Data Acquisition and Transmission System 48

3.4.1 Configuration of DATS

Sensors generate analogue or digital signals that represent the physical parameters being monitored. Data acquisition is an intermediate device between the sensors and computers, which collects the signals generated by the sensors, converts them, and transmits the signals to the computers for processing. For a small laboratory based experiment, the above function can be achieved with a card based data acquisition unit in a PC. However, configuration of a data acquisition and transmission system (DATS) in a long-term bridge monitoring system is generally much more complicated. It usually consists of local cabling network, stand-alone data acquisition units (DAUs) or substations, and global cabling network, as illustrated in Figure 3.2. The local cabling network refers to the cables connecting the distributed sensors to the individual DAUs, and the global cabling network refers to the cables connecting the DAUs to central database servers.

Appropriate deployment of DAUs plays a significant role in assuring the quality and fidelity of the acquired data in long-span bridges. Long distance wires cause noise and significant loss in analogue signals (especially for voltage signals) because the distributed sensors are far from the central control office (usually hundreds or thousands of metres away). It is also inefficient to wire all sensors to one central server. Therefore, DAUs are assigned at a few cross-sections of the bridge to collect the signals from surrounding sensors, condition the signals, and transmit the digital data into the central database server.

It is noted that some proprietary sensors such as GPS, video cameras, fibre optic sensors, and corrosion sensors have their specific data acquisition systems. These united systems capture corresponding information, transform the information into digital data, and directly connect to the central data server for processing (Figure 3.2).

For such a system as illustrated in Figure 3.2, it is desirable to have a uniform platform to assure the scalability, functionality, and durability of the system.

Figure 3.2 Configuration of a data acquisition and transmission system

3.4.2 Hardware of DAUs

A DAU generally comprises a number of electronic components including signal conditioners, memory and data storage unit, microcontroller, communication device, uninterruptible power supply, fan/air conditioner, lightning conductor, and GPS time synchronizer. All these components are integrated in a waterproof, rugged enclosure or cabinet for the long-term monitoring purpose.

It is common that different types of sensors with different output signals and different sampling rates are included in one DAU. Consequently a DAU can facilitate this flexibility and may have more than one signal conditioner. A signal conditioner manipulates an analogue signal such that it meets the requirements of further processing. Signal conditioning usually includes amplification, filtering, analogue-to-digital (A/D) conversion, and isolation. Amplification serves to amplify the analogue signal before A/D conversion to utilize the full range of the A/D converter, thus increases the signal-to-noise ratio and resolution of the input signal. For example, acceleration in an ambient vibration may be very low and the output may be in milli-volt level. An amplifier can multiply the voltage signal to up to that required by the A/D converter (0−10 V, for instance). The amplifier is preferably placed near to the sensor, but sometimes this is difficult and it is integrated inside the DAU. Filtering is used to remove the unwanted frequency components, for example, alternating current (AC). Most signal conditioners

Sensor Sensor Sensor DAU 1 Sensor Sensor Sensor Sensor Sensor Sensor

United sensor system DAU 2

DAU n

United sensor system

Central database servers

employ low-pass filters. Isolation is used to isolate the possible ground loop and protect the hardware from damage. As the sampling rate in the measurement data of bridges is usually low, a single A/D converter can perform A/D conversion by switching between several channels (i.e., multiplexing). This is much less expensive than having a separate A/D converter for each channel, and thus adopted in most practical SHM systems.

The internal memory serves as a temporary buffer of data for transmission. It is usually integrated with the microcontroller. The data storage unit can save the measurement data relatively longer in case the global cabling network does not work appropriately. The data can be retrieved manually to the external storage devices or automatically to the database server when the global cabling network recovers.

The microcontroller consists of internal electronic circuitry to execute commands sent by the users and to control other hardware components. For example, the sampling rates and acquisition duration of the sensors can be changed by the users.

The communication device is responsible for communication between the DAU and the computer. Usually Ethernet interface is employed.

The power supply provides power to the data acquisition system and to some sensors that require external power supply. An uninterruptible power supply provides instantaneous or near-instantaneous protection from unexpected power interruption or unstable input voltage.

A fan or air conditioner is used to cool the temperature of the DAU. A lightning conductor can provide protection of the DAU from lightning damage.

Previously the DAUs were synchronised through a synchronisation signal sent from the central station regularly. Nowadays GPS time synchronisers have become more popular as they can provide an easy way to keep the DAUs and united sensor systems accurately synchronised.

3.4.3 Network and Communication

In the DATS, a uniform network communication is crucial to assure the data can be transmitted over the entire system. Various communication network technologies such as Ethernet, RS-232, RS-485, IEEE-1394 can be employed for the common network.

In an SHM system for a long-span bridge, the distance between the DAUs to the central control office may be as far as a few kilometres, and fibre optic cabling is desirable. In fibre optic communication, there are basically two types of fibre optic cables: single mode and multi-mode fibres. Multi-mode fibres generally have a larger core diameter, and are used for shorter distance communication. Single mode fibres are used for communication links longer than 600 metres, thus preferable for long-span bridges.

Wireless communication and networking have been rapidly developed and employed for data transmission, whereas its transmission speed and accuracy are still not comparable with the cable-based network at the moment. Nevertheless, the wireless network is promising in the near future. It has advantages in some situations, particularly for construction monitoring when the cable network is not ready.

3.4.4 Operation of Data Acquisition and Transmission

After the hardware has been installed, the DATS should be tested or verified through field tests, for example, controlled load tests. This is because the actual performance of the hardware is uncertain under long-term exposure to the harsh conditions. Moreover, it is difficult in practice to identify, repair, and change the damaged facilities after the bridge is put into service, which is usually operated by a different sector from the construction stage. The field verification can help identify problems in hardware, installation, cabling, and software, such that the problems can be fixed before normal operation.

During normal operation, data acquisition and transmission are carried out in a systematic and organised manner. Depending on the nature of the monitored parameters, some sensors may work continuously while others may work in the trigger mode, in which the sensor signals are collected only when the parameters are above certain threshold values, usually some events occur (for example, earthquakes or typhoons). These two modes can be operated simultaneously in one data acquisition system.

The sampling rate (or sampling frequency) is an important factor affecting the data acquisition speed. It relies on the variation speed of the monitored parameters and can be programmed by the users. For example, ambient temperature usually changes slowly with time and can be treated as a static measurand with a low sampling rate. Acceleration, on the other hand, varies more rapidly with time and is usually regarded as a dynamic parameter with a higher sampling rate. If a parameter is not sampled fast enough, which is known as under- sampling, the resulting digitised signal will not represent the actual signal accurately, and this error is called as aliasing error. To avoid this error, the Nyquist criterion requires that the sampling rate should be more than twice the highest frequency component of the original signal. In bridge monitoring exercises, the sampling rate of most signals is usually no higher than 100 Hz as the fundamental frequencies of long-span bridges are relatively low, unless some special measurements like acoustic methods or guided-wave methods are employed.

For the low sampling rate, multiplexed sampling, rather than simultaneous sampling can be employed. The multiplexed sampling allows different channels to share one A/D converter and to be sampled sequentially. This can reduce cost compared with the simultaneous sampling, in which each signal channel has an individual A/D converter.

After operation for a period in the adverse environment, DAUs are inevitably subject to error or malfunction. It is preferable to carry out periodical calibrations. As mentioned previously, the DAUs should be accessible for maintenance.