Using LabVIEW to Create Gas Sensor Characterization System

Author(s):
Chan Chui Chai - Nanyang Technological University

Industry:
Semiconductor

Products:
LabVIEW,

The Challenge:
Characterizing thick and thin film gas sensors that are fabricated using hybrid technology with an automated system.

The Solution:
Building a PC-based system using GPIB and data acquisition (DAQ) boards controlled by LabVIEW.

Introduction
The gas sensor characterization system (GSCS) block diagrammed in the figure, is designed to test the electrical characteristics and performance of gas sensor subjects in response to various stimuli, including resistance, capacitance, D-loss factor temperature, and gas flow rate.

The system is PC-based. The computer monitors and controls all system equipment. The AT-GPIB/TNT (PnP) interface board communicates with the measuring instruments, namely the HP 4192 impedance analyser, HP 4282 precision LCR meter, and source measure unit. The second and third serial ports, provided by a plug-in serial I/O board, communicate between the MKS Type 232 Interface and the TMS 93 temperature controller, respectively. The Lab-PC-1200AI DAQ board acquires temperature readings at the test chamber via the temperature board.

The control instrument - MKS Type 232 interface - serves as the communicating platform between the system controller and the MKS Type 247C 4-channel readout. The 4-channel readout provides a remote (via PC) or manual mode to control and acquire the gas flow rate of the mass flow controllers (MFCs) for each channel. The various gases from the MFCs are premixed in a chamber to ensure homogeneity of the test gas during measurement.

A separate control instrument - TMS 93 temperature controller - adjusts the temperature in the test chamber. Similar to the 4-channel readout, the temperature controller provides modes to control the temperature remotely and manually. The temperature board measures low-level thermocouple signals. Together with the DAQ board, the temperature board acquires the signals without an external conditioning board. The vacuum pump is used to discharge the gases out of the test chamber.

Software Architecture
LabVIEW version 4.1 was used extensively in interfacing, monitoring, and controlling data acquisition. Five main modules were developed - three modules for the LCR meter, one Keithley 236 module, and one HP 4192 module. All modules are initialized when activated. Entries can be made on the controls, which occupy the left-half portion of the front panel, prior to pressing the <Start> button.

Four of the five modules - LCR time sampling, LCR DC bias sweeping, LCR frequency sweeping, and HP 4192 - measure AC characteristics of the gas sensor based on time, DC bias, and frequency. In addition, DC characterizations of the gas sensor can be measured by the Keithley 236 module. Concerning the looping mechanism, acquisition is performed as many times as the user specifies. Entering a new loop, the software modules act as the controller, setting various parameters to these desired values. Next the modules switch to talker mode to acquire the necessary data. Eventually, the modules become listeners to record the data produced by the equipment. LabVIEW continuously triggers itself through the functional stages until the last condition isachieved or the module is terminated by pressing the <Stop> button.

Features have been incorporated into the modules to enhance their user-friendliness. The features include the automated graph clearing mechanism and the tiling concept.The automated graph clearing mechanism reshapes the arrays that pass values into the graph to a zero dimension at the beginning of the experiment. Thus, the graph palette is no longer necessary. The tiling concept groups together controls and indicators to clean up the overcrowded front panel. The visibility of a few overlapping clusters is controlled by their attribute nodes. Only one cluster is shown at a time.

Results
The LCR time-sampling module prototype was completed within two weeks. By sharing subVIs, development time for the rest of the modules was around two days. However, all of the modules have been refined and enriched through modification.

For more information, contact:

Chan Chui Chai and Ooi Seong Thean

Mr O. K.Tan and Dr W. Zhu

School of Electrical & Electronic Engineering

Nanyang Technological University Nanyang Avenue

Singapore 639798

Tel: 065-790-5411

Fax: 065-791-2687

E-mail: eoktan@ntu.edu.sg

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