Rotary actuator

A method of controlling an actuator system comprising the steps of:

• providing an actuator system comprising:
• a controlled element configured for rotary movement about a first axis relative to a reference structure;
• a linkage system connected to said element and said reference structure;
• a first actuator configured and arranged to power a first degree of freedom of said linkage system;
• a hold device configured and arranged to selectively lock a second degree of freedom of said linkage system, said first degree of freedom and said second degree of freedom being independent degrees of freedom;
• said linkage system having a first link configured for rotary movement about a second axis relative to said reference structure;
• said first axis and said second axis not being coincident;
• said linkage system having a second link configured for rotary movement about a third axis relative to said reference structure;
• said first link and said second link coupled such that rotation of said first link about said second axis in a first direction causes rotation of said second link about said third axis in a second direction;
• said linkage system configured and arranged such that a first angle of rotation between said element and said reference structure may be driven independently of a second angle of rotation between said first link and said reference structure;
• wherein said hold device is configured and arranged to lock said second degree of freedom when said first actuator is operational and to unlock said second degree of freedom when said first actuator is operatively locked;
• and
• providing power to said first actuator and said hold device simultaneously such that said hold device link locks said second degree of freedom of said linkage system, and said first actuator applies a desired torque to said controlled element.