Position control arrangement, especially for a surveying instrument, and a surveying instrument

A controller for controlling a direct drive motor having a rotating shaft, the controller comprising:

• input means for receiving an intended rotational position of said rotating shaft of said motor;
• position detecting means for detecting an actual rotational position of said rotating shaft;
• control means for controlling said rotating shaft to rotate from said actual rotational position of said rotating shaft to said intended rotational position of said rotating shaft;
• and
• friction control means for causing said motor to produce a torque resisting a manually forced rotation of said rotating shaft while said rotating shaft is manually rotated to a new rotational position, wherein
• said control means automatically connects said motor to said friction control means in response to a detection of said manually forced rotation of said rotating shaft to said new position, and automatically disconnects said motor from said friction control means when said new rotational position is reached.

A survey instrument comprising:

• a housing;
• a movable unit rotatable back and forth around a first axis relative to said housing, and including survey optical equipment;
• and
• a first position control system for controlling the angular position of said movable unit, said first position control system comprising a first motor to rotate said movable unit about said one axis and a control circuit selectively to operate said first motor in one of a normal mode, in which said first motor rotates said movable unit to a desired position, and a friction mode in which said first motor applies a torque to said movable unit resisting change of angular position of said movable unit caused by incident force while permitting incident force to rotate said movable unit.