Control principle
PLC control:
PLC (Programmable Logic Controller) is an electronic system for digital computing operations. It uses programmable memory to store programs internally, executes user-oriented instructions such as logic operations, sequential control, timing, counting and arithmetic operations, and controls various types of machinery or production processes through digital or analog input/output. It is mainly based on logic control, such as controlling output signals (such as controlling motor start, stop, valve opening and closing, etc.) according to pre-written programs by making logical judgments on input signals (such as button presses, sensor signals, etc.).
For example, in an automated packaging production line, PLC can control the start and stop of the conveyor belt motor according to the product position signal detected by the photoelectric sensor to ensure that the product enters the packaging station accurately.
Servo control:
Servo control is mainly used to accurately control the position, speed and torque of the motor. It is a closed-loop control system that includes a servo drive, a servo motor and a feedback device (such as an encoder). The servo drive drives the servo motor according to the given command signal (position, speed or torque command), and the feedback device detects the actual operating status of the motor (such as actual position, actual speed) in real time and feeds it back to the drive.
The drive continuously adjusts the output of the motor by comparing the difference between the given command and the actual feedback signal (called error), so that the motor can track the command signal with high precision. For example, on CNC machine tools, servo motors can accurately control the position and movement speed of the tool through servo control to produce high-precision parts.
Control accuracy
PLC control:
The accuracy of PLC control mainly depends on the controlled equipment and application scenarios. For some simple switch quantity control (such as controlling the start and stop of the motor, the on and off of the light, etc.), the accuracy requirements are relatively low. When performing analog quantity control, its accuracy is generally within a certain range. For example, in temperature control, the accuracy may be around ±1℃, which is acceptable for some process controls that do not require extremely high accuracy.
Servo control:
Servo control has very high accuracy. In terms of position control, its positioning accuracy can reach a very high level. For example, the positioning accuracy of some high-precision servo systems can reach ±0.01mm or even higher. In terms of speed control, the speed accuracy is also very high, and the speed of the motor can be accurately controlled. For example, in situations where constant speed operation is required, the speed fluctuation can be controlled within a very small range.
Application scenarios
PLC control:
It is widely used in various fields of industrial automation, such as sequential control, logic control, and process control of automated production lines. In the automation systems of chemical, metallurgical, and construction industries, PLC is used to control the operating sequence and logical relationship of various equipment. For example, in a sewage treatment plant, PLC controls various process flows of sewage treatment, including the sequence and time control of sewage inlet, sedimentation, aeration, and drainage.
Servo control:
It is mainly used in situations where high-precision position, speed, or torque control is required. In the field of robotics, servo control is used to control the movement of robot joints, so that the robot can accurately complete various complex actions, such as welding and assembly. In printing equipment, servo control is used to accurately control the delivery of paper and the rotation of printing cylinders to ensure printing quality.
Programming method
PLC control:
Programming languages such as ladder diagram, instruction table, function block diagram, etc. are usually used. Ladder diagram is a programming language similar to electrical schematic diagram, which is intuitive and easy to understand and suitable for electrical engineers and technicians. The control logic is realized by connecting various logic elements (such as normally open contacts, normally closed contacts, coils, etc.) in the ladder diagram.
Servo control:
Programming mainly involves parameter setting of servo drive and programming of motion instructions. When setting the parameters of servo drive, it is necessary to configure parameters such as motor model, control mode (position, speed or torque mode), speed loop and position loop gain. Motion instruction programming is based on specific application requirements, through the upper computer software or the driver’s own programming interface, to write the program of position, speed or torque instructions, such as writing G code for servo control in CNC equipment.
