Analog signal transmission and analog signal standardization

zidonghua01

Analog signal transmission and analog signal standardization [Copy link]

For thermal control instruments, we should strive to achieve universality and mutual compatibility during design. Universality means that the same instrument can display or control different parameters, although the measured parameters vary greatly, regardless of their range of variation, although the sensors, transmitters, and converters used are different, but the display instruments are exactly the same. At present, these technologies have been maturely applied to display instruments. Mutual compatibility (also known as interchangeability) means that instruments from different series or different manufacturers can be used together in the same detection and control system, cooperate with each other, and jointly realize the functions of the system. If universality and mutual compatibility can be achieved, the interchangeability of control instruments and their components can be improved, spare parts can be reduced, and when the production process changes, most of the original instruments can be fully utilized, saving the investment in reconstruction. Analog signal yunrun.com.cn/tech/463.html The previously widely used electric or pneumatic unit combination instrument has good versatility. Not only are its display unit and control unit universal, but even the transmitter unit such as differential pressure transmitter can be used to measure various signals such as pressure, flow, liquid level, density and force. For example, the intelligent temperature transmitter is designed to be equipped with either thermocouples or thermal resistors. It can measure the temperature of one point or several points (usually called "temperature remote monitoring instrument") or temperature difference. Its measurement range and zero point can be freely adjusted. Universalization not only reduces the variety and specifications of instrument manufacturers, but also brings users the benefits of flexibility, convenience and multi-purpose. In a thermal power plant, the DCS system installed in the control room needs to be connected to the transmitters and actuators installed at the production site. To achieve universality and compatibility, the signal format between instruments must be unified. Both analog and digital signals should be standardized. The signal system refers to the unified communication signal used for the input and output signals of each instrument in a complete set of series instruments. Only by using a unified signal can any connection between instruments become possible. 1. Standards for analog pneumatic signals GB/T 777-2008 "Analog pneumatic signals for industrial automation instruments" stipulates that the lower limit of analog pneumatic signals is 20kPa and the upper limit is 100kPa 2. Standards for analog DC current signals GB/T 3369.1-2008 "Analog signals for process control systems Part 1: DC current signals" stipulates analog DC current signals (load resistance is 0-300Ω) as shown in Table 1 Table 1 Analog DC current signal

Lower limit (mA)	Lower limit (mA)	Description
4	20 *	*Preferred
0	20 **	**Non-preferred, will be cancelled in the future

3. Electrical analog signal Electrical analog signal includes DC current, DC voltage, AC current and AC voltage. From the signal range, the lower limit can start from zero or not (that is, there is a live zero point), and the upper limit can be high or low. How to determine the type and range of unified signal has a direct impact on the technical and economic performance of the whole instrument. Below, Yunrun Instrument analyzes and compares DC current and DC voltage. 3.1 DC current signal DC current signal is the most widely used signal type at present, and it has the following advantages: ① DC has less interference than AC AC is easy to generate interference from alternating electromagnetic fields, which affects nearby instruments and circuits. After the external AC interference signal is mixed in, it is in the same form as the useful signal and is difficult to filter; DC signals do not have this disadvantage. In signal transmission, DC is not affected by AC induction, and it is easy to solve the anti-interference problem of the instrument. ② DC signals have simple requirements for loads AC has frequency and phase problems, and is sensitive to the inductive reactance or capacitive reactance of the load, which increases the number of influencing factors and complicates the calculation; DC circuits only consider resistance. DC is not affected by the inductance, capacitance and load properties of the transmission line, and there is no phase shift problem, which simplifies the wiring. ③ The use of DC signals facilitates analog-to-digital conversion and unified signals, making it easier to use field instruments with digital instruments. ④ DC signals are easy to obtain reference voltages. Therefore, countries around the world use DC current and DC voltage as unified signals. 3.2 DC voltage signals When using DC voltage signals as communication signals, if the output voltage of a transmitting instrument is to be transmitted to several receiving instruments at the same time, the several receiving instruments should be connected in parallel, as shown in Figure 1. Two points need to be noted when using DC voltage signals: ① In order to reduce transmission errors, the internal resistance r0 of the transmitting instrument and the wire resistance Rcm are required to be small enough. If the voltage signal is transmitted over a long distance, the increased Rcm value will inevitably put too high a requirement on the receiving instrument resistance ri, and the high input impedance will easily introduce interference, so the DC voltage signal is not suitable for long-distance transmission signals. ② The higher the input impedance of the receiving instrument, the smaller the error. When there are more instruments connected in parallel, it is equivalent to a decrease in the total input impedance and an increase in the error. Therefore, the more instruments are connected in parallel, the greater the input impedance of each instrument is required. When using DC voltage as a communication signal, since the instrument is connected in parallel, its main advantages are: it is relatively simple in design and installation; adding or reducing a certain instrument will not affect the work of other instruments; the withstand voltage requirements for the instrument output stage can be reduced, thereby improving the reliability of the instrument. From the above analysis, it can be seen that DC current signal transmission and DC voltage signal transmission have their own characteristics. DC current signals are suitable for long-distance transmission, and voltage signals enable instruments to be connected in parallel. Therefore, in foreign electric instrument systems and my country's DDZ-Ⅲ instruments, the transmission signals entering and leaving the control room use DC current signals, and the communication signals between the instruments in the control room use DC voltage signals, that is, the connection characteristics are current transmission and parallel reception of voltage signals. 4. Comparison of upper and lower limits of signals In Table 1, the lower limit value (zero point) current of the transmission signal is different, namely 4mA and 0mA. The former is called a live zero signal, and the latter is called a true zero signal. The lower limit of the true zero signal starts from zero, which is convenient for mathematical operations such as addition, subtraction, multiplication, division, and square root of analog quantities. It is very convenient to handle, but it is difficult to distinguish the lower limit value under normal circumstances from circuit faults (such as disconnection), which is easy to cause misunderstanding or misoperation; the live zero signal is 4mA under normal circumstances. Once 0mA appears, it must be a disconnection, short circuit or power outage. The fault can be discovered in time, which is extremely beneficial to production safety. The use of live zero signal not only creates working conditions for two-wire transmitters, but also avoids the initial nonlinearity of the transistor characteristic curve, which is also the advantage of 4-20mA signal over 0-10mA signal. Before calculating and processing the live zero signal, the signal value corresponding to the zero point must be subtracted, and the zero signal must be added to the result of the calculation. Although it is not very convenient, it is very easy to subtract or add a few constants in instruments that widely use operational amplifiers and microprocessors, so the use of live zero is not a problem. The upper limit of the signal current range is limited by the power of the circuit and should not be too large to avoid inconvenience in the design of the output circuit and the selection of devices. Moreover, excessive signal current can easily cause the wires and components to heat up, which is not conducive to safety and explosion-proof. Foreign electric instruments. Generally, the upper limit of the signal does not exceed 50mA. From the perspective of reducing the power loss of DC current signals in the transmission line, reducing the volume of the instrument, and improving the explosion-proof performance of the instrument, it is better to have a smaller upper limit value of the current signal. However, it is not good to choose a signal upper limit value that is too small, because weak signals are easily interfered with, and the receiving instrument is required to have a higher sensitivity, which brings design difficulties to the input and amplification circuit of the instrument. When it is determined to use a live zero signal, the ratio of the upper limit value to the lower limit value is preferably 5:1, so that it has the same ratio as the upper and lower limits of the pneumatic analog signal. In this way, the DC current signal and the air pressure signal have a one-to-one correspondence and are easy to convert to each other. Therefore, the 4-20mA and auxiliary communication signals 1-5V and 20-100kPa stipulated in our country have the same upper and lower limit ratios. These are the origins of the determination of the upper and lower limit range of the 4-20mA signal.