Industrial Control Devices and Relays

Industrial Control Devices

Upon completion of this lesson material, students will be able to: 1. Understand mechanical switch types, configurations, and

terms.

2. Describe the types and operation of electromechanical relays. 3. Understand the characteristics of solid-state relays and the

advantages and disadvantages of solid-state relays compared with electromechanical relays.

MODULE

OBJECTIVES

1. Industrial Control Devices

a. Primary and pilot control devices b. Switching Devices c. Transducers and sensors

2. Relays

a. Electromechanical Control Relays b. Solid State Relays c. Timing Relays d. Latching Relays e. Relay logic

TOPICS

Brian S. Elliot, Electromechanical Devices and Components Illustrated Sourcebook, McGraw-Hill Co.

Killan, Modern Control Technology Components and Systems, Delmar Publishing

Modules in Industrial Electronics and Controls, Nanyang Polytechnic - School of Engineering , Singapore

Sinclair, I and Dunton, J (2007). Practical Electronics Handbook 6th Edition. Great Britain: Elsevier Ltd.

Giblisco, S. (2005). Teach Yourself Electricity and Electronics 3rd Edition. McGraw Hill.

Floyd, T (2005). Electronic Devices Conventional Current Version 7th Edition. Pearson Prentice Hall.

Boylestad, R and Nashelsky, L (2006). Electronic Devices and Circuit Theory 9th Edition. Pearson Prentice Hall.

Rashid, M. H. ,editor-in-chief (2001). Power Electronics Handbook. Academic Press, San Diego California.

Clayton, G., Winder, S (2003). Operational Amplifiers 5th Edition. Newness Publication.

Kasparis, T., PhD (2005) Laboratory Manual for Electronics II (Revised). University of Central Florida, Department of Electrical and Computer Engineering

REFERENCES:

This lesson material covers the more common electrical devices used in power and control circuits. Indicator Lights Indicator lights show the position of mount components or the status of switches, solenoids, fuses, and control and power circuits. Fuses The fuse is the simplest form of a circuit protective device. It consists of a metal alloy fusible element that melts at a predetermined value of current. Thus, if a circuit draws more current than the rated value of the fuse, the fuse opens (blows) and the circuit components are protected. Switches A switch is a device used for making, breaking, or changing the connections in an electric circuit. It is the simplest digital signal sensor. In addition, an electro-mechanical switch is a vital part of any sensing and transducing equipment. Any switch designed to be operated by a person is generally called a hand switch. Some switches are specifically designed to be operated by a motion of a machine or something in a machine rather by the hand of the human operator. These types are called control switches An essential function of any switch is to maintain a good, low-resistance contact when the switch is closed. A poor connection between switch elements produces considerable resistance. This resistance results in overheating the contact area. When heavy current is being carried by the switch and the switch contacts are opened, an arc is produced. Therefore, switches should be opened and closed quickly to minimize arcing. Usually, they are designed to have snap action. An electro-mechanical switch is an electrical device that can open or close, thereby allowing a current to flow or not.

a. Toggle switch b. Push-button switch c. Limit switch d. Rotary switch e. Relay switch

Switches are classified by the number of poles, by the number of throw, and the pole contact schemes used. The pole of a switch is its movable blade or contactor. A switch may have one or several poles single pole (SP), double pole (DP), and triple pole (TP). The throw of a switch identifies the number of possible actuated positions and consists of either one, single throw (ST), or two, double throw (DT). Either type can be used with single-, double-, or triple throw configurations. The number of positions a switch has is the number of places at which the operating device (toggle, shaft, plunger, and soon) will come to rest and, at the same time, open or close a circuit. In pole-contact schemes, switches are designed to have either open or close contacts when NOT actuated, hence

may be normally open (NO) that are closed whenever the switch is actuated; or, normally closed (NC) that opens when the switch is actuated.

Transducers and Sensors

What is a sensor? A device for measuring some quantity The sensor usually converts from the measurement space to an electrical

signal Industrial Sensors

Proximity o Mechanical o Optical o Inductive/Capacitive

Position/Velocity o Potentiometer o Linear variable differential transformer (LVDT) o Encoders o Tachogenerator

Force/Pressure Vibration/acceleration

Example of Industrial Sensors

Start & Stop Proximity Sensor, Electronics, Software & Hardware The sensors are pre-wired with connectors to plug in and integrate with Lasercheck electronics and software to automate starting, stopping, and saving measurements. They will sense when surfaces to be measured are in position under the Lasercheck and stop measurements when surfaces move out of position. Particularly useful in high volume operations where parts can be conveyor fed past the Lasercheck for 100% automated surface inspection.

Proximity Sensors The Bulletin 871ZT ToughWeld 3-wire DC family of proximity sensors. These sensors are offered in 12, 18, and 30mm barrel diameters in both shielded and unshielded microconnector versions. This new family features equal sensing capabilities as well as extended ranges for unshielded models.

Sample Proximity Sensor

Definition of Sensor Parameters

Accuracy: The agreement between the actual value and the measured value Resolution: The change in measured variable to which the sensor will respond Repeatability: Variation of sensor measurements when the same quantity is measured several times Range: Upper and lower limits of the variable that can be measured Sensitivity and Linearity Proximity Sensors

Widely used in general industrial automation o Conveyor lines (counting, jam detection, etc) o Machine tools (safety interlock, sequencing)

Usually digital (on/off) sensors detecting the presence or absence of an object Consist of:

o Sensor head: optical, inductive, capacitive o Detector circuit o Amplifier o Output circuit: TTL, solid state relay

Mechanical Proximity Switches Essentially a mechanical switch On/off operation only Two general modes

o Normally Open (NO) o Normally Closed (NC)

Come in a wide variety of mechanical forms For a wide range of uses

Mechanical Proximity Sensor

Example Mechanical Proximity Switches

When to Use Mechanical Proximity Switches Where physical contact is possible Where definitive position is required In operation-critical or safety-critical situations Where environment conditions preclude the use of optical or inductive

sensors Applications and Use of Mechanical Proximity Switches

Easy to integrate into machinery of all types Requires contact (thus wear) Range of voltages: DC 0-1000V, AC, etc. Very robust (explosion proof if required) Usually used as:

o Limit switch o Presence/absence indicator o Door closed/open

Optical Proximity Sensors

Consist of a light source (LED) and light detector o (phototransistor)

Modulation of signal to minimize ambient lighting o conditions

Various models: 12-30V DC, 24-240V AC, power Output: TTL 5V, Solid-state relay, etc. Modulator Power

Operational Modes

Through Beam: o Long range (20m) o Alignment is critical!

Retro-reflective o Range 1-3m

o Popular and cheap Diffuse-reflective

o Range 12-300mm o Cheap and easy to use

Optical Proximity Sensor

Through beam sensors Emitter and receiver are in two separate housings facing each other. The

sensor switches whenever the light beam is interrupted.

Through beam sensor

Use of Through beam Sensors Advantages

Through beam sensors offer the largest sensing ranges. The switching point is independent of the surface nature of the object. Due to the narrow effective beam, through beam sensors have excellent

repeatability. Application

Monitoring doors and gates. Counting and monitoring of objects over large distances.

Application of through beam sensor

Retro-reflective sensors Description

With the emitter and receiver in the same housing this sensor transmits a pulsed infrared or red light beam which is reflected back from a "triple prism" reflector or reflective tape. The sensor switches when the light beam is interrupted. These devices recognize objects independent of their surface qualities, as long as they are not too shiny. Advantages

Large sensing range Matte finished objects are recognized independent of their surface

properties. Application

Height detection of stacked objects. Control of randomly positioned objects on a conveyor.

Retro-reflective sensors application

Retro-reflective sensors with polarization filter Description

Only one orientation of the polarized emitted light passes through the polarization filter. The light is rotated and depolarized after reflection from a triple prism. Only part of this reflected light is able to pass through the second polarization filter in front of the receiver. When a shiny target breaks the beam, it reflects the light without changing the plane of polarization and the light is not seen by the receiver. In this manner, the receiver can distinguish between light received from a shiny object and light received when the beam is unbroken. Advantages

Detection of shiny objects, such as aluminum cans Application

Monitoring shiny cans on a conveyor belt. Standard diffuse sensors with intensity difference Description

The emitter (S) and receiver (E) are in the same housing. The emitter sends out a beam of pulsed infrared or red light, which is reflected directly by the target. When the beam of light hits the target (at any angle), it is diffused in all directions and some light is reflected back. The receiver sees only a small portion of the original light, switching the sensor when a target is detected within the reflective distance.

Advantages

Using the targets own reflective properties Suitable to distinguish between black and white Relatively large active distance Easy mounting (only one sensor)

Diffuse sensors application

Diffuse sensors with background or foreground suppression Description

Diffuse sensors with background or foreground suppression which work with triangulation do not only sense the light reflected from the target, but also sense the distance of the object to the sensor. In the diagram above the receiver R1 receives the light from the object T1 and R2 from the background. Within the fully adjustable distance, objects which are at least as big as the diameter of the light beam are recognized independently of color and surface properties (minimal reflectivity of 6 %). The background distance should be at least 10% larger than the sensing distance.

Advantages

The distance can be precisely adjusted The active distance is smaller than a diffuse sensor and repeatability is

therefore better

Foreground suppression Object detection without blind region Targets are recognized independent of color and surface in front of a

defined background

Background suppression Targets are recognized largely independent of color and surface

properties even in front of changing background conditions The background is largely suppressed, independent of color and surface

properties

Relays Specific Objectives:

1. Explain the operation of electromechanical relays, time-delay relays, counters, and sequencers.

2. Explain the purpose and operation of a ladder diagram. 3. Explain the operation of a relay-based controller.

A relay is simply an electromagnetically operated switch. Relays are designed to open or close a circuit when the current through its coil is applied and removed, or varied in magnitude. The main parts of a relay are a coil wound on an iron core and an armature that operates a set of contacts.

The most important point to understand about the relay is that the electrical circuit that energizes the coil is not connected electrically with the contacts.

The contacts are moved by a magnetic force, so they are electrically isolated from the coil voltage.

Relay

CoilRelay

Contact

SpringMovable

Arm

Figure 5.1 Schematic Diagram of an Electromagnetic Relay

Figure 5.2 Physical view of an Electromagnetic Relay

When activated: ALL Normally-OPEN contacts will CLOSE ALL Normally-CLOSED contacts will OPEN

Concept of Normally Close and Normally Open contact The contacts for relay and contactor can be normally open or normally closed. The word 'normally' in this case indicates the position of the contacts when no power is applied to the coil. Generally, normally open or normally closed contacts are selected to make the circuit logic function properly. The contacts are identified as NC for normally closed, NO for normally open and C / COM for common. Relays are available with a variety of types of contacts for many types of applications. These types of contacts include fast acting contacts for logic circuits and interface to solid state controls or programmable logic controllers, power contacts for switching currents up to 30A, sealed contacts such as reed contacts for power and logic applications, latched and unlatched contacts for logic applications and control circuits, and auxiliary contacts that can be added to a relay after initial installation.

For the unlatching relay as shown below, there is only one coil that needs to be activated. When that coil is activated (by supplying a 24V DC voltage at the

Figure 5.3 The electromechanical relay consists of an electromagnet which uses solenoid action to move a set of electrical contacts from the open to the closed position or vice versa. The basic principle involved in the operation of a relay is similar to contactors and motor starters.

terminals 13 and 14 of the coil in the relay), all the NO contacts will close and all the NC contacts will open. When the coil is de-activated, all the contacts return to their normal state.

For the latching relay, a special mechanical latch holds the relay in the energized state even when the coil voltage is interrupted. This allows the coil to be energized with a pulse of current. A second coil is used to de-energize the contacts. The coil used to de-energize the contacts is called the reset coil and the coil used to latch the relay is called the set coil. The following figure shows a latching relay and its terminal arrangement diagram.

Note: Throughout this topic, we will be using only the UNLATCHED RELAY.

ALL LATCHING CIRCUITS refer to the unlatched relay, which uses only the SET COIL. Electric power is needed to keep these relays activated.

UNDERSTANDING NORMALLY-OPEN/ CLOSED STATES AND OPEN/CLOSED STATES When relay is not activated: Control circuit is open.

relay coil not activated.

relay in normal state

relay contacts 1-2 is NC

relay contacts 1-4 is NO

Power circuit ON for green light. connected to NC relay contact 1-2 Power circuit OFF for red light. connected to NO relay contact 1-4 Relays contacts are in their normal state Normally open and Normally closed.

When relay is activated: Control circuit is closed.

relay coil is activated.

relay in activated state

relay contacts 1-2 is Open

relay contacts 1-4 is Closed

Power circuit OFF for green light. connected to Open relay contact 1-2 Power circuit ON for red light. connected to Closed relay contact 1-4 Relay contacts are in their activated state - Normally open contacts Closed Normally closed contacts Open.

When one purchase relays, one generally have to look at the following criteria:

1. The voltage and current that is needed to activate the armature. 2. The maximum voltage and current that can run through the armature

and its contacts. 3. The number of armatures (generally 1 or 2). 4. The number of contacts for the armature. 5. (Generally 1 or 2 - the relay shown here has 2, one of which is

unused. The one used is Normally Open, NO). 6. Number of Normally Open (NO) or Normally Closed (NC) contacts.

Relay in Normal state.

Relay in Activated state. (not Normal)

Example 1: You have a spotlight (Fig. E1b below with terminal points S1 and S2) that operates on 220V AC. You need to be able to switch on and off this light using a 24V DC unlatched relay, as shown in Fig. E1a below. Use one normally-opened as ON pushbutton and one normally-closed pushbutton as OFF pushbutton. Number the points across the relay coil and the relay contacts in your diagram. (a) Draw the control circuit. (b) Draw the power circuit. (c) Solution:

Figure E1a. 24Vdc Unlatched Relay Figure E1b. 110V Spotlight

220V

Example 2: The electric forklift truck is able to hoist objects between the upper and lower limits as shown in Figure E2. The hoisting mechanism is powered by a 240V DC electric motor. The direction of lift is controlled by the direction of current flow in the electric motor using an H-bridge.

(a) Design a relay circuit to simulate the control of the hoist movement. Draw only the control circuit using only two unlatching relays and , two pushbuttons (for hoisting up) and (for hoisting down), and one stop pushbutton to inhibit the motor movement.

Label the diagram clearly and include in the control circuit,

(i) A green pilot light to indicate if any of the relay coils is being energized

(ii) A red pilot light to indicate that both relay coils are de-energized

(b) Using the above control circuit 2(a), draw only the power circuit to show how a simple H-bridge can be implemented to control the direction of armature current flow, and hence the direction of the movement of the lift.

EXERCISES: 1. With the aid of a simple sketch, describe briefly how an

electromechanical relay works, stating the purpose of the relay coil and the relay contacts.

Upper Limit

Lower Limit

Fig E2: Electric forklift truck

2. Explain briefly how the relay circuit below works, when you close the

switch. Explain also the difference between the control circuit and the power circuit.

3. Show how a 220V AC motor can be switched ON and OFF by using a 24V DC unlatched relay shown in Fig. 1. Design the control circuit and the power circuit.

1 5

9

1

3

4

8

1

2

1

4

Normally 1-9 NC 5-9 NO When activated 1-9 Opened

5-9 Closed

Normally 4-12 NC 8-12 NO When activated 4-12 Opened

8-12 Closed

1-9, 5-9, 4-12, 8-12 are all switches,

called relay contacts

Relay Coil

Relay Contacts

Control Switch

Control Circuit

Power Circuit

Fig. 1: 24V DC unlatching relay

4. A two-way interlock circuit is shown below in Fig. 2. Describe how it works.

5. Having understood how the two-way interlock circuit in Fig. 2 above

works, design a three-way interlock circuit using three unlatching relays and three pilot lights.

6. The direction of rotation of a DC permanent-magnet motor can be

reversed by changing the polarity of the armature coil voltage, the field being constant. The power circuit of a DC permanent-magnet motor is as shown in Fig. 3 below.

a) Draw a basic interlock diagram using only two unlatching relays

FWD and REV, and its relay contacts to realize the control circuit.

b) Include three pilot lights: (i) a green light - to indicate forward direction of rotation, (ii) a blue light - to indicate reverse direction of rotation and (iii) a red light - to indicate the halted (stop) state of the motor.

c) Re-draw the power circuit below in your solution, in the circuit style of Q4.

Fig. 3: Power Circuit of the DC Motor

M

R1

R1

R2

R2

+_

24V

PB1 R1

R2

PB2 R2

R1

R1 R2

R1 R2

L1 L2

Relay Contacts

Relay Coil

Fig. 2: 2-way interlock circuit

Pilot Light or Lamp

7. Automatic Escalator System : The automatic escalator system shown in Fig. 4 can either be moving upwards or downwards depending on whether the UP or DOWN pushbutton is selected.

When the UP pushbutton is pressed, a 3-phase AC motor will turn clockwise (FWD), and when the DOWN pushbutton is pressed, the same motor will turn anticlockwise (REV). The escalator can be stopped by pressing a STOP pushbutton.

a) Design a control circuit for the escalator system such that the

motor must always be stopped before it can be reversed in either direction.

b) Design a power circuit for the 3-phase AC motor.

8. Automatic Escalator System: The escalator starts moving when someone steps on the pressure sensor on the escalator landing. This could either be the top-landing pressure-sensor switch, TOP_SW or the bottom-landing pressure-sensor switch, BTM_SW. a) Design an improved control circuit (to replace the one in part Q7

a)) to incorporate the pressure-sensor switches BTM_SW, and TOP_SW. Assume that after the person leaves the escalator system, the escalator will stop automatically a while later. All sensors used are normally-open and PNP type. Both sensors and relays operate on 30V DC.

Fig. 4: Automatic Escalator System

TOP_SW

BTM_SW

UP DOWN STOP

Laboratory Experiment Electromagnetic Relays Objectives:

* To understand the basic function of an electromechanical relay. * To understand the latching circuit. * To understand the interlock circuit.

* To understand control circuits and power circuits. * To be able to design circuits involving electromechanical relays.

Introduction:

The electromechanical relay consists of an electromagnet which uses solenoid action to move a set of electrical contacts from the open to the closed position or vice versa. Fig. 1 below shows the construction of an electromechanical relay.

It is useful to note that the electrical solenoid circuit that energizes the coil has no electrically connection with the contacts. The contacts are moved by a magnetic force, so they are electrically isolated from the coil voltage. EQUIPMENT:

(1) Relay Panel training set (2) Pushbutton Panel with Indicator Lights (3) 6 Ampere DC Power Supply -- Goodwill GPC-3030

Fig. 1 Electromechanical Relay

Relay contacts 1-2 NC 1-4 NO

Relay coil

A1-A2

PROCEDURE: In this lab, we are going to experiment with some hard-wired basic control circuit. Each basic circuit serves a unique function. At the end, we will see how these basic circuits could be combined to form complex sequential control circuit. Connect up and test the circuits below. Draw truth table for the circuits 1 to 4. 1. Direct Circuit (using Normally-Open Pushbutton) 2. Inverse Circuit (using Normally-Closed Pushbutton) 3. OR Circuit

PB

LAMP

24V

0V

Normally-Closed (NC)

PB Normally Open (NO)

LAMP

24V

0V

Pushbutton

Indicator Light

PB2

LAMP

24V

0V

PB1

4. AND Circuit 5. Latching Circuit

Modification: Can you turn off the indicator light (lamp) after you momentarily pressed the pushbutton? How would you modify the circuit to enable the indicator light to be turned off?

6. SET-RESET Circuit (SET Priority)

PB1

LAMP

24V

0V

PB2

R1

PB1

24V

R1

LAMP

A1

A2 0V

0V

SET

24V

R1

LAMP

RST

R1

7. SET-RESET Circuit (RESET Priority) Be able to show the difference between the above two circuits when you have connected each of them. 8. Interlock Circuit Connect the above circuit modularly. a. Connect the vertical R1 circuit, and test. b. Connect the vertical R2 circuit, and test. c. Connect the vertical L1 circuit, and test. d. Connect the vertical L2 circuit, and test. e. Connect the R1 latch to the R1 vertical circuit, and test. f. Connect the R2 latch to the R2 vertical circuit, and test. g. Show that when PB1 is pressed, L1 turns on, but L2 cannot be turned

on when you press PB2. h. Press Stop PB. i. Show that when PB2 is pressed, L2 turns on, but L1 cannot be turned on

when you press PB1. j. Hence show that when Circuit R1 (& hence L1) is on, Circuit R2 ( & hence

L2) will be locked, and vice versa.

R1 SET

RST

24V

0V

R1

R1

PB1

24V

0V

R1

R2

PB2 R2

R2 R1

R1 R2

L2 L1

Latch

k. Include a red light which lights up to indicate when both R1 and R2 are not activated.

9. Three phase motor control circuit. Explain how the control circuit works.

Control Circuit

Power Circuit

FOR

STOP

FMC

REV RMC

RMC FMC

FMC FMC

AMBER

RMC

RMC

RED BLUE

RMC FMC

THR

THR

FUSE

L

N GREEN

Legend: THR: Thermal Overload xMC: Motor Contactor / Relay MCB: Miniature Circuit Breaker FOR: Forward REV: Reverse