FIELD DATA LOGGER

Sigra's field data logger is a versatile device designed to monitor four types of sensor. These are vibrating wire, frequency, voltage and resistance.

The device has a short range radio link with a line of site range of approximately 500 m with a whip aerial (greater range can be acheived using a Yagi antenna). Each data logger may be used to gather data and as a transceiver. The radio link used in these loggers is a low power device that does not require any licence to use.

Each logger may be connected via an RS232 connection directly to a computer, modem or industrial cellphone. This gives the potential for data to be gathered into the telephone system. The loggers may also be paired to powerful radio transmitters to give greater radio range.

The use of radio communication affords significant ease of use. In its simplest form it saves the user from having to get physically to the logger to retrieve information. This may save climbing a hill, getting out of a car in rain or crossing a road. In this mode the user might sit in a car near the logger and simply download information into a laptop computer via a logger used as a transceiver and then drive on to the next site. This application is shown in drawing D7.

The next configuration is a fan pattern of loggers. Here a logger is used as a base station and is connected to the telephone network via a direct line or cell phone. This logger may be fitted with a whip or a directional Yagi antenna. It communicates with other loggers in the surrounding area. Such a system could be used slope monitoring as shown in drawing D25.

The third configuration is where a base station logger communicates with distant loggers in a daisy chain with intermediate loggers serving to relay information. This system slows rapidly with an increased number of loggers in the chain.

Combinations of configurations may include fan and daisy chaining. It should however be taken into consideration that whilst data is being transmitted sensors are not being logged; thus overly large networks become inefficient.

One of the advantages of the radio system is that loggers are not connected by cable and sensor cables can be short. This means that cable burial is not required. The systems susceptibility to lightning strike is also greatly diminished. While a strike will probably destroy directly hit loggers, the avoidance of cable interconnection will prevent the destruction of the entire monitoring system.

The loggers are intelligent. They can be programmed to decide whether to store data or not. This is based on setting a data change threshold. If the absolute difference between the data just read and the last stored value does not exceed a preset threshold then the data will not be stored. If the data does exceed the threshold then it will be stored. A "heartbeat" data gathering interval can be specified which ensures that a data reading is stored at least every specified interval. This is done to give the user confidence that the system is working in the event of no significant change in sensor values.

The approach of storing record only if the data changes saves downloading and data processing time. This is important in reducing radio transmission periods to transfer data.

The loggers have an on board capability to correct readings for transducer offset changes. Thus for example a vibrating wire transducer typically gives an output from the vibrating wire in terms of frequency and a resistance value from a thermistor. The thermistor resistance is converted within the logger to a temperature offset value. This value may then be used by the logger to calculate an engineering value from the frequency of the vibrating wire transducer.

It should be noted that while loggers may be programmed to display engineering units the actual data is stored in raw format. Thus if the incorrect calibration data is programmed into the logger this may be subsequently corrected and data loss avoided.

The loggers may also serve as warning devices. They can be used to examine whether a sensor value exceeds a threshold, exceeds a rate of change or a combination of these. It can be specified that these warning levels must be held for one to five readings before the warning is transmitted. This avoids isolated electrical spikes causing a warning to occur. The warning is either transmitted directly to a host computer or to the logger which is being used as a transceiver which then makes a telephone connection to a specified number to convey that a warning event has occurred.

This warning function is extremely valuable. It was conceived for landslide monitoring where a number of piezometric or movement sensors may be connected to loggers. In the event of groundwater level rise or movement acceleration a warning would be transmitted. This use could be adapted to monitor river level, pollution level or any other need.

A six channel quarter bridge resistance strain gauge interface is available for the field data logger.

The computer software used with the data loggers may operate under Microsoft Windows operating systems.