MME 3112 LECTURE 5MME 3112 LECTURE 5DISCRETE CONTROL WITH PROGRAMMABLE LOGIC DISCRETE CONTROL WITH PROGRAMMABLE LOGIC

CONTROLLERS (PLCS):CONTROLLERS (PLCS):

LOGICAL SENSORSLOGICAL SENSORS

INSTRUCTOR: ISRAD HAKIM JAAFARDEPARTMENT OF MANUFACTURING & MATERIALS

ENGINEERING

MANUFACTURING AUTOMATION

Discrete Control

Review from last lecture.

Depends on Depends on ApplicationApplication

Handled by Logical Sensors & Handled by Logical Sensors & ActuatorsActuators

Discrete Control (2)

PLC makes decision to update outputs to

actuators.

Binary signal

Binary signal

Sensors: Sensors: read

current state of system

ActuatorsActuators::

Drive system to new states

Discrete Control (3)

Purpose of the Controller coordinate the various actions of the physical systemthe physical system solve the programmed logic programmed logic based on the inputs and gives

a resultant output

Examples of actions of physical system: transferring parts onto the work holder feeding the machining work head separating parts on a conveyer belt

Discrete process control can be divided into: Logic controlLogic control, for event-driven changes in the system; and Sequential controlSequential control, for time-driven changes in the system.

Sensors and Actuators in Discrete Control

Logical (discrete) Logical (discrete) Sensors and Actuators typically used.

Sensor “1”/”0” Actuator “1”/”0”

Limit Switch Contact/No Contact

Motor On/Off

Photodetector

On/Off Control Relay Contact/No Contact

Push-Button switch

On/Off Light On/Off

Timer On/Off Valve Closed/Open

Control Relay Contact/No Contact

Clutch Engaged/Not Engaged

Circuit Breaker

Contact/No Contact

Solenoid Energized/Not Energized

Common Interpretation of Binary Signals for Various Logical Sensors & Actuators

Discrete Control ExampleDiscrete Control ExampleDiscrete Sensors and Actuators Discrete Sensors and Actuators

Motor control with PLC controller Discrete sensors (push button switches) Discrete Actuator (motor)

An Initial Example: Motor Control with PLC

Typical Push Buttons: Normally Closed (NC)

Switch is in the ON position until you press it. When we press, it will be in the OFF position. When we release, it will be back in the ON

position

Normally Open (NO) Switch is in the OFF position until you press

it. When we press, it will be in the ON position. When we release, it will be back in the OFF

position.

Schematic Symbol

An Initial Example: Motor Control with PLC

Push buttonsensors

Motor actuatorOverload

relay sensor

We’ll talk about relays later.

An Initial Example: Motor Control with PLC

StartStart button (sensorsensor) pressed. Controller receives logic “11” from input I0.0

All three inputs are now a logic “11”. First rung is connected.

Controller sends logic “11” to Motor starter (actuatoractuator) at output O0.0

Motor starts.

An Initial Example: Motor Control with PLC

The output status bit for O0.0 is now a “11”. Finger released but O0.0 is still connected via

2nd rung. Motor remains running.

An Initial Example: Motor Control with PLC

Stop button pressed. Controller received logic “00” from I0.1 It disconnects the rung. Power cannot go through to output

O0.0.Controller sends logic “00” to Motor starter (actuatoractuator) at

output O0.0 Motor stops.

Logical SensorsLogical Sensors

Logical Sensors

Sensors allow a PLC to detect the state of a process.

Logical sensors can detect a state that is either TRUE or FALSE.

It measures a physical quantity, & converts it to a logical signal which can be read by the PLC

“0”

“1” “1” in this case may mean a fire/heat is present

Logical Sensor

Sensor Wiring

When sensor detects a logical change, it signals that change to the PLC

“0”

“1”

Sensors provide input to PLC. Output of a Sensor is Input to a PLC.

Signal change “0” to

“1”

Sensor Wiring (2)

How this signal is relayed to the PLC depends on the sensor wiring.

Several types:1. Plain Switches

Voltage switched ON or OFF

2. Transistor-to-Transistor Logic (TTL) Uses 0V for “OFF”, 5V for “ON”

3. Sinking and Sourcing Current switched ON or OFF

4. Solid State Relays Switches AC to DC or DC to AC

1. Plain Switches

Simplest example of logical sensor.

Sensor must be separately powered V+ and V-connected

separately to 24 Vdc supply.

When physical phenomenon detected, an internal switch (a relay) will close allowing V+ to be connected to PLC Input 06 shown. (“1”, ON, TRUE)

We’ll talk about relays later.

Electrical Wiring Diagram showing connection of 24 Vdc power supply, Push Button (NO), and Plain Switch sensor to a PLC Input Card

2. Transistor-Transistor Logic (TTL)

TTL is based on 2 voltage levels. 0 V = FALSE 5 V = TRUE

Susceptible to Noise (when ~0V, ~5V or in “ambiguous range”).

Improved by employing Schmitt TriggerSchmitt Trigger.

Schematic symbol for Schmitt Trigger

2. TTL - Schmitt Trigger

Output retains the same value until input changes sufficiently to “trigger” a change in the output.

Eliminates influence of noise to PLC input.

5V

0V

LTL

HTL

2. TTL - Schmitt Trigger (2)

•If input above HTL 5 V •If input below LTL 0 V•If voltage is in an ambiguous range, about 1.5-3.5V it will be ignored.

3. Sinking & Sourcing

Sinking Allows current to flow into sensor, and to the voltage

common

Sourcing Allows current to flow out of the sensor from a positive

source.

Emphasis is on current flow, not voltage.

Sinking Sensor (NPN)

Sensor has part that detects physical phenomena Sensor needs supply voltage supply (V+ and V-) to operate Phenomenon detected Active Line triggered Active Line connected to NPN transistor If voltage to transistor on Active Line is at 0V Transistor OFF,

current does not flow through. Phenomenon detected. Voltage to transistor on Active Line is

triggered to High Transistor ON, current flows into NPN, through to the voltage common V-.

NPN transistor always has arrow pointing away from center

Sinking Sensor (2)

Schematic symbol for NPN sensor

Sinking Sensor (3)

Electrical Wiring Diagram showing connection of 24VDC power supply and NPN sensor connected to a PLC Input Card

When PLC Input card does not

have a common (COM) but it has a V+ instead, it can be used for NPN sensors. Current flows out of card and “sinks” into the NPN sensor when sensor is

ON.

Sinking Sensor (4)

Electrical Wiring Diagram showing connection of 24VDC power supply and NPN sensor to a PLC Input Card

Current flowCurrent flow when NPN sensor

is “ON”, i.e. phenomenon

detected.Notice current current

fl0ws into NPN fl0ws into NPN outputoutput. NP

N

“Sinking” sensor with “Sourcing” PLC Input Card

V+

V-

Sourcing Sensor (PNP)

No phenomenon detected, Active Line stays at V+ value Transistor OFF, this is like an open switch.

Phenomenon detected, sensor is activated. Active Line Triggered to 0V Transistor ON, allow current to flow out of PNP.

PNP transistor always has arrow pointing towards center

Sourcing Sensor (2)

Schematic symbol for PNP sensor

Sourcing Sensor (3)

Electrical Wiring Diagram showing connection of 24VDC power supply and PNP sensor connected to a PLC Input Card

When PLC Input card has common

(COM) it can be used for PNP

sensors. Current flows out or

“sourced” from PNP sensor

“sinks” into the common when sensor is ON.

PNP

V+

V-

Sourcing Sensor (4)

Electrical Wiring Diagram showing connection of 24VDC power supply and PNP sensor to a PLC Input Card

Current flowCurrent flow when PNP sensor

is “ON”, i.e. phenomenon

detected.Notice current current

fl0ws out of PNP fl0ws out of PNP outputoutput.

“Sourcing” sensor with “Sinking” PLC Input Card

PNP

V+

V-

Sinking and Sourcing Sensors Sinking and Sourcing Sensors to turn ON a Lightbulbto turn ON a Lightbulb

Sensor Wiring: Sinking Sensor / NPN

LOAD

Sensor detects phenomena NPN output turns “ON”

NPN output “sinks” or “pulls” current through the load to the common, V-

Light turns ONLight turns ON

Sensor Wiring: Sourcing Sensor / PNP

Sensor detects phenomena PNP output turns ON

PNP outputs “sources” or “pushes” current through the load to the common, V-

Light turns ON

LOAD

Question

Why is a PNP or NPN sensor not like a Plain Switch?

Example ProblemExample Problem

EXAMPLE PROBLEMSketch the connections required for the PLC

input and output cards shown. The input is a NO push button and an optical

sensor that has both NPN and PNP outputs. The output is 24Vdc light.

EXAMPLE PROBLEM: Solution

V+

PNP

V-

4. Solid State Relays

An electronic switching device in which a small control signal controls a larger load current or voltage.

It has a voltage or current sensor which responds to an appropriate input (control signal), a solid-state electronic switching device of some kind which switches power to the load circuitry either ON or OFF, and some coupling mechanism to enable the control signal to activate this switch without mechanical parts.

The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relayelectromechanical relay, but has no moving partsno moving parts.

We’ll talk about relays later.

MME 3112 Assignment 1

Sketch the electrical wiring for diagrams for PLC Input cards that have: 3 NO push buttons 1 NO thermal relay 3 sinking sensors 1 sourcing sensors

Sourcing Sensor

Sinking Sensor

JIC Schematic Symbols

MME 3112 Assignment 1

Submit next Monday (5 March) in Class.Write NameName, Matric #Matric #, Assign #Assign #.