About
A
data logger (or datalogger) is an electronic instrument that records data over
time or in relation to location.
Increasingly, but not necessarily, they are based on a digital processor
(or computer). They may be small, battery powered and portable and vary between
general purpose types for a range of measurement applications to very specific
devices for measuring in one environment only.It is common for general purpose
types to beprogrammable. Standardisation of protocols and data formats is
growing in the industry and XML is increasingly being adopted for data
exchange. The development of the Semantic Web is likely to accelerate this
trend. A smart protocol, SDI-12, exists that allows some instrumentation to be
connected to a variety of data loggers. The use of this standard has not gained
much acceptance outside the environmental industry.
The Extensible Markup
Language
XML
is a text-based markup language that is fast becoming the standard for data
interchange on the Web. As with HTML, you identify data using tags (identifiers
enclosed in angle brackets, like this: <...>). Collectively, the tags are
known as "markup". But unlike HTML, XML tags identify the data,
rather than specifying how to display it. Where an HTML tag says something like
"display this data in bold font" (<b>...</b>), an XML tag
acts like a field name in your program. It puts a label on a piece of data that
identifies it .
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Data Logging Versus Data Acquisition
The
terms data logging and data acquisition are often used interchangeably.
However, in a historical context they are quite different. A data logger is a
data acquisition system, but a data acquisition system is not necessarily a
data logger.
Data loggers are implicitly stand-alone
devices, while typical data acquisition system must remain tethered to a
computer to acquire data. This stand-alone aspect of data loggers implies
on-board memory that is used to store acquired data. Sometimes this memory is
very large to accommodate many days, or even months, of unattended recording.
This memory may be battery-backed static random access memory, flash memory or
EEPROM.
About The Modbus
Protocol
Modbus
is a serial communications protocol published by Modicon in 1979 for use with
its programmable logic controllers (PLCs). It is has become a de facto standard
communications protocol in industry, and is now the most commonly available
means of connecting industrial electronic devices.
About The Sdi-12
Protocol
SDI-12
stands for serial data interface at 1200 baud. It is a standard to interface
battery powered data recorders with micro-processor based sensors designed for
environmental data acquisition (EDA).
Usage Of
Micro-Processor Based Sensors
A
micro-processor in the sensor may calibrate the sensor, control sensor
measurements, and convert raw sensor readings into engineering units. The
micro-processor also controls the SDI-12 interface. It accepts and decodes
instructions received from the data recorder, starts the measurements, controls
all timing, and uses the SDI-12 protocol to communicate with the data recorder.
Online Analysis
This
step includes any analysis you would like to do before storing the data. A
common example of this is converting the voltage measurement to meaningful
scientific units, such as degrees Celsius. You can complete these complex
calculations and data compression before logging the data. Controlling part of
a system based on current measurements -- for example, a kill switch -- is also
part of online analysis.
Conclusion
Data
Loggers are changing more rapidly now than ever before. The original model of a
stand alone data logger is changing to one of a device that collects data but
also has access to wireless communications for alarming of events and automatic
reporting of data. Dataloggers are beginning to serve web pages for current
readings, email their alarms and FTP their daily results into databases or
direct to the users. Technically
speaking, a data logger is any device that can be used to store data.
