increased number of sensing units and electrode terminals, which in turn means increased cost and extra difficulty in electrode signal extraction, increased signal interference, and greater difficulty in data processing. To address this issue, a simplified tactile sensor configuration has been proposed, whereby position of a fingertip engaged on the tactile sensor surface can be determined by the output ratio from two pairs of electrodes. However, the working mechanism of this proposed tactile sensor depends on the contact- separation at a single touch point, which means that the user’s fingertip has to be separated from the tactile sensor surface after each individual engagement to be able to detect the touch position. Clearly, detection of a continuous 2D trajectory of a moving object or 2D pattern drawn by a fingertip is not possible by this method. For certain applications such as robotics or control of unmanned vehicles, motion and position control must be achievable in three dimensions. Previously proposed tactile sensors have failed to adequately address this issue. It would be desirable to overcome or alleviate one or more of the above difficulties, or at least to provide a useful alternative. SUMMARY Disclosed herein is a triboelectric sensor comprising: a substrate; at least one grid structure disposed in or on the substrate; and at least one electrode for collecting triboelectric charges that are generated by sliding of an object over a surface of the substrate; wherein the at least one grid structure is configured such that motion of the object is detectable from a signal generated by crossing of the object over at least part of the grid structure.