Since real manipulation tasks imply interaction with the environment, the robot end-effector motion is constrained at the
contact. Proper execution of constrained motion tasks can be achieved using compliant control systems which attempt to
accommodate external forces. A new approach to the force/position control problem has been proposed; namely the parallel
control approach. The method combines simplicity and robustness of impedance control with the ability of simultaneously
handling both position and force variables typical of the hybrid control approach. This goal is achieved by using two
independent controllers and managing conflicting situations by means of a priority strategy with dominance of the force
controller over the position controller; automatic recovery from unplanned impacts is made possible by exploiting sensor
measurements. Developments of this control concerning edge-following tasks have been worked out. The use of ultrasonic
range sensing in constrained motion control has been investigated. A computationally cheaper controller has been proposed
that is based on a PID force/position action together with gravity compensation; global stability of this controller has been
shown via a Lyapunov argument, and the scheme has been extended to avoid velocity measurements. In the case of
imperfect gravity compensation, the controller can be made adaptive with respect to a suitable set of parameters in the
gravity term. A passivity-based control scheme has been proposed which ensures tracking of the unconstrained components
of the desired end-effector trajectory with regulation of the desired contact force along the constrained direction; adaptation
with respect to dynamic parameters has also been achieved. A nonlinear observer has been designed. An adaptive
force/position control scheme in case of contact with unknown stiffness has been proposed. More recently, the problem of
spatial impedance control have been addressed. In order to perform six-dof tasks, not only is a representation of end-
effector orientation required, but also a suitable definition of end-effector orientation displacement to be related to the
contact moment should be sought. Instead of the usual minimal representation based on three Euler angles, the
geometrically meaningful angle/axis representation has been adopted. Among the various angle/axis representations, a
singularity-free parameterization has been shown to be very compact and effective, i.e. the unit quaternion. Such a
formulation has also been used to develop a controller for contact moment regulation and orientation tracking for a
manipulator interacting with a compliant environment. Experimental tests of impedance control and force control have been
carried out on the Comau robots in the PRISMA Lab, as well as on a Kuka robot at DLR.