Ch2 Measurement Analysis and Actuation

8/10/2019 Ch2 Measurement Analysis and Actuation

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ME 445

INTEGRATED MANUFACTURING SYSTEMS

MEASUREMENT,

ANALYSIS,&

ACTUATION

2004


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In an integrated manufacturingenvironment,

information is gathered through computer

networks to make decisions on time.

Among the information to be gathered is the

measurementand sensingof physical objectsand events.


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Typical situations are:

Computer controlof 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 toolneed to sense the position of the

tool to follows the path instructed by the program.

Adaptive controlof machine tools requires that

conditions can be sensed to perform adaptive

reaction.

A shop f loor control systemassumes that on-line

information can be gathered in real time on which

timely decisions can be based.

Manufacturing cell controllerrequires informationon the sequence of operations.

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All these situations assume that control

follows the sequence of

1. sense current conditions in a process,

2. analyze those conditions,

3. effect conditions through changes to theprocess.


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In turn, this assumes that theconditions can be detected and the

information can be transmitted to

the analysis device.

Once the decision is made the

decision should be executed bysome sort of actuators.

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EXAMPLE:

Consider 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 axes

2. tachometers sense the speed of the axis

motors

3. resolver measures axis motor shaft position

4. ac motors provide motion to the spindle

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5. limit switches sense when the milling tableisapproaching its maximum allowable bounds

for preventing overtravel

6. 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 usingsensors

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What is a Sensor?

A sensor is a device that

receives a signal orstimulus and respondswith an electrical signal.

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Main Characteristics of Sensors

Sensitivity

Sensitivity Error

Linearity

Range

Accuracy

Resolution

Precision

Dynamic RangeHysteresis

Response time


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Sensitivity

The sensitivity of the sensor is definedas the slope of the output characteristic

curve or, more generally, the

minimum input of physical parameter

that wil l create a detectable outputchange.


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Range

The range of the sensor is the

maximum and minimum valuesof applied parameter that can be

measured.


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Resolution

This specification is the smallest

detectable

incremental change of inputparameter that

can be detected in the output

signal


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Precision

The concept of precision refers to the

degree ofreproducibilityof a measurement


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Hysterisis

A sensor should be capable offollowing the changes of the input

parameterregardless of which direction the

change is made;

hysteresisis the measure of thisproperty


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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 brokendown into four categories:

1. Object detection

2. Object identification

3. Condition detection

4. Sensing for machines and robots

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Object detection:

a) Limit switches:

A 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 aconveyor will close the contacts

just by its own weight

Limit switches can be:

normally closed(NC)normally open(NO)


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Normally Closed NC

Normally Opened NO

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b) Photoelectric Sensors:

A typical photoelectric sensorprovides 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 sensors:

1. Through scanning2.Retro-reflective scanning

3.Diffuse (proximity) scanning


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An important characteristic to be

noted when considering

photoelectric sensing is

hysteresis, which the percent ofthe received light beam that must

be broken to cause an on oroff condition.

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This prevents the possibility of

multiple sensings if the part

vibrates while moving throughthe beam.

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c) Proximity Sensing:

Proximity detectors are electrical

or electronic sensors that respond

to the presence of a material.

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The major categories are:

inductive:

for detection of

steelchrome-nickel

stainless steel

brass

aluminum

copper parts

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magnetic:

for the detection offerrous materials

capacitive:

for detection of

steelwater

wood

glassplastics

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Object Identification:

1. Bar codes

2. Magnetic stripes

3. 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 Conditions:

To measure the environmental conditions

transducersare used. A transducerinputs 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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L inear displacement measurements:

Strain gage

Linear potentiometerLinear encoder

Capacitive transducer

LVDT (linear voltage differential transformer)

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LVDT (linear variable differential tranformer)

is a transducer for converting positionalinformation into an electrical signal. It is

wound as a transformer with a single primary

and two secondary wound around acylindirical 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

measurements:

Strain gage

Angular potentiometer

Rotary encoderCapacitor

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Force and pressure measurements:

Diaphragm-strain gage

Strain gageBellow-potentiometer

Piezo-electric

LVDT (linear voltage differentialtransformer)

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Temperature measurements:

ThermocoupleOptical pyrometer

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Machine Tool Sensing:

The 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 Measurement:

Other than the positioning of the tool, the

positioning system of the machine tools maybe 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 emittedfrom 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 measurement:

Tactile probe

Force measurement on the tool

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Robot Sensing:

Tactile sensing

Force sensing

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Ch2 Measurement Analysis and Actuation

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