  Contact force is a natural way for humans to interact with the physical world
around us. However, most of our interactions with the digital world are largely
based on a simple binary sense of touch (contact or no contact). Similarly,
when interacting with robots to perform complex tasks, such as surgery, richer
force information that includes both magnitude and contact location is
important for task performance. To address these challenges, we present the
design and fabrication of WiForce which is a 'wireless' sensor, sentient to
contact force magnitude and location. WiForce achieves this by transducing
force magnitude and location, to phase changes of an incident RF signal of a
backscattering tag. The phase changes are thus modulated into the backscattered
RF signal, which enables measurement of force magnitude and contact location by
inferring the phases of the reflected RF signal. WiForce's sensor is designed
to support wide-band frequencies all the way up to 3 GHz. We evaluate the force
sensing wirelessly in different environments, including through phantom tissue,
and achieve force accuracy of 0.3 N and contact location accuracy of 0.6 mm.
