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Smartwheela (left) and Optipushb (right). Both wheels measure three-dimensional forces and torques on the handrim, combined with the angle under which the wheel is rotated. The local coordinate systems are defined as: x- and y-axis, orthogonal to each other in the plane of the handrim, z axis orthogonal to the plane of the handrim and pertruding from the wheel axle. The direction of positive torque is mirrored for the wheels so that they measure positive torque when pushing forward.
