Motion control refers to the process of controlling physical quantities such as position and velocity of mechanical transmissions using servo systems. For instance, controlling the conveyor belts and tools of a machine tool to achieve accurate workpiece cutting. A motion control system mainly consists of motion controllers, servo drives, servo motors, and encoders. The motion controller is either the CPU of a PLC with motion control functionality or a dedicated motion control module. The servo drive receives commands from the motion controller and controls the motion of the servo motor. The servo motor acts as the actuator to drive the process axis in motion. The servo motor has a built-in encoder that feeds back the motor's position to the servo drive or motion controller, enabling closed-loop control.
The Siemens SIMATIC S7-1200 series PLC integrates motion control functionality and can control servo drives in various ways. In this article, we will discuss the motion control methods available for the S7-1200.
For S7-1200 CPUs with firmware versions greater than or equal to V4.1, there are three methods to control servo drives:
（1）PROFIdrive: PROFIdrive is a drive technology standard based on the PROFIBUS or PROFINET bus, included in the international standard IEC 61800-7. PROFIdrive defines a motion control model that includes various devices. Data exchange between devices is conducted through predefined interfaces and messages, referred to as PROFIdrive message frames. Each message frame has a standard structure, and different message frames can be selected according to specific applications. Control words, status words, setpoints, and actual values can be transmitted via PROFIdrive message frames. The motion control schematic based on PROFIdrive for the S7-1200 is as follows:
In the above diagram, there are four ways in which the signals from the servo motor's built-in encoder can be fed back:
- Feedback to the servo drive.
- Direct feedback to the CPU via the bus.
- Connection to distributed process modules, with the CPU reading data via the bus.
- Direct connection to the high-speed counter channels of the CPU.
Regardless of the method used, PROFIdrive can form closed-loop control.
(2) PTO Method: PTO stands for "Pulse Train Output." It controls the speed of the servo motor by sending a pulse train with a duty cycle of 50% to the servo drive. Taking CPU1215C as an example, it supports a total of four pulse train outputs (Pulse1~Pulse4), with each pulse signal supporting four PTO modes:
- 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)
See the diagram below:
Among these, PTO (Pulse A and direction B) is a commonly used method. This PTO method utilizes two outputs of the CPU: one (A) generates a high-speed pulse train, while the other (B) controls the direction of motion of the servo motor. By controlling the frequency of the pulse train generated, the motor's speed can be controlled.
PTO mode can operate in open-loop control, or the motor's encoder can be connected to the high-speed counter of the CPU for position counting, thereby forming a closed-loop system. The schematic diagram of motion control based on PTO for S7-1200 is as follows:
(3) Analog Mode: In this method, an analog signal serves as the setpoint signal for the servo drive. The speed of the servo motor is controlled by the changes in the analog signal. Taking SINAMICS V90 as an example, it can accept a ±10V speed setpoint signal. We can use the S7-1200 signal board "AQ 1x12 BIT" to output a ±10V voltage signal. By connecting the output of the signal board to the speed setpoint input of the V90, motion control can be achieved using analog signals.
Analog motion control also requires the formation of a closed-loop system. The signal from the encoder can be fed back to the CPU using either the high-speed counter or the bus. The schematic diagram of motion control based on analog signals for S7-1200 is as follows:
Alright, that concludes the introduction to the motion control methods for S7-1200. In future articles, we will delve into the specifics of each control method.