Design of an optical tri-axial force sensor

Optics offers great possibilities for the design of cheap force sensors. One of the applications where these sensors are needed, is a computer pen. There, the force sensor is used to measure the contact forces between the pen tip and the paper. The optical tri-axial force sensor presented in this paper is designed for this purpose. The sensor is based on a flexible structure, which converts the force into a displacement. This displacement is measured optically, with a LED, a photodiode and a moving plate between these two components. Tests show that this simple measuring principle reaches a resolution of 60nm and a linearity of 1% in the range of 500mum. Based on these experiments, a mechanical structure is designed. Special attention is spent to get an equal stiffness and, therefore, an equal sensitivity in all directions. In addition each axis is provided with an emergency stop, to protect the sensor against overloading. The design results in a sensor with the size of 8.4mm x 8.4mm x 44.2mm. After production and assembly, this sensor is tested. It has a resolution of 0.01N, a sensitivity of 40mV/N and a linearity of 2%. Moreover, the sensor is insensitive to temperature variations, due to an extra dummy pair of LED and photodiode.