Page 355 - Sensors and Control Systems in Manufacturing

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The vision subsystem is assumed to be able to recognize the objects to
be avoided and manipulated and to generate data on the coordinates
of the objects from sent positions in the field-of-view reference frame.
The new method can be implemented in two steps:
1. The close-up stereoscopic cameras are set initially to view a
small region that contains an object of interest. The end effec-
tor is commanded to move to a nominal position near the
object and within the field of view. Typically, the manipulator
stops at a slightly different position, which is measured by
the cameras. Then, the measured error in position is used to
compute a small corrective motion. This procedure is designed
to exploit the fact that small errors in relative position can be
measured accurately and small relative motions can be com-
manded accurately.
2. The approximate direct mapping between the visual coordi-
nates and the manipulator joint-angle coordinates can be
designed without intermediate transformation to and from
absolute coordinates. This is, in effect, a calibration, but it
requires fewer points than does a conventional calibration in
an absolute reference frame over the entire work space. The
calibration is performed by measuring the position of a target
(in field-of-view coordinates) when the target is held rigidly by
the manipulator at various commanded positions (in manipu-
lator joint-angle coordinates) and when the cameras are placed
at various commanded positions. Interpolations and extrapo-
lations to positions near the calibration points are thereafter
performed by use of the nonlinear kinematic transformations.

6.24 Force and Optical Sensors Controlling Robotic
Grippers for Agriculture and Manufacturing
Applications
A robotic gripper operates in several modes to locate, measure, recog-
nize (in a primitive sense), and manipulate objects in an assembly
subsystem of a robotic cell that is intended to handle geranium cut-
tings in a commercial greenhouse. The basic concept and design of
the gripper could be modified for handling other objects—for exam-
ple, rods or nuts—including sorting the objects according to size. The
concept is also applicable to real-time measurement of the size of an
expanding or contracting part gripped by a constant force and to
measurement of the size of a compliant part as a function of the
applied gripping force.
The gripper is mounted on an industrial robot. The robot posi-
tions the gripper at a fixed distance above the cutting to be processed.