Five degree of freedom measuring arm for resolving spatial relationships within TFTR vacuum vessel

To optimize power loading on the first wall components in the TFTR (Tokamak Fusion Test Reactor), parts must be carefully aligned with the toroidal magnetic field (TF) surfaces. First, the location of a template with six fixed positions was established with respect to the TF using measurements of magnetic field strength at several positions on the template. Next, a measuring arm was installed at each of these locations and manually positioned in order to resolve relative and absolute coordinates of the in-vessel components with respect to the TF. The measuring arm is a flexible linkage consisting of six links and five joints instrumented with absolute optical encoders. Prior to use in the vacuum vessel, the arm was calibrated to determine the best fit for the 23 unknowns involving link lengths, encoder zero positions, and shaft-to-link angles. The position of the indicator point with respect to the measuring arm base is known to ±0.75 mm after this calibration process. During operation, the shaft encoder positions are processed by an algorithm to determine the relative and global coordinates of the indicator point for real-time use and stored for later detailed analysis. Data obtained with this device are shown