Bimanual Manipulation

Two arms can carry heavy or long objects that one arm cannot, and can hold something while working on it. But once both hands grip the same rigid object, they are no longer free: every motion of one hand must match the other's, or they will fight through the object. Planning for both arms at once is called bimanual manipulation.

Task Bar center x Bar center height Bar tilt Coordinate the two arms

The closed chain

When both hands hold a rigid bar of length $\ell$, the two hand poses are tied to the bar's pose $T_b$:

$$\mathbf{p}_{L} = T_b\begin{bmatrix} -\ell/2 \\ 0 \end{bmatrix} \qquad \mathbf{p}_{R} = T_b\begin{bmatrix} \ell/2 \\ 0 \end{bmatrix}$$

So the planner works on the bar's pose and derives both arms' joint angles from it, with inverse kinematics for each arm.

What to notice

With coordination on, both arms follow the bar exactly, and the bar stays the same length. Turn coordination off and each arm moves on its own toward a slightly wrong target, so the hands pull apart or squeeze together, shown as a strain on the bar. On a real robot that internal force can damage the object or the arms. Handovers add timing: the receiving hand must close before the giving hand opens.