Application of multi-channel digitizer PCI-9846 in ultrasonic inspection system

Abstract

ltrasonic inspection is the key nondestructive inspection adopted by contemporary industries now. A multichannel ultrasonic inspection system is made with ADLINK's PCI-9846 digitizer. Controlled by a computer, this singlechip microcomputer connected to the USB port processes reflected ultrasonic signal for data transformation by the digitizer and displays and analyzes data with the software system. The system is proved to feature high inspection accuracy, powerful analysis functions, and good reliability, and is suitable for application in industrial automatic inspections.

Keywords

Ultrasonic inspection, digitizer, data collection

Introduction

Regular inspection is one of the essential measures in managing pressurized vessels against critical failures. The manufacturing and use of pressurized vessel leads to inevitable mechanical defects including cracks, gas apertures, and poor soldering seams. Nondestructive inspection is especially important to the use of pressurized vessels.Ultrasonic inspection is an important nondestructive inspection technique for its deep penetration, reliable and accurate inspection, and convenience and ease of use. It is now widely adopted in industrial and advanced technology industries.

Early ultrasonic inspection employs simulation signals for flaw detection. As its results rely on subjective personal experiences, a series of problems of poor consistency and accuracy exists. With the application of advanced computing and signal processing techniques, research on digitized ultrasonic inspection is getting popular when more and more ultrasonic flaw detectors are adopting digitized collection and analysis. In general, resolution is a key indicator of ultrasonic inspection. It is directly associated with measurement accuracy and detection of tiny defects. With advanced material technology, pressurized vessels' walls are getting thinner This leads to more demanding requirements in ultrasonic inspection resolution. Ultrasonic inspection is, in essence, an attempt to measure the time difference of reflection waves between two surfaces in the material thickness direction to learn about material thickness. With regard to tiny flaws and thin walls, higher resolution means more frequent detection via ultrasonic inspection. This, in turn, leads to more demanding requirements in subsequent signal sampling and processing. Taking these and features of ADLINK's PCI-9846 multichannel digitizer into account, we designed a four channel ultrasonic inspection system, featuring high precision and powerful analysis functions, composed of a PCI-9846 digitizer and relevant software and hardware.

System design and structure

The ultrasonic inspection system is composed of a PCI-9846 digitizer, C8051F340 single chip microcomputer, power supply, transmitting circuit, and programmable amplification circuit. See Figure 1 for its structure.

The computer initiates the measuring process by triggering the singlechip microcomputer C8051F340 connected to the USB port, which, in turn, generates the relevant on/off signal. The transmitting circuit then generates a high voltage narrow pulse to excite the transmission probe for ultrasonic generation. The receiving probe transforms the received ultrasonic, with acousto-electric transformation, back to signals to the programmable amplified set by the computer for sampling by the PCI-9846 digitizer. The digitizer then exchanges data with the computer through the PCI port for the latter's display and processing. The power circuit and singlechip microcomputer's peripheral circuit contains a watchdog, chip oscillator and reset circuit. The design requires four probes for concurrent inspection.

2.1 Selection of digitizer

A digitizer is an A/D device in essence. Accompanied by relevant ports and software it comes to be a standalone device for signal collection and analysis. The data collector of this system requires the following:

1. Accept 4 lines of input and run A/D conversion concurrently. Workpieces move along the production line and complete required processing at the same time. The system is required to detect flaws at 4 major positions concurrently and so must have digitizing equipment with four inputs.
2. Higher resolution Ultrasonic travels 10 times faster in steel material than in air. The receiving end calculates the thickness of the steel object by comparing bounced echoes from the ultrasonic pulses transmitted. For flaw detection, the higher the frequency is the more valuable the collected signals are for later processing. If an ultrasonic frequency of 5MHz is selected as required by the transmitting probe's maturity level and application requirements, then a 10M sampling rate is needed. However, the actual operation may need a sampling rate above 30M.
3. Larger cacheA/D converted data is then processed by the computer. Data exchanges between the two through a PCI bus. With 4 lines of collection this system generates a large amount of sampling data. A larger cache can lessen demands to the computer's real time processing requirements.
4. Easy to use and develop. A computer communication device requires relevant drivers and scores of software supports.

ADLINK's PCI-9846 digitizer is adopted taking the above factors, price, and performance into account as in-house circuit development may lead to long development time, difficulty in driver development, and higher costs.The PCI-9846 digitizer [5] is selected for several reasons. Its four inputs meet the requirements of input terminals. Its sampling rate of 40MS/s at each line meets the requirements of digitization processing. Its 512MB internal memory relieves the asynchronism issue among ports between different devices. Last but not the least, ADLINK's Matlab, VC++, and .net development drivers and SDK enable us to focus on the application rather than software coding.

Key circuits and implementation

3.1 The ultrasonic transmission circuit

Transmission circuit is the key part of an ultrasonic inspection system. It dictates the final target it may achieve. A well-designed transmission circuit determines later transmission power and waveform duplication. An ultrasonic transducer is usually required to generate instant high-power sharp pulse waveforms with an excitation pulse amplitude above 100V and width at the us level. The actual figures vary with the target of the transmission probe.Conventional excitation circuits are made by way of instantaneous discharge, pulsed excitation and resonance. Though the last two methods are more popular, all three suffered because of high voltage power and transformation. An inductive pulser is employed here for pulse excitation with the circuits shown in Figure 2[1].

Pulses are input from 12 of U1. IR2110 must be chosen with special care. The ideal high voltage pulse drive may not be achieved on account of isolation and drive in case IRF7450 is driven by pulses directly. IRF7450 is a MOSFET of Vds=500V的MOSFET. With current up to around 20A, it meets the requirements of fast switching on ideal switches. The energy storage coil generates high voltage at the control of the switch. Diode Documents and Development here is for circuit protection. Ultrasonic transducer V1, GE's 014LJM, may be replaced with other models as required.

See Figure 3 for the high voltage pulse generated. A pure resistive 25 folds attenuator head is added to the oscillator when taking measurements. Units on the Y-axis and X-axis are 2V and 100ns respectively. With transient excitation level synchronization the pulses generated are of amplitudes around 200V and width 300ns.

3.2 Programmable receiving circuit

Amplitude of signal accepted and transformed by the ultrasonic receiving probe is very weak, around a couple of micro volts, and requires amplification before digitization. The amplification ratio of the circuit should be set by the computer to meet flaw detection and auto testing requirements of individual situations. There are a lot of integrated operational amplifiers available. Here we need one with band width greater than 5MHz as the device is designed to process signals of frequency centered at 5MHz. TI's OPA2300 is employed here. It features two operational amplifiers, each with its own prohibition end. The accumulated gain band width of 150M far outpaces requirements of this signal amplification. Its 3nV/Hz noise coefficient satisfies low noise amplification needs. Its single +5V power does not require a dual power supply. Gains can be adjusted by selecting different resistors in the gain circuit. As the control signal is a digital signal given by the computer, a DAC is needed for digital simulation signal conversion. Here we choose the 8-bit D/A conversion chip AD7524. The singlechip microcomputer feeds it with 8-bit data to adjust gains by controlling the gain resistor with the relevant diode after conversion.

3.3 Selection of system control

The computer starts the system for testing. An external controller, connecting both the computer and probes, is required to ensure detection signals transmitted by all four probes are in specific timing relation. This is done via the singlechip microcomputer. The computer and singlechip microcomputer were connected with RS-232 ports in the past. With technology advances, the USB port is now replacing the RS-232 because of its speed, expansion capacity, bus power supply, and PnP support.Here we choose Silicon's C8051F340 singlechip microcomputer as the external controller for communication with the computer through the USB port.

The C8051F340 is a new singlechip microcomputer compatible with 8051. It features low power consumption, high performance, and a high speed pipelined structure. It is embedded with 64K flash and 4352 byte RAM and requires no extra storage device in this application. The C8051F340 contains four 16-bit counters to concurrently control four probes' collection timing and supports 16-bit interrupt to meet the collection control needs of this application. It has forty I/O lines for direct +5V connection without electric level conversion at port. The best is that it integrates a USB controller and offers relevant SKD for easy development.

Software design

The system software contains control, data receiving and signal analysis sections. The control software determines operation of the system. It communicates with the USB port connected singlechip microcomputer to control the whole system's transmission and receiving process and controls the data receiving software for data receiving and display. The data receiving software saves data sampled by the PCI-9846 digitizer to given positions while the signal analysis software displays and analyzes signal samples. See Figure 4 for software structure.

The singlechip microcomputer software development contains both host and singlechip microcomputer sections. The host section is developed with Keil V3 for the control of C8051F340 in C language. The development is special in the USBXpress device drivers needed to contain relevant firmware library in the head file and call relevant DLLs to use the included API functions. The primary functions are USB device interrupt, device initialization, database read/write, and USB device On/Off.

Data receiving, graph display, and signal analysis functions are developed with .net. This requires the installation of drivers, especially the DAQPilot, included with the PCI-9846 digitizer. ADLINK offers plenty of sample codes for .net development. Just call the appropriate ones to read in data as needed.The signal analysis function covers amplitude waveform analysis, FFT analysis, threshold setup, and flaw formation and positioning. The goal is to process and record signal playback as well as to evaluate the workpiece status.

See Figure 5 for the display interface for the flaw detection signals acquired by the ultrasonic inspection system with four probes against the same workpiece. This detection is conducted with liquid media, transmission by transmitting probe #1, and receiving by four probes. The received signals look the same in amplitude and waveform while differing slightly in time.The noise of the whole probe #2 and rear part of probe #3 looks a little bigger as they are placed at the rear of the workpiece and near the processing equipment. Analyzing these signals may reveal the exact position of a crack.The digitization platform may be used for the flaw detection signal's subsequent spectrum analysis and flaw forecasting to implement theoretical analysis on flaw formation and projection.

Conclusions

The proposed system, when applied in a specific inspection system, features simple implementation, reliable performance, easy operation, and precision measurement. With PCI-9846 digitizer's high sampling rate and accuracy this system can detect internal cracks and flaws precisely. It works especially well with subsequent data processing software in positioning cracks in workpieces based on the shapes of the specimen. Its signal analysis approach also enables the quantitative projection for tiny flaws in a workpiece.

Reference

Related ADLINK Links:

• More about ADLINK Modular instrument (digital instrument)
• More about PCI-9816/9826/9846