1、 Main concepts

What is a transmitter? So first, what is a sensor? Sensors are a general term for devices or equipment that can be measured according to regulations and converted into usable output signals according to certain rules. They are usually composed of sensitive elements and conversion elements.

When the output of the sensor is a specified standard signal, it is called a transmitter. Transmitter: In addition to sensing function, it also has amplification and shaping function, and outputs standard control signals. For example: 4-20mA

2、 Main features (taking two-wire system as an example)

Two wire system refers to the use of only two wires for on-site transmitters and control room instruments, which serve as both power and signal lines. Compared with the two-wire system (one positive power line, two signal lines, one of which shares GND) and the four wire system (two positive and negative power lines, two signal lines, one of which shares GND), the advantages of the two-wire system are:

1. Not easily affected by parasitic thermocouples and resistance drop and temperature drift along the wire, can be used with very cheap and thinner wires; Can save a lot of cable and installation costs;

2. When the output resistance of the current source is large enough, the voltage induced in the wire loop through magnetic field coupling will not have a significant impact because the current caused by the interference source is extremely small. Generally, twisted pair cables can be used to reduce interference; Three wire and four wire systems must use shielded wires, and the shielding layer of the shielded wire must be properly grounded.

3. Capacitive interference can cause errors related to receiver resistance. For a 4-20mA two-wire loop, the receiver resistance is usually 250 Ω (sampling Uout=1-5V), which is small enough to produce significant errors. Therefore, the allowable wire length is longer and farther than that of a voltage telemetry system;

4. Each individual reading device or recording device can be switched between different channels with varying wire lengths, without causing differences in accuracy due to differences in wire lengths, achieving decentralized collection. The benefits of decentralized collection are: decentralized collection and centralized control ...

5. Using 4mA for zero level makes it very convenient to determine open and short circuits or sensor damage (0mA state).

6. It is very easy to add one or two lightning and surge protection devices at the two-wire output port, which is beneficial for safe lightning and explosion prevention.

Both three wire and four wire transmitters do not have the above advantages and are about to be replaced by two-wire transmitters. This can be inferred from the industry trends abroad and the supply and demand of transmitter chips. Current transmitters need to be installed on the power line of on-site equipment during use, while monitoring systems with microcontrollers as the core are located in monitoring rooms far away from the equipment site, usually tens to hundreds of meters or even farther apart. The on-site environment of the equipment is relatively harsh, and strong electrical signals can generate various electromagnetic interferences. Lightning induction can produce strong surge pulses. In this situation, a difficult problem encountered in microcontroller application systems is how to reliably transmit small signals over long distances in harsh environments.

The emergence of two-wire transmission devices has effectively solved this problem. We have designed a small and inexpensive through-hole two-wire current transmitter with DH4-20 transmission module as the core. It has the characteristics of low offset voltage (<30 μ V), low voltage drift (<0.7 μ V/C °), and ultra-low nonlinearity (<0.01%). It isolates and converts the current of the power line of the on-site equipment into a standard current signal that varies linearly from 4 to 20 mA, and then sends it to the input interface of the monitoring system through a pair of twisted pair cables. The twisted pair cables also send the 24V working power supply located in the monitoring system to the current converter. The measurement signal and power are transmitted simultaneously on twisted pair cables, eliminating the need for expensive transmission cables. Additionally, the signal is transmitted in the form of current, greatly enhancing the anti-interference ability

3、 Classification and Comparison

A device that converts physical measurement signals or ordinary electrical signals into standard electrical signals or outputs them in a communication protocol. Generally divided into: temperature/humidity transmitter, pressure transmitter, differential pressure transmitter, liquid level transmitter, current transmitter, electricity transmitter, flow transmitter, weight transmitter, etc.

4、 Example analysis

Using a current transmitter with a measurement range of 0-100A as an example for description. For transmitters with an output of 0-20mA, the 0mA current corresponds to the input 0A value, while for transmitters with an output of 4-20mA, the 4mA current corresponds to the input 0A value. Both types of sensors have a 20mA current corresponding to the 100A value.

For transmitters with an output of 0-20mA, in terms of circuit design, we only need to choose a suitable step-down resistor, and directly convert the 0-V or 0-10V voltage on the resistor into a digital signal at the input interface of the A/D converter. Circuit debugging and data processing are relatively simple. But the disadvantage is that it cannot distinguish the damage of the transmitter, and cannot distinguish between open and short circuits in the transmitter output.

For transmitters with an output of 4-20mA, circuit debugging and data processing are quite cumbersome. But this type of transmitter can determine whether the circuit has malfunctioned and whether the transmitter is damaged by detecting the normal range of current (even the smallest value is 4mA under normal circumstances) when the transmitter circuit is not connected, short circuited, or damaged. Therefore, it has been widely used.

Because when the 4-20mA transmitter outputs 4mA, the voltage on the sampling resistor is not zero, and the digital quantity directly converted by the analog-to-digital conversion circuit is also not zero. The microcontroller cannot directly use it, and calculating it through formulas is too complex. Therefore, the general processing method is to eliminate the voltage drop generated by 4mA on the sampling resistor through hardware circuits, and then perform A/D conversion. This type of hardware circuit, RCV420, is a precision I/V conversion circuit, as well as an I/V conversion circuit built on LM258. This circuit forms a current loop by generating 4-20mA current from a two-wire current transmitter, 24V, and a sampling resistor, resulting in a 1-5V voltage drop across the sampling resistor, which is input to the third pin of amplifier LM258. The resistor divider circuit is used to generate a fixed voltage value at pin 2 of the integrated circuit LM258, which is used to offset the voltage drop caused by a 4mA current on the sampling resistor. So when the maximum value of the two-wire current transmitter is 4mA, the voltage difference between pin 3 and pin 2 of LM258 is basically 0V. LM258 and the resistor connected to it form an adjustable voltage amplification circuit, which amplifies the voltage value of the two-wire current transmitter current on the sampling resistor and outputs it to the analog/digital conversion circuit through pin 1 of LM258 for the microcontroller CPU to read in. Through data processing methods, the 4-20mA current of the two-wire current transmitter is displayed on the LCD/LED screen in the form of 0-100A values.