Popular Science
What is repeatability?
In precision motion platforms, repeatability (or repeatability) refers to a statistical quantity of the deviation from the actual position when the motion table moves to the same nominal position multiple times. Different statistical calculation methods will be used depending on the test standards, such as peak-valley value, 2σ, 3σ, etc.
Figure 1. Yacobes VRG Series Platform
Repeatability is affected by a variety of complex factors, including those inherent to the motion table, such as backlash, friction, cable disturbance, servo jitter, and stiffness, as well as environmental factors, such as temperature drift, foundation vibration, and environmental noise. It is almost impossible for us to provide a quantitative explanation of the impact of each factor on repeatability.
For example, backlash and irreversible elastic deformation in a structure often lead to significant positive and negative deviations in positional error during the forward and reverse movements of the motion table, a phenomenon known as the "gap." The causes of backlash and irreversible elastic deformation are not fully understood, but backlash is generally believed to be caused by the connections between parts. Irreversible elastic deformation is the result of the combined effects of static friction, cable forces, and motor forces; we collectively refer to the structural deformation caused by these two factors as irreversible deformation, as illustrated in Figure 2. In practice, it is difficult to separate the effects of these two factors in the test results.
Figure 2. Deformation of the structure that cannot be reset.
Although the impact of each factor on repeatability cannot be quantified, the test results can be interpreted using statistical theory and phenomenological perspectives. Therefore, we can statistically categorize the factors influencing repeatability as shown in Figure 3.
Figure 3. Factors influencing statistical repeatability
experiment
Factors affecting repeatability accuracy
Figure 4 Air-floating single-axis motion table
To explore the factors affecting repeatability accuracy, we first randomly selected a typical air-bearing platform for repeatability testing to identify potential influencing factors. The test results are shown in Figure 5, which clearly demonstrates the impact of random jitter, non-recoverable deformation, and long-term position drift on the repeatability results.
Figure 5. Typical repeatability test results of air-float platform
To address these potential influencing factors, we will next use test data from Jacobs' air-bearing platform to specifically analyze the impact of different factors on repeatability accuracy. To more accurately determine the degree of influence of each factor, the air-bearing platform was tested in both cable-chain and cableless versions for repeatability testing.
Table 1. Repeatability test results according to Renishaw 2012 standard (without cable chain)
Based on the Renishaw 2012 testing standards, we obtained the repeatability test data in Table 1. However, it is difficult to directly identify the factors affecting the repeatability of the motion table from the test data; therefore, we need to decompose the test data.
Figure 6. Decomposed repeatability test data (without cable chain)
The decomposition yielded the results shown in Figure 6. It can be seen that although the test data varied with each repetitive test, the effects of random noise and non-recoverable gaps remained relatively stable. However, the inconsistency in the test results was mainly due to temperature drift.
Since air-bearing platforms have almost no guiding friction, cables and cable chains become the main sources of noise. We added cable chains to the same test air-bearing platform for comparison.
Figure 7 Typical test data for cableless and cable-connected systems.
Typical test data with and without a cable chain are shown in Figure 7. With a cable chain, the noise exhibits a distinct wavy pattern, while without a cable chain, it presents a linear pattern. Comparing the test data, the noise level without a cable chain is approximately ±15nm, while with a cable chain, the noise level is approximately ±40nm, a significant increase. Therefore, for motion platforms requiring ultra-high repeatability, the disturbance caused by the cable chain cannot be ignored.
Figure 8 Test data after disassembly without cable chain
Figure 9 shows the test data of the cable chain after disassembly.
Conclusion
Improving repeatability accuracy is crucial.
It is a systematic project
Repeatability is one of the most critical indicators for all motion platforms, as the platform's repeatability determines the precision level of the final product. Whether it's semiconductor testing equipment, lithography machines, placement platforms, or machining tools, all prioritize repeatability.
At the same time, improving repeatability is also difficult. The position of the scale, the position of the motor, the friction of the guide rail, etc. need to be considered. Even cable interference can affect repeatability. In short, improving repeatability is a systematic project.
Akribis possesses a deep understanding of repeatability and can provide customers with customized motion platforms offering ultra-high repeatability. Simultaneously, the company also conducts targeted development based on customers' specific application scenarios, helping them solve technical challenges in motion control, including speed, accuracy, and force control. Leveraging our expertise and technological accumulation in the direct drive field, we contribute to the upgrading of China's manufacturing industry.
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