Field emission is used as a displacement sensing method, exploiting the exponential relation between field emission current and electrode gap. Atomic force microscopy (AFM) probes have been used as field emission source to measure I/V characteristics which were found to correspond well to theory. The field emission sensor was operated in a more linear regime by using feedback on the position of the probe in order to maintain a constant current. The sensitivity of the sensor for displacement was found to be 0.26 V/nm at a range of ~100 nm. From the experimental data, typical parameters for the Fowler-Nordheim equation were deduced and used to model the sensor performance. The measurements confirm that field emission can be applied to sense the distance between a probe tip and sample with <20 nm resolution.