What DAQ System Should We Use?

April 21, 2003
Readers help a reader solve this control problem. Next month: How can we measure load profiles?

A Reader Writes:

We are adding instrumentation to a number pieces of standalone equipment. Recorders, programmable digital multimeters, and PCs with DAQ cards have each been proposed. How should we decide? What are the fundamental differences in the capabilities of these types of systems? Are there other approaches we should consider?

--from February 2003 CONTROL


It Depends

The answer is basically "it depends!" There really needs to be more information before valid selection can be made. For instance:

1. How much data is going to be recorded or is it going to forward the data somewhere immediately?

2. Do these devices need to be Internet/Ethernet capable?

3. How fast is the data entering the device and at what resolution?

4. What is the format of the data - ASCII, binary, analog, etc?

5. Are there any size and/or cost constraints?

6. How many data sources does a single device have to read?

7. Do the devices require an HMI of any kind?

...There are probably more questions but these are a few that come to mind immediately.

Larry Cicchinelli, Tech Support Manager


Question Leads to Questions

The question leaves many other questions unanswered. To properly solve the problem you must also ask:

1. Where are the standalone devices located?

2. Are they to be connected together to one central analysis system?

3. How many inputs and what sensors are to be used?

4. How is the data to be analyzed?

5. What format are the results to be displayed?

6. What is the budget?

...In other words, define the project, then design or define the steps necessary for the end result. At each step you must evaluate the results desired against the cost and time to produce the results.

...A simple approach is to use our CF2 with a PDA. It can function as a recorder, a multimeter, oscilloscope, data logger and many others. It can be connected to a wired or wireless network to centralize collection of the data. It comes with virtual instrument software for collection and the resultant data can be evaluated using software customized to produce the desired results.

Jerry Davis, President

ADPI, www.adpi.com

Recorders Are Best

The best solution based on your application description would be a paperless data acquisition system (recorder). Although all would more than likely provide adequate data collection, the paperless recorder would integrate seamlessly into a standalone platform while giving the appearance of being part of the original equipment design. The end user would realize the added benefit of a local, color LCD display with multiple configuration formats as well as flexible storage media options including Secure Digital, 250 MB Zip, Compact Flash, 1.44 MB floppy, and PCMCIA. Furthermore, the built-in 10/100 Base-T network functionality provides a simple method of remotely monitoring and transferring all data points over a local area network (LAN), intranet, or the Internet using off-the-shelf software.

Don Hite, National Sales Manager

Thermo Electron Process Instruments, www.thermo.com/process

Be Aware of Your Needs

The task of selecting a DAQ system can be daunting. There are basically two areas to address: the hardware needed to collect the signals, and what to do with the data. You must decide if you want to just capture data and print it out or to archive and analyze the data and/or send it to other software programs. If just capturing and printing is acceptable, then chart recorders may be fine. While they are relatively easy to set up, they have their limitations - namely the display area, ease of adding points, and inability of doing comparative analysis. Other devices, such as digital multimeters, are even less useful. If archiving and analysis are desired, a PC-based system would be much more flexible and functional; this system would provide long-term storage of data and would lend itself to exporting data to Excel or other programs.

...If your decision is a PC-based system, the next area to focus on is the hardware needed to collect the data. If your machine has a PLC, this would be the most proven and stable factory floor solution for the data inputs. Perhaps some of the data points may already be brought into the PLC but if not, adding a point is usually very easy and virtually any signal can be brought in. The downside of this is requiring someone with programming experience to configure these new inputs.

...If you do not have a PLC then a good solution would be to use Ethernet I/O modules. These are a good choice since no control is being performed. They are designed for the factory floor environment and can be easily added as your requirements grow. A simple Ethernet cable back to your computer is all that is needed. Other options such as DAQ cards in computers can be used, but are limited if more points are needed and can be cumbersome for wiring. Another advantage of using either PLC or Ethernet modules and a PC-based system is the ability to network the data points from your equipment back to a central data collection source. It also allows easy expansion.

...Next you can turn your attention to selecting one of the many software packages that are available. There are many software packages that can be used however attention must be given as some packages are expensive and require the user to have knowledge of communications protocols as well as perform their own configuration or hire an integrator to do this for them. Despite this, many have found that a PC-based system offers the most flexibility for storage, analysis of data and ease of adding points to their system.

...Factory Information Technologies offers a product called FIN (Factory Intelligence Network), that provides real time and historical trending, long-term data storage, easy exporting of data to Excel, powerful report features, and true client-server operation, and comes completely configured so it works out of the box. The FIN data collection package can collect data from existing PLCs or Ethernet I/O modules.

Ron Iannacone, President

Factory Information Technologies, www.factoryintelligence.com

Consider Technical Requirements First

There is a wide variety of methodologies available for data acquisition today, and there are is an equally wide range of applications that need to be satisfied. Choosing the most suitable system is, to some extent, predicated by the type and quantity of data to be recorded, but there are far more aspects that should influence the choice. This article is a very brief, broad-brush overview considering just three technologies: instrumentation recorders, programmable digital voltmeters, and PCs with DAQ cards.

...There are many applications where all three of the above technologies will provide successful data acquisition, and in such cases, the choice will probably come down to cost and personal preference. In applications where this is not so, the first step is to establish a concise specification about the data to be acquired, including details of where that data is physically being obtained from, where the acquired data is intended to be finally located and what analysis or other post-acquisition operations must be performed before the end product is supplied to users. Choosing the most appropriate data acquisition device requires consideration of the whole system, from obtaining data from sensors through providing processed results to users in their requested format.

...There are many parameters that will affect choice, the most important being technical capability. The highest bandwidth analog signal and the highest bit rate digital input need to be noted. Also the aggregate bandwidth (i.e. the sum of all the input signal bandwidths and bit rates) should be calculated. The mission-minimum duration must be defined, with this in turn yielding the size of storage media required for the mission [aggregate data rate multiplied by mission duration]. The operating environment should be defined, including issues such as portability, mobility, and extremes of temperature, humidity, pressure, vibration, shock, and EMC/RFI.

...Programmable voltmeters operate at very low bandwidths and can only be used for slowly changing signals. Typically, they will sample the input channels sequentially. In some circumstances, this will create problems with the time alignment of data from different channels, although this is less likely to be a concern for low bandwidth data. The signal type is more or less only one type - voltage. There are a wide variety of methods for transferring data from programmable voltmeters to analysis platforms; potential users should ensure that the method is appropriate to their needs and is accompanied by desirable analysis and display tools. Methods for distribution of the data to several users need to be considered. These devices can be portable and suitable for mobile use, though environmental capability may be limited.

...PC DAQs can acquire data over a range of rates, from a few Hertz to megahertz, and can handle from one to many channels. Some DAQs share a single analog-to-digital converter [ADC] between several channels; this will create time misalignment between channel data. If phase relationships are an issue, ensure that the channel interfaces contain one ADC per channel and the sampling clock is synchronized across all ADCs.

...A wide variety of DAQs interface to a variety of signal types like analog voltage, bridges, gauges, logic levels, and others. PC-based systems are generally not suitable for use in non-benign environments outside of typical laboratory use, although certain manufacturers provide rugged and/or portable systems. PC-based DAQs are frequently supplied with and closely coupled to comprehensive analysis and visualization tools. PC-based systems rely on the robustness of the PC operating system for reliability of acquisition, this sometimes being problematic if the acquisition application is run concurrently with other applications. The acquired data will typically end up on a hard drive, and the user will have to make arrangements to copy data to tape or use network file transfers if the data is to be distributed among several users/customers.

...Instrumentation recorders like those manufactured by Heim Data Systems are purpose-built platforms for data acquisition and, as such, provide the widest selection of signal input types and the widest range of bandwidth (up to many megahertz), typica lly have the most economical solution for high aggregate bandwidth applications, and have standard solutions for non-benign environments. The signal sampling systems are designed to ensure accurate timing of data both between channels and in absolute time. They appear in laboratory, portable, mobile, and airborne packages. Their purpose-designed operating systems are optimized for robustness, speed, and user operation.

...Recorders can use many different types of media: tape is a low-cost media that can be removed from the data acquisition system for easy transfer from acquisition site to analysis platform and later distribution. Hard drives provide an economical solution for high aggregate data rate, long-duration applications. Solid-state Flash memory drives provide the most robust solution for non-benign environment operation. Although the price of flash memory continues to come down, it is presently much more expensive than tape and hard drive solutions.

...Other media are available. Except for tape, many of the media are not removable and the user should consider if and how they meet operating requirements. Heim recorders can provide all the media drives in removable cartridges, thus converting all media types to "removable" and providing an enhanced ability to support the media and exchange it for other types to accommodate specific mission requirements. Recorders tend to rely on third-party applications for display and analysis, particularly for more complex analyses. To this end, they will be fitted with high-speed interfaces [such as SCSI or fiber channel] to transfer the data from recorded media to analysis platform in a format compatible with analysis and display applications.

...This a very brief comparison and cannot cover the many facets of three data acquisition technologies that provide significant overlap in technical performance, environmental capability, user facilities and cost. In general programmable voltmeters will be suitable for simple low-bandwidth applications; PCs with DAQs will be suitable for medium-bandwidth, medium-channel-count applications in benign environments; and instrumentation recorders are most suited to high-bandwidth and non-benign environment applications.

Richard Bond, National Sales Manager

Heim Data Systems

PLC/Ethernet/HMI Is an Easy Answer

Many industrial plants are moving toward new communication protocols such as Ethernet because they allow them to store machine-monitoring data on the corporate network, then access and display the information anywhere. Object linking and embedding for process control (OPC) is the most reliable way to move this data over Ethernet, and the most common method of moving this type of data between Windows programs. HMI software with OPC Client capability is typically used to store and display data from the PLC.

...PLCs are rugged, reliable, and available in a wide range of prices. They provide more functionality than recorders and programmable digital multimeters. They are also more reliable than DAQ cards in PCs, are better understood by plant technicians, and are more likely to receive extended manufacturer support.

...A wide range of HMI programs are available for machine monitoring, ranging from costly plant automation packages to simple operator interface software, such as our Infilink HMI software. Available at about one-third the cost of a more complicated HMI program, this type of software incorporates standard HMI features, such as trending, datalogging, and easy-to-use setup screens, and is widely used in process monitoring, HVAC monitoring, and by machine manufacturers to access and display data.

...The Ethernet/HMI solution is a practical and easy-to-implement approach and consists of the following steps:

1. Data collection: Determine which signals need to be monitored, i.e. 4-20 mA analog inputs, RTD temperature inputs, and 24 VDC discrete inputs. Select a PLC that can handle these inputs, and ensure that the PLC has an Ethernet port. Connect the PLC to the corporate Ethernet network.

2. Data storage: Purchase an OPC Server program that includes a driver for your PLC's Ethernet port, and install an HMI software program that integrates data logging, trending, and OPC Client capability.

3. Computer: The OPC server and HMI software runs on a dedicated PC. If the PC is to be used on the plant floor, you should consider an industrial panel PC with touchscreen. If not, a commercial PC and monitor are acceptable.

4. Put it together: Wire the instrument inputs to the PLC. Load a program into the PLC to define the inputs and configure the Ethernet port. Configure the OPC server and HMI software to connect to the PLC to store and display the data.

Ron Dawson, Operator Interface Product Manager

Kessler Ellis Products, www.kep.com

Answers Lead to Answers

There are several questions that readers should ask themselves before deciding which products they are interested in. The answers to these questions will help them cut down the large number of choices out there.

1. What data do I need to collect? Do I need to collect single discrete points, do I want dynamic data taken over time for averages, maximums, or minimums? Do I want profiling with post processing do I also need attribute information? This is a big question. If there is more than one set of data that you would like to collect, you may need more that one type of data acquisition. The number of different methods for collecting data is very large. They include LVDTs, temperature sensors, ultrasonic, vision systems, laser, digital hand tools, pressure and motion sensors and many more. The data could be continuous, in groups, as single discrete datapoints, or subjective. The data may be collected into a computer system using a card plugged into the PCI or ISA bus, a USB device, serially, a network device, or others. Each of these scenarios has its positive and negative points, but a few rules of thumb are shown in the accompanying table.

2. Are there data collection time constraints? Do I need to read the data quickly when I get the trigger? Do I need to associate the location of the reading with where or when the reading was taken from the process? Unfortunately, very fast reaction time can only truly be realized electronically. A trigger is made directly to the device collecting the data. This method is a factor of 10 faster than running a trigger through a PC application to collect data. Make sure that the device you are looking for has a triggering mechanism, such as a foot switch port, mounted directly on it if you need very fast reaction times.

3. Will closed-loop actions need to be taken from the processed data? Closed loop on a DAQ system usually implies that additional processing of the data will be needed to feed it back to a device managing a process. Make sure that the software and interfacing is available on your DAQ system to allow closed-loop communications. Usually the logic includes some Event/Action capability, plus some communication capability would be needed (usually serial, but I/O plus network is good, too). Be aware that proprietary bus communications are not supported by many of the devices you may want to control.

4. What am I going to do with the actual data after it is taken? Is it for control, pass/fail, analysis, distributed analysis, or all of the above? Do I need SPC? If this data is selected only for control, storage and analysis tools won't be needed. Often PLC or PC-based PLC systems are good for this type of DAQ usage. If the data is to be analyzed to get a handle on a process, consider a good math or real-time SPC package. But be aware, many analysis packages do not do data acquisition, control, and analysis. If these things interest you, they all can be had in one package.

5. How much data do I need to collect and what do I need to retain? Who will need to access it? If the collected data needs to be retained and shared with many people, consider storage in a central database on the company network. You may find that getting a very standardized system is the best bet. A standard SQL server on a standard PC network is easy to maintain and is usually cheaper.

6. What is the cost-to-benefit ratio confining the project? Basically, figure out what your budget is. Take into account the training and expertise needed to configure and maintain the system.

7. How difficult can this be to setup and maintain? Does our company have the expertise to configure or maintain this, or will we need to outsource? There are many good product out there that are very reliable, reasonably priced and have good features. A good thing to also take into account is how easy it is to set up and maintain. As an example, a USB device takes basically no setup time, it may be plugged in and unplugged at will. No protocols or baud rates need to be set up, it just works.

8. Would a piece-by-piece configuration work or are we better off with a single supplier and a turn-key package? Many companies sell components piece-by-piece for data acquisition and some provide all the components plus the configuration and installation. Determine what make the most sense in your case.

...After these questions are answered as well as possible, the road to a decision on which products are the best fit becomes much more obvious. Kurt Electronics sells multiple hardware and software packages for data acquisition, control, and distributed analysis, both real-time and post-process. We sell components or completely configured systems as specified by the customer. Products include KurtSPC, KurtUSB, the KurtICS, MagNUM CMMs, and the KurtCheck systems.

Jon Baller, Software Engineering Manager

Kurt Electronics, www.kurtelectronics.com

Define Must-Have Data

The first step in selecting a data logging system is to understand the required results. You should have a good understanding of the minimal amount of critical data required 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 available.

...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 is 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, saving 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.

...The next step is to understand the operating constraints. Self-imposed constraints include deadlines, test environment, available power sources, persons analyzing the data, and persons operating the equipment. Company-imposed constraints include budget, deadline, authorized supplier, communications methods, and government-imposed: FDA, EPA, OSHA, or other regulations 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.

...Third, consider available solutions and their functional differences. A quick run-down of the available solutions includes portable standalone data loggers, PC-hosted data acquisition boards/systems, chart recorders (paper and electronic), SCADA systems, and portable standalone data loggers.

...Low-cost options include instruments designed for low power consumption to operate from batteries and which have internal memory for data storage. These are standalone data recorders that operate independent of a PC, but will require a PC to either program or analyze the stored data. They are intended for long-term unattended data collection.

...Systems in this category are typically more rugged and able to withstand more abuse, both environment and shock and vibration, for the demands of remote and plant floor applications. Operating temperature ranges and environmental tolerances are wide. Low-cost units 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, i.e. temperature, relative humidity, DC voltage or current, or event recording. Don't look for accuracy or a lot of flexibility, but they are great for the right application.

...Mid-priced systems offer more flexibility, input channels, accuracy and programming flexibility. Accuracy will be 12-18 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 standalone 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-plus bit), and higher channel count. These units may also be more ruggedized for a specific application, for example, automotive or extreme temperatures. The new breed of data loggers built into multimeters fall into this category as well. Increased flexibility comes from the addition of the data logger to a multimeter, making 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 data acquisition instruments are used in combination with 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 that are connected to the PC card. Application software is loaded and run on the computer, allowing the operator to create programs controlling 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 it can be sent in 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 can be divided into two segments. The very common circular chart recorder 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 videographic, which uses a video display for presentation of the data, typically in a horizontal manner instead of circular. Data is now stored electronically in memory, on a disc, or in a 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 3x1.5 in. monochromatic to 10-in. 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 at the location. These units are not battery-operated due to the demand for power from the display.

...Supervisory control and data acquisition (SCADA) systems consist of three important components: the central host, usually called a master station or master terminal unit (MTU); one or more remote devices called remote stations or remote terminal units (RTUs) that gather field data; and software, either a standard 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, so once a system is up and running in a manufacturing facility there is reluctance to modify it.

...Finally, there are exotic systems: custom systems made specifically for a 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.

Mark Albert, Sales & Marketing Mgr.

Logic Beach

PC-Based Systems High on Flexibility

Programmable digital multimeters are inherently different than recorders and PC DAQ systems. DMMs usually get better resolution than the alternatives, but have limited acquisition rates and include only one input channel. Expanding your channel count requires front-end switching, which further limits your acquisition rates. For low-speed measurements in limited channel-count systems, DMMs are a good fit. To measure more than a couple of channels or to achieve sampling rates higher than 1,000 samples per second requires a recorder or PC DAQ system.

...Even though built-in processing and data storage have made recorders more powerful, it is still intrinsically less expensive to leverage the state-of-the-art processors and hard drives available in today's PCs. To accomplish this, some recorders support high-speed data transfer across a USB or Ethernet bus. The limitation of recorders now is not speed, but functionality. The fact that recorders have signal conditioning, digitizers, processors, memory chips, and displays all in a single box usually means that they are either too much or too little for any one application. For example, expanding the channel count or adding a measurement type not supported by the recorder requires adding an additional system. On the other hand, if you are using the PC to analyze and display your data, the built-in processors and displays on a recorder are overkill.

...The most versatile and cost-effective way to acquire and log data is to use a PC-based data acquisition system. Plug-in DAQ vendors such as National Instruments offer boards starting at $395, with models capable of sampling up to 800 kS/s at 16-bit resolution. The true benefit of the PC-based DAQ system is the flexibility that it provides. You can configure each piece of your system, from the speed and disk space of the PC to the signal conditioning for your specific measurements. This unique capability allows you to select the functionality you need to exactly meet your application requirements.

...Another often overlooked benefit of PC-based systems is the ease of expanding or upgrading the system. To expand a DMM or recorder requires buying an additional instrument (duplicating your system). Modular signal conditioning, such as our SCXI, allows you to increase your channel count or add a measuremen

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