Force Control and Hybrid Position/Force Control

To wipe a table, polish a surface, or insert a part, a robot must control two things at once: where it moves along the surface, and how hard it presses into it. Hybrid control splits the directions: position control along the surface, force control into it.

Controller Desired contact force Surface bumps Surface stiffness

Splitting the directions

A selection matrix $S$ marks which directions are position-controlled. The rest are force-controlled:

$$\boldsymbol\tau = J^\top\!\left(S\,\mathbf{F}_{\text{position}} + (I - S)\,\mathbf{F}_{\text{force}}\right) \qquad F_{\text{force}} = F_d + K_f\,(F_d - F_{\text{measured}})$$

What to notice

Under pure position control, the tool follows a planned height. Where the surface rises, it digs in and the force spikes; where the surface dips, it lifts off and the force drops to zero, so the wiping is uneven. Under hybrid control, the vertical direction tracks force instead, so the tool rides up and down over the bumps while pressing with nearly constant force. A stiffer surface makes position control much worse, since a tiny height error becomes a large force.