9. A triboelectric sensor according to claim 7 or claim 8, wherein the second electrode grid is on the same side of the substrate as the first electrode grid. 10. A triboelectric sensor according to any one of claims 5 to 9, comprising a cover layer formed over the first and/or second electrode grids. 11. A triboelectric sensor according to any one of the preceding claims, wherein the substrate is flexible. 12. A triboelectric sensor according to any one of the preceding claims, wherein the electrodes are formed from a conductive polymer material. 13. A triboelectric sensor according to claim 12, wherein the conductive polymer material comprises a hydrogel PDMS elastomer. 14. A triboelectric control system comprising a motion control interface, said motion control interface comprising at least one triboelectric sensor according to any one of the preceding claims. 15. A triboelectric control system according to claim 14, wherein the motion control interface comprises a further triboelectric sensor configured to detect motion in a single direction, such that motion detected by the triboelectric sensor and the further triboelectric sensor is translatable into a movement control signal in three dimensions. 16. A triboelectric control system according to claim 15, wherein the further triboelectric sensor comprises an elongate substrate having an electrode at each end. 17. A triboelectric control system according to claim 16, wherein the elongate substrate is formed from an elastic material. 18. A triboelectric control system according to claim 17, wherein the elastic material is silicone rubber.