Selection Skills of Yaskawa Robot Position Encoder

The calculation method of the position resolution in the follow-up file in the Yaskawa Robot Manual is:

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In formula (1): △L is the moving distance, the unit is millimeters, and Data2-Data1 represents the number of pulses corresponding to the moving distance. Then there is a problem. Since the smallest unit of position resolution is 0.01μm, if the calculation result in the above formula is 4 digits after the decimal, for example, 0.3125μm/pulse, then the value you enter can only be 0.31 or 0.32 , Which means that the follow-up speed of the robot will be faster or slower than the actual speed of the conveyor belt. This situation is very fatal for some precision equipment.

This article discusses a more direct method of calculating the robot position resolution.

For example, the servo drive system is used in the field, the lead of the drive screw is 25mm, the reduction ratio of the reducer is 10:1, and the field follow-up encoder is 2000rpp.

We use the calculation method of the robot manual, and the actual result we calculated is 0.3125μm/pulse, then we can only use a value of 0.31 or 0.32, which will cause the robot to follow the process and the actual speed of the conveyor belt is inconsistent. But it is difficult for us to find out where the problem lies.

If we first calculate the moving distance of the conveyor belt in one revolution of the servo motor: 25÷10=2.5mm, we can know that the conveyor belt needs to feedback 2000 pulses to move 2.5mm, then the calculated position resolution is 2.5÷(2000×4) =0.3125μm, the 4 in the formula is because the robot will do 4 times the frequency when it is connected to the encoder, so the number of pulses received by the robot is 4 times the actual feedback pulse.

From the above two algorithms, we can see that the value of the position resolution we calculated is the same, but the second algorithm allows us to find a solution to the problem. In other words, we can change the lead of the lead screw, the reduction ratio of the reducer or the model of the follow-up encoder according to the actual situation, and then the value of the position resolution can be changed to ensure the accuracy to the smallest unit of the position resolution. Still the above example, if we change the follow-up encoder to 500rpp, then the position resolution we calculated is: 2.5÷(500×4)=1.25μm, which can ensure that the follow-up speed of the robot is absolutely consistent with the speed of the conveyor belt . Of course, if we require the resolution of the encoder to be as high as possible, then we can change the lead of the lead screw or change the reduction ratio of the reducer to achieve the same effect.

In this way, we can give priority to the consistency of the robot's follow-up in the early design, and avoid the passive situation of re-engineering in the later stage.