10.1109/ULTSYM.2014.0356
FPGA Embedded System for Ultrasound Particle Manipulation with Sonotweezers Han Wang, Yongqiang Qiu, Christine Demore, Sandy Cochran Institute for Medical Science and Technology, University of Dundee, Dundee, United Kingdom
[email protected] Abstract—Recent research in acoustic tweezing has mainly focused on the behaviour of devices for specific applications and the fundamental theories of the forces exerted on the target particles. However, compared to optical tweezers, there have been only limited reports about developments and applications of acoustic tweezers at the system level. This paper outlines a novel design of a multichannel electronics system for ultrasonic particle manipulation applications. The development process of this 16channel embedded-FPGA ultrasonic signal generation system is described and experimental results are presented based on its use with array-based acoustic tweezing devices (Sonotweezers). Keywords—FPGA, Electronic Sonotweezer, Array, Acoustic Manipulation
I.
INTRODUCTION
Acoustic particle manipulation technology has drawn great interest from researchers who are studying methods for contactless particle manipulation. Termed “acoustic tweezing” or “sonotweezing”, acoustic manipulation approaches use either ultrasound standing waves or ultrasound travelling waves to exert acoustic radiation forces to trap and manipulate relatively large particles (diameters over 10 μm) or particle agglomerates (dimensions of hundreds of micrometres) in a medium such as air or a fluid. In this arrangement, particles or particle concentrations are focused either at the local pressure maxima (pressure anti-nodes) or local pressure minima (pressure nodes), depending on their acoustic properties, and can be manipulated by moving these features in the field. This paper focuses on the approach using ultrasound standing wave (USW) devices which is being widely researched [1]. The fundamental principle of the device for particle manipulation comes from the mechanism of varying the positions of pressure nodes and anti-nodes. For USW devices, the pressure nodal plane can be controlled by two principal approaches. One is by means of variation of the chamber geometry that confines the effective acoustic field. A typical example is in multilayer resonator devices [2]. By tuning the thickness of the chamber, defined between the transducer and reflector, the position of the pressure nodal and anti-nodal planes can be varied close to or further away from the reflector. Another approach to control the position of the pressure nodes/anti-nodes within USW devices is to vary the electrical signals which excite the ultrasound transducers if multiple transducers are present.
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Electrical signal frequency, amplitude and phase are the three main factors that make direct contributions to the acoustic field that is created. It has been reported that by adjusting the transducer excitation signal frequency, the pressure nodes / anti-nodes in USW devices can be manipulated, as a result of acoustic wavelength variation [3]. It has also been demonstrated that, by changing the phases of the excitation signal, in USW devices the positions of the pressure nodes / anti-nodes can be controlled [4]. Ultrasound transducer array technologies that are conventionally used in medical imaging applications have recently have been adopted in acoustic particle manipulation research. The advantages of using arrays over single element transducers are that they bring the capability to construct complex patterns of acoustic fields with the multiple transducers in the array. In using such devices, demands arise for multichannel electrical systems that allow the possibility of constructing dynamic and reconfigurable acoustic fields to improve particle manipulation dexterity. Although such instruments have been studied extensively in medical ultrasound imaging, there are limited reports of the design and development of systems optimised for acoustic tweezing applications. This paper outlines the development of a low-cost, fully programmable, multichannel electronic ultrasound array driving system that allows dynamic reconstruction of the acoustic field for particle manipulation with array-based acoustic tweezing devices (Sonotweezers). The present system incorporates an FPGA (Field Programmable Gate Array) as a reconfigurable digital core, and low-cost, low power consumption 16-channel buffer amplifier array for excitation of the transducers. This bespoke system allows full control of the output signal frequency, amplitude and phase for up to 16 independent channels, with many more possible through straightforward expansion. Experimental demonstration of the system with Sonotweezers is discussed. II. SYSTEM DESIGN AND DEVELOPMENT A. User Interface Design Depending on the complexity of the acoustic models for different Sonotweezer devices, the user interface could be designed in a more complex form at PC level with graphical features, or in a simpler form at hardware level with input
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devices such as switches and potentiometers. In this paper, the demonstration interface at PC level was designed in the MATLAB environment (R2012b, The MathWorks Ltd, Cambridge, UK). As versatile computational software, MATLAB is ideal for this situation, permitting easy integration of the physical - mathematical acoustic models with a Graphical User Interface (GUI) design in a single package. B. Digital Core Configuration For the purpose of building a reconfigurable and programmable electronic system, FPGA technology was adopted in the present development. With extensive applications in almost all types of industry, the FPGA is recognized to provide high performance, reusability, fast time to market, and cost-effectiveness. In this paper, an off-the-shelf FPGA development board (Spartan 3a Starter Kit, Xilinx, Inc., San Jose, CA, USA) was used for ease of rapid prototyping of different functional designs with a relatively low cost. A picture of the development board is shown in Fig. 1, with key features for the present application highlighted.
2) Frequency Control The FPGA output signal frequency is determined both by a customized Digital Frequency Synthesizer (DFS) VHDL module and the phase resolution in bits defined previously. The output of the frequency synthesizer module is a signal with a frequency corresponding to the FPGA global frequency, FREQ_FPGA, divided by a fractional factor, DIV_FACTOR equal to M.L/N, where L, M and N are integers and M ≥ 2, L, N ≥ 1. The synthesized frequency can be calculated as: _
_
(2)
_
This synthesized frequency is used to clock the single-cycle phase data for a continuous waveform. The signal output frequency, FREQ_OUT, is defined as: _
_ _
_
(3)
The duty cycle of the waveform is non-uniform as a result of the non-integer frequency divider and, for continuous wave output, the equivalent frequency is achieved as a mathematical weighted average of different sub-frequency components.
Fig. 1. FPGA-board with complete 16-channel system assembly inset
An FPGA soft-core was designed in VHDL (Very Highspeed integrated circuit Hardware Description Language) as part of the 16-channel array driving system. The main function of the FPGA core in this case is to alter signal frequency and phase. Fig. 2 is a block diagram of the core logic design showing the signal input and output ports and internal signal flow. The core was designed to generate frequency synthesized signals with quantized phase values. 1) Phase Control Signals are digitized in order to allow representation with different phase values. A series of binary codes combined with a few digits of “0” and the same number of digits of “1” in the form such as “000111” is used as a single-cycle signal with 50% duty cycle. The total number of digits in one cycle is defined as the phase resolution in bits, PHA_RES_BIT. The phase resolution in degrees, PHA_RES_DEG, is defined as: _
_
°
_
_
(1)
A phase look-up table with fixed phase resolution is built after synthesizing the VHDL code, and the phase value for each channel is ready to be assigned by an external phase select signal input outside the FPGA, as shown in Fig. 2. For each channel, the desired phase value is picked up from the look-up table and converted into a continuous waveform through a waveform generation module.
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Fig. 2. Logic flow diagram of the FPGA core design
C. Analogue Electronics Design The output from the FPGA electronics is an alternating digital signal with a small fixed range of 0 - 3.3 Vpp. However a bipolar excitation signal is preferred for piezoelectric transducers. As a result a differentiator circuit was created to alter the unipolar signal to bipolar. In addition, for each channel, a potentiometer was added to enable manual adjustment of the final signal amplitude. All channels were constructed on a single printed circuit board (PCB) for signal conditioning. Because of the small dimensions of the piezoelectric transducers in typical Sonotweezers devices, their electrical impedance values are normally hundreds or thousands of ohms, requiring power amplification units to generate useful transducer surface displacements. A customized 16-channel amplifier array was designed with operational amplifiers (AD811, Analog Devices, Inc., Norwood, MA, USA) and current buffers (BUF634, Texas Instruments, Inc., Dallas, TX, USA). For each channel, with ± 15 V power supply, the operational amplifier can provide up to 30 Vpp, and the current buffer is able to generate up to 250 mA output.
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D. Complete System Assembly The complete system was assembled in two layers in a casing of 334 mm × 289 mm × 117 mm. The top layer has the FPGA development board connected with the 16-channel signal conditioning PCB, and the bottom layer has the power amplifier circuit constructed on two identical PCBs with eight channels each. A 6 mm × 6 mm fan was mounted on the back wall of the case for cooling to enable stable amplifier performance, and a 60 W switch-mode power supply unit (TXL 060-0533TI, TRACOPOWER, Baar, Switzerland) was used as a global power source for the electronics. III.
B. Analogue Electronics Characterisation Fig. 4 shows an output signal from the complete system. For each channel, this was a near sinusoidal, continuous signal with a voltage amplitude up to 24 Vpp without waveform distortion for open load conditions. In Fig. 4, the Fast Fourier Transform (FFT) of a single channel output shows that as a weighted average result from the frequency synthesizer in FPGA, accurate frequency output can be achieved with minimal other frequency components. The -3 dB bandwidth of the channel output signal was measured as 10 MHz.
SYSTEM CHARACTERISATION
A. Digital Electronics Characterisation At PC level, the desired signal frequency and phase values for each channel were calculated in MATLAB based on acoustic models for different Sonotweezer devices. The data was then transmitted from the PC to the FPGA through serial port communication. The resulting digital signals outputs from the FPGA development board were tested with an oscilloscope (InfiniiVision MSO-X 3014A, Agilent Technologies, Inc., Santa Clara, CA, USA). A screenshot of a typical 16-channel test waveform is shown in Fig. 3. The waveforms generated by all channels are mutually independent with relative phase shifts. The phase resolution was pre-defined in the FPGA core and the phase value for each channel was configured through MATLAB. The signal frequency is directly proportional to the FPGA clock frequency and inversely proportional to the phase resolution in bits. In the present test, with an on-board 133.33 MHz FPGA clock, for 16-bit of phase resolution, according to Equations (2) and (3), the maximum signal frequency is 4.167 MHz and, according to Equation (1), the minimum phase resolution is 360°/16 = 22.5°. The frequency synthesizer is able to generate frequencies with a resolution of 0.001 MHz, which is sufficient for acoustic tweezing applications. This is especially useful for multilayer resonator devices which require precise frequency control to generate homogeneous pressure nodal plane [2].
Fig. 4. Single channel output waveform (yellow) and FFT of the signal (purple)
IV.
EXPERIMENTAL DEMONSTRATION
The Sonotweezer device used for demonstration was a 16element circular ultrasound array with a central fluid chamber of diameter 10.8 mm surrounded by a ring of piezoelectric transducer elements [4]. In principle, by applying relative phases between the array elements, the device is able to create a reconfigurable acoustic field in a 1st order Bessel function shape for particle trapping and manipulation. The experimental setup is shown in Fig. 5(a). In the experiment, 10 μm diameter fluorescent polystyrene microspheres (Fluoresbrite, Polysciences, Inc., Warrington, PA, USA) in a water suspension were introduced into the fluid chamber. With a relative phase shift of 360°/16 = 22.5°, a stationary concentric circular acoustic field was generated. Polystyrene beads were concentrated in the central pressure node as well as in other circular pressure nodes, as shown in Fig. 5(b). At PC level, based on the circular array acoustic model, the phase profile across all 16 transducer elements for each desired geometric position of the central pressure node was calculated and stored in a data matrix in MATLAB. Then each phase profile was transmitted in sequence to the FPGA through a RS232 serial port with a baud rate of 19200 bit/s. Output signals for all 16 channels with amplitudes of 20 Vpp and frequency 2.35 MHz were generated simultaneously to excite the transducers. Consequently, this allowed particles concentrated in the central pressure node, as well as those in surrounding circular pressure nodes, to be manipulated in realtime, synchronized with the update rate of the phase profiles.
Fig. 3. 16 channel signal output waveforms with relative phase shifts
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Fig. 5. (a) Experimental setup with circular array Sonotweezer. (b) Trapping of ø10 μm polystyrene beads with a stationary acoustic field.
For the present data transmission rate, the maximum particle manipulation update rate was 100 Hz. As shown in Fig. 6(a), the coordinates of 60 geometric positions within the circular chamber were pre-defined in MATLAB, each corresponded to a specific phase profile for the array elements. As the transducers were excited, the pressure nodes represented by concentrated particles in the chamber moved in 60 corresponding steps. The movements of the particles were recorded through a microscope camera (Moticam 2500, Wetzlar, Germany) and the video was post-processed using the open source software, ImageJ, to depict the trajectory of the central pressure node, as shown in Fig. 6(b). Considering the inhomogeneities introduced by the array transducer fabrication process, the shape of the trajectory has a desirable good correspondence with the pre-defined coordinates in the acoustic model calculations. V.
CONCLUSIONS AND FURTHER WORK
A. Conclusions This paper has demonstrated the feasibility of developing a fully programmable, low-cost multichannel electronics system optimised for use with array-based Sonotweezer devices for microparticle manipulation applications. The current system has 16 independent channels that can provide continuous nearsinusoidal signal outputs with amplitudes up to 24 Vpp and current up to 250 mA. The output signal frequency can be adjusted with an accuracy of 0.001 MHz. This system is able to generate reconfigurable complex acoustic fields, making it useful for experimental characterisation of devices in acoustic tweezing research. Furthermore, compared with, for example, costly and space-consuming optical tweezing systems, the Sonotweezers electronics system developed has a relatively low cost ($78 per channel) and dimension of only 334 mm × 289 mm × 117 mm, which could be easily reduced further. B. Further Work Further improvements of the system will emerge from work on diverse applications. Currently the number of output channels is limited by the logic gate capacity of the Xilinx Spartan 3a FPGA. With a high-end device, the channel count
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Fig. 6. (a) Coordinates pre-defined in MATLAB for the positions of central pressure node. (b) The trajectory of the central pressure node in the circular array acoustic field formed approximate same as in (a).
could be increased significantly. The maximum output signal frequency could also be further improved with a faster FPGA clock speed, as well as improved analogue electronics for higher signal bandwidth. More accuracy in the signal amplitude control could be explored, for example with digitally-controlled potentiometers, for applications which requires complex beam forming [5]. ACKNOWLEGEMENTS The authors thank their colleagues in the former Sonotweezers team (Universities of Bristol, Glasgow and Southampton) and Martin Curran-Gray, Gerry Owen and Tony Kirkham from Agilent Technologies, Inc. for their helpful advice. REFERENCES [1]
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Y. Qiu, H. Wang, C. Demore, D. Hughes, P. Glynne-Jones, S. Gebhardt, A. Bolhovitins, R. Poltarjonoks, K. Weijer, A. Schönecker, M. Hill, and S. Cochran, “Acoustic Devices for Particle and Cell Manipulation and Sensing”, vol. 14, no. 8. 2014, pp. 14806–14838. P. Glynne-Jones, C. E. M. Démoré, C. Ye, Y. Qiu, S. Cochran, and M. Hill, “Array-controlled ultrasonic manipulation of particles in planar acoustic resonator,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 59, no. 6, pp. 1258–66, Jun. 2012. X. Ding, J. Shi, S.-C. S. Lin, S. Yazdi, B. Kiraly, and T. J. Huang, “Tunable patterning of microparticles and cells using standing surface acoustic waves”, Lab Chip, vol. 12, no. 14, pp. 2491–7, Jul. 2012. C. R. P. Courtney, B. W. Drinkwater, C. E. M. Demore, S. Cochran, A. Grinenko, and P. D. Wilcox, “Dexterous manipulation of microparticles using Bessel-function acoustic pressure fields”, Appl. Phys. Lett., vol. 102, no. 12, p. 123508, 2013. C. E. M. Demore, Z. Yang, A. Volovick, S. Cochran, M. P. MacDonald, and G. C. Spalding, “Mechanical Evidence of the Orbital Angular Momentum to Energy Ratio of Vortex Beams”, Phys. Rev. Lett., vol. 108, no. 19, p. 194301, May 2012.
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