Page 357 - Sensors and Control Systems in Manufacturing

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Cha p te r
S i x

The gripper subsystem includes a PC/AT-compatible industrial
computer; a gripper mechanism, actuated by two DC servomotors,
with an integrated load cell; discrete input/output components; and
two fiber-optic analog-output distance sensors. The computer
includes a discrete input/output circuit card, an 8-channel A/D con-
verter circuit card, a motor-control circuit card with A/D compo-
nents, two serial ports, and a 286 processor with coprocessor.
A geranium cutting comprises a main stem and several petioles
(tiny stem leaves). The individual outputs from the fiber-optic sen-
sors can be processed into an indication of whether a stem or a petiole
is coming into view as the gripper encounters the cutting. Conse-
quently, the gripper can be commanded to grasp a stem but not a
petiole. The axial centerline of a stem can also be recognized from the
outputs of the five optic sensors. Upon recognition of a centerline, the
gripper signals the robot, and the robot commands the gripper to
close.
The motor-controller circuit card supplies the command signals
to the amplifier that drives the gripper motors. This card can be oper-
ated as a position control with digital position feedback or as a force
control with analog force feedback from the load cell mounted in
the gripper. A microprocessor is located on the motor control card.
Buffered command programs are downloaded from the computer to
this card for independent execution by the card.
Prior to a controlled force closure, the motor-control card controls
the gripper in position-servo mode until a specified force threshold is
sensed, indicating contact with the cutting. Thereafter, the position-
servo loop is opened, and the command signal to the amplifier is cal-
culated as the difference between the force set point and the force
feedback from the load cell. This distance is multiplied by a program-
mable gain value, then pulse-width-modulated with a programmable
duty cycle of typically 200 percent. This technique provides integral
stability to the force-control loop. The force-control loop is bidirec-
tional in the sense that, if the cutting expands between the fingertips,
the fingers are made to separate and, if the cutting contracts, the fin-
gers are made to approach each other.

6.25 Ultrasonic Stress Sensor Measuring Dynamic
Changes in Materials
An ultrasonic dynamic vector stress sensor (UDVSS) has recently been
developed to measure the changes in dynamic directional stress that
occur in materials or structures at the location touched by the device
when the material or structure is subjected to cyclic load. A strain
gauge device previously used for the measurement of such a stress
measured strain in itself, not in the part being stressed, and thus pro-
vided a secondary measurement. Other techniques, such as those that