What DAQ System Should We Use?

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

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...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.

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