Design methods and steps for industrial robot manipulators

When designing a new robot, many factors should be fully considered, and more reference should be made to advanced models of similar products at home and abroad, and their design parameters. After repeated research and comparison, the characteristics of the required mechanical parts should be determined and a design plan should be finalized.

1. Design principles of the whole machine

(1) Principle of minimum inertia Since the manipulator has many moving parts and the motion state changes frequently, impact and vibration will inevitably occur. The principle of minimum inertia can increase the stability of the manipulator and improve the dynamic characteristics of the manipulator. Therefore, when designing, attention should be paid to reducing the mass of the moving parts as much as possible while meeting the strength and stiffness requirements, and attention should be paid to the configuration of the center of mass of the moving parts relative to the rotating shaft.

(2) Principle of scale planning optimization When the design requirements meet certain workspace requirements, the smallest arm size is selected through scale optimization, which will help improve the rigidity of the manipulator and further reduce the motion inertia.

(3) Principles for selecting high-strength materials Since the wrist, forearm, upper arm and base of the manipulator act as loads in sequence, it is necessary to use high-strength materials to reduce the weight of components.

(4) Principles of stiffness design In the design of operating machines, stiffness is a more important issue than strength. To maximize stiffness, it is necessary to properly select the cross-sectional shape and size of the rod, improve the support stiffness and contact stiffness, reasonably arrange the forces and moments acting on the arm, and minimize the deformation of the rod.

(5) Reliability principle The reliability of robot manipulators is particularly important due to their complex structure and many links. Generally speaking, the reliability of the robot should be higher than the reliability of its components, and the reliability of components should be higher than the reliability of the whole machine. The reliability of parts or structures that meet the requirements can be designed through probabilistic design methods, and the reliability of the manipulator system can be evaluated through the comprehensive system reliability method.

(6) Manufacturability principle The robot manipulator is a high-precision, high-integration automatic mechanical system. Good processing and assembly manufacturability is one of the important principles to be reflected in the design. Only reasonable structural design without good manufacturability will inevitably lead to reduced performance and increased cost of the manipulator.

2. Design method and steps of the manipulator

(1) Determine the work object and work tasks Before starting to design the operating machine, you must first determine the work object and work tasks.

1) Task: If the work object is a car or an object with a complex curved surface, and the work task is to perform arc welding or spot welding on it, the robot's manufacturing precision is required to be very high. The arc welding task has high requirements on the robot's trajectory accuracy, posture accuracy and speed stability. The spot welding task has high requirements on the robot's posture accuracy. Both tasks require the robot to have the function of arc swinging, and to be able to move freely in a small space and have anti-collision function. Therefore, the robot has at least six degrees of freedom.

2) Painting task: If the work object is a car or an object with a complex curved surface, and the work task is to spray the interior and doors of the car or the surface of a complex curved object, the robot wrist must be flexible, able to move freely in a small space, and have anti-collision function; the robot must be able to work continuously, stably and reliably for a long time; at the same time, the robot must have a smooth streamlined outer surface, and the paint and air pipelines should preferably pass through its cross arm and wrist, so that the robot surface is not easy to accumulate paint and dust, and will not spray the work object, and the paint and air pipelines are not easy to be damaged; because the painting robot works in a flammable and explosive working environment, it must have an explosion-proof function. At the same time, there are also high requirements for the robot's trajectory accuracy, posture accuracy and speed stability. The robot should have at least six degrees of freedom.

3) Handling tasks: If the work object is heavy, the work task is fixed-point handling, and the positioning accuracy is high, then the robot's carrying capacity and positioning accuracy are high. If the work object is light, the work task is also fixed-point handling, but it is required to be handled with care, and the positioning accuracy is high, then the robot's speed stability and positioning accuracy are high.

4) Assembly tasks: high requirements are placed on the robot’s speed stability and position accuracy.

Some robots can complete multiple tasks, such as the MOTOMAN-SKI20 series robots, which can be used for both handling and spot welding, and are fast, precise, powerful and safe. Another type of MOTOMAN-SK6/SK16 series robots can complete multiple tasks such as arc welding, handling, gluing, glazing and assembly, and are high speed, precise and reliable.

When designing a new robot, we must fully consider the above factors, refer to advanced models of similar products at home and abroad, refer to their design parameters, and after repeated research and comparison, determine the characteristics of the required mechanical parts and formulate a design plan.

The following describes the design process using a six-degree-of-freedom AC servo general-purpose robot as an example.

(2) Determine design requirements

1) Load: Determine the load of the robot based on the requirements of the work object and work task, and refer to the advanced models of similar products at home and abroad. Generally, the load of painting and arc welding robots is 5-6kg.

2) Accuracy: According to the user's work object and work task requirements, and referring to advanced models of similar products at home and abroad, determine the maximum composite speed of the robot end and the maximum angular velocity of each single axis of the robot.

3) Accuracy: According to the requirements of the user's work object and work task, refer to the advanced models of similar products at home and abroad, determine the repeat positioning accuracy of the robot, such as the repeat positioning accuracy of the arc welding robot is ±0.4mm, and the repeat positioning accuracy of the Model 5003 painting robot developed by ABB is ±1mm. At the same time, it is necessary to determine the accuracy of the parts that constitute the robot, the dimensional accuracy of the arm body, the shape and position accuracy, and the clearance of the transmission chain, such as the accuracy of the gears and the transmission clearance; it is also necessary to determine the accuracy of the components used in the robot, such as the transmission accuracy of the reducer, the accuracy of the bearings, etc.

4) Teaching method: Determine the robot's teaching method based on the user's work object and work task requirements. Generally, there are several teaching methods for robots:

①Offline teaching (offline programming);

②Teaching with teaching box;

③ Manual step-by-step teaching.

If it is a painting robot, it should have the function of manual step-by-step teaching, while for other robots, the first two functions are enough.

5) Workspace: Determine the size and shape of the robot's workspace based on the user's work object and work task requirements, and refer to advanced models of similar products at home and abroad.

6) Size planning: According to the requirements for the workspace, refer to the advanced models of similar products at home and abroad, determine the arm length and arm angle of the robot, and optimize the size.

(3) Analysis of robot motion For most non-direct drive robots, the motion of the front joint will cause additional motion of the rear joint, resulting in a motion coupling effect. For example, if all six axes are installed in the robot's turret, and other joints are driven by chains, connecting rods or gears, or if the wrist joint is driven by a concentric gear sleeve, a motion coupling effect will occur. In order to decouple, when programming the robot's kinematic control, the rear joint must rotate an additional corresponding number of revolutions to compensate. For a robot with six degrees of freedom, if there is motion coupling between the 2nd and 3rd axes, and there is motion coupling between the 3rd, 4th, 5th and 6th axes, then the motors of the 3rd, 4th, 5th and 6th axes must rotate the corresponding number of revolutions (sometimes forward, sometimes reverse, depending on the structure) to eliminate the influence of motion coupling. The 3rd axis must eliminate the 2nd axis, the 4th axis must eliminate the 2nd and 3rd axes, and so on. If they all rotate forward, when it comes to the 6th axis, the motor must have a very high speed to eliminate the influence of so many axes. Sometimes the motor speed will not be enough, and there are too many axes with motion coupling, which will make the kinematic analysis and control of the robot very troublesome. Therefore, when designing a six-degree-of-freedom AC servo robot, in general, the motion of the first four axes is designed to be relatively independent, and the motion coupling only occurs between the 4th, 5th and 6th axes, that is, the motion of the 5th axis is affected by the motion of the 4th axis, and the motion of the 6th axis is affected by the motion of the 4th and 5th axes. In this way, the compactness of the mechanical structure can be ensured, and the number of axes with coupling relationships will not be too large.