Traditional CEM DAS systems interface to the analysis instruments using a CEM data logger or a traditional PLC. How does CEMView acquire data from the analysis instrument and how is this different from a traditional CEM data logger?
CEMView connects directly to the analysis instrument using the serial RS-232/422/485 or Ethernet connection and an instrument specific OPC Server. It communicates directly with the analyzer to acquire the measured pollutant information. There is no analogue signal drift of the data values using this technique.
The CEM data logger or the traditional PLC interface with the analysis instrument's analogue current (4-20 mA) or voltage (0-10 VDC) signals that represent the measured pollutant. There is no way to eliminate the analogue signal drift of the data values inherent in using this technique.
Analogue signal drift of the analyzer output signal digital-to-analog (D/A) conversion process and the data logger input signal analog-to-digital (A/D) conversion process are an unavoidable fact-of-life. The bit resolution of the D/A and A/D converters also contribute directly to magnitude the signal drift. The lower the bit resolution, the higher the discrepancy, hence, expensive 16-bit resolution D/A and A/D converters should be used. Often with some inexpensive PLC models, 16-bit analog input modules are simply not available.
Additional benefits of the direct connection to the analyzer include advanced instrument diagnostic and parameterization capability that is simply not possible with the traditional legacy discrete and analog signal interconnect of traditional data logger and PLC based approaches.
But, the data logger or PLC is more robust than a PC.
A data logger or a PLC is effectively a single board computer (SBC) which is housed in an industrially hardened chassis or enclosure. If the data logger or PLC fails, it must be fixed or replaced.
Nexus Solutions offers more than just one choice as a solution to the needs for a robust CEM DAS. With CEMView, this need is balanced with the advantages of directly communicating with the instrument and the inherent benefits that arise in diagnosing and fixing problems with the analyzers of the CEM system. With a data logger or PLC, advanced instrument diagnostics are simply not available to the CEM DAS. Our solutions include:
The High Availability CEMView system is available for applications that require the CEMView DAS to be always available. The redundant system is more than just two computers running the CEMView application in parallel. As a redundant system, it is designed to handle numerous failure modes with automatic failover and recovery capability.
The CEMView Data Buffer Programmable Automation Controller (DB-PAC) is a single board computer (SBC) with a Windows XP Embedded operating system. This is effectively equivalent to a data logger or PLC and it communicates directly with the instrument.
Finally, the typical CEMView PC that Nexus Solutions strongly recommends is greatly enhanced to increase robustness. It contains a RAID 1 mirrored hard drive array mounted in vibration isolation drive bays. Solid state Compact Flash based solutions and highly vibration resistant micro-drive options are also available.
True, if the PC which CEMView is running on fails, then it must be fixed or replaced. But then again, if the data logger or PLC fails, it too must be fixed or replaced.