The arm structure of industrial robots includes a large arm and a small arm, which not only support the wrist and hand, but more importantly, ensure that the robot can move from one position to another according to precise trajectories. The structure, working range, flexibility, load capacity, and positioning accuracy of the arm directly affect the performance of the robot.
1. Arm characteristics
Characteristics: The arm of industrial robots generally has 2-3 degrees of freedom, including extension, rotation, pitch, or elevation. The arm of a specialized robotic arm generally has 1-2 degrees of freedom, including extension, rotation, or straight-line movement. (2) The weight of the arm is relatively large, and the force is generally complex. During movement, it directly bears the static and dynamic loads of the wrist, hand, and workpiece (or tool), especially during high-speed movement, which will generate a large inertial force, causing impact and affecting the accuracy of positioning. (3) The arm of industrial robots is generally installed on the body together with the control system and drive system.
2. Arm design requirements
The structural form of the arm must be determined based on factors such as the robot's motion form, grasping weight, degree of freedom of movement, and motion accuracy. When designing, the following requirements should be noted:
(1) The stiffness should be high and there should be sufficient load-bearing capacity. When the arm is working, it is equivalent to a cantilever beam. To prevent excessive deformation of the arm during movement, the cross-sectional shape of the arm should be designed reasonably. Generally, hollow shafts are used to make the arm rod and guide rod, and I-beams and channel steel are used to make the support plate.
(2) Good guidance. To prevent relative rotation of the arm along the axis of motion during linear movement and ensure the correct direction of the hand, guide devices or arm rods in the form of squares, splines, etc. should be installed.
(3) The weight should be light. To improve the movement speed of the robot, it is necessary to minimize the weight of the moving parts of the arm as much as possible, in order to reduce the rotational inertia of the entire arm to the rotation axis.
(4) The movement should be smooth and the positioning accuracy should be high. Due to the higher speed and weight of arm movement, the greater the impact before positioning caused by inertial force, which can result in unstable movement and low positioning accuracy. Therefore, the weight of the arm movement should be minimized as much as possible to make the structure compact and lightweight, while also taking some form of buffering measures.
3. Arm mechanism of industrial robots
The arm of a robot consists of a large arm, a small arm, or multiple arms. The driving methods of the arm mainly include hydraulic driving, pneumatic driving, and electric driving, among which the electric driving form is universal. And its arm mechanism is also quite rich, including telescopic mechanism, pitching mechanism, and arm rotation and lifting mechanism.
(1) Arm telescopic mechanism
The telescopic motion of the robot arm is divided into linear motion, and the specific operation method varies according to the length of the stroke. When the travel is short, the oil (steam) cylinder is used for direct drive. When the travel is long, one can choose to use a doubling mechanism that combines oil (steam) cylinders with rack and pinion transmission, or use stepper motors and servo motors for driving. You can also consider using screw nuts or ball screws for transmission.
In order to improve the rigidity of the arm and prevent it from rotating around the axis or deforming during the stretching and contracting process, it is necessary to add a guiding device to the arm structure or design the arm into a square or spline shape. Common guiding devices include single guiding rods and double guiding rods.
In the telescopic structure of the dual guide arm section, the arm and wrist are installed at the upper end of the lifting hydraulic cylinder through a connecting plate. When the two chambers of the double-acting hydraulic cylinder are filled with pressure oil, it pushes the piston rod (i.e., the arm) to make a linear reciprocating motion. The guide rod moves inside the guide sleeve to prevent the arm from rotating, and at the same time serves as the oil pipeline for the wrist rotation cylinder and the hand clamping hydraulic cylinder. Due to the fact that the telescopic hydraulic cylinder is located between two guide rods, the guide rod bears bending force, while the piston rod only experiences tensile pressure. Therefore, the structure is simple in terms of stress, smooth in transmission, neat and beautiful in appearance, and compact in structure.
(2) Arm pitch mechanism
The arm pitch motion of robots is generally achieved through piston hydraulic cylinders and connecting rod mechanisms. The piston cylinder used for the pitch motion of the arm is located below the arm, and its piston rod is connected to the arm with a hinge. The cylinder body is connected to the column using tail earrings or middle pin shafts, as shown in the following figure.
The following diagram shows the mechanism of the articulated piston cylinder for achieving arm pitch. By using articulated piston cylinders 5 and 7 and a connecting rod mechanism, the small arm 4 can achieve pitch motion relative to the large arm 6 and the large arm 6 can achieve pitch motion relative to the column 8.
(3) Arm rotation and lifting mechanism
There are various structural forms available for achieving the rotational motion of robot arms, including lade-type rotary cylinders, gear transmission mechanisms, sprocket transmission mechanisms, and linkage mechanisms. Let's take the piston cylinder and gear rack mechanism in the gear transmission mechanism as an example to illustrate the rotation of the arm.
In the gear transmission mechanism, the gear rack mechanism drives the gear connected to the arm to perform reciprocating rotary motion through the reciprocating movement of the gear rack, thereby achieving the rotation of the arm. This gear rack mechanism can be driven by pressure oil or compressed gas. The structure of lifting and rotating motion is shown in the diagram [Structure of arm lifting and rotating motion].
The two chambers of the piston hydraulic cylinder are respectively filled with pressure oil, which drives the rack piston 7 to move back and forth (see section A-A). Rack 7 meshes with gear 4, causing gear 4 to undergo reciprocating rotational motion. Due to the fact that gear 4, arm lifting cylinder body 2, and connecting plate 8 are all connected by screws, and the connecting plate is firmly connected to the arm, it is possible to achieve the rotational motion of the arm.
The piston rod of the lifting hydraulic cylinder is connected and fixed to the machine base 6 through the connecting cover 5, and the cylinder body 2 moves up and down along the guide sleeve 3. Due to the guide sleeve on the outside of the lifting hydraulic cylinder, this structure has good rigidity and smooth transmission.
It is the diligent research of a large number of researchers that has led to the continuous implementation of projects involving industrial robots. Depth has played a huge role in practical applications. Industrial robots play a crucial role in manufacturing, healthcare, and even space exploration. Different arm structures are suitable for different industrial scenarios, driving the development of modern industry and improving efficiency and safety.