Design & Development of 4-channel Phased Array Control & Amplifier for EMAT based Phased Array UT System for Weld Joints

Transcription

1 Design & Development of 4-channel Phased Array Control & Amplifier for EMAT based Phased Array UT System for Weld Joints S.K.Lalwani 1,a, G.D.Randale 1, T.V.Shyam 2 and P.Jyothi 1 1 Electronics Division, BARC, Mumbai, India 2 Reactor Engineering Division, BARC, Mumbai, India a skl@barc.gov.in Abstract. Weld joints in Austenitic steel are very difficult to inspect using conventional UT technique employing piezoelectric transducers. EMAT (Electro Magnetic Acoustic Transducer) based UT technique is more suited for such applications by generating Shear Horizontal (SH) wave mode. Due to lower efficiency of EMAT, ultrasound is focused using multiple elements for increasing the SNR. A four element phased array EMAT system is under development at BARC, Mumbai. Two constituent units of this system i.e. 4-channel phased array control and 4-channel high gain amplifier have been designed and developed at Electronics Division, BARC. The control section generates four sets of square wave tone burst signals for the pulser. It is based on four synchronously operating DDS chips, which facilitate user programmable precise frequency (up to 10MHz with 1Hz resolution) and phase control (in steps of 2Π/4096) of the output bursts. Programmable features include precise relative delay between the four trigger signals in steps of 1ns, signal frequency, no. of cycles in the burst, PRF and pulse height control. The amplifier section has four independent programmable gain amplifier channels with instrumentation amplifier (10MHz BW) input stage, protection up to +/-300V, On board HP/BP filter on each channel and Programmable gain of more than 100dB. Both the control and amplifier section have been implemented on a USB based unit which works with a GUI based software. The unit has been tested and interfaced to a four channel high current tone burst type pulser and excitation signals for the EMAT sensors have been generated. Amplifier section has been tested using low level differential signal inputs using a signal generator. This paper describes the design features of 4-channel phased array control and amplifier unit developed at Electronics Division, BARC and results of testing. Keywords: Ultrasonic Testing, EMAT, Phased Array, DDS, Amplifier. Introduction Ultrasonic inspection of Austenitic steel by conventional ultrasonics is a formidable task due to skewing and scattering of ultrasonic beam due to columnar structure of grains in Austenitic steel. Horizontally polarized shear wave is a mode which is uniquely produced by EMATs in PPM (Periodic Permanent Magnet) configuration. The horizontally polarized shear waves have the advantage of propagation of beam in Austenitic steel without skewing and further there is no mode conversion at interfaces especially is dissimilar weld joints. The EMAT transduction is highly inefficient phenomenon. To reinforce energy of the beam it is required to operate the EMATS in phased array configuration. The angle of propagation for the individual EMAT element is decided by the track wavelength of the PPM and frequency of excitation of the EMAT coil. Track wavelength is the distance between two adjacent magnets in PPM. The phased array EMAT probes have the advantage of reinforcing beam in forward direction when EMAT segments are fired with stipulated delay time.

2 A four element phased array EMAT based UT system is under development at BARC, Mumbai for any angle weld inspection in reactor components. Fig. 1 shows the system components. 4-channel phased array control and 4-channel high gain amplifier components of the system have been designed and developed at Electronics Division-BARC; 4-element EMAT sensors at Reactor Engineering Division-BARC and 4- channel high current pulser (EMAT driver) at M/s. Point R Technologies, Thane. This paper deals with the design and development of the control & amplifier unit and result of its testing. Description of 4-Channel Phased Array Control & Amplifier Fig. 2 shows the block diagram of the four channel phased array control and amplifier unit. It comprises of two distinct blocks of 4-channel phased array control section and 4-channel high gain amplifier section. Fig. 1: Four element EMAT based phased array UT system components Fig. 2: Block diagram of 4-channel phased array control & amplifier unit

3 4-Channel Phased Array Control Section: The EMAT driver needs a pair of square wave tone burst signals, for each channel, to generate the desired excitation signals for the four EMAT sensor elements. The two signals of the pair are shifted by half cycle with respect to each other. This section generates four sets of such signals for the driver. It is based on four synchronously operating DDS channels. Each DDS channel can be programmed by the user for desired frequency and relative phase with respect to other channels. This facilitates user programmable precise frequency (up to 10MHz with 1Hz resolution) and phase control (in steps of 2Π/4096) of the output bursts. Each DDS channel comprises of a DDS chip to generate required frequency & phase sine wave, filter to remove harmonics, amplifier to improve slew rate and comparator to generate square wave from the sine wave signal. FPGA collects the continuous square wave signals and generates gated pair of bursts with user programmable number of cycles in the burst. These bursts are generated either on the internally generated PRF trigger or an external trigger input. The tone burst signals thus generated are galvanically isolated and converted to LVDS type before sending them to the EMAT driver. Other control signals for the driver include pulse height control, arm etc. The pulse height control signal is in the form of a user programmable frequency, which is generated in the FPGA. EMAT driver provides its status on the status lines to this unit. Opto-isolators have been provided for sensing these lines by FPGA. 4-channel high gain amplifier section: The output of the four receiver (Rx) sensor elements are connected to this section. The four channel amplifier section has four independent programmable gain amplifier channels with instrumentation amplifier (10MHz BW) input stage, protection up to +/-300V, On board HP/BP filter on each channel and Programmable gain of more than 100dB. The first stage of the amplifier is a programmable gain amplifier with software gain control. The second Fig. 3: Photograph of 4-channel phased array control &amplifier board

4 stage is configurable fixed gain amplifier, the gain of which is hardware configured as per the requirement. Both the above sections i.e. 4-channel phased array control and 4-channel amplifier have been incorporated on a single PCB with USB interface to PC/laptop. There is galvanic isolation between the two sections on the PCB. Fig. 3 shows the photograph of this board. Other features on this board include: a) Generation of clock and trigger signals for the interface of external 4-channel DAQ hardware: Provision has been made to interface two different DAQ boards available commercially. For this purpose LVPECL as well as 1Vpp clocks are generated on this board. b) Suitable connectors have been provided for implementing add-on hardware for on board DAQ. GUI based Control Software: Windows based control software has been developed for control of all the features on this board. Fig. 4 shows the screen print of the GUI. Technical Specifications Phased Array Control: No. Of Channels : 4 Frequency Range : 1 khz to 10 MHz (1Hz Step) No. Of cycles : 1 to 32 Relative Delay : Upto 255us (in steps of 1ns) Pulse Output : 4 pairs of tone burst signals Output signals : LVDS PRF : 1Hz to 1KHz Pulse Height Control: 50V to 500V in steps of 10V Amplifier: No. of Channels : 4 Gain : 40dB to 100dB Bandwidth : 4MHz Filter : Highpass/Bandpass Interface : USB2.0 Power supply : 230VAC Dimensions : 19 x 3U x 350mm Fig. 4: GUI based software for 4- channel phased array control & amplifier unit Testing and Results The 4-channel phased array control & amplifier unit has been tested for generation of the required control signals for the EMAT driver. Fig. 6 shows trigger command pairs generated by the unit which are used to drive the high current pulser. Fig. 5: Photograph of 4-channel phased array control & amplifier unit

5 (a) (b) (c) Fig. 6: Trigger commands generated by phased array control & amplifier unit for driving the high current pulser. a) Pair of tone burst signal for one channel, b) two pairs of the control signals generated on two of the four channel outputs with relative delay between the two, c) two control outputs (only one signal from each pair) with 50ns relative delay, d) four control signal outputs of 700kHz and two cycles with relative delay of 200ns each at 100Hz repetition rate. The amplifier section of the unit was tested using a signal generator, converting the single ended output of the signal generator to differential signals and the output of the amplifier was observed on an oscilloscope. Fig. 7 shows amplifier output with about 100dB gain when a continuous sine wave of 1MHz was applied at input. (d) The unit has been interfaced with the high current pulser designed and developed by M/s. Point R Technologies. Four trigger pair commands were generated with other necessary control signals and applied to the pulser. The frequency, number of cycles in the burst, PRF and relative delays were varied and the four outputs of the pulser were monitored. Fig. 8 Fig. 7: Amplifier output with 1MHz input low level signal and 100dB gain

6 shows four excitation signals generated at the output of pulser and applied to four EMAT sensor elements. The high power signals were stepped down before applying to the oscilloscope for measurements. Fig. 8: four excitation signals generated at the output of high current pulser Conclusion A 4-channel phased array control and high gain amplifier unit has been designed and developed at Electronics Division, BARC for development of EMAT based 4-channel phased array UT system. The control & amplifier unit provides LVDS compatible trigger commands and control signals for the high current pulser for generation of precise frequency and phase delays between the excitation signals for the 4 EMAT sensor elements. The control section employs four synchronously operating DDS channels which facilitate user programmable precise frequency and phase control of the output bursts. The amplifier section has four independent programmable gain amplifier channels for amplifying the weak signals from the receiving EMAT sensors. This USB based unit has been tested and interfaced to a four channel high current tone burst type pulser and excitation signals for the EMAT sensors have been generated. Acknowledgements Authors are thankful to Shri C.K. Pithawa, Director E&I and A&M Groups; Dr. P. K. Vijayan, Director, Reactor Design and Development Group, Dr. T.S. Ananthakrishnan, Head, Electronics division; Shri K. Madhusoodnan, Head, Reactor Coolant Channel Section, Reactor Engineering Division and Shri Gopal Joshi, Head ACS Section, Electronics Division - BARC for their encouragement and guidance for this development. Authors are also thankful to M/s Point R Technologies, Thane for providing support during testing of the unit.

7 References [1] C.F.Vasile and R.B. Thompson; Excitation of horizontally polarized shear elastic waves by electromagnetic transducers with periodic permanent magnets ; J. of Applied Physics, 50(4), April [2] K. Sawaragi et. al; Improvement of SH-wave EMAT phased array inspection by new eight segment probes ; Nuclear Engineering and Design 198(2000), [3] S.K. Lalwani, G.D. Randale, P. Jyothi and S.S. Pandey; Development of USB based integrated Tone Burst Generator, Receiver Amplifier & 100MSPS Digitizer for Ultrasonic NDT and other applications ; Proceedings of National Symposium on Nuclear Instrumentation (NSNI- 2013), Nov 19-21, 2013 organized at Anushakti Nagar, Mumbai, India. [4] All about Direct Digital Synthesis, Analog Devices Inc., [5] S.K. Lalwani, Reetesh Chaurasia, Alok Agashe and V. M. Joshi; Development of USB bus based compact Tone Burst Generator/Receiver, proceedings of IINC-2005 held at IIT, Bombay from st Dec