What Is A Plc

Published Date: 02 Nov 2017

PLC is an abbreviation for programmable logic controller; it is used for automation of electromechanical processes, mainly used in industries for control of machinery on factory assembly lines it is used widely in all sorts of places such as steel works, oil rigs or amusement rides. They are not alike general computers they are designed for multiple input and output variations, temperature variations they aren’t affected by electrical noise.

A PLC looks like a small box with screws on both ends and has small lights on it; they are like this because they are built to withstand harsh environments.

They are made by many different companies and they have no universal way of programming each company has a different way. Two examples of two major PLC makers are Siemens and Allen Bradley.

History of PLC’s

Before PLC’s companies had to use hundreds or thousands of relays, drum sequencers and cam timers for sequencing, assembly lines and safety interlock logic. To update the line for new models the process could take up to as long as a year and was very expensive with a high demand for electricians to rewire each relay individually.

Early computers needed specialist programmers for power quality, cleanliness and environmental control for temperature. Using general computer for process control required protection for the computer due to harsh environment or floor conditions. An industrial control computer would be built to handle the floor conditions, support input and output (bit form), it wouldn’t require years of training to use.

In roughly 1968 hydramatic requested for an electronic replacement for hard wired relay systems. Shortly after we had our first modular digital controller

Development

Early PLCs were designed to replace relay logic systems. These early PLCs and modern PLCs are programmed in "ladder logic", which works the same as a schematic diagram of relay logic but in a different language. This type of programming notation was chosen to reduce the amount of training needed for the existing technicians.

Types of PLC’s

Unitary- are compact, they are also connected direct to the machine or equipment that they are controlling. Each unitary module will have one purpose and cannot be altered after it has been constructed.

Unitary PLC's are most useful if used in an industry which does not or very rarely changes its production techniques and does not require much use of PLC use for small tasks.

Unitary plcs are not practical if future changes are due

Unitary PLC's aren’t as expensive as others and are more compact than others.

Modular- are easily adapted so are useful in industries where change is required. it has a rail on a cabinet where the system is placed upon. This type of system has a power supply, CPU and input and output control. This system is not alike the unitary system it is easily adapted.

it can have upgrades of power supplies or CPU’s and modules added to different sections.

You can mount these systems on rails or in cabinets where they will be safe and secure.

Because modular PLC’s are multi-functional they prove to be very practical, the downfall of the modular system is that they are a lot bigger and more costly than unitary PLC’s

Rack mounted- Rack mounted PLC's are pretty much compact mounted modular PLC’s. They are multi-adaptive and are modified easily, similar as to modular PLC's.

The main difference between the two is in the name and that is that rack mounted PLC’s can be mounted to racks as seen on the diagram below.

The rack mounted PLC’s are the most advantageous of all three types these advantages are:

Multi adaptive features

Very compact

The biggest downfall of the rack mounted system is that they are far more expensive than the other systems.

Input and output devices

Inputs to, and outputs from a PLC are needed to control and monitor processes. Outputs to actuators tell a PLC to activate and this activation makes something happen, depending on what has been programmed to the plc. A list of common actuators is given below.

Solenoid Valves -once given a signal can switch a pneumatic or hydraulic valve for change of flow.

Lights – lights can be used for warnings or just power signals.

Motor Starters – because motors draw a large amount of current when they are started, they need a motor starter this takes some of the load these are pretty much just like large relays.

Inputs come from sensors that translate physical activity into electrical signals. Examples of common sensors that are used today are listed below.

Proximity Switches – can use a variety of things to detect an object it can be set to use light, capacitance or inductance

Switches – either open or close

In smaller PLC’s the inputs are specified when purchasing with the inputs built in. this is different for larger PLCs on these the inputs are purchased separately as cards, or modules, with 8 or 16 inputs of the same type on each card dependant an what is needed. I will show these inputs as if they have been bought as cards. The list below shows the ranges for input cards.

12-24 Vdc

12-24 Vac/dc

200-240 Vac

24 Vac

Output modules

Output modules rarely supply any power, but they act as switches. An external power supply must be connected to the output card and the card will switch the power from on/off for each output. Again typical voltages used are listed below

24 Vdc

12-48 Vac

12-48 Vdc

230 Vac

These cards have 8-16 outputs of the same type and can also be purchased with different current ratings. When choosing output cards a common choice is transistors and relays. Relays are the most flexible output devices. They are flexible because they are capable of switching both AC and DC outputs. The downfall of them is that they are slower, they are bigger, they are more expensive, and they will eventually wear out because of the constant switching. Transistors are limited to DC outputs are limited to AC outputs. Transistor outputs are called switched outputs. Switched outputs are fast, they are solid state circuitry.

Input devices

Input devices are used for many things sensors are widely used here is a list of the things that they are used for

temperature

speed

pressure

quantity

weight

depth

density

flow rate

voltage

power

torque

position (angular and linear)

Some of the sensors determine if something is on or off

simple switches (start and stop)

relays

proximity switches

voltage sensing relays

micro switches

To control the position of actuators (pneumatic, hydraulic, electrical) sensors may be placed on or on the machine they move. These detect the intended position has been reached (e.g. a safety switch to ensure the guard is in place) if the controls are electrically (solenoid) operated, simple mechanically operated switches may be used (micro switches)

Switches and valves maybe normally open

Or normally closed i.e. the emergency stop button

In many cases it is best to fit the sensor to the actuator. Cylinders are often fitted with reed switches; these are activated by a magnet fixed in the piston. These will only work if the barrel is made up of a non-magnetic material

There are a range of devices which switch on when something comes close to them. These are called proximity switches. The switching signal is turned on or off when the sensor is activated. Some will detect any material, there are others that only detect iron or just metals these may be handy in food factories to ensure nothing has worn into products etc.

A very similar sensor uses light beams and sensors. Often the light used is infra-red. These sensors switch on or off when the light is interrupted. These might be used for an item passing on a conveyor belt. They can be used to activate a cylinder to push and are widely used to count the number of objects passing.

Voltage sensing relays

These are used with analogue devices that produce a voltage representing the variable (e.g a D.C tachometer for measuring speed) the unit is adjusted to trip when a certain point is reached i.e. when a motor has reached its correct speed

Input voltages

Typical input voltages are 12v and 24v

They may accept A.C or D.C no two PC’s are the same so you must take care to check the input rating

Output devices

Output devices are switched on by the PLC. This can be anything that is electrical such as the following:

D.C motor (to start a conveyor belt)

A.C motor (to start a pump)

Linear electric actuator

Solenoid valve in hydraulic or pneumatic systems

Lights (i.e. traffic lights)

Alarms (fire alarm)

Heating elements (glue heater for packaging)

Typical switches are 12V, 24V, 110 and 240V. In many cases, the PLC cannot switch the device directly because of the high voltage or current needed. In this case power switching relays or transistors are used.

Relays

Some output devices cannot switch high currents directly because the module would be damaged by high currents.

They have to be interfaced to the hard ware by relays. a relay is used to allow small current to operate devices with high current ratings. The relay is a mechanical switch the contacts are moved by a solenoid.

Communicating

A PLC must be able to communicate with other devices. This is needed to link them to programming devices and to other equipment. Many modern instruments send and receive information digitally so they are connected to the PLC by some sort of network. Dependant on the place of work some PLC’s may form part of a larger system controlled by a mainframe computer. The PLC’s must be linked to each other and to the computer by a network.

the above diagram connecting a mainframe computer to a series of PLC’s.

Links may be made through cables using serial data or parallel data through a ribbon cable (e.g. the ribbon cable linking a disc drive to a motherboard in a computer). Serial data only requires two wires (e.g. a modem) although often many more are used

Twisted pair-when information is sent down two wires, a twisted pair is used. An example of this copper wire connects your landline telephone to the network.

To reduce picking up noise the two insulated copper wires are twisted around each other, more than one twisted pair may be placed inside an outer insulated layer and sometimes the cable is screened or shielded by a grounded outer layer. Twisted pairs come with each pair uniquely colour coded when it is packaged in multiple pairs. Different uses such as analogue, digital and Ethernet require different pair multiples.

Co axial cables-is used to connect your TV to an aerial. It is also used to connect telephone exchanges to telephone poles near to users. It is also used to connect computers and PLC’s with systems such as Ethernet and other types of local area network.

The cable has an inner conductor surrounded by concentric conductor (coaxial with it) made from copper mesh and separated by a layer of insulation. The outer layer is usually grounded. They can carry information for a great distance.

Ribbon cable

Fibre optics- fibre optics are basically thin strands of glass. When light is shone into one end of a strand (eg by a laser) it is carried over enormous distances without losing its strength. The light can be pulsed to carry digital information at incredible speed rates. Optical fibre carries much more information than copper and in general is not subject to electromagnetic interference. Many strands can be bundled together to give many more channels. Computers needing high data transmission usually have fibre optic links to server.

Programming of drilling rig system

Attached to this document is a ladder logic programme matching a drilling rig system. Within this system there is a start button which activates the process there are 3 cylinders, cylinder one pushes an object under the drilling unit, cylinder 2 the drilling unit extends to its maximum then returns to its up position, once cylinder 2 is fully up, cylinder 3 can retract a let the object fall out, Cylinder 3 and 1 return to their home position. The system has a reset ready for the next activation of the start button. If the stop is activated the unit must hold in position. If the reset is activated the unit must sequentially return to its home position.

On the ladder logic programme

Firstly you will notice two parrellel lines travelling down the page these are your power lines travelling left to right, positive to nogeative. Withing these lines are a sequence of inputs and outputs that meet the process of the drilling rig system.

Line 1

On this line there is a start, stop a N/C reset contact and a reset coil once the start is pressed power will pass from left to right energising coil R1 this will then latch contact R1 so your then able leave go of the start button while the process is carrying on this is an example of latching.

Line 2

Here once R1 has latched in and proxy switch sw4 sw1 and sw5 are met s1 will be energised which is the left hand cylinder this will move the drilling piece underneath the drill head these conditions have to be met before the solenoid can be energised.

Line 3

There are another 3 switches here sw2 sw4 and sw5 must all be made to activate s5. There is a lockout contact here as well these are used to drop out certain sequences in the programme so the same sequences can be used again further down for the reverse process.

Line 4

Here sw6 and sw1 are accompanied by the lockout coil when once energised will open the previous contact on line 3 cutting out that sequence.

Line 5

When sw6 is made on this line it will activate sol6 retracting the main drill head which was pulled down on line 3.

Line 6

This line you have a N/O lockout contact which will close once the lockout coil is energised which it has been. Then once sw2 sw4 and sw5 are made s3 will be activated, retracting the right hand side solenoid. You may have notice a N/C lockout contact labelled 2 this is our second lockout coil this is there for the same as the first to let us use these conditions again further down the programme.

Line 7

Here our lockout coil N/O would be made so when sw2 sw5 and sw3 are made it will activate s4 pulling out the right hand side cylinder.

Line 8

Here sw3 has been latched so once the power has passed sw3 and sw1 and the lockout coil 2 is energised sw3 will latch so lockout coil 2 will remain energised.

Line 9

Once the two N/O contacts are made which are lockout coil and lockout coil 2 and sw2 sw5 and sw4 are made s2 can be energised retracting the left hand cylinder which is then the end of the process.

Reset system

You may have noticed reset contacts and coils dotted on parts of the programme these have been inserted mostly before solenoid valves, you are able to stop the machine anytime through its process and reset it, so wherever the drilling rigs position the cylinders and solenoids will react to bring it to its home position so the drilling rig can be restarted and the process can continue.