Keywords: industrial uses of plcs, characteristics of plcs, plc design
Overview
Before PLC's were created many establishments like the automobile industry used thousands of relays to regulate their processes. This was very frustrating and expensive, sometimes resulting in a two calendar year change around between different products because electricians could have had to independently rewire every relay to improve a development system for an alternative line of cars etc.
Programmable Reasoning Controllers or PLC's were first designed in 1968 when Standard Motors decided a replacement for this type of process was needed. PLC's are designed for multiple inputs and outputs.
A PLC is essentially a little computer which is employed for mainly industrial purposes but also offers other uses.
Industrial uses could be:
- Packaging lines
- Robots
- Hydraulic machines
- Pneumatic machines
- Other uses could be;
- Traffic lights
- Car auto parking barriers
- Signalling systems
PLC Units
There are three types of programmable logic controllers (PLC's).
They are Unitary, Modular and Rack installed.
All three types have different features both bad and the good which make each kind suitable for a number of applications.
Unitary PLC Design and characteristics
A Unitary PLC consists of all the features of a simple system in one compact unit, the features include;
- A power.
- The main component which includes a central handling device (CPU).
- The input component.
- The output component.
Unitary PLC's are equipped directly to the aspect or machine they are controlling.
The advantages of a unitary PLC:
- They are small and compact.
- They keep all the essential components in a single unit.
- They allow lightweight and easy access.
- They are usually the cheapest type of PLC.
The disadvantages of an unitary PLC:
- They cannot be expanded.
- If any feature fails then your whole unit needs to be replaced.
- They are extremely simple and basic.
Applications
Unitary PLC's can be utilized for any request that does not require a great deal of inputs or outputs. For instance a car area barrier, this would not use many inputs/ outputs and would only require a simple program.
Modular PLC Design and Characteristics
Modular PLC's are a system of modules that can be slotted together to develop a system. The basic modules contain;
- A power supply.
- The main component which consists of a central processing unit (CPU).
- The input module.
- The output component.
Other types of module can be fastened as well as extra source and output modules to boost the capacity or to handle changes in hardware system.
The features of a modular PLC:
- The amount of suggestions and result terminals can be widened to cope with any changes to the hardware system.
- If any feature fails then only that part must be changed cutting down on cost.
The disadvantages of any modular PLC:
- They are expensive in comparison to unitary PLC's.
Applications
Modular PLC's are useful for applications where there would be a whole lot of inputs and outputs because more insight/output modules can be added if needed. This type of PLC is trusted in the processing industry to regulate process lines.
Rack-Mounted PLC Design and Characteristics
The design and characteristics of a rack mounted system are extremely similar to a modular system however these modules are on standard cards which then slot machine jointly into a rack in the control pantry. These modules connect via the rack.
The benefits of a rack-mounted PLC:
- They are easily modified or extended.
- They have more I/O details than some other type of PLC.
- If any feature fails then only that part has to be changed saving on cost.
The disadvantages of the rack-mounted PLC:
- They are usually the most expensive of the three PLC's
Applications
Like the modular PLC this type of system is widely used in the processing industry. That is mainly as a result of amount of source/output tips that the machine can contain but also since it has the capacity to talk to other systems.
Internal Workings of an PLC
CPU
The CPU- Central Control Unit operates as the brain of the PLC. It contains a memory device into which the PLC program is written into.
It is actually used to process the info it will get from inputs and bears out instructions to the outputs in line with the logic programmed into it.
This process is called the scan routine and it occurs every 5ms. The scan circuit is shown below.
(Ref 2)
When an application is written on a programming device like a laptop it is uploaded to the CPU, once it has been submitted it is then written into the CPU's storage. The coding device can then be disconnected without shedding any data from the PLC because the program is now saved into the storage of the CPU.
Only the unitary PLC has a CPU built into it, on the modular and rack-mounted PLC's the CPU comes as another module. On modular PLC's the CPU would be the first component whereas on rack-mounted PLC's the CPU is governed by the quantity of inputs and outputs that are being used.
Input and Productivity Devices
In PLC's data is accumulated from inputs which are connected to the type modules and sent to the CPU, this data is then carried out within the program reasoning and then sent to the productivity devices via the output modules to perform a specific task.
The inputs and outputs are allocated a guide point in the PLC in order that they are often recognisable when programming.
An suggestions device is any peripheral piece of hardware that is utilized to send data information and control impulses to any information processing system, in cases like this the PLC.
In conditions of PLC's there are two different types of inputs, analogue and digital. An example of an analogue input will be a sensor or transmitter and a good example of a digital source would be a switch, force button etc.
Inductive closeness sensor
An inductive closeness sensor will find metal surfaces or devices without coming into contact with them. The sensor face comprises of windings from the oscillator. These form an alternating magnetic field round the sensor face named an inductance loop. The inductance of the loop changes if a steel is placed within it because metals are much more effective inductors than other styles of material. Once the inductance loop is improved the change is diagnosed by sensing circuitry which then signals to another device like a PLC.
Capacitive sensor
A capacitive sensor will detect any surface or device without getting into contact with it.
A capacitive sensor uses an alternating voltage which in turn causes the positions of the charges to constantly reverse. This then creates an alternating electric energy which is diagnosed by the sensor face. The sensor face is made up of capacitors from the oscillator.
The amount of current movement depends upon the capacitance, and the capacitance is determined by the region and proximity of the conductive objects.
Therefore the larger and closer the thing then the better the current will be and vice versa.
Photo Electric Sensor
A photoelectric sensor uses light to use. When a preset degree of light is found by the sensor the sensor switches. This type of sensor is employed to identify moving objects, for example containers over a conveyor are discovered because of the containers breaking the line of light. After the collection has been broken the switch signal is fed in to the electrical power control circuit which provides a corrective action.
There are three types of photo electric sensor. These are Reflective, retro-reflective and individual type.
Optical Reflective Sensor
The light is mirrored off the top of bottle back again to the recipient.
Optical Retro-reflective Sensor
The light is sent and demonstrates off a reflective surface back again to the device.
Optical Individual Sensor
Outputs
An result device is actually any device that is controlled via the result module of any PLC.
In terms of PLC's there are two different kinds of outputs, analogue and digital.
Analogue devices are devices that the productivity can be mixed.
Digital outputs are being used to control two condition devices, this means that they are really either on or off.
An example of an analogue end result would be motor swiftness; valve position etc and a good example of a digital outcome would be a relay, solenoid valve etc.
Indicators
Indicator light
The simplest of the PLC outputs, the sign light simply shows that a certain function has worked effectively, for example if the machine was in run mode then a green sign light may be activated to inform providers etc that the machine is working.
Indicator buzzer
Acts in the same process as the light but uses audio; it works more effectively at alerting you to definitely a problem especially if that person does not have a line of sight of the machine. For instance if there is one that has brought on a machine to avoid then the buzzer would sound.
Solenoid valve
A solenoid valve can be operated electrically and pneumatically. In the case of a normally available 3/2 valve it uses one of the to change it from off to on.
Solenoids are named after the amount of says or ports they have got and also what function they do. For example a 3/2 spring go back solenoid has 3 slots, 2 states as soon as it's been deactivated it returns back again to its natural position via a spring.
The 3/2 valve below is shown in two periods.
Stage 1 is where the valve is in its natural point out where air is flowing from 'P' (mid-air source) to 'b' (the exhaust). Whereas in stage 2 the valve is activated allowing air to move from 'P' to 'a' where in fact the air is sent to another device for example a cylinder.
Stage 1
(Ref 1)
Stage 2
(Ref 2)
Signalling
Signalling identifies the utilization of alerts for controlling marketing communications.
Signals can be either digital or analogue.
With digital impulses the transmission is logic therefore it is either a '1' (True) or a '0' (False).
With analogue signals the entity is continuous where the sign is constantly differing based on real-time. Including the amount of light a sensor obtains is an analogue signal since it can be any value within the number that the sensor can pick up.
(Ref 1)
In some systems both digital and analogue signalling is needed. For example a house phone shares the same network connection as a personal computer. In cases like this a tool called a modem is used to convert the transmission between your two.
(Ref 2)http://geekrich. com/wp-content/uploads/2011/09/home-network-diagram. gif
Networks
A PLC must talk to other devices to be able to operate. These devices are connected to the PLC with a network.
There are many types of network such as:
Bus Network
A bus network is a long cable behaving as a backbone which links devices along, the backbone has nodes connected to it via a single brand. The signal journeys in both directions until these are terminated at either end.
The amount of nodes that can be attached however is limited to the strength of the sign.
http://www. olson-technology. com/mr_fiber/images/bus_network. gif
(Ref 1)
LAN- GEOGRAPHIC AREA Network
A LAN network is used to connect different hosts in a tiny area such as an office or a little building. Data is sent in the form of the data packet which includes the address of both the sender and the vacation spot.
http://berbagisolusi. data files. wordpress. com/2011/12/lanwan11. gif
(Ref 1)
Programming
There are three main types of encoding that are used in PLC's. They are;
Functional block
Ladder logic
Ladder reasoning is one of the most common styles of PLC encoding and it is utilized in most production companies. It really is called ladder logic simply because the style resembles a ladder as shown below where in fact the two vertical rails are linked by way of a rung.
Rung
Rails
(Ref 1)
It is popular due to its similarity to relay logic which allows most program designers to easily understand the idea.
Ladder reasoning is written using logic symbols that happen to be shown below and is also read from left to right.
Ladder logic symbols
(Ref 1)
Normally open
Normally closed
Normally wide open immediate
Normally finished immediate
NOT
Positive transition
Negative transition
Output
Output immediate
Set
Set immediate
Reset
Reset immediate
Set-dominate bistable
Reset-dominate bistable
No operation
Example of Ladder logic
Below is a straightforward circuit made to mimic a drilling process where 3 cylinders are being used to drill a part.
Cylinder 'a' when long places a part below cylinder 'b', whilst cylinder 'c' has already been extended possessing the part safely set up and also stopping the part from falling through the difference below cylinder 'b'. Cylinder 'b' then boils down, drills the part, goes back up and cylinder 'c' retracts allowing the part to fall through the distance.
(Ref 1)
Cylinder B
Cylinder A
Cylinder C
The ladder diagram because of this circuit is shown below.
(Ref 1)
As is seen from above the machine uses sensors showing whether each cylinder is long or retracted. This information is then used to stimulate each cylinder.
For example in the first area of the programme which includes been highlighted;
Relay R1 is triggered when the beginning button is pressed considering that the stop button is in the normally sealed position.
Sensor 5 (S5) is off which is exhibiting that cylinder 3 is lengthened. When this is off it allows Relay 1 to stay permanently on because one of the contacts from R1 can then complete the circuit and develop a latch.
R1 is then used to stimulate solenoid 1(Sol1) to extend cylinder 1 so long as S2 and R4 are off.
Structured text
Structured words is a word based dialect that uses assertions to identify what needs to be performed.
Structured text message uses statements such as:
IF. . . THEN. . .
CASE. . . OF. . .
FOR. . . DO. . .
WHILE. . . DO. . .
REPEAT. . . UNTIL. . .
People who are trained in computer terminology find structured words better to understand than ladder reasoning because they are of an identical format.
