PREFACE

<TODO> Some sections are still in point form. The last major task of this book will be to write the preface to reflect the book contents and all of the features.

Control systems apply artificial means to change the behavior of a system. The type of control problem often determines the type of control system that can be used. Each controller will be designed to meet a specific objective. The major types of control are shown in See Control Dichotomy.

Control Dichotomy

· Continuous - The values to be controlled change smoothly. e.g. the speed of a car.

· Logical - The value to be controlled are easily described as on-off. e.g. the car motor is on-off. NOTE: all systems are continuous but they can be treated as logical for simplicity.

e.g. "When I do this, that always happens!" For example, when the power is turned on, the press closes!

· Linear - Can be described with a simple differential equation. This is the preferred starting point for simplicity, and a common approximation for real world problems.

e.g. A car can be driving around a track and can pass same the same spot at a constant velocity. But, the longer the car runs, the mass decreases, and it travels faster, but requires less gas, etc. Basically, the math gets tougher, and the problem becomes non-linear.

e.g. We are driving the perfect car with no friction, with no drag, and can predict how it will work perfectly.

· Non-Linear - Not Linear. This is how the world works and the mathematics become much more complex.

e.g. As rocket approaches sun, gravity increases, so control must change.

· Sequential - A logical controller that will keep track of time and previous events.

The difference between these control systems can be emphasized by considering a simple elevator. An elevator is a car that travels between floors, stopping at precise heights. There are certain logical constraints used for safety and convenience. The points below emphasize different types of control problems in the elevator.

Logical:

1. The elevator must move towards a floor when a button is pushed.

2. The elevator must open a door when it is at a floor.

3. It must have the door closed before it moves.

etc.

Linear:

1. If the desired position changes to a new value, accelerate quickly towards the new position.

2. As the elevator approaches the correct position, slow down.

Non-linear:

1 Accelerate slowly to start.

2. Decelerate as you approach the final position.

3. Allow faster motion while moving.

4. Compensate for cable stretch, and changing spring constant, etc.

Logical and sequential control is preferred for system design. These systems are more stable, and often lower cost. Most continuous systems can be controlled logically. But, some times we will encounter a system that must be controlled continuously. When this occurs the control system design becomes more demanding. When improperly controlled, continuous systems may be unstable and become dangerous.

When a system is well behaved we say it is self regulating. These systems don't need to be closely monitored, and we use open loop control. An open loop controller will set a desired position for a system, but no sensors are used to verify the position. When a system must be constantly monitored and the control output adjusted we say it is closed loop. A cruise control in a car is an excellent example. This will monitor the actual speed of a car, and adjust the speed to meet a set target speed.

Many control technologies are available for control. Early control systems relied upon mechanisms and electronics to build controlled. Most modern controllers use a computer to achieve control. The most flexible of these controllers is the PLC (Programmable Logic Controller).

Purpose

· Most education focuses on continuous control systems.

· In practice most contemporary control systems make use of computers.

· Computer based control is inherently different than continuous systems.

· The purpose of this book is to address discrete control systems using common control systems.

· The objective is to prepare the reader to implement a control system from beginning to end, including planning and design of hardware and software.

Audience Background

· The intended reader should have a basic background in technology or engineering.

A first course in electric circuits, including AC/DC circuits is useful for the reader, more advanced topics will be explained as necessary.

Editorial notes and aids

Sections labeled Aside: are for topics that would be of interest to one discipline, such as electrical or mechanical.

Sections labeled Note: are for clarification, to provide hints, or to add explanation.

Each chapter supports about 1-4 lecture hours depending upon students background and level in the curriculum.

Topics are organized to allow students to start laboratory work earlier in the semester.

sections begin with a topic list to help set thoughts.

Objective given at the beginning of each chapter.

Summary at the end of each chapter to give big picture.

significant use of figures to emphasize physical implementations.

worked examples and case studies.

problems at ends of chapters with solutions.

glossary.

Platform

This book supports Allen Bradley micrologix, PLC-5s, SLC500 series

TODO LIST

- Finish writing chapters

* - structured text chapter

* - FBD chapter

- fuzzy logic chapter

* - internet chapter

- hmi chapter

- modify chapters

* - add topic hierachies to this chapter. split into basics, logic design techniques, new stuff, integration, professional design for curriculum design

* - electrical wiring chapter

- fix wiring and other issues in the implementation chapter

- software chapter - improve P&ID section

- appendices - complete list of instruction data types in appendix

- all chapters

* - grammar and spelling check

* - update powerpoint slides

* - add a resources web page with links

- links to software/hardware vendors, iec1131, etc.

- pictures of hardware and controls cabinet

PROGRAMMABLE LOGIC CONTROLLERS

INTRODUCTION

Control engineering has evolved over time. In the past humans were the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.

PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer.

· Cost effective for controlling complex systems.

· Flexible and can be reapplied to control other systems quickly and easily.

· Computational abilities allow more sophisticated control.

· Trouble shooting aids make programming easier and reduce downtime.

· Reliable components make these likely to operate for years before failure.

Ladder Logic

Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.

Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in See Simple Relay Layouts and Schematics. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting).

Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in See A Simple Relay Controller. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on.

A Simple Relay Controller

The example in See A Simple Relay Controller does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. See A PLC Illustrated With Relays shows a more complete representation of the PLC. Here there are two inputs from push buttons. We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types.

A PLC Illustrated With Relays

Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in See A Seal-in Circuit is an example of this, it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labelled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if input A goes off. After B is turned on the output B will not turn off.

A Seal-in Circuit

Programming

The first PLCs were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLCs today. An example of ladder logic can be seen in See A Simple Ladder Logic Diagram. To interpret this diagram imagine that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are combinations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or closed in the right combination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can come from a sensor, switch, or any other type of sensor. An output will be some device outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are normally open and normally closed. Which means if input A is on and input B is off, then power will flow through the output and activate it. Any other combination of input values will result in the output X being off.

A Simple Ladder Logic Diagram

The second rung of See A Simple Ladder Logic Diagram is more complex, there are actually multiple combinations of inputs that will result in the output Y turning on. On the left most part of the rung, power could flow through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom if both E and F are true. This would get power half way across the rung, and then if G or H is true the power will be delivered to output Y. In later chapters we will examine how to interpret and construct these diagrams.

There are other methods for programming PLCs. One of the earliest techniques involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal. An example of mnemonics is shown in See An Example of a Mnemonic Program and Equivalent Ladder Logic. In this example the instructions are read one line at a time from top to bottom. The first line 00000 has the instruction LD (input load) for input 00001. This will examine the input to the PLC and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or false). The next line uses an LDN (input load not) statement to look at the input. If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the reverse of the LD). The AND statement recalls the last two numbers remembered and if the are both true the result is a 1, otherwise the result is a 0. This result now replaces the two numbers that were recalled, and there is only one number remembered. The process is repeated for lines 00003 and 00004, but when these are done there are now three numbers remembered. The oldest number is from the AND, the newer numbers are from the two LD instructions. The AND in line 00005 combines the results from the last LD instructions and now there are two numbers remembered. The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1, otherwise the result is a 0. This result replaces the two numbers, and there is now a single number there. The last instruction is the ST (store output) that will look at the last value stored and if it is 1, the output will be turned on, if it is 0 the output will be turned off.

An Example of a Mnemonic Program and Equivalent Ladder Logic

The ladder logic program in See An Example of a Mnemonic Program and Equivalent Ladder Logic, is equivalent to the mnemonic program. Even if you have programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the PLC. In the past mnemonic programming was the most common, but now it is uncommon for users to even see mnemonic programs.

Sequential Function Charts (SFCs) have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. The example seen in See An Example of a Sequential Function Chart is doing two different things. To read the chart, start at the top where is says start. Below this there is the double horizontal line that says follow both paths. As a result the PLC will start to follow the branch on the left and right hand sides separately and simultaneously. On the left there are two functions the first one is the power up function. This function will run until it decides it is done, and the power down function will come after. On the right hand side is the flash function, this will run until it is done. These functions look unexplained, but each function, such as power up will be a small ladder logic program. This method is much different from flowcharts because it does not have to follow a single path through the flowchart.

An Example of a Sequential Function Chart

Structured Text programming has been developed as a more modern programming language. It is quite similar to languages such as BASIC. A simple example is shown in See An Example of a Structured Text Program. This example uses a PLC memory location N7:0. This memory location is for an integer, as will be explained later in the book. The first line of the program sets the value to 0. The next line begins a loop, and will be where the loop returns to. The next line recalls the value in location N7:0, adds 1 to it and returns it to the same location. The next line checks to see if the loop should quit. If N7:0 is greater than or equal to 10, then the loop will quit, otherwise the computer will go back up to the REPEAT statement continue from there. Each time the program goes through this loop N7:0 will increase by 1 until the value reaches 10.

An Example of a Structured Text Program

PLC Connections

When a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators, as shown in See The Separation of Controller and Process. The process is a real process that will change over time. Actuators will drive the system to new states (or modes of operation). This means that the controller is limited by the sensors available, if an input is not available, the controller will have no way to detect a condition.

The Separation of Controller and Process

The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any computer this does not happen instantly. See The Scan Cycle of a PLC shows the basic operation cycle of a PLC. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. For example, if the PLC backup battery is low and power was lost, the memory will be corrupt and this will result in a fault. If the PLC passes the sanity check it will then scan (read) all the inputs. After the inputs values are stored in memory the ladder logic will be scanned (solved) using the stored values - not the current values. This is done to prevent logic problems when inputs change during the ladder logic scan. When the ladder logic scan is complete the outputs will be scanned (the output values will be changed). After this the system goes back to do a sanity check, and the loop continues indefinitely. Unlike normal computers, the entire program will be run every scan. Typical times for each of the stages is in the order of milliseconds.

The Scan Cycle of a PLC

Ladder Logic Inputs

PLC inputs are easily represented in ladder logic. In See Ladder Logic Inputs there are three types of inputs shown. The first two are normally open and normally closed inputs, discussed previously. The IIT (Immediate InpuT) function allows inputs to be read after the input scan, while the ladder logic is being scanned. This allows ladder logic to examine input values more often than once every cycle.

Ladder Logic Inputs

Ladder Logic Outputs

In ladder logic there are multiple types of outputs, but these are not consistently available on all PLCs. Some of the outputs will be externally connected to devices outside the PLC, but it is also possible to use internal memory locations in the PLC. Six types of outputs are shown in See Ladder Logic Outputs. The first is a normal output, when energized the output will turn on, and energize an output. The circle with a diagonal line through is a normally on output. When energized the output will turn off. This type of output is not available on all PLC types. When initially energized the OSR (One Shot Relay) instruction will turn on for one scan, but then be off for all scans after, until it is turned off. The L (latch) and U (unlatch) instructions can be used to lock outputs on. When an L output is energized the output will turn on indefinitely, even when the output coil is deenergized. The output can only be turned off using a U output. The last instruction is the IOT (Immediate OutpuT) that will allow outputs to be updated without having to wait for the ladder logic scan to be completed.

Ladder Logic Outputs

A CASE STUDY

Problem: Try to develop (without looking at the solution) a relay based controller that will allow three switches in a room to control a single light.

SUMMARY

· Normally open and closed contacts.

· Relays and their relationship to ladder logic.

· PLC outputs can be inputs, as shown by the seal in circuit.

· Programming can be done with ladder logic, mnemonics, SFCs, and structured text.

· There are multiple ways to write a PLC program.

PRACTICE PROBLEMS

1. A PLC can effectively replace a number of components. Give examples and discuss some good and bad applications of PLCs.

(ans. A PLC could replace a few relays. In this case the relays might be easier to install and less expensive. To control a more complex system the controller might need timing, counting and other mathematical calculations. In this case a PLC would be a better choice._

2. Give an example of where a PLC could be used.

(ans. to control a conveyor system)

3. Why would relays be used in place of PLCs?

(ans. for simple designs)

4. Give a concise description of a PLC.

(ans. A PLC is a computer based controller that uses inputs to monitor a process, and uses outputs to control a process. A simple program is used to set the controller behavior.)

5. List the advantages of a PLC over relays.

(ans. less expensive for complex processes, debugging tools, reliable, flexible, easy to expend, etc.)

6. Explain the trade-offs between relays and PLCs for control applications.

(ans. tradeoffs include: cost, complexity, easy of debugging, etc.)

7. Explain why ladder logic outputs are coils?

(ans. the ladder logic outputs were modelled on relay logic diagrams. The output in a relay ladder diagram is a relay coil. This is normally drawn as a circle.)

8. In the figure below, will the power for the output on the first rung normally be on or off? Would the output on the second rung normally be on or off?

(ans. off, on)

9. Write the mnemonic program for the Ladder Logic below.

(ans. LD 100, LD 101, OR, ST 201)

PLC HARDWARE

INTRODUCTION

Many PLC configurations are available, even from a single vendor. But, in each of these there are common components and concepts. The most essential components are:

Power Supply - This can be built into the PLC or be an external unit. Common voltage levels required by the PLC (with and without the power supply) are 24Vdc, 120Vac, 220Vac.

CPU (Central Processing Unit) - This is a computer where ladder logic is stored and processed.

I/O (Input/Output) - A number of input/output terminals must be provided so that the PLC can monitor the process and initiate actions.

Indicator lights - These indicate the status of the PLC including power on, program running, and a fault. These are essential when diagnosing problems.

The configuration of the PLC refers to the packaging of the components. Typical configurations are listed below from largest to smallest as shown in See Typical Configurations for PLC.

Rack - A rack is often large (up to 18" by 30" by 10") and can hold multiple cards. When necessary, multiple racks can be connected together. These tend to be the highest cost, but also the most flexible and easy to maintain.

Mini - These are similar in function to PLC racks, but about half the size.

Shoebox - A compact, all-in-one unit (about the size of a shoebox) that has limited expansion capabilities. Lower cost, and compactness make these ideal for small applications.

Micro - These units can be as small as a deck of cards. They tend to have fixed quantities of I/O and limited abilities, but costs will be the lowest.

Software - A software based PLC requires a computer with an interface card, but allows the PLC to be connected to sensors and other PLCs across a network.

Typical Configurations for PLC

INPUTS AND OUTPUTS

Inputs to, and outputs from, a PLC are necessary to monitor and control a process. Both inputs and outputs can be categorized into two basic types: logical or continuous. Consider the example of a light bulb. If it can only be turned on or off, it is logical control. If the light can be dimmed to different levels, it is continuous. Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control. As a result most controls applications (and PLCs) use logical inputs and outputs for most applications. Hence, we will discuss logical I/O and leave continuous I/O for later.

Outputs to actuators allow a PLC to cause something to happen in a process. A short list of popular actuators is given below in order of relative popularity.

Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow.

Lights - logical outputs that can often be powered directly from PLC output boards.

Motor Starters - motors often draw a large amount of current when started, so they require motor starters, which are basically large relays.

Servo Motors - a continuous output from the PLC can command a variable speed or position.

Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs. Continuous outputs require special output cards with digital to analog converters.

Inputs come from sensors that translate physical phenomena into electrical signals. Typical examples of sensors are listed below in relative order of popularity.

Proximity Switches - use inductance, capacitance or light to detect an object logically.

Switches - mechanical mechanisms will open or close electrical contacts for a logical signal.

Potentiometer - measures angular positions continuously, using resistance.

LVDT (linear variable differential transformer) - measures linear displacement continuously using magnetic coupling.

Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs. Sourcing and sinking inputs are also popular. This output method dictates that a device does not supply any power. Instead, the device only switches current on or off, like a simple switch.

Sinking - When active the output allows current to flow to a common ground. This is best selected when different voltages are supplied.

Sourcing - When active, current flows from a supply, through the output device and to ground. This method is best used when all devices use a single supply voltage.

This is also referred to as NPN (sinking) and PNP (sourcing). PNP is more popular. This will be covered in more detail in the chapter on sensors.

Inputs

In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8 or 16 inputs of the same type on each card. For discussion purposes we will discuss all inputs as if they have been purchased as cards. The list below shows typical ranges for input voltages, and is roughly in order of popularity.

12-24 Vdc

100-120 Vac

10-60 Vdc

12-24 Vac/dc

5 Vdc (TTL)

200-240 Vac

48 Vdc

24 Vac

PLC input cards rarely supply power, this means that an external power supply is needed to supply power for the inputs and sensors. The example in See An AC Input Card and Ladder Logic shows how to connect an AC input card.

An AC Input Card and Ladder Logic

In the example there are two inputs, one is a normally open push button, and the second is a temperature switch, or thermal relay. (NOTE: These symbols are standard and will be discussed in chapter 24.) Both of the switches are powered by the hot output of the 24Vac power supply - this is like the positive terminal on a DC supply. Power is supplied to the left side of both of the switches. When the switches are open there is no voltage passed to the input card. If either of the switches are closed power will be supplied to the input card. In this case inputs 1 and 3 are used - notice that the inputs start at 0. The input card compares these voltages to the common. If the input voltage is within a given tolerance range the inputs will switch on. Ladder logic is shown in the figure for the inputs. Here it uses Allen Bradley notation for PLC-5 racks. At the top is the location of the input card I:013 which indicates that the card is an Input card in rack 01 in slot 3. The input number on the card is shown below the contact as 01 and 03.

Many beginners become confused about where connections are needed in the circuit above. The key word to remember is circuit, which means that there is a full loop that the voltage must be able to follow. In See An AC Input Card and Ladder Logic we can start following the circuit (loop) at the power supply. The path goes through the switches, through the input card, and back to the power supply where it flows back through to the start. In a full PLC implementation there will be many circuits that must each be complete.

A second important concept is the common. Here the neutral on the power supply is the common, or reference voltage. In effect we have chosen this to be our 0V reference, and all other voltages are measured relative to it. If we had a second power supply, we would also need to connect the neutral so that both neutrals would be connected to the same common. Often common and ground will be confused. The common is a reference, or datum voltage that is used for 0V, but the ground is used to prevent shocks and damage to equipment. The ground is connected under a building to a metal pipe or grid in the ground. This is connected to the electrical system of a building, to the power outlets, where the metal cases of electrical equipment are connected. When power flows through the ground it is bad. Unfortunately many engineers, and manufacturers mix up ground and common. It is very common to find a power supply with the ground and common mislabeled.

One final concept that tends to trap beginners is that each input card is isolated. This means that if you have connected a common to only one card, then the other cards are not connected. When this happens the other cards will not work properly. You must connect a common for each of the output cards.

There are many trade-offs when deciding which type of input cards to use.

· DC voltages are usually lower, and therefore safer (i.e., 12-24V).

· DC inputs are very fast, AC inputs require a longer on-time. For example, a 60Hz wave may require up to 1/60sec for reasonable recognition.

· DC voltages can be connected to larger variety of electrical systems.

· AC signals are more immune to noise than DC, so they are suited to long distances, and noisy (magnetic) environments.

· AC power is easier and less expensive to supply to equipment.

· AC signals are very common in many existing automation devices.

Aside: PLC Input Circuits

Output Modules

As with input modules, output modules rarely supply any power, but instead act as switches. External power supplies are connected to the output card and the card will switch the power on or off for each output. Typical output voltages are listed below, and roughly ordered by popularity.

120 Vac

24 Vdc

12-48 Vac

12-48 Vdc

5Vdc (TTL)

230 Vac

These cards typically have 8 to 16 outputs of the same type and can be purchased with different current ratings. A common choice when purchasing output cards is relays, transistors or triacs. Relays are the most flexible output devices. They are capable of switching both AC and DC outputs. But, they are slower (about 10ms switching is typical), they are bulkier, they cost more, and they will wear out after millions of cycles. Relay outputs are often called dry contacts. Transistors are limited to DC outputs, and Triacs are limited to AC outputs. Transistor and triac outputs are called switched outputs.

- Dry contacts - a separate relay is dedicated to each output. This allows mixed voltages (AC or DC and voltage levels up to the maximum), as well as isolated outputs to protect other outputs and the PLC. Response times are often greater than 10ms. This method is the least sensitive to voltage variations and spikes.

- Switched outputs - a voltage is supplied to the PLC card, and the card switches it to different outputs using solid state circuitry (transistors, triacs, etc.) Triacs are well suited to AC devices requiring less than 1A. Transistor outputs use NPN or PNP transistors up to 1A typically. Their response time is well under 1ms.

Aside: PLC Output Circuits

Caution is required when building a system with both AC and DC outputs. If AC is accidentally connected to a DC transistor output it will only be on for the positive half of the cycle, and appear to be working with a diminished voltage. If DC is connected to an AC triac output it will turn on and appear to work, but you will not be able to turn it off without turning off the entire PLC.

A major issue with outputs is mixed power sources. It is good practice to isolate all power supplies and keep their commons separate, but this is not always feasible. Some output modules, such as relays, allow each output to have its own common. Other output cards require that multiple, or all, outputs on each card share the same common. Each output card will be isolated from the rest, so each common will have to be connected. It is common for beginners to only connect the common to one card, and forget the other cards - then only one card seems to work!

The output card shown in See An Example of a 24Vdc Output Card (Sinking) is an example of a 24Vdc output card that has a shared common. This type of output card would typically use transistors for the outputs.

An Example of a 24Vdc Output Card (Sinking)

In this example the outputs are connected to a low current light bulb (lamp) and a relay coil. Consider the circuit through the lamp, starting at the 24Vdc supply. When the output 07 is on, current can flow in 07 to the COM, thus completing the circuit, and allowing the light to turn on. If the output is off the current cannot flow, and the light will not turn on. The output 03 for the relay is connected in a similar way. When the output 03 is on, current will flow through the relay coil to close the contacts and supply 120Vac to the motor. Ladder logic for the outputs is shown in the bottom right of the figure. The notation is for an Allen Bradley PLC-5. The value at the top left of the outputs, O:012, indicates that the card is an output card, in rack 01, in slot 2 of the rack. To the bottom right of the outputs is the output number on the card 03 or 07. This card could have many different voltages applied from different sources, but all the power supplies would need a single shared common.

The circuits in See An Example of a 24Vdc Output Card With a Voltage Input (Sourcing) had the sequence of power supply, then device, then PLC card, then power supply. This requires that the output card have a common. Some output schemes reverse the device and PLC card, thereby replacing the common with a voltage input. The example in See An Example of a 24Vdc Output Card (Sinking) is repeated in See An Example of a 24Vdc Output Card With a Voltage Input (Sourcing) for a voltage supply card.

An Example of a 24Vdc Output Card With a Voltage Input (Sourcing)

In this example the positive terminal of the 24Vdc supply is connected to the output card directly. When an output is on power will be supplied to that output. For example, if output 07 is on then the supply voltage will be output to the lamp. Current will flow through the lamp and back to the common on the power supply. The operation is very similar for the relay switching the motor. Notice that the ladder logic (shown in the bottom right of the figure) is identical to that in See An Example of a 24Vdc Output Card (Sinking). With this type of output card only one power supply can be used.

We can also use relay outputs to switch the outputs. The example shown in See An Example of a 24Vdc Output Card (Sinking) and See An Example of a 24Vdc Output Card With a Voltage Input (Sourcing) is repeated yet again in See An Example of a Relay Output Card for relay output.

An Example of a Relay Output Card

In this example the 24Vdc supply is connected directly to both relays (note that this requires 2 connections now, whereas the previous example only required one.) When an output is activated the output switches on and power is delivered to the output devices. This layout is more similar to See An Example of a 24Vdc Output Card With a Voltage Input (Sourcing) with the outputs supplying voltage, but the relays could also be used to connect outputs to grounds, as in See An Example of a 24Vdc Output Card (Sinking). When using relay outputs it is possible to have each output isolated from the next. A relay output card could have AC and DC outputs beside each other.

RELAYS

Although relays are rarely used for control logic, they are still essential for switching large power loads. Some important terminology for relays is given below.

Contactor - Special relays for switching large current loads.

Motor Starter - Basically a contactor in series with an overload relay to cut off when too much current is drawn.

Arc Suppression - when any relay is opened or closed an arc will jump. This becomes a major problem with large relays. On relays switching AC this problem can be overcome by opening the relay when the voltage goes to zero (while crossing between negative and positive). When switching DC loads this problem can be minimized by blowing pressurized gas across during opening to suppress the arc formation.

AC coils - If a normal relay coil is driven by AC power the contacts will vibrate open and closed at the frequency of the AC power. This problem is overcome by adding a shading pole to the relay.

The most important consideration when selecting relays, or relay outputs on a PLC, is the rated current and voltage. If the rated voltage is exceeded, the contacts will wear out prematurely, or if the voltage is too high fire is possible. The rated current is the maximum current that should be used. When this is exceeded the device will become too hot, and it will fail sooner. The rated values are typically given for both AC and DC, although DC ratings are lower than AC. If the actual loads used are below the rated values the relays should work well indefinitely. If the values are exceeded a small amount the life of the relay will be shortened accordingly. Exceeding the values significantly may lead to immediate failure and permanent damage.

· Rated Voltage - The suggested operation voltage for the coil. Lower levels can result in failure to operate, voltages above shorten life.

· Rated Current - The maximum current before contact damage occurs (welding or melting).

A CASE STUDY

(Try the following case without looking at the solution in See Case Study for Press Wiring.) An electrical layout is needed for a hydraulic press. The press uses a 24Vdc double actuated solenoid valve to advance and retract the press. This device has a single common and two input wires. Putting 24Vdc on one wire will cause the press to advance, putting 24Vdc on the second wire will cause it to retract. The press is driven by a large hydraulic pump that requires 220Vac rated at 20A, this should be running as long as the press is on. The press is outfitted with three push buttons, one is a NC stop button, the other is a NO manual retract button, and the third is a NO start automatic cycle button. There are limit switches at the top and bottom of the press travels that must also be connected.

Case Study for Press Wiring

The input and output cards were both selected to be 24Vdc so that they may share a single 24Vdc power supply. In this case the solenoid valve was wired directly to the output card, while the hydraulic pump was connected indirectly using a relay (only the coil is shown for simplicity). This decision was primarily made because the hydraulic pump requires more current than any PLC can handle, but a relay would be relatively easy to purchase and install for that load. All of the input switches are connected to the same supply and to the inputs.

ELECTRICAL WIRING DIAGRAMS

When a controls cabinet is designed and constructed ladder diagrams are used to document the wiring. A basic wiring diagram is shown in See A Ladder Wiring Diagram. In this example the system would be supplied with AC power (120Vac or 220Vac) on the left and right rails. The lines of these diagrams are numbered, and these numbers are typically used to number wires when building the electrical system. The switch before line 010 is a master disconnect for the power to the entire system. A fuse is used after the disconnect to limit the maximum current drawn by the system. Line 020 of the diagram is used to control power to the outputs of the system. The stop button is normally closed, while the start button is normally open. The branch, and output of the rung are CR1, which is a master control relay. The PLC receives power on line 30 of the diagram.

The inputs to the PLC are all AC, and are shown on lines 040 to 070. Notice that Input I:0/0 is a set of contacts on the MCR CR1. The three other inputs are a normally open push button (050), a limit switch (060) and a normally closed push button (070). After line 080 the MCR CR1 can apply power to the outputs. These power the relay outputs of the PLC to control a red indicator light (040), a green indicator light (050), a solenoid (060), and another relay (080). The relay on line 080 switches a relay that turn on another device drill station.

A Ladder Wiring Diagram

In the wiring diagram the choice of a normally close stop button and a normally open start button are intentional. Consider line 020 in the wiring diagram. If the stop button is pushed it will open the switch, and power will not be able to flow to the control relay and output power will shut off. If the stop button is damaged, say by a wire falling off, the power will also be lost and the system will shut down - safely. If the stop button used was normally open and this happened the system would continue to operate while the stop button was unable to shut down the power. Now consider the start button. If the button was damaged, say a wire was disconnected, it would be unable to start the system, thus leaving the system unstarted and safe. In summary, all buttons that stop a system should be normally closed, while all buttons that start a system should be normally open.

JIC Wiring Symbols

To standardize electrical schematics, the Joint International Committee (JIC) symbols were developed, these are shown in See JIC Schematic Symbols, See JIC Schematic Symbols and See JIC Schematic Symbols.

JIC Schematic Symbols

SUMMARY

· PLC inputs condition AC or DC inputs to be detected by the logic of the PLC.

· Outputs are transistors (DC), triacs (AC) or relays (AC and DC).

· Input and output addresses are a function of the card location and input bit number.

· Electrical system schematics are documented with diagrams that look like ladder logic.

PRACTICE PROBLEMS

1. Can a PLC input switch a relay coil to control a motor?

(ans. no - a plc OUTPUT can switch a relay)

2. How do input and output cards act as an interface between the PLC and external devices?

(ans. input cards are connected to sensors to determine the state of the system. Output cards are connected to actuators that can drive the process.)

3. What is the difference between wiring a sourcing and sinking output?

(ans. sourcing outputs supply current that will pass through an electrical load to ground. Sinking inputs allow current to flow from the electrical load, to the common.

4. What is the difference between a motor starter and a contactor?

(ans. a motor starter typically has three phases)

5. Is AC or DC easier to interrupt?

(ans. AC is easier, it has a zero crossing)

6. What can happen if the rated voltage on a device is exceeded?

(ans. it will lead to premature failure)

7. What are the benefits of input/output modules?

(ans. by using separate modules, a PLC can be customized for different applications. If a single module fails, it can be replaced quickly, without having to replace the entire controller.

8. (for electrical engineers) Explain the operation of AC input and output conditioning circuits.

(ans. AC input conditioning circuits will rectify an AC input to a DC waveform with a ripple. This will be smoothed, and reduced to a reasonable voltage level to drive an optocoupler. An AC output circuit will switch an AC output with a triac, or a relay.)

9. What will happen if a DC output is switched by an AC output.

(ans. an AC output is a triac. When a triac output is turned off, it will not actually turn off until the AC voltage goes to 0V. Because DC voltages don't go to 0V, it will never turn off.)

10. Explain why a stop button must be normally closed and a start button must be normally open.

(ans. If a NC stop button is damaged, the machine will act as if the stop button was pushed and shut down safely. If a NO start button is damaged the machine will not be able to start.)

11. Describe what could happen if a normally closed start button was used on a system, and the wires to the button were cut.

12. Describe what could happen if a normally open stop button was used on a system and the wires to the button were cut.

13. For the circuit shown in the figure below, list the input and output addresses for the PLC. If switch A controls the light, switch B the motor, and C the solenoid, write a simple ladder logic program.

14. We have a PLC rack with a 24 VDC input card in slot 3, and a 120VAC output card in slot 2. The inputs are to be connected to 4 push buttons. The outputs are to drive a 120VAC lightbulb, a 240VAC motor, and a 24VDC operated hydraulic valve. Draw the electrical connections for the inputs and outputs. Show all other power supplies and other equipment/components required.

15. You are planning a project that will be controlled by a PLC. Before ordering parts you decide to plan the basic wiring and select appropriate input and output cards. The devices that we will use for inputs are 2 limit switches, a push button and a thermal switch. The output will be for a 24Vdc solenoid valve, a 110Vac light bulb, and a 220Vac 50HP motor. Sketch the basic wiring below including PLC cards.

16. Add three pushbuttons as inputs to the figure below. You must also select a power supply, and show all necessary wiring.

17. Three 120Vac outputs are to be connected to the output card below. Show the 120Vac source, and all wiring.

18. Sketch the wiring for PLC outputs that are listed below.

- a two way hydraulic solenoid valve

- a 24Vdc lamp

- a 120 Vac high current lamp

- a low current 12Vdc motor

19. a) For the input and output cards below, add three output lights and three normally open pushbutton inputs. b) Redraw the outputs so that it uses relay outputs.

20. Draw a wiring ladder diagram for PLC outputs that are listed below.

- a two way hydraulic solenoid valve

- a 24Vdc lamp

- a 120 Vac high current lamp

- a low current 12Vdc motor

PREFACE

TODO LIST

PROGRAMMABLE LOGIC CONTROLLERS

INTRODUCTION

Ladder Logic

Programming

PLC Connections

Ladder Logic Inputs

Ladder Logic Outputs

A CASE STUDY

SUMMARY

PRACTICE PROBLEMS

PLC HARDWARE

INTRODUCTION

INPUTS AND OUTPUTS

Inputs

Output Modules

RELAYS

A CASE STUDY

ELECTRICAL WIRING DIAGRAMS

JIC Wiring Symbols

SUMMARY

PRACTICE PROBLEMS

LOGICAL SENSORS

INTRODUCTION

SENSOR WIRING

Switches

Transistor Transistor Logic (TTL)