Data Acquisition Instrumentation

GUIDELINES FOR DATA LOGGER SELECTION

OVERVIEW

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 be.

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 and money.

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 CONSTRAINTS?

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 your decision.

AVAILABLE SOLUTIONS 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 cost range.

Computer 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 functionality.

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.

 

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