Capacitance detection system for pipetting steel needle and capacitance signal processing method

Abstract

The invention belongs to the technical field of detection, and particularly relates to a capacitance detection system for a pipetting steel needle and a capacitance signal processing method. The system comprises a pipetting needle, a capacitance voltage division circuit, a frequency selection circuit, a peak detection circuit and a subtraction amplification circuit which are connected in sequence. The capacitance voltage division circuit is connected with a signal generation circuit; the subtraction amplification circuit comprises a processing unit circuit, the processing unit circuit comprises a signal processor, and the signal processor is an MCU unit with an ADC-1, an ADC-2 and a DAC; the peak detection circuit outputs an S0 signal, and the S0 signal is accessed to ADC-1 of the processing unit circuit; a bias voltage VBias is subtracted from the S0 signal output by the peak detection circuit at the same time to obtain a V1 signal, and the bias voltage VBias is given by a DAC (Digital-to-Analog Converter); and the V1 signal from which the VBias is subtracted is further amplified. The method is high in detection response speed and can be applied to high-speed instruments.

Description

Technical Field

[0001] The invention belongs to the field of detection technology, and in particular relates to a capacitance detection system for a pipetting steel needle and a capacitance signal processing method. Background Art

[0002] In IVD instruments, the main methods of sample or reagent pipetting are TIP pipetting and steel needle pipetting. TIP pipetting avoids cleaning operations by replacing disposable tips, thus completely avoiding carryover contamination during the pipetting process. At the same time, due to the reduction of cleaning mechanisms, the instrument design is also simpler. However, on the other hand, the use of disposable TIPs will lead to increased costs during use.

[0003] The steel needle pipetting system has a clear cost advantage because it does not consume TIP. However, when using the steel needle solution, a cleaning mechanism must be designed, and the cleaning effect largely determines a key performance indicator of the entire machine - carryover contamination. In addition, as an industry trend, the throughput of analytical instruments is getting higher and higher, which compresses the cleaning time and puts higher speed and reliability requirements on the reliability of various action details such as aspiration and injection.

[0004] When using a steel needle to aspirate liquid, the steel needle needs to be inserted into the liquid to a certain depth. A too shallow depth may cause the steel needle to absorb air and cause insufficient sample aspiration. A too large depth will cause the outer wall of the steel needle to be contaminated with more samples, which will have an adverse effect on subsequent cleaning and cause carryover contamination problems. Therefore, the pipetting system needs to have a liquid level detection function. In addition, since bubbles may exist on the surface of the liquid after the sample or reagent is transferred or mixed, which will affect the accuracy of the liquid level detection, the pipetting system should provide such abnormal detection functions.

[0005] The accuracy of liquid level detection is crucial to the pipetting system and even the entire machine function. Common liquid level detection methods include resistance method, air pressure method, laser method, ultrasonic method, and capacitance method. Among them, the capacitance method has the advantages of small size, simple implementation and high sensitivity, and is more suitable for use in the pipetting system. However, the traditional capacitance method has some disadvantages, such as being susceptible to external interference and poor EMI characteristics.

[0006] Common capacitance detection circuits use an active crystal oscillator to generate an excitation signal of a fixed frequency (such as 8MHz). C1, C2 on the circuit board and the measured capacitance Ct form a voltage divider circuit. The frequency selection circuit is essentially a bandpass filter that only retains signals of the same frequency as the excitation source. The signal after frequency selection generally presents the characteristics of the local oscillator signal of the oscillation circuit after being amplitude-adjusted by the Ct capacitance. The amplitude-modulated signal enters the peak detection circuit, which filters out the oscillation signal while retaining its amplitude information. At this time, the change in the amplitude signal has represented the change in the measured capacitance to a certain extent. However, due to the small change in the measured capacitance, the amplitude change and driving ability are not enough to drive the post-stage sampling circuit, so an amplifier circuit is designed. At the same time, because it is hoped to only amplify the signal change part, an automatic bias circuit is also specially designed to ensure that the op amp will not be saturated and only retain the signal at the time of the measured signal jump. For this reason, the output of the amplifier circuit, as the input of the MCU, its signal characteristics are pulsed. When the steel needle enters the liquid surface or exits the liquid surface, the circuit will output two pulse signals to the MCU, but the polarity of the two signals is opposite. The steady-state signal at the moment of circuit stability does not contain information and only serves as a stable working point of the circuit. This circuit can also be called relative capacitance detection, that is, only the change of capacitance is detected. The relative capacitance scheme has certain advantages. The pulse signal output greatly reduces the difficulty of software processing. Generally, only threshold comparison is required as the criterion for liquid level detection. At the same time, the addition of the automatic bias circuit allows the system to always tend to enter a steady state. This design can reduce the impact of slowly changing environmental factors such as temperature and humidity on the signal. During the years of use, the relative capacitance solution has gradually exposed some defects, mainly manifested as: 1. Poor sensitivity, unable to detect small amounts of liquid (<200ul); 2. Easily affected by interference, occasional false triggering; 3. Loss of capacitance change details, relying only on a single threshold detection; 4. Slow detection response speed, not suitable for high-speed instruments.

[0007] In order to solve the defects of the above relative capacitance solution, the present invention hopes to provide a capacitance detection system and a capacitance signal processing method for a pipetting steel needle. Summary of the invention

[0008] The object of the present invention is to provide a capacitance detection system for a pipetting steel needle and a capacitance signal processing method to solve the problems raised in the above-mentioned background technology.

[0009] To achieve the above object, the present invention provides the following technical solutions: A capacitance detection system for a pipetting steel needle, comprising a pipetting needle, a capacitance voltage divider circuit, a frequency selection circuit, a peak detection circuit and a subtraction amplifier circuit connected in sequence, wherein the capacitance voltage divider circuit is connected to a signal generating circuit; The subtraction amplifier circuit includes a processing unit circuit, which includes a signal processor, and the signal processor is an MCU unit with ADC-1, ADC-2 and DAC; the peak detection circuit outputs an S0 signal, and the S0 signal is connected to ADC-1 of the processing unit circuit; the S0 signal output by the peak detection circuit will simultaneously subtract a bias voltage VBias to obtain a V1 signal minus the bias voltage VBias, and the bias voltage VBias is given by the DAC of the processing unit circuit; the V1 signal minus VBias is further amplified by 10 times.

[0010] Preferably, the S0 signal output by the peak detection circuit is directly connected to ADC-1 of the processing unit circuit, and the processing unit circuit adjusts the bias voltage VBias according to the S0 signal to ensure that the voltage range of the S1 signal after subtracting the bias voltage VBias and amplifying is within the input voltage range of ADC-2 of the processing unit circuit.

[0011] Preferably, the subtraction amplifier circuit includes a low-pass filter circuit, which filters out high-frequency interference from the amplified V1 signal; the filtered V1 signal is read by ADC-2 of the processing unit circuit and then judged.

[0012] Preferably, the MCU unit provides ADC-1 conversion and ADC-2 conversion to convert the capacitance analog quantity into a digital signal.

[0013] Preferably, the MCU unit provides an external communication interface to receive control instructions and report working status.

[0014] Preferably, the MCU unit provides two I / Os (Ready / Start) as synchronization signals, which are respectively used to start liquid level detection and liquid level detection status feedback.

[0015] A capacitance signal digital processing method for a pipetting steel needle capacitance detection system comprises the following steps: S1. Determination of capacitance characteristics: Before aspirating, the pipette needle is located above the aspirating position until the pipette needle touches the liquid surface. During this process, the pipette needle does not touch the liquid, the environmental capacitance changes, and the capacitance signal rises slowly; after the pipette needle touches the liquid, the capacitance signal rises rapidly. At this time, it is determined that the pipette needle touches the liquid surface, and the pipette needle stops moving vertically; during the aspirating process, the capacitance signal enters a stable period; after the aspirating is completed, the pipette needle moves up and leaves the liquid surface, and the capacitance signal decreases to the original level; S2. Liquid entry detection: When the pipette needle enters the liquid, it is judged based on the slope characteristics of the capacitance signal change; the value of the capacitance signal is differentially calculated to obtain the slope information, and the slope information is filtered, and the filtering is processed by a one-stage or two-stage sliding average filtering method; when the slope signal after filtering is greater than the slope threshold (Sth), it is determined that the pipette needle has touched the liquid surface; S3. Second confirmation: To avoid empty suction caused by liquid surface shaking, read the capacitance signal twice after the liquid surface is triggered, and compare the capacitance signal with the capacitance signal before contact; if the difference is less than the trigger threshold, it is considered a false trigger; S4. Detection of detachment from liquid: After the pipette needle has completed the liquid aspiration, it starts to withdraw the needle, and the pipette needle detaches from the liquid, and the voltage value of the capacitance signal decreases; the capacitance signal characteristics of the pipette needle detaching from the liquid are compared with the capacitance signal characteristics of the pipette needle entering the liquid. The capacitance signal change time of the pipette needle detaching from the liquid is longer, and the slope characteristics of the capacitance signal of the pipette needle detaching from the liquid appear to be small at first and then large.

[0016] Preferably, the presence of bubbles can easily lead to abnormal capacitance. When abnormal capacitance caused by bubbles occurs, the amplitude of the capacitance signal is directly used for detection, and the capacitance signal is subjected to sliding average filtering and compared with the amplitude threshold T after subtracting the background. When it exceeds the amplitude threshold T, it is considered that liquid is detected.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention provides a capacitance detection system and capacitance signal processing method for a pipette needle. According to the circuit temperature drift characteristics, a temperature drift of 30 degrees Celsius will cause a DC signal drift of about 150mV. If a 30-fold amplification is used, the 4.5V output signal of the operational amplifier will drift, which will actually cause the operational amplifier to saturate. Therefore, a 10-fold amplification is selected to reduce the impact of temperature drift on the signal while meeting the sensitivity. (2) The present invention provides a capacitance detection system and capacitance signal processing method for a pipette needle, wherein the output signal S0 of the peak detection circuit is directly connected to another ADC-1 of the processing unit, and the processing unit can automatically set the bias voltage according to the size of S0, so that the processing unit can automatically determine the size of the bias voltage; (3) The capacitance detection system and capacitance signal processing method for a pipetting needle provided by the present invention set up a secondary confirmation process, so that the detection process is not easily disturbed and false triggering is not likely to occur; (4) The present invention provides a capacitance detection system and capacitance signal processing method for a pipetting needle, which has a fast detection response speed and can be applied to high-speed instruments. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1It is a block diagram of the absolute capacitance detection scheme of the present invention; Figure 2 A schematic diagram of a capacitance signal during the liquid transfer process of the present invention; Figure 3 This is a schematic diagram of data processing for detecting the pipette needle entering the liquid according to the present invention; Figure 4 This is the waveform diagram when the pipette needle probes the liquid surface under the interference of bubbles; Figure 5 The flowchart of the processing unit circuit of the present invention is shown in FIG. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Example

[0020] 1. Subtraction amplifier circuit The output S0 of the peak detection circuit can only reach 30mV / pF level even after optimization, so it needs to be amplified. At the same time, considering that the output of the peak detection circuit has a large DC level, DC amplification of the signal can easily lead to output saturation of the op amp. If only AC amplification is performed, the DC signal will be completely lost, resulting in the same problem as the relative capacitance solution.

[0021] Therefore, this solution first subtracts a bias voltage VBias from the output of the peak detection circuit. The bias voltage is given by the processing unit through the DAC. The processing unit should adjust the bias voltage to ensure that the final output voltage range of the circuit is within the ADC input voltage range.

[0022] Then, the amplifier circuit is applied to amplify the signal after the bias voltage is subtracted. The selection of the amplification factor needs to take into account that the DC signal is easily affected by temperature. According to the circuit temperature drift characteristics, a temperature drift of 30 degrees Celsius will cause the DC signal to drift by about 150mV. If a 30x amplification is used, it will cause the 4.5V output signal of the op amp to drift, which will actually cause the op amp to saturate. Therefore, a 10x amplification is selected to reduce the impact of temperature drift on the signal while meeting the sensitivity.

[0023] In order to improve the anti-interference ability of the circuit, a low-pass filter circuit is applied to the amplified signal to filter out high-frequency interference. The filtered signal V1 can be read by ADC-2 for judgment by the processing unit.

[0024] In order to enable the processing unit to automatically determine the bias voltage, the output signal S0 of the peak detection circuit is directly connected to another ADC-1 of the processing unit, and the processing unit can automatically set the bias voltage according to the size of S0.

[0025] 2. Processing unit circuit In order to process the signal and transmit the detection results to a relatively distant main control circuit by communication, and to set the bias voltage of the subtraction amplifier circuit, an MCU (such as STMicroelectronics' STM32F103 series) with ADC-1, ADC-2 and DAC is selected as the signal processor. The MCU plays the following roles in this circuit: (1) Provide 2-way ADC conversion to convert the capacitance analog quantity into digital signal, and provide software for signal processing to identify the liquid level signal; (2) Provide the bias voltage of the subtraction amplifier according to the size of the previous stage (unamplified signal) signal to ensure that the subtraction amplifier circuit is not saturated; (3) Provide external communication interface, receive control instructions and report working status; (4) Provide two digital I / O (Ready / Start) as synchronization signals, which are used to start liquid level detection and liquid level detection status feedback respectively; (5) The sensitivity of the MCU circuit to capacitance detection depends mainly on the number of bits of the ADC in addition to the noise of the external signal. The AD on a general MCU chip is 12 bits. When the reference voltage is 3.3V, the theoretical resolution of the AD is 0.8mV / LSB. If the amplifier gain is 10 times and the peak detection circuit output is 15mV / pF, the theoretical resolution of capacitance detection can reach 0.005pF. The actual minimum capacitance detection also needs to consider factors such as the signal-to-noise ratio and the ENOB of the AD, which will be worse than this data.

[0026] Digital Processing of Capacitive Signals V1. Capacitance characteristic determination: The change of capacitance signal during the pipetting process of a steel needle pipette is shown as follows Figure 2 As shown in the figure, before aspiration, the pipette needle is located above the aspiration position, and then the vertical motor descends to start liquid level detection. During this process, although the steel needle does not touch the liquid, the capacitance signal may have begun to rise slowly due to changes in environmental capacitance (usually caused by the liquid container). When the steel needle touches the liquid, the capacitance signal rises rapidly. At this time, the system determines that the steel needle has touched the liquid surface and stops moving vertically, and then starts to aspirate. During the aspiration process, since the liquid level height does not change much and is relatively slow, the capacitance signal will enter a stable period. After the aspiration is completed, the pipette needle begins to rise and detach from the liquid, and the capacitance signal value will decrease again and return to the level before entering the liquid.

[0027] S2. Liquid entry detection: (For liquid entry detection data processing, see Figure 3 ) From the observation and analysis of the change characteristics of the capacitance signal when the pipette needle enters the liquid, it can be seen that the signal changes rapidly when the steel needle contacts the liquid, the slope characteristics are more obvious, and the slope has the characteristics of first small and then large. Therefore, a relatively simple slope recognition logic can be used to realize liquid entry detection. That is, the slope information is obtained by differential operation of the signal value. In order to improve the anti-interference ability, the slope signal is filtered, and the filtering can be realized by a one-stage or two-stage sliding average filtering method. When the filtered slope signal is greater than the set slope threshold (Sth), it is considered that the pipette needle touches the liquid surface.

[0028] In the case of bubbles on the surface of some liquids (reagent or sample abnormality), simply using slope detection may result in an error of being unable to detect the liquid. The reason for this abnormality is that the presence of bubbles slows down the slope of the capacitance signal, and even if the steel needle passes through the bubble and touches the liquid surface, the signal cannot change significantly. The abnormal capacitance waveform in the presence of bubbles is as follows: Figure 4 When this abnormal situation occurs, the capacitance signal amplitude is directly used for detection, that is, the capacitance signal is subjected to sliding average filtering and the background is subtracted before comparison with the amplitude threshold T. When the amplitude threshold T is exceeded, it is considered that liquid is detected.

[0029] S3. Secondary confirmation: When the reagent disk is running at a high speed and acceleration, the liquid in the reagent bottle will shake. If the liquid level detection is started without standing still, the steel needle will easily stop after detecting the liquid. However, due to the shaking, the liquid level will eventually separate from the steel needle. If the liquid is sucked at this time, empty suction will occur. Therefore, this abnormality needs to be detected. In order to identify such abnormalities, the liquid level detection algorithm will read the capacitance value twice after the liquid level is triggered and compare it with the capacitance value before the trigger. If the difference does not reach the trigger threshold, it is considered a false trigger. This logic is called secondary confirmation logic.

[0030] S4. Liquid detachment detection: After the steel needle has finished absorbing liquid, it starts to withdraw the needle. The steel needle detaches from the liquid, and the voltage value of the electrical signal changes from high to low. Compared with the signal characteristics of the signal entering the liquid, although the amplitude of the signal detaching from the liquid changes consistently, the time required for the change is significantly longer than that of the signal entering the liquid. In addition, from the perspective of the signal slope, the signal slope shows a characteristic of first small and then large. The core idea of ​​the liquid detachment detection algorithm is basically the same as that of the liquid entry detection. It is supplemented by the background threshold trigger logic based on the slope detection. Since the signal slope is relatively slow, the interval time for statistical slope should be longer than the time interval for liquid entry detection. At the same time, since the capacitance signal is relatively stable after the steel needle enters the liquid, a fixed background should be used instead of a sliding background.

[0031] Software working logic and process The main working process of the processing unit circuit is as follows Figure 5 As shown in the figure, after receiving the liquid level detection instruction, the processing unit will first collect the current capacitance value as the background, then set the liquid level detection threshold and start ADC acquisition. After that, the processing unit will continuously monitor the capacitance of the pipette needle and perform the following operations on the capacitance data: Figure 3 The data processing shown.

[0032] When the capacitance data meets the trigger threshold condition, the detection data processing flow will report that the liquid level detection is triggered. If the pipette needle has moved the maximum distance vertically downward but still does not meet the trigger threshold condition, the detection data processing flow will report that the liquid level detection has failed.

[0033] In order to avoid false detection caused by liquid shaking, a secondary confirmation logic is added here, that is, the liquid level detection algorithm will read the capacitance value again after the liquid level is triggered and compare it with the capacitance value before the trigger. If the difference meets the threshold condition, the liquid level detection is considered to be triggered. If the difference does not reach the trigger threshold, it is considered a false trigger.

[0034] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A capacitance detection system for a pipetting needle, characterized in that: It includes a pipetting needle, a capacitor voltage divider circuit, a frequency selection circuit, a peak detection circuit and a subtraction amplifier circuit connected in sequence, wherein the capacitor voltage divider circuit is connected to a signal generating circuit; The subtraction amplifier circuit includes a processing unit circuit, which includes a signal processor, and the signal processor is an MCU unit with ADC-1, ADC-2 and DAC; the peak detection circuit outputs an S0 signal, and the S0 signal is connected to ADC-1 of the processing unit circuit; the S0 signal output by the peak detection circuit will simultaneously subtract a bias voltage VBias to obtain a V1 signal minus the bias voltage VBias, and the bias voltage VBias is given by the DAC of the processing unit circuit; the V1 signal minus VBias is further amplified by 10 times.

2. A capacitance detection system for a pipetting steel needle according to claim 1, characterized in that: The S0 signal output by the peak detection circuit is directly connected to the ADC-1 of the processing unit circuit. The processing unit circuit adjusts the bias voltage VBias according to the S0 signal to ensure that the voltage range of the S1 signal after subtracting the bias voltage VBias and amplifying is within the input voltage range of the ADC-2 of the processing unit circuit.

3. A capacitance detection system for a pipetting steel needle according to claim 1, characterized in that: The subtraction amplifier circuit includes a low-pass filter circuit, which filters out high-frequency interference from the amplified V1 signal; the filtered V1 signal is read by ADC-2 of the processing unit circuit and then judged.

4. A capacitance detection system for a pipetting needle according to claim 1, characterized in that: The MCU unit provides ADC-1 conversion and ADC-2 conversion to convert the capacitance analog quantity into a digital signal.

5. A capacitance detection system for a pipetting steel needle according to claim 1, characterized in that: The MCU unit provides an external communication interface to receive control instructions and report working status.

6. A capacitance detection system for a pipetting steel needle according to claim 1, characterized in that: The MCU unit provides two I / Os as synchronization signals, which are respectively used to start liquid level detection and liquid level detection status feedback.

7. A capacitance signal digital processing method for a pipetting needle capacitance detection system according to claim 1, characterized in that: The following steps are involved: S1. Determination of capacitance characteristics: Before aspirating, the pipette needle is located above the aspirating position until the pipette needle touches the liquid surface. During this process, the pipette needle does not touch the liquid, the environmental capacitance changes, and the capacitance signal rises slowly; after the pipette needle touches the liquid, the capacitance signal rises rapidly. At this time, it is determined that the pipette needle touches the liquid surface, and the pipette needle stops moving vertically; during the aspirating process, the capacitance signal enters a stable period; after the aspirating is completed, the pipette needle moves up and leaves the liquid surface, and the capacitance signal decreases to the original level; S2. Liquid entry detection: When the pipette needle enters the liquid, it is judged according to the slope characteristics of the capacitance signal change; the value of the capacitance signal is differentially calculated to obtain the slope information, and the slope information is filtered, and the filtering is processed by a one-stage or two-stage sliding average filtering method; when the slope signal after filtering is greater than the slope threshold, it is determined that the pipette needle has touched the liquid surface; S3. Second confirmation: To avoid empty suction caused by liquid surface shaking, read the capacitance signal twice after the liquid surface is triggered, and compare the capacitance signal with the capacitance signal before contact; if the difference is less than the trigger threshold, it is considered a false trigger; S4. Detection of detachment from liquid: After the pipette needle has completed the liquid aspiration, it starts to withdraw the needle, and the pipette needle detaches from the liquid, and the voltage value of the capacitance signal decreases; the capacitance signal characteristics of the pipette needle detaching from the liquid are compared with the capacitance signal characteristics of the pipette needle entering the liquid. The capacitance signal change time of the pipette needle detaching from the liquid is longer, and the slope characteristics of the capacitance signal of the pipette needle detaching from the liquid appear to be small at first and then large.

8. The capacitance signal digital processing method according to claim 7, characterized in that: The presence of bubbles can easily lead to abnormal capacitance. When abnormal capacitance caused by bubbles occurs, the amplitude of the capacitance signal is directly used for detection. The capacitance signal is processed by sliding average filtering and compared with the amplitude threshold T after subtracting the background. When it exceeds the amplitude threshold T, it is considered that liquid is detected.

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