Force, Torque, and Tactile Sensors

Vision tells a robot where things are. Touch tells it how hard it is pushing and where contact happens. Tactile skins give a pressure map, and a force-torque sensor in the wrist gives the total push and twist on the hand.

Contact shape Press force Tool tip force direction Tool tip force size Tool length

Drag on the tactile grid to move the contact.

Tactile sensing

A tactile array measures pressure $p_i$ at each small cell $i$ of area $A$. Summing gives the total force, and the weighted average gives the center of pressure:

$$F = \sum_i p_i\,A \qquad \mathbf{c} = \frac{\sum_i p_i\,\mathbf{x}_i}{\sum_i p_i}$$

Force-torque sensing

A six-axis force-torque sensor reports three forces and three torques. A force $\mathbf{F}$ at a tool tip offset $\mathbf{r}$ from the sensor also creates a torque:

$$\boldsymbol{\tau} = \mathbf{r} \times \mathbf{F}$$

What to notice

On the tactile grid, a fingertip gives a round blob and an edge gives a long strip. Two fingers give two blobs, but the center of pressure sits between them where nothing is touching, which is a reminder that one summary number can hide the shape. On the wrist sensor, a longer tool makes the same tip force produce a bigger torque. Robots use these readings to grip gently, to tell when a part is seated, and to stop when they bump into a person.