Monitoring Compressor Performance Using NI Data Acquisition Boards

Author(s):
Jason Swoboda - Quantum Controls, Inc

Industry:
Automotive

Products:
Compact FieldPoint, High-Precision DAQ, LabVIEW, PXI/CompactPCI, FieldPoint

The Challenge:
Designing a gas compressor data acquisition system capable of synchronizing samples from multiple sensors to the rotational position of the pistons’ crankshaft and performing measurements from field mounted meters and thermocouples.

The Solution:
Combining high-speed data acquisition and signal conditioning to precisely measure of compressor performance, using a FieldPoint network to acquire data from the thermocouples, pressure meters, and flow meters located throughout the gas loop and LabVIEW to process the data, calculate performance results, log data, and generate reports.

Introduction
Using a PC, we acquire data using the National Instruments PCI-6023. The data acquisition sequence includes a data collection process, followed by generation of a pressure-volume (PV) curve, and calculation of vibration analysis information. After we collect the data, it processes immediately and displays the results on the PC screen.

The sequence of acquiring compressor pressure data is dependent on an analog trigger and an external scan clock - based on an optical encoder. The optical encoder returns 1440 pulses per revolution. This results in a resolution of one data point for every degree of revolution. The system records 1440 data points from each of the desired number of cycles to sample the data collection. The built-in LabVIEW data acquisition tools helped eliminate a complex timing and triggering problem.

User Configuration
After the user logs in, a list of five prerequisite safety items appears with blank check boxes accompanying each of them. We must check each box and press continue to proceed to the data acquisition program. The final item of the list of prerequisite safety items is a message confirming which type of compressor the user selected. The program will continue on to the data acquisition portion of the program after the user checks all of the prerequisite safety boxes and presses the confirm button.

We can use multiple sensors during data acquisition, dependent on the range of the unit under test (UUT). As a result, the system requires input of the physical specifications of the compressor test frame before a test can begin. The system presents a list of pressure transducer serial numbers alongside each of the pressure data acquisition locations. Next, we select the serial number of the pressure transducer at each of those locations.
Once we select a serial number, the system displays the operating range of the selected transducer. Each pressure transducer has a configuration file that contains the transducer serial number, last calibration date, operating range, offset, and gain.

After we select and confirm the serial numbers of all pressure transducers, we have the option to calibrate the transducers. We employed a two-point, linear calibration method. If the user elects not to calibrate the transducers, the program will skip the calibration routine and use the current gain and offset values found in the configuration file.

With the new configuration system, we can easily switch sensors from one test to another. This represents a marked improvement, since the documentation of the sensors and their calibration information is built into the test system.

Dramatically Reducing Development Time
We selected LabVIEW software to perform the processing of average torque, displacement, volumetric efficiency, flow, rod load, and vibration. In addition to analysis resident in LabVIEW, ActiveX control tools access external analysis and reporting programs that were already in use. The ActiveX tools send data to the program and feed results back to LabVIEW. As a result of LabVIEW and the ActiveX tools, we saved two months of development time through the reuse of existing analysis algorithms.

Alarming
When the system detects an alarm situation the alarm digital output signal is set to the true state. This alerts the control panel of an alarm situation. Alarm conditions include vibration over predefined limit, bearing temperature over predefined limit, rod load over predefined limit, and oxygen content over predefined limit.

Test Data Logging
The log file, in ASCII format, contains a header that reports the current date, current time, and the user entered physical specifications for the type of compressor selected. Below the header is a section containing the description, gain, and offset for each of the pressure transducers used in the test, which is taken directly from the ini file where the system stores that information. The purpose of log file is to provide longterm trending and analysis information. We did not intend for it to provide detailed information at any particular time in the system. It may, however, provide insight to the general operations and performance of the compressor. The data in the file includes all current sensor readings and values displayed on the screen at a predetermined rate.

A second file generated is the main data file. We can log data to this file by pressing the "log data to file" button. Data includes the most current sensor readings, display values, raw data used to create the PV diagram, and vibration analysis charts. We use this most often for post-test report generation.
A short-term circular buffer file exists, which allows us to assess the operation of the compressor prior to a failure or shut down situation. The system stores data to this file once every minute. The file only retains the last 30 minutes of data - with the most recent data replacing the oldest data.

Test Reports
Once we acquire data from the compressor system, we can press the "test report" button to generate a report file. The header information from the log file is found at the top of this report. The report file has a name composed of the test date, time, and log file number. Data in the report file includes discharge pressure, suction pressure, internal head end cylinder pressure, internal crank end cylinder pressure, torque meter data, and accelerometer data.

Conclusion
This system required acquisition of a variety of signal types across a broad range of acquisition speeds. The use of high-speed data acquisition and flexible field sensor monitoring was made easier by using multiple National Instruments data acquisition platforms.
We can now monitor data precisely acquired with respect to the system’s operations. Now we can automate and present calculations and report generation in an easy-to-use and clear format.

For more information, contact:

Jason Swoboda

Systems Integrator

Quantum Controls, Inc.

2500 Wilcrest, Suite 630

Houston, TX 77042

Tel: 713-355-3900

Fax: 713-339-1199

E-mail: swoboda@QAutomation.com

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