In industrial environments, sensors for pressure, displacement, temperature, flow, and rotational speed use different design techniques. Sensor power and signal wiring are mainly two-wire or four-wire. Common analog outputs include 0-20mA, 4-20mA current, and voltage signals such as 0-75mV, 0-5V, and 1-5V.
Signal Types, Powering, and Data Acquisition
To acquire sensor signals into PLCs, DCS, FCS, MCUs, FA controllers, or PCs, the sensor and data acquisition system must be matched based on functional and technical characteristics. Also consider differences in supply arrangements and EMC interference at the site. Sensor outputs are usually isolated, amplified, and converted before being sent to the PLC or other acquisition system.
PLCs read analog or digital signals via signal wiring and then process them. If a sensor outputs an analog signal, the PLC must use an analog input module; if the sensor outputs a digital signal, the PLC must use a digital input module.
Discrete (switch) sensors act like contactless switches and provide PLC discrete inputs, typically used for on/off control of devices and machinery. Analog sensors convert physical quantities (pressure, flow, temperature) into analog signals (4-20mA current or 1-5V voltage) and are used for process control via PLC analog input modules. Digital sensors refer to analog sensors equipped with A/D conversion modules so they output digital or encoded signals; these typically include amplifier, A/D converter, microprocessor (CPU), memory, and communication interface circuits.
Two-wire vs Four-wire Analog Sensors
Common analog sensors are two-wire or four-wire. Both have two signal wires, but differ in function: two-wire sensors use the two wires for both power and signal, while four-wire sensors have separate power and signal wiring. Typically, two-wire passive sensors rely on the load to provide 16–24V supply, while four-wire sensors are active and provide their own 24VDC supply.
When a PLC analog input channel is configured for a four-wire sensor, the PLC simply samples the analog signal at the input terminal. When the channel is configured for a two-wire sensor, the PLC analog input channel must also supply a 24V DC excitation to power the two-wire sensor.
The 4-20mA standard is defined by the International Electrotechnical Commission (IEC) for process-control analog signals. China began adopting this international standard starting with the DDZ-III electric instrument series: instrument transmission uses 4-20mA while signaling uses 1-5VDC. Using 4mA as the live zero provides loop power for the transmitter and separates the electrical zero from the mechanical zero, which helps detect open-circuit or power-failure faults.
1. Select Sensors by Analog Signal and Performance
1. Two-wire current/voltage output sensors (passive, load provides 16–24V supply; output 4-20mA or 0-5V)
1.1 Two-wire passive 4-20mA input sensors can be powered via a current-isolated distribution module before connecting to the PLC. For example, weight or distance sensors have their positive connected to 16–24VDC and their negative output a 4-20mA current.
1.2 Two-wire passive voltage-output sensors are supplied via a voltage distribution module before connecting to the PLC. For displacement and electronic scale sensors, the positive and negative terminals connect to 16–24VDC.
For strain gauges and load-cell bridge sensors, the positive and negative terminals are supplied with 16–24VDC.
1.3 Two-wire sensors that output a 4-20mA loop are connected to the PLC via a loop isolator. For example, pressure and flow sensors have their positive connected to 9–32VDC and the negative outputs 4-20mA.
1.4 Two-wire sensors outputting 4-20mA may be connected through isolated conditioning modules to resolve input/output conflicts before connecting to the PLC. For example, temperature and speed sensors have positive connected to 12–24VDC and negative output 4-20mA.
2. Four-wire current/voltage sensors (have own 24VDC supply; I/O: 4-20mA or 0-5V)
2.1 Four-wire current-output sensors are connected to the PLC via analog isolation and amplification. For example, temperature and humidity sensors have their positive connected to 24VDC and their negative output 4-20mA.
Pressure and speed sensors can be connected with I/V conversion before the PLC, with the sensor positive at 24VDC and negative output 4-20mA.
2.2 Four-wire voltage-output sensors are connected via isolation and amplification. For example, pressure and speed sensors powered by 24V may output 0-5V.
Similarly, liquid level and flow sensors powered by 24V can output 0-5V and be isolated before the PLC input.
II. PLC and Sensor Connection Matching and Wiring Examples
PLC wiring examples (example: terminal 2 is positive, terminal 3 is negative):
- Two-wire: Terminal 2 provides 24VDC, terminal 3 receives current. One wiring option is: terminal 2 to sensor positive, terminal 3 to sensor negative; set the PLC jumper to two-wire current. Another option is: leave terminal 2 open, connect terminal 3 to sensor negative, and connect the sensor positive to a cabinet 24VDC supply; set the PLC jumper to two-wire current.
- Four-wire: Terminal 2 receives current, terminal 3 is negative. One wiring option is: terminal 2 to sensor positive, terminal 3 to sensor negative, and set the PLC jumper for four-wire current. The advantage is that if the sensor negative and cabinet common (M) are at different potentials, accuracy is less affected because the loop current is local to the sensor.
- Connecting a four-wire sensor when the PLC is set for two-wire jumpers: connect the sensor negative to the cabinet M common. Connect the sensor positive to PLC terminal 3 and leave terminal 2 open; set the PLC jumper to two-wire current.
- Voltage signals: connect terminal 2 to sensor positive and terminal 3 to sensor negative; set the PLC jumper to voltage signal.
