Analyze control requirements
Determine the type of control task
If it is a simple logic control and sequential control, such as the selection of goods, payment change and shipping process of a vending machine, or the start-stop sequence control of equipment in a factory production line, then PLC control is mainly considered. PLC can easily implement logic such as “execute action A first, then execute action B, and execute action D when condition C is met”.
If it is a task that requires high-precision position, speed or torque control, such as tool positioning and motion control of CNC machine tools, and joint motion control of industrial robots, it is necessary to focus on the servo system.
Clarify the control accuracy requirements
For applications with low control accuracy requirements, such as liquid level control with errors at the centimeter level and opening and closing control of warehouse doors, general PLC control combined with ordinary motors can meet the requirements.
For occasions with micron-level or even higher precision requirements, such as wafer handling and processing equipment in semiconductor manufacturing equipment, optical lens grinding equipment, etc., high-precision servo systems must be selected, and factors such as the encoder accuracy of the servo motor and the control algorithm of the driver must be considered.
Consider the response speed requirements
In some applications that do not require high response speed, such as timed irrigation systems (the response time can be within a few seconds or even minutes), the response speed of the PLC is usually sufficient.
For high-speed packaging machinery (which may need to complete multiple packaging actions per second), high-speed punching machines and other equipment that require fast response, it is necessary to select a servo system with fast response speed, and consider factors such as the bandwidth of the servo drive and the electromechanical time constant of the motor.
Key points for PLC selection
Number of I/O points
Determine the number of I/O points of the PLC according to the number of input and output signals of the control object. For example, a small material sorting system has several sensors for detecting the position, color, etc. of the material (input signals), and several motors and cylinders for sorting actions (output signals). It is necessary to count the number of these signals and then select a PLC with slightly more I/O points than the actual demand to facilitate system expansion.
Program storage capacity
For complex control logic and a large number of data processing tasks, a larger program storage capacity is required. If it is an automated production line with multiple product production modes, the PLC needs to store the production process programs of different products, which requires the PLC to have sufficient storage space to store these programs and related data.
Scan cycle
In situations where there are certain requirements for response speed, the PLC scan cycle should be considered. For fast process control, such as high-speed counting or high-frequency pulse output applications, a PLC with a short scan cycle should be selected. You can understand the range of its scan cycle by looking at the PLC’s technical specifications.
Communication function
If the PLC needs to communicate with other devices (such as host computers, touch screens, other PLCs or intelligent instruments), a PLC with corresponding communication interfaces and protocol support should be selected. For example, in a distributed control system, the PLC needs to transmit data with the host computer in the central control room through industrial Ethernet or fieldbus, and the PLC needs to support the corresponding communication protocol.
Servo system selection points
Motor type and power
Select the type of servo motor (such as permanent magnet synchronous servo motor, AC asynchronous servo motor, etc.) and power according to the nature of the load (such as inertia, friction, load torque, etc.) and motion requirements. For example, for the crossbeam movement of a gantry machining center with a large inertia load, a servo motor with high power and good torque characteristics needs to be selected.
Encoder accuracy
For high-precision position control applications, the accuracy of the servo motor encoder should be considered. For example, in a high-precision electronic device placement machine, the motor needs to be able to accurately locate the position of the placement head, so a servo motor with a high-resolution encoder should be selected, such as an absolute encoder with millions of pulses per revolution.
Driver performance
The control algorithm of the driver (such as advanced adaptive PID control, vector control, etc.) will affect the performance of the servo system. In applications that require good dynamic performance, such as high-speed motion control of robots, a servo driver with a high-performance control algorithm needs to be selected to ensure that the motor can respond to control instructions quickly and accurately.
Compatibility with PLC
Because servo systems usually need to be used in conjunction with PLCs, the compatibility between them should be considered. This includes whether the communication interface matches (such as whether they both support communication protocols such as CANopen and EtherCAT), whether the signal levels are consistent, etc. This ensures that the PLC can easily send control instructions to the servo system and receive feedback information.
