Common sense problems in first grade

Common sense problems of sensor 1. Problems in sensor application

I'll just talk about piezoelectric effect, photoelectric effect 1. Piezoelectric effect can be divided into positive piezoelectric effect and reverse piezoelectric effect.

The positive piezoelectric effect means that when the crystal is subjected to an external force in a fixed direction, polarization occurs inside, and the two surfaces simultaneously produce charges with opposite signs; When the external force is removed, the crystal returns to the uncharged state; When the direction of external force changes, the polarity of charge also changes; The amount of charge generated by the force on the crystal is proportional to the magnitude of the external force. Piezoelectric sensors are mostly made of positive piezoelectric effect.

Inverse piezoelectric effect refers to the phenomenon of mechanical deformation of crystals caused by alternating electric field, also known as electrostrictive effect. The transmitter made of inverse piezoelectric effect can be used in electroacoustic and ultrasonic engineering.

There are five basic forms of stress deformation of piezoelectric sensing elements: thickness deformation, length deformation, volume deformation, thickness shear and plane shear (see figure). Piezoelectric crystals are anisotropic, and not all crystals can produce piezoelectric effects in these five states.

For example, the timely crystal has no volume deformation piezoelectric effect, but has good thickness deformation and length deformation piezoelectric effect. A kind of sensor based on dielectric piezoelectric effect is called piezoelectric sensor. 2. The photoelectric effect (1) summarizes the effect of emitting electrons from the metal surface under the action of light irradiation, and the emitted electrons are called photoelectrons.

Only when the wavelength of light is less than a certain critical value can electrons be emitted, that is, the limit frequency and the limit wavelength. The critical value depends on the metal material, and the energy of emitted electrons depends on the wavelength of light, which has nothing to do with the intensity of light and cannot be explained by the fluctuation of light.

And the fluctuation of light is also contradictory, that is, the instantaneity of photoelectric effect. According to the fluctuation theory, if the incident light is weak and the irradiation time is long, the electrons in the metal can accumulate enough energy and fly out of the metal surface. But the fact is that as long as the frequency of light is high and the ultimate frequency of metal is high, no matter whether the brightness of light is strong or weak, the generation of photons is almost instantaneous, not exceeding the ninth power of minus ten.

The correct explanation is that light must be composed of strictly defined energy units (i.e. photons or optical quanta) related to wavelength. This explanation was put forward by Einstein.

The photoelectric effect was discovered by German physicist Hertz in 1887, which is the foundation of the development of quantum theory. Under the irradiation of light, the phenomenon that electrons in an object are ejected is called photoelectric effect. The photoelectric effect can be divided into photoelectron emission, photoconductive effect and photovoltaic effect.

The former phenomenon occurs on the surface of an object, which is also called external photoelectric effect. The latter two phenomena occur inside the object, which is called internal photoelectric effect.

(2) Explain ① the experimental law of photoelectric effect. A. The number of photoelectrons emitted by the cathode (metal material emitting photoelectrons) is directly proportional to the illumination intensity.

B the initial velocity of photoelectrons leaving the object is related to the frequency of the irradiated light, but has nothing to do with the luminous intensity. That is to say, the initial kinetic energy of photoelectrons is only related to the frequency of irradiated light, and has nothing to do with the luminous intensity.

C only when the light frequency of the irradiated object is not less than a certain value can the object emit photoelectrons. This frequency is called the limit frequency (or cutoff frequency) and the corresponding wavelength λ. It is called the red limit wavelength.

Limit frequency of different substances ". And the corresponding red limiting wavelength λ.

Is different. Red limit wavelengths of several metal materials: Au, Cs, Na, Zn, Ag, Pt, red limit wavelength (Angstrom) 6520 5400 3720 26001960 d. It is known from experiments that the process of generating photocurrent is very fast, generally less than lOe-9 seconds; When the light irradiation stops, the photocurrent stops immediately.

This shows that the photoelectric effect is instantaneous. ② Einstein equation explains photoelectric effect: According to Einstein theory, when a photon irradiates an object, its energy can be completely absorbed by an electron in the object.

After electrons absorb photon energy hυ, the energy increases, and there is no need to accumulate energy. If the energy hυ absorbed by an electron is large enough to overcome the energy (ionization energy) i needed to leave an atom and the work function (or work function) w when leaving the surface of an object, then the electron can escape from the surface of the object and become a photoelectron, which is the photoelectric effect.

Einstein's equation is hυ=( 1/2)mv2+I+W, where (1/2)mv2 is the initial kinetic energy of photoelectrons leaving the object. There are a lot of free electrons in metals, which is the characteristic of metals. So for metals, the I term can be omitted, and the Einstein equation becomes hυ=( 1/2)mv2+W If hυ.

For a certain metal, the minimum optical frequency (limit frequency) υ0 that produces photoelectric effect. Is determined by h υ 0 = w.

The corresponding red limit wavelength is λ 0 = c/υ 0 = HC/W. The increase of luminous intensity increases the number of photons irradiated on the object, so the number of photoelectrons emitted is proportional to the intensity of irradiated light.

③ photomultiplier tubes can be manufactured by photoelectric effect. The photomultiplier tube can convert the flash into amplified electric pulses, which are then sent to electronic circuits and recorded.

Formula The following formula is used to quantitatively analyze the photoelectric effect according to Einstein's way: photon energy = energy required to remove an electron+kinetic energy algebraic form of emitted electrons: where h is Planck constant, f is the frequency of incident photons, which is the work function, the minimum energy required to remove an electron from an atomic bond is the maximum kinetic energy of emitted electrons, and f0 is the threshold frequency of photoelectric effect. M is the rest mass of emitted electrons, and vm is the velocity of emitted electrons. Note: If the energy (hf) of photons is not greater than the work function (φ), electrons will not be emitted. The work function is sometimes marked w.

When this formula is inconsistent with the observation (that is, there is no electron emission or the kinetic energy of electrons is less than expected), it may be because the system is not completely efficient and some energy is lost in the form of heat energy or radiation. Einstein won the Nobel Prize in Physics for his discovery of photoelectric effect.

2. Key points of sensor fault diagnosis

1, the failure of the computer power cord will make the performance of the automobile engine worse and the economy lower, so you should check the computer power cord before replacing the automobile computer. (The power cord should include the ground wire and be regarded as a complete power cord).

2. If the voltage signal of the oxygen sensor is higher than the standard value, it may be that the sensor is polluted, which will make the air-fuel ratio rich in many cases.

3. If the voltage signal of the oxygen sensor is lower than the standard value, it may be that the sensor is out of order, which will lead to the lean air-fuel ratio of the engine.

4, check the oxygen sensor must use digital multimeter or oscilloscope.

5. If the oxygen sensor heater is faulty, it may prolong the open-loop working time of the engine and increase fuel consumption.

6. Engine coolant temperature sensor can check its performance with digital meter or analog meter.

7. In some computer ECT circuits, an internal resistor is controlled to change the voltage on the sensor at a certain temperature of the engine. If the voltage at this time is abnormal during the measurement, it does not mean that the sensor is faulty.

8. Testing the engine coolant temperature sensor and the air temperature sensor can use exactly the same operating procedure. The only thing to note is that their temperature change curves are different, so there will be no same voltage signal at the same temperature.

9. When the throttle is opened and the voltage signal of the throttle position sensor is checked, the stability of the sensor can be checked by vibration with appropriate force. This method is very effective for virtual connection faults of some circuits.

10, many four-wire throttle position sensors include an idle position switch, which is used to provide the engine control unit with the working state information when the throttle is in the idle position.

1 1. In some cases, when the throttle is in the idle position, you can loosen the fixing screw of the throttle position sensor and rotate the sensor housing to adjust the voltage signal.

12. If the absolute pressure sensor of the intake manifold outputs a frequency signal, it can't be detected by a common multimeter.

13. Many intake manifold absolute pressure sensors output voltage signals converted from atmospheric pressure. This signal can be checked by turning on the ignition switch. (This method can only prove that the sensor can still work. If the output accuracy drops, this method can't be detected. )

14. When checking the output voltage signal of the absolute pressure sensor of the intake manifold, there should be a certain degree of vacuum in the sensor. In most cases, it can be judged by detecting its output signal every 10 kPa.

15. When measuring the voltage signal of the vane-type intake air flow sensor, you can check when the vane of the sensor turns from fully closed to fully open, and observe the voltage value and continuity of the output signal.

16. Some intake air flow sensors with thermal resistors or hot wires are provided with voltage signals of different frequencies by the engine computer. This kind of sensor can only check its voltage with a multimeter that can test the frequency.

17. The voltage signal of the exhaust gas recirculation valve position sensor will change from 0.8V when the valve is closed to 4.5V when the valve is fully opened.

18, the computer uses the signal of the speed sensor to control the clutch of the torque converter, shift gears while driving, and collect data from the driving computer.

In fact, there are many things in our work that deserve our recollection and summary. The above is just a typical phenomenon in practical application, and I hope it can help you.

If a friend has encountered a special sensor at work, he might as well send it to remind everyone so as not to take many detours.

3. What are the characteristics of the sensor?

Sensor characteristics:

First, the sensor is static.

The static characteristic of sensor refers to the relationship between output and input of sensor for static input signal. Because the input and output have nothing to do with time, the relationship between them, that is, the static characteristics of the sensor can be described by an algebraic equation without time variables, or by a characteristic curve drawn with the input as the abscissa and the corresponding output as the ordinate. The main parameters that characterize the static characteristics of the sensor are linearity, sensitivity, hysteresis, repeatability and drift.

1, linearity: refers to the degree to which the actual relationship curve between sensor output and input deviates from the fitting straight line. It is defined as the ratio of the maximum deviation between the actual characteristic curve and the fitted straight line to the full-scale output value within the full-scale range.

2. Sensitivity: Sensitivity is an important indicator of the static characteristics of the sensor. It is defined as the ratio of the output increment to the corresponding input increment that causes the increment. Use s to indicate sensitivity.

3. Hysteresis: In the process of changing the input from small to large (positive stroke) and the input from large to small (reverse stroke), the phenomenon that the input-output characteristic curves of the sensor do not coincide is called hysteresis. For the same input signal, the output signals of the sensor before and after the stroke are not equal, and this difference is called hysteresis difference.

4. Repeatability: Repeatability refers to the degree to which the obtained characteristic curves are inconsistent when the input of the sensor changes continuously for many times in the same direction.

5. Drift: The drift of the sensor means that the output of the sensor changes with time when the input is unchanged, which is called drift. There are two reasons for the drift: one is the structural parameters of the sensor itself; The second is the surrounding environment (such as temperature and humidity, etc.). ).

6. Resolution: When the input of the sensor slowly increases from a non-zero value, the output changes significantly after exceeding a certain increment, which is called the resolution of the sensor, that is, the minimum input increment.

7. Threshold: When the input of the sensor slowly increases from zero, the output changes significantly after reaching a certain value, which is called the threshold voltage of the sensor.

Second, sensor dynamics.

The so-called dynamic characteristics refer to the characteristics of the output of the sensor when its input changes. In practical work, the dynamic characteristics of the sensor are often expressed by its response to some standard input signals. This is because the response of the sensor to the standard input signal can be easily obtained by experiments, and there is a certain relationship between its response to the standard input signal and its response to any input signal, and the latter can often be inferred by knowing the former. The most commonly used standard input signals are step signal and sine signal, so the dynamic characteristics of the sensor are also commonly expressed by step response and frequency response.

Third, linearity.

Usually, the actual static characteristic output of the sensor is a curve rather than a straight line. In practical work, in order to make the instrument have a unified calibration reading, a fitting straight line is often used to approximate the actual characteristic curve, and linearity (nonlinear error) is a performance index of this approximation.

There are many ways to choose a fitting straight line. For example, the theoretical straight line connecting zero input and full-scale output points is used as a fitting straight line; Or the theoretical straight line with the smallest sum of squares of deviations of each point on the characteristic curve is regarded as the fitting straight line, which is called the least square fitting straight line.

Fourth, sensitivity.

Sensitivity refers to the ratio of output change △y to input change △x of the sensor under steady-state working conditions.

It is the slope of the output-input characteristic curve. If there is a linear relationship between the output and the input of the sensor, the sensitivity S is constant. Otherwise it will change with the change of input.

The dimension of sensitivity is the ratio of the dimensions of output and input. For example, when the displacement of the displacement sensor changes 1mm and the output voltage changes by 200mV, its sensitivity should be expressed as 200 mv/mm.

When the output and input of the sensor are the same size, the sensitivity can be understood as the magnification.

Improve the sensitivity and obtain higher measurement accuracy. But the higher the sensitivity, the narrower the measuring range and the worse the stability.

Verb (short for verb) solution

Resolution refers to the sensor's ability to feel the smallest change being measured. That is, if the input quantity changes slowly from a non-zero value. When the input change value does not exceed a certain value, the output of the sensor will not change, that is, the sensor cannot distinguish the change of this input. Only when the input changes beyond the resolution will its output change.

Usually, the resolution of each point of the sensor is different in the full-scale range, so the maximum change value of the input quantity that can make the output change step by step in the full-scale range is often used as the index to measure the resolution. If the above indicators are expressed as a percentage of full scale, it is called resolution. The resolution is negatively correlated with the stability of the sensor.

4. What problems should be paid attention to when selecting vibration sensors?

Even for the most experienced engineers, it is an arduous task to choose the best acceleration sensor in predictive maintenance.

This process can usually be filtered into nine questions. Question 1: What do you want to test? What do you really want to measure? In other words, what do you want to do? What do you hope to get? What are you going to do with the data? Acceleration sensors can monitor vibration and provide raw vibration data, while vibration transmitters provide root mean square (RMS) values.

It is useful to analyze the original vibration data because it contains all the information of the vibration signal, the true peak amplitude and vibration frequency. Because the total value or peak value of RMS is a continuous 4-20 mA signal, it is very useful in PLC, DCS, SCADA system and PI control system.

Some applications use both signals at the same time. By determining various signals required by the application, the search range can be greatly narrowed.

In addition, is acceleration, velocity or displacement used to measure vibration? Have you considered that some industrial sensors can output vibration and temperature simultaneously? Finally, in some field applications, such as vertical pumps, it is best to monitor the vibration of more than one shaft. Does your field application require uniaxial, biaxial or triaxial measurement? Question 2: What is the range? The maximum amplitude or vibration range to be measured determines the sensor range to be used.

Typical acceleration sensor sensitivity is 100 mV/ g, standard application (50g range) and low frequency or low amplitude application of 500mv/g (10g range). The range of 0- 1 in/sec or 0-2 in/sec is usually used for 4-20mA transmitters in general industrial applications.

Question 3: What is the vibration frequency? For different excitation frequencies, the physical structure and dynamic system have different responses. Vibration sensors are no exception.

The properties of piezoelectric materials are like high-pass filters, so even the best piezoelectric sensor is limited by the low frequency of about 0.2Hz. As a single-degree-of-freedom dynamic system, the sensor has natural vibration frequency.

The signal is greatly amplified at the natural vibration frequency, resulting in a significant change in sensitivity, which is likely to be out of range. Most industrial accelerometers have one or two RC filters to eliminate the excited * * * vibration frequency.

It is very important to choose the available frequency range of the sensor, including the frequency you are interested in. Question 4: What is the ambient temperature? For ICP acceleration sensor and 4-20mA transmitter, extremely high ambient temperature will pose a threat to internal electronic equipment.

The acceleration sensor in charging mode can be used in very high ambient temperature. It has no built-in electronics, but uses a remote charge amplifier. The charging mode acceleration sensor is equipped with an integrated hard-wired cable, which can be applied to the environment where the temperature exceeds 260℃, such as vibration monitoring of gas turbines.

Question 5: Will it be submerged in liquid? Industrial acceleration sensors equipped with integral polyurethane cables can be permanently installed by immersion in liquid. For high-pressure applications, it is best for the sensor to be pressure tested for one hour.

Fully submerged applications require integrated cables. It is also necessary to integrate cables in the case of spraying rather than full immersion, such as cutting fluid for machine tools.

Question 6: Will it be exposed to potentially harmful chemicals or debris? The industrial acceleration sensor can be made of corrosion-resistant and chemical-resistant stainless steel. In the environment of harmful chemicals, PTFE corrosion-resistant connecting cable is considered for the sensor.

It is strongly recommended to check the chemical compatibility chart of any suspicious chemicals. The integrated armored cable can provide good protection for the environment that the chip may be exposed to.

Question 7: Do you need to eject, deflect and compress the link? Finally, the sensor needs to be installed in the available space of the device. The shape of the sensor has little influence on its performance, but the safe installation and maintenance operation on site should be considered.

The compact acceleration sensor designed with lock nut can be fixed in any direction, but it is very convenient if it is equipped with integrated cable. Question 8: Do you use high-precision or low-cost sensors? There are two main differences between low-cost and high-precision acceleration sensors.

First of all, the precision cell is usually completely calibrated, which means that the sensitivity response measurement is plotted within the available frequency range. The low-cost acceleration sensor is calibrated at a single point, and the sensitivity is only measured at one frequency.

Secondly, high-precision acceleration sensors have strict tolerances in some specifications, such as sensitivity and frequency range. For example, the nominal sensitivity of a high-precision acceleration sensor is1100mv/g 5% (95mv/g to 105mV/g), while the nominal sensitivity of a low-cost acceleration sensor is100mv/g10% (90mv/g

Customers can set the calibration sensitivity of sensors in the data acquisition system, so that low-cost sensors can also provide accurate and repeatable data. As for frequency, the maximum deviation of high-precision acceleration sensors is usually 5%, while low-cost sensors can provide a frequency range of 3 dB.

However, low-cost sensors can provide excellent frequency response. Question 9: Do you need a special authentication code? CSA and ATEX certified acceleration sensors and 4-20 mA transmitters can be used in dangerous areas.

Compare sensor certifications to ensure that they meet your needs. The answers to the nine questions can greatly narrow your search scope and find the best solution for your application.

Remember that the combined answers may be mutually exclusive, that is, there is no solution that meets all the criteria. For example, a specific model used in hazardous areas may not have ATEX certification.

In addition, there may be other considerations for specialized field applications.