joint friction

Joint friction is the quiet resistance that fights motion wherever a robot's parts rub or slide against each other — inside bearings, gears, and seals. It is the same drag you feel trying to slide a heavy book across a table: the surfaces grip one another, and some of your effort is wasted as heat instead of going into useful motion. In a robot, this drag steals part of every motor's output and makes joints behave less perfectly than the neat textbook equations predict.

Friction comes in three flavors that engineers treat separately. Static friction is the stubborn grip that must be broken before a still joint will begin to move at all — like the extra shove needed to get that book sliding from a dead stop. Kinetic friction is the steadier drag once the joint is already moving; a common simple model treats it as roughly constant no matter how fast the joint goes. Viscous friction is the speed-dependent drag from lubricants and seals: the faster the joint turns, the harder this kind pushes back, much like how water resists a hand swept quickly through it.

Friction is a mixed blessing. It wastes energy, adds heat, and makes precise control harder — especially the jerky start-stop behavior near zero speed, where a joint can stick and then suddenly break free. Yet it also helps a parked robot hold its pose without burning power, and a little of it can quietly damp out unwanted wobble. Good controllers measure and model joint friction so they can add just enough extra motor effort to cancel it out, letting the arm move as smoothly as the idealized math assumes.