Constant Current AC Source using Improved Howland Pump for Exciting Eddy Current Testing (ECT) Probe

Constant Current AC Source using Improved Howland Pump for Exciting Eddy Current Testing (ECT) Probe Sumayya Abbas, Uzair Gillani†, Shayan Ahmed‡, Salar B Javed§, Iqra Sajidς Electronic and Power Eng.

Constant Current AC Source using Improved Howland Pump for Exciting Eddy Current Testing (ECT) Probe Sumayya Abbas*, Uzair Gillani†, Shayan Ahmed‡, Salar B Javed§, Iqra Sajidς Electronic and Power Engineering Department Pakistan Navy Engineering College, National University of Sciences and Technology*†‡§ς Karachi, Pakistan Email: *sumayyaabbas@hotmail.com, †uzair.gilani20@gmail.com, ‡shayanahmedhassan@gmail.com, § salarbinjaved@gmail.com, ςiqrasajidwazirkhan@hotmail.com Abstract—an endeavor has been done to produce a cost effective Eddy Current Testing System For this purpose, study was carried out on using the Improved Howland Pump as a constant current AC source for exciting inductive load transducer i.e the Eddy Current Testing (ECT) probe Experimental validation was performed by fabricating the constant current AC pump and using it for excitation purposes in the eddy current testing system As a result, through-hole surface cracks were successfully detected which proved the working of the Improved Howland Current pump as a constant current AC source It is a valuable technique for exciting Eddy Current Probes I INTRODUCTION Nondestructive testing (NDT) is a wide group of analysis techniques in science and industry to evaluate the properties of a material, component or system without causing damage [1] In this study, a constant current AC source is designed to be used as an excitation source in an Eddy Current Testing system Eddy Current Testing is a nondestructive testing technique that is based on the principle of Faraday’s law of Electromagnetic Induction to inspect cracks/flaws in metallic non- magnetic materials [2] If a crack in the material disturbs the eddy current circulation, the magnetic coupling with the probe is changed and a defect signal can be read by measuring the coil impedance variation [2] [3] The magnetic field generated in the coil is directly proportional to current A constant current AC source is needed so that a constant magnetic field is generated in the coil and remains the same throughout the inspection Only that can offer uniform and precise testing of the metal specimen The aim of the study on Eddy Current Testing system is to devise a system that would detect surface cracks on aerospace structures The basic objective is to fabricate an affordable system These systems are very expensive An effort has been made to produce a cost effective solution at laboratory level so that further improvements can be easily done The waveforms acquired at every step of instrumentation section prove that harmonics are not an issue They are very stable 508 and are not affected throughout the running of the system That is why no harmonic filter has been used Eddy Current Testing system has been designed by using the constant current AC source designed and developed through the Improved Howland Current Source with modifications for the particular study In this paper, we describe the need for constant current AC source in Eddy Current Testing This is a must for Eddy Current Testing and is the key behind the whole concept It has been further explained in II The hardware design and its implementation using a sinusoidal oscillator is then explained Moreover, protection circuitry has been designed for driving inductive load (the ECT probe/coil) which is the main purpose of the constant current AC source Finally, results are shown using the designed Constant Current AC source These results prove the feasibility of the design for the said purpose and have been experimentally confirmed as well II CONSTANT CURRENT AC SOURCE The concept of eddy current testing is based on the principles of Electromagnetic Induction The Maxwell’s Ampere Law states that an alternating current sets up a time varying magnetic field [4], which is known as a primary magnetic field When a coil excited with a constant peak alternating current source is placed in close proximity to a conducting surface, the alternating magnetic field will induce a secondary time varying magnetic field of fixed amplitude in the nearby surface as a result of which an EMF is produced [5] This induced voltage produces eddy currents in the material which by Lenz’s Law oppose the primary magnetic field [6] This mutual inductance causes a change in the impedance of the coil The path of these eddy currents is circular in nature and is disturbed due to the presence of a crack in its way As a result the impedance of the coil changes [7] In the case of Eddy Current Testing, if the current source providing the excitation to the eddy current coil is not constant, the presence of cracks will not be detected It is necessary that the primary magnetic field remains constant and uniform throughout the course of inspection in order to detect surface cracks on metal specimen This is so because the phase shift that is detected by the demodulator compares the two input and output _ ISBN: 978-1-4799-5754-5/14/$26.00 ©2014 IEEE waveforms of the Howland circuit That is why the primary magnetic field (input waveform) should remain constant at all times so that the output signal can be measured in comparison with it The current source using Improved Howland Pump has been designed for the same purpose; Eddy Current Testing The working and results are discussed ahead III METHODOLOGY A Sinusoidal Oscillator An oscillator is required to provide input to the excitation source This is a sinusoidal oscillator with frequency of 30 kHz and Vpp amplitude The design chosen is the phase shift oscillator [9] It offers a stable frequency and absolutely no noise to drive the source The schematic of the circuit can be seen below in Fig developed on MultiSim software v13.0 The project Eddy Current Testing system consists of two main parts; hardware and the software The basic hardware flowchart is shown below in Fig [8] Fig Phase Shift Oscillator Schematic The working formula of the above circuit is shown below in (2) where R= resistance, C= capacitance [9] Fig Hardware Flowchart √ The oscillator drives the current pump that excites the eddy current probe The probe is used for detecting surface and through-hole cracks Whenever a crack is present, the ECT probe undergoes a change in impedance, which in turn causes a phase shift between the input and output signals of the probe The output from the probe is prepared for demodulation in order to recover the message signal, which is basically the crack signal Once the signal is demodulated or extracted from the carrier, it is sent to the PC through a Data Acquisition (DAQ) card for analysis and real-time plotting The excitation portion of an eddy current system consists of signal generation amplifiers to drive the transducer (inductive load in this case) The signal generator is basically an oscillator that provides the Sine Wave excitation for the eddy current coil The design emphasis at this stage is to produce a signal of adequate frequency accuracy, stability and low distortion levels for reliable testing [8] If frequency is inaccurate, the depth of penetration given in (1), is not as expected to get the desired results In (1), ς= skin depth, µ= permeability and σ = conductivity (2) Using for this circuit V=5V, R=2.2 kΩ and C=101µF, the frequency output and voltage were approximately 30 kHz and Vpp These frequency and voltage levels are not large enough as to produce significant heat in the circuit As a result, the output signal is not affected by this reason All of the designs have been experimentally tested and no phase difference occurs due to heat in the system The only heat losses produced are by the operational amplifier OPA548 but the output remains stable due to high input impedance of the amplifier A PCB was manufactured and can be seen below in Fig (1) Since excitation is the beginning of the signal train of the instrument, it is necessary that this signal is accurate and stable In simpler words, we require an alternating current source that has a constant maximum/peak value This is a difficult task as there is a need of a purely sinusoidal current output that produces efficient and reliable magnetic field for eddy current testing purposes The excitation source is the fundamental requirement of the system Fig Phase Shift Oscillator PCB Testing was done by using a digital oscilloscope for amplitude and frequency Pure sinusoidal waveform can be seen in Fig 509 Amplitude (V) -2 -4 20 40 60 80 Time (us) 100 120 140 160 Fig Phase Shift Oscillator Output Waveform B Constant Peak Alternating Current Source A constant peak alternating current (AC) source is needed to excite the eddy current probe of above 100µH inductance The design shown in Fig is used for this purpose [10] It basically uses the concept of voltage controlled current source [11] [12] It changes voltage signal into current signal Fig Improved Howland Current Pump Schematic For the Howland current pump, a power amplifier OPA548 is used due to the following advantages • • • • Fig Improved Howland Current Pump For the circuit shown in Fig 5, the load current is given by (3) where Vl = load voltage and Vi = input voltage (3) The circuit works on the principle that all four resistors should be perfectly matched and the operational amplifier should have a large gain in order to keep the current directly proportional to input voltage and the current sense resistor (R2B) We have used 0.1% precision resistors in order to keep the current to remain constant throughout The voltage at the two inputs of the operational amplifier remains the same due to presence of these resistors The circuit also introduces a separate resistor R6 for current flow control and negligible current flows through the four precision resistors As a result power dissipation due to them is also least and only one power resistor is required in the circuit which is the ‘current sense’ resistor It is used of only 1Ω and so I2R losses are not an issue The design was simulated on MultiSim software v13.0 along with the protection for driving inductive load of eddy current coil Given below in Fig is the schematic of the circuit 510 high current output current limit feature controllable through RC capable of driving transducers TO-220 transistor packaging allows for efficient heat sinking and easy use The PCB was designed and maximum output current limit has been set at 1A by using the relevant resistance Other resistance values used in the Howland Current Pump are given below in TABLE I TABLE I IMPROVED HOWLAND PUMP RL VALUES S No Label R1 R2A R3 R4 R2B Rl Xl Value 10 kΩ kΩ 10 kΩ kΩ 1Ω 1Ω 115.7 µH All precision resistances used are of 0.1% tolerance level Fig has been taken from the datasheet [13] that shows the safe operating region of OPA548 Fig Improved Howland Current Pump with Snubber The main component of the snubber is the capacitor that is comparative to the inductance of the eddy current probe During turn off and turn on periods, the capacitor does not allow sudden change of voltage across it And hence the inductor discharges slowly The time constant is set by using a resistance in series with the capacitor The hardware circuit for the Howland Current Pump can be seen below in Fig 10 Fig Safe Operating Area Graph (OPA548) Values of voltage and current were set accordingly shown in Table II TABLE II VOLTAGE AND CURRENT PARAMETERS Parameter Frequency (f) Vin Io Vs-Vo Value 30 kHz +/-15V 1A max limit 22V 1) Improved Howland Pump Protection for Inductive Load Since the transducer is inductive in nature, protection against inductive loads is also necessary A simple RC Snubber Circuit [14] [15] has been used in this case that incorporates a resistor and capacitor in series with each other and parallel to the load As seen in Fig 8, the voltage spikes at turning on/off of supply is greater than times the output voltage Fig 10 Improved Howland Current Pump PCB The PCB in Fig 10 shows an external heat sink with 11 fins at both left and right sides It allows for efficient heat removal from the amplifier as air passes across the heat sink at all times The only two power resistors in the circuit are that of 1Ω and hence other heat losses is not a problem except that generated by the IC The output of the circuit can be seen below in Fig 11 The yellow waveform is that of input and the blue waveform is that of output which is being sent to the eddy current probe to excite it for the detection of cracks 20 Amplitude (V) S No 10 -10 -20 20 40 60 80 Time (us) 100 120 140 160 Fig 11 Improved Howland Current Pump Output Waveform Fig Improved Howland Current Pump without Snubber Hence a snubber circuit has been used for this purpose This means that any kickback of the inductor would circulate through the snubber circuit and the IC would remain safe The result can be seen below in Fig Since the input is pure sinusoidal, the output follows showing that the input impedance of OPA548 operational amplifier is very high The Howland current pump has been made using the inverting amplifier configuration And hence the input and output signals are 180 degrees out of phase from one another as clear from Fig and Fig 11 IV RESULTS AND FUTURE DIRECTION This constant current AC source has been used for the purpose of eddy current testing The Eddy Current Probe [8] [16] is shown in Fig 12 and with its specifications in TABLE III 511 The probe then moved/dragged over the specimen for its inspection In terms of future work, efforts are being made to design a very portable Eddy Current System using the same Improved Howland current pump as a constant current AC source And to improve the sensitivity of the system so that it can detect minor surface and sub-surface cracks as well V A constant current AC source has been designed by using the Improved Howland Pump and results prove their usefulness in Eddy Current Testing System The constant current AC source gives a very stable output even on different values of the inductive load This study allowed detection of through-hole cracks and by improving the sensitivity of the ECT Probe, smaller cracks can also be detected The overall system is cost effective as compared to the internationally available systems being imported to Pakistan and also acts as a precursor to further NDT endeavors within the country Fig 12 Eddy Current Probe TABLE III EDDY CURRENT PROBE PARAMETERS S No Parameter Coil Radius Coil Inductance No of turns Value 0.4in 115.75 µH 100 The eddy current probe was used to test through-hole surface cracks on an Aluminum sheet of thickness 2-3mm and dimensions of 12’*12’ The signal from the probe was demodulated and filtered to get the required signal of the crack The demodulated signal can be seen below in Fig 13 ACKNOWLEDGMENTS The authors pay special thanks to Dr Tariq Mairaj, our Advisor; Dr Faisal Amir, the Co-Advisor; Mr Taha Ali, Research Assistant at NDT Center, PNEC; and the rest of the NDT team for their continuous support and technical advices Amplitude (V) 20 10 -10 REFERENCES 20 40 60 80 Time (us) 100 120 140 160 Fig 13 Demodulator Output Waveform The output of the demodulator is a square wave whose pulse width changes when a crack is detected But the change was very minute and could not be picked up visually So, this signal was filtered out and the result can be seen below Amplitude (V) Voltage level in Air Voltage Impulse on crack -1 Time (s) 10 12 14 16 Fig 14 Low Pass Filter Output Waveform In this step, the square wave has been filtered out to get pure dc voltage It was observed that the voltage level dropped when the probe was moved from air to the metal specimen and then a rising impulse occurred when the probe passed over a crack This is clearly depicted in the waveform of filter output i.e Fig 14 In this section, it is important to note that the position of the probe is changed uniformly over the test specimen so that all cracks can be detected Moreover, the probe is touched with the specimen to avoid liftoff errors while testing Otherwise, cracks may be missed or wrong indications may be received Hence, it is very important to keep the probe very close to the metal specimen in order to concentrate the magnetic field on it 512 CONCLUSION [1] E.P Mix (1987), ‘Introduction to nondestructive testing: a training guide’ New York, (2nd Ed) Wiley [2] Hagemaier, D J (1990), ‘Fundamentals of eddy current testing’, Columbus, OH: American Society for Nondestructive Testing [3] H L Libby (1979), ‘Introduction to Electromagnetic Nondestructive lest Methods’, Wiley [4] R E Beissner P HÃller (1988), ‘Theory of eddy current characterization of magnetic conductors’, Proc 3rd Int Symp Nondestructive Characterization of Materials, Saarbrück, pp.541 -548 [5] R.C McMaster (1958), ‘Nondestructive testing handbook edited for the Society for Non-destructive Testing’, New York: Ronald Press Co [6] J Garcia-Martin, J Gomez-Gil, E Vasquez-Sanchez (2011), ‘Non-destructive techniques based on Eddy current testing sensors’ [7] Kitchin, C., ‘Analog devices: Amplifier circuits’, Analog Dialog, p41 [8] E James, J David and J Eric, ‘Electromagnetic testing’, chapter 6, pp 172-187 [9] V P Ramon, ‘RC phase shift oscillators’, Universidad Nacional De Ingenieria [10] R.A Pease (2008), ‘A comprehensive study of the Howland current pump’ [11] D X Chen, X Deng and W Q Yang (2010), ‘Comparison of three current sources for singleElectrode Capacitance Measurement’, Review of Scientific Instruments, Vol 81, No 3, pp 1-3 [12] Apex Micro technology Inc (2012), ‘Voltage to Current Conversion’, Apex [13] Texas Instruments Inc (2012), ‘High-voltage, highcurrent Operational Amplifier’ [14] William McMurray (1972), ‘Optimum Snubbers for power semiconductors’, IEEE IAS transactions, Vol IA-8, No 5, pp 593-600 [15] S Rudy, ‘Design of Snubbers for power circuits’ [16] A Ayad, A Semmah, A Lahcen and Y Ramdani, ‘Numerical and analytical analysis of eddy current Non-destructive testing in JSAEM benchmark’, JEE Journal of Electrical Engineering [17] D X Chen, X Deng and W Q Yang (2010), ‘Comparison of Three Current Sources for SingleElectrode Capacitance Measurement’, Review of Scientific Instruments, Vol 81, No 513 ... surface and sub-surface cracks as well V A constant current AC source has been designed by using the Improved Howland Pump and results prove their usefulness in Eddy Current Testing System The constant. .. specimen for its inspection In terms of future work, efforts are being made to design a very portable Eddy Current System using the same Improved Howland current pump as a constant current AC source. .. This constant current AC source has been used for the purpose of eddy current testing The Eddy Current Probe [8] [16] is shown in Fig 12 and with its specifications in TABLE III 511 The probe