In the field of vision measuring, clients often ask: why do some servo-driven machines maintain precision even at high speeds, while others show deviation? The answer lies in whether the motion control system uses a robust feedback design. Based on years of hands-on experience, I can confidently say: dual-loop control combined with a high-precision grating scale is the key to maintaining long-term accuracy in large-travel vision measuring machines.

In such machines, motor shaft positioning does not always equal platform positioning. Errors may occur due to screw backlash, belt slip, or thermal expansion. A motor encoder only confirms the motor angle, not the true platform position. By installing a grating scale directly on the guide rail, the platform’s actual displacement is measured and fed back into the system. This second feedback loop ensures accuracy.

The grating scale acts like the “eyes” of the system. With resolutions as fine as 0.5 μm or even 0.1 μm, it provides precise, non-contact, and wear-free feedback. Its fast response allows real-time corrections, keeping the platform on target even during high-speed motion.

I once worked with a client whose machine suffered positioning errors exceeding 10 μm during medium-to-high-speed scanning. Their system relied only on a single-loop encoder. After adding an open grating scale and enabling dual-loop control, the error dropped to under 3 μm, fully meeting production requirements. This reinforced my belief that dual-loop control is not optional—it is essential.

The advantages of dual-loop control can be summarized in three areas:

Speed and Accuracy – Unlike single-loop systems that must slow down to ensure precision, dual-loop control allows both speed and accuracy.

Long-Term Stability – Over time, screws and belts wear out, leading to accumulated errors. Dual-loop feedback automatically compensates for these effects.

Resistance to Interference – Temperature changes, vibration, and dust can disrupt mechanics, but dual-loop feedback minimizes their impact on precision.

For clients, the value is clear: accurate measurements and high efficiency. In batch inspections, dual-loop control ensures fast positioning without losing accuracy. Over years of operation, it reduces the need for recalibration and maintenance, lowering long-term costs. I often remind clients: the true value of equipment is not just in the specs on a brochure, but in whether it still delivers reliable results after years of use.

In essence, the motor encoder ensures the motor “rotates correctly,” while the grating scale ensures the platform “moves accurately.” Together, they allow large-travel vision measuring machines to maintain precision over time. That’s why high-end models adopt dual-loop systems, while low-cost single-loop machines often lose accuracy after extended use.

Ultimately, equipment value lies not in marketing claims, but in lasting measurement reliability. Dual-loop control and grating scale feedback are the technologies that make such reliability possible.