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This post was last edited by yunrun on 2018-4-17 at 18:39. The international standards for continuous electrical signals are a direct current of 4–20 mA and a direct voltage of 1–5 V. Also, in order to facilitate differentiation from power-off conditions, 0mA and 0V are intentionally excluded; this also avoids the initial non-linear region of the transistor devices, allowing the signal values to have a more linear relationship with the magnitude of the parameter being measured. For instruments that receive 4-20mA DC current signals, their input impedance should not be too high; it should not exceed 750Ω. At the input of the regulator, a 250Ω resistor is typically used to convert the current signal into a voltage signal before further processing. Regulators that accept DC voltage signals in the range of 1–5V should have an input impedance that is not too low, to avoid errors caused by excessive current draw; generally, operational amplifiers or field-effect transistors are used as the input stage. For instruments that transmit signals using air pressure, there is also an international standard for air pressure signals, namely 0.02–0.1 MPa. Sensors that output a standard signal are called transmitters, and they come in electric and pneumatic types. Regardless of whether the output is a standard signal or not, all sensors that produce continuous outputs should have a stable \"zero point\" and \"range.\" The \"zero point\" corresponds to the value at the starting point of the measurement range, while the \"range\" refers to the span of values from the starting point to the ending point of that range. When both of these values remain constant, the performance of the sensor or transmitter is stable. Stability is a prerequisite for accuracy; if stability cannot be achieved, and the output signal keeps fluctuating even when the value being measured remains unchanged, then accuracy is certainly not possible. Furthermore, it is generally also desired that sensors or transmitters have good “linearity”. In other words, within the entire measurement range, there is a strict proportional relationship between the output signal value and the value being measured. Of course, to meet these requirements, reasonable design, high-quality materials, and advanced manufacturing processes are necessary. Apart from those that output in the form of resistance, capacitance, or inductance, the majority of sensors or transmitters used in continuous beams are active. Based on the type of signal they generate, they can be divided into analog and digital types; currently, most of them are analog, and all sensors or transmitters that are activated also operate in an analog manner. Classified by the type of parameter being measured, analog types can be further divided into displacement, force, rotational speed, temperature, pressure, flow rate, liquid level, etc. The principle involved is to use appropriate physical phenomena to convert various non-electrical variables into electrical signals. Even if the parameter being measured is already in the form of current or voltage, if its value is too high or too low, it is converted through circuitry into signals that are suitable for measurement and control. The simplest digital sensor is the encoder, which is a sensor for measuring angles. It uses photoelectric elements and a disc with special transparent patterns to convert the angle of rotation of the axis being measured into binary digits (for reliability, \"Gray code\", which is slightly different from ordinary binary, is sometimes used). Since there is a one-to-one correspondence between the output digits and the angle size, independent of the direction of rotation, and in the event of a power outage during operation, the same values are retained once power is restored again, this is known as a \"positional\" sensor. There is another method for measuring angle, which also involves installing a code disk on the axis being measured. However, the disk does not have black and white marks in binary format; instead, it has alternating black and white stripes. The number of stripes that have passed by is read using photoelectric methods. A higher value indicates more stripes that have passed, meaning a larger angle of rotation. The figure obtained through this method represents the total number of photoelectric pulses generated during rotation, and it depends on the direction of rotation – it increases during clockwise rotation and decreases during counterclockwise rotation. If there is a power outage during operation, the accumulated figures will be lost (unless data protection measures are in place). This is called an “incremental” sensor. Sensors that measure parameters such as frequency and rotational speed reflect the measured values in terms of pulse frequency; some sensors or transmitters for measuring flow rate also output signals based on pulse frequency. It should be noted that all digital sensors or transmitters convert continuous quantities into discrete digital values. Sampling on a periodic basis, performing analog-to-digital conversion on a periodic basis, refreshing display values and communications on a periodic basis, as well as outputting in discrete digital form – all of these factors mean that it cannot continuously reflect the parameter being measured. Strictly speaking, even if the discretization issue is not considered for the time being, its output signal is also the average value over a sampling period rather than an instantaneous value. However, since the sampling period of digital instruments is very short and the changes in the parameters being measured are slow, they can be approximated as instantaneous values.