plc experiment 1

8/11/2019 PLC experiment 1

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Notre Dame UniversityFaculty of Engineering

ECCE Department

EEN 365Programmable Logic Controller Laboratory

Experiment 1Introduction to industrial sensors

Eng. Yves Mansour


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1. Objective: the object of this experiment is to describe the construction andcharacteristics of the different types of industrial sensors and their application, suchas mechanical switches, magnetic, inductive, capacitive, optical and fiber opticsensors.

2. Background information:

2.1 Push button and control switches

Switches are installed in circuits to apply a current to a load or to interrupt the circuit. Theseswitches are divided into pushbuttons and control switches.

1. Control switches are mechanically detented in the selected position. The switch position remains

unchanged until a new switch position is selected. Example: Light switches in the home.

2. Push button switches only maintain the selected position as long as the switch is actuated

(pressed). Example: Bell push.

In the case of a normally open contact, the circuit is open if the switch isNormally open contact inits initial position (not actuated). The circuit is closed by pressing the(make) push button currentflows to the load. When the plunger is released, the spring returns the switch to its initial position,interrupting the circuit.

In the case of a normally closed contact, the circuit is closed when the switch is in its initial position.Pressing the pushbutton interrupts the circuit.


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In the case of a changeover contact, the changeover contact combines the functions of thenormally open and normally closed contacts in one device. Changeover contacts are used to closeone circuit and open another in one switching operation. The circuits are momentarily interruptedduring changeover.


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2.2 Displacement and Pressure Sensors

Sensors have the task of measuring information and passing this on to the signal processing part ina form that can easily be processed.In electro pneumatic controllers, sensors are primarily used for the following purposes:

1. To detect the advanced and retracted end position of the piston rod in cylinder drives

2. To detect the presence and position of work pieces

3. To measure and monitor pressure

A limit switch is actuated when a machine part or work piece is in a certain position. Normally, acam effects actuation. Limit switches are normally changeover contacts. They can then beconnected as required

As a normally open contact, normally closed contact or changeover contact.


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2.3 Proximity switches

In contrast to limit switches, proximity switches operated contactlessly (non-contact switching) andwithout an external mechanical actuating force. As a result, proximity switches have a long servicelife and high switching reliability.The following types of proximity switch are differentiated:

1. Reed switch (magnetic sensors)2. Inductive proximity switch3. Capacitive proximity switch4. Optical proximity switch

2.3.1 Reed switch (magnetic sensor)

Reed switches: are magnetically actuated proximity switches. They consistof two contact reeds in a glass tube filled with inert gas. The field of a magnet causes the two reedsto close, allowing current to flow. In reed switches that act as normally closed contacts, smallmagnets close the contact reeds. This magnetic field is overcome by the considerably stronger

magnetic field of the switching magnets.Reed switches have a long service life and a very short switching time (approx. 0.2 ms). They aremaintenance-free, but must not be used in environments subject to strong magnetic fields (forexample in the vicinity of resistance welders).

Inductive, optical and capacitive proximity switches are electronic sensors. They normally havethree electrical contacts.1. Contact for supply voltage2. Contact for ground


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3. Contact for output signal

In these sensors, no movable contact is switched. Instead, the output is either electricallyconnected to the supply voltage or to ground (= output voltage 0 V).There are two types of electronic sensor with regard to the polarity of the output voltage.

1. In positive switching sensors, the output voltage is zero if no part issensors detected in the

proximity. The approach of a work piece or machine part leads to switchover of the output,applying the supply voltage.

2. In negative switching sensors, the supply voltage is applied to the output if no part isdetected in the proximity. The approach of a work piece or machine part leads to switchoverof the output, switching the output voltage to 0 V.

2.3.2 Inductive proximity sensors

An inductive proximity sensor consists of an electrical oscillator (1), a flip-flop (2) and an amplifier(3). When a voltage is applied, the oscillator generates a high-frequency alternating magnetic fieldthat is emitted from the front of the sensor. If an electrical circuit is introduced into this field, the

oscillator is attenuated. The downstream circuitry, consisting of a flip-flop and an amplifier,evaluates the behavior of the oscillator and actuates the output. Inductive proximity sensors can beused for the detection of all good electrical conductors (materials). In addition to metals, theseinclude, for example, graphite.


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2.3.3 Capacitive proximity sensor

A capacitive proximity sensor consists of a capacitor and an electrical resistance that togetherforms an RC oscillator, and a circuit for evaluation of the frequency. An electrostatic field isgenerated between the anode and the cathode of the capacitor. A stray field forms at the front ofthe sensor. If an object is introduced into this stray field, the capacitance of the capacitor changes.The oscillator is attenuated. The circuitry switches the output. Capacitive proximity sensors not only

react to highly conductive materials (such as metals) but also to insulators of high dielectricstrength (such as plastics, glass, ceramics, fluids and wood).

2.3.4 Optical proximity sensor

Optical proximity sensors use optical and electronic means for object detection. Red or infraredlight is used. Semiconductor light-emitting diodes (LEDs) are particularly reliable sources of red orinfrared light. They are small and rugged, have a long service life and can be simply modulated.Photodiodes or phototransistors are used as a receiver. Red light has the advantage that the light

beam can be seen during adjustmentof the optical axes of the proximity switch. Polymer optical fibers can also be used because of theirlow attenuation of light of this wavelength.Three different types of optical proximity switch are differentiated:1. One-way light barrier2. Reflective light barrier3. Diffuse reflective optical sensor


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The one-way light barrier has spatially separate transmitter and receiver units. The parts aremounted in such a way that the transmitter beam is directed at the receiver. The output is switchedif the beam is interrupted.

In the reflective light barrier, the transmitter and receiver are mountedtogether in one housing. Thereflector is mounted in such a way that the light beam transmitted by the transmitter is practicallycompletely reflected to the receiver. The output is switched if the beam is interrupted.

In the diffuse reflective optical sensor, the transmitter and receiver are mounted together in oneunit. If the light hits a reflective object, it is redirected to the receiver and causes the output of the

sensor to switch. Because of the functional principle, the diffuse reflective optical sensor can onlybe used if the material or machine part to be detected is highly reflective (for example polishedmetal surfaces, bright paint).


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2.4 Relays

A relay is an electromagnetically actuated switch. When a voltage is applied to the solenoid coil, anelectromagnet field results. This causes the armature to be attracted to the coil core. The armatureactuates the relay contacts, either closing or opening them, depending on the design. A returnspring returns the armature to its initial position when the current to the coil is interrupted


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Rotary encoder

Whenever mechanical rotary motions have to be monitored, the encoder is the mostimportant interface between the mechanics and the control unit. Encoders transform rotarymovement into a sequence of electrical pulses. A number of bright/dark segments on adisc are photo electrically scanned. The number of segments determine the resolution of

the movement and hence the accuracy of the position. Because only increments of rotationare detected with a single channel, an additional signal, phase shifted by 90 degrees, isgenerated by inserting a grid diaphragm. This 90 degrees out of phase signal enables,together with the initial signal, the direction of rotation to be determined. Once perrevolution a reference signal may also be generated.

Pulse disc:


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Exercise 2: Inductive proximity sensors

Equipments used:

1. Power supply2. Sequence Indicator Module (SIM)3. Inductive proximity sensor

ConnectionsConnect 0V of the Sequence Indicator Module to 0V of the power supply,For the inductive sensor the wiring connections is as follow:Brown (bn): to 24VDCBlue (bl): to 0VDCBlack (bk): to the SIM

Application

By placing a metallic object in front of the magnetic sensor the corresponding LED in the SIM willlight on and offTest this sensor on non-metallic objects, such as wood, plastic, paper. Note the results

Exercise 3: Capacitive proximity sensor

Equipments used:

1. Power supply2. SIM3. Capacitive proximity sensor

Connections

Connect 0V of the Sequence Indicator Module to 0V of the power supply,For the capacitive sensor the wiring connections is as follow:Brown (bn): to 24VDCBlue (bl): to 0VDCBlack (bk): to the SIM

ApplicationBy placing a metallic object in front of the magnetic sensor the corresponding LED in the SIM willlight on and offTest this sensor on non-metallic objects, such as wood, plastic, paper. Note the results

Exercise 4: Optical proximity sensor

Equipments used:

1. Power supply2. SIM3. One-way light barrier4. Reflective light barrier5. Diffuse reflective optical sensor


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Connections

Connect 0V of the Sequence Indicator Module to 0V of the power supply,For the optical proximity sensor the wiring connections is as follow:Brown (bn): to 24VDCBlue (bl): to 0VDC, for the reflective light barrier the white wire should be connectedBlack (bk): to the SIM, for the reflective light barrier the black wire should not be connected

Note that (except for the reflective light barrier) the white color wire if connected to 0VDC theoutput of the sensor is normally open (NO), if connected to 24VDC the output of the sensor isnormally closed (NC)

Application1. In the one-way light barrier place a metallic object between the transmitter and the receiver, andnote the result.Test this sensor on non-metallic objects, such as wood, plastic, paper. Note the results

2. In the reflective light barrier place a metallic object between the sensor and the reflector, andnote the result.Test this sensor on non-metallic objects, such as wood, plastic, paper. Note the results

3. In the diffuse reflective optical sensor place a metallic object in front of the sensor, and note theresult.Test this sensor on non-metallic objects, such as wood, plastic, paper. Note the results

Exercise 5: Fiber optic sensor

Equipments used:1. Power supply2. SIM3. Fiber optic sensor4. Fiber optic cable

ConnectionsConnect 0V of the Sequence Indicator Module to 0V of the power supply,For the the wiring connections is as follow:Place the fifer optic cable in its position in the fiber optic sensorBrown (bn): to 24VDCBlue (bl): to 0VDCBlack (bk): to the SIMNote that the white color wire if connected to 0VDC the output of the sensor is normally open (NO),if connected to 24VDC the output of the sensor is normally closed (NC)

Application

In the fiber optic sensor application place a metallic object between the transmitter and the receiver,and note the result.Test this sensor on non-metallic objects, such as wood, plastic, paper. Note the resultsMake a black mark on a white paper fix the fiber optic sensor at a fixed distance than move thepaper up and down, comment on the result.


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plc experiment 1

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