ME 445 INTEGRATED MANUFACTURING SYSTEMS MEASUREMENT, ANALYSIS,

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ME 445INTEGRATED MANUFACTURING
SYSTEMSMEASUREMENT, ANALYSIS, ACTUATION
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In an integrated manufacturing environment,
information is gathered through computer
networks to make decisions on time. Among the
information to be gathered is the measurement and
sensing of physical objects and events.
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Typical situations areComputer control of
processes requires analog input/outputs as well
as digital input/output.Programmable logic
controllers (PLC) require information to produce
the required output control signal.
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CNC machine tool need to sense the position of
the tool to follows the path instructed by the
program.Adaptive control of machine tools
requires that conditions can be sensed to perform
adaptive reaction.A shop floor control system
assumes that on-line information can be gathered
in real time on which timely decisions can be
based.Manufacturing cell controller requires
information on the sequence of operations.
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All these situations assume that control follows
the sequence of1. sense current conditions in a
process,2. analyze those conditions,3. effect
conditions through changes to the process.
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In turn, this assumes that the conditions can be
detected and the information can be transmitted
to the analysis device. Once the decision is
made the decision should be executed by some sort
of actuators.
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EXAMPLEConsider the necessary information that
must be obtained in a CNC machining center
(milling machine) and required actuation to
perform the functions of the center
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1. dc motors provide the motion along axes2.
tachometers sense the speed of the axis motors
3. resolver measures axis motor shaft
position4. ac motors provide motion to the
spindle
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5. limit switches sense when the milling table
isapproaching its maximum allowable bounds for
preventing overtravel6. stepping motor
positions the tool exchanger 7. tactile probe
measures the dimensions of the workpiece
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Sensing of physical conditions is obtained by
using sensors
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What is a Sensor?A sensor is a device that
receives a signal or stimulus and responds with
an electrical signal.
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Main Characteristics of Sensors Sensitivity Sensi
tivity Error Linearity Range Accuracy Resolution P
recision Dynamic Range Hysteresis Response time
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Sensitivity
The sensitivity of the sensor is defined as the
slope of the output characteristic curve or, more
generally, the minimum input of physical
parameter that will create a detectable output
change.
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Range
The range of the sensor is the maximum and
minimum values of applied parameter that can be
measured.
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Accuracy
The accuracy of the sensor is the maximum
difference that will exist between the actual
value (which must be measured by a primary or
good secondary standard) and the indicated value
at the output of the sensor
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Resolution
This specification is the smallest detectable
incremental change of input parameter that can
be detected in the output signal
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Precision
The concept of precision refers to the degree of
reproducibility of a measurement
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Hysterisis
A sensor should be capable of following the
changes of the input parameter regardless of
which direction the change is made hysteresis
is the measure of this property
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Response Time
Time required for a sensor output to change from
its previous state to a final settled value
within a tolerance band of the correct new value
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Sensing and measurements may be broken down into
four categories1. Object detection2. Object
identification3. Condition detection4.
Sensing for machines and robots
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Object detectiona) Limit switchesA limit
switch has on/off characteristics. It changes
the position automatically when an object forces
closure of the switch contact
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• The limit switch may be pressure-sensitive, so
  that an object on a conveyor will close the
  contacts just by its own weight
• Limit switches can benormally closed
  (NC)normally open (NO)

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b) Photoelectric SensorsA typical photoelectric
sensor provides a voltage of 10 volts DC, if a
light beam is not interrupted by some objects.If
the beam is interrupted, then no voltage is
provided. Thus the voltage is the equivalent of
an on/off switch.
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There are three possible models for photoelectric
sensors1. Through scanning2. Retro-reflective
scanning3. Diffuse (proximity) scanning
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An important characteristic to be noted when
considering photoelectric sensing is hysteresis,
which the percent of the received light beam
that must be broken to cause an on or off
condition.
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A typical hysteresis has 10 blocked for an
on condition and 95 unblocked for an off
state. When part blocks 10 of the through beam,
the sensor output will change the state. When the
part has moved a sufficient distance to unblock
95 of the light beam, the sensor output will
revert back to its original state.
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This prevents the possibility of multiple
sensings if the part vibrates while moving
through the beam.
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c) Proximity SensingProximity detectors are
electrical or electronic sensors that respond to
the presence of a material.
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The major categories areinductive for
detection of steel chrome-nickel stainle
ss steel brass aluminum copper parts
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magnetic for the detection of ferrous
materialscapacitive for detection
of steel water wood glass plastics
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Object Identification1. Bar codes2. Magnetic
stripes3. Camera with pattern recognition
software4. Radiofrequency (RF) devices
(transponders)5. Optical Character Reading
(OCR)6. Voice Recognition System (VRS)
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Measurement of ConditionsTo measure the
environmental conditions transducers are used. A
transducer inputs one form of energy or
characteristics and has an output a form of
energy or characteristic that is different from
the input.
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Linear displacement measurementsStrain
gageLinear potentiometerLinear
encoderCapacitive transducerLVDT (linear
voltage differential transformer)
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LVDT (linear variable differential tranformer) is
a transducer for converting positional
information into an electrical signal. It is
wound as a transformer with a single primary and
two secondary wound around a cylindirical bobin.
A movable core is positioned inside the windings
and it is the movement of this core which is
measured
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Angular displacement measurementsStrain
gageAngular potentiometerRotary
encoderCapacitor
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Force and pressure measurementsDiaphragm-strain
gageStrain gageBellow-potentiometerPiezo-elect
ricLVDT (linear voltage differential
transformer)
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Temperature measurementsThermocoupleOptical
pyrometer
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Machine Tool SensingThe most important sensing
required in a machine tool is the position of the
tool. Therefore the position transducers
determine the accuracy of a machine tool.
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Part MeasurementOther than the positioning of
the tool, the positioning system of the machine
tools may be used to check the dimension of the
machined part and the amount of the tool wear by
utilizing a tactile probe.
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Linear array Parallel light beams are emitted
from one side of the object to be measured to a
photo optical diode array that is mounted on the
objects opposite side. Diameters are measured by
the number of array elements that are blocked. 1
mm resolution may be possible.
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Tool wear measurementTactile probeForce
measurement on the tool
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Robot Sensing Tactile sensing Force sensing
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