SIMPLE SENSOR INTERFACE
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A
simple sensor is basically one that has its signal originate in some
peripheral hardware or device.
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Peripheral
hardware is defined as the equipment used in the design of a work cell with an
industrial robot present.
•
The
communication with the robot must be by discrete signals only (on or off).
–
The
group does not include peripheral devices such as disk drives, printers, or
equipment using higher-frequency binary-coded signals with a data format.
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Equipment in this group does include all the discrete
sensors, such as limit switches, proximity sensors, and photoelectric sensors.
•
All
discrete process signals necessary to control the cell are included.
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These
would be signals from machine tools, welders, and material handlers, plus the
signals that the PLC, cell con-troller, or robot originates to operate these
peripherals.
•
The
standard logic signal levels for this interface are 0 volts for the low level,
and 110 volts ac or 24 volts dc for the high level. Some robot controllers use
logic levels of 0 volts and 5 volts, however, and are identified as TTL signal
devices. The input/output interface uses optical coupling between the PLC, cell
controller, or robot and the peripheral to assure isolation of power and
grounds between the different systems.
•
Electrical
noise from industrial machines and contact bounce on the sensor switch contacts
can cause input/output interface problems in the system.
•
No
standard connector type or size is recommended for the simple sensor interface.
•
The
complex sensor interface is used with sensors that require some signal
conditioning before the data are trans-ferred to the PLC, cell controller, or
robot controller.
•
A
complex sensor is defined as one that requires some type of preprocessor to
perform analog-to-digital conversion, scaling, filtering, formatting, analysis,
or coordinate transformation on the raw data before they are presented to a
higher level controller.
•
A
complex sensor communicates with the complex sensor interface using either
digital or analog signals, but the complex sensor interface communicates with
the higher level controller using only digital signals.
•
The
complex sensor system gathering the data frequently must perform computational
operations on the data to do the following:
–
Convert
from analog values to digital values that are compatible with the higher
level
controller
–
Detect
features or recognize patterns present in the information.
–
Compare
measured data with values previously stored in the controller or sensor
interface.
–
Transform
measured values or images into x, y, z, coordinate values
referenced to the work cell equipment.
•
Data
from vision and torque/force sensors illustrate the way in which complex
computation must be performed on the sensory information before it can be used
by the higher level controller.
•
For
example, an image of a part captured by a vision camera is processed by the
vision interface electronics to produce a coordinate value for use by a robot
controller. The robot controller uses the part location coordinate values to
change the programmed pickup point to the actual part location captued by the
camera.
•
In
addition to the computation requirements placed on the complex sensor
interface, it must also operate at high speed.
•
The
very nature of the job to be accomplished, for example, trajectory control,
requires that the closed-loop response time of the interface be very short.
•
Decisions
cannot be made and errors corrected until the data are presented to the higher
level controller.
•
Delays
of more than a few milliseconds do not permit high-performance closed-loop
operation.
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Bidirectional
data communication is another important characteristic of the complex sensor
interface.
–
Not
only must the interface talk to the PLC, cell controller, or robot controller,
but the PLC and controllers must communicate with the interface. In the most
complex interfaces, the two-way communication includes control signals, serial
or parallel data, and addressing information.
–
For
example, in vision applications, the robot controller could signal the vision
system when a part needs to be analyzed, and the vision system sends the
coordinates and orientation of the part to the controller.
•
The
choice of complex sensor interface is dictated by the type of complex sensory
information present.
•
The
system may require one sensor per interface if processing speed is important,
or it may permit one interface to handle many sensors.
•
At
present, vision, torque/force, and remote positioning tables
require the most complex interfaces.
• In one of the interface modules, the
vision sensor requires a minicomputer with memory for storage of data and
vision system programs.
• In another, a pressure sensor needs an
analog-to-digital converter to change data in a analog representation, for
example, zero to five volts, to the equivalent digital value in binary code. In
a third, a digital-to-analog converter is used to drive the heating unit by
converting the digital or binary data output from the controller to an analog
(0 to 24 volts) output voltage.
• As automated work cells become more
complex, the demands on sensor interfacing will increase. In some current work
cells, for example, more than 1000 sensors are required for control of the
production line
system.
VIDEO: THERMAL SENSOR, SIMPLE OR COMPLEX INTERFACE??
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