Page 463 - Sensors and Control Systems in Manufacturing

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8.12.2 Vision Sensor Modules
The vision module consists of two Primagraphics VME boards. One
board contains the frame image of 768 × 576 pixels and 128 gray val-
ues. A frame-transfer solid-state camera is connected to the system.
Force sensing is available and can measure three force and three
torque components. Forces up to 200 N are measured with a resolution
of 0.1 N. Torques up to 4 N⋅m is measured with a resolution of 0.002
N⋅m. The data rate ranges from 1.8 to 240 Hz. The torque-force sensor
is controlled by a Texas Instruments 9995 microprocessor. Besides a
high-speed RS-232 link 38400 board, an analog output is available.
An ultrasonic range finder sensor has been interfaced to the G64
bus. A tactile array based on the piezoelectric material PVDF has been
developed. A stereo vision system is incorporated to the system.

8.12.3 Software Structure
Because an important aim of the testbed is to investigate the use of
sensors in an assembly process, it is essential that an optimal environ-
ment is present for program development and real-world testing. A
user-friendly program environment is provided by the UNIX V oper-
ating system. Real-time operation is provided by the special process-
ing units. Programs developed under the UNIX system can be down-
loaded to the SPUs via the VME bus.
The assembly process consists of a sequence of assembly stages. A
succeeding assembly stage is entered when certain entrance conditions
are met. This means that the assembly process can be controlled by a
state machine. An assembly stage may consist of a number of states.
With this approach, a modular and hierarchical control structure is
obtained in which certain processes are executed at the lowest level.
The control software for the assembly task consists of three different
levels: (1) state machine level, (2) executor level, and (3) driver level.
At the highest level, a command interpreter activates certain
states of the machine. The state machine activates another (nested)
state machine or a process (executor). Executors may run in parallel.
For instance, the vision system may analyze an image of a product
while the robot is assembling. The synchronization of the processes is
obtained from the required entrance conditions for the next state of
the state machine. In general, this level requires a moderately fast
response and may run under the UNIX system.
At the second level, the executors are activated by the state machine.
An executor performs a certain subtask and requires a real-time
response. Examples are robot movements and image analysis rou-
tines. The executors reside in the SPUs and in the robot controller
(robot programs). An executor is the only program that communi-
cates directly with a driver.
The drivers for the different subsystems form the third level and
also reside in the SPUs. The drivers take care of the protocol conversion
and of the error checking in the communication with the subsystems.