Technical specifications of the sensor
1. Technical specifications of the sensor
Due to the wide range of applications of sensors, many principles, structures and types, and different requirements for use, it is difficult to list a unified index for comprehensively measuring the quality of sensors. Table 1-2 lists the technical performance indicators of the sensor, among which some basic parameter indicators and more important environmental parameter indicators are often used as the basis for testing, use, and evaluation of the sensor.
For a specific sensor, not all indicators are required. Hoping to make a sensor all the indicators are good, not only the design and manufacture is difficult, but also in practice is not necessary. Therefore, do not choose a "universal" sensor to apply to different use occasions. On the contrary, the main indicators should be guaranteed according to actual needs, and the remaining indicators can meet the basic requirements. Even the main indicators, it is not necessary to blindly pursue the overall excellence of a single indicator, but mainly should be concerned about its stability and change law, so that it can be compensated and corrected on the circuit or using the computer, so that many sensors can be applied at low cost and high precision.
2. Ways to improve sensor performance
The following technical approaches can be taken to improve the performance of the sensor.
2.1. Differential technology
Differential technology is widely used in sensors. Its application can significantly reduce the influence of temperature change, power supply fluctuation and outer boundary interference on the accuracy of the sensor, reduce the nonlinear error and increase the sensitivity. This technique is also widely used to eliminate or reduce common mode errors due to structural reasons (such as temperature errors). The principle is as follows:
For a specific sensor, not all indicators are required. Hoping to make a sensor all the indicators are good, not only the design and manufacture is difficult, but also in practice is not necessary. Therefore, do not choose a "universal" sensor to apply to different use occasions. On the contrary, the main indicators should be guaranteed according to actual needs, and the remaining indicators can meet the basic requirements. Even the main indicators, it is not necessary to blindly pursue the overall excellence of a single indicator, but mainly should be concerned about its stability and change law, so that it can be compensated and corrected on the circuit or using the computer, so that many sensors can be applied at low cost and high precision.
2. Ways to improve sensor performance
The following technical approaches can be taken to improve the performance of the sensor.
2.1. Differential technology
Differential technology is widely used in sensors. Its application can significantly reduce the influence of temperature change, power supply fluctuation and outer boundary interference on the accuracy of the sensor, reduce the nonlinear error and increase the sensitivity. This technique is also widely used to eliminate or reduce common mode errors due to structural reasons (such as temperature errors). The principle is as follows:
A sensor is provided with an output of
y₁=ao+a₁x+a₂x2+a₃x3+a₄x⁴+ …
With another identical sensor, its input sign is opposite (for example, the displacement sensor makes it move in the opposite direction), then its output is
Y₂=ao-a₁x+a₂x2-a3x3+a4x⁴- …
Subtract the two outputs, i.e
△y=y₁-y₂=2(a₁x+a₃x3+ …)
Therefore, the total output eliminates the zero output and even nonlinear term, and obtains a fairly wide approximate linear range symmetrical to the origin, which reduces the nonlinearity, doubles the sensitivity, and cancels the common mode error. In the sensor, the full scale of the outside being measured often only causes a small amount of change in a single sensitive element, in order to remove this small amount of change, remove the invariant part, it is necessary to use differential technology in the sensitive part.
2.2. Average technology
The average technique utilizes the average effect and can reduce the random error in measurement. Common averaging techniques include error averaging and data averaging.
(1) Error averaging effect The principle of the error averaging effect is to use n sensor units to feel being measured at the same time, and its output will be the sum of the output of these units. If the possible error δ0 of each element is treated as a random error, the total error △ will be reduced to
For example, when n=10, the error △ can be reduced to 31.6% of δ0. If n=500, the error is reduced to 4.5% of δ0.
The error averaging effect has obtained obvious effect in grating, induction synchronizer, magnetic gate, capacitive gate and other sensors. In other sensors, the error averaging effect can also make up for the errors caused by some process defects. (2) Data average processing Similarly, if the measurement under the same conditions is repeated n times or sampled n times, and then the data average processing is carried out, the random error will also be reduced by times. Therefore, data averaging can be used to reduce random errors in all measurements that allow multiple repeated measurements (or samples). For the intelligent sensor with microcomputer chip, the realization is particularly convenient. The principle of the above error averaging effect and data averaging processing can be adopted in the design and application of sensors, and the entire measurement system should be regarded as an object when applied. The common multi-point measurement scheme and multi-sampling average method can reduce the random error, increase the sensitivity and improve the measurement accuracy. 2.3. Zero indication, differential and closed-loop techniques When designing or applying sensors, zero indication, differential and closed-loop techniques may be used to eliminate or reduce system errors. (1) Zero method It can eliminate the error caused by the inaccurate indicator. In this method, the action of the measured on the indicator is balanced with the action of the known standard quantity on it, so that the indicator represents zero, and the measured quantity is equal to the known standard quantity. The mechanical balance is a typical example of the zero indication method. The balanced bridge is an example of the application of zero indication in sensor technology.
(2) Differential method Differential method is developed on the basis of zero indication method. Since the zero indication method requires that the measured and the standard quantity should be exactly equal, the standard quantity is required to be continuously variable, which is often not easy to do. However, if the difference between the standard quantity and the measured quantity is reduced to a certain extent, the error effect of the indicating instrument can be greatly weakened due to their reciprocal effect, which is the principle of the differential method.
The micrometers widely used in geometric measurement, such as inductance micrometer and optical comparator, are examples of differential method. When measuring with this method, the standard quantity can be measured by measuring block or standard workpiece, and the measuring accuracy is greatly improved.
(3) Closed-loop technology When the sensor is required to have a wide frequency response, large dynamic range, high sensitivity, resolution, accuracy and high stability, repeatability and reliability, the open-loop sensor composed of sensitive components, conversion components, measurement circuits and other links will be difficult to meet the requirements, and the use of feedback technology to make the sensor form a closed-loop balanced sensor. The closed-loop feedback measurement system can meet the above requirements. Closed-loop sensor is widely used in process parameter detection technology.
Tracking technology also belongs to closed-loop technology idea. In addition to the tracking of the equilibrium point, it can also track some specific value points (often extreme value points) and comprehensive index parameters, and the amount of feedback has one or multiple dimensions. Tracking technology has a wide range of applications, such as star tracking, radar multi-target tracking, navigation inertial platform tracking and so on.
2.4. Shielding, isolation and interference suppression
Most sensors are installed in the field, and the field conditions are often poor, sometimes even extremely bad. All kinds of external factors will affect the accuracy of the sensor and the related performance. In order to reduce the measurement error and ensure its original performance, we should try to weaken or eliminate the influence of external factors on the sensor. Mainly from two aspects to achieve, one is to reduce the sensitivity of the sensor to the influencing factors; The second is to reduce the actual effect of external factors on the sensor.
For electromagnetic interference, shielding (electric field shielding, electromagnetic shielding and magnetic shielding), isolation measures, can also be suppressed by filtering and other methods. For such as temperature, humidity, mechanical vibration, air pressure, sound pressure, radiation and even airflow, appropriate isolation measures can be used, such as heat insulation, sealing, vibration isolation, etc., or after converting into electricity to separate or suppress the interference signal to reduce its impact. Circuit measures such as filtering, adding decoupling capacitance and correct grounding can also be adopted in the circuit.
2.5. Segmentation and subdivision technology
For large size and high precision geometric measurement problems, the segmented measurement scheme can be adopted. The measuring range is divided into several sub-sections, in which the local subdivision is carried out. This technique requires that the scale be divided into sections as closely as possible under the condition of technological economy. The measurement process starts at zero, records the number of segments experienced, and then subdivides the segments using simulation methods. Two sensors are commonly used to complete the function of segment counting, simulation subdivision and resolution of the direction of motion. The distance between the two sensors differs by 1/4 segment after subtracting the segment integral multiple, that is, the two sensors emit sine and cosine signals respectively when the motion is measured.
Segmentation and subdivision technology are used in laser interferometer, induction synchronizer, grating, magnetic gate, capacitive gate and other sensor technologies. CCD photoarray is also used to measure spot position. In this technique, multiple sensors are often used to cover multiple segments, and spatial averaging is used to improve the measurement accuracy.
2.6. Compensation and correction techniques
Compensation and correction techniques have been widely used in sensors. The use of this technology is mainly for two situations, one is for the characteristics of the sensor itself, and the other is for the working conditions or external environment of the sensor.
For the characteristics of the sensor, the variation law of the error can be found, or its size and direction can be measured, and appropriate methods can be used to compensate or correct.
Error compensation for sensor working conditions or external environment is also a powerful technical measure to improve sensor accuracy. Many sensors are sensitive to temperature, due to temperature changes caused by the error is very considerable, in order to solve this problem, if necessary to control the temperature, but often the cost is too high, or the use of the site is not allowed. It is often feasible to introduce temperature error compensation into the sensor. At this time, we should find out the law of the influence of temperature on the measured value, and then introduce temperature compensation measures.
Compensation and correction can be solved by electronic circuit (hardware), or can be realized by software with microcomputer.
2.7. Stability treatment
As a device for long-term measurement or repeated use, the stability of the sensor is particularly important, and its importance even exceeds the precision index, especially for those occasions that are difficult or unable to be regularly verified.
The reason for the instability of the sensor performance is mainly that with the passage of time and the change of environmental conditions, the performance of various materials and components that constitute the sensor will change.
In order to improve the stability of the sensor performance, the necessary stability treatment should be carried out on the material, components or sensor as a whole. Such as aging treatment of structural materials, cold treatment, time aging of permanent magnet materials, temperature aging, mechanical aging and AC magnetic stabilization treatment, aging screening of electrical components, etc.
When using sensors, if the measurement requirements are high, additional adjustment elements and key components of subsequent circuits should also be aged if necessary.
