A wrench-sensitive touch pad based on a parallel structure

Many different robotic in-parallel structures have been conceived as six-component force sensors. In general, they perform well for most applications but, when accuracy is a must, two main limitations arise. First, in most designs, the legs are connected to the base and the platform through ball- and-socket joints. Although the dry friction in each of these joints can be individually neglected, the integrated effect of twelve such elements becomes noticeable. Second, dynamical measurements might not be very accurate because the natural resonance frequency of the used structures is quite low even for relatively small dimensions. This dynamical response can be obviously modified with a proper mechanical design, but this increases the complexity of the sensor. This paper discusses the design and implementation of a touch pad based on a 6-axis force sensor and shows how the above limitations degrade its behavior. Moreover, it is shown how using a tensegrity structure both problems could be alleviated because ball-and- socket joints can be substituted by point contacts and the resonance frequency of the structure can be controlled by adjusting the static tensions of the tendons.