Wireless analog transmission system

In industrial control, it is often necessary to convert various non-electrical physical quantities such as temperature, pressure, speed, and angle into analog electric signals for transmission.Among them, the 4-20mA current signal is a widely used analog signal. This transmission mode has simple communication mode, strong anti-interference ability, and relatively reliable fault diagnosis function.The reason for using the current signal is that it is not easy to be interfered, and the internal resistance of the current source is infinite, and the resistance of the wire in series in the loop does not affect the accuracy, and it can transmit hundreds of meters on the ordinary twisted pair.

The upper limit is 20mA because of explosion-proof requirements: the spark energy caused by the on-off of the 20mA current is not enough to ignite the gas.

The lower limit of 4mA is to avoid interference and provide the static working current of the circuit for the sensor.

In addition, because when the transmission line is broken due to a fault, the loop current drops to 0, so 0mA is used for disconnection detection, and 2mA is usually taken as the disconnection alarm value.

The instrumentation interface that uses 4-20mA current signal is a two-wire system. The two wires are power lines and need to provide 24vDC power supply to power the instruments; they are also signal lines that output 4-20mA current signals to the instruments.

However, the 4-20mA current signal needs to be transmitted through wires. In practical applications, sometimes the problem of wiring inconvenience is encountered.

If the wired signal is replaced with a wireless signal, since the wireless signal interface is mostly a digital interface, it is not compatible with the original two-wire interface of 4-20mA current signal.

Advantages of wireless analog transmission system:

This system can replace the wired transmission of the 4-20mA current signal with the wireless transmission. The interface also uses the two-wire interface of the original instrument and meter, and the cost of modification is low and the use is convenient.

The solution provided by NiceRF:

This system is divided into transmitting end and receiving end. The transmitting terminal is the 4-20mA signal input terminal, and the receiving terminal is the 4-20mA signal output terminal. The working process is that the transmitter converts the input two-wire 4-20mA signal into a digital signal through the conversion chip, and sends it out wirelessly. After receiving the signal, the receiving end restores the digital signal to an analog signal and outputs it through the two-wire system.

Steps for transmitting:

Step 1: Connect the input 4-20mA current signal to the ADC conversion circuit inside the transmitter to convert the current signal into a voltage signal.

Step 2: The ADC conversion circuit connects the voltage signal to the AD detection pin of the microcontroller, and converts the voltage signal into data.

Step 3: The microcontroller transmits the data to the LoRa wireless module and controls the LoRa wireless module to transmit the data.

Step 4: Repeat steps 1~3.

Transmitter block diagram:

Steps for receiving:

Step 1: The receiving end always detects the wireless signal.

Step 2: Repeat step 1 when no wireless signal is detected.

Step 3: When the signal is received, skip to step 4.

Step 4: The receiving end restores the wireless data to a voltage signal and outputs it to the signal conversion circuit.

Step 5: The signal conversion circuit converts the voltage signal into a 4-20mA current signal.

Step 6: The receiver skips to step 1.

Receiver block diagram: