The challenge:

To develop a system that monitors the health of various sub-systems of the Unit Under Test (UUT) under various test conditions, displays the system status on eight PCs connected across a network and calculates health related parameters real-time.

The Solution:

Utilize the ruggedness, high-performance signal conditioning & programmability of NI SCXI modules and the graphical features of LabVIEW to build a configurable, expandable functional test bench which will minimize testing time and maximize productivity.

Introduction:

The test system developed is used to monitor eight sub-systems of the UUT. Each sub-system of the UUT is subjected to various test conditions. The health of the UUT is then monitored from

• Measured parameters like strain, pressure, temperature
• Derived parameters (calculations of which are specified by the user).

Data for each sub-system is displayed on individual display systems connected across a network.
The main requirements of the test system are:

• Maintain a database of different types of sensors and their calibration details
• Perform three different types of chain calibrations – physical, electrical and DC
• Automatically switch resistances across the fourth arm of a strain gauge in the electrical calibration
• Acquire data in one PC and display data across multiple PCs (distributed display)
• Compactness, Ruggedness, accuracy and reliability
• Provision for future expansion
• Ability to store test data into a database
• Generate reports from test data

National Instrument’s SCXI system provided the required compactness, ruggedness, accuracy and reliability. SCXI 1127 module is used for switching a resistor bank. The test software is developed using LabVIEW 7.1 whose excellent graphical capabilities came handy while creating user interface. Database connectivity toolkit for LabVIEW is used to connect to MS Access Database Management System. Reports were generated using NI Report Generation Toolkit.

System Architecture:

The fig - 1 shows the system architecture. There are eight systems, one each for monitoring a different sub-system of the UUT. Data communication between the systems is done through TCP/IP. One master system acquires all the data and communicates it to the client systems.

Application:

The developed system is broadly divided into four parts

Sensor Database Maintenance

The sensor specific details, their calibration details and test configuration are stored in a database. MS Access is the database management system selected and NI Database Connectivity toolkit is used to communicate between the application and the database.

Calibration

All the sensors that are used for measuring the parameters are calibrated before the test. Different types of calibration are used for different types of sensors.

• Physical Calibration – the sensor is connected to the UUT. The physical load is applied in steps and data is acquired. Then the calibration constants are calibrated. Other properties like hysterisis, non linearity and repeatability of the sensor are also calculated.
• Electrical Calibration – this is done only on bridge type sensors. The physical load is simulated by changing the resistance in the fourth are of the sensor and voltage is measured. The resistance is switched through software using the NI SCXI 1127 switching module. Then the calibration constants are calculated.
• DC Calibration – this is done only for sensors for which application of the physical load is not possible. The entire line without the sensors is calibrated by giving a known voltage from a power supply and acquiring the voltage and calculating the constants. The power supply is programmed through GPIB to source the required voltage

Health Monitoring

The UUT is subjected to various test conditions by the user. When the user initiates health monitoring, data is acquired from all the configured sensors into the master system. Various parameters like strain, pressure, temperature are acquired through the SCXI modules during monitoring and are logged into the master system.

During acquisition data is segregated as required for each subsystem by the master system. The data for each sub-system is then communicated to corresponding client system through TCP/IP. (The client system assignment for various subsystems is configurable by the user.)

The health of each individual sub-system is monitored independently on each of the clients assigned for them. The customized calculations also are done independently on each client system.

Report

The acquired data is logged in the master system for later use. The user can generate different types of reports from the data.

Software Features:

Conclusion:

The NI SCXI system helped immensely due to its programmable hardware and its high accuracy. The selected modules also helped in reducing manual intervention to a large extent. LabVIEW 7.1 with its rich GUI features provided for a very easy and user friendly interface. It also effectively reduced the development time because of tight integration with NI products through DAQmx. All this was possible because of the configurable solution offered through the Virtual Instrumentation approach.