Generally, robotic arms have three degrees of freedom, that is, the arm's telescoping, left-right rotation and lifting (or pitching) movement. The rotation and lifting of the arm are realized by the pillar of the machine base, and the lateral movement of the pillar is the lateral movement of the arm. The various movements of the arm are usually realized by the driving mechanism and various transmission mechanisms. The three degrees of freedom of the arm can be combined with different degrees of motion (degrees of freedom). Usually, it can be designed in five forms as shown in Fig. 2-34.


(1) Cylindrical coordinate type


As shown in Fig. 2-34 (a), the motion form is a motion system consisting of one rotation, two movements and three degrees of freedom. The workspace figure is cylindrical. Compared with the rectangular coordinate, under the same working space, the body occupies a smaller volume and has a larger range of motion.


(2) Cartesian coordinates are shown in Fig. 2-34 (b). Cartesian coordinates are composed of three perpendicular linear movements, and the workspace is cuboid. It can read the moving distance of each axis directly on each coordinate axis. It is intuitive and easy to program and calculate the position and attitude. It has high positioning accuracy and simple structure, but the space occupied by the body is large and the flexibility is poor.

(3) The spherical coordinate type is shown in Fig. 2-34 (c), also known as the polar coordinate type. It consists of two rotations and a straight line movement, i.e. one rotation, one pitch and one telescopic motion. Its workspace figure is a sphere. It can perform the up-down pitch action and grasp the workpiece on the ground or at a lower position. It has compact structure and large workspace, but its structure is light and complex.


(4) As shown in Fig. 2-34 (d), the joint type is also called the rotary coordinate type. The arm of this kind of robot is similar to the upper limb of human body. The first three joints of this kind of robot are rotary joints. This kind of robot generally consists of a column and a small arm. The column and six arms form shoulder joints together. The elbow joint between the big arm and the small arm can make the big arm work. Rotating motion theta 1 and making the big pipe swing in pitch theta 2 and the small arm swing in pitch theta 3. Its characteristics are wide working space, flexible movement, strong versatility, and can grasp objects close to the seat.


The planar joint type is shown in Fig. 2-34 (e). Two revolving joints and one moving joint are used; the two revolving joints control the movement of front, back, left and right, while the moving joints realize up and down motion, and have the trajectory graph of the workspace. Its longitudinal section is a rectangular revolving body, and the height of the longitudinal section is the travel length of the moving joints. The rotation angle of the rotary joint determines the size and shape of the cross section of the rotary body. This form is also called SCARA assembly robot. This paper introduces one of the working principles of P29-30.


Typical Mechanism of Robot Arm


(1) Arm linear and rotary motion mechanism


The telescoping and lateral movement of the robot arm belong to linear motion. There are many kinds of mechanisms to realize the reciprocating linear motion of the arm, such as piston oil (steam) cylinder, gear rack mechanism, screw nut mechanism and connecting rod mechanism. Because the piston oil (steam) cylinder is small in size and light in weight, it has been widely used in the arm structure of robots.


(2) arm pitching mechanism


Piston oil (steam) cylinder and connecting rod mechanism are usually used to realize the pitching motion of robot arm. The piston cylinder for arm pitching movement is located below the arm. The piston rod and arm are connected by hinges. The cylinder body is connected with the column by means of tail earrings or middle pin shafts.