How is the design of the mechanical motion system carried out in the design of robot systems?

The motion system ranges from simple planar platforms to complex mechanical arms with multiple degrees of freedom. The design and development of robot systems require rich experience, and a deep understanding of the problems they usually generate and better related solutions is needed.

For mechanical design, five key considerations are proposed in the following text.

1. Motor selection

When choosing a motor, the two most crucial elements are its torque and rotational speed. Based on the magnitude of the force required to operate the target acceleration and velocity of this motion system, the above data can be easily calculated. Another element that is more easily overlooked but crucial is the ratio of inertia between the motor rotor and the load to be rotated. If the load inertia is much greater than the motor inertia, it is almost impossible to achieve a control system with a fast stabilization time and stability.

2 Open-loop and closed-loop control

Simple motion systems usually use stepper motors as their main drive, such as 3D printers. Stepper motors are typically used in open-loop control configurations, where the controller sends commands to the motor to make it move at a certain speed to a certain position. The motor does not provide feedback on completed commands. This is acceptable if the system can meet the performance targets and the accuracy of the system's movement is not crucial.

If the performance requirements exceed the open-loop control configuration of the stepper motor system, then a servo with closed-loop PID control needs to be used? Motor. In a closed-loop system, the controller commands the motor to run at a certain acceleration speed to a certain position. The encoder on the motor shaft sends a report to the controller, providing feedback on its accurate speed and position. The controller decides whether to increase or decrease the motor based on errors in position and speed? Power to meet the position and speed requirements of the command.

A PID control loop can achieve a faster and more accurately positioned motion system, while meeting the realization of command motion. But compared with the simple open-loop system, its controller is more complex.

The mechanical design control avoids backlash and free movement

In a mechanical system with obvious backlash and free movement, it makes no sense to develop an advanced control system and specify the range encoder between the motor and the top, because the result is inaccurate system, low repeatability, and neither looks nor looks.

In a motion system, the two fundamental reasons for the formation of free motion are the arrangement of bearings and the backlash of gears (other system-specific factors). The backlash of gears can be avoided through proper component selection. For example, strain wave gears have no backlash, and planetary gear sets have extremely small backlash. Moreover, although it is not applicable in all situations, the direct drive system can also completely avoid these two problems.

The free movement from the bearing joint can be avoided through the accurate specification of the components and the application of preload. The use of preloaded angular contact ball bearings can eliminate all free movement of the articulated arm, and the use of spring-loaded polymer bearings can make the linear slider and guide rail feel sturdy.

Mechanical strength and alignment: The higher a motion system is or its performance is, the more attention should be paid to the mechanical strength of components and the geometric tolerances of assembly to ensure its performance. For instance, in a simple XY motion system, close-fitting polymer bearings are adopted on a pair of linear guides. Even minor deviations in these tracks will cause interference to the system, resulting in deviations during manufacturing, assembly, or deformation due to normal working loads.

4. Friction, fatigue and service life

Component wear caused by relative motion will lead to changes in their performance over time, and even catastrophic failures. The design of both metal roller bearings and polymer bearings emphasizes bearing life to determine maintenance intervals and assess service life. When designing systems with a long service life and infrequent maintenance, polymer bearings have greater advantages than steel bearings. Steel bearings rely on lubrication to limit or eliminate wear on the metal surface. When the lubricant is contaminated and friction increases, performance will decline. Dry-running polymer bearings can maintain the same friction characteristics throughout their service life.

Better suggestions for the design of the motion system

When choosing a motor, the inertia coefficient between the motor rotor and the load should also be taken into account.

2. Open-loop and closed-loop control systems: Is the open-loop system sufficient to meet the performance requirements? Is it necessary to rely on a closed-loop system to achieve higher speed and positioning requirements?

3. To avoid free movement and backlash caused by common elements such as bearings and gears in the system, the mechanical design should be carefully planned.

4. Consider the mechanical strength, geometric tolerance and matching of components to achieve better system performance;

5. When designing the system, understand the friction, fatigue and service life of the components. It is necessary to consider factors such as whether a solution with a long service life and high performance needs to be implemented, and whether the maintenance of this system is frequent.