PLCs were developed based on logic control and sequential control. As a replacement for heavy relay control cabinets and AC contactor control cabinets, they have effectively realized the automation and intelligence of industrial control.
Sensors are essentially front-end data acquisition modules that convert the operating status of field equipment, such as pressure and liquid level, into standard PLC-controllable signals to provide input control for the PLC.
I. Sensor signal types:
1. Analog signals: voltage or current signals! Used to provide analog input signals to devices such as PLCs.
For example, pressure transmission signals, liquid level transmission signals, temperature transmission signals, differential pressure transmission signals, valve opening feedback signals, etc., are all analog signals.
Additional information: Standard voltage signals are typically 1V-5V, and standard current signals are typically 4mA-20mA. Of course, there are other non-standard voltage and current signals as well.
2. Digital signals
Digital signal sensor types include: valve open/close signals, proximity switch on/off signals, infrared sensing signals, etc.
Digital signals typically have two states, 0 and 1, which represent on or off, and are used to provide on/off signals to devices such as PLCs.
II. Corresponding PLCs also have different input/output ports:
1. Analog input port: Accepts analog signals from sensors and other devices.
2. Digital input port: Accepts digital signals from sensors and other devices.
3. Analog output port: Used to control analog outputs, such as the opening degree of a valve.
4. Digital output port: used to control the on/off state of the circuit, etc.
III. In industrial automation control: sensors are front-end signal measurement devices, PLCs centrally process control signals, and there are also output actuators, etc.
1. The analog sensor is connected to the analog input port of the PLC, and the corresponding output port can be either an analog signal or a digital control signal.
2. The digital sensor is connected to the digital input port of the PLC, and the corresponding output port is generally a digital signal.
IV. Precautions for connecting sensors and PLC:
1. The sensor signal type must be the same as the PLC input port type.
2. There are many types of PLCs. Generally, PLCs have digital input ports that can be directly connected to digital signals.
3. If the PLC does not have analog input ports, an analog expansion module of the same model is required. Generally, a PLC comes with 1-2 sets of analog input ports.
4. When using the sensor, pay attention to the power supply and signal; do not reverse them, otherwise the PLC may be damaged.
5. There are many types of sensors, including two-wire, three-wire, four-wire, etc. You must distinguish them when wiring. If necessary, you may need to install a signal isolator.
6. When using a PLC, pay attention to the power input/output terminals and signal input/output terminals, and use it safely.
