In the world of PLCs (Programmable Logic Controllers), the "three fundamentals"—discrete signals, analog signals, and pulse signals—are the cornerstones of building various automated control systems. Understanding and mastering the characteristics and applications of these three signal types are crucial for deeply understanding PLC technology.
I. Discrete Signals
- Definition and Characteristics of Discrete Signals
Discrete signals refer to signals with only two states, "on" and "off," typically represented by 0 and 1. Their characteristics include clear states, ease of processing, and control. - Discrete Signal Input and Output
Common inputs for discrete signals include pushbuttons, proximity switches, and limit switches; outputs include relays, contactors, and indicator lights. - Practical Applications of Discrete Signals
- Motor Start/Stop Control: Using a button as an input, when the PLC receives a start signal (discrete), it closes the control relay, powering the motor; conversely, a stop signal opens the relay, stopping the motor.
- Lighting Switch Control: Utilizing switch input signals to control the lighting on/off, simple and direct.
II. Analog Signals
- Concept and Representation of Analog Signals
Analog signals are quantities that change continuously, such as temperature, pressure, and speed. In PLCs, they are usually represented by a range of digital values, for example, 0-4095 corresponding to a 0-10V voltage range. - Analog Signal Input and Output
Specialized analog input modules are required to convert external analog signals into digital signals for PLC processing. Analog output modules convert computed digital values into corresponding analog outputs. - Analog Signal Processing and Conversion
Inside the PLC, operations like scaling and filtering are needed to obtain accurate analog values, and to convert control outputs into corresponding analog signals. - Examples of Analog Signal Applications
- Temperature Control System: The analog input from temperature sensors is processed by the PLC, which then outputs control signals to adjust heating or cooling equipment, maintaining the temperature within a set range.
- Pressure Regulation: Analog signals from pressure transducers are input to the PLC, compared against set points, and control valve openings to stabilize pressure.
III. Pulse Signals
- Definition and Generation of Pulse Signals
Pulse signals are characterized by a rapid change over a short period, returning quickly to their initial value. They can be generated by encoders, pulse generators, etc. - Measurement and Counting of Pulse Signals
PLCs use high-speed counters to count pulses, thereby obtaining information on position, speed, etc. - Application Scenarios for Pulse Signals
- Speed Measurement with Encoders: As an encoder rotates with a shaft, it generates pulses; the PLC calculates the shaft speed based on the pulse count per unit time.
- Stepper Motor Control: The PLC outputs pulses at specific frequencies and quantities to control the rotation angle and speed of the stepper motor.
IV. Integrated Use of the Three Signal Types
- Coordinated Work in Complex Control Systems
In a real automation system, handling discrete, analog, and pulse signals simultaneously is often necessary for precise and efficient control. - Case Analysis: Comprehensive Control in Automated Production Lines
For instance, on a production line, discrete signals control the start/stop of equipment, analog signals monitor parameters like temperature and pressure during production, and pulse signals control the speed and position of conveyor belts precisely.
V. Conclusion
Mastering the "three signals" of PLCs is key to achieving automated control. Through deep understanding and flexible application of discrete, analog, and pulse signals, one can design control systems that meet various complex requirements, driving innovation and development in industrial automation.