Developing Stimulating and Monitoring System Responses using LabVIEW

Author(s):
S. Mitchell - Systems Engineering & Assessment Ltd.

Industry:
Electromechanics/ Electrotechnics

Products:
LabVIEW

The Challenge:
Providing continuous high-speed and high-resolution waveform reproduction and sampling to accurately characterize black box systems.

The Solution:
Creating an equipment tester that incorporates high precision National Instruments digital-to-analogue and analogue-to-digital converter cards driven by a sophisticated LabVIEW-managed environment.

Introduction

Systems Engineering and Assessment Ltd. (SEA) has developed a flexible real-time stimulation and acquisition tool that provides users with a cost-effective, high-performance aid to both the characterisation and emulation of black box systems. A black box system refers to an undefined, not easily understood function/operation on a box system (a type of electronic equipment, system, or sensor). The operation/function may not be easily understood because the information is not available, the box (i.e. system, sensor, electronic equipment) is not performing properly, or it may be a competitor’s box. Characterisation and emulation means the user may have a system that he or she wishes to characterise. The equipment can simulate known, either measured or modelled, characteristics of a system or subsystem and simultaneously record the resulting responses of the system under test. We can analyse these results together with the prompting stimulus to establish the relationships between stimulus and response. We can use this information to modify the behaviour of the system under test, identify problems in a test scenario, or to design systems that benefit from a detailed knowledge of the behaviour of a sealed or “alien” system.

 

Stimulator Design

The heart of the stimulation and monitoring system is a PC-based, fully-automated test system running National Instruments LabVIEW. The stimulation and acquisition elements, which provide the required accuracy and data rates, are four PCI-4452 4-channel 16-bit acquisition cards with programmable anti-aliasing, and four PCI-6111E 2-channel 16-bit analogue output cards. We apply additional conditioning to the inputs and outputs, providing optimum performance for the system. Notably the 16-bit attenuator on the outputs provides an overall dynamic range of 190 dB.

A set of LabVIEW VIs provides system control. The user can employ these to build a bespoke application, which uses the supplied modular blocks that control and interface to all elements, or to run the stimulation and monitoring system standard VIs, which reproduce stored data and acquire input data and preselectable data rates. Key features to the overall performance are the dual 550 MHz Pentium processors and the 3 by 18 Gbyte removable fast disk drives, which provide the input, output, and control data. These removable drives also provide ease of data insertion and allow removal for security, if necessary.

Applications

Typical applications for the stimulation and monitoring system include:

• “Black box” characterisation
• System functional and performance testing
• Development aid for system or artificial intelligence tailoring
• Production spread recording or testing
• System recording for playback
• Event or occurrence recording
• Arbitrary waveform generation and response capture

Stimulator Configuration

The stimulator essentially uses eight 16-bit analogue digital to analogue converter channels – each followed by a 16-bit attenuator and differential amplifier. This configuration enables a stimulation range of ±10 V (±20 V differential) full scale to ±150 mV (±300 mV differential) full scale at 16-bit resolution, independently adjustable for each channel. The differential output amplifier ensures balance of the signal for optimum analogue performance using twisted screened test cables. This configuration also allows the use of single-ended outputs of either polarity with zero volt return.

Sixteen 16-bit high-input impedance acquisition inputs capture the response of the system under test. The system records these sample inputs simultaneously for later analysis. We can individually select data rates for each channel to optimise results storage, ensuring that we use the most favourable bandwidth for the signal in question. Actual sampling performs at the least common multiple sample rate, ensuring the highest timing integrity. Digital filtering provides the required data sets with the demanded bandwidth. In addition, we can also record digital inputs.

Control system LabVIEW VIs enable us to rapidly retrieve the stimulation information from disk to provide real-time waveform generation and simultaneous data acquisition. Eighteen GB of stimulation data and 18 GB of acquisition store are standard. However, we can easily increase this by using larger disk capacities or by adding disks to the spare removable disk bays.

Additionally, 16 uncommitted digital I/O signals supply the equipment we use for synchronously inputting or outputting control or status information.

Design Challenges

Maintaining high data rates, high precision, and low noise were among the biggest challenges for the equipment design. The selection of seamlessly integrated commercial off-the-shelf components drove the design towards the National Instruments data acquisition devices controlled through the LabVIEW environment. This, coupled with the use of the Dell RAID disk module used as independently accessed, high-data rate disks controlled by a Dell highperformance computer, provided the integrated compatibility required to add the necessary throughput to the system performance.

Results

Now, we can post process the sampled, filtered results with the influence waveforms, to establish the system under test characteristics and performance. The performance and software support drivers of LabVIEW provided a solution for us at the right price.

For more information, contact:

 S. Mitchell

Head of the Simulator and Simulation

Systems Engineering and Assessment, Ltd.

Sea House, P.O. Box 800

Fishponds, Bristol, UK BS 16 154

Tel: +44(0) 1373 852 000

Fax: +44(0) 117 969 1177

E-Mail: info@sea.co.uk

 

 

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