Sensor is a general term for devices or devices that can be measured and converted into usable output signals according to certain rules, usually composed of sensitive components and conversion components. When the output of the sensor is a specified standard signal, it is called a transmitter.
The concept of a transmitter is an instrument that converts non-standard electrical signals into standard electrical signals, and a sensor is a device that converts physical signals into electrical signals. In the past, physical signals were used, but now other signals are available. The primary instrument refers to the on-site measuring instrument or the base control meter, and the secondary instrument refers to the instrument that uses the primary meter signal to complete other functions: such as control, display and other functions.
Sensors and transmitters are the concepts of thermal instruments. The sensor converts non-electrical physical quantities such as temperature, pressure, liquid level, material, gas characteristics, etc. into electrical signals or directly sends physical quantities such as pressure, liquid level, etc. to the transmitter.
The transmitter is a signal source that amplifies the weak electrical signal collected by the sensor to transmit or start the control component, or converts the non-electrical input from the sensor into an electrical signal and amplifies it for remote measurement and control. The analog quantity can also be converted into a digital quantity as required. The sensor and the transmitter together form an automatically controlled monitoring signal source. Different physical quantities require different sensors and corresponding transmitters. There is also a transmitter that does not convert physical quantities into electrical signals, such as a "differential pressure transmitter" of a boiler water level gauge. On both sides of the bellows of the transmitter, the differential pressure on both sides of the bellows drives the mechanical amplifying device to indicate the water level with a pointer. Of course, there are also those that convert electrical analog quantities into digital quantities, which can also be called transmitters. The above is just a conceptual illustration of the difference between a sensor and a transmitter.
Characteristics of various sensors
First, the definition of sensor
The national standard GB7665-87 defines a sensor as: "A device or device that can sense the specified measured and convert it into a usable signal according to a certain law, usually composed of sensitive components and conversion components." A sensor is a detection device that can sense the measured information, and can transform the sensed information into electrical signals or other required forms of information output according to certain rules, so as to meet the requirements of information transmission, processing, storage, Display, record and control requirements. It is the first link to realize automatic detection and automatic control.
Second, the classification of sensors
At present, there is no unified classification method for sensors, but the following three are commonly used:
1. According to the physical quantity of the sensor, it can be divided into displacement, force, speed, temperature, flow, gas composition and other sensors
2. According to the working principle of the sensor, it can be divided into resistance, capacitance, inductance, voltage, Hall, photoelectric, grating, thermocouple and other sensors.
3. According to the nature of the sensor output signal, it can be divided into: switch-type sensor whose output is switch value ("1" and "0" or "on" and "off"); output is analog sensor; output is pulse or Code digital sensor.
Third, the static characteristics of the sensor
The static characteristic of the sensor refers to the relationship between the output of the sensor and the input of the static input signal. Because the input and output are independent of time at this time, the relationship between them, that is, the static characteristics of the sensor, can be an algebraic equation without time variables, or the input is used as the abscissa, and the corresponding output is The characteristic curve drawn by the ordinate is described. The main parameters that characterize the static characteristics of the sensor are: linearity, sensitivity, resolution and hysteresis.
Fourth, the dynamic characteristics of the sensor
The so-called dynamic characteristics refer to the characteristics of the output of the sensor when the input changes. In practical work, the dynamic characteristics of the sensor are often represented by its response to some standard input signals. This is because the response of the sensor to the standard input signal is easy to obtain experimentally, 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 sinusoidal signal, so the dynamic characteristics of the sensor are also commonly expressed by step response and frequency response.
5. Linearity of the sensor
Typically, the actual static characteristic output of the sensor is a curve rather than a straight line. In practical work, in order to make the meter have a uniform scale reading, a fitted straight line is often used to approximately represent the actual characteristic curve, and the linearity (non-linear error) is a performance indicator of this approximation.
There are many ways to select the fitting line. For example, take the theoretical straight line connecting the zero input and the full-scale output point as the fitting straight line; or take the theoretical straight line whose sum of squares of deviations from each point on the characteristic curve is the smallest as the fitting straight line, and this fitting straight line is called the least squares fitting. Line up.
6. Sensitivity of the sensor
Sensitivity refers to the ratio of the output change △y to the input change △x under the steady state working condition of the sensor.
It is the slope of the output-input characteristic curve. If there is a linear relationship between the sensor output and the input, the sensitivity S is a constant. Otherwise, it will vary with the amount of input.
The dimension of sensitivity is the ratio of the dimensions of output and input. For example, for a displacement sensor, when the displacement changes by 1mm, the output voltage changes by 200mV, then its sensitivity should be expressed as 200mV/mm.
When the dimensions of the output and input of the sensor are the same, the sensitivity can be understood as the magnification.
Improve the sensitivity, can get higher measurement accuracy. However, the higher the sensitivity, the narrower the measurement range and the worse the stability.
Seven, the resolution of the sensor
7. Resolution refers to the ability of a sensor to perceive the smallest change in the measurand. That is, if the input quantity changes slowly from some 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 the input quantity. The output changes only when the input quantity changes beyond the resolution.
Usually, the resolution of each point in the full-scale range of the sensor is not the same, so the maximum change value in the input quantity that can produce a step change in the output quantity in the full-scale range is often used as an index to measure the resolution. If the above indicators are expressed as a percentage of full scale, it is called resolution.
