9 parameters of co-robot selection

2020-09-15 14:50:54
For veterans in the automation industry and experienced mechanical and electrical engineers, choosing the right "robot" may be a simple task. But for designers or factories who are preparing to buy and import robots for the first time, they may be a little confused.

The following discusses how to choose a suitable co-robot from 9 professional parameters.

1. Applications

First of all, the most important source is to evaluate the imported robot, for what application and what process.

If the application process needs to be completed by the machine by the side of the manual, for the usual man-machine hybrid semi-automatic line, especially the situation that needs to frequently change the station or shift the displacement line, and the occasion of the new torque sensor, the co-robots should be a good option.
In the following discussion, we will focus on joint multi-axis robots. This robot can be adapted to a very wide range of applications. From taking, unloading to stacking, as well as spraying, deburring, welding, and other special processes. Now, co-robot manufacturers have corresponding robot solutions for each application process. All you have to do is to make it clear which job you want the robot to do for you and choose the most suitable model from the different types.
2. Effective load

The effective load is the maximum load that the robot can carry in its workspace. From, for example, 1.5Kg to 20Kg.

If you want the robot to complete the transfer of the target workpiece from one station to another, you need to pay attention to adding the weight of the workpiece and the weight of the robot's gripper to its workload.

Also, special attention should be paid to the load curve of the robot. The actual load capacity will vary at different distances in the space range.

3. Degree of freedom (number of axes)

The number of axes configured by the robot is directly related to its degrees of freedom. If it is for a simple straight-and-go situation, such as picking and placing from one belt line to another, a simple 4-axis robot is sufficient.

However, if the application scene is in a small workspace and the robot arm requires a lot of twisting and rotation, a 6-axis or 7-axis robot will be the best choice.

The number of axes generally depends on the application. It should be noted that under the premise of cost permitting, selecting a larger number of axes is not a problem in terms of flexibility. This facilitates the subsequent reuse of the modified robot to another application process, which can adapt to more work tasks instead of finding that the number of axes is insufficient.

Robot manufacturers tend to use slightly different axis or joint naming. The first joint (J1) is the one closest to the robot base. The next joints are called J2, J3, J4, and so on until they reach the end of the wrist.

4. Maximum operating range

When evaluating the target application, you should know the maximum distance the robot needs to reach. Choosing a robot is not just based on its payload-it also requires comprehensive consideration of the exact distance it reaches. Each company will give the corresponding robot's operating range diagram, from which it can be judged whether the robot is suitable for a specific application. The horizontal movement range of the robot, pay attention to the non-working area near and behind the robot.

The maximum vertical height of a robot is measured from the lowest point the robot can reach (often below the robot base) to the maximum height that the wrist can reach (Y). The maximum horizontal operating distance is the distance (X) from the center of the robot base to the center of the farthest point that the wrist can reach horizontally.
5. Repeatability

Similarly, this factor also depends on your application. Repeatability can be described as the robot's ability to complete routine tasks to reach the same position every time.

 Generally between ±0.05mm to ±0.02mm, even more precise. For example, if you need your robot to assemble an electronic circuit board, you may need a robot with super precision repeatability. If the application process is relatively rough, such as packaging, palletizing, etc., the co-robot does not need to be so precise.

On the other hand, the selection requirements of robot accuracy for assembly engineering are also related to the transfer and calculation of dimensions and tolerances of each link of assembly engineerings, such as the positioning accuracy of incoming materials, and the repeated positioning accuracy of the workpiece itself in the jig. This indicator is expressed as plus or minus ± from the 2D perspective. In fact, since the robot's motion repetition point is not linear but moves in 3D space, the actual situation of this parameter can be any position in the spherical space within the tolerance radius.

Of course, the current motion compensation with the current machine vision technology will reduce the robot's requirements and dependence on the accuracy of incoming materials, and improve the overall assembly accuracy.

6. Speed

This parameter is closely related to each user. It depends on the Cycle Time that needs to be completed in the job. The specification table lists the maximum speed of this model of robot, but we should know that considering the acceleration and deceleration from one point to another, the actual operating speed will be between 0 and the maximum speed. The unit of this parameter is usually measured in degrees/second. Some robot manufacturers also mark the maximum acceleration of the robot.

7. Bodyweight

The weight of the robot body is an important factor when designing the robot cell. If the co-robot must be installed on a custom machine or even on a rail, you may need to know its weight to design the corresponding support.

8. Braking and moment of inertia

Every robot manufacturer provides information about their robot braking system. Some robots are equipped with brakes for all axes, while other robot models are not equipped with brakes for all axes. To ensure precise and repeatable positions in the work area, a sufficient number of brakes are required.

In another special case, when an accidental power failure occurs, the axis of the load-bearing robot without brakes will not lock up, which may cause accidents.

At the same time, some robot manufacturers also provide the moment of inertia of the robot. This will be an additional guarantee for the safety of the design. You may also notice the applicable torques on the different shafts. For example, if your action requires a certain amount of torque to complete the job correctly, you need to check whether the maximum torque applied on the shaft is correct. If the selection is incorrect, the robot may be down due to overload.

9. Protection level

According to the use environment of the robot, select a standard that reaches a certain protection level (IP level). Some manufacturers provide product series with the same manipulator with different IP protection levels for different occasions. If the robot is working in food-related products, medicines, medical appliances, or flammable and explosive environments, the IP level will be different.