Journal of KIIT. Vol. 13, No. 3, pp. 17-24, Mar. 31, 2015. pISSN 1598-8619, eISSN 2093-7571 17 http://dx.doi.org/10.14801/jkiit.2015.13.3.17
Development of a Transducer Power Control System for Ultrasonic Cleaning System Tran Trung Tin*, Dang Van Chien*, Joon-Ik Sohn**, Hyo-Cheol Kang***, and Jong-Wook Kim*
This work (Grants No.C0142588) was partly supported by Business for Cooperative R&D between Industry, Academy, and Research Institute funded by Korea Small and Medium Business Administration in 2014 and also supported by the Human Resources Development program (No. 20134010200550) of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.
Abstract The goal of this paper is to implement a systematic controller for transducer power used in an ultrasonic cleaning system to save its consumption power in changing objects to be cleaned. To estimate and observe the current which is produced from the generator, root-mean-square (RMS) values of current signals in the transducer circuit are processed to produce moving average and moving standard deviation values by sliding a window for the most recent data. In this paper, we propose the controller scheme which is implemented with a digital signal processing (DSP) board based on the digital signal controller (DSC) chip to control the transducer power through digital proportional-integral (PI) controller and also applied to a commercial ultrasonic cleaning system for validation under change of cleaning load. The overshoot and undershoot of the current RMS have been decreased by 52.2% and 70.7%, respectively, with faster recovery to the initial current level by the proposed control system.
요 약
본 논문에서는 초음파 세척기의 소비전력을 절약하기 위해 세척 작업 조건의 변화에 따라 초음파 세척기의 발진부 출력을 조절하는 제어기를 설계한 내용을 소개한다 발진부의 출력전류를 측정 및 관찰하기 위해 초음 파 진동부의 교류 전류를 측정하여 실효치 전류를 계산하고 최신 전류 실효치 데이터에 이동 창을 씌워 이동 평균과 이동표준편차를 계산했다 본 논문에서는 발진 출력 제어기를 위해 디지털 신호제어 칩을 기반으로 한 보드를 사용해서 디지털 비례 적분 제어기를 설계했다 그리고 이를 상용 초음파 세척기에 적용하여 세척 물 부하 변화 시 발진부 출력전류의 실효치를 측정한 결과 오버슛과 언더슛은 각각 와 감소했으 며 부하 변화 이전의 전류 값으로 좀 더 빨리 회복됨을 확인했다 .
,
.
DSP
-
.
52.2%
70.7%
.
Keywords ultrasonic cleaning, transducer power, digital signal processing, proportional-integral controller
* Dept. of Electronic Engineering, Dong-A University ž Received: Nov. 30, 2014, Revised: Jan. 25, 2015, Accepted: Jan. 28, 2015 ** President in BIT Technology, Inc. ž Corresponding Author: Jong-Wook Kim *** President of Sonitec Co., Ltd. Dept. of Electronic Engineering, Dong-A University, 37 Nakdong-Daero 550beon-gil, Busan, 604-714, Korea Received: Nov. 30, 2014 Tel: +82 51 200-7714, Email:
[email protected] Revised: Jan. 25, 2015 Accepted: Jan. 28, 2015
18 Development of a Transducer Power Control System for Ultrasonic Cleaning System
Ⅰ. Introduction
approach the high efficiency of UCS, the method of the matching electrical generator vibratory systems
High-intensity applications and potential applications
with electrical oscillators is also mentioned[6].
of ultrasonic energy consist of cleaning, soldering, spot
The goal of our study is automatic adjustment of
welding of metals, foam control, material forming,
electric current supplied to transducer in order to
plastic bonding and so on. However, ultrasonic
increase the performance of the UCS. An energy
cleaning system (UCS) is probably the best known
saving effect during replacement of the contaminated
application as well as distance measurement with
object to be cleaned is also mentioned.
ultrasonic wave[1]. The main function of ultrasonic
Section 2 outlines the system structure of the
cleaner is based on formation of the cavitation bubbles
existing UCS and describes a current detector circuit
in the liquid medium[2]. The collision between
to measure and compute RMS value of current.
contaminated surface and cavitation bubbles as well as
Section 3 explains the proposed digital PI controller
the violent activity of cavitation bubbles make the
for controlling transducer power. Section 4 provides
particles of contaminant removed from the surface
experiment results carried out for UCS, and Section 5
immediately. A scrubbing action which promotes the
concludes the paper with future research topic.
removal of contaminant is provided by agitation from the transducers equipped on the bottom of the
Ⅱ
. System Configuration and Design of
cleaning tank. In order to evaluate the effectiveness of an ultrasonic cleaner, there are some factors that affect the cleaning performance. One of the factors is effectiveness in providing sufficient energy at the
Current Detector Circuit
2.1 Ultrasonic generator configuration The common UCS is formed by the main parts
contaminated surface and the cleaning fluid to promote
including
the desired cleansing[2]. Moreover, the amount of
transducers, and the cleaning tank or vessel. In a
power available to the cleaning solution is determined
cleaning vessel filled with cleaning fluid, objects with
by the conversion efficiency from the electrical
contaminated surfaces are immersed to be cleaned by
generator to transducer. Therefore, maintaining steady
the acoustic power.
ultrasonic
power
generator,
a
set
of
state of energy supply to transducer will be an
The ultrasonic transducer converts the electrical
important role in determining efficiency of ultrasonic
energy from the ultrasonic generator into mechanical
cleaner.
vibrations. A method to obtain the maximum cleaning
In addition to the other factors to be considered, a
by the vibration is activated by a current whose
study concerning water cavitation intensity allows to
frequency equals the mechanical resonance frequency
evaluate the effectiveness of cleaning with implosion
of transducer[7]. Sandwich-type piezoelectric ceramic
force of particular bubbles[3]. Intensities of cavitation
transducers are equipped in the present UCS as
and the liquid agitation caused by the pressure of
packaged and laid at the bottom of the tank with
ultrasonic radiation have been compared with 10
electrical
selected solvents[4]. Temperature is also one of the
transducer is set at 28kHz with power of 50W, and
factors affecting the cleaning efficiency of UCS.
24 transducer units are installed in the package.
isolation.
Resonant
frequency
of
the
Surveying temperature dependence of the cavitation
A smart generator with a negative feedback,
pressure is presented in[5]. On the other hand, to
guarantees that the optimal excitation frequency is
Journal of KIIT. Vol. 13, No. 3, pp. 17-24, Mar. 31, 2015. pISSN 1598-8619, eISSN 2093-7571 19 proposed[8]. Fig. 1 shows an overall block diagram of the present ultrasonic generator. Its structure consists
2.2 Design of current detection circuit
of several components such as power circuit, oscillator
The power level which is supplied for the
circuit, control circuit, amplifier, and microcontroller
transducer is required to achieve stability during its
unit (MCU) which processes analog-to-digital conversion
operation. In practice, however, instability of electric
(ADC) and displays instantaneous current and power
current is observed in inserting and removing the
percentage. The used ultrasonic generator converts
contaminated objects into the cleaning vessel. At this
electrical energy at 60Hz to electrical power at
time, the energy supply for the transducer fluctuates
ultrasonic frequency approximately 28kHz and operates
within the range around the set value.
with energy of 1200W. As can be seen from the
In the present work, a new power controller circuit
block diagram, reference power level is set by the
has been developed to maintain the transducer current
operator by knob or button on control board.
level, where a current detection circuit is composed of AD737 and TL084 as shown in Fig. 2. AD737 converts AC current signals with corresponding RMS value into DC current which is the input of DSP controller, simultaneously. Observing the RMS signal allows to monitor and estimate the changing of instantaneous AC current signal when the contaminated parts are immersed into the tank. TL084 is a type of operational amplifier whose applications [9] are used
Fig. 1. Block diagram of ultrasonic generator
as amplifier, buffer or low pass filter. In this circuit, TL084 amplifies the current signal and the conditioned signal is transferred to DSP controller circuit.
Ⅲ 3.1 Transducer power control modeling
. Automatic Transducer Power Control
In general, transducer power level must be set stable while the objects to be cleaned are inserted into the cleaning medium and removed after a sufficient time. In addition, during the period of replacing with new parts, transducer power needs to be minimal to save the electric consumption power of UCS. In our study, realtime RMS value pattern of transducer current is continuously observed, and in case of the object change, transducer power is changed and recovered according to deviation in the current magnitude.
Fig. 2. Current RMS detector circuit
Currently,
in
order to
improve the cleaning
efficiency of UCS, there are many acoustic power
20 Development of a Transducer Power Control System for Ultrasonic Cleaning System
controlling program implemented on MCU[10]-[12].
The signal conditioning module in Fig. 3 detects
Fig. 3 shows an overall system configuration with
AC current with a hall sensor, smooths the waveform,
transducer power controller based on a DSC chip such
amplifies with two operational amplifiers. It computes
as TMS320F28335 for the economic operation of
and converts the RMS into DC current to transducer
UCS.
power controller as shown in Fig. 4. The output
The preset power is set between 0 and 100% of the rated power using a knob or buttons according to
signal of converter block is amplified by the second amplifier to yield the signal of 3V peak amplitude.
amount of objects to be cleaned. For transducer power
Fig. 5 is the manufactured control board for
control, PI control is employed using extended PWM
transducer power, and its main features are to identify
(EPWM) on the TMS320F28335 chip. The pulse
the power level with initial value set by a knob or
width of EPWM depends on the magnitude of current
button, to condition and convert the ac current signal
signal which is processed in the current detector
into RMS values, to perform analog to digital
circuit shown in Fig. 2.
conversion of the current signal, and to remove the difference of value between initial set current and instantaneous current.
3.2 Digital PI controller In this paper, digital PI controller is designed and applied to transducer power control. The continuous
Fig. 3. Power controller configuration
time PI control rule is described with the following equation:
(1)
where and represent proportional and integral
Fig. 4. Block diagram of signal conditioning
gain, and is error signal between the reference and output of the current signals. For digital PI controller, differentiation of (4) is written as follows
(2)
Applying the backward difference method to (2), one can obtain digital PI control equation at the -th discrete time, , such that
Fig. 5. Digital processing controller board
where is sampling time.
(3)
Journal of KIIT. Vol. 13, No. 3, pp. 17-24, Mar. 31, 2015. pISSN 1598-8619, eISSN 2093-7571 21
3.3 Software algorithm for automatic transducer power control
The software configuration is based on TMS320 F28335 chip with clock frequency of 10kHz and 12-bit for ADC with the sampling time approximately
The developed software for transducer power
1ms. By using TMS320F28335 chip running at
control to be run on DSC TMS320F28335 chip has
150MHz system clock, it obtains a resolution of
the following features; cleaning-power control by
66.66Hz of EPWM pulse frequency for UCS.
EPWM pulse, ability to regulate the electric current of
Ⅳ
transducer and saving consumption power, communi-
. Experiment Results
cation with PC to observe the realtime data as the system operates.
Fig. 7 shows the prototype automatic transducer
Automatic transducer power control algorithm is
power regulator with DSP controller board which is
mainly composed of the system initializing program,
connected to the oscillator circuit of the present
ADC subprogram and EPWM interruption routine as
generator. Experiment results attained by cleaning with
shown in Fig. 6 as a flow chart. In the main program
detergent are shown in Fig. 8 and Fig. 9. The level
of our system, the EPWM interruption routine is
of power is set 65%, and UCS operates in inserting
depicted with its function that is normalizing, filtering
and removing the contaminated parts into the tank.
the initial current value, and driving the present current by EPWM pulse.
To monitor the operation condition, a simple and reliable approach is proposed by sliding a window of size on recent current magnitude values in computing average, , and standard deviation, as follows
(4)
(5)
where is sampling time.
Fig. 6. Flow chart of automatic transducer power algorithm
Fig. 7. Snapshot of prototype ultrasonic generator with automatic transducer power circuit
22 Development of a Transducer Power Control System for Ultrasonic Cleaning System
Table 1. Comparison of current control performance System Overshoot Undershoot Standard deviation Conventional 13.6% 25.6% 0.0260 Proposed 6.5% 7.5% 0.0145 Decrement 52.2% 70.7% 44.2% Table 1 summarizes and compares the performance of the proposed PI controller with the conventional system. Consequently, it is validated that the proposed
Fig. 8. Profiles of average of current magnitudes, and standard deviation in inserting and removing parts in liquid medium
control system improves cleaning performance with almost constant supply of power against various operation changes. V. Discussion and Conclusion
The present work proposes a power control system for transducers in UCS by feedback of transducer output current magnitude. To this end, we developed the DSP board to control the transducer powser through digital PI controller and also applied to a commercial
Fig. 9. Profiles of average of current magnitudes, and standard deviation in inserting and removing parts in liquid medium using the proposed control system
UCS.
The
control
performance
and
effectiveness has been quite improved and reliable under changes of cleaning load such as inserting and removing the object to be cleaned. The overshoot and undershoot of transducer current magnitude have been
Fig. 8 is the cleaning result attained by the
decreased to 52.2% and 70.7%, respectively.
conventional UCS under insertion and removal of the contaminated object, and Fig. 9 is the case controlled
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24 Development of a Transducer Power Control System for Ultrasonic Cleaning System
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