2014 IEEE Workshop on Electronics, Computer and Applications

The Study and Implementation of Connected Liquid Level Detection Technology Applying to Medical equipment

Yang Xu Automobile Electronic Research Center, Chongqing

University of Posts and Telecommunications Chongqing, China 342870@qq.com

Ahstract-A liquid level detection technology for medical devices is introduced. Based on the principle of capacitance

detection, the design uses a single needle as the liquid level

detection probe to achieve automatic liquid level detection. In

the design, a 32-bit high-performance processer, STM32 is

used for the system controller. The detection system consists of

detection circuit and signal processing unit. The detection

circuit includes a capacitance detection circuit, a band pass

filter and a differential amplification circuit. The system has

been employed in a fully automatic chemiluminescence

immune analyzer. A lot of experiments prove that the design can achieve tOllL liquid level detection in a diameter of tcm

micro tube and it has a better practicability and stability.

Keywords liquid level detection; pi petting needle; capacity; differential amplification circuit;

I. INTRODUCTION

With the continuous development of automation technology, many medical devices which are manual or

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Liquid level detection is the main part of the automatic medical equipments which involved liquid transfer (such as the Blood Detector, the Immune System Analyzer, the Urine Analyzer, etc). Liquid level signal as a judging condition of the medical device determines whether the transmission mechanism (stepper motor system) to be operated, so the accuracy of the signal ensure the entire medical device to work safely and effectively [21. In medical device, there are many problems caused by liquid level detection. Such as., the pipetting needle couldn't suck the testing sample because of it don't dip into the testing sample, or the pipetting needle dips into the tube too deep and it is stuck, or the pipetting needles don't clean up cause sample crossed infection, etc. These problems have caused a lot of problems and make medical equipment can't work long-term.

II. THE PRINCIPLE OF L1QIUID LEVEL DETECTION SYSTEM

There are three relationships between the pipetting needle and the liquid level as shown in Fig.I. Fig.l (a) shows the pipetting needle is under the liquid level, Fig 1 (b) shows the needle is just at the liquid level, and Fig.1 (c) shows it is above the liquid level. If the liquid level detection system didn't detect the liquid level in Fig 1 (a), or it had detected the liquid level in Fig I (c).That is an error for the system.

978-1-4799-4565-8/14/$31.002014 IEEE 333

Zhaoxi Wang Automobile Electronic Research Center, Chongqing

University of Posts and Telecommunications Chongqing, China

wangzxhome@ 126.com

Either error will bring unexpected consequences for normal operation of the entire device. The detected signal is correct only and can be used as a valid signal shown in Fig.1 (b). There are two mainstream technologies of liquid level detection. The one is contact detection technology and the other is non-contact detection technology [31. The non-contact liquid level detection technologies include laser retlection method, ultrasonic method [41, imaging method [51, etc. The contact detection technologies are: resistance method, capacitors method, mechanical vibration method, atmospheric pressure method, etc.

(a) (b) (el Figure I. The relationship between pipetting needle and liquid level.

The capacitors method is used to detect the liquid level in this paper. By converting the capacitance between the pipetting needle and the ground to voltage signal, the system achieves liquid level detection. The method treats the pi petting needle as one capacitor's pole plate and the earth as the other one. The area of the movable plates is suddenly increased and the distance between the capacitor's plate decreases as soon as the pipetting needle touches the liquid level. According to the relationship between the capacitance of condenser C with the distance of two pole plate D and the pole plate area S shown in Equation (1), where k is electrostatic force constant and E is dielectric constant. The value of C will increase suddenly. By detecting the changes of the capacitance value, system can achieve the liquid level detection.

ES c= --

4n:kD (1)

III. THE IMPLEMENTATION OF LIQUID LEVEL DETECTION SYSTEM

STM32Fl03ZET6, a 32 high-performance and lowpower dissipation processors is used as the system controller.

2014 IEEE Workshop on Electronics, Computer and Applications

The signal that is processed by liquid level detection circuit is collected and processed by using a 12-bit precision ADC in the processor. The result of detection will be output to actuators for execution. The design of the whole system is divided into two aspects, hardware design and software design. Fig.2 shows the structure of the design.

Liquid Level Detection System Power K=:;:;:::====::::;::;:====;::1 Management

Figure 2. Relationship between pipetting needle and liquid level

A. The hardware design of liquid level detection system

1) The C/V conversion circuit As show in Fig 3, an oscillation frequency of 21 KHz,

Wien-bridge oscillator is acted as the oscillation source to measure the capacitance. The oscillating circuit consisted of UIA connects to the positive input of the follower circuit constituted by the OP213 (UIB) through a resistor of IMQ. Simultaneously, it was connected to a pipetting needle through a lK resistor. The large resistor R8 is used to isolate the oscillating circuit from the subsequent circuit. It ensures the stable frequency of the output signal of the oscillation circuit. In addition, the follower circuit has high input impedance and low output impedance, which is also obtained a good isolation between pre-stage circuit and the post-stage circuit. In order to avoid or reduce an abrupt change of the capacitance caused by the outside interference, a resister of lK in series with the pipetting needle is added, and then they are in parallel with a large capacitance C 10. With the oscillation frequency of 21 KHz, the value of R9 is much smaller than the capacitance value in pipetting needle. So it can be neglected. The circuit equaled that the capacitance value in pipetting needle is in parallel with the capacitor C I 0 whose value is large than the capacitance value formed at the time of the needle touches the liquid level. So it can buffer the voltage changes caused by outside interference [6]

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Figure 3. Connection diagram of oscillation and pipetting needle

2) Band-passfilter circuit (BFP)

334

In order to reduce or prevent mechanical jitter and outside interference sources affect the measuring result of the liquid level detection system, a band-pass filter to filter the signal outputted from the follower circuit is necessary. The band-pass filter is consisted of a 25 KHz low-pass filter and 15 KHz high-pass filter, as shown in Fig 4.The filter limits the pass frequency nearby the center frequency of the oscillation. It can filter the high-frequency and lowfrequency interference from the outside. In the design, we use the multisim software to simulate the band-pass filter. The spectrogram, as show in Fig 5, shows that the center frequency of the filter is around 20 KHz and can meet the requirements of the system.

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Figure 4. The band-pass filter(BPF)

Figure 5. The spectrogram of the BPF

3) Differential amplifier

IK

A lot of experiments show that because of the different samples or the different liquid level height of the same samples, the change of the capacitance is different when the pipitting needles touch the liquid level. It results in the level of the output voltage of pass state circuit is different. For example, when the liquid level height is lower, the change of the capacitance is smaller and the change of the output voltage is also smaller. Small voltage variation is easy to be interfered and then it brings difficulties for pass state circuit to deal with it. In view of above reasons, a differential amplifier is design to deal with the signal outputted from the filter. As shown in Fig 6, the differential circuit can filter out the interference introduced by the power supply ripple, etc. Moreover, in order to facilitate the letter processing, the negative input of the differential amplifier circuit is connected to an adjustable voltage divider and the positive input connected to the output of the BPF. Adjusting the potentiometer and makes the output of the differential amplifier circuit is approach to zero when pipetting needle is above the liquid level. The positioning of the potentiometer is set as the calibration values. The capacitance at the

2014 IEEE Workshop on Electronics, Computer and Applications

position of pipetting needle increases and its captance decreases when the pi petting needle touched the liquid level. So the voltage at the negative port decrease with it, and the differential amplifier would output positive voltage. If the signal duration is more than SOms, it means that the system detect the liquid level.

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2014 IEEE Workshop on Electronics, Computer and Applications

that the minimum detectable volume of liquid up to 10uL in a diameter of 1 cm micro tube. The test environment shows in Fig 10.

Figure 10. The experimental environment

Figure II. The response time of the system

As in Fig 11 show, the response time of the system is shorter than 5ms when the pipetting needle touched the liquid level. It guarantees that the system has high response sensitivity and it can be qualified for the real-time requirements in the medical device.

Table 1 shows the outputted voltage data from 12 experiments of the liquid level detection system with serum samples. The result of the experiment testifies that the system can work well under the rugged environment with shaking.

TABLE I THE RESULT OF 12 EXPERIMENTS

Sequence voltage before voltage after detection(V) detection(V)

1 0.012 3.263 2 0.012 3.182

3 0.012 3.122

4 0.013 3.283

5 0.013 3.323 6 0.013 3.336

7 0.011 3.212

8 0.012 3.314

9 0.014 3.411

10 0.012 3.392 11 0.010 3.258

12 0.013 3.273

336

V. CONCLUSIONS

A system is applied to micro-biological pipette for liquid level detection is presented in this paper. The system which bases on the capacitance detection principle takes advantage of the band-pass filter, differential amplifier circuit to preprocess the detecting signal. The processed signal is collected by the inherent ADC part of STM32, and then through the software filtering, the system achieves liquid level detection ultimately. A great amount of experiments show that the system has high reliability and stability for different samples and different test tubes. So it can be applied in the automatic chemical immunity analyzer successfully.

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[3] Zhu Xianfeng,Zeng Sisi,Zhang Kuo. Automatic liquid level detection technology progress in clinical laboratory instruments [J]. Journal of biomedical engineering, 2010,27(4):949-953.

[4] Cosense Inc.ML series micro measurement system [EB/OLl (2008-10-10)[2009-07-1 Olhttp://www. cosense. conlimages/ DataSheetsl miseries datasheet.Pdf

[5] May F. A method for liquid level detection under complicated boundary conditions [J].Journal of Vibration and Acoustics, 2006, 128(4)535-539.

[6] Prochert J.Liquid level detection (LLD) in high-through-put screening appl ications [J].lnternational Biotechnology Laboratory,2000, 18( I 0): 44-45