The MTG satellite series will comprise four imaging (MTG-I) and two sounding satellites (MTG-S) that will capture data from the geostationary earth orbit. Both of these applications require that the actuators are individually controlled by stiff servo-loops using absolute encoder feedback. Renishaw’s Resolute encoder system with ZeroMet (FeNi36) Rela linear scale is installed on each strut, providing high-precision metrology with near-zero thermal expansion scales, five nanometre resolution, minimal noise, low cyclic error, and high translational speeds. Matthieu Cuq, principal mechanical engineer at Symetrie explained the choice: “We chose Renishaw’s Resolute absolute encoders because of their high level of metrology performance. These absolute encoders work reliably in high vacuum conditions.

Moreover, the encoder scale is made of low expansion ZeroMet material, which makes our hexapods less sensitive to temperature variations. Thermal stability was a critical requirement for these space optics projects.” One of the key advantages of having the Resolute includes a dependable and reliable data output due to its high tolerance to dirt, scratches and grease that can cause other encoders to miscount. An independent position-checking algorithm also ensures that any potential problem is flagged before any servo control problem can develop.

As hexapods need high precision and stability, the low noise, of typically less than 10 nanometre, RMS helps to improve displacement sensitivity and increases the hexapod stability via servo-loop stiffness. Due to their parallel kinematic structure, hexapods necessitate a particularly complex control system. Resolute’s accuracy combined with fast encoder response time as a result of the BiSS interface can help to optimise both the controller and hexapod performance. Resolute as a single-track absolute optical encoder, also allows for wide installation tolerances and its integral setup LED also simplifies installation during hexapod manufacture and servicing.