With zero coupling losses and exceptional repeatability, the REXM20 ultra‑high accuracy angle encoder achieves better than ±1 arc second total installed accuracy. Like the RESM20 encoder, the REXM20 is a stainless steel ring with the scale graduations marked axially onto the periphery, but with a number of differences to improve upon RESM20’s already impressive accuracy. REXM20 has a thicker cross-section, to ensure that the only significant installation error is eccentricity. Eccentricity is easily removed using two readheads, either with Renishaw’s DSi (Dual Signal interface), or by combining the signals inside the host controller. The only errors remaining are graduation errors and readhead SDE, both of which are so small they are often negligible. As a non-contact encoder, REXM20 offers dynamic performance advantages, eliminating coupling losses, oscillation, shaft torsion and other hysteresis errors that plague enclosed encoders. Combining two readheads is easy with the DSi, which also offers an angularly repeatable reference position (propoZ ™) which is unaffected by bearing wander or power cycling. REXM20 total installed accuracy grades: Designed for axes that are limited to partial rotation, REXT20 rings have two reference marks, oriented diametrically opposed, for use with partial arc versions of DSi. DSi processes these reference marks to give a single, angularly-repeatable propoZ reference output.