The point control system is a position servo system.
The Point Control System is a comprehensive control system that uses a combination of electronic technology, computer technology, automatic control, and detection to ensure that the controlled terminal reaches its destination according to the specified trajectory and speed.
The system typically consists of several components, including a final mechanical actuator, a mechanical transmission mechanism, a power component, a controller, a position measuring device, etc. The mechanical actuator is the component that ultimately carries out the desired function, such as the arm of a welding robot or the work table of a CNC machine. In a broader sense, the actuator may also include components such as guide rails, which play a crucial role in maintaining positioning accuracy.
Point control systems have a wide range of applications in the field of mechatronics and the robotics industry. Currently, typical applications include the contour tracking of parts by CNC machine tools in the mechanical manufacturing industry, the fingertip trajectory control of industrial robots, and the path tracking of walking robots.
Significance
Point Control has a diverse range of applications in the fields of mechatronics and robotics. Currently, some of the typical applications include the tracking of part contours by CNC machine tools in the mechanical manufacturing industry, fingertip trajectory control of industrial robots, and path tracking of walking robots.
In recent years, with the advancement of science and technology, wafer processing equipment and the use of medical surgical robots have emerged as new applications of Point Control, which is now considered a key technology with both innovative significance and high practical value.
Point control system classification
The Point Control System is essentially a Position Servo System. While they share similar basic structure and composition, their focus differs. The control complexity of each system also has its own advantages.
In general, the Position Servo System places emphasis on stability, fast response with no overshoot, small following error, a wide speed range, high precision, and high dynamic characteristics. On the other hand, the Point Control System requires high positioning accuracy and short positioning time.
For Point Control Systems with trajectory requirements, it includes almost all the requirements of the Position Servo System.
The Point Control System can be classified based on the feedback method into:
- Closed Loop System
- Semi-Closed Loop System
- Open Loop System
Closed-loop control system of point control system
The Closed-Loop Control System places a position detector on the final output shaft (or platform) to obtain the most accurate position information of the terminal, and it achieves high-precision positioning through closed-loop feedback.
If the mechanical transmission system has a high stiffness or very low inertia, the natural frequency of the mechanical transmission system is much higher than the natural frequency of the motor system. In this case, the frequency characteristics of the positioning system are determined by the frequency characteristics of the speed loop, and the positioning system can be modeled as a second-order system.
The complexity of the design can be reduced by simplifying the system based on the characteristics and requirements of the positioning system. In theory, the closed-loop system is the most ideal control method, as it eliminates both the error of the electrical control system and various errors in the drive chain.
However, implementing this control idea can be challenging due to the presence of non-linear factors such as electrical and mechanical failures, transmission gaps, nonlinear friction characteristics, and variable transmission stiffness, which can cause the system to experience resonance, creep, and other unstable factors. Furthermore, changes in the load can also affect the friction characteristics, mechanical inertia, etc., making it difficult to set the system.
Semi-closed loop control system of point control system
The Semi-Closed Loop System is characterized by the fact that the mechanical transmission system is not part of the feedback loop and the position measuring element is typically located at the end of the motor shaft.
Semi-Closed Loop Systems are widely used, and these systems are often referred to as Closed Loop Systems. The feedback link does not include the mechanical transmission system, which makes the system easier to implement by avoiding the introduction of many nonlinear factors.
However, this system cannot compensate for the transmission error of the external transmission system or factors such as gap error, leading to reduced control accuracy. To improve control accuracy, certain compensation measures can be taken, such as calculating the transmission error and reverse gap in advance and incorporating these errors into the control system.
However, the transmission error and reverse clearance can vary due to factors such as friction, mounting position, and stiffness, and changes in temperature, lubrication, and load can also affect these factors. This limits the system’s ability to improve accuracy.
Open loop control system of point control system
The Open Loop System is a relatively basic control system. It is widely used due to its simple control method, low system cost, and mature technology.
The Open Loop System does not have a position detection device and relies on the performance of the controller and the motor and transmission system to achieve accuracy. The most common example of this system is a Position Servo System using a stepper motor.
In such systems, a one-way operation mode is often employed when the positioning requirements are high to overcome the influence of the reverse gap. Anti-backlash devices, such as gear-driven systems, can also be used in case of reversal, but they have certain limitations, such as increased structural complexity, and they are difficult to adjust. Most anti-backlash devices cannot eliminate gap backlash, and they also increase friction, which exacerbates wear.
For loads with large inertia, large acceleration and deceleration will have a significant impact on positioning accuracy due to the stiffness of the transmission chain. The transmission chain is generally a second or similar second-order oscillation, and it must be handled carefully due to the oscillation that can be caused by the impact of motor starting or stopping or by a constant value input from the motor (step function).
