  In this paper, we propose a preliminary definition and analysis of the novel
concept of sensor observability index. The goal is to analyse and evaluate the
performance of distributed directional or axial-based sensors to observe
specific axes in task space as a function of joint configuration in serial
robot manipulators. For example, joint torque sensors are often used in serial
robot manipulators and assumed to be perfectly capable of estimating end
effector forces, but certain joint configurations may cause one or more
task-space axes to be unobservable as a result of how the joint torque sensors
are aligned. The proposed sensor observability provides a method to analyse the
quality of the current robot configuration to observe the task space. Parallels
are drawn between sensor observability and the traditional kinematic Jacobian
for the particular case of joint torque sensors in serial robot manipulators.
Although similar information can be retrieved from kinematic analysis of the
Jacobian transpose in serial manipulators, sensor observability is shown to be
more generalizable in terms of analysing non-joint-mounted sensors and other
sensor types. In addition, null-space analysis of the Jacobian transpose is
susceptible to false observability singularities. Simulations and experiments
using the robot Baxter demonstrate the importance of maintaining proper sensor
observability in physical interactions.
