SCRUB

SCRUB (Surface Contact Regulation Using the Body) is my primary graduate research project in the ALMaS Research Group at WPI, advised by Dr. Mahdi Agheli. The goal: let a quadruped with no arm do useful contact work — pressing a body-mounted tool against a surface and sliding it along while regulating the contact force, all while the robot keeps balancing on four legs.

This is a fundamentally different regime from locomotion or pick-and-place. The contact is persistent rather than intermittent, the interaction force must be actively regulated rather than avoided, and every stance transition of the gait perturbs the tool. Think of wiping a wall, sanding a hull, or inspecting a pillar. SCRUB does this blind — with no model of the surface — so the surface normal has to be recovered from the contact itself.

How it works

• A friction-independent normal cue. The out-of-plane component of the sliding drag vanishes exactly at the true surface normal. The cue holds whatever the friction coefficient, so the normal can be found without knowing the friction.
• Surface-frame estimation. An online estimator tracks the surface frame at the tool, so the controller can regulate force along the normal and slide along the surface — on a flat wall or around a curved column.
• Centroidal MPC over whole-body control. A 100 Hz centroidal MPC (OCS2, SQP) runs over a 1 kHz whole-body-control QP on a Unitree Go1.

Hardware results

Unitree Go1 hardware, 10 N normal-force target:

Baselines with a fixed normal or with friction compensation completed 0/5 column orbits (p = 0.008, Fisher exact test).

Status

Manuscript (2026): S. Selvaraj and M. Agheli, “SCRUB: Surface Contact Regulation Using the Body for Armless Quadruped Force–Motion Control.”

Video: youtu.be/R2P-cebOBVM