A capacitance detection system for a pipetting steel needle and a capacitance signal processing method

Abstract

The present invention belongs to the field of detection technology, and particularly relates to a capacitance detection system for a pipetting steel needle and a capacitance signal processing method. The system includes a pipetting needle, a capacitance voltage division circuit, a frequency selection circuit, a peak detection circuit, and a subtraction and amplification circuit connected in sequence. A signal generation circuit is connected to the capacitance voltage division circuit; the subtraction and amplification circuit includes a processing unit circuit, and the processing unit circuit 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 is simultaneously subtracted by a bias voltage VBias to obtain a V1 signal, and the bias voltage VBias is given by the DAC; further, the V1 signal after subtracting VBias is amplified. The detection reaction speed of the present invention is fast and can be applied to high-speed instruments.

Description

Technical Field

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

[0002] In IVD instruments, the mainstream pipetting methods for samples or reagents are divided into two types: TIP pipetting and steel needle pipetting. TIP pipetting avoids the cleaning operation by replacing disposable tips, thus completely avoiding carry-over contamination during pipetting. At the same time, due to the reduction of the cleaning mechanism, the instrument design is more concise. On the other hand, due to the use of disposable TIPs, the cost will increase during use.

[0003] The steel needle pipetting system has an obvious cost advantage because it does not require the consumption of TIPs. However, when choosing the steel needle solution, the design of the cleaning mechanism must be increased, and the cleaning effect largely determines a key performance index of the whole machine - carry-over contamination. In addition, as an industry trend, the throughput of analytical instruments is getting higher and higher, which compresses the cleaning time and puts forward higher speed and reliability requirements for the reliability of each action detail such as liquid aspiration and liquid dispensing.

[0004] When using a steel needle for liquid aspiration, the steel needle needs to be inserted into the liquid to a certain depth. Too shallow a depth may cause the steel needle to suck in air, resulting in insufficient sample aspiration. Too large a depth will cause more samples to adhere to the outer wall of the steel needle, which will have an adverse effect on subsequent cleaning and cause carry-over contamination problems. Therefore, the pipetting system needs to have a liquid level detection function. In addition, since there may be bubbles on the liquid surface after the sample or reagent is transported and mixed, which will affect the accuracy of liquid level detection, the pipetting system should provide a detection function for such abnormalities.

[0005] The accuracy of liquid level detection is crucial for the pipetting system and even the functions of the whole machine. Common liquid level detection methods include the resistance method, the air pressure method, the laser method, the ultrasonic method, and the 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 easily affected by external interference and having poor EMI characteristics.

[0006] Common capacitance detection circuits use active crystal oscillators to generate an excitation signal with a fixed frequency (such as 8 MHz). Capacitors C1, C2 on the circuit board and the capacitor Ct to be measured form a voltage division circuit. The frequency selection circuit is essentially a band-pass filter that only retains signals with the same frequency as the excitation source. The signal after frequency selection generally exhibits the characteristics of the local oscillator signal of the oscillator circuit after being amplitude-adjusted by the Ct capacitor. The amplitude-modulated signal enters the peak detection circuit, filtering out the oscillation signal while retaining its amplitude information. At this time, the change in the amplitude signal already represents the change in the measured capacitor to a certain extent. However, since the change in the measured capacitor is small, the amplitude change and driving ability are not sufficient to drive the subsequent sampling circuit. Therefore, an amplification circuit is designed. At the same time, since it is desired to only amplify the changing part of the signal, an automatic bias circuit is specifically designed to ensure that the operational amplifier does not saturate and only retains the signal at the moment when the measured signal jumps. For this reason, the output of the amplification circuit, which is also the input to the MCU, has a pulsed signal characteristic. When the steel needle enters or exits the liquid surface, the circuit will output two pulsed signals to the MCU, but the polarities of the two signals are opposite. The steady-state signal at the moment of circuit stability does not contain information and only serves as a stable operating point of the circuit. This circuit can also be called relative capacitance detection, that is, only the change in capacitance is detected. The relative capacitance scheme has certain advantages. The pulsed signal output greatly reduces the software processing difficulty. Generally, only threshold comparison is required to serve as the criterion for liquid surface detection. At the same time, the addition of the automatic bias circuit makes the system always tend to enter the steady state. This design can reduce the influence of slow-changing environmental factors such as temperature and humidity on the signal. During years of use, the relative capacitance scheme has successively exposed some defects, mainly manifested as: 1. Poor sensitivity, unable to detect small liquid volumes (<200 ul); 2. Prone to interference, occasionally mis-triggering occurs; 3. Losing the details of capacitance changes, relying only on a single threshold detection; 4. The detection response speed is relatively slow and not very suitable for high-speed instruments.

[0007] To solve the above defects of the relative capacitance scheme, 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 purpose of the present invention is to provide a capacitance detection system and a capacitance signal processing method for a pipetting steel needle to solve the problems raised in the above background technology.

[0009] To achieve the above purpose, the present invention provides the following technical solutions:

[0010] A capacitance detection system for a pipetting steel needle includes a pipetting needle, a capacitance voltage division circuit, a frequency selection circuit, a peak detection circuit, and a subtraction amplification circuit connected in sequence. A signal generation circuit is connected to the capacitance voltage division circuit;

[0011] 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.

[0012] 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.

[0013] 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.

[0014] Preferably, the MCU unit provides ADC-1 conversion and ADC-2 conversion to convert the capacitance analog quantity into a digital signal.

[0015] Preferably, the MCU unit provides an external communication interface to receive control instructions and report working status.

[0016] 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.

[0017] A capacitance signal digital processing method for a pipetting steel needle capacitance detection system comprises the following steps:

[0018] 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;

[0019] S2. Enter liquid detection: When the pipetting needle enters the liquid, it is judged according to the slope characteristics of the change in the capacitance signal; the differential operation is performed on the value of the capacitance signal to obtain the slope information, and the slope information is filtered. The filtering is processed by a first-level or second-level moving average filtering method; when the filtered slope signal is greater than the slope threshold (Sth), it is determined that the pipetting needle touches the liquid surface;

[0020] S3. Secondary confirmation: To avoid the situation of air aspiration caused by the shaking of the liquid surface, the values of the capacitance signal are read twice after the liquid surface is triggered, and the values of the capacitance signal are compared with the values of the capacitance signal before contact; if the difference is less than the trigger threshold, it is considered a false trigger;

[0021] S4. Detachment from liquid detection: After the pipetting needle completes liquid aspiration, the needle pulling action starts. When the pipetting needle detaches from the liquid, the voltage value of the capacitance signal decreases; the capacitance signal characteristics when the pipetting needle detaches from the liquid are compared with the capacitance signal characteristics when it enters the liquid. The capacitance signal change time when the pipetting needle detaches from the liquid is longer, and the slope characteristics of the capacitance signal when the pipetting needle detaches from the liquid show the characteristic of first small and then large.

[0022] Preferably, the presence of bubbles easily leads to abnormal capacitance. When the abnormal capacitance caused by the presence of bubbles occurs, the amplitude of the capacitance signal is directly used for detection. The capacitance signal is processed by moving average filtering, and after subtracting the background, it is compared with the amplitude threshold T. When it exceeds the amplitude threshold T, it is considered that the liquid is detected.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] (1) A capacitance detection system and a capacitance signal processing method for a pipetting steel needle provided by the present invention. According to the circuit temperature drift characteristics, a temperature drift of 30 degrees Celsius will cause a drift of about 150 mV in the DC signal. If 30 times amplification is used, it will cause an output signal drift of 4.5 V in the operational amplifier, which will actually cause the operational amplifier to saturate. Therefore, 10 times amplification is selected to reduce the influence of temperature drift on the signal while meeting the sensitivity;

[0025] (2) A capacitance detection system and a capacitance signal processing method for a pipetting steel needle provided by the present invention directly connect the output signal S0 of the peak detection circuit to another ADC-1 of the processing unit. 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;

[0026] (3) A capacitance detection system and a capacitance signal processing method for a pipetting steel needle provided by the present invention set up a process of secondary confirmation, making the detection process not easily interfered and not easily mis-triggered;

[0027] (4)The capacitance detection system and capacitance signal processing method for a pipetting steel needle provided by the present invention have a fast detection reaction speed and can be applied to high-speed instruments. Description of the Drawings

[0028] Figure 1 It is a block diagram of the absolute capacitance detection solution of the present invention;

[0029] Figure 2 It is a schematic diagram of the capacitance signal during the pipetting process of the present invention;

[0030] Figure 3 It is a schematic diagram of the detection data processing when the pipetting needle enters the liquid of the present invention;

[0031] Figure 4 It is a waveform diagram when the pipetting needle probes the liquid level under the interference of bubbles;

[0032] Figure 5 It is the working process of the processing unit circuit of the present invention. Detailed Embodiments

[0033] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention. Embodiment

[0034] I. Subtraction Amplification Circuit

[0035] Although the output S0 of the peak detection circuit can only reach the level of 30 mV / pF after optimization, it is necessary to perform amplification processing. At the same time, considering that the output of the peak detection circuit has a large DC level, directly amplifying the signal in DC is likely to cause the output saturation of the operational amplifier. If only AC amplification is performed, the DC signal will be completely lost, resulting in the same problem as the relative capacitance solution.

[0036] Therefore, in this solution, the output of the peak detection circuit is first subtracted by a bias voltage VBias, which is given by the processing unit through the DAC. The processing unit should adjust the magnitude of the bias voltage to ensure that the final output voltage range of the circuit is within the ADC input voltage range.

[0037] After that, an amplifier circuit is applied to amplify the signal after subtracting the bias voltage. The selection of the amplification factor needs to consider that the DC signal is easily affected by temperature. According to the temperature drift characteristic of the circuit, the temperature drift at 30 degrees Celsius will cause a drift of about 150 mV in the DC signal. If 30 times amplification is used, it will cause a drift of the output signal of the operational amplifier by 4.5 V, which will actually cause the operational amplifier to saturate. Therefore, 10 times amplification is selected to reduce the influence of temperature drift on the signal while meeting the sensitivity.

[0038] 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 then be read by ADC-2 for the processing unit to judge.

[0039] To enable the processing unit to automatically determine the magnitude of 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 magnitude of S0.

[0040] II. Processing Unit Circuit

[0041] In order to process the signal and transmit the detection result to a relatively far main control circuit by means of communication, and at the same time to set the bias voltage of the subtraction amplifier circuit, a MCU (such as the STM32F103 series of STMicroelectronics) with ADC-1, ADC-2 and DAC is selected as the signal processor. The main functions of the MCU in this circuit are as follows:

[0042] (1) Provide 2-channel ADC conversion to convert the capacitance analog quantity into a digital signal and supply it to the software for signal processing to identify the liquid level signal;

[0043] (2) Provide the bias voltage of the subtraction amplifier according to the magnitude of the pre-stage (unamplified signal) to ensure that the subtraction amplifier circuit does not saturate;

[0044] (3) Provide an external communication interface to receive control instructions and report the working status;

[0045] (4) Provide two digital I / Os (Ready / Start) as synchronization signals, which are used to start the liquid level detection and feedback the liquid level detection status respectively;

[0046] (5) The capacitance detection sensitivity of the MCU circuit mainly depends on the number of bits of the ADC except for the noise of the external signal. The on-chip AD of general MCUs is 12 bits. When the reference voltage is 3.3 V, the theoretical resolution of the AD is 0.8 mV / LSB at this time. Assuming that the amplification factor of the amplifier is 10 times, if the output of the peak detection circuit is 15 mV / pF, the theoretical resolution of the capacitance detection can reach 0.005 pF. The actual minimum capacitance detection still needs to consider factors such as the signal-to-noise ratio and the ENOB of the AD, and it will be worse than this data.

[0047] Digital Processing of Capacitive Signals

[0048] V1. Capacitance characteristic determination:

[0049] 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.

[0050] S2. Liquid entry detection: (For liquid entry detection data processing, see Figure 3 )

[0051] 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.

[0052] 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.

[0053] S3. Secondary confirmation: When the reagent disk runs at a relatively high speed and acceleration, the liquid in the reagent bottle will shake. If the liquid level detection starts without standing still, it is easy for the steel needle to stop after detecting the liquid. However, due to the shaking, the liquid level will eventually move away from the steel needle. If liquid aspiration is performed at this time, air aspiration will occur. Therefore, it is necessary to detect this abnormality. 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 triggering. If the difference is less than the trigger threshold, it is considered a false trigger. This logic is called the secondary confirmation logic.

[0054] S4. Liquid separation detection: After the steel needle finishes liquid aspiration, the needle pulling action starts, and the steel needle separates from the liquid. The voltage value of the electrical signal changes from high to low. Comparing the signal characteristics when entering the liquid, although the amplitude change of the signal when separating from the liquid is the same, the time required for the change is significantly longer than when entering the liquid. Moreover, from the perspective of the signal slope, the signal slope shows the characteristic of being small first and then large. The core idea of the liquid separation detection algorithm is basically the same as that of the liquid entry detection, supplemented by the background threshold trigger logic on the basis of slope detection. Since the signal slope is relatively gentle, the statistical slope time interval should be longer than the liquid entry detection time interval. At the same time, since the capacitance signal is relatively stable after the steel needle enters the liquid, a fixed background rather than a sliding background should be used for the background.

[0055] Working logic and process of the software

[0056] The main working process of the processing unit circuit is as Figure 5 shown. 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 pipetting needle and perform data processing on the capacitance data as Figure 3 shown.

[0057] When the capacitance data meets the trigger threshold condition, the detection data processing process will report the triggering of liquid level detection. If the pipetting needle has moved down vertically by the maximum distance but still does not meet the trigger threshold condition, the detection data processing process will report the failure of liquid level detection.

[0058] To avoid false detection caused by liquid shaking, the 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 triggering. If the difference meets the threshold condition, it is considered that the liquid level detection is triggered. If the difference is less than the trigger threshold, it is considered a false trigger.

[0059] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A digital processing method for capacitance signals of a pipetting steel needle capacitance detection system, characterized in that : The system comprises a pipette needle, a capacitive voltage divider circuit, a frequency selection circuit, a peak detection circuit and a subtraction amplifier circuit which are connected in sequence, and the capacitive voltage divider circuit is connected to a signal generating circuit; The subtraction amplifier circuit includes a processing unit circuit, the processing unit circuit 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 the ADC-1 of the processing unit circuit; the S0 signal output by the peak detection circuit will also 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; 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 the bias voltage VBias is subtracted and amplified is within the input voltage range of the ADC-2 of the processing unit circuit. 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; The method 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 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. Liquid separation detection: After the pipette needle has completed the liquid aspiration, the needle begins to be withdrawn, and the pipette needle is separated from the liquid, and the voltage value of the capacitance signal decreases; the capacitance signal characteristics of the pipette needle separated 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 separated from the liquid is longer, and the slope characteristics of the capacitance signal of the pipette needle separated from the liquid appear to be small at first and then large; The existence of bubbles can easily lead to abnormal capacitance. When abnormal capacitance caused by the existence of bubbles occurs, the amplitude of the capacitance signal is directly used for detection. The capacitance signal is subjected to moving average filtering, and after subtracting the background, it is compared with the amplitude threshold T. When it exceeds the amplitude threshold T, it is considered that the liquid is detected.

2. The capacitance signal digital processing method for a pipetting steel needle capacitance detection system 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.

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

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

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