The purpose of a data logger or data acquisition system is to acquire
information about a particular system in order to test, characterize
or confirm the operation of the system. The wide variety of tests performed
is vast requiring a similar number of solutions. Understanding the final
results desired is the only way to determine the best solution for obtaining
those results. There is usually a number of good solutions for each
application with selection of the “best” system dependent
upon the individual requirements. The following is a brief guide to
assist you with the task.
WHAT ARE THE RESULTS
YOU WISH TO ACQUIRE?
The first step in the selection of a data logging system is to understand
the required results. You should have a good understanding of the minimal
amount of critical data required in order to accomplish the decision
task at hand. A list of the “must have” data vs. the “nice-to-have”
data will establish the priorities. Data overload is costly and typically
unnecessary but frequently occurs when surplus data is availability.
Determine the number and types of signal types and the period of time
over which the recording will take place. This will result in a count
for the total number of samples which will help to determine amount
of memory required to store the resulting data. Then think about all
those numbers and ask yourself if they are all necessary. Some may not
Probably the most important need to understand is the speed at which
you must acquire the information and the accuracy of the measurements
taken. The type of input and rate of change of that input will determine
the acquisition speed. Room temperatures change much more slowly than
the load on a shock absorber. You may think that the acquisition speed
is self evident but another review of the system under test as well
as the instrumentation may reveal alternative methods of analysis resulting
in less data to sort through, resulting in a subsequent savings in time
Assessment of other instrument functionality is another step in the
decision process. Many new systems offer an array of features that,
at first glance, appear mandatory. A review of the process and goal
will provide an understanding of the necessary added features such as
expanded memory, multiplexing channels, isolation of inputs, communication
options and alarming features. Buy what you think you will need within
the next two years.
WHAT ARE YOUR
Now that we understand what we need to measure, the next step is to
understand the constraints under which we will be operating. These constraints
could be self-imposed: deadlines, test environment, available power
sources, persons analyzing the data, persons operating the equipment,
company-imposed: budget, deadline, authorized supplier, communications
methods, government-imposed: FDA, EPA, OSHA or other regulatory agencies
requiring data integrity or security. Know what your constraints are
prior to looking for a solution as this will limit the field and expedite
AND FUNCTIONAL DIFFERENCES
A quick run-down of the available solutions includes: portable stand-alone
data loggers, PC hosted data acquisition boards/systems, chart recorders;
paper and electronic, SCADA systems and Portable stand-alone data loggers:
Price range - Under $100 to thousands. This category is defined as instruments
designed for low power consumption in order to operate from batteries
and which have internal memory for data storage. These are stand-alone
data recorders that operate independent of a any PC, but will require
a PC to either program or analyze the stored data. They are intended
for long-term unattended data collection.
Portable Stand-alone Data Loggers: Systems in this
category are typically more rugged and able to withstand more abuse,
both environment and shock and vibration, due to the demands of remote
and plant floor applications. Operating temperatures and environmental
tolerance are greater within this product category. Logic Beach Incorporated
is a well known manufacturer of this type of data logging system.
Low–cost units in the category typically are designed for a specific
input type with low channel count. One to four channels with dedicated
temperature, humidity or contact closure channels is the norm. Factory
options include voltage, current or RTD inputs. A range of samples rates
are preselected from the factory and programming flexibility is limited.
Accuracy is 8-12 bits and acquisition speeds are in the seconds to minutes
range. They are typically considered disposable and are very likely
task specific. You will receive a device limited to the measurement
of temperature, relative humidity, DC voltage or current, or event recording.
Don’t look for accuracy or a lot of flexibility here. They are
great for the right application.
Mid-priced systems offer more flexibility, input channels, accuracy
and programming flexibility. Accuracy will be 12 to 18 bit with available
faster speeds in the sub-second area. This is the first area where flexibility
is a product feature. Expect to be able to configure input channels
for different signal types and levels. Additional features, include
communication options of modem, RF, Cellular and Ethernet are possible.
While mid-priced units offer more means for communication they are still
stand-alone instruments and capable of conditional and intelligent logging
scenarios. Alarm functionality is greatly expanded as well.
High-priced units will offer faster sample rates slightly higher accuracy
(18+ bit) and higher channel count. These units may also be more ruggedized
for a specific application, for example, automotive or extreme temperature.
The new breed of data loggers built into a multimeters fall into the
portable category as well. Increased flexibility comes from the addition
of the data logger to a multimeter, making for a good digital multimeter
and a limited functionality data logger. Memory, programmability and
flexibility are the limiting factors of this segment within the low
Hosted: This segment of data acquisition instruments is used
in combination with a data acquisition cards or data collection nodes
and specialized software. The simpler system will consist of a data
acquisition card inserted into a PC. The card will be selected for the
required types and number of inputs which are connected to the PC card.
Application software is loaded and run on the computer allowing the
operator to create programs controlling the data collection. Additional
data acquisition cards may be added for increased channel count and
Another computer hosted system makes use of computer communication protocols.
Signal conditioning nodes can be placed on an Ethernet, RS-232, USB
or RS-485 bus into which the signals are fed and conditioned. The data
may remain in memory in the remote signal conditioning node for the
host computer to retrieve later at a scheduled time or the data can
be sent real-time to the host PC for storage. Sample rates will determine
the best method for collection.
These systems are found in the lab or on a plant floor and are typically
faster than portable instruments due to the processing power available
from the PC. That also means they are more potentially more fragile,
susceptible to environmental conditions and data backbone issues, and
power hungry. Systems that rely on laptop based units will also find
battery life a constraint.
Chart Recorders: Chart recorders can be divided into
two segments. The very common circular chart record that uses pen on
paper to record information. These are slowly being replaced by electronic
methods although there is still a continued legacy requirement. The
more current method is the videographic method with utilizes a video
display for presentation of the data, typically in a horizontal manner
as compared to a circular. Data is now stored electronically in memory,
or on a disc or PCMCIA card and either reviewed/analyzed on the display
or downloaded into a computer.
These systems vary widely in capabilities but have in common a graphic
display. Displays can vary from 3 in. x 1.5 in. monochromatic to 10–inch
circular color displays designed to mimic the paper chart recorders
they are trying to replace. Similar to portable systems chart recorders
are available in many sizes with a varying number of inputs and acquisition
speeds. The more you spend the faster you can go, or the more channels
you can monitor. Application for these products is typically a permanent
installation where real-time or near real-time display is important
to have immediately available at the location. These units are not battery
operated due to the demand for power from the display.
SCADA Systems: Supervisory Control and Data Acquisition:
A SCADA system can be defined as an industrial measurement and control
system which consists of three important components. The first component
is the central host (usually called a master station, master terminal
unit or MTU). The second part of the system includes one or more remote
devices call remote stations, remote terminal units or RTUs that gather
field data. The third and final part of the system is software, either
a standard and/or custom program designed to monitor and control remotely
located field data elements. SCADA systems are typically custom designed
for a specific facility and the functionality of that facility. Installation
and programming are a major component of a SCADA system. These systems
are designed to monitor and facilitate the operation of a, typically,
very large system. They are discussed here as a possible alternative
method for collecting data. They are not portable and generally very
difficult to modify for simple tests since once a system is up and running
a manufacturing facility there is reluctance to modify it.
Exotic Systems: These are custom made systems specifically
for unique application. From the ultra- fast system to a worldwide distributed
system these applications are not solved by off-the-shelf hardware and
software. Expensive and complex they are designed to do a single job.
In summary, there
are a wide variety of data logging instruments available to an individual
for use in collecting data. Price, application and desired results will
drive the product selection. Awareness of viable solutions is now much
easier with the use of the Internet and the range of data logging solutions
now available should allow the user to find an product that meets most
of their needs.
Logic Beach Inc.
designs, manufactures and supplies portable data logging and acquisition
systems to a wide variety of industries and users. The entire range
of portable data loggers operate with Logic Beach HyperWare™ configuration
software, providing the industry’s most powerful programming capability
with an intuitive graphical interface. Specializing in ruggedized, programmable
systems, Logic Beach Inc. offers a variety of modular data logging solutions
for system integration to stand alone applications.