1. School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton Campus, Edinburgh EH14 4AS, UK e-mail:
2. School of Engineering and Physical Sciences, Heriot-Watt University, Riccarton Campus, Edinburgh EH14 4AS, UK e-mail:
3. Renishaw plc, Research Park North, Riccarton, Edinburgh EH14 4AP, UK e-mail: