The Plcs And Fluid Power Systems

Published Date: 02 Nov 2017

Marc Joseph

Contents

Design

Unitary

The term Unitary PLC describes a PLC which is a singular unit.  This would include the power supply, CPU, file storage and all relevant I/O.  While the design is considered old fashioned in industry they still have several advantages, cost being key.http://www.plcedge.com/image-files/plc-fixed-architecture.jpg

However this also comes with some significant drawbacks as a single I/O slot failing can cause the entire unit to become useless, leading to costly repairs or replacement.  Another factor is the time it takes to replace the unit, rewire all of the I/O and also reprogram the PLC.  These costs typically mean that Unitary PLCs are not used for critical plant or machinery which could cost a business a lot of money if they are offline for a long time.

For these reasons Unitary PLCs aren’t really used in industry any more, a majority of modern industries have upgraded to Rack Mounted PLCs for the benefits seen there.

Advantages

Allow automation in industry

Disadvantages

Expensive to purchase

Needed to train workforce in their operation

In case of failure the whole PLC would need to be replaced

Modular

Modular PLCs are a more modern version of Unitary PLCs, they contain everything that a Unitary PLC would contain but also allow for the amount of inputs and outputs to be increased through adding larger I/O racks.  The amount of I/O that can be added however is limited by the CPU of the PLC, if more I/O is needed that the PLC will allow it may be necessary to replace the PLC itself.

Similar to Unitary PLCs but are upgradable, they are typically cheaper than full Rack Mounted PLCs but are more flexible.  They also can allow some components to be replaced.

Modular PLCs typically don’t include their own power supply which means that a separate power supply must be used to power the PLC.  Fortunately these power supplies are quite compact meaning that can easily be installed in the same cabinet as the PLC itself.http://media4.rsdelivers.cataloguesolutions.com/LargeProductImages/R468422-01.jpg

Advantages

Can upgrade the number of I/O slots for upgrades

Some components can be replaced if faulty

Disadvantages

Expensive when compared to unitary PLCs

Rack Mounted

A rack mounted PLC is exactly what it sounds like, all the necessary components for the PLC are mounted onto the back plate. The back plate carries power and and communications to wherever it I needed on the PLC, Cards such as the CPU, I/O or Power supply are attached to this back plate.

Typically a rack will have 8 or 16 I/O cards, a Power supply and a PSU but some models might have backup units to help prevent failures. The CPU and PSU used in a rack mounted PLC are decided based upon the requirements of the system and how many I/O cards are expected to be installed. These separate cards can be replaced or upgraded very quickly and simply meaning that maintenance times can be kept to a minimum.

http://www.pjboner.com/wp-content/uploads/wpsc/product_images/rx3i.jpg

Advantages

Upgradeable CPU and Power Supply

Expandable I/O rack

Faulty components can be replaced quickly

Disadvantages

High initial costs

Components

Back plate

The back plate is the rack that the cards are slotted into, it is sometimes referred to as the spine of the PLC as any problems with the back plate can cause problems for the whole PLC.

The two main functions of the back plate are firstly, to carry power from the power supply to the various PLC cards, but also it contains a data-bus which sends data between the CPU and I/O cards.https://lh4.googleusercontent.com/Z09NnO1Vo0JZaMwsmnOSSStTuIBxyu_cn54hx9WFR6J9ZBrlRW08sCcSkIC5-hIuSVXpUTqRYNeRZvzQU-HBAQkzXn4Yoyz-patlUvY8sFEInjQGm3UE

Power Supplies

Power supplies used in PLCs are specially designed to be very stable, reliable sources of dc power. Any ripple or power surges could cause serious damage to the CPU so power supplies are typically relatively expensive units.  They are typically supplied with 100 to 240VAC and step it down to a 24VDC supply for the PLC.

The current output of a given power supply should be chosen based upon the amount of I/O connected to the PLC, for example a cheap 0.5A power supply would be sufficient for several I/O cards but a more powerful 1.3A power supply would likely be enough to power as much I/O as you would be likely to put on a single PLC rack.

CPU Cards

The CPU (or central processing unit) is the brain of the PLC, it is where I/O signals are sent, where the ladder logic is stored and communications ports to allow the program to be charged or backed up.  Every PLC must contain a CPU to perform it’s job.  The diagram below shows the various components that make this possible.

MEMORY

The memory is split into two main sections, the ROM stores the ladder program, this can be read by the processor and altered by a technician if necessary.  There is also a short term memory which allows the CPU to make complex calculations and remember bits of information.https://lh6.googleusercontent.com/kBFjgMeb-EsZZKH8WYZ2VrbXKE2D3wwszoko9yASYyHaHYmRukpp9tmI1dLLEmNkMem6rZ2pC3_HaIshWsOh3i5gz0_JIgjE2SjM7ZU0Hm14K4rrlCvT

Microprocessor

The microprocessor works its way through the ladder of the PLC, making decisions and calculations; this component makes the PLC possible.

The microprocessor works based on a scan cycle that can be broken up into three main steps, reading the status of input devices, interpreting the ladder logic and then controlling the outputs of the PLC.

During the reading input devices stage the PLC determines the state of it’s inputs, the second stage the PLC reads the ladder of the PLC and determines whether it needs to take any action during the third stage of the cycle. In this third stage the PLC updates all of it’s output signals to reflect what they should be according to the ladder.

This cycle is extremely quick and happens many times a second, typically as high as once every few milliseconds however this can be affected by the size of the program and the clock speed of the PLC’s processor.

Battery Backup

The battery backup is exactly what it sounds like, a backup power supply for the PLC that can allow the processor to operate for around 30 minutes if it’s power supply fails. Anything longer and information about the plant is likely to be lost.  The battery will supply enough current to allow I/O to operate during this period. Very rarely (around once every 3 years) a PLC’s battery will fail and need to be replaced, this will likely be indicated on a HMI screen.

Communication Card

The communications card in a PLC is used to link the PLC either with backup/redundant PLCs or other equipment such as variable speed drives.  Multiple communications cards can be connected to each PLC rack to allow them to communicate with multiple drives or PLCs.

Most communications cards contain multiple communications ports, typically labelled Channel A, Channel B, ect.  These ports will perform independently and can  carry out completely different jobs simultaneously.

For more information about the types of links that are found in communications cards see the "PLC Information & Communication Techniques" section found later in this manual.

I/O Cards

I/O cards can be split into 4 main categories: Digital Input, Digital Output, Analogue Input & Analogue Output.  Some manufacturers produce analogue cards which can be used as either outputs or inputs but most produce two separate cards for these functions.

For some more examples of what might be connected to an I/O card or the internals of the card refer to the "I/O" section found later in the manual.

Digital Inputs

Probably the most common card used in a PLC, the digital input covers items like pushbuttons, proximity switches, relays and limits. The significance of these pieces of equipment is that they are either on or off, they have no in-between states (hence digital).

The signals being sent to the digital input cards are typically extra low voltage levels, switching between 0 and 5 or 10 volts. The PLC will read this as either a 0 or a 1 respectively.http://upload.wikimedia.org/wikipedia/commons/thumb/1/10/Optoisolator_topologies_both.svg/220px-Optoisolator_topologies_both.svg.png

The digital input cards themselves must be protected from potential current surges or electrical faults and so opto-isolators are used to electrically isolate the cards. Opto-isolators have two main components inside them, a Light-Emitting Diode (LED) and a photo sensor which is used to detect the light energy and controls an electrical current which is then used as a signal. This effectively isolates the PLC from the sensor.

Analogue Inputs

An analogue input card allows variable measurements to be taken by the PLC. This allows for a lot more complex operations to take place, an analogue input cards might use a level sensor to gauge the level of water in a tank, a RTD to gauge temperature or a lux meter to measure light levels.

The signals sent back to the analogue input cards are usually in the 0 to 10 volts or 4 to 20 mA ranges. This depends on the instrument and application. The advantage to using current over voltage is that volt drop is no longer an issue however a transducer may be required for the conversion. The advantage of not using 0 as the lower end of the scale is that cable disconnection faults can be detected by the PLC instead of the PLC assuming that the minimum valve is being read constantly.J:\PLCs\1-s2.0-S0376738810007295-gr9.jpg

The signal sent back to the input card is not necessarily a linear representation of the output from the sensor. The transmitter or PLC may instead use a different scale such as square root as the example to the right does.

Digital Outputs

A digital output is a low voltage signal sent from the PLC. This is usually as low as a 5 volt signal so cannot be used to control much more than a lamp or relay.

As this is the case digital outputs are usually connected to relays or contactors which allow much larger voltages to be activated. However the only possible outputs from a digital output card are on or off meaning that control is limited to these two states, if more control than this is needed an analogue output card should be used.

Opto-isolators are also used to protect the digital outputs in much the same way that they are used in digital inputs.

Analogue Outputs

Predictably the analogue output card does the opposite of the analogue input card; instead of receiving a signal from sensors it instead sends a signal to equipment. This is typically a 4 to 20 mA signal.

This type of outputs is used to control devices with much finer control than digital outputs are capable of. These signals can be used a positional references for valves, speed references for motors and many other useful applications.

Networking Card

The networking card allows programs and technicians to communicate with the PLC. This card can be found as a part of the PLC but in some cases it is installed onto the PLC’s rack as a separate unit. This is so the card enjoys the benefits of the rack design such as easy replacement and upgradeability

These are most commonly used as an aid for fault finding on plant, a technician with a laptop can interrogate the PLC and discover the nature of a fault without having to check each instrument on site.

The networking card also allows the PLCs to be networked, connecting them to servers and more complex systems which allow operators to make changes and requests of the PLC. It also allows the operators to be aware of plant conditions at all times, keeping them informed and make them able to make better decisions.

I/o devices

Below are some examples of various types of sensor, their applications and limitations. This is not an exhaustive list but an introduction to some of the most common types of I/O connected to a PLC.

Sensors

There are many different types of sensors that can be used with a PLC, they can be either digital or analogue depending on the sensor type and application.

Proximity Sensors

A proximity sensor is used to detect the presence of objects. This can use many different methods, most commonly an electromagnetic field or inductive system. These are commonly used to detect the presence or absence of an object, typically items on a production line. The type of sensor must be carefully considered when choosing and testing these sensors, an electromagnetic sensor may only pick up metallic objects

Photoelectric Sensor

Photoelectric sensors are used to detect the presence of an item by using a light transmitter and receiver; these can either be separate devices or one discreet unit. http://www.omron-ap.com/product_info/E3G/photoelectric_sensor_e3g_application_3_small.jpg

Most commonly there will actually be three main components to a photoelectric sensor, a light source, a light sensor and a separate control panel that contains the electronics such as amplifiers and relays. The advantage to this system is that the sensor can now be very small and inexpensive to replace so can be left in more hazardous areas without risk to the more expensive control gear.

Another consideration when choosing a photoelectric sensor is what objects the sensor is likely to be detecting. A very shinny or reflective material could reflect the beam instead of refracting it tricking the sensor into now seeing the object at all.

More complex photoelectric sensors are capable of measuring the distance an object is away from the sensor meaning that positional data can be obtained without relying on mechanical devices which can bend or break.

Mechanical Switcheshttp://www.rapidonline.com/catalogueimages/module/M030255P01WL.jpg

Mechanical switches are very common in industry and range from simple toggle switches to more complex magnetic reed switches.

Toggle switches

Toggle switches are used where operators are likely to need to adjust machinery manually. They range from simple On-Off type switches to potentiometors that allow analogue control of machinery by varying the current in a loop.

(Needs Stuff) Read Switches

(Needs Stuff) Relays

Micro switches

(Needs Stuff) Signalling

Analogue digi signals

Tele uses ana

Programming & logic

Ladder Logic Symbols

Ladder logic symbols are designed to be easily recognisable to those familiar with electrical circuits, maybe of the symbols are shared or very similar to their electrical counterparts. However each PLC manufacturer may have their own variation of a symbol so it can be worth checking the documentation for the particular PLC being used.

Below are some examples of common symbols used and their meaning.

A normally open (NO) contact, allows current to pass if the coil controlling the contact is in the on position.

A normally closed (NC) contact allows current to pass if the coil controlling the contact is in the off position.

An output or coil, if current reaches this coil then either the assigned output is energised or the programs coil is energised.

Simple Ladder Logic

PLC’s make decisions based on their programming; this is typically abstracted away into what is known as ladder logic. Ladder logic uses blocks of simple instructions and inputs that can be placed in a certain order to perform more complex tasks, the name ladder logic comes from the shape of the programming, divided up into multiple layers of instructions known as rungs.

The diagram above shows two examples of ladder logic, Fig. A shows an AND circuit where both Input 1 and Input 2 must be on for the Output 1 to be energised. Fig. B shows an OR circuit where either Input 3 or Input 4 must be energised for Output 2 to be energised. This shows how in ladder logic the positioning of the various blocks can be as important as which blocks are used.

Fig. C below shows an example of how a retaining contact can be used to keep a contact powered. The Start push button is used to start the pump. This then energises the Input labelled Pump. While the pump is running the Input Pump keeps the pump running until the normally closed contact labelled Stop is pushed, this would break the connection to the circuit, disconnect the Pump and also switch off the Input Pump preventing the pump from switching back on until the Start push button is pressed again.

In application the Stop push button would be left as a normally open Input with the push button itself being wired as normally closed. This is done so that the push button itself is fail safe, if a connection to the pushbutton the motor would stop safely. If the push button was wired as a normally open as shown in Fig. C above then a break in the wires connecting it to the PLC would mean that the pump would be unstoppable in an emergency and therefore unsafe.

Worked Example of ladder logic

This example shows how a PLC could be programmed to perform a manufacturing process. In this example there are 3 pistons being used, the first to position an object, the second representing a drill and the third removing the object.

The sequence of operation is this:

First piston moves object into position

Drill (represented by the second piston) descends fully and then retracts

Third piston retracts to remove the piece and then returns to it’s start position

First piston returns

Sequence restarts

NB: At all times an emergency stop must be able to stop the sequence.

System Layout

Annotated program

[Type the sidebar content. A sidebar is a standalone supplement to the main document. It is often aligned on the left or right of the page, or located at the top or bottom. Use the Drawing Tools tab to change the formatting of the sidebar text box.]The E-Stop here represents the emergency stop which can halt the operation at any time by cutting the supply to the circuit. The second contact in this second is used to reset the circuit once the program has run its course.

This first rung of the ladder contains the start pushbutton and a relay which is used to activate the next rung of the ladder.

This rung contains the output of the starter relay and the relay which starts the first piston moving. S2 is a normally closed contact which opens when the piston hits the s2 sensor (see the System Layout subheading for positional information)

This rung uses sensor S2 to activate the second piston (the drill), this continues to push the drill down until it activates the S4 sensor which opens the contact and stops the piston trying to push down.

This rung uses a retaining contact in the relay to remember when contact S4 has been hit. The purpose of this contact is to prevent the second piston from trying to return once it has fully extended. This is reset once the first piston has returned to its original position.

This rung retracts the second piston once it has extended fully.

This rung pushes the third piston into position as soon as the system is energized. This ensures that it is in the correct position when it is needed. It also pushes the piston back into position once the sequence has been completed.

This rung is used to remember when sensor S5 has been hit (using the same principle used in rung 5). However it also sensor s4 to have been activated first. This ensures the program activates in the correct sequence.

This rung pulls the third piston back in so long as the other 2 pistons are in the correct positions and have been allowed to carry out their operations (drilling ect)

This rung is used to retract the first piston once the sequence has been completed.

This final rung is used to reset the circuit once all operations have been completed. This is the second contact mentioned in the first bullet point of Annotated Program. This means that the system is now ready to be activated again as soon as the start button is pressed.

Functional Block Programming

This is an example taken from a simple automatic trash screen switchover operation from the granulated coal injection plant at Tata Steel. It uses a pressure sensor to detect when a screen has become blocked so it can automatically switchover to a bypass line.

The example being shown here is the alarm that initiates the switchover procedure. The pressure is being scaled by the PLC so 1 bar is taken as analogue input with 1 bar represented as 100, 6 bar represented as 600.

There are 2 greater than blocks in this rung that both need to be activated to allow the timer to begin counting to 10. The purpose of this is to prevent the switchover being activated by a pressure spike or minor blockage.

Once this timer has counted to 10 seconds it activates a retaining contact and closes a contact which activates the alarm itself, beginning the switchover. The reason for there being 2 greater than blocks (GRT) is that the second greater than block is used to cancel the alarm if the pressure drops below 0.75 bar again, this usually indicates that whatever was blocking the trash screen has been blown through or destroyed.

Possibly safety if I can be fucked