The Challenge

Developing a reliable, cost effective, portable data acquisition and event monitoring system that distributes the computational load between multiple networked systems

The Solution:

Using National instruments PXI platform, NI DAQmx and LabVIEW real time software, we have provided a high performance, completely synchronized and distributed network of data acquisition and measurement systems.

Introduction:

The distributed measurement system consists of three data acquisition units that are separated from each other by more than 10m. These units are connected through Ethernet link and the acquired data is processed and stored locally during tests. There are four display units to which data can be transmitted both during tests and offline.

The main requirement was to perform complete functional tests for distributed electronic assemblies. These functional tests include detecting and identifying commands generated from various command generation systems in these electronic assemblies.

The measurement system was required to perform the following tasks:

• Detect and identify events through optically isolated digital input lines with a resolution of 100 milliseconds
• Detect and identify analog events through analog inputs with a resolution of 100 micro seconds
• Maintain separate tables of detected analog and digital events for each acquisition system
• Transmit test data through network to remote display unit during tests and offline
• Merge test data from various acquisition systems and generate reports

We decided to build the test system based on the open industry standard PXI architecture. Also the NI range of PXI products provided us with the hardware for all of our signal types. LabVIEW was the language of choice for programming the system due to its excellent graphical capabilities, distributed system design features and tight integration with NI PXI products using NI DAQmx.

System Configuration:

The Distributed Event Monitoring System consists of the following two sections:

• The Remote Data Acquisition Systems (RDAS) based on PXI platforms
• Data Display Systems based on laptop computers running Windows.

The Remote Data Acquisition Systems (RDAS) communicate with display systems through Ethernet link. Each PXI consists of two 6254M 12 bit resolution DAQ Cards. The 10 MHz clock routed through the PXI back plane is used to synchronize both the cards. Each RDAS can monitor up to 96 isolated digital inputs and 64 analog inputs. The system provides a maximum sampling rate of 10 kHz per analog channel.

The system lay out is as shown below.

Depending on the complexity and size of the electronic assemblies to be tested, our measurement systems could work in one of the following modes:

• Stand alone mode for testing a small subsystem with a single RDAS and display unit.
• As part of a Network which should be able to transfer data to any of the displays as shown in the fig 1.0

Just as in a single-computer system, the components of a distributed system need to share data with each other in an easy and transparent way. The challenge was to implement a communication scheme that was best suited to link distributed components together.

System Implementation:

New features and functions in LabVIEW 8.0 especially related to data communications made it very easy to develop the distributed measurement system. The application software thus developed is mainly divided into three modules namely Display module, RT module and Network module. The display module runs on all laptop computers and takes care of all user interface actions. RT Module runs on all Remote PXI systems. The network module runs on both and interacts with these two modules. The interface and communication between different modules are shown in the below figure.

The GUI in the display module consists of user friendly software panels. The user has options to configure the following:

• Digital and analog channels for event and pulse acquisition.
• Giving legend names and order of displaying the events in the test panel.
• Deciding pass fail criteria by selecting event time window, threshold and range for pulse currents.
• Setting sensor parameters like sensitivity and offset.

The tests are initiated by the display units. Test could be started on remote systems simultaneously or one after the other. Synchronization was provided by using a common hardware trigger line for all remote systems. The real time systems continuously keep monitoring for events for a duration of 3 to 4 hours. These systems perform following functions during tests

• Analog Event Monitoring with 100us resolution
• Digital Event Monitoring with 100ms resolution
• Comparison of analog and digital event details with the user configured look up tables.
• Determining pass/fail criteria and generating result tables.
• Updating a software mimic panel corresponding to 96 digital channels
• Event based logging
• Battery voltage monitoring to alarm the user for any drastic voltage variations.

The analog event details received by the display system will be as shown in Figure 3

Some of the other features provided by the software are

• Utility to transfer configuration and test data files from display to RT systems and vice versa.
• Offline analysis to analyze the previous test results
• Merging the files from different RT systems.
• Generating quick reports both during online and offline
• Security settings for administrator and user logging in

The distributed system developed was flexible as each individual component can be upgraded or modified without affecting the other parts of the system. Computing capacity and functionality can be added by upgrading hardware components or by adding computer nodes.

Conclusion:

We were capable of designing and developing a test system that enabled our client to maximize the throughput and capabilities of the time critical acquisition system. NI LabVIEW development environment and the modularity extended by the PXI architecture and networking features equipped us in providing a flexible and reliable solution.