Resistance sensor

Metal bodies have a certain resistance, and the resistance value varies depending on the type of metal. The thinner or thinner the same material, the greater the resistance value. When an external force is applied, if the metal becomes thinner and longer, the resistance increases; if it becomes thicker and shorter, the resistance decreases. If a metal resistor is mounted on the object where the strain occurs, when the object expands and contracts, the metal body also expands and contracts according to a certain ratio, and the resistance value changes accordingly. [1]

Resistive sensors [2] can be divided into: resistance strain type; potentiometer type; thermal resistance type; semiconductor thermal resistance sensor, etc. according to its working principle.

Structure: It consists of two basic parts: a resistive element and a brush (active contact). The movement of the brush relative to the resistive element can be linear, rotational and helical, and thus linear or angular displacement can be converted to a resistive or voltage output in a functional relationship thereto.

Potentiometer structure and materials

(1) Resistance wire: Constantan wire, platinum-iridium alloy, gamma wire, etc.

(2) Brush: Commonly used metals such as silver, platinum rhodium, platinum rhodium

(3) Skeleton: Commonly used materials are ceramics, phenolic resin, cloth bakelite and other insulating materials. The structure of the skeleton is many, and the rectangle is commonly used.

Measuring instruments such as measuring force, pressure measuring, weighing, measuring displacement, acceleration and torque composed of resistive sensors and corresponding measuring circuits are automatically weighed by metallurgy, electric power, transportation, petrochemical, commercial, biomedical and national defense departments. One of the indispensable tools for process inspection and automation of production processes. [3]

(1) There is a large nonlinearity and the output signal is weak, but certain compensation measures can be taken. It is therefore widely used in automated test and control technology.

(2) The strain gauge in the resistance strain sensor has the strain effect of the metal, that is, mechanical deformation under the action of external force, so that the resistance value changes accordingly.

(1) There is a large nonlinearity for large strain, but the output signal is weak.

(2) The material and device properties that make up the sensor change over time and environment. Therefore, it is not suitable for long-term monitoring, because time drift, temperature drift is large, and long-term measurement may not be able to obtain real and effective data.

(3) It is susceptible to electric fields, magnetic fields, vibration, radiation, air pressure, sound pressure, air flow, and the like.

(1) Photosensitive resistance sensor structure

A photoresistor sensor is controlled by converting a change in light intensity into a change in an electrical signal. Its basic structure includes a light source, an optical path, and a photoelectric element. It first converts the measured change into a change in the optical signal. The optical signal is then further converted into an electrical signal by means of a photovoltaic element.

Since the photoresistor sensor relies on the relationship between the object to be measured and the photoelectric element and the light source to achieve the measurement purpose, the light source of the photoresistor sensor plays an important role, and the power source of the photoresistor sensor is a constant light source. The design of power supply stability is very important. The stability of the power supply directly affects the accuracy of measurement. The common light sources are as follows:

A light emitting diode is a semiconductor device that converts electrical energy into light energy. It has the advantages of small size, low power consumption, long life, fast response, high mechanical strength, and can be matched with integrated circuits. Therefore, it is widely used in computers, instrumentation, and automatic control equipment.

Silk bulb This is one of the most commonly used light sources with a rich infrared light. If the selected photoelectric element is sensitive to infrared light, the color filter can be added to filter the visible light of the tungsten light bulb, and only the infrared light is used as the light source, so that interference of other light can be effectively prevented.

(2) Working principle of photoresistor sensor

Since the maximum output voltage of the photosensitive battery is only 0.6V even under strong light, it cannot make the next-stage transistor have a large current output, so it is necessary to add a forward bias. In order to reduce the impedance change of the transistor base circuit, Try to reduce the reverse bias that the photocell can withstand when there is no light. A resistor can be connected in parallel across the photocell. Or use the forward voltage drop generated by the é”—[Note 1] diode and the voltage generated when the photocell is illuminated, so that the voltage between the electrodes e and b of the silicon tube is greater than 0.7V, and the conduction is turned on.

An integrated operational amplifier can also be used for the photoelectric conversion circuit of the semiconductor photovoltaic element. The silicon photodiode can obtain a larger output amplitude by integrating the op amp. When the photocurrent generated by the illumination is the output voltage, in order to ensure that the photodiode is reverse biased, a negative voltage is applied to the positive electrode due to the short circuit current of the photocell. It has a linear relationship with the illumination, so it is connected between the positive and negative input terminals of the op amp. By using the characteristics that the potential difference between the two ends is close to zero, better results can be obtained.

(3) Characteristics and application of photoresistor sensor

With the development of science and technology, people have higher requirements for measurement accuracy, which makes the photoelectric sensor have to be updated with the pace of the times. The main means to improve the performance of photoelectric sensors is to use new materials and new technologies to manufacture photoelectrics with superior performance. element. For example, the prototype of today's photoelectric sensor is a small metal cylindrical device. The transmitter has a calibration lens that focuses the light toward the receiver. The receiver cable connects the device to a vacuum tube amplifier in a metal cylinder. A sturdy incandescent lamp sensor with a small incandescent lamp as a light source. Due to various defects in this type of sensor, it gradually disappeared in the field of measurement. When the fiber appeared, because of its superior performance, the passive components used in the fiber and the sensor appeared. In addition, the fiber was not interfered by any electromagnetic signals, and the electronic components of the sensor were isolated from other electrical interference. . Because of this, photoelectric sensors have other sensors that cannot replace the superiority, so its development prospects are very good, and the application will become more and more extensive.

(1) Thermistor sensor structure

The ordinary type of thermistor is composed of a temperature sensing element (metal resistance wire), a bracket, a lead wire, a protective sleeve, and a junction box. In order to avoid the inductance component, the thermal resistance wire is often wound by a double wire to form a non-inductive resistor.

(2) The working principle of the thermistor sensor

In the metal, the carrier [Note 2] is a free electron. When the temperature rises, although the number of free electrons is substantially constant (when the temperature variation range is not large), the kinetic energy of each free electron will increase. Under the action of a certain electric field, the directional movement of these disordered electrons will encounter greater resistance, resulting in an increase in the metal resistance value with increasing temperature. Thermal resistance is measured by the fact that the resistance increases with increasing temperature.

The thermistor is a new type of semiconductor temperature measuring element. It is carriers that participate in conduction in the semiconductor. Since the number of carriers in the semiconductor is much smaller than the number of free electrons in the metal, its resistivity is large. As the temperature increases, more valence electrons in the semiconductor are excited by the thermal transition to a higher energy level to generate a new electron-hole [Note 3] pair, so the number of carriers participating in the electric charge increases, and the resistivity of the semiconductor It is also reduced (increased conductivity). Since the number of carriers increases exponentially with increasing temperature, the resistivity of the semiconductor decreases exponentially with increasing temperature. A thermistor is a temperature-sensitive component made by utilizing the characteristics of a semiconductor such that the number of carriers changes with temperature. When the temperature changes by 1 °C, the resistance change of some semiconductor thermistors will reach (3-6)%. Under certain conditions, the temperature change is obtained according to the change of the measured thermistor value.

(3) Thermistor sensor characteristics and applications

Thermistor sensor mainly measures the temperature and temperature-related parameters by using the characteristic that the resistance value changes with temperature. The main uses are temperature measurement, temperature compensation, overheat protection, and liquid level measurement.

This type of sensor is suitable when the temperature detection accuracy is relatively high. The thermal resistance sensor has the characteristics of large temperature coefficient of resistance, good linearity, stable performance, wide temperature range, and easy processing. It is used to measure temperatures in the range of -200 ° C to +500 ° C.

Ordinary resistance sensors are oriented toward high precision, easy to use, and labor-saving. The resistance sensor is a sensitive component that directly converts the strain signal into an electrical signal. Therefore, it is suitable for making various sensors. The resistance sensor is mainly used for measuring force, pressure, acceleration, displacement, torque and the like. In the past, resistance sensors were mainly used for experimental research, but now they are often used for industrial testing and weighing and control of production lines. And the use of medicine and bioengineering has also increased. In the course of use, the sensor usually has the characteristics of stable electrical signal output, fast response, small size and light weight, and the resistance sensor can satisfy such conditions. However, the resistance sensor still has shortcomings in various functions such as temperature, creep, hysteresis, and self-compensation of elastic modulus. However, with the advent of materials such as excellent phenolic glue, epoxy phenolic aldehyde and polyimine rubber, the performance of the more perfect resistance sensor will have a better prospect.