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PROXIMITY SENSORS

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					            ME 445 INTEGRATED MANUFACTURING TECHNOLOGIES
                             EXPERIMENT 1
                          "PROXIMITY SENSORS"

OBJECTIVE

Increasing automation of complex production systems necessitates the use of components
which are capable of acquiring and transmitting information relating to the production
process. Sensors fulfill these requirements and have therefore in the last few years become
increasingly important components in measuring and in open and closed loop technology.
Sensors provide information to a controller in the form of individual process variables.

Proximity sensors are the most basic data acquisition devices in automation. They measure /
detect physical input such as temperature, pressure, force, length, and proximity of an object.
Transducers are typically a sensorial system capable of signal processing, equipped with
electronic instrumentation Position sensors give a yes or no response according to the place of
the object.

The aim of this experiment is to illustrate the aspects of different types of proximity sensors,
their properties, and to compare them. For this, a setup table containing Mechanical, Magnetic
,Inductive, Capacitive, Optical, and Ultrasonic sensors is used. A positioning slide coupled
with a vernier caliper is used to measure switching distances.




                             Figure: Proximity sensors setup table
GENERAL INFORMATION
Sensors are the first of the four milestones of Automation:
1. Sensing
2. Signal Processing
3. Planning and Response
4. Memory

They usually convert some physical data into a voltage difference for further processing by a
Computer, PLC or I/O Card. The advantages of proximity sensors are:
 They determine the geometrical positions automatically and sensitively.
 They do not need of a direct contact with the workpiece.
 They do not have movable parts that can wear out.
 They are usually equipped with electronic circuits for failure protection.
 They have various types that can be used under different situations.
 They provide the secure working of the process.
 They are used for the system failure-analysis.

Their typical usage areas are:
 Automotive industry,
 Packaging industry,
 Printing and paper industry
 Ceramic industry
 Wood-working industry
 Food processing industry

CATEGORIES
According to I/O processing:
 Binary: Convert a physical measurement value to a binary code (in the form of ON/OF
   signals in a selected voltage range)
 Analog: Convert a physical measurement in to an analog signal (e.g. temperature readings
   to variable voltage differences)

According to physical considerations:
 Mechanical switches
 Magnetic (with/without contacts, pneumatic output)
 Inductive (inductive sensors)
 Capacitive (capacitive sensors)
 Optical (light barriers, reflection sensors)
 Ultrasonic (ultrasonic barriers, ultrasonic sensors)
 Pneumatic (back-pressure nozzles, air reflection sensors, air barriers)
TYPICAL USAGE

Detecting whether an object exists in a defined position:




Positioning of an object:




Counting the number of parts:
Determining the rotational speed




Determining the linear speed
TYPES

1. Mechanical switches:
Mechanical switches are simple GO/NoGO indicators. They have physical contact with the
object, usually coupled with relays and contactors to drive a circuit. Widely used in the
industry to mark the end-start points of cylinders, pistons, linear- and rotary drives, to sense
doors. They are less sensitive and have lower maximum switching frequency compared to
proximity switches. Because of the physical contact with the object, they require maintenance
and replacement.

2. Magnetic Proximity Switches:
Magnetic switches (also called as Reed-contacts) use the distortion of the magnetic field. If a
ferromagnetic material (Fe-Ni compound) comes in the vicinity, the magnetic field distorts
and gives an input to the switch. Thus, they are only sensitive to ferromagnetic materials and
magnetic fields. Dirt and humidity is of little importance. They preserve high hysterisis
(undefinite range of physical input). They are widely used in pairs of machine parts such as
piston-cylinder arrangements.

3. Inductive Proximity switches:
Inductive proximity switches also work on the principles of magnetic fields and induction.
They response to conductive materials, typically metals. The tabular data on switching
sensitivity depends on mild steel (usually Fe37); thus, a reduction coefficient must be defined
for different metals. For the metals such as Cr-Ni, brass, aluminum, and copper this value
must be modified with the experimental reduction coefficient found usually in the range of
0.25-0.9. Also the reduction coefficient depends on the size of the measured object. They are
widely used in the mass production lines and conveyors to detect metallic workpieces,
moving parts of machinery, for measuring linear-, rotational speeds, presses, and encoders.

4. Capacitive Proximity switches:
Unlike the magnetic and inductive types, capacitive proximity switches response to all types
of materials. The reduction coefficient is determined experimentally found in the range of 0.1
to 1 (metals =1 and water =1). Note that liquids can also be detected by capacitive switches.
They are very sensitive to environmental factors such as dust, dirt and humidity. Therefore
they can be used to distinguish object properties such as color, thickness, water column
height, and vibration. Sample application areas are in production lines and conveyors to count
workpieces, sense packaging defects etc.

5. Optical Proximity switches:
Optical proximity switched use the presence of visible (with wavelength of 660nm -red-) or
invisible (with wavelength of 880nm -ultra-red-), light for input. They give a NPN or PNP
output to the circuit. Here, instead of the reduction coefficient the operation reserve is defined
as the ratio of signal intensity in the input of the sensor to the required intensity for switching.
Note that in correct working conditions, operation reserve must have a value of greater than
one. The operation reserve depends on ambient conditions such as dust, dirt, ambient light
color and intensity, distance from part, reflect-angle etc.
Optical sensors are divided into two main parts:
 Light sensors (can be equipped with fiber-optic cabling for long distance transmission,
    may use ambient light or the light produced in a coupled unit)
   Reflected light sensors (can be equipped with fiber-optic cabling for long distance
    transmission, uses the reflected light produced in the same unit from the part or a reflector
    sheet)

Optical sensors have a relatively greater switching distance. Therefore they may be used in
detecting surface irregularities, failure detection, detection of transmissive surfaces, colors
etc. Fiber optic cabling for transmission also gives a flexibility to use small units at difficult
locations.

6. Ultrasonic Proximity switches:
They use the reflected sound power for input. Note that above the sensors stated here,
ultrasonic proximity switches have the greatest switching distance and frequency. Therefore,
they are used to detect distant objects with very high speeds. They are usually insensitive to
ambient conditions and should be preferred in very extreme conditions, while they are very
expensive.

7. Pneumatic Proximity switches:
They use the reflected back-pressure supplied from a nozzle at or distant from the switch unit.
Generally preferred in the areas of:

   Very dirty and dusty places,
   At high temperatures,
   In the vicinity of explosive materials where electrical currents may be dangerous,
   At places where intensive magnetic fields are present, in the vicinity of big motors,
    pumps, turbines etc.
   The sensor unit and nozzle unit may be built in one package or as different units. Can be
    used to drive a pneumatic piston directly.
SELECTION CRITERIA
PROTECTION CLASSES

The protection classes of the mechanical elements are defined in DIN 40050. For example,
IP67 represents a device with protection against contact and foreign material according to 6
(Table A1) and against water and humidity 7(Table A2).

 First   Protection Class
 digit
 0       No special protection
 1       Protection against solid objects larger than 50 mm diameter. Unprotected against
         forced contacts (eg. via hand). Should be kept apart from the body
 2       Protection against solid objects larger than 12 mm diameter. Should be kept apart
         from the fingers
 3       Protection against solid objects larger than 2.5 mm diameter. Should be kept apart
         from the devices (wire, hand tools etc.)
 4       Protection against solid objects larger than 1 mm diameter. Should be kept apart
         from the devices (wire, hand tools etc.)
 5       Protection against hazardous dust accumulation. Dust protection is not totally
         achieved, but inner dust accumulation does not affect functioning of the device. Full
         protection against forced contact.
 6       Full protection against dust accumulation. Full protection against forced contact.
                       Table: Protection against dust & forced contact.

Second      Protection Class
digit
0           No special protection
1           Protection against vertically tipping water. The water has no hazardous effects
            (tipping water).
2           Protection against vertically tipping water at 15  to the normal of the device
            surface. The water has no hazardous effects (inclined tipping water).
3           Protection against water tipping at 60 to the normal of the device. The water
            has no hazardous effects (sprinkling water)
4           Protection against water from any direction to the device. The water has no
            hazardous effects (flowing water)
5           Protection against water from a nozzle coming from any direction to the device.
            The water has no hazardous effects (flowing water)
6           Protection against water forced water coming from any direction to the device.
            The water has no hazardous effects (forced water)
7           Protection against water in case of immersion at certain pressure for a specific
            time Leakage of the water into the device is avoided.
8           Full protection against water in case of immersion for a predetermined period of
            time (permanent immersion).
                              Table: Protection against water
DEFINITIONS

Object material: The material of the object to be sensed. Note the under non-ideal
circumstances reduction factors are defined. All tabular data about the properties of the sensor
are based on identifying the indicated object under ideal circumstances.

Switching Voltage: The operating supply/output voltage of the sensor. The sensor must
definitely be operated at the permitted voltage range. For most industrial applications
typically 5V DC, 12-24V DC, 110-220 V AC.

Switching Distance: The maximum distance of the object to be sensed from the head of the
sensor. Reduction factors about the environment and object properties not applied.

Max. Current: The maximum allowable current at the sensor output. To avoid excess
currents a protection circuit may be necessary.

Protection Class: The physical protection of the industrial device against foreign material,
dust, water and humidity. Defined in DIN 40050. Generally related with the construction.

Life: The theoretical life of the device. Indicated as time or in operating cycles.

Switching Frequency: The maximum occurrence of the object material at the switching
distance of the sensor in one second.

Reduction factor: The ratio of switching distance of metals (typically Fe37) of a to other
materials sensor at the same ambient conditions. Some guide values are given in the table:

 Material                                          Reduction factor
 All metals                                        1.0
 Water                                             1.0
 Glass                                             0.3 to 0.5
 Plastic                                           0.3 to 0.6
 Cardboard                                         0.3 to 0.5
 Wood (depends on humidity)                        0.2 to 0.7
 Oil                                               0.1 to 0.3

                           Table: Reduction factor of some materials

Hysteresis: The distance between swich-on and switch-off position of a sensor.
EXPERIMENTAL DATA

The following equipment is contained on the setup table. In the experiment, you may use this
list as a reference to distinguish between equipment.

Component                                                                      Designation
Proximity Sensor, non-contact, inductive-magnetic                              167055
Reed switch                                                                    167056
Optical proximity sensor with fiber optic connector, block shaped (2 pieces)   167065
Diffuse reflective optical sensor, block shaped                                167068
Optical sensor with fiber optic connector, cylindrical, M18                    167166
Inductive Proximity Sensor, cylindrical, M12                                   177464
Inductive Proximity Sensor, cylindrical, M18                                   177466
Capacitive proximity switch, cylindrical, M18                                  177470
Ultrasonic proximity sensor, cylindrical, M18                                  184118

                                      Table: List of sensors

Component                                                                      Designation
Reflector unit for reflex light barrier                                        150504
Optical fiber for one-way light barrier (2 pieces)                             150505
Optical fiber for diffuse reflective optical sensor                            150506
One way light barrier, transmitter                                             167064
One way light barrier, receiver                                                167067

                             Table : List of optical fibers & barriers

Component                                                                      Designation
Set of test objects                                                            034083
Graph paper, mm grid                                                           034085
Positioning slide                                                              034094
Adapter set                                                                    035651
Vernier caliper                                                                035653
Digital multimeter                                                             035681
Ruler                                                                          035697
Distributor unit                                                               162248
Counter unit                                                                   162252
Rotary unit                                                                    167097

                               Table: List of auxiliary equipment
Part no   Material, Dimensions (mm)
1         Magnet 1
2         Magnet 2
3         Mild steel (St 37), 90 x 30
4         Stainless steel, 90 x 30
5         Aluminium, 90 x 30
6         Brass, 90 x 30
7         Copper, 90 x 30
8         Cardboard, 90 x 30
9         Rubber, 90 x 30
10        Plastic, transparent, 90 x 30
11        Mild steel (St 37), 30 x 30
12        Mild steel (St 37), 25 x 25
13        Mild steel (St 37), 20 x 20
14        Mild steel (St 37), 15 x 15
15        Mild steel (St 37), 10 x 10
16        Mild steel (St 37), 5 x 5
17        Kodak gray card, 100 x 100
18        Plastic, transparent, 100 x 100
19        Plastic, red, 100 x 100
20        Plastic, blue, 100 x 100
21        Plastic, black, 100 x 100
22        Cardboard, white, 100 x100
23        Plastic, 2.0 mm thick, 90 x 30
24        Plastic, 3.0 mm thick, 90 x 30
25        Plastic, 4.0 mm thick, 90 x 30
26        Plastic, 8.0 mm thick, 90 x 30
27        Plastic, 11.0 mm thick, 90 x 30
28        Plastic, 14.0 mm thick, 90 x 30
29        Plastic, 17.0 mm thick, 90 x 30
30        Holder for fiber optic cable
31        Base plate with gear wheels
32        Holding bracket for liquid level measurement, through-beam sensor
33        Beaker
34        2 test screws
35        Valve housing
36        Screw driver

                               Table: List of test objects
PART 1 (Switching characteristics of a contacting magnetic proximity sensor)

The objective of the experiment is to learn about the switching characteristics of a contact
based magnetic proximity sensor (Reed contact) as a function of position and orientation of a
magnet.

Setup
Mount the distribution plate (1), the positioning slide (2), and the magnetic Reed sensor (3,
Designation 167056) on the assembly board. Mount the magnetic sensor laterally offset by 5
cm to the center of the positioning slide. Plug in the electrical power supply and connect the
sensor to the distribution plate. Note that the red color represents (+24V), the blue (0 V or
natural) and the black is the sensorial output (either +24V or 0, ON/OFF). Mount the test
object (Magnet 1) on the positioning slide. Adjust the stroke from -50 to + 50 mm by 5 mm
increments and the distance from +2 to +38 mm with 4mm increments. Enter the response
points into the diagram and the graph paper provided. Repeat the same procedure with the test
object 2 (Magnet 2).




                                   Figure: Setup for part 1
Conclusion
When working with magnetic proximity sensors, one has to take into account that there may
be several switching areas. This can lead to multiple counting when counters are employed.
This effect depends on the field strength of the permanent magnet used, and/or the distance of
the magnet to the proximity sensor.

As can be seen from the response diagram, two or even three switching areas may be
observed, depending on the orientation of the axis of the magnetic poles. This ambiguity of
the output signals can be prevented by attaching the magnet with the correct orientation of the
axis and, given a specific field strength, at the correct distance.

Discussion
Which orientation of the magnet would be appropriate if the magnet is located on a wheel and
for each rotation it should count only once? Is there a similarity of the response diagram and
magnetic field lines, why?
Data sheet for Part 1




X          Y            X    Y           X          Y   X   Y




                            Table: Response positions
PART 2 (Switching characteristics of different types of sensors)

The objective of the experiment is to learn about the switching characteristics of different
types of sensors, their interaction with material, thickness, color. The reduction factors and
hysteresis will be investigated.

Setup
Mount the distribution plate (1), the positioning slide (2) on the assembly board. In this
experiment you will use all other sensors (3) available:




                                   Figure: Setup for Part 2
Data sheet for Part 2


Component                   Workpiece         Switch-On      Switch-Off    Hysteresis
                                              Point          Point
Inductive       Proximity   Mild    Steel
Sensor, cylindrical, M12    (St 37), Part
(177464)                    3
Inductive       Proximity   Mild    Steel
Sensor, cylindrical, M18    (St 37), Part
(177466)                    3


Component                   Workpiece         Switch-     Switch-   Hystere-   Reduction
                                              On          Off       sis        Factor
                                              Point       Point
Inductive       Proximity   Mild      Steel                                    1.0
Sensor, cylindrical, M18    (St 37),
(177466)                    Part 3
""                          Stainless
                            Steel,
                            Part 4
""                          Aluminium,
                            Part 5
""                          Brass,
                            Part 6
""                          Copper,
                            Part 7
Component                        Workpiece         Switch-On    Switch-Off      Hysteresis
                                                   Point        Point
Optical sensor with fiber        Kodak grey
optic connector, cylindrical,    card, white
M18 (167166)                     side, part 17
""                               Kodak grey
                                 card,      grey
                                 side, part 17
""                               Plastic,
                                 transparent,
                                 part 18
""                               Plastic, red
                                 part 19
""                               Plastic, blue,
                                 part 20
""                               Plastic, black
                                 part 21
""                               Cardboard,
                                 white, part 22
""                               Mild      steel
                                 (St37), part 3
""                               Rubber,
                                 part 9

Component                       Workpiece          Switch-On    Switch-Off      Hystere-sis
                                                   Point        Point
Capacitive        proximity Mild Steel (St
switch, cylindrical, M18, 37), Part 3
(177470)
""                            Stainless Steel,
                              Part 4
""                            Aluminium,
                              Part 5
""                            Brass, Part 6
""                            Copper, Part 7
""                            Cardboard,
                              Part 8
""                            Rubber,
                              part 9
""                            Plastic,
                              transparent,
                              part 10
Discussion
Industrial solutions are highly problem dependent so that the selection of sensor for particular
cases is very important. Which sensor would you prefer in an installation ıf you were to count:
1. Automobile wheels,
2. Tiny industrial metallic chips,
3. Plastic cups,
4. Bottles to determine either filled or empty.
PART 3 (Determining the rotational speeds)

The objective of the experiment is to learn about the differences and the application criteria of
rotational speed detection with optical and inductive proximity sensors.
Setup
Mount the distribution plate (1), the counting unit (2), rotary unit (3), Optical sensor with
fiber optic connector, cylindrical, M18, (4, 167166) and Inductive Proximity Sensor,
cylindrical, M12 (177464) on the assembly board. Mount the Optical fiber for diffuse
reflective optical sensor (150506) to the Optical Sensor. The inductive sensor unit is to be
mounted approximately 2mm away from the perforated disc. You will use the counting unit to
read the output pulse frequency and to determine the speed. Use the digital multimeter to read
the motor voltage. Adjust the speed such that the motor voltage is increased in 2V intervals.




                                   Figure 6. Setup for Part 3
The rotational speed is determined by the formula:

       60  f s
RS 
         n

where,
RS: Rotational speed (rpm)
n: Number of actuations per rotation (=8 pulse/rpms for the disc)
fs: Pulse frequency of the output signal (pulse/s)

Data sheet for Part 3

Motor         Output pulse frequency Output pulse frequency Speed RS (rpm)
Voltage       of    optical      sensor of   inductive     sensor
(V)           (167166) (pulse/s)        (177464) (pulse/s)
4
6
8
10
12
14
16
18
20
22
23
INSTRUCTIONS FOR THE EXPERIMENT

Before the Experiment
1. Read your lab manual carefully.
2. You can use the data sheets in your manual provided, or you take photocopies of the data
   sheets and fill them.

During the Experiment
1. Note that the experiment will be conducted by the group members, so be prepared and
   familiar with the setup. The assistant should not answer all your questions or mount items
   to help you.
2. You should take notes in the experiment to prepare a good report.
3. Time is short, be quick to finish everything required.

Grading
1. Your individual contributions in the laboratory will be assessed and graded.
2. Prepare a lab report having Theory, Results, Discussion and Conclusion sections, provide
   your Data Sheets in the appendix.
3. Submit your report one week after the lab date until 17.00 to your assistant.

				
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