Application of sensor technology in micro-injection pump

0 Introduction
The microinjection pump is a commonly used instrument for long-term uniform microinjection in clinical medicine and life science research. The difficulties faced by microinjection pumps at home and abroad today are insufficient precision and relatively high cost. Domestic similar products use software to control the accuracy of injection, which results in poor instrument fault tolerance and can only use syringes from a single manufacturer. Foreign similar products use potentiometers to control the accuracy of injection, and the requirements for potentiometers are very high to achieve relatively high accuracy. A sensor
is a device or device that can sense or respond to a specified measured physical quantity and convert it into a usable signal output according to a certain rule. It can convert input variables into electrical signals that can be detected, and send various parameters to the computer system for intelligent monitoring and control. It is a pre-component in the measurement system. In recent years, the application of sensors is developing in two directions. One is the development of single-function sensors towards the comprehensive application of multi-purpose sensors; the other is the interface between sensors and microprocessors, which not only improves the measurement accuracy and reliability of sensors, but also improves the calculation accuracy of microprocessors. The two complement each other. At present, sensors have been widely used in various fields such as industry, agriculture, transportation, energy, space, resource development, environmental protection, natural disaster forecasting, medical care, and cancer diagnosis.
This paper studies the application of several sensors in measurement, selects sensors such as capacitive sensors for the design of micro-injection pump system, successfully improves the injection accuracy, and is compatible with syringes from multiple manufacturers, enhancing the function of micro-injection pump. This paper will explain the application of these sensors in micro-injection pump.

1 Capacitive grating sensor
Capacitive grating sensor is a capacitive digital sensor based on the variable area working principle that can measure large displacement. Compared with other digital displacement sensors, such as gratings and inductive synchronizers, it has outstanding characteristics such as small size, simple structure, high resolution and accuracy, fast measurement speed, low power consumption, low cost, and low requirements for the use environment. Therefore, it occupies a very important position in electronic measurement technology. With the development of measurement technology towards precision, high speed, automation, integration, intelligence, economy, non-contact and multifunctionality, the application of capacitive grating sensors is becoming more and more extensive.
This system mainly measures linear displacement, so a linear capacitive grating sensor is used. The structure of the capacitive grating sensor is very similar to a parallel plate capacitor. It is composed of a group of parallel plate capacitors arranged in a grid structure in parallel. If the periodic signal that changes with time is controlled by an electronic circuit and loaded on each grid of the sequentially arranged grid capacitor with different phase distributions at the same moment, then on the other common plate, the induction signal generated at any moment will have the same phase distribution as the excitation signal loaded at that moment.

The equivalent circuit of the capacitance formed between the moving grid and the fixed grid plates of the capacitive grid sensor is shown in Figure 1. Let C1(x), C2(x), C3(x)...C8(x) be the capacitance formed by the 48 plates on the moving grid and the corresponding plates on the fixed grid. It is a function of the displacement x. Assuming that the capacitance between the small emitter plate and the reflector plate is C0 when they are completely covered, and the width of each small emitter plate is w, it can be seen from the figure that when 0≤x≤w, C8(x)=C0(x)／w, C1(x)=C2(x)=C3(x)=C0, C4(x)=C0(1-x／w), c5(x)=c6(x)=c7(x)=0. From this, it can be concluded that the variation law of the capacitance between the two plates with the displacement x in the entire range.

As can be seen from Figure 1, when x is any value, there is always a part of the 48 plates on the moving grid that forms a capacitor with the "ground" (shielding plate). The corresponding input signal source is directly connected to the "ground", which has no effect on the output signal of the sensor. However, in order to derive a unified formula for the continuous change of φ(x) (φ(x) is the phase displacement of the output signal of the sensor relative to a certain driving signal) with the displacement x, the capacitance formed by these plates on the "ground" is not considered in the derivation, but they are still regarded as forming capacitance on the fixed grid plate, except that their capacitance is zero at this time. Since these capacitances are zero, their impedance is infinite. The corresponding signal sources all fall on these capacitors, and similarly, they have no effect on the output signal of the sensor.
If the emission voltage V1~V8 applied to each group of emitter plates of the capacitive grid sensor is a sinusoidal alternating voltage with 8 frequencies, the same amplitude and a phase difference of π/4 between adjacent small plates, there is a voltage Vf on the emitter and a voltage Vr on the receiver. Applying the theory of AC circuits and Kirchhoff's current law, the equivalent circuit of Figure 1 is interpreted as follows:

If Vo is used to represent the amplitude of each emitter voltage, and the phase of the first signal among the 8 signals is taken as the reference value, then:

Where φ0 is the phase angle of V1. Substituting the above quantities and Ci(x) (i=1, 2, ..., 8) into the above two equations, we get It can be seen that the output voltage of the capacitive sensor is a sinusoidal voltage with the same frequency as the transmitting voltage. Its amplitude varies within a very small range and can be approximately regarded as a constant, while the phase ratio V1 is π/4+φ(x). The phase displacement φ(x) can be measured by a phase-comparison measurement circuit to obtain the relative displacement x. It can be seen that the capacitive sensor is a phase tracking displacement sensor. This sensor is insensitive to the amplitude change of the input signal, so it has good anti-interference ability. In the entire measurement system, the main function of the capacitive sensor is to convert the mechanical displacement into the phase change of the electrical signal, and then send it to the measurement circuit for data processing. The capacitive sensor is controlled by the precision voltage comparator TLC354, powered by a relay, and provided by the CPU89C52 with the required excitation signal. At the same time, it receives its induction signal and measures the phase difference between the excitation signal and the induction signal through the phase-comparison circuit. After a series of changes, the length of the piston movement can be obtained. 2 Photoelectric switch In the design, in order to be compatible with syringes from multiple manufacturers, we specifically considered the problem of measuring the diameter of the syringe. Initially, we chose a high-precision CCD optical sensor, but considering that its main function is to detect the diameter of the syringe, and the diameters of different models of syringes have a step-like characteristic, in order to reduce costs, we replaced it with an optical

A photoelectric switch is a kind of electrical quantity sensor, which converts the change of light intensity between the transmitting end and the receiving end into the change of current, that is, converts the electrical signal → optical signal → electrical signal, so as to achieve the purpose of detection. Since the output circuit and input circuit of the photoelectric switch are electrically isolated (ie, electrically insulated), it can be used in many occasions. Its working principle is shown in Figure 2.

In this system, we use the photoelectric switch model HY-301-05. When the valve that locks the syringe is lifted to different heights, the diameter of the syringe is read by blocking the photoelectric switch.

3 Pressure sensor
Pressure sensor is the most commonly used sensor in industrial practice, and the pressure sensor we usually use is mainly a piezoelectric sensor that uses the piezoelectric effect. At present, there are many types of pressure sensors, including vibration cylinder type, quartz Bourdon tube type, piezoresistive type, strain gauge type, etc. The resistive pressure sensor used in this system is a sensor whose working principle is to convert the measured non-electrical quantity into a resistance value, and to measure the non-electrical quantity by measuring this resistance value. The traditional resistive strain pressure sensor is a sensor composed of a sensitive grid and an elastic sensitive element, as shown in Figure 3.
The strain gauge is attached to the elastic sensitive element with an adhesive. When the elastic sensitive element is subjected to external pressure, strain will be generated. The resistance strain gauge converts them into resistance changes, and then outputs electrical signals through the bridge circuit and compensation circuit. In
the product, we use the CZA-102 pressure sensor. The chip of this sensor has the characteristics of high precision, small drift, and large test range. When the resistance of the needle of the syringe is greater than the normal value, the pressure sensor can detect the change of the voltage signal, and then amplify it, and then convert it into a digital signal through analog-to-digital conversion.
Figure 4 is the application circuit diagram of the pressure sensor. The bridge used in this circuit forms the measurement voltage, which is a measurement method with high sensitivity. When there is no differential pressure, the two arms of the bridge are equal. The differential pressure signal is added to the four ceramic varistors, and the resistance of the varistors changes with the differential pressure, causing the bridge to be unbalanced. The imbalance of the bridge causes the change of current, which is amplified by the operational amplifier LM2904 and then connected to the ADC0834 analog-to-digital conversion chip to convert the analog signal into a digital signal, which is then transmitted to the CPU for processing.

4 Conclusion

This paper studies some new applications of sensors in micro-injection pumps based on the comparison of the shortcomings of existing micro-injection pumps at home and abroad. Through these applications, the injection accuracy has been successfully improved and the cost has been reduced. Its wide range of applications and comprehensive functions make the market prospects very broad.