The difference between PLC and various sensor connection methods

In industrial sites, various analog sensors such as pressure, displacement, temperature, flow, and speed are designed with different technical methods. The working power distribution methods of sensors are mainly divided into two-wire and four-wire systems, and the analog signals they output are also different. The common ones are 0-20mA, 4-20mA current signals and 0-75mV, 0-5V, 1-5V voltage signals.

To successfully collect various sensor analog signals into PLC/DCS/FCS/MCU/FA/PC systems, it is necessary to match and select sensors based on the functions and technical characteristics of the data acquisition system. At the same time, the power supply differences between industrial field sensors and PLC and other data acquisition systems and the influence of various EMC interferences must also be considered. Usually, the analog signals output by the sensors are isolated, amplified, converted, and then sent to PLC and other data acquisition systems.

The PLC collects the analog or digital signals of the sensor through the signal line and then processes them. If the sensor is an analog output, the PLC must be connected to the analog input interface. If the sensor is a digital signal output, the PLC must be connected to the digital input interface.

A switch sensor is a contactless switch. It can be used as a switch input signal for PLC. It is generally used for switch-controlled equipment, machine tools, machines, etc. Analog sensors convert different physical quantities (such as pressure, flow, temperature) into analog quantities (4-20MA current or 1-5V voltage). Analog sensors are used as input signals for the analog input module of PLC and are generally used for process control. Digital sensors refer to sensors that add or modify A/D conversion modules to traditional analog sensors to make their output signals digital (or digitally encoded). They mainly include: amplifiers, A/D converters, microprocessors (CPUs), memory, communication interface circuits, etc.

Commonly used analog sensors are divided into two-wire and four-wire systems. Both the two-wire and four-wire systems have only two signal wires. The main difference between them is that the two signal wires of the two-wire system not only supply power to the sensor or transmitter, but also provide current and voltage signals; while the two signal wires of the four-wire system only provide current signals. Usually, sensors or transmitters that provide two-wire current and voltage signals are passive, while sensors or transmitters that provide four-wire current signals are active.

Therefore, when the template input channel of a data acquisition system such as a PLC is set to connect a four-wire sensor, the PLC only collects analog signals from the terminals of the template channel. When the template input channel of a data acquisition system such as a PLC is set to connect a two-wire sensor, the channel of the PLC's analog input template must also output a DC 24V power supply to drive the two-wire sensor to work.

4-20mA is related to electrical standards. The 4-20mA signal system is the analog signal standard for process control systems of the International Electrotechnical Commission (IEC). my country began to adopt this international standard signal system from the DDZ-Ⅲ electric instrument. The instrument transmission signal uses 4-20mA, and the communication signal uses 1-5VDC, that is, a signal system that uses current transmission and voltage reception. Because the signal starting current is 4mA, it provides a static working current for the transmitter. At the same time, the instrument electrical zero point is 4mA, which does not coincide with the mechanical zero point. This active zero point is conducive to identifying faults such as power failure and disconnection.

1. Search based on the performance of analog signal sensors

1. Two-wire current/voltage output sensor (no power supply, 16-24V power distribution provided by the load, output 4-20mA/0-5V).

1.1. Two-wire passive 4-20mA input sensor is connected to PLC after being powered by current isolation distributor. As shown in Figure 1, the positive terminal of the weighing and distance sensor is connected to 16-24VDC, and the negative terminal outputs 4-20mA current.

Figure 1 Typical application diagram of two-wire 4-20mA isolated distributor

1.2. Two-wire passive voltage signal input type sensor is connected to PLC after being distributed by voltage distributor. As shown in Figure 2 and Figure 3, the positive and negative terminals of displacement and electronic ruler sensors are connected to 16-24VDC voltage.

Figure 2 Non-isolated two-wire passive voltage distributor

Figure 3 Isolated voltage distributor

As shown in Figures 4 and 5, the positive and negative terminals of the stress gauge and weighing bridge sensor are connected to a 16-24VDC voltage.

Figure 4 Isolated voltage signal conversion and amplification

Figure 5 Isolated bridge voltage distributor

1.3. The two-wire sensor outputs a 4-20mA current loop that is isolated by an isolator and connected to the PLC. As shown in Figure 6, the positive end of the pressure and flow sensor is connected to 9-32VDC, and the negative end outputs 4-20mA current.

Figure 6 Typical application diagram of two-wire 4-20mA current loop isolator

1.4. The two-wire sensor outputs 4-20mA current and is connected to the PLC after isolation conditioning and matching (to resolve the conflict between input and output). As shown in Figure 7, the positive end of the temperature and speed sensor is connected to 12-24VDC, and the negative end outputs 4-20mA current.

Figure 7 Typical application diagram of two-wire 4-20mA signal and PLC matching isolation conditioning

2. Four-wire current/voltage (input/output) type sensor (with its own power supply 24VDC, input/output: 4-20mA or 0-5V).

2.1. The four-wire current output sensor is connected to the PLC after analog signal isolation and amplification. As shown in Figure 8, the positive terminal of the temperature and humidity sensor is connected to 24VDC, and the negative terminal outputs 4-20mA current.

Figure 8 Typical application diagram of connecting the four-wire sensor signal I/I isolation amplification and PLC

As shown in Figure 9, the positive terminal of the pressure and speed sensor is connected to 24VDC, and the negative terminal outputs 4-20mA current.

Figure 9 Typical application diagram of connecting four-wire sensor signal to PLC after I/V conversion

2.2, the four-wire voltage output sensor is connected to the PLC after analog signal isolation and amplification. As shown in Figure 10, the positive and negative terminals of the pressure and speed sensors are connected to a 24V power supply, and the output terminal outputs a 0-5V voltage.

Figure 10 Typical application diagram of connecting four-wire sensor signal to PLC after V/I conversion

As shown in Figure 11, the positive and negative terminals of the liquid level and flow sensors are connected to a 24V power supply, and the output terminals output a 0-5V voltage.

Figure 11 Typical application diagram of connecting four-wire sensor signal with PLC after V/V isolation

2. Connection matching and selection between PLC and sensor

Select PLC (for example, 2# connector is positive and 3# connector is negative)

1. In two-wire system, positive pole 2 outputs 24VDC voltage, and 3 receives current. So when encountering a two-wire sensor, one connection method is to connect 2 to the positive of the sensor and 3 to the negative of the sensor; the jumper is a two-wire current signal. The other connection method is to leave 2 suspended and connect 3 to the negative of the sensor, and at the same time, the positive of the sensor should be connected to the 24vdc in the cabinet; the jumper is a two-wire current signal. (Take 2 positive and 3 negative as an example)

2. In the four-wire system, the positive pole 2 is the receiving current, and 3 is the negative pole. (The advantage of the four-wire system is that when the negative pole signal of the sensor is at a different level from M in the cabinet, it will not affect the accuracy greatly, because it is the current loop of the sensor itself.) When encountering a four-wire sensor, one method is to connect 2 to the positive pole of the sensor, 3 to the negative pole of the sensor, and the PLC jumper is a four-wire current. (Take 2 positive and 3 negative as an example)

3. Four-wire sensor and PLC two-wire jumper connection method: connect the negative signal line to the M line in the cabinet. Connect the positive sensor to the 3 of the PLC, and leave 2 hanging. The jumper is a two-wire current. (Take 2 positive and 3 negative as an example)

4. Voltage signal: 2 connects to the positive of the sensor, 3 connects to the negative of the sensor, and the PLC jumper is a voltage signal. (Take 2 positive and 3 negative as an example)