Induction defectoscope based on uniform eddy

Induction defectoscope based on uniform eddy current probe with GMRs Octavian Postolache

Artur Lopes Ribeiro

Helena Geirinhas Ramos

Instituto de Telecomunicações/IST, Lisboa, Portugal e-mail: [email protected]



Abstract— Defect detection in conductive plates represents an important issue. The present work proposes an induction defectoscope that includes a uniform eddy current probe with a rectangular excitation coil and a set of giant magnetoresistance sensors (GMR). The excitation current, the acquisition of the voltages delivered by the GMR and the signal processing of the acquired signal are performed by a real-time control and processing unit based on a TMS320C6713 digital signal processor (DSP). Different tests were carried out regarding the excitation coil position versus crack orientation and also regarding the GMR position inside the coil and the best response concerning the crack detection for a given aluminum plate specimen. Embedded software was developed using a NI LabVIEW DSP module including sinusoidal signal generation, amplitude and phase extraction using a sine-fitting algorithm and GUI for the induction defectoscope. Experimental results with probe characterization and detection of defects were included in the paper.

magnetoresistances (GMRs) positioned inside a rectangular coil that provides a uniform excitation field applied on the plane of the conductive plates to be tested. Based on the measurement of the magnetic field components tangential to the sample, defects such as flaws and holes in the specimen are detected. A practical approach concerning the optimal position of the GMR to assure accurate detection of the defects was considered. An important part of the work is represented by descriptions of the conditioning circuits and of the embedded architecture based on the DSP (TMS320C6713) that assures the excitation signal control, the acquisition, processing and defect signaling as parts of the proposed induction defectoscope. Embedded LabVIEW software description and experimental results regarding the UECP tests and defect detection are also provided.

Keywords-uniform eddy current probe, magnetoresistances sensors, non-destructive testing, digital signal processor

The uniform eddy current probe (UECP), described on Figure 1, consists of a large wide tangential exciting coil (rectangular profile) and a set of three giant magnetoresistance sensors (GMR1, GMR2, GMR3) whose sensitive axis are perpendicular to the excitation field (Hex).

I. INTRODUCTION Eddy current testing has the important advantage of noncontact and fast testing of conductive plates. Thus, different eddy current probe (ECP) architectures to detect flaws are reported in the literature [1-4]. An excitation coil and detection coils usually form the eddy current probe. The use of giant magneto resistors as sensing components of ECPs has greatly increased its sensitivity [5]. Some drawbacks of this kind of architecture are the large noise and distortion due to the lift-off effect and to the electromagnetic material characteristics. Uniform eddy current probes are mentioned in literature as a solution that provides higher immunity to the noise associated with the lift-off effect. Uniform eddy current probe (UECP) architecture is reported by Koyama [6] including a tangential excitation coil having differential coil detection disposed inside the excitation coil. Good results were obtained using this probe in the area of crack detection in a weld zone. NDT system based on uniform eddy currents is also part of the United States Patent 4594549 US patent [7]. Taking into account the advantages of uniform eddy current probes (UECP) and of the GMR sensor characteristics the authors developed novel architectures that combine rectangular coils for uniform magnetic field generation and a high sensitivity magnetometer based on GMR sensors [8]. This work continue presents a novel UECP and proposes a sensing architecture that includes a set of giant

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II.

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UNIFORM EDDY CURRENT PROBE ARCHITECTURE

H ex

permanent magnet

magnetic sensor sensitive axis N

S

rectangular excitation

H ex GMR1

conductive plate under test GMR3 GMR2

Figure 1. Uniform eddy current probe based on a rectangular excitation coil and a 3x GMR for defect detection in conductiove plates

The induction defectoscope system based on the use of a wide tangential excitation coil that induces uniform eddy currents presents an interesting solution to detect flaws and other nonuniformities, because due to its geometry it provides higher response immunity when compared with the generally used pancake solution. Due to a better signal to noise ratio (S/N), it provides an increased accuracy in the defect detection. The geometrical characteristics of the used rectangular coil are 20 mm width, 20 mm height, 40 mm length, number of turns 80, and diameter of wire 0.5 mm. The rectangular coil was electrically characterized using an RLC-meter and the impedance and phase characteristics are presented in Figure 2.

Figure 2. The electrical characteristics of the used rectangular excitation coil

In order to obtain high sensitivity from the uniform eddy current probe a practical approach concerning the excitation coil position and GMR sensitive axis position versus the crack axis was carried out. The coil position corresponding to eddy currents parallel and perpendicular to the crack have been imposed, and the output signals were measured for different amplitudes and frequencies of the excitation current (100≤Iex≤300 mA and 1≤fIex≤10 kHz).Using the xy motion system, different aluminum plate specimens with induced cracks were scanned considering different distances between the excitation coil and the plate. Taking into account the crack depth, special attention was given to the excitation signal frequency taking into account the dependence between the frequency f and the aluminum plate standard penetration depth, δ:

δ=

1

(1)

π ⋅ f ⋅σ ⋅ µ

where µ is the magnetic permeability and σ is the electric conductivity of the plate. For the particular case of the

aluminum plate to be tested, Al-2024-T3, µ = µ0 and σ = 1.75×107 S/m. Referring to the eddy current detection the GMR1, GMR2 and GMR3 (Figure 1) are located inside the rectangular coil and detect the minor changes on the magnetic fields. The used GMR sensors (AA002 from NVE) present high sensitivity that is independent of the used frequency. Each one of the used GMR sensors includes four 5 kΩ GMR resistors configured in a Wheatstone bridge as presented in Fig. 1. The magnetic characteristics of the sensor used as eddy current detector are: saturation field Hs=15 Oe, linear operation range between 1.5 Oe and 10.5 Oe, (1 Oe=79.577 A/m) and sensitivity of 4 mV/V⋅Oe. In order to detect the optimal mounting of the GMR sensors to attain a high sensitivity of the probe, different positions of the magnetic sensors axes related to the magnetic field detection and to the position of the crack were verified. Several geometrical scenarios are presented in Fig. 1 where the excitation magnetic field on the coil axis is perpendicular to the sensing axis of the GMR sensors package. In these cases the excitation magnetic field produced by the rectangular coil has no effect on the sensor output. However, the imperfections of the geometrical setup lead to a sensor output voltage different from zero even when the used aluminum plate specimen doesn’t present any flaws. Due to the GMR magnetic field sensor output V-shaped characteristic, presented in Fig. 4, a dc magnetic field generated by a small permanent magnet (AlNiCo type with Hc=51 kAm-1) was applied for biasing. A study concerning the position of magnet versus GMRs and detected eddy current magnetic field level and the distortion values was included in the present work. Referring to the scanning experiences of the aluminum plate specimens the uniform eddy current probe motion was assured using a xy-automated positioning system with high resolution (up to 50 µm) that receives the positioning commands from a laptop PC through an RS-232 interface (Fig.3). The UECP tests were done using a PXI system that was described in [9]. III.

UECP CONDITIONING CIRCUITS

The UECP conditioning circuit includes two main modules that include a voltage to current conversion module (V-I conv) and a programmable gain amplifier (PGA) module (Figure 3). The V-I converter is designed to provide an excitation current with appropriate values of amplitude and frequency. It is driven by a sinusoidal signal from the line out of the real-time control processing and communication units (RT-CPCU) based on TMS320C6713 and AIC23 codec. The voltage to current converter is implemented using an OPA2604 operational amplifier and SPP03N60C3 cool MOS power transistor. A set of precision, low power instrumentation amplifiers (INA118 from Burr-Brown) materializes the PGA that is used to amplify the GMR sensors voltage output (VGMRi). The gain selection is done using a set of resistances and a switching

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circuit based on DG303 from Maxim. The switching circuit control is performed using one of the digital output lines of (RT-CPCU) based on TMS320C6713 DSK. Filtering capacitors were connected to the GMR sensors and PGA power supplies in order to reduce the noise. \

RT-CPCU Embedded JTAG

DSK6 HPI (USB)

USB DSP

SDRAM

USB/ RS232 conv RS232

TMS320C6713

FLASH CPLD

AIC23Codec DAC

Dx

DIO

Rotra Motion

Dy

ADC

x y MUX

UECP

PGA

Iex≈ VRA

R

based on the XR-2206 monolithic function generator. Taking into account the limited memory resource of the RT-CPCU boards and the timing requirements, the second solution assures better performances regarding the quality of the injected sinusoidal signal, with no lost data events during the specimen scanning process. A drawback of the second sinusoidal generation scheme is the impossibility to choose by software the signal frequency for a given bandwidth (100Hz5kHz) in the present case. However, using a switching procedure working under the DO3 digital output control, two frequency values can be imposed (e.g.: 1 kHz, 5 kHz). The PGA and VRA output voltage signals are acquired using the stereo 16bit ADC, a part of AI23 codec, which supports sampling rates up to 48kS/s. The maximum working voltage for analog inputs is 600mV. The limited capabilities of the ADC impose a limited number of values when the excitation frequency goes up to 5kHz, in order to assure enough number of samples for period that will be processed by software in order to obtain the eddy current amplitude and phase informations. Regarding the UECP position on the scanned plate two digital input lines, DI2 and DI3, are used to receive the digital information (Dx,Dy) provided by the Rotra Motion controller. Thus, for each step on the “x” direction the “1” logic value is received by the DI2 line, while for each step on the “y” direction the “1” logic value is received by the DI3 line.

V-I conv

V.

Figure 3. The main blocks of the induction defectoscope based on uniform eddy current probe with GMR and real-time control, processing and comunication unit ( PGA- programable gain amplifier, V-I conv – voltage to current converter, MUX-multiplexer, RT-CPCU- real-time control processing and communication unit, DSK6XXXHPI – host port interface, VRA resistance voltage amplifier, Iex- excitation current )

Additionally to extract the phase difference between the output voltage of the UECP and the excitation current Iex a 10 Ω sensing resistor (Rs) is serial connected to the coil, the resistor voltage, VR being amplified using an INA118 instrumentation amplifier. The amplified voltage VR is applied to the second channel of the AIC23 codec. IV.

EXCITATION CONTROL, ACQUISITION AND SIGNAL PROCESSING UNIT

The analog signals associated with the UECP outputs are acquired using the ADC in the AIC23 codec (Figure 3) included in the RT-CPCU board based on TMS320C6713 DSP DSK operating at 225MHz [11]. This board including an AIC23’DAC and a digital output port (DIO) also controls the sinusoidal signal (VS) applied to the V-I conv. Two VS generation schemes were considered as part of the induction defectoscope. The first scheme is based on the AIC23’ DAC (16bit, maximum 48kS/s) that provides the sinusoidal signal, while the second scheme uses an external sinusoidal generator

SYSTEM SOFTWARE

The induction defectoscope software (IDS) used the code embedded in the RT-CPCU TMS320C6713 board the LabVIEW DSP module [11], while for Rotra Motion control LabVIEW software was implemented. The main IDS components are: a) sinusoidal signal generation or digital selection of the external signal generator frequency, b) UECP signal acquisition, c) UECP position data reading, d) data processing for defect detection, e) defect signaling. The sinusoidal generation component is implemented using a simulated signal and the elemental I/O node function of the LabVIEW DSP module. The trigger event is based on the changes of Dx or Dy values. Thus Dx=“1” or Dy=“1” conducts to sinusoidal signal generation and UECP signal acquisition startup. For the usage of hardware sinusoidal generator scheme, when the Dx or Dy value are “1” the digital line DI4 is “clear” or “set” according to the imposed frequency for a given NDT test (e.g. DI4=”0” for fex=1kHz, “DI4=”1” for fex=4.8kHz). The UECP signal acquisition component is mainly related to the used frequency of the excitation signal (fex). Taking into account the limitation of the system memory (on chip RAM 32Kx32, Flash 512Kx8) the maximum number of acquired points associated with single acquisition frame up to 1024 points that permits to acquire about 100 periods of the generated signal for an imposed frequency of 4.8kHz that means 10 samples for each period of the VUECP signal. The acquired samples are processed in order to extract the signal characteristics as amplitude and phase and to analyze the

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Thus, in order to obtain the static and dynamic characteristics of the used GMR (NVE AA002), positive and negative values of current (|Iex|=0-500 mA) provided by Fluke5700 calibrator were injected in a solenoid where the GMR was disposed and properly oriented. Results concerning the characterization of the GMR are presented in Figure 5

Vout[mV]

amplitude and phase variation during the scanning of the conductive plate specimen (Al plate). A three-parameter sine fitting software algorithm [12] and a function taking into account the hard constrains concerning matrix multiplication and 2D data representation were implemented using the LabVIEW DSP module. After amplitude Aij and phase ϕij calculation, for each (xi, yj) location of the UECP on the scanned plate, a set of arrays Γj are filled with the amplitude and phase values. First and second derivative calculations are carried out as defects signature. The embedded software flowchart of the UECP position reading, acquisition and signal processing, as so as the defect signaling software components is presented in Figure 4. start

n

B=“1”

stop

y n

H[Am-1]

Dx=“1”

y x

Figure 5. GMR sensor experimental characteristics (H – applied magnetic field intensity, Vout – GMR output voltage)

xi=xi+Dx,

The UECP crack detection capabilities were experimentally proved using an automatic measurement system described in [9] that permits to perform 2D scanning of the conductive plates, excitation signal generation, UECP output signal acquisition and data processing. Different tests were done for aluminum plates and for different induced cracks. For the particular case of a specimen with an induced crack characterized by 10 mm length, 0.5 mm width and 1mm depth the obtained 2D magnetic field distribution caused by eddy currents and measured by GMR1 and GMR2 are presented in Figures 6-7.

VUECP(t)|xi,yj, acquistion (0