitoring of several bridges located mostly

in Portugal but also overseas.

- In the early days, monitoring was carried out by manual measuring campaigns;

- In the recent years semi-automatic monitoring was implemented.The most recent developments in electronics, computation and information technologies include:

Introduction

- wireless remote communication with high transfer rates;

- great improvements in dataloggers and industrial compact computers;

Development of automatic structural health monitori ng systems which must be capable of, autonomously, acquire, pr ocess and

transfer data (transmission and on-line availabilit y)

Industrial computer

Cellularrouter

Cellularrouter Server

Overview

LocationsConnections

Wifirouter

Database

Data loggers

Cellularrouter

Industrialcomputer

Data loggers

Data loggers

Cellular Network

Internet

Overview

Programming Languages:• G Programming Language (LabVIEW) for implementing the main procedures and

routines;• Each datalogger’s programming language for writing data aquisition procedures;• Hayes Command Set (AT Command), language used for controlling

communication units such as modems and routers;communication units such as modems and routers;• Command line scripting for controlling the operating system and managing files

and folders;• SQL (Structured Query Language) for controlling Database Management

Systems.

Data acquisition

Data acquisition is carried out by drivers , whose operation include the following main steps:

1. Send adequate instructions to data loggers;

2. Acquire electrical measures;

3. Process data;

4. Repeat the last three steps as many times as needed according to predefined criteria;

5. Locally store the data acquired.

Data acquisition

Driver’s behavior

Control and acquisition rate Network and communication constraints

(slow networks: RS485, RS232, radio frequency)

FIFO qeues

Parallel independent loops

(high data rate networks: Ethernet or Wifi)

Parallel synchronized loops

(for high data rate networks and modal analysis)

Query driver

(slow acquistion rate andgreater control)

Data Stream driver

(Pre-programming and highacquisition rate)

Real time processingStatistical Analysis

Conversion from electrical to structural data

Filtering of data

The preferred method for statistical processing is the modified z-score method,

( )i

ixxmedianmedianc

x~xz

−×−

=- Rousseuw and Croux identifier ( )jiji2 xxmedianmedianc −×

- Outlier identification is made by comparison with reference values (Little and Rubinproposal for a 95% CI) ( )

( )

→α>→α≤

=?outlier,Nz

value"true",Nzzi

- In the cases where zi > z (N,α) the routine analyses the whole sample array

( ) ( )

→<→≥−→α>

value"true"%5

outlier%5

x~xx

,Nzz minmáxi

- The sample is considered valid if less than 20% of the sample elements are removed.

Real time processing

1. Performed on electrical measures immediatly after acquisition;

Statistical processing is carried out in two distinct phases of the acquisition process.

2. The results obtainded in the previous processing are then grouped for new analysis;

Data storage is carried out bearing in mind:

- robustness of the routines, which can not block and must generate errors reports;

- the possibility of new data crushing old stored data;

- data size and format: ASCII and binary.

Data Storage

Start

Date_timefolder

Filenamearray_of_data

array_of_headers

Folder exists?

Create folderF File

exists?

T

Create fileF

Is there a header line equal to the

array_of_headers?

T

Write array_of_headers to file

Write date_time + array_of_headers to file

F

Is there a different header line below the array_of_headers?

T

T

F

Automatic Data Transfer

Adequate remote control and transfer of data, in real time, requires:

1. Wireless connections with “unlimited” range;

2. High data transfer rates;

3. Programmable and autonomous communication units;3. Programmable and autonomous communication units;

4. Static IP addresses;

5. “Closed” and secure private network.

Cellular Network

Internet

Automatic sending of data is carried with the following concerns:

1. Minimize the volume of data with compact file format (binary or .zip files);

2. Avoid crushing of data: creation of temporary file and folder systems with

reference to date and time;

Automatic Data Transfer

3. Temporary file deletion only after the “transfer OK” sign from the server;

4. Maintain backup copies of data, in a compact format, on site.

Alarm Triggering

Alarms based on real time data acquisition

Garanteeing the monitoring system’s functioning

Change the monitoring system’s behavior during it’s execution

Early warning accident occurrence

Event

- Alarm triggering based on a modification of the Schmitt trigger comparator circuit;

- Setting of conditions “AND” or “OR” to include groups of data measurements;

Upper trigger

Low threshold

High threshold

10

Lower trigger10

Low threshold

High threshold

Application to Existing Structures

Application to Existing Structures

São João Bridge:- Framed bridge;- 250m central span;- 7 datalogger network;- 242 measuring devices;

Salgueiro Maia Bridge:- Cable Stayed Bridge;

- 242 measuring devices;

New Railway Crossingover Sado:- Bow String bridge;- Three 160m spans;- 5 datalogger network;- 250 measuring devices;

- Cable Stayed Bridge;- 250m span;- 10 datalogger network;- 243 measuring devices;

International Bridge overGuadiana:- Cable Stayed Bridge;- 324m span;- 4 datalogger network;- 85 measuring devices;

Application to Existing Structures

Bridge over Kwanza:- Framed bridge;- 120m central span;- 3 datalogger network;- 128 measuring devices;- 128 measuring devices;

Bridge over Catumbela:- Cable Stayed bridge;- 160m central span;- 4 datalogger network;- 70 measuring devices;

- The monitoring system developed in LNEC carries out the tasks for which it was

idealized: automatic acquisition, storage, long distance data transfer, real time processing

and alarm triggering.

- The system is installed and running flawlessly, up to the moment, in four bridges.

Concluding Remarks

- The system is installed and running flawlessly, up to the moment, in four bridges.

- The system’s architecture appears to be adequate for structures and infrastructures

monitoring systems whose purpose is to control safety and maintenance in real time.

- So far, the analysis of the acquired data suggests that the developed system is

successful in removing outliers.

MONITORING OF ROAD STRUCTURES REAL TIME ACQUISITION AND

CONTROL OF DATA