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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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

Solutions:

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

Z-World

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.

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