Page 28 - Rapid Learning in Robotics

14 The Robotics Laboratory

with environment interaction need quick response and therefore require,
a very high frequency of the digital force control loop. The bottleneck
is given by the Puma controller structure. The realizable force control in-
cludes a fast inner position loop (joint micro controller) with a slower outer
force loop (involving the Sun “druide”). But still, by generating the robot
trajectory setpoints on the external Sun workstation, we could double the
control frequency of VAL II and establish a stable outer control loop with
65 Hz.

Fig. 2.3 sketches the two-loop control scheme implemented for the mixed
force and position control of the Puma. The inner, fast loop runs on the
joint micro controller within the Puma controller, the outer loop involves
the control task on the Sun workstation “druide”. The desired position
X des and forces F des are given for a specified coordinate system (here writ-
ten as generalized 6 D vectors: position and orientation in roll, pitch, yaw

(see also Fig. 7.2 and Paul 1981) X des p x y p z p and generalized
mThe
force F des f x y f z f x m y m z ). control law transforms the force
deviation into a desired position. The diagonal selection matrix elements
in S choose force controls (if 1) or position control (if 0) for each axis, fol-
2
lowing the idea of Cartesian sub-space control . The desired position is
transformed and signaled to the joint controllers, which determine appro-
priate motor power commands. The results of the robot - environment in-
is monitored by the force-torque sensor measurement and
teraction F meas
transformed to the net acting force F trans after the gravity force compu-
tation. The guard block checks on specified sensory patterns, e.g., force-
torque ranges for each axes and whether the robot is within a safe-marked
work space volume. Depending on the desired action, a suitable controller
scheme and sets of parameters must be chosen, for example, S, gains, stiff-
ness, safe force/position patterns). Here the efficient handling and access
of parameter sets, suitable for run-time adaptation is an important issue.

2
Examples for suitable selection matrices are: S=diag(0,0,1,0,0,0) for a compliant mo-
tion with a desired force in z direction, or b S=diag(0,0,1,1,1,0) for aligning two flat sur-
faces (with surface normal in z). A free translation and z-rotational follow controller in
Cartesian space can be realized with S=diag(1,1,1,0,0,1). See (Mason and Salisbury 1985;
Schutter 1986; Dücker 1995).