The so-called “motion control” refers to the process of using the servo system to control the physical quantities such as the position and speed of mechanical transmission. For example, control the conveyor belt and cutting tools of the machine tool to complete accurate workpiece cutting. The motion control system mainly includes: motion controller, servo driver, servo motor, encoder and other components. The motion controller is a CPU of PLC with motion control function or a special motion control module; The servo driver is used to receive the command of the motion controller and complete the motion control of the servo motor; Servo motor is an actuator, which is used to drive the process shaft to move; The built-in encoder of the servo motor can feed back the position of the motor to the servo driver or motion controller to form a closed-loop control.
Siemens SIMATIC s7-1200 series PLC integrates motion control function and can control servo driver in a variety of ways.
1. There are three methods of motion control:
1) Pulse stepping motor;
2) Upper computer + PLC + encoder + speed regulating motor;
3) PLC + position switch + ordinary (speed regulating) motor; 2. What motion and what motion control method are used:
1) For example, embroidery technology is suitable for: pulse stepping motion control mode;
2) For example, the worktable of universal milling machine is used for front and back, up and down, left and right and rotation: PLC + position switch + ordinary (speed regulating) motor
3) For example, the manipulator is applicable to: PLC + position switch + ordinary (speed regulating) motor, upper computer + PLC + encoder + speed regulating motor; 3. If PLC + position switch + ordinary (speed regulating) motor is enough for your system, you can use “PLC + position switch + ordinary (speed regulating) motor”. This system has the advantages of rapid conversion, agile action, convenient and simple use, operation and maintenance, stable and reliable work;
Let’s talk about the motion control methods of s7-1200.
For firmware version greater than or equal to v4 The s7-1200 CPU of 1 has three methods to control the servo driver:
（1） Profidrive: profidrive is a driving technology standard based on PROFIBUS or PROFINET bus, which is included in the international standard iec61800-7. Profidrive defines a motion control model, which contains a variety of devices. Data exchange is carried out between devices through preset interfaces and messages, which are called profidrive message frames. Each message frame has a standard structure, and different message frames can be selected according to specific applications. Through the profidrive message frame, the control word, status word, set value and actual value can be transmitted. The motion control diagram of s7-1200 based on profidrive is as follows:
In the figure above, the signal of the built-in encoder of the servo motor can have four feedback modes:
Feedback to servo driver;
Directly feed back to CPU through bus;
Connected to the distributed process module, and the CPU reads the data through the bus;
High speed counter channel directly connected to CPU;
No matter which method is used, profidrive can form closed-loop control;
（2） PTO mode: the full name of PTO is “pulse train output”, that is, “pulse train output”. The motion controller controls the speed of the servo motor by sending a pulse train with a duty cycle of 50% to the servo driver. It supports four pulse trains (pulse E1 ~ pulse E1, for example), and each supports four pulse trains:
PTO (pulse a and direction B);
PTO (count up a and count down b);
PTO (A / B phase shifted);
PTO (A / B phase shifted fourfold);
As shown below:
Among them, PTO (pulse a and direction B) is a commonly used method. This PTO mode uses two outputs of CPU: one (a) generates high-speed pulse train and the other (b) controls the movement direction of servo motor. By controlling the frequency generated by the pulse train, the speed of the motor can be controlled.
The motor can be connected to the CPU to form a closed-loop counting system, and the motor can also be connected to the open-loop counting system. The schematic diagram of s7-1200 PTO based motion control is as follows:
（3） Analog mode: this mode takes the analog signal as the given signal of the servo driver, and controls the speed of the servo motor through the change of the analog signal. Taking SINAMICS V90 as an example, it can accept the speed given signal of ± 10V. We can use the signal board “AQ 1×12 bit” of s7-1200 to output ± 10V voltage signal. By connecting the signal board output with the signal setting of V90, we can use analog quantity to control the motion.
The analog motion control mode must also form a closed-loop system, and the signal of the encoder can be fed back to the CPU by means of high-speed counter or bus. The schematic diagram of s7-1200 motion control based on analog quantity is as follows:
Well, the motion control mode of s7-1200 is introduced here first.