unit 2 (plc)

UNIT 2

LOGICAL SENSORS AND ACTUATORS

LOGICAL SENSORS

STUDENTS LEARNING OUTCOME

At the end of the lesson, students will be able

• to define logical sensor used in Industry.

• To identify contact and proximity type of sensors.

• To identify limit switch and its function.

• To identify capacitive proximity sensor and its’

function.

STUDENTS LEARNING OUTCOME

At the end of the lesson, students will be able

• To identify inductive proximity sensor and its’

function.

• To identify different type of optical sensors and

its’ function.

• To identify ultrasonic sensor and its’ function.

LOGICAL SENSOR DEFINITION

Logical sensors are devices used to detect the state of a process and return the state either in true or false condition.

Examples of physical phenomena that are typically detected are :

• inductive proximity - is a metal object nearby?• capacitive proximity - is a dielectric object nearby?• optical presence - is an object breaking a light beam or reflecting light?

• mechanical contact - is an object touching a switch?

TYPE OF LOGICAL SENSOR

Generally logical sensors can be classified into :

• Contact type• Contact implies that there is mechanical contact and a resulting force between

the sensor and the object.• limit switch, tactile sensor

• proximity type• Proximity indicates that the object is near, but contact is not required.

• Capacitive sensor, optical sensor

CONTACT TYPE SENSOR

CONTACT SWITCH

Contact or Limit switches are used to indicate end and beginning position of a moving mechanism

It is operated by depressing the lever on the switch to activate the switching action.

Usually the switch will provide a set of contacts which include NC and NO type.

Roller lever type

hinge lever type

PROXIMITY TYPE SENSOR

OPTICAL ( PHOTOELECTRIC) SENSOR

Optical sensors require both a light source (emitter) and detector

Emitters will produce light beams in the visible and invisible (infra red) spectrums using LEDs

PROXIMITY TYPE SENSOR

OPTICAL ( PHOTOELECTRIC) SENSOR

Detectors are typically built with photodiodes or phototransistors

The oscillating light wave is used so that the sensor can filter out normal light in the room. The freq is in a range of KHz

PROXIMITY TYPE SENSOR

OPTICAL ( PHOTOELECTRIC )SENSOR

When the detector receives the light it checks to make sure that it is at the same frequency as is transmitted

An advantage of the frequency method is that the sensors can be used with lower power at longer distances

PROXIMITY TYPE SENSOR

THROUGH BEAM TYPE OPTICAL SENSOR

This sensor needs two separate components; the transmitter is separated with the detector.

Emitter is set up to point directly at a detector

An object is detected if it interrupts the beam

PROXIMITY TYPE SENSOR

RETROREFLECTIVE TYPE

Transmitter and detector are housed in the same enclosure

Requires the use of a separate reflector or reflective tape mounted across from the sensor to return light back to the detector.

An object is detected if it interrupts the beam

PROXIMITY TYPE SENSOR

DIFFUSE TYPE

Diffuse sensors detect light reflected from object rather than light interrupted by object in the case of through beam and retroreflective.

Easiest to set up, but they require well controlled conditions

For example if it is to pick up light and dark coloured objects problems would result.

PROXIMITY TYPE SENSOR

CAPACITIVE SENSOR Act similar to a simple capacitor.A metal plate, in the end of the sensor, is electrically connected to the oscillator.The object to be sensed acts as a second plate.

When power is applied to the sensor, the oscillator senses the external capacitancebetween the target and the internal sensor plate

This forms a part of the feedback capacitance in the oscillator circuit.

If the distance is near enough, the oscillation amplitude is higher than the threshold voltage of the trigger circuit, the output of the sensor will switch state from logic 0 to logic 1, indicating and object is present nearby.

PROXIMITY TYPE SENSOR

CAPACITIVE SENSOR

Capacitive sensor can detect all materials object nearby with vary sensing distance from material to material.

The output circuit of the sensor can be of type NPN , PNP or contact

PROXIMITY TYPE SENSOR

INDUCTIVE SENSOR

• The oscillator creates a radio frequency that is emitted from the coil away from the face of the sensor.

• If a metal plate enters this radiated field, eddy currents circulate within the metal

• The oscillator requires energy to maintain the eddy currents in the metal plate.

• As the plate approaches the sensor, the eddy currents increase and cause a greater load on the oscillator.

• The oscillator stops when the load becomes too great.

• The trigger circuit senses when the oscillator stops, then changes the state of the switching device

PROXIMITY TYPE SENSOR

ULTRASONIC SENSOR

Operates by sending ultrasonic sound waves (above 18,000 hertz) toward the target and measuring the time it takes for the pulses to bounce back

The time taken for this echo to return to the sensor is directly proportional to the distance or height of the object because sound has a constant velocity.

Very effective for applications such as fluid levels in tanks and crude distance measurement as well as object orientation and sorting of object with different height.

Solids, fluids, granular objects and textiles can be detected by an ultrasonic sensor.

SENSOR WIRING

Introduction

When a sensor detects a logical change it must signal that change to a controller such as PLC in a control system by switching current or voltage ON or OFF.

The output circuit of the sensor will determine the method of switching.

Type of Sensor output circuit

Switches

• The simplest examples of sensor outputs are switches and relays.

• The sensor is powered separately, by connecting power to V+ and V- of the sensor.

• When the sensor is active ,the internal switch (probably a relay) will be closed allowing current to flow and the positive voltage will be applied to input 06.

Type of Sensor output circuit

Current sinking output

• This type of output allow current to flow into the sensor to the voltage common.

• NPN transistor is used for the sinking output to switch current flow into the sensor.

(sinking)

• When sensor is active, it turns on the transistor and allow current to flow into the sensor

• When sensor is inactive, it turns off the transistor and thus disallow current to flow into sensor.

Type of Sensor output circuit

Connect sinking output to PLC

• The positive terminal of the supply is connected to the sensor positive terminal and the common terminal of PLC.

• The negative terminal of the supply is connected to the sensor negative terminal only.

• The output terminal of the sensor is connected to the input terminal of PLC ( 00)

Type of Sensor output circuit

Connect sinking output to PLC

• The dashed line in the figure represents current flow path when the sensor is active

• This current will flow through optocoupler and use light to turn on a phototransistor to tell the computer in the PLC that the input current is flowing

Type of Sensor output circuit

Current sourcing output

• This type of output allow current to flow out from the sensor to the voltage common through the load .

• PNP transistor is used for the sourcing output to switch current flow out from the sensor to the load

(sourcing)

• When sensor is active, it turns on the transistor and allow current to flow out from V+ through the transistor and to the load.

Type of Sensor output circuit

Connect sourcing output to PLC

• The negative terminal of the supply is connected to the sensor negative terminal and the common terminal of PLC.

• The positive terminal of the supply is connected to the sensor positive terminal only.

• The output terminal of the sensor is connected to the input terminal of PLC ( 00)

Type of Sensor output circuit

Connect sinking output to PLC

• The dashed line in the figure represents current flow path when the sensor is active

• This current will flow through optocoupler and use light to turn on a phototransistor to tell the computer in the PLC that the input current is flowing

LOGICAL ACTUATORS

STUDENTS LEARNING OUTCOME

At the end of the lesson, students will be able

• To describe function of solenoid, valves, and cylinder used in Industry.

• To explain the operation of solenoid, valves and cylinder

• To describe function of motor.

• To identify type of motor

• To describe the operation of dc motor, induction motor , synchronous

motor.

• To describe the operation of stepper motor and its various drive

method.

LOGICAL ACTUATOR

DEFINITION

Actuators converts electrical ,pneumatic or hydraulic energy into some form of mechanical motion.

LOGICAL ACTUATOR

SOLENOID

LOGICAL ACTUATOR

SOLENOID

• The principle of operation is a moving ferrous core (a piston) that will move inside wire coil.

• Normally the piston is held outside the coil by a spring.

• When a voltage is applied to the coil, a magnetic field is built up that attracts the piston and pulls it into the center of the coil.

• The piston can be used to supply a linear force.

• Applications of these include pneumatic valves and car door openers.

LOGICAL ACTUATOR

VALVES

1. Valve Body

2. Inlet Port

3. Outlet Port

4. Coil / Solenoid

5. Coil Windings

6. Lead Wires

7. Plunger

8. Spring

9. Orifice

LOGICAL ACTUATOR

VALVES

• The flow of fluids and air can be controlled with solenoid controlled valves

• The solenoid is mounted on the side, When actuated it will drive the central spool left.

• The top of the valve body has two ports that will be connected to a device such as a hydraulic or pneumatic cylinder

• The bottom of the valve body has a single pressure line in the centre with two exhausts to the side

LOGICAL ACTUATOR

VALVES

In the top drawing the power flows in through the centre to the right hand cylinder port. The left hand cylinder port is allowed to exit through an exhaust port.

In the bottom drawing the solenoid is in a new position and the pressure is now applied to the left hand port on the top, and the right hand port can exhaust.

LOGICAL ACTUATOR

VALVES

LOGICAL ACTUATOR

VALVESA 3 ports 2 positions Normally close

valve controlling a single acting spring return cylinder

LOGICAL ACTUATOR

CYLINDERS

• Cylinder uses pressurized fluid or air to create a linear force/motion

• Fluid is pumped into one side of the cylinder under pressure, causing that side of the cylinder to expand , and advancing the piston.

• The fluid on the other side of the piston must be allowed to escape

LOGICAL ACTUATOR

CYLINDERS

The force the cylinder can exert is proportional to the cross sectional area of the cylinder.

For Force:

P=pressure on the fluidF=force acting on the pistonA=area of the piston

AFP

PAF

LOGICAL ACTUATOR

CYLINDER

Single acting cylinders apply force when extending and typically use a spring to retract the cylinder.

Double acting cylinders apply force in both direction.

ELECTRIC MOTOR

Introduction

• Motor is a continuous actuator allow a system to position or adjust outputs over a wide range of values.

• Electric motor is composed of a rotating centre core, called the rotor and a stationary outside, called the stator.

• These motors use the attraction and repulsion of magnetic fields to induce forces, and hence motion.

• Typical electric motors use at least one electromagnetic coil, and sometimes permanent magnets to set up opposing fields.

• When a voltage is applied to these coils the result is a torque and rotation of an output shaft.

ELECTRIC MOTOR

Type of electric motors

AC motors - rotate with relatively constant speeds proportional to the frequency of the supply power.

induction motors - squirrel cage, wound rotor - inexpensive, efficient.synchronous motor- fixed speed, efficient

DC motors - have large torque and speed ranges

permanent magnet - variable speedwound rotor and stator - series, shunt and compound (universal)

Hybridbrushless permanent magnet -stepper motors

ELECTRIC MOTOR

Basic brushed DC motor

• The structure of a dc motor contains a current carrying armature(rotor) which is connected to the supply through commutator segments and brushes and placed within the north south poles of a permanent or an electro-magnet(stator )

• The magnetic field produced by current flows through the armature interacts with the field of the stator and produces a force acting on the armature in a direction that can be determined by Fleming’s left hand rule.

• This force rotates the rotor in clockwise or anticlockwise direction according to the apply voltage polarity

ELECTRIC MOTOR

Basic brushed DC motor

• The speed of rotation will vary with the applied voltage amplitude. A higher voltage will make the rotation faster.

.

• The motor can be started or stopped through using contactor or relay. The speed of rotation can be control by using Pulse Width Modulation(PWM) technique.

ELECTRIC MOTOR

Speed control using PWM

• (PWM) signal produces an effective voltage that is relative to the time that the signal is on.

• The percentage of time that the signal is on is called the duty cycle

• When the voltage is on all the time the effective voltage delivered is the maximum voltage.

• if the voltage is only on half the time, the effective voltage is half the maximum voltage.

• The frequency of these waves is normally above 20KHz

AC MOTOR

Induction Motor

• in a three phase induction motor, when three phase supply is given to three phase stator winding, a rotating magnetic field is produced.

• The rotor of an induction motor is either wound type or squirrel cadge type.

• Whatever may be the type of rotor, the conductors on it are shorted at end to form closed loop.

• Due to rotating magnetic field, the flux passes through the air gap between rotor and stator, cuts the rotor conductor. Hence induced current in the closed rotor conductors.

AC MOTOR

Induction Motor

• As per Lenz law the rotor will try to reduce the every cause of producing current in it.

• Hence the rotor rotates and tries to achieve the speed of rotating magnetic field to reduce the relative speed between rotor and rotating magnetic field.

• The speed of rotation of the rotor is always slower than the rotating magnetic field. The difference is called the SLIP.

Sine wave current in each of the coils produces sine varying magnetic field on the rotation axis.    

Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector of resulting rotating magnetic field.

Rotating magnetic field

• The operation of a synchronous motor is due to the interaction of the magnetic fields of the stator and the rotor.

• The stator winding, when excited by a poly-phase (usually 3-phase) supply, creates a rotating magnetic field inside the motor.

• The rotor locks in with the rotating magnetic field and rotates along with it. Once the rotor locks in with the rotating magnetic field, the motor is said to be in synchronization.

• The speed of rotation is same as the speed of the rotating magnetic filed

N

S

S

N

Synchronous Motor

HYBRID MOTOR

STEPPER MOTOR

HYBRID MOTOR

STEPPER MOTOR• The stepper motors consists of a stator an a rotor

• The rotor carries a set of permanent magnets, and the stator has the coils.

• Basic design of a stepper motor have 4 coils with 90o angle between each other fixed on the stator

• The motor has 90o rotation step. The coils are activated in a cyclic order, one by one

• The rotation direction of the shaft is determined by the order that the coils are activated.

• Have the advantage in position control, no feedback is needed because the motor rotates with a fixed degree of step. Some step angle can be as small as 1.8o.

TYPE OF STEPPER MOTOR DRIVE

• Only one coil is energized each time

• This method is only when power saving is necessary.

• It provides less than half of the nominal torque of the motor, therefore the motor load cannot be high.

• This motor will have 4 steps per full cycle.

Wave drive or Single-Coil Excitation

TYPE OF STEPPER MOTOR DRIVE

• Most often used method

• The coils are energized in pairs.

• Depend on the connection of the coils (series or parallel) the motor will require double the voltage or double the current to operate than operation in Single-Coil Excitation.

• it produces 100% the nominal torque of the motor.

• This motor also have 4 steps per full cycle.

Full step drive

TYPE OF STEPPER MOTOR DRIVE

• A way to achieve double the accuracy of a positioning system, without changing anything from the hardware

• The coils are energized in the sequence of first a single coil, then adjacent coil and the first coil together.

• This will cause the rotor to rotate to position in between the energized coils.

• Next the first coil is de-energized, leaving only the second coil remains in energized state.

• The rotate will advance another half step to align with the second coil.

• The sequence is repeated to cause the rotor to rotate a cycle with 8 steps instead of 4 steps.

Half stepping ( single coil excitation )

TYPE OF STEPPER MOTOR DRIVE

• Another way to achieve double the accuracy of a positioning system, without changing anything from the hardware

• Instead of one two, one two, the coils are energized in sequence of 2,4,2,4.

• It consumes more power than the single coil excitation method.

• This motor also have 8 steps per full cycle.

Half stepping ( double coils excitation )

Using a sorting device, parts are to be transferred from conveyor belt. By pressing the pushbutton switch, the piston rod of a single-acting cylinder pushes the part off the conveyor belt. When the pushbutton is released, the piston rod returns to the retracted end position

Application