High-Precision Control of Servomechanism Using Traction Drive

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Traction drive has excellent transmission characteristics compared with reduction gear. Backlash of traction drive is negligible and the frequency components of transmission error are small in high frequency range. Actually, traction drive contains only transmission error that has low frequency. This is significant for obtaining high-precision servomechanisms because the transmission error can be compensated more easily compared with gear reducers. Therefore, EMURA applied traction drive to a constant-velocity servomechanism. We developed a high-precision servomechanism and demonstrated the performance of traction drive for high-precision servomechanisms by experimental results.Traction drive transmits torque by viscous force of fluid film formed between two rolling rollers. Torque transmission of traction drive is accompanied with slip between the two rollers. The torque transmission characteristics versus slip are very important to the servomechanisms. Applying traction drive to posi … More tioning servomechanisms, we have to carry out feedback control of rotary angle of output axis. This means that the sliptakes place inside feedback loop and affects the performance of the servomechanisms. Therefore it is important to know torque transmission characteristics versus slip. However, most of reported results are only about static characteristics and the dynamic characteristics have not yet clarified, because it is difficult to measure the dynamic characteristics, On the other hand, the dynamic characteristics of torque transmission are very important to the dynamic performance of servomechanisms.Therefore, we investigated the transmission characteristics versus slip by experiments. Slip between the two rollers is very small so that it is difficult to detect with high accuracy by using conventional methods, in particular, at a high-velocity rotation. Emura proposed to use non-sinusoidal 2-phase type PLL for high-resolution detection of slip velocity. This method used two encoders mounted on input axis and output axis respectively. Byinterpo lating the phase of the two encoders, we are able to detect the difference of rotary velocity between the two rollers with high accuracy and high resolution. However, the eccentricity and run-out of rollers induce large ripples to the velocity difference, and the output of detection circuits contains large error. Therefore, in this study, cross correlation method was used to remove such error for precise investigation of the frequency characteristics of torque transmission.The experimental results showed that the torque transmission characteristics have large nonlinearity and the linear range is very small. The nonlinearity of torque transmission induces unstableness like oscillations to servomechanisms and the robustness for load fluctuation becomes poor. To improve the stability and load characteristics, we proposed a slip-velocity feedback control to the servomechanism based on the measured torque transmission characteristics. The experimental results showed that the stability and load characteristics of the servomechanism were significantly improved. Less