Paper Title (use style: paper title) - International Journal of Advanced ...

ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering (IJARCSEE) Volume 3, Issue 12, December 2014

Frequency-Controlled Power Ultrasound Unit for Battery-Powered UVC-LED Based Disinfection System Andrej Gross, Felix Stangl, Katharina Hoenes, Martin Hessling, Rainer Brucher Ulm University of Applied Sciences Institute of Medical Engineering and Mechatronics Albert-Einstein-Avenue 55, 89081 Ulm Abstract — A battery operated, energy- and costeffective power ultrasound unit is developed. It is used for purification of contaminated water in combination with an UVC-LED based disinfection system by utilizing acoustic cavitation. Thus biofilm formation is prevented and synergistic effects with UVC-rays enhance disinfection efficiency [1]. The ultrasound unit consists of a Langevin type ultrasonic transducer driven by a specific ultrasound generator optimizing power consumption. It delivers up to 3 watts acoustical output using an excitation voltage amplitude of 100 Vpp. Furthermore the generator is capable of producing a maximum electrical power output of about 23 watts. Drifts of the transducer’s resonance frequency would decrease the power efficiency of the system [2]. In order to reduce this effect an algorithm was implemented for adjusting the optimum driving frequency of the transducer after well- defined time intervals. Acoustical output power was verified with an ultrasound power meter and the generation of transient cavitation was demonstrated by disintegrating aluminum foil. The ultrasound unit delivers low-frequency (~20-100 kHz) power ultrasound with intensities up to 1.7 W/cm² for the generation of acoustic cavitation. Index Terms – ultrasound generator, resonance tracking algorithm, ultrasonic transducer design, disinfection.

I.

INTRODUCTION

Properties of power ultrasound as a sole or synergistic disinfection method for water or food have been subject to many research papers [1, 3]. The basic concept of most biotechnological and cleaning applications with power ultrasound is acoustic cavitation. This physical effect describes the generation and collapse of thousands of micro bubbles in a liquid, triggered by ultrasonic waves and the generated pressure oscillations [4]. The collapse of these bubbles can cause extreme local physical conditions such as high shear forces of up to 50.000 kPa, temperature peaks of 5.000° Kelvin as well as the formation of .O, .H and .OH radicals [2, 4]. The appearing of such mechanical forces are able to damage the membrane of microorganisms or lead to organic defects [5]. So power ultrasound with intensities above 2 W/cm² can basically be applied as a sole disinfectant [3, 5, 6]. However the disinfection process needs considerably more energy compared to classic methods such as UVC or chlorine germicides and is usually not employed for primary disinfection [5, 7]. Therefore the aim of this application

is to obtain homogenizing of solved particles or declumping of bacteria, which needs less energy and therefore can be used to enhance the disinfection rates of UVC-radiation [1, 8]. UV-disinfection of water is a well established and very energy efficient method for the inactivation of pathogen microorganisms directly breaking up the DNA and preventing further reproduction cycles [9]. Here a disadvantage is that UVC-rays are attenuated by large particles, which can also offer shadowing areas for microorganisms [8]. By applying power ultrasound the size of these particles and therefore the shadow protected areas can severely be reduced and thus the disinfection efficiency is increased remarkably [1, 8]. Furthermore power ultrasound is often used for cleaning applications. If cavitation bubbles collapse close to a surface structure, a jet or micro stream is created which encounters the surface with flow velocities of up to 400 km/h and removes agglomerations efficiently [4, 10]. Commercially available ultrasonic cleaning systems are usually designed for larger sampling volumes and work with high input powers up to several kilowatts [11]. Therefore a miniaturized ultrasound system has to be designed, which is able to create transient cavitation in sampling volumes of 20 – 30 ml and works with input powers of less than 10 watts. The ultrasound unit should consist of two main parts: an ultrasonic transducer and an electronic circuit, which are specially designed for such an application focusing low power consumption. II.

DISINFECTION SYSTEM - OVERVIEW

For getting a better comprehension of the aspired ultrasound unit and its specifications, Fig. 1 shows a schematic representation of the complete UVC-LED based disinfection system. Its main parts are: a solar system based on a battery power supply, the reactor itself consisting of a quartz tube with integrated ultrasonic transducer and UVC-LED [Type: SETI UVLUX275-HL-3, 1,4 mW, wavelength 280 nm] and the electronics containing the control system for effective power consumption. The device is designed as stand-alone system and powered by a solar module (Fosera LSHS). Due to this concept water can be disinfected at any time of the day and no consumables are necessary for the disinfection. Even weather changes do not affect functionality. Especially people who live 476

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ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering (IJARCSEE) Volume 3, Issue 12, December 2014 in rural areas often have no access to electricity or germfree water from sewage treatment plants and therefore would profit from such an autonomous disinfection system [12]. Because of the limited power supply of the solar system it is essential that the ultrasound unit has to reduce its power consumption as much as possible. Furthermore the acoustical energy has to travel down from the transducer along the quartz tube efficiently, where the actual disinfection takes place.

Electrodes

Piezoelectric Elements

Bolt

Mass 1

Mass 2

+ Fig. 2 Langevin bolt-clamped type transducer

Water Tank

Contaminated Water Quartz Tube

UVC-LED

Ultrasonic Transducer Solar Panel

Photodiode

Disinfected Water

Control-Unit

Ultrasonic Generator

LED-Driver

Power Supply

To determine the correct resonance frequency f0, the size of these masses can approximately be calculated by using the following physical quantities: the angular frequency (𝜔0 = 2𝜋 ∗ 𝑓0 ), length (𝑙𝑖 ) of mass i (= 1, 2), thickness ( 𝑡𝑃𝑖𝑒𝑧𝑜 ) of both piezoelectric elements, the sound velocity ( ci ) of mass i and the piezoelectric elements (cPiezo ), the acoustical impedance of mass i ( 𝑍𝑖 = 𝜌𝑖 ∗ 𝑐𝑖 ∗ 𝐴𝑖 ) and the piezoelectric elements (𝑍𝑃𝑖𝑒𝑧𝑜 = 𝜌𝑃𝑖𝑒𝑧𝑜 ∗ 𝑐𝑃𝑖𝑒𝑧𝑜 ∗ 𝐴𝑃𝑖𝑒𝑧𝑜 ) [15]. Where A is the cross-sectional area of mass i and the piezoelectric element respectively and 𝜌 is the corresponding density.

Fosera LSHS Solar Module

tan

Fig. 1 Disinfection system overview

In parallel the quartz tube works as a light guide for the UVC-rays. By this approach most of the optical power is used effectively for disinfection. In addition during long term applications the optical properties of the quartz tube would decrease due to increasing agglomerations and biofilm formation, which absorb UVC-energy and by this decrease disinfection efficiency. The cleaning effects of power ultrasound therefore ensure the functionality of the disinfection quality, by removing these agglomerations. III.

ULTRASOUND UNIT – DESIGN PROCESS

A. Ultrasonic Transducer High-intensity ultrasound power can be created either by piezoelectricity or magneto-resistance. Piezoelectric elements offer the advantage that electric energy is directly converted into mechanical energy. Therefore very high efficiency rates of up to 90% are achievable [13]. A so called Langevin transducer is often used for the generation of power ultrasound. Fig. 2 shows such a transducer, consisting of two piezoelectric elements (material type Lead Zirkonate Titanate PZT4), which are clamped between two metal masses and prestressed by a bolt [14]. The piezoelectric elements are electrically connected in parallel and the masses are fixed by the bolt. Usually the front mass 2 is made out of duralumin or titanium whereas the back mass 1 consists of stainless steel [13]. The dimensions of the metal masses shift the natural resonance frequency of the piezoelectric elements in that way, that driving frequencies of 20 kHz to 100 kHz can be realized [13].

𝜔 0 ∗𝑙𝑖 ci

∗ tan

𝜔 0 ∗𝑡 𝑃𝑖𝑒𝑧𝑜 2∗c Piezo

=

𝑍𝑃𝑖𝑒𝑧𝑜

(1)

𝑍𝑖

And solving equation (1) is giving approximately 𝑙𝑖 : 𝑙𝑖 =

𝑐𝑖 𝜔0

∗ arctan

(2)

𝑍𝑃𝑖𝑒𝑧𝑜 ∗𝐴𝑃𝑖𝑒𝑧𝑜 𝑍𝑖 ∗𝐴𝑖 ∗tan

𝜔0 2∗𝑐 𝑃𝑖𝑒𝑧𝑜

∗𝑡 𝑃𝑖𝑒𝑧𝑜

However equation (2) is just an approximation and the transducer‟s resonant frequency has to be verified by appropriate methods. Measuring the transducer‟s impedance depending on excitation frequency delivers the exact series resonant frequency showing the electrical impedance in active resistance with phase shift tending to zero [13]. According to equation (2) a small ultrasonic transducer is designed. Mass 1 has the dimensions of Ø 15 mm x 12 mm and is made of stainless steel. Mass 2 has the dimensions of Ø 15 mm x 18 mm and is made of aluminum. The resulting driving frequency would theoretically be expected at about 70 kHz. Furthermore the transducer was mechanically pre-stressed with about 30 MPa using the bolt. B. Ultrasound Generator The purpose of an ultrasound generator is to provide and control the resonance frequency signal driving the piezo elements with high voltage amplitude. Here the designed generator is powered by a battery based DC voltage of 12 V of the solar system. Fig. 3 shows a block diagram of the designed electrical circuit. It can be separated into two parts: 1.

A control logic for frequency modulation (via PWM – pulse width modulation), the recording of the current through the ultrasonic transducer and the automatic resonance frequency tracking function. To realize the control functions, an 8-bit 477

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2.

microcontroller type PIC18f26k22 from Microchip was assembled. A power stage which boosts the supply voltage up to 60 V supplying the H-bridge, which generates alternating output voltages of up to ±60 V, with frequencies between 20 kHz and ~100 kHz (max. 1 MHz). Current Sampling

5 V AC

Low-Pass Filter

Driver

Power Stage

Amplifier

H-Bridge

Voltage Boost

Impedance Matching

Transducer Water Load Fig. 3 Block diagram of ultrasound unit with electrical driver

Langevin type transducers for high-intensity ultrasound applications are usually driven in their serial resonance frequency to maximize acoustical power output. Therefore, if high power outputs are required, it is necessary to deliver high voltage amplitudes. To achieve high driving voltages over 100 Vpp, a boostconverter type LT1171 from Linear Technology is implemented. By integrating such a boost-converter no additional transformer is needed and a small and lightweight generator can be realized. In its basic application as boost-converter, the maximum output power of the LT1171 can be calculated by equation (3) from the LT1171 design manual [16]. 𝑃𝑂𝑢𝑡 = 𝑉𝐼𝑁 ∗ 𝐼𝑃 ∗ 1 − 𝐼𝑃 ∗ 𝑅 ∗ = 23.39 𝑤𝑎𝑡𝑡𝑠

1 𝑉 𝐼𝑁

−

1

𝑃 𝑂𝑈𝑇 𝑃 𝐼𝑁

=

𝐼𝑂𝑢𝑡 ∗𝑉 𝑂𝑢𝑡 𝐼𝑂𝑢𝑡 ∗𝑉 𝑂𝑢𝑡 +𝑃 𝐼𝐶 +𝑃 𝐷

= 95%

(4)

This means the LT1171 is capable of delivering output powers up to 23 watts with an energy-efficiency of over 90%, at an input voltage of 12 V and an output voltage of 50 V. The voltage of the boost-converter now supplies an H-bridge driver, consisting of a HIP4080A from Intersil and four n-channel MOSFETs type IRF510 from Vishay. The H-bridge generates an alternating voltage with the given frequency of the PWM signal from the control logic and the voltage amplitude of the boost converter (12-60 V). This enables frequency modulated output voltages up to ±60 V (or 120 Vpp) with up to 1 MHz. Fig. 4 shows the circuit of the designed driver stage using the described boost converter at connectors with 12-60 V. The driver module can be separated in three different parts:

12 - 60 V AC

Shunt Resistor

𝐸=

𝐼𝑂𝑢𝑡 : max. output current (here: 0.5 A) PIC: power loss due to “on” switch resistance and switch driver (here 0.4 W) PD: power loss due to external diode (here: 0.66 W)

Control Logic

PWM

efficiency of the boost-converter is determined by the “on” switch resistance, the switch driver and the external diode [16] type UF4004 of the LT1171. It is given by equation (4).

(3)

𝑉 𝑂𝑈𝑇

𝑉𝐼𝑁 : input voltage (here: 12 V) 𝑉𝑂𝑢𝑡 : output voltage (here: 50 V) 𝐼𝑃 : max. switching current LT1171 (2 A) R: “on” switch resistance (0.5 Ω)

1.

2. 3.

A HIP4080A high frequency H-bridge driver for frequencies up to 1 MHz. This part of the circuit performs the switching of the MOSFETs in the Hbridge up to 80 V. The H-bridge itself that is powered by the voltage from the LT1171. An operational amplifier measuring the voltage signal proportional to electrical current through a shunt resistor with implemented low-pass filter with a cut-off frequency of 1 kHz for reducing noise (Fig. 4: see pining „CURRENT_SAMPLE‟).

The driver stage can be controlled completely by the PIC18f26k22 using four signal pins: An ENABLE and DISABLE-Pin to activate or deactivate the driver stage, and two PWM pins. Here the application software with its implemented algorithms delivers the control signals for tracking the drifting resonance frequency based on current measurements.

One of the most important parameter of this application is energy-efficiency. Therefore the

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Fig. 4 H-bridge driver and current sampling

C. Implemented Resonance Tracking Function During ultrasound application shifts in the transducer‟s resonance frequency can appear due to heating of the piezoelectric elements or load changes of the cavitation area [2]. Thus to compensate these changes and optimize the power output of the ultrasonic unit an algorithm was implemented, which automatically measures and adjusts the resonance and driving frequency in intervals of 90 seconds during operation.

This time period proved to be best suited here, because the functionality of the ultrasound unit is not disrupted and appearing resonance shifts are compensated quickly. The working principle of the designed algorithm is shown in the flow chart in Fig. 5. Basically the current through the transducer is rectified, low-passed filtered and then sampled over a defined frequency interval starting at „Freq_Low‟ and ending at „Freq_High‟. After each recorded current value a new frequency is calculated and set by changing the „PR2‟ register of the PIC18. During serial resonance the impedance of the transducer has its minimum. By sampling and analyzing the current at different frequencies, the highest current determines the lowest impedance value and therefore the series resonance frequency keeping voltage amplitude constant [2]. With this algorithm the power output of the unit is optimized efficiently, because it is able to automatically adapt to the actual resonance frequency. D. Coupling of Ultrasound In order to transfer the acoustic power to the disinfection system, the mounting support from Fig. 6 was designed. Steel Plate, 0.5 mm

Flange

Ultrasonic Transducer

Fig. 6 Coupling of ultrasonic transducer to disinfection system Fig. 5 Flow chart of resonance tracking function


ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering (IJARCSEE) Volume 3, Issue 12, December 2014 The ultrasound is transmitted through a stainless steel plate of 0.5 mm thickness showing approximately acoustic impedance matching. The transducer itself is therefore glued to the steel plate with epoxy resin and additionally fixated by a flange and four M3 plastic screws as a protection against mechanical stresses. If malfunctions appear and the transducer has to be replaced this mechanic-acoustic assembly offers an easy handling with low effort.

(4) a theoretical efficiency of 95% (for the boostconverter) was calculated. The real efficiency of the complete system was measured at about 80%. The other 15% of the inserted electrical power are probably lost in the H-bridge due to heating effects of the MOSFETs. The conversion of electrical input power to acoustical output power of the transducer itself was measured between 50 – 60%.

RESULTS AND DISCUSSION

To ensure the functionality of the system and the stated requirements in optimal power consumptions the acoustical output power of the developed hardware was measured with an ultrasonic power meter (UPM-DT1AV) from Ohmic Instruments. The results are presented in Fig. 7. The measured power levels differ from ~0.35 watts (or 0.19 W/cm²) at ±15 V driving voltage up to ~3 watts (or 1.7 W/cm²) at ±50 V.

Efficiency [%]

100 IV.

80 60 40 20 0 20

30 40 Driving Voltage [±V]

3.5

Fig. 9 Measured efficiency of the ultrasound generator (Vin = 12 V; RLoad = 510 Ω)

3.0 Acoust. Power [W]

50

2.5

V.

2.0 1.5 1.0 0.5 0.0 15 20 Linear

25 30 35 40 Driving Voltage [±V]

45

50

Fig. 7 Acoustical output power at different driving voltages

To ensure the generated ultrasound is capable of producing cavitation and supporting UV disinfection a simple test was carried out using aluminum foil at the maximum acoustical output power of 3 watts and a sample volume of distilled water of about 50 ml. The result can be seen in Fig. 8. After a sonication interval of 60 seconds the aluminum foil shows several holes proving the occurrence of cavitation in the sample volume.

The ultrasound unit, designed for low power consumption is well suited for the implementation in the UVC-LED based disinfection system, as it is capable of achieving acoustic cavitation and therefore creates a purification effect as well as synergistic effects with UVC-radiation inside the disinfection system [1, 8]. This unit was successfully designed for electrical input powers of less than 10 watts designated to a battery based solar system. The generator demonstrates electrical efficiency rates of about 80%. Another advantage is that the generator is able to run completely autonomously with the implemented control logic realizing resonance tracking function implemented on the microcontroller PIC18f26k22, so that basically no user intervention is necessary. VI.

ACKNOWLEDGEMENTS

The authors would like to thank the University of Applied Sciences in Ulm, as well as Fosera and Epigap GmbH for supporting this project. VII. [1]

[2]

[3] Fig. 8 Aluminum foil (left) before and (right) after 60 seconds of sonication time with 3 watts and 50 ml sampling volume (distilled water)

Finally Fig. 9 shows the measured efficiency of the designed ultrasound power generator. Using equation

CONCLUSION

[4] [5]

REFERENCES

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Andrej Gross received his B. Eng. and M. Eng. degree in medical engineering from the University of Applied Sciences in Ulm in 2013 and 2014, respectively. He is currently working on the design of an autonomous disinfection system for rural areas at the Institute of Medical engineering and Mechatronics in Ulm. Felix Stangl received his B. Eng. degree in Medical Engineering from the University of Applied Sciences in Ulm in 2014. Currently he is pursuing his degree M. Eng. in medical engineering and is working as a laboratory engineer. His primary research area is biotechnology. Katharina Hoenes received her B. Eng. degree in Bioengineering from the University of Applied Sciences in Munich in 2013. Currently she is pursuing her degree M. Eng. in medical engineering and is working in the laboratory for biotechnology at the University of Applied Sciences in Ulm. Martin Hessling is professor at the University of Applied Sciences in Ulm, Institute of Medical Engineering and Mechatronics. His research interests include biophotonics, biotechnology, process engineering, optics/optoelectronics and electrical engineering. Rainer Brucher is professor at the University of Applied Sciences in Ulm, Institute of Medical Engineering and Mechatronics. His research interests include systems analysis, medical electronics, ultrasound for diagnostics and therapy, as well as patient monitoring and signal-processing.
