Robot head comprising spindle drive

A back-drivable surgical robot head comprising:

• a frame;
• an arm for carrying a tool the position of which is to be controlled;
• a manually-graspable driving member on said arm;
• and
• a first rotation control mechanism for rotating the arm about a first axis with respect to said frame, said first rotation control mechanism comprising:
• a first lead screw having a rotational motor coupled at one end thereof, said lead screw and motor being mounted at said one end to pivot with respect to said frame;
• a bearing which moves longitudinally of said first lead screw as it rotates, said bearing being coupled to an offset crank of or secured to said arm, said lead screw taking up a zero pivotal position when said bearing is at said one end of said lead screw, said lead screw pivoting away from the zero position as the bearing moves along said lead screw to a maximal pivotal position in which the bearing is part way along the lead screw, and returning to the zero position as the bearing reaches an extreme position at an end of the lead screw opposite said one end;
• said head being back-drivable wherein manual forces applied to said driving member by a user grasping said driving member cause said arm to rotate to a desired position, said motor responding to said manual forces to ensure that said arm moves smoothly to said position with constant low resistance in an unconstrained region and with increasing resistance towards a constraint boundary.