What is a Robotic Arm?

Manipulator refers to a complex system with high accuracy, multiple inputs and multiple outputs, high nonlinearity and strong coupling. Because of its unique operating flexibility, it has been widely used in industrial assembly, safety and explosion protection.

The robot system is composed of a vision sensor, a robot arm system and a main control computer. The robot arm system includes a modular robot arm and a smart hand. The construction model of the entire system is shown in Figure 1. ^[2]

Uncertainty is mainly divided into two main types: structured and unstructured uncertainty.Unstructured uncertainty is mainly due to measurement noise, external interference, and sampling delays and rounding in calculations. Uncertainty caused by factors other than the controlled object itself, such as entry errors. Structural uncertainty is related to the modeling model itself, which can be divided into

Robot arm research background

In recent years, with the development of robot technology, the application of robot structures with high speed, high accuracy, and high load-to-weight ratio has attracted attention in the industrial and aerospace fields. Due to the increased flexibility effects of joints and connecting rods during movement, the structure is deformed and the accuracy of task execution is reduced. Therefore, the flexible characteristics of the structure of the robotic manipulator must be considered, and the dynamic characteristics of the system must also be considered to achieve high-precision and effective control of the flexible manipulator. The flexible manipulator is a very complex dynamic system, and its dynamic equations are non-linear, strongly coupled, and real variable. It is extremely important to establish a model for the study of the dynamics of flexible arms. The flexible manipulator is not only a rigid-flexible coupled nonlinear system, but also a nonlinear system in which the dynamics and control characteristics of the system are coupled to each other, that is, electromechanically. The purpose of dynamic modeling is to provide basis for control system description and controller design. The description of the general control system (including the description of the state space in the time domain and the description of the transfer function in the frequency domain) is closely related to the positioning of the sensor / actuator, the information transfer from the actuator to the sensor, and the dynamic characteristics of the robotic arm. ^[3]

Manipulator Modeling Theory

The establishment of the dynamic equation of the flexible manipulator mainly uses the two most representative equations, Lagrange equation and NeWton-Euler equation. In addition, the methods of variational principle, virtual displacement principle and Kane equation are more commonly used. The description of flexible body deformation is the basis for modeling and controlling flexible manipulator systems. Therefore, firstly, a certain method is chosen to describe the deformation of the flexible body, and the description of the deformation is closely related to the solution of the system dynamics equation. ^[3]

The description of the deformation of the flexible body mainly includes the following:

1) Finite element method;

2) Finite segment method;

3) Modal synthesis method;

4) centralized quality method;

The establishment of the dynamic equation of the manipulator

Whether it is a continuous or discrete dynamic model, its modeling methods are mainly based on two basic methods: vector mechanics and analytical mechanics. More widely used and more mature are the Newton-Euler formula, Lagrange equation, variational principle, virtual displacement principle, and Kane equation. ^[3]

Manipulator control strategy

The control of the flexible robot arm generally has the following methods,

1) Rigidization. The effect of the elastic deformation of the structure on the rigid body motion is completely ignored. For example, in order to prevent excessive elastic deformation from damaging the stability and end positioning accuracy of the flexible robotic arm, the maximum angular velocity of NASA's remotely controlled space hand movement is 0.5 deg / s.

2) Feedforward compensation method. The mechanical vibration caused by the flexible deformation of the robotic arm is regarded as a deterministic interference to the rigid movement, and the feedforward compensation method is used to offset this interference. German Bernd Gebler studied feedforward control of industrial robots with elastic rods and elastic joints. Zhang Tiemin studied the method of removing the dominant poles and system instability of the system by adding zeros, and designed a feedforward controller with time delay and PID controller to eliminate the residual vibration of the system more obviously. Seering Warren P. Other scholars have conducted in-depth research on feedforward compensation technology.

3) Acceleration feedback control. Khorrami FarShad and Jain Sandeep studied the end trajectory control of flexible robotic arms using end acceleration feedback.

4) Passive damping control. In order to reduce the influence of the relatively elastic deformation of the flexible body, various energy-consuming or energy-storing materials are used to design the structure of the arm to control the vibration. Or the use of a damper, a damping material, a composite damping metal plate, a damping alloy, or a viscoelastic large damping material to form an additional damping structure on a flexible beam is a passive damping control. In recent years, the use of viscoelastic large damping materials for vibration control of flexible manipulators has attracted great attention. RoSSi Mauro and Wang David investigated the passive control of flexible robots.

5) Force feedback control method. The force feedback control of the vibration of the flexible robotic arm is actually a control method based on inverse dynamics analysis, that is, the torque applied to the driving end is obtained by a given motion of the end of the arm according to the inverse dynamics analysis, and the driving torque is feedbacked by motion or force detection make up.

6) Adaptive control. Combined adaptive control is used to divide the system into joint subsystem and flexible subsystem. The parameter linearization method is used to design adaptive control rules to identify the uncertain parameters of the flexible manipulator. The design of a tracking controller for a flexible manipulator with nonlinear and parameter uncertainties is carried out. The controller design is a robust and adaptive control design based on the Lyapunov method. The system is divided into two subsystems through state transitions. Adaptive control and robust control are used to control the two subsystems respectively.

7) PID control. As the most popular and widely used controller, PID controller is widely used for rigid robotic control because of its simplicity, effectiveness and practicality. It is often used to adjust the controller gain to form a self-tuning PID controller or other control methods. Combined to form a composite control system to improve PID controller performance.

8) Variable structure control. Variable structure control system is a discontinuous feedback control system, of which sliding mode control is the most common variable structure control. Its characteristics: On the switching surface, there is a so-called sliding mode. In the sliding mode, the system remains insensitive to parameter changes and disturbances. At the same time, its trajectory is on the switching surface. The sliding phenomenon does not depend on system parameters and has a stable nature. The design of the variable structure controller does not require an accurate dynamic model of the robotic arm, and the boundaries of the model parameters are sufficient to construct a controller.

9) Fuzzy and neural network control. It is a language controller, which can reflect the characteristics of people's thinking when performing control activities. One of its main characteristics is that the control system design does not require the mathematical model of the controlled object in the usual sense, but requires the operator's or expert's experience and knowledge, operating data, etc. ^[3]

Significance of robotic arm research

Compared with rigid manipulators, flexible manipulators have the characteristics of light structure and high load / self-weight ratio, so they have lower energy consumption, larger operating space and high efficiency.The response is fast and accurate, and there are many potential Advantages, occupy a very important position in industrial, defense and other applications. With the rapid development of the aerospace industry and the robot industry, more and more flexible body systems composed of several flexible members are used. The traditional analysis methods and control methods of multi-rigid body dynamics cannot meet the requirements of dynamic analysis and control of multi-flexible body systems. As the simplest non-trivial multi-flexible body system, flexible manipulators are widely used as multi-flexible body systems Research model. ^[4]