Influence of radar targets on the accuracy of FMCW radar distance measurements

Distance measurement tasks in micromachine tools need to be performed with micrometer accuracy. For such tasks, frequency-modulated continuous-wave (FMCW) radars with a combination of frequency and phase evaluations are a good choice. However, the accuracy cannot be indefinitely increased as there are constraints on the target size and placement imposed by the limited space inside micromachines. This paper investigates the influence of target geometry and position on the accuracy of its range estimation using a W-band FMCW radar with a bandwidth of 25 GHz. A relation between target geometry and accuracy is established through the Cramér-Rao lower bound (CRLB). Based on the measurements of different targets, an optimal shape and size is proposed, which provides an average accuracy in the single digit micrometer range. Furthermore, the influence of different bandwidths on the accuracy is investigated. It is also demonstrated how the CRLB can be used to optimize the size of a target, when a certain accuracy is needed. In addition, antenna field regions are analyzed for suitable target placements. Finally, the radar system is implemented in a machine tool and measurements with accuracies in the micrometer range are carried out.