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Original Authors: Arun Joy, Captronic Systems Pvt Ltd
Edited by Cyth Systems
The Challenge
Developing a real-time monitoring system to accurately detect and analyze the vibrations and movements an object undergoes using a noncontact inspection technique.
The Solution
Using NI PXI Express hardware and the NI LabVIEW Real-Time Module to create a SONAR data acquisition and processing system that sends ultrasonic pulses and analyzes the echo from the object to capture the minute variations in the distance to the object caused by vibratory movement.
In the system we created for this application, the critical component is accurately detecting the minute vibratory movements (on the order of a few millimeters) in real time for objects where in-contact measurement methods, such as using a linear variable differential transformer (LVDT), is not possible.
SONAR describes the technique of sending ultrasonic pulses and listening to their echoes from an object. By analyzing the echoes, we can obtain details such as distance to the object, size, shape, and structure. We chose a SONAR system for this application because it delivers more accurate results than other methods such as LVDT, and it provides a noncontact inspection method useful for inspecting objects on which we cannot mount sensors.
In this application, ultrasonic transducers repeatedly pulse at a pulse repetition frequency of 40 Hz to 100 Hz. The system captures corresponding echoes and precisely measures the time taken to receive each echo for an accurate distance calculation. Any vibration or movement introduces a variation in the time intervals.
For our solution, we needed to:
Develop a high-speed data acquisition system to simultaneously acquire ultrasonic frequency signals from four ultrasonic transducers (multiple transducers are mounted at different angles to the object of inspection)
Create a high-speed bus to handle the large amount of data acquired at high sampling rates (on the order of MS/s per channel)
Design a real-time processing unit to implement the algorithm for precise time and distance measurement
Synchronize the pulsing of all transducers
Ensure precise synchronization to detect each pulse or echo
System Overview
The system is made of an ultrasonic transducer, a pulse receiver, a digital output module, a digitizer, a real-time controller, and application software.
The ultrasonic transducer is pulsed, sending out an ultrasonic wave. The subsequent echoes generate a voltage in the transducer, which is sent back to the pulser-receiver. The pulser-receiver provides the high-voltage pulses necessary to excite the transducer. It receives the echo signal from the ultrasonic transducer and amplifies it before feeding it to the digitizer.
The digital output module provides digital output signals to synchronize the pulser-receiver instrument, and the digitizer converts the waveform sent from the pulse-receiver from voltage to bits using an analog-to-digital converter.
The real-time controller controls the acquisition and processing and sends the data to the monitoring station (user interface) for display. The application software processes analyzes, and presents the data from the digitizer according to the user-defined parameters.
We completely developed the application software in LabVIEW. It has two parts: the GUI running on the monitoring station PC (host PC) and the processing unit running on the NI PXIe-8133 real-time controller. A LabVIEW TCIP/IP protocol facilitates communication between the real-time controller and the host PC (user interface). The user can configure and control the system using the software GUI.
The NI PXIe-1071 chassis contains the real-time controller, digitizers, and digital output module. The real-time controller controls all module operations. The digital output module provides digital outputs for the synchronized pulsing of pulser-receivers. The ultrasonic echo signals amplified by pulser-receivers are acquired using NI high-speed digitizers. The acquired signal undergoes signal processing, such as averaging and filtering, to remove any waveform degradation.
Left: Ultrasonic Pulse and Echo, Right: Acquisition and Processing—Result Display
The algorithm for measuring the time of flight (time taken to receive the echo) of the ultrasonic signal is implemented using the LabVIEW Real-Time Module and the LabVIEW Advanced Signal Processing Toolkit. The processing unit in the real-time controller executes this algorithm deterministically, to detect the vibration with high accuracy. The application also provides a data-logging feature for future data analysis. The results as well as acquired data are transmitted to the host PC in real time for monitoring.
The system measures the time and distance for each pulse. The monitoring is simultaneously carried out for signals from four ultrasonic transducers located at different angles to the object under inspection. It can even precisely measure the minute vibratory movement of the object this way.
Application Software
The system has an easy-to-use GUI for the host PC, developed in LabVIEW. Users can configure the system per their requirements and give commands to control the acquisition. It also provides a feature to record the acquired data and results to analyze later. With a configuration utility, users can enter the signal parameters and digitizer settings to configure the acquisition. The acquisition and processing panel contains the controls to start and stop acquisition, and to record data and results.
The graphical display indicators make monitoring and analysis easy with the help of LabVIEW analysis tools. The display and analysis options include A-Scan and B-Scan display of digitizer data and the frequency and time domain analysis of results.
Advantages of the SONAR System
The SONAR system we created provides a noncontact method to monitor the object characteristics. There are no limitations on the surface, object shape, or environment where the object is kept. The system gives far more accurate results than other methods, such as LVDT, even for vibration with a maximum 1 mm peak-to-peak amplitude. By simultaneously acquiring data from multiple transducers, the system can capture movements in more than one axis. The system also has a feature to record data and results for future analysis.
We used NI hardware and software to develop a precise monitoring system to monitor the vibratory movements of objects in real time. NI products such as the LabVIEW development environment and real-time PXI hardware made it easy and efficient.
With the real-time PXI controller and the LabVIEW Real-Time Module, our system can simultaneously acquire and process from all channels with a high degree of determinism. The NI PXIe-1071 chassis gave us sufficient bandwidth to handle the large amount of data acquired by the digitizers, and the digitizer’s NI-TClk feature gave us precise synchronization.
The built-in advanced signal processing tools in LabVIEW offer an efficient way to implement the algorithm for measurement and calculations. We used the built-in communication protocols, such as LabVIEW TCP/IP, for data transfer between the real-time target and the host PC. Efficient file options, such as technical data management solution data storage, helped us manage our large amount of data, and the LabVIEW platform made development easy, fast, and effective with a powerful GUI.
Original Authors: Arun Joy, Captronic Systems Pvt Ltd
Edited by Cyth Systems
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