Input - Output Devices for PLC

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Presentation on
Input – Output Devices used with PLC
Presented By:
Name: Arko Saha
Roll: 138014
Department of Mechatronics Engineering
Rajshahi University of Engineering & Technology
Course No. : MTE 4101
Course Name : Automation

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PLC Structure
Input
Central
Processing
Unit
Output

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PLC Structure
Input
Central
Processing
Unit
Output

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I/O Module
• The I/O interface section of a PLC connects it to external field devices.
• The main purpose of the I/O interface is to condition the various signals received from or
sent to the external input and output devices.
• Input modules converts signals from discrete or analog input devices to logic levels
acceptable to PLC’s processor.
• Output modules converts signal from the processor to levels capable of driving the
connected discrete or analog output devices.
O : 2 / 00
Input Module
Number
Terminal
Number
Output Module
Number
Terminal
Number
I : 1 / 00

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Sensors which give digital/discrete, i.e. on/off signal to the input ports of the PLC.
• Accuracy
• Range
• Response Time
• Rise Time
• Sensitivity
• Stability
• Repeatability
• Reliability
Input Devices

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Mechanical Switch:
A mechanical switch generates an on−off signal or signals as a result of some mechanical
input causing the switch to open or close.
Input Devices
• Switches are available with normally open
(NO) or normally closed (NC) contacts.
• NO switch has its contacts open in the
absence of a mechanical input and the
mechanical input is used to close the
switch.
• NC switch has its contacts closed in the
absence of a mechanical input and the
mechanical input is used to open the
switch.

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Input Devices
Limit Switch:
A switch which is used to detect the presence or passage of a moving part. It can be
actuated by a cam, roller or lever.

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Input Devices
Proximity switches:
Proximity switches are used to detect the presence of an object without making contact with it. There are a
number of forms of such switches, some being only suitable for metallic objects.
Eddy Current type:
It has a coil which is energized by a constant alternating current and produces a
constant alternating magnetic field. When a metallic object is close to it, eddy
currents are induced in it.
The magnetic field due to these eddy currents induces an e.m.f. back in the coil
with the result that the voltage amplitude needed to maintain the constant coil
current changes. The voltage amplitude is thus a measure of the proximity of
metallic objects.

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Reed switch:
Consists of 2 overlapping, but not touching strips of a springy ferromagnetic
material sealed in glass or plastic casing. When the magnet is brought close to
the switch, the strips become magnetized and are attracted to each other,
causing the contacts to close.
Capacitive proximity switch:
A proximity switch that can be used with metallic and non-metallic
objects. The sensor of the capacitive proximity switch is just one of
the plates of the capacitor, the other plate being the metal object
whose proximity is to be detected. Thus the proximity of the object is
detected by a change in capacitance.
Input Devices

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Inductive proximity switch:
Consists of a coil wound round a ferrous metallic core. When one end of this core is
placed near to a ferrous metal object there is effectively a change in the mount of metallic
core associated with the coil and so its inductance changes. Inductance change can be
measured by resonant circuit and can be made to activate a switching circuit.
Input Devices

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Input Devices
Photoelectric switch:
1. Transmissive type
2. Reflective type
Encoder:
The term encoder is used for a device that provides a digital
output as a result of angular or linear displacement.
• An increment encoder detects changes in angular or
linear displacement from some datum position.
• An absolute encoder gives the actual angular or linear
position.

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Fig: Incremental Encoder Fig: Absolute Encoder
Input Devices
Encoder:

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Input Devices
 Temperature sensors
Thermocouple
LM3911N circuit for on–off control
Bi-Metallic Strip
RTD

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Input Devices
Figure: LVDT
 Displacement Sensor:

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Input Devices
 Strain Gauge
 Pressure Sensors

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Input Devices
 Liquid Level Sensors
 Orifice Flow Meter

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 Smart Sensors
• The term smart sensor is used for a sensor which is integrated with the required buffering
and conditioning circuitry in a single element.
• The circuitry with the element usually consists of data converters, a processor and
firmware, and some form of non-volatile EEPROM memory.
• Such sensors can be programmed for specific requirements.
• The IEEE 1451.4 standard requires the non-volatile EEPROM embedded memory to hold
and communicate data which will allow a plug-and-play capability. It thus would hold
data for the identification and properties for the sensor and might also contain the
calibration template, so facilitating digital interrogation.

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Output devices
 Relay
• The digital signal from the output channel of a PLC is used to control an actuator
which in turn controls some process.
• The term Actuator is used for the device which transforms the electrical signal into
some more powerful action which then results in the control or action in the process.
 D.C. Motor

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Output devices
 Stepper Motor

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Output devices
 DCV
 Solenoid Operated Valve

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Examples of applications
 Conveyor belt  Liquid Level Monitoring
 Lift
 Robot Control System
 Industrial Applications

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References:
Programmable Logic Controllers
Fourth Edition
W. Bolton

Input - Output Devices for PLC

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