The Challenge:

To develop a GUI rich, user friendly, automated control system to calibrate and test high precision pressure transducers (up to 16 numbers at a time).

The Solution:

To build a configurable, upgradeable and flexible system using Lab VIEW 7.0, PCI DAQ Card and PCI GPIB Card to communicate to the various controlling instruments and implement the calibration and test system for high precision pressure transducers which minimizes test time.

Introduction:

The high precision pressure transducers to be tested are used under varying atmospheric conditions. It is required that the transducers parameters are consistent and within specified tolerance limits. The test system developed, analyses the behavior of the transducers under pressure varying and temperature varying conditions and checks if the obtained parameters are within the specified limits.

Application:

An initial screening test called the pre calibration test is done to check if the thickness of the fabricated sensor is within allowable limits. Pressure applied to the transducer is varied in predefined steps and the hysterisis shown by the transducer gives an indication of the transducer thickness. If the transducer does not pass this test, it is discarded.

Following the screening test, a Temperature Drift test is done to observe the drift in the transducer parameters as temperature changes. Depending on the results, appropriate compensation is provided.

Once compensation for drift is done, a final calibration test is done for a final check. Then a thermal soak test is done. Readings are taken at ambient temperature. The transducer is then subjected to temperature variation. The temperature is brought back to ambient conditions and readings are again taken. This checks the repeatability of the transducer.

Schematic Diagram

The above figure shows how each instrument is interfaced to the PC and the communication standard used.

Software Implementation:

This section can be broadly classified into four parts.

Channel Selection

The test system allows for the user to select the required channels. Against each channel the serial number of the transducer connected needs to be specified by the user to indicate which transducer is connected at each one of them. The test to be performed is also selected here.

Communication with interfaced instruments

The test system developed using NI integrated solutions communicates with the pressure controller and power supply using GPIB 488.2 communication standard and with the environment chamber using RS 232 standard. Communicating with these instruments include writing commands to set required values, to read back the value and check if set value has reached and is stabilized, to take appropriate actions. Data from multiple transducers is read and displayed to the user and is simultaneously logged into files for future reference. Once the test completes the software calculates the required parameters from the test data and creates a report file. The software also checks if the acquired and calculated values fall within the specified limits and decides if the transducer meets the requirements.

Data acquisition

At each pressure point data is acquired for all the configured transducers and tabulated on the front panel. After the entire data acquisition is over, calculation if various parameters like sensitivity, hysterisis, drift etc. is done. The decision as to whether the transducer passed or failed the test is taken based on the obtained values.

Report Generation

Each test has a specific report format. Separate reports are generated and saved for every transducer in the required format. The user can opt to print it immediately or print it later.

The calibration procedure was previously implemented manually. The power supply, pressure controller and the environment chamber were operated manually. The output values from the transducers were read individually using a digital multimeter. To generate reports for all the experiments, calculations from the test data had to be done by the user. These, along with the test data had to be entered by the user and printed out. During the experiment in case of any emergency condition, the user had to take care to shut down all the instruments and document it in the reports. All these require the users attention during the entire test procedure and since some of the test durations run in hours a lot of user time is wasted.

The current NI Lab VIEW based test system does not require the user to monitor the test procedure. The user has to initially configure the system for the required test and “start” the test. In case of emergency shut down conditions, the software generates an audible alarm for the benefit of the user and takes necessary steps to safely shut down the system.

Conclusion:

The developed system proved to be a time saving and simple alternative to previous methods. The system helped the customer to increase the number of transducers tested. The simple and user-friendly front panel made it easy for the user to handle the system. This was made possible due to the rich graphical features provided by Lab VIEW.