PLC Projects

Motor Control Circuits Using PLC Part II

2010-03-11T13:10:00.003+07:00

Based from part I, now, if either forward or reverse circuits are latched, they may be "unlatched" by momentarily pressing the "Stop" pushbutton, which will open either forward or reverse circuit, de-energizing the energized contactor, and returning the seal-in contact to its normal (open) state. The "Stop" switch, having normally-closed contacts, will conduct power to either forward or reverse circuits when released.

So far, so good. Let's consider another practical aspect of our motor control scheme before we quit adding to it. If our hypothetical motor turned a mechanical load with a lot of momentum, such as a large air fan, the motor might continue to coast for a substantial amount of time after the stop button had been pressed. This could be problematic if an operator were to try to reverse the motor direction without waiting for the fan to stop turning. If the fan was still coasting forward and the "Reverse" pushbutton was pressed, the motor would struggle to overcome that inertia of the large fan as it tried to begin turning in reverse, drawing excessive current and potentially reducing the life of the motor, drive mechanisms, and fan. What we might like to have is some kind of a time-delay function in this motor control system to prevent such a premature startup from happening.

Let's begin by adding a couple of time-delay relay coils, one in parallel with each motor contactor coil. If we use contacts that delay returning to their normal state, these relays will provide us a "memory" of which direction the motor was last powered to turn. What we want each time-delay contact to do is to open the starting-switch leg of the opposite rotation circuit for several seconds, while the fan coasts to a halt.

Figure 3

If the motor has been running in the forward direction, both M1 and TD1 will have been energized. This being the case, the normally-closed, timed-closed contact of TD1 between wires 8 and 5 will have immediately opened the moment TD1 was energized. When the stop button is pressed, contact TD1 waits for the specified amount of time before returning to its normally-closed state, thus holding the reverse pushbutton circuit open for the duration so M2 can't be energized. When TD1 times out, the contact will close and the circuit will allow M2 to be energized, if the reverse pushbutton is pressed. In like manner, TD2 will prevent the "Forward" pushbutton from energizing M1 until the prescribed time delay after M2 (and TD2) have been de-energized.

The careful observer will notice that the time-interlocking functions of TD1 and TD2 render the M1 and M2 interlocking contacts redundant. We can get rid of auxiliary contacts M1 and M2 for interlocks and just use TD1 and TD2's contacts, since they immediately open when their respective relay coils are energized, thus "locking out" one contactor if the other is energized. Each time delay relay will serve a dual purpose: preventing the other contactor from energizing while the motor is running, and preventing the same contactor from energizing until a prescribed time after motor shutdown. The resulting circuit has the advantage of being simpler than the previous example:

Figure 4

Motor Control Circuits Using PLC Part I

2010-03-11T13:07:00.001+07:00

The interlock contacts installed in the previous section's motor control circuit work fine, but the motor will run only as long as each pushbutton switch is held down. If we wanted to keep the motor running even after the operator takes his or her hand off the control switches, we could change the circuit in a couple of different ways: we could replace the pushbutton switches with toggle switches, or we could add some more relay logic to "latch" the control circuit with a single, momentary actuation of either switch. Let's see how the second approach is implemented, since it is commonly used in industry:

Figure 1

When the "Forward" pushbutton is actuated, M1 will energize, closing the normally-open auxiliary contact in parallel with that switch. When the pushbutton is released, the closed M1 auxiliary contact will maintain current to the coil of M1, thus latching the "Forward" circuit in the "on" state. The same sort of thing will happen when the "Reverse" pushbutton is pressed. These parallel auxiliary contacts are sometimes referred to as seal-in contacts, the word "seal" meaning essentially the same thing as the word latch.

However, this creates a new problem: how to stop the motor! As the circuit exists right now, the motor will run either forward or backward once the corresponding pushbutton switch is pressed, and will continue to run as long as there is power. To stop either circuit (forward or backward), we require some means for the operator to interrupt power to the motor contactors. We'll call this new switch, Stop:

Figure 2

To be continue in Part II

Normally Open and Normally Closed Contacts

2010-03-11T12:59:00.003+07:00

Since we frequently meet with concepts "normally open" and "normally closed" in industrial environment, it's important to know them. Both terms apply to words such as contacts, input, output, etc. (all combinations have the same meaning whether we are talking about input, output, contact or something else). Principle is quite simple, normally open switch won't conduct electricity until it is pressed down, and normally closed switch will conduct electricity until it is pressed. Good examples for both situations are the doorbell and a house alarm.

If a normally closed switch is selected, bell will work continually until someone pushes the switch. By pushing a switch, contacts are opened and the flow of electricity towards the bell is interrupted. Of course, system so designed would not in any case suit the owner of the house. A better choice would certainly be a normally open switch. This way bell wouldn't work until someone pushed the switch button and thus informed of his or her presence at the entrance.

Home alarm system is an example of an application of a normally closed switch. Let's suppose that alarm system is intended for surveillance of the front door to the house. One of the ways to "wire" the house would be to install a normally open switch from each door to the alarm itself (precisely as with a bell switch). Then, if the door was opened, this would close the switch, and an alarm would be activated. This system could work, but there would be some problems with this, too. Let's suppose that switch is not working, that a wire is somehow disconnected, or a switch is broken, etc. (there are many ways in which this system could become dysfunctional). The real trouble is that a homeowner would not know that a system was out of order. A burglar could open the door, a switch would not work, and the alarm would not be activated. Obviously, this isn't a good way to set up this system. System should be set up in such a way so the alarm is activated by a burglar, but also by its own dysfunction, or if any of the components stopped working. (A homeowner would certainly want to know if a system was dysfunctional). Having these things in mind, it is far better to use a switch with normally closed contacts which will detect an unauthorized entrance (opened door interrupts the flow of electricity, and this signal is used to activate a sound signal), or a failure on the system such as a disconnected wire. These considerations are even more important in industrial environment where a failure could cause injury at work. One such example where outputs with normally closed contacts are used is a safety wall with trimming machines. If the wall doors open, switch affects the output with normally closed contacts and interrupts a supply circuit. This stops the machine and prevents an injury.

Concepts normally open and normally closed can apply to sensors as well. Sensors are used to sense the presence of physical objects, measure some dimension or some amount. For instance, one type of sensors can be used to detect presence of a box on an industry transfer belt. Other types can be used to measure physical dimensions such as heat, etc. Still, most sensors are of a switch type. Their output is in status ON or OFF depending on what the sensor "feels". Let's take for instance a sensor made to feel metal when a metal object passes by the sensor. For this purpose, a sensor with a normally open or a normally closed contact at the output could be used. If it were necessary to inform a PLC each time an object passed by the sensor, a sensor with a normally open output should be selected. Sensor output would set off only if a metal object were placed right before the sensor. A sensor would turn off after the object has passed. PLC could then calculate how many times a normally open contact was set off at the sensor output, and would thus know how many metal objects passed by the sensor.

Concepts normally open and normally closed contact ought to be clarified and explained in detail in the example of a PLC controller input and output. The easiest way to explain them is in the example of a relay.

Figure 1

Normally open contacts would represent relay contacts that would perform a connection upon receipt of a signal. Unlike open contacts, with normally closed contacts signal will interrupt a contact, or turn a relay off. Previous picture shows what this looks like in practice. First two relays are defined as normally open , and the other two as normally closed. All relays react to a signal!

First relay (00) has a signal and closes its contacts. Second relay (01) does not have a signal and remains opened. Third relay (02) has a signal and opens its contacts considering it is defined as a closed contact. Fourth relay (03) does not have a signal and remains closed because it is so defined. Concepts "normally open" and "normally closed" can also refer to inputs of a PLC controller. Let's use a key as an example of an input to a PLC controller. Input where a key is connected can be defined as an input with open or closed contacts. If it is defined as an input with normally open contact, pushing a key will set off an instruction found after the condition. In this case it will be an activation of a relay 00.

If input is defined as an input with normally closed contact, pushing the key will interrupt instruction found after the condition. In this case, this will cause deactivation of relay 00 (relay is active until the key is pressed). You can see in picture below how keys are connected, and view the relay diagrams in both cases.

Figure 2

Normally open/closed conditions differ in a ladder diagram by a diagonal line across a symbol. What determines an execution condition for instruction is a bit status marked beneath each condition on instruction line. Normally open condition is ON if its operand bit has ON status, or its status is OFF if that is the status of its operand bit. Normally closed condition is ON when its operand bit is OFF, or it has OFF status when the status of its operand bit is ON.

When programming with a ladder diagram, logical combination of ON and OFF conditions set before the instruction determines the eventual condition under which the instruction will be, or will not be executed. This condition, which can have only ON or OFF values is called instruction execution condition. Operand assigned to any instruction in a relay diagram can be any bit from IR, SR, HR, AR, LR or TC sector. This means that conditions in a relay diagram can be determined by a status of I/O bits, or of flags, operational bits, timers/counters, etc.

Timers and Counters in PLC

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Timers and counters are indispensable in PLC programming. Industry has to number its products, determine a needed action in time, etc. Timing functions is very important, and cycle periods are critical in many processes. There are two types of timers delay-off and delay-on. First is late with turn off and the other runs late in turning on in relation to a signal that activated timers. Example of a delay-off timer would be staircase lighting. Following its activation, it simply turns off after few minutes. Each timer has a time basis, or more precisely has several timer basis. Typical values are: 1 second, 0.1 second, and 0,01 second. If programmer has entered .1 as time basis and 50 as a number for delay increase, timer will have a delay of 5 seconds (50 x 0.1 second = 5 seconds).

Timers also have to have value SV set in advance. Value set in advance or ahead of time is a number of increments that timer has to calculate before it changes the output status. Values set in advance can be constants or variables. If a variable is used, timer will use a real time value of the variable to determine a delay. This enables delays to vary depending on the conditions during function. Example is a system that has produced two different products, each requiring different timing during process itself. Product A requires a period of 10 seconds, so number 10 would be assigned to the variable. When product B appears, a variable can change value to what is required by product B.

Typically, timers have two inputs. First is timer enable, or conditional input (when this input is activated, timer will start counting). Second input is a reset input. This input has to be in OFF status in order for a timer to be active, or the whole function would be repeated over again. Some PLC models require this input to be low for a timer to be active, other makers require high status (all of them function in the same way basically). However, if reset line changes status, timer erases accumulated value. With a PLC controller by Omron there are two types of timers: TIM and TIMH. TIM timer measures in increments of 0.1 seconds. It can measure from 0 to 999.9 seconds with precision of 0.1 seconds more or less. Quick timer (TIMH) measures in increments of 0.01 seconds. Both timers are "delay-on" timers of a lessening-style. They require assignment of a timer number and a set value (SV). When SV runs out, timer output turns on. Numbers of a timing counter refer to specific address in memory and must not be duplicated (same number can not be used for a timer and a counter).

Ladder Diagram of PLC

2010-03-06T07:39:00.002+07:00

There are several languages designed for user communication with a PLC, among which ladder diagram is the most popular. Ladder diagram consists of one vertical line found on the left hand side, and lines which branch off to the right. Line on the left is called a "bus bar", and lines that branch off to the right are instruction lines. Conditions which lead to instructions positioned at the right edge of a diagram are stored along instruction lines. Logical combination of these conditions determines when and in what way instruction on the right will execute. Basic elements of a relay diagram can be seen in the following picture.

Most instructions require at least one operand, and often more than one. Operand can be some memory location, one memory location bit, or some numeric value -number. In the example above, operand is bit 0 of memory location IR000. In a case when we wish to proclaim a constant as an operand, designation # is used beneath the numeric writing (for a compiler to know it is a constant and not an address.). Based on the picture above, one should note that a ladder diagram consists of two basic parts: left section also called conditional, and a right section which has instructions. When a condition is fulfilled, instruction is executed, and that's all!

Picture above represents an example of a ladder diagram where relay is activated in PLC controller when signal appears at input line 00. Vertical line pairs are called conditions. Each condition in a ladder diagram has a value ON or OFF, depending on a bit status assigned to it. In this case, this bit is also physically present as an input line (screw terminal) to a PLC controller. If a key is attached to a corresponding screw terminal, you can change bit status from a logic one status to a logic zero status, and vice versa. Status of logic one is usually designated as "ON", and status of logic zero as "OFF". Right section of a ladder diagram is an instruction which is executed if left condition is fulfilled. There are several types of instructions that could easily be divided into simple and complex. Example of a simple instruction is activation of some bit in memory location. In the example above, this bit has physical connotation because it is connected with a relay inside a PLC controller. When a CPU activates one of the leading four bits in a word IR010, relay contacts move and connect lines attached to it. In this case, these are the lines connected to a screw terminal marked as 00 and to one of COM screw terminals.

CPM1A PLC Controller

2010-03-06T07:34:00.001+07:00

Each PLC is basically a microcontroller system (CPU of PLC controller is based on one of the microcontrollers, and in more recent times on one of the PC processors) with peripherals that can be digital inputs, digital outputs or relays as in our case. However, this is not an "ordinary" microcontroller system. Large teams have worked on it, and a checkup of its function has been performed in real world under all possible circumstances. Software itself is entirely different from assemblers used thus far, such as BASIC or C. This specialized software is called "ladder" (name came about by an association of program's configuration which resembles a ladder, and from the way program is written out).

Specific look of CPM1A PLC controller can be seen in the following picture. On the upper surface, there are 4 LED indicators and a connection port with an RS232 module which is interface to a PC computer. Aside from this, screw terminals and light indicators of activity of each input or output are visible on upper and lower sides. Screw terminals serve to manually connect to a real system. Hookups L1 and L2 serve as supply which is 220V~ in this case. PLC controllers that work on power grid voltage usually have a source of direct supply of 24 VDC for supplying sensors and such (with a CPM1A source of direct supply is found on the bottom left hand side and is represented with two screw terminals. Controller can be mounted to industrial "track" along with other automated elements, but also by a screw to the machine wall or control panel. This is the figure of the PLC diagram.

Controller is 8cm high and divided vertically into two areas: a lower one with a converter of 220V~ at 24VDC and other voltages needed for running a CPU unit; and, upper area with a CPU and memory, relays and digital inputs. When you lift the small plastic cover you'll see a connector to which an RS232 module is hooked up for serial interface with a computer. This module is used when programming a PLC controller to change programs or execution follow-up. When installing a PLC it isn't necessary to install this module, but it is recommended because of possible changes in software during operation. The figure for the PLC OMRON.

To better inform programmers on PLC controller status, maker has provided for four light indicators in the form of LED's. Beside these indicators, there are status indicators for each individual input and output. These LED's are found by the screw terminals and with their status are showing input or output state. If input/output is active, diode is lit and vice versa.

PLC Controller Input Lines

2010-01-28T19:29:00.005+07:00

Different sensors, keys, switches and other elements that can change status of a joined bit at PLC input can be hooked up to the PLC controller inputs. In order to realize a change, we need a voltage source to incite an input. The simplest possible input would be a common key.

As CPM1A PLC has a source of direct voltage of 24V, the same source can be used to incite input (problem with this source is its maximum current which it can give continually and which in our case amounts to 0.2A). Since inputs to a PLC are not big consumers (unlike some sensor where a stronger external supply must be used) it is possible to take advantage of the existing source of direct supply to incite all six keys.

How PLC Controller Works

2010-01-28T19:16:00.002+07:00

Basis of a PLC function is continual scanning of a program. Under scanning we mean running through all conditions within a guaranteed period. Scanning process has three basic steps:

Step1.
Testing input status. First, a PLC checks each of the inputs with intention to see which one of them has status ON or OFF. In other words, it checks whether a sensor, or a switch etc. connected with an input is activated or not. Information that processor thus obtains through this step is stored in memory in order to be used in the following step.

Step2.
Program execution. Here a PLC executes a program, instruction by instruction. Based on a program and based on the status of that input as obtained in the preceding step, an appropriate action is taken. This reaction can be defined as activation of a certain output, or results can be put off and stored in memory to be retrieved later in the following step.

Step3.
Checkup and correction of output status. Finally, a PLC checks up output status and adjusts it as needed. Change is performed based on the input status that had been read during the first step, and based on the results of program execution in step two. Following the execution of step 3 PLC returns to the beginning of this cycle and continually repeats these steps. Scanning time is defined by the time needed to perform these three steps, and sometimes it is an important program feature.

PLC Controller Output Lines

2010-01-28T19:14:00.002+07:00

Aside from transistor outputs in PNP and NPN connections, PLC can also have relays as outputs. Existence of relays as outputs makes it easier to connect with external devices. Model CPM1A contains exactly these relays as outputs. There a 4 relays whose functional contacts are taken out on a PLC controller housing in the form of screw terminals. This the really of the figure for the PLC.

With activation of phototransistor, relay comes under voltage and activates a contact between points A and B. Contacts A and B can in our case be either in connection or interrupted. What state these contacts are in is determined by a CPU through appropriate bits in memory location IR010. One example of relay status is shown in a picture below. A true state of devices attached to these relays is displayed.

Introduction PLC OMRON

2010-01-25T00:00:00.004+07:00

It is a huge company which has high quality and by our standards inexpensive controllers. Today we can say almost with surety that PLC controllers by manufacturers round the world are excellent devices, and altogether similar. Nevertheless, for specific application we need to know specific information about a PLC controller being used.

Therefore, the choice fell on OMRON companies and it is PLC of micro class CPM1A. Adjective "micro" itself implies the smallest models from the viewpoint of a number of attached lines or possible options. Still, this PLC controller is ideal for the purposes of this book, and that is to introduce a PLC controller philosophy to its readers.

Output Lines

2010-01-24T23:53:00.002+07:00

PLC controller output lines usually can be:
-transistors in PNP connection
-transistors in NPN connection
-relays

The following two pictures display a realistic way how a PLC manages external devices. It ought to be noted that a main difference between these two pictures is a position of "output load device". By "output load device" we mean some relay, signalization light or similar.
How something is connected with a PLC output depends on the element being connected. In short, it depends on whether this element of output load device is activated by a positive supply pole or a negative supply pole.

Input Lines

2010-01-24T23:38:00.005+07:00

Explanation for the PLC controller input and output line has up to now been given only theoretically. In order to apply this knowledge, we need to make it a little more specific. Example can be connection of external device such as proximity sensor. Sensor outputs can be different depending on a sensor itself and also on a particular application.

This pictures display some examples of sensor outputs and their connection with a PLC controller. Sensor output actually marks the size of a signal given by a sensor at its output when this sensor is active. In one case this is +V (supply voltage, usually 12 or 24V) and in other case a GND (0V). Another thing worth mentioning is that sinking-sourcing and sourcing - sinking pairing is always used, and not sourcing-sourcing or sinking-sinking pairing.

If we were to make type of connection more specific, we'd get combinations as in following pictures (for more specific connection schemas we need to know the exact sensor model and a PLC controller model).

Input Adjustment Interface

2010-01-18T05:46:00.003+07:00

Adjustment interface also called an interface is placed between input lines and a CPU unit. The purpose of adjustment interface to protect a CPU from disproportionate signals from an outside world. Input adjustment module turns a level of real logic to a level that suits CPU unit (ex. input from a sensor which works on 24 VDC must be converted to a signal of 5 VDC in order for a CPU to be able to process it). This is typically done through opto-isolation, and this function you can view in the following picture.

Opto-isolation means that there is no electrical connection between external world and CPU unit. They are "optically" separated, or in other words, signal is transmitted through light. The way this works is simple. External device brings a signal which turns LED on, whose light in turn incites photo transistor which in turn starts conducting, and a CPU sees this as logic zero (supply between collector and transmitter falls under 1V). When input signal stops LED diode turns off, transistor stops conducting, collector voltage increases, and CPU receives logic 1 as information.

Sinking-Sourcing Concept

2010-01-18T05:40:00.003+07:00

PLC has input and output lines through which it is connected to a system it directs. Input can be keys, switches, sensors while outputs are led to different devices from simple signalization lights to complex communication modules.

‘This is a very important part of the story about PLC controllers because it directly influences what can be connected and how it can be connected to controller inputs or outputs. Two terms most frequently mentioned when discussing connections to inputs or outputs are "sinking" and "sourcing". These two concepts are very important in connecting a PLC correctly with external environment. The most brief definition of these two concepts would be:

SINKING = Common GND line (-)
SOURCING = Common VCC line (+)

First thing that catches one's eye are "+" and "-" supply, DC supply. Inputs and outputs which are either sinking or sourcing can conduct electricity only in one direction, so they are only supplied with direct current.

According to what we've said thus far, each input or output has its own return line, so 5 inputs would need 10 screw terminals on PLC controller housing. Instead, we use a system of connecting several inputs to one return line as in the following picture. These common lines are usually marked "COMM" on the PLC controller housing.

PLC Controller Inputs

2010-01-18T05:40:00.001+07:00

Intelligence of an automated system depends largely on the ability of a PLC controller to read signals from different types of sensors and input devices. Keys, keyboards and by functional switches are a basis for man versus machine relationship. On the other hand, in order to detect a working piece, view a mechanism in motion, check pressure or fluid level you need specific automatic devices such as proximity sensors, marginal switches, photoelectric sensors, level sensors, etc. Thus, input signals can be logical (on/off) or analogue. Smaller PLC controllers usually have only digital input lines while larger also accept analogue inputs through special units attached to PLC controller.

One of the most frequent analogue signals are a current signal of 4 to 20 mA and milivolt voltage signal generated by various sensors. Sensors are usually used as inputs for PLCs. You can obtain sensors for different purposes. They can sense presence of some parts, measure temperature, pressure, or some other physical dimension, etc. (ex. inductive sensors can register metal objects).

Other devices also can serve as inputs to PLC controller. Intelligent devices such as robots, video systems, etc. often are capable of sending signals to PLC controller input modules (robot, for instance, can send a signal to PLC controller input as information when it has finished moving an object from one place to the other.)

Programming a PLC Controller

2010-01-18T05:31:00.000+07:00

PLC controller can be reprogrammed through a computer (usual way), but also through manual programmers (consoles). This practically means that each PLC controller can programmed through a computer if you have the software needed for programming. Today's transmission computers are ideal for reprogramming a PLC controller in factory itself.

This is of great importance to industry. Once the system is corrected, it is also important to read the right program into a PLC again. It is also good to check from time to time whether program in a PLC has not changed. This helps to avoid hazardous situations in factory rooms (some automakers have established communication networks which regularly check programs in PLC controllers to ensure execution only of good programs).

Almost every program for programming a PLC controller possesses various useful options such as: forced switching on and off of the system inputs/ouputs (I/O lines), program follow up in real time as well as documenting a diagram. This documenting is necessary to understand and define failures and malfunctions. Programmer can add remarks, names of input or output devices, and comments that can be useful when finding errors, or with system maintenance. Adding comments and remarks enables any technician (and not just a person who developed the system) to understand a ladder diagram right away.

Comments and remarks can even quote precisely part numbers if replacements would be needed. This would speed up a repair of any problems that come up due to bad parts. The old way was such that a person who developed a system had protection on the program, so nobody aside from this person could understand how it was done. Correctly documented ladder diagram allows any technician to understand thoroughly how system functions.

Power Supply of PLC

2010-01-11T04:48:00.002+07:00

Electrical supply is used in bringing electrical energy to central processing unit. Most PLC controllers work either at 24 VDC or 220 VAC. On some PLC controllers you'll find electrical supply as a separate module. Those are usually bigger PLC controllers, while small and medium series already contain the supply module.

User has to determine how much current to take from I/O module to ensure that electrical supply provides appropriate amount of current. Different types of modules use different amounts of electrical current.

This electrical supply is usually not used to start external inputs or outputs. User has to provide separate supplies in starting PLC controller inputs or outputs because then you can ensure so called "pure" supply for the PLC controller.

With pure supply we mean supply where industrial environment can not affect it damagingly. Some of the smaller PLC controllers supply their inputs with voltage from a small supply source already incorporated into a PLC.

Memory of PLC

2010-01-11T04:48:00.001+07:00

System memory (today mostly implemented in FLASH technology) is used by a PLC for an process control system. Aside from this operating system it also contains a user program translated from a ladder diagram to a binary form. FLASH memory contents can be changed only in case where user program is being changed.

PLC controllers were used earlier instead of FLASH memory and have had EPROM memory instead of FLASH memory which had to be erased with UV lamp and programmed on programmers. With the use of FLASH technology this process was greatly shortened. Reprogramming a program memory is done through a serial cable in a program for application development.

User memory is divided into blocks having special functions. Some parts of a memory are used for storing input and output status. The real status of an input is stored either as "1" or as "0" in a specific memory bit. Each input or output has one corresponding bit in memory. Other parts of memory are used to store variable contents for variables used in user program. For example, timer value, or counter value would be stored in this part of the memory.

PLC Controller Components

2010-01-11T04:47:00.000+07:00

PLC is actually an industrial microcontroller system (in more recent times we meet processors instead of microcontrollers) where you have hardware and software specifically adapted to industrial environment.

Special attention needs to be given to input and output, because in these blocks you find protection needed in isolating a CPU blocks from damaging influences that industrial environment can bring to a CPU via input lines. Program unit is usually a computer used for writing a program (often in ladder diagram).

First Programmable Controllers

2010-01-11T04:38:00.000+07:00

"General Motors" is among the first who recognized a need to replace the system's "wired" control board. Increased competition forced auto-makers to improve production quality and productivity. Flexibility and fast and easy change of automated lines of production became crucial! General Motors' idea was to use for system logic one of the microcomputers (these microcomputers were as far as their strength beneath today's eight-bit microcontrollers) instead of wired relays.

Computer could take place of huge, expensive, inflexible wired control boards. If changes were needed in system logic or in order of operations, program in a microcomputer could be changed instead of rewiring of relays. Imagine only what elimination of the entire period needed for changes in wiring meant then. Today, such thinking is but common, and then it was revolutionary!

Everything was well thought out, but then a new problem came up of how to make electricians accept and use a new device. Systems are often quite complex and require complex programming. It was out of question to ask electricians to learn and use computer language in addition to other job duties. General Motors Hidromatic Division of this big company recognized a need and wrote out project criteria for first programmable logic controller ( there were companies which sold instruments that performed industrial control, but those were simple sequential controllers û not PLC controllers as we know them today).

Specifications required that a new device be based on electronic instead of mechanical parts, to have flexibility of a computer, to function in industrial environment (vibrations, heat, dust, etc.) and have a capability of being reprogrammed and used for other tasks. The last criteria was also the most important, and a new device had to be programmed easily and maintained by electricians and technicians. When the specification was done, General Motors looked for interested companies, and encouraged them to develop a device that would meet the specifications for this project.

"Gould Modicon" developed a first device which met these specifications. The key to success with a new device was that for its programming you didn't have to learn a new programming language. It was programmed so that same language ûa ladder diagram, already known to technicians was used. Electricians and technicians could very easily understand these new devices because the logic looked similar to old logic that they were used to working with. Thus they didn't have to learn a new programming language which (obviously) proved to be a good move.

PLC controllers were initially called PC controllers (programmable controllers). This caused a small confusion when Personal Computers appeared. To avoid confusion, a designation PC was left to computers, and programmable controllers became programmable logic controllers. First PLC controllers were simple devices. They connected inputs such as switches, digital sensors, etc., and based on internal logic they turned output devices on or off. When they first came up, they were not quite suitable for complicated controls such as temperature, position, pressure, etc. However, throughout years, makers of PLC controllers added numerous features and improvements. Today's PLC controller can handle highly complex tasks such as position control, various regulations and other complex applications. The speed of work and easiness of programming were also improved. Also, modules for special purposes were developed, like communication modules for connecting several PLC controllers to the net. Today it is difficult to imagine a task that could not be handled by a PLC.

PLC System

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A process control system is made up of a group of electronic devices that provide stability, accuracy and eliminate harmful transition statuses in production processes. Operating systems can have different arrangements and implementation, from energy supply units to machines. As technology quickly progresses, many complex operational tasks have been solved by connecting programmable logic controllers and a central computer.

Beside connections with devices (e.g., operating panels, motors, sensors, switches, valves, etc.) possibilities for communication among instruments are so great that they allow a high level of exploitation and process coordination. In addition, there is greater flexibility in realizing a process control system. Each component of a process control system plays an important role, regardless of its size. For example, without a sensor, the PLC wouldn't know what is going on during a process. In an automated system, a PLC controller is usually the central part of a process control system.

With the execution of a program stored in program memory, PLC continuously monitors status of the system through signals from input devices. Based on the logic implemented in the program, PLC determines which actions need to be executed with output instruments. To run more complex processes it is possible to connect more PLC controllers to a central computer.

History of PLC

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The first introduction of PLC is in 1960. PLC has built for decrease lost pay for changing the control system that using relay. MODICON (Modular Digital Controller) 084 is the first PLC for commercial.

Based on increasing necessity in production processing is causing the system must changing in some period. If the system that used is mechanic relay, it will make big problem. In 1970, the great of PLC technology is sequencer and CPU using bit-slice.

Introduction of PLC (Programmable Logic Control)

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In industrial world, increasing quality and productivity from product result has needed to control process of industrial machines and monitoring the machine process. General, controlling the machine is setting up by some group electronics devices and preventing transition at production processing.

In last year, automatic industry has only using electronic board as control system. This board is needed many interconnection between relay to make the system can work. In other word, for this connection has many cables for connect each other relay. Relay that using in this connection is hundreds and called Ladder Schematic.

Ladder schematic is displaying the switch, sensor, motor, and relay. All devices is connection as one wiring. The problems for this wiring, if one of the relay has broken, so the process production will be stopped automatically. The solution for this problems is using a electronics devices that is called as Programmable Logic Control or PLC unit.