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Wednesday, January 26, 2011

SCADA : What?, Why?

              The acronym SCADA stands for Supervisory Control And Data Acquisition.  In reality, the primary purpose of SCADA is to monitor, control and alarm plant or regional operating systems from a central location.  While override control is possible, it is infrequently utilized; however control set points are quite regularly changed by SCADA.

In a municipal context, "plant" systems are typically water and wastewater treatment facilities, while "regional" systems include intake and/or effluent structures, pumping stations, chlorination stations, control valve stations and the like.

What makes up a SCADA system?

There are three main elements to a SCADA system, various RTU's (Remote Telemetry Units), communications and an HMI (Human Machine Interface).

Each RTU effectively collects information at a site, while communications bring that information from the various plant or regional RTU sites to a central location, and occasionally returns instructions to the RTU.

The HMI displays this information in an easily understood graphics form, archives the data received, transmits alarms and permits operator control as required.

Communication within a plant will be by data cable, wire or fiber-optic, while regional systems most commonly utilize radio.  The HMI is essentially a PC system running powerful graphic and alarm software programs.

Why is SCADA so popular?

The major attraction of SCADA to a municipality is the ability to significantly reduce operating labor costs, while at the same time actually improve plant or regional system performance and reliability.  Information gathering within a plant no longer requires personnel to spend time wandering all over the site, and correspondingly the frequency of field site inspections required in a regional system can be minimized.

Costly after-hours alarm call-outs can often be avoided since a SCADA system will indicate the nature and degree of a problem, while the ability to remotely control site equipment may permit an operator at home to postpone a site visit till working hours.  SCADA based alarming is also very reliable since it is in-house and tied directly to process control.

A significant feature of a SCADA system, often not fully appreciated, is the trending of data and nothing comes close for speed and ease of operation. When graphically displayed, accumulated operating data often will indicate a developing problem, or an area for process improvement. Reports can easily be generated from this data utilizing other common software programs.
It should be appreciated that while a SCADA system is often complex to configure - it is extremely easy to operate!

What is involved?

There are five phases to creating a functional SCADA system:

Phase 1 
The DESIGN of the system architecture. This includes the all-important communication system, and with a regional system utilizing radio communication often involves a radio path survey. Also involved will be any site instrumentation that is not presently in existence, but will be required to monitor desired parameters.
Phase 2
The SUPPLY of RTU, communication and HMI equipment, the latter consisting of a PC system and the necessary powerful graphic and alarm software programs.

Phase 3
The PROGRAMMING of the communication equipment and the powerful HMI graphic and alarm software programs.

Phase 4
The INSTALLATION of the communication equipment and the PC system. The former task is typically much more involved.

Phase 5
The COMMISSIONING of the system, during which communication and HMI programming problems are solved, the system is proven to the client, operator training and system documentation is provided.

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Tuesday, January 25, 2011

Sensors used in industrial automation

            You find here an overview of the most common industrial automation sensors used in industrial automation. They have been sorted by function principal.Sensors are used for data acquisition by industrial automation controllers, to control industrial processes.

There are industrial automation sensors for every possible application.

* Inductive Sensors: have the ability to sense metal. For example, they can detect copper.

* Magnetic Field Sensors: are used to detect a magnetic field. They can detect a magnet placed in a cylinder.

* Capacitive Sensors: are able to detect all things that are not metal. With these industrial automation sensors you are able to see wood.

* Photoelectric Beam Sensor: is being used to detect light. These sensors are able to detect an object, with the reflected light.

* Fiber Optic Sensor:this kind of sensor is often used when the place where an object needs to be detected is either small, or is very hot or cold.

* Automation Optical Sensors: Optical sensors are one of the most widely used type in industrial automation. Here are some solution / application examples for you.

* Ultrasonic sensors: have ability to hear ultrasonic sounds. This sensor can detect the level of corn in a container. 

* Flow Sensors: are able to detect a the flow of a liquid. These sensors are used to detect if water is still running. 

* Pressure Sensors: are used to detect pressure. They can detect steam pressure. 

* Temperature sensors: have the ability to detect changes in temperature. These sensors can detect the temperature inside an oven. 

* Level Sensors and Liquid Level Sensors: are able to detect levels of different media. One common application is to detect the water level in a tank. 

* Rotary Position Sensors: are able you give information about a position. For example, this type of sensor can give you the position of an elevator. 

* Inclination Sensors: are able to give you the degree of inclination of an object. A typical application is the detection of the degree of inclination on an off-road vehicle.

* Laser Sensors: have the ability to measure a distance. The are used in welding robots, to control the welding process.

* Pick To Light Sensors: are able to detect if a person has picked the right object. These sensors are used to control the quality in assembly applications.

* Magnetostrictive Sensors: have the ability to give an exact position. These sensors are commonly used in hydraulic cylinders, to measure their position. Popular are Temposonics from MTSsensors. 

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Lecture Series on Industrial Automation and Control

 The video given below is 'Lecture Series on Industrial Automation and Control by Prof. S. Mukhopadhyay, Department of Electrical Engineering, IIT Kharagpur.


Industrial Automation Using PLCs

I.       Introduction  and Requirements of  Industrial  Automation

1.     Industries go through  a certain process  or processes  to manufacture their   products.
2.     Process involve various physical and chemical treatments.
3.     They involve control of physical parameters like speed, force and torque, power, energy, pressure, temperature, time etc.
4.     Not only that the parameters are to be controlled, they have to be controlled in an appropriate sequence so that the process is effectively controlled.
5.     In a process plant, engineers like the mechanical (thermal), metallurgical, chemical engineers are involved in the actual process know-how; electrical engineers are involved extensively in the control and the actual implementation of the process.
6.     The  electrical engineers have at their disposal, various electrical and electronic items – equipments, components as well as other electrically operated devices enabling  them to implement, operate, control monitor and trouble-shoot the above process controls.
7.     The normal  devices  are motors, motor control equipment, power on/off devices, types of solenoids, pumps, fans, compressors, heaters, refrigeration system etc.
8.     The above are the work-horses or the equipment of the action in the control process.  These actions are getting monitored through various sensors or transducers for the parameters like the speed, current, voltage, pressure, flow, temperature, position, time etc and if necessary modified.  These modifications can either be on-off type or of continuous regulation.
9.     All these methods need to be controlled as per an appropriate strategy or program which is unique for each process.
10.      Also modifications are to be done to improve the    process efficiency or changing the process for different product requirements should be done fast and effectively.
11.      There must be methods to visually monitor whether the process is proceeding according to the desired requirements.
12.      There must be trouble shooting methods in case of problems; There must be preventive maintenance methods.
13.      Maintaining records of all the process issues as follows for a reasonable time.
-         Process parameter logging.
-         History of energy consumption.
-         History of problem and solutions.
-         History of revisions etc.

14.      Other facilities as follows are also required.
-         Process scheduling
-         Test schedules
-         Maintenance Scheduling
-         Energy consumption calculations.  
The above form the requirement for industrial automation.

II.      Typical  Industrial  Electrical Equipment:
      The following  are the  various  electrical equipment for the  implementation of the  Process. The actual items  present   will  vary  from  plant to plant  depending the       individual requirements.
1.     HT power distribution through HT breakers / isolators including their protective relaying and monitoring through CT-s / PT-s
2.     HT cables and bus ducts.
3.     Various Transformers  including  their protective relaying and monitoring.
4.     LT cables  and bus ducts.
5.     LT power distribution and control panels.
-         Power distribution boards with monitoring.
-         Local isolation Transformers  for voltage and fault level  matching
-         Motor control centers with starters for fixed frequency AC motors.
-         Power conversion panels for variable speed drive panels.
-         Output  power distribution panels for individual motors
6.     Various types of motors.
-         HT  AC motors and their starters.
-         Main and auxiliary DC / AC motors ( for  the VSD-s) as well as   
-         Fixed speed AC motors for conventional applications.
7.     Other common electrical utilities like
-         Pumps; - fans; - compressors; - heaters; - lighting systems,
-         Material handling equipment like cranes etc.
8.     Sensors
-         Motor mounted devices called MMD-s
-         Field mounted devices called FMD-s for instrumentation, feedback and controls.
9.     Central relay and PLC based control panels for (earlier these functions were carried out by relays; now they are extensively carried out by PLC-s)
-         Providing power suppliers for all the PLC inputs and outputs as well as  other devices, sensors, solenoids and such actuators.
-         For process sequencing
-         Fault monitoring and protection
-         Special REAL TIME controls like coordinated controls.
-         Communication networking for the 1) control signals  and  2) Information signals through separate control bus and information bus.
-         Fault annunciation. requirements
-         Status annunciation requirements.
-         Human Machine Interface terminals for Total mill visualization and controls (these are normally located in the control pulpit for the operator’s requirement)
-         Event history recorders and Data loggers.

10.      Control desks and posts with PB-s, Lamps, selector switches, meters, LCD panels as well as HMI-s for total mill visualization and controls.
11.      Other miscellaneous items like, UPS, CCTV-s, fire alarm, lighting, material handling cranes, communication systems etc.

III.       Functions of Central relay or PLC based control panels:

1.     They form the central set of control, interlock and protection panels.
2.     In earlier  days, with relay panels the exchanges were related to mainly commands and status ( called  discrete  or digital signals) only. With PLC, these signals can now be either digital or analog (control values or variables).  The analog set points or feedbacks can  also be processed as required.
3.     They interface with all other equipment in the plant and exchange various signals as enumerated below
i)                   For HT panels the status indication, fault annunciation, feed back information  for instrumentation such as input voltage, current, pf and energy input.
ii)                For transformers, the status, temperature figures, fault annunciation etc.
iii)              For PDB-s, status indication, voltage current and pf feedback and hence the energy of a section.
iv)              For drive control panels the relevant signals  are, status signals, healthy signals, on/off command, forward/reverse commands,  fault annunciation, voltage, current  speed and such relevant control parameters and  also the  required generation of the appropriate reference signals. 
v)                For MCC-s the on/off, forward/ reverse commands, fault annunciation and other relevant indications.
vi)              For motors, signals related to temperature, cooling, motor protection relay inputs, insulation levels and also  vibration level signals.
vii)           With utility panels, they ensure proper operation of these equipment and they receive status of hydraulic,  pneumatic and water utility systems of the plant like, pressure, flow, temperature, on/off conditions and if required redundancy operation for reliability.
viii)         For MMD-s and FMD-s the various feedbacks and status signals are received.  The required power supplies for these are also provided from these panels.

ix)              The central relay and PLC based control panels interface with the devices in the control desks in control pulpits  like the various PB-s, switches (on / off or selector type), lockable selector switches, mushroom head PB-s, foot operated switches etc. for the inputs or commands  and with  the  lamps, PB lamps, meters, hooters etc for the outputs.  Most of these devices are oil tight types. Also HMI is normally mounted in a control desk for the human machine interface with PC-based SCADA for visualization and control with industrially  ruggedized PC and  membrane type keyboard.  In older control pulpits individual fault annunciation windows on the panels used to be always present ; In the present day  plants these are only provided  optionally as these  functions  can be provided in the HMI PC itself.

 Thus the relay and PLC based control and protection panels interface with all the mill equipment and functions and hence  is  truly  a central panel. The enclosed  drawing  provide  both the power flow  and the  signal exchange  flow  with  this central panel in  a typical plant. 

IV     Relaying  Functions  of the Conventional  central relay and control panels or called the  mill  interlock panels

          The conventional  central   relay and control panels  generally  provide the  relaying  ( sequencing  ) functions only. In addition they are used for  contact  multiplication  for distribution of the  commands and  outputs.

1.       Industrial Relays

          Even though PLC-s have replaced the relays effectively, the knowledge of the elector-mechanical relays are necessary because they are still used for contact multiplication, voltage and current matching  of the output etc. along with the  PLC I/O-s themselves.

       i.      Relays  have coils with appropriate windings wound on a hollow former through which the plunger moves when energized.  The moving contacts are connected to the plunger. The coil is energized by a DC or AC voltage.  24V DC, 110V AC, 220V AC are some of the standard voltages. 
     ii.      The relay can be individually front mounted with the coils and terminals accessible from front or socket mounted with the wiring done to the socket.
  iii.      The DC relay coils are normally connected with a free wheeling diode across the coil.  This provides protection to the coil against high voltage breakdown, when the coil current is interrupted.
  iv.      The contacts are N/O or N/C type or C/O type.  The individual front mounting type relays have normally 2 N/O + 2 N/C contacts.  Sometimes additional contacts can be added to the basic relay.
  v.      In the socket mounting type relay, the contacts are C/O type, with one pole and  1NC and 1 No contacts.  Normally 3 C/O contacts are present  in such a relay.
  vi.      If more contacts are required to implement a logic may have to add additional contact multiplication relays. This is one of the  major disadvantages of the  conventional relay panels. Physical   contacts must be present for each function.  In contrast in the PLC panels the status of  the  input or output ( called input and output images ) or intermediate  flags  can be used any  number  of times  as these  are soft signals when implementing the logic
vii.      The relays   for the  logic build-up  can be same type  with minimum current rating. However  the relays   for the  outputs be selected such that the contact current rating should take  care of the load current.
viii.      The relays along with their contact blocks can be wired in such a way that the appropriate sequencing is taken care of.

2.       Timing Relays

   i.      Till recently it was  implemented using a pneumatic timing head  which was getting added to a relay to provide the time delays.
     ii.      This time delay could be during on-time or during off time or some times they are with on/off time delays.
  iii.      Now a days solid state timers are available for use and pneumatic timers are hardly in use.
  iv.      The timer  relays  are  normally provided with  adjustment for setting the time delays.
The above relay contacts ( under 1  and 2 )  go back  to the normal conditions when the power is disconnected or made off.

3.       Latching or memory relay

•        This has got 2 coils – one latching coil and the other un-latching coil.
•        The latching coil when energized , closes and latches the relay   by a mechanical means.  With this, the relay maintains its position even when the power is off.
•        To de-energize the relay, a de-energizing coil need to be momentarily energized.

4.       Mechanically latched type
          These are two individual relays with an additional mechanical latching arrangement to ensure that when one relay is on, the other is necessarily off.  One will not be able to energize the second  relay.  This will be useful for applications like forward / reverse control of a motor where only one function is normally required.

The above discussions on relays provide the idea of control functions which can be  executed with  the conventional relay panels  and which need to be executed when PLC-s are used.

These conventional panels  are built up  depending on the logic  to be implemented after the  mill  builder  provides the overall operational  method  which is  elaborated   further   by the  electrical supplier  as per the  actual implementation.

While  the functionality  of the  mill interlock panel  is similar in  the PLC panels, the concept defining the  number of inputs  and outputs  came into the use with the implementation with  PLC-s.

  V     PLC based control and inter lock panels

As compared  to the central  relay and interlock panels or mill interlock panels , the modern day  PLC based  interlock panels  are able  to handle   much more of   functionalities  as explained under  III .  Since the PLC-s  themselves are  micro- processor or controller  based they  have the same  working  architecture  like  the  standard  computer  architecture with  the following  elements. They are  the  i) Central processing unit or CPU-s  along with their hard-ware, firm-ware and   application software, ii) memory iii) communication with other PLC-s, HMI-s ,  other  devices etc. iv) power supply  as well as  the racks with back plane  for mounting the cards and the  v)  inputs  and  vi ) outputs. The inputs and the outputs form the bulk of the  hardware depending on the plant requirement  and capacity.

While the PLC CPU, instruction sets, software,   memory, communication and other capabilities .  are dealt with separately  under the  next section, this section deals  with  inputs and outputs. Apart from the  functions  involving Boolean  logic ( normally done  by electro-mechanical relays ), PLC-s  handle many other functions like, counters,  timers, real  time clock,  communication with other PLC-s  and devices, handling math  functions etc. The functionalities  differ  from PLC to PLC. The appropriate and optimal type  has to be selected to address the application requirements and to get the best value for the cost. 

Apart from the functionalities , the PLC type is primarily decided based on the  number and the type of inputs and outputs.

The defining of the inputs  and the  outputs  got into the use   with the  use of  PLC-s  for   the  control  and interlock   functions.  This is because in the PLC panels the status of  the  inputs or outputs ( called input and output images ) or intermediate  flags  can be used any  number  of times  as these  are soft signals when implementing the logic.  The advantage of this method  is that  even at the beginning of a project ( even during the  cost estimation stage )  one can estimate  the number of inputs and  outputs and  decide the type of the  PLC.  One  need not even be fully clear  about the logic  or the total functionalities to  estimate the PLC-s.

An example for   selecting the  number of  inputs  and outputs is given in one of the following sections . The types of  industrial inputs and outputs are   discussed  first . 
1        Input Devices:

          1.1     Push Buttons: 

          They are the commonly used input devices  They have basically two parts – operating buttons (actuators) and the contact units.  The contact unit can be N/O or N/C types;

       i.      Normally 2 N/O and / or 2 N/C contact blocks are common when used with the conventional relay based control panels.  With PLC based system, the input block need only be 1 N/O or 1 N/C.  This input along with their complement can be used any number of times in the PLC  ladder diagrams .
     ii.      The operator buttons are of different types;  i.  flushed or extended head ii. Mushroom head
  iii.      The buttons are normally with different colours, like the indication lamps to be described  under the outputs. Green is normally used for ‘on’ functions and ‘Red’ is used for ‘off’ function; other colors like yellow, black etc are used for other functions like start, stop, jog, thread etc. functions.
  iv.      Sometimes the mushroom headed red coloured off (emergency) PB actuators get mechanically latched when pressed; They need to be turned in one direction to release the same.
     v.      Sometimes other types of actuators replace the conventional push button heads, like the following : i) locking type (through key )PB actuator ii) selector type push button actuators ; They are selected for certain locking type control functions like enable / disable or forward / reverse selection type respectively.
  vi.      Some times the PB actuator unit has a transparent  head(with appropriate coloured lens) with in built indication lamp, apart from the contact blocks.  This can be used for an application like starting a motor by processing the PB which initiates closing a contactor for starting the motor and at the same time provide a feed back through this contactor auxiliary contact block (a N/O contact getting closed) which lights up the inbuilt lamp in the PB head.  These are called PB lamps.  For a PB lamp, the lamp will be fed as the output of a relay or PLC.
vii.      Push buttons are extensively used as input devices for the control relay and PLC panels giving the required ‘operator commands’

1.2     Limit switches:

          These are normally used to sense the reaching of the required physical limit by a machinery or a motion control system.  On reaching the required limit, the limit switch closes and provides an input to the central relay and PLC panels for initiating an action;
eg stopping a motor or reversing an operation etc.  The following types limit switches are there:

i)  Mechanical / lever type limit switch :
          The limit switch operates on physical contact and closes a switch provided with N/O and N/C contacts.

ii)  Rotary programmable cam limit switches:

          Here the rotary limit switch arrangement is coupled to the motor or the actual load shaft driving an operation.  The coupling is through an appropriate gear box so that for number of revolution on  the equipment shaft side, the limit switch shaft rotates once.  On the limit switch shaft are number of cams and the cam actuation positions are set at the required appropriate angles.  These cam settings are adjustable programmable.  With this arrangement the individual cams operate the associated limit switches.  The operation of these limit switches can be associated with the reaching of appropriate limits in a linear travel; eg. travel of a cage (carriage) carrying the ore and other materials into a blast  furnace.  At the various operations of the limit switches various functions as follows can be initiated :
Position i.   Bottom position – start at low speed
Position ii.  Acceleration to full speed
Position iii. Signal to small bell to open
Position iv. Signal to close small bell
Position v.  Deceleration to low speed
Position vi. Stop ; top position for tipping the material  into furnace.

iii)  Rotary programmable encoder type limit switch:

          This functions exactly like ii) but this is done by counting of the pulses generated by the encoder and comparing  the accumulated count with the preset counts.  This type is much more flexible as compared to ii) and is easily settable.  However they  require external power supplies.

iv)  Non-contact type proximity switches:

1.     They also function as limit switches but without physical contact.
2.     There are two types – called inductive proximity switches or capacitive proximity switches which operate due to changes in the magnetic field or the capacitance value under the proximity of the part or item being sensed.  These switches also require external power supply and the outputs are normally open collector transistor type to which an external relay can be connected or can be a direct input to PLC input card.
3.     These proximity sensors can also be used to measure the speed of a motor or drive by non-contact method.

1.3     Other type of sensors
Other type of sensors include rotary pulse tacho-generator  (incremental encoder) for speed measurement, absolute encoder for position measurement, light sensors, ultra sonic linear velocity sensing unit, temperature measuring sensor, pressure measuring sensor, flow measuring sensor, on-off type (discrete levels) temperature monitor , pressure monitor, flow monitor etc.

Of the above mentioned inputs, many of them are on/off type (either potential free contacts or open collector type) and are  called as digital inputs and some of them are providing analog inputs for continuous controls.

There are also other soft inputs like the keyboard inputs etc. but they do not form hard wired inputs.

2.       Output devices:

The above mentioned devices are input devices for a relay and PLC control system.  There are also number of output devices which get connected to a relay or PLC based control system.  They are as follows:

2.1     Contactors

                   i.      The functioning of contactors are similar to relays; Additionally contactors have 3 or 4 power N/O contacts (normally) which is used to switch on the power.
                 ii.      The contactor coils are energized through  PLC output or another relays contact and through the coil power supply (110VAC, 220VAC, 415VAC etc).  DC supplied coils are also present some times.
              iii.      The contactors are normally used to switch 3 phase AC power to say a motor or a power modulator or such an application and they are extensively used.
              iv.      Contactors are normally available in various sizes (size 0 to size 16  – corresponding to 16 A to 630 A and more).
                 v.      Most of the contactors are for AC power switching.
              vi.      There are also DC power switching contactors available.  They require special arc- chutes for lengthening  the arc.
            vii.      Hence these DC contactors are more expensive and need to be carefully selected.
         viii.      All contactors have auxiliary contacts for inter locking and relaying functions.  Normally 2 No + 2 N/C contacts are provided for these auxiliary functions.
              ix.      Even though relays, times etc. are replaced by the soft logic of PLC to undertake logic function, the contactors continue to be used for feeding the final load like a motor etc.  Hence the contactors are normally on-off devices for the other electrically operated equipment and hence it is essentially an output device.  Only their auxiliary contacts take part in the relaying functions.
                 x.      Apart from the selection of contactors to match with output load current and voltage, they need to be selected, also considering the numbers of operating cycles per hour.

2.2     Solenoids

             i.      Like a relay and contactor, the solenoid is an electro-mechanical device.  In this the electrical energy is used to magnetically cause a mechanical movement.
           ii.      The solenoid; like the contactor has a frame, plunger and the coil.  The coil is energized by AC or DC voltage.  Upon the application of the voltage to the coil, the corresponding plunger is pulled back through a spring in case the coil voltage is interrupted. 

        iii.      The AC solenoid draws a large in-rush current on energizing, when the plunger is fully out.  The current drops to minimum value when the plunger is fully in. Due to this, it is important to ensure that the solenoid is fully energized i.e. the plunger is fully in.  Otherwise the solenoid coil will take more than rated current continuously resulting in the burn-out of the coil.
        iv.      As against this, DC solenoid takes a constant coil current; but AC solenoid has superior initial pull.
           v.      For many industrial applications, 24V DC solenoids are normally preferred.
        vi.      While the contactor as output devices are selected based on the load current and voltage requirement, the solenoids are normally selected to handle appropriate pressure, force or weigh to be lifted etc.  Accordingly the solenoid size varies.
      vii.      Depending on the application, there are different types of solenoids
1.     Single solenoid (with one motion) with one coil.  When it is energized it moves against the spring and when de-energized comes back to original position.
2.     Single solenoid (with one motion) but with two coils.  This is similar to above but with latching feature.  One coil is for switching on and another is for switching off.
3.     Double solenoid (with two motions, say up / down) and with two coils. When one coil is energized, the motion is upwards and downwards when the other coil is energized. It goes to neutral position when both the coils are not energized.
4.     As it can be understood the number of coils to be energized decide the number of outputs per solenoid to be considered by the PLC.
5.     There are different types of solenoids available like solenoids for gas, for lubricants, for water or for emulsion (water + oil) etc and depending upon the application or the force the type is selected.

2.3     Proportional solenoids / solenoid valves:

          Normally the above solenoids are on/off devices; some times it is  required to provide a motion which is proportional to the supply current amplitude.  They will be normally supplied with currents in the range of 4 mA to 20 mA for opening a valve from 0% to 100%.  Proportional valves are used in a closed loop systems to correctly adjust position, pressure/force of a control system eg for automatic gauge control of a cold rolling mill.

There are also continuously adjustable devices – e.g screw down in a rolling mill, measuring gauge adjustment for a cut length etc which are controlled through a proportional control system based on  electrical motors or hydraulic valves .

2.4     Other output devices

Apart from the above, there are number of other output devices, such as  air / oil / gas circuit breakers, clutch with a coil for mechanical equipment like shears, brake and brake coil for stopping the machine precisely, magnetic lifts etc.

Other commonly used outputs are indication lamps with different colours ( like the colours  for the input PB-s ), fault annunciation windows, hooters  and howlers  for the faults,  various indication meters.

Of the above mentioned outputs, many of them are on/off type ( through either potential free contacts or open collector type outputs ) and are  called as digital outputs and some of them are providing analog outputs  for continuous controls.

There are also other soft inputs like the keyboard inputs etc. but they do not form hard wired outputs.

The above  discussions provide information on the various  types of  inputs and outputs seen by  a PLC.

3. PLC  inputs and outputs :  For  a PLC, normally  there are separate input and output cards ( there are also  mixed type of cards having both inputs and outputs );  these  cards are normally  with  number of  inputs or outputs –for 8 or  16 or  32 digital inputs  or outputs  and  4 or 8 or  16  analog inputs or outputs. All the digital inputs  are normally opto- isolated and analog inputs  are either  single  ended type  or the   differential  type. The differential inputs have better noise immunity.  The digital outputs are normally open collector type; some of the digital output cards have  potential free contacts  of  a  relay. Similarly the  analog  outputs are single  ended or  differential  type.

Most of the input  and output  cards  are  suitable   for 24 V DC  power supplies. This is the most preferred  power supply. Other   voltages like 110 V DC or AC  are also common The analog  input  and output cards are capable  of  accepting  and providing ± 10 V inputs  and outputs respectively. 

These cards are generally located in the racks. Low end PLC-s have brick type construction and are DIN-Rail mounted.  Some of the micro PLC-s are available in a compact version as a single module with all the functions interconnected. They can be in the main PLC rack itself or in the extension racks located in the PLC panel exclusively for these  I/O-s. These input cards access the PLC in the parallel mode and hence their access times are  quite small. Normally all the inputs and outputs   which  require fast access are located in the PLC   or their extension racks. Similarly  all time critical analog  inputs  and outputs  are  normally located in these racks. These I/O-s are called  parallel I/Os.

While the main PLC rack  will be located in  the control  room with controlled ambience – dust free  and at controlled  temperature ,  the I/ O-s  can  also be  located all over the plant. These are called remote I/O-s. These are  located in the  vicinity of  an area where there are  number of  I/O-s. E.g in the   pulpit there will be number  of  I/O-s   and  they   are  terminated into  a  panel or box  which house  the remote I/Os. These are also  located in the  racks.  But they are different from the  racks in the PLC  panel.  They are called  remote racks with their own  separate  power supplies etc. This arrangement helps to minimize the  length of cabling for majority of the  I/O-s  in  the  plant. From the  remote rack, only a  single  serial cable  communicates with the main CPU in the PLC  room through  special  remote communication cards at  both ends at speeds  of the order of  56 or 112 KBPS .  Since the remote I/O-s   communicate  over the serial cables the response  or through-put of these I/O-s  will be slower than the parallel I/O-s. 

Depending on the application, the PLC I/o  racks  are  to be selected ( parallel type  or remote  serial type ) and used considering the saving  in the cabling cost  on one hand  and the required  responses on the other hand.

4.  Selection of  the PLC based on inputs   and outputs :

i)  PLC as a   standard programmable  controllers  are   being offered by many important multinational  electrical product companies such as  i)  General electric company of USA, ii)  Siemens, iii) ABB iv)  Alstom  v ) Allen Bradley ( Rock-well automation)   vi) Mitsubhishi  vii)  Hitachi  and  some other  companies. In India  Siemens and Allen- Bradley  PLC-s  are the  most popular. All these companies have range of  PLC-s  from micro PLC-s  ( addressing less than 10 digital inputs  and 10 digital outputs  for  simple sequencing ( Boolean ) capabilities )   to high-end  large scale PLC-s ( addressing  more than  10 K  inputs and outputs – both digital  and analog types-and with  complex  instructional capabilities and handling  high speed calculations  involving real or floating point values ).

i.      The I/O-s (which as mentioned earlier) form the bulk of the PLC hardware, are decided based on the motor and component list normally prepared by the machine supplier or mill builder.

ii.      This is the starting document for deciding the specification of all the electrical equipments.  Refer the enclosed ‘typical motor & Component list’.

iii.      This list provides information such as:

1.              Name of the application and its 5 No of the mechanical supplier.
2,3.           Quantity and type of motor or other actuator required – DC separately excited, AC slip ring or AC squirrel cage, single solenoid, double solenoid etc.
4,5.           kW and RPM of the motor.
6.              Duty class of operation S 1 to S 9
7,8.           Mounting, protection class and cooling method for the motor and make.
9.              Input voltage supply available or preferred like 3 ph, 415V, 50Hz etc.
10,11.      Type of control of the motor- Reversing / non-reversing etc and quantity of power circuit.
12.            Open loop or closed loop etc.
13.            Location of the controls.
14,15.      Quantity and types of other accessories (sensors and transducers like proximity switches, infrared sensors, light sensors, pressure transducers) etc.
16.            Remarks

iv.      The above information is used to decide the final number of I/o-s for the Mill automation.


Friday, January 21, 2011

Virtual Instrumentation - Changing the Face of Design, Measurement and Automation

         From testing cars in automotive companies to controlling production and quality in manufacturing plants, the must need for engineers and scientists is to have a flexible cost-effective solutions for test and measurement. Around 30 years ago, to address these needs, a different way to solve the test and measurement problem was evolved, called "virtual instrumentation". Today, virtual instrumentation has reached mainstream acceptance and is used in thousands of applications around the world in the industries such as automotive, electronics, and oil and gas.

      The concept of virtual instrumentation is, an engineer can use software running on a computer combined with instrumentation hardware to define a custom, built-to-order test and measurement solution. The vision of virtual instrumentation revolutionized the way engineers and scientists work, delivering solutions with faster development time, lower costs, and greater flexibility.

Components of Virtual Instrumentation

      The heart of any virtual instrument is flexible software. Every virtual instrument is built on this flexible and powerful software. Innovative engineer or scientist will apply his domain expertise to customize the measurement and control application as per the requirement. The result is a user-defined instrument specific to the application needs. With such software, engineers and scientists can interface with real-world signals; analyze data for meaningful information, and share results and applications. NI LabVIEW, the productive software component of the virtual Instrumentation architecture, is the graphical development platform for test, design and control applications.

Virtual instrumentation combines productive software, modular I/O, and scalable platforms.

          The second virtual instrumentation component is the modular I/O for measurements that require higher performance, resolution, or speeds. Advanced Modular Instrument hardware use the latest I/O and data processing technologies, including Analog to Digital Converters (ADC), Digital to Analog Converters, Filed Programmable Gate Arrays (FPGAs), and PC busses to provide high resolution and throughput for measurements from 7 1/2 digit DC to 2.7 GHz. In combination with powerful software, engineers can create custom-defined measurements and sophisticated analysis routines.

Visual: NI LabVIEW, the graphical development platform along with NI Modular Hardware provides among the most reliable virtual instrumentation architectures.

          The third virtual instrumentation element is - popular and commercially available computing platform (PC or Server) to run the software and connect to I/O module, often enhanced with accurate synchronization - ensures that virtual instrumentation takes advantage of the very latest computer capabilities and data transfer technologies. This element delivers virtual instrumentation on a long-term technology base that scales with the high investments made in processors, buses, and more.

      Together, these components empower engineers and scientists world over to create their own solutions with virtual instrumentation.

        Virtual instrumentation has gradually increased addressable applications through continuoussoftware innovation and hundreds of measurement hardware devices. Having influenced millions of test and automation professionals, today it is winning over experts in the control and design domains. Virtual Instrumentation is rapidly revolutionizing the functions of control design, distributed control, data logging, design verification, prototyping, simulation and more.

 Virtual Instrumentation for Test

      Test has been a long-proven field for virtual instrumentation. More than 25,000 companies (the majority being test and measurement companies) use virtual instrumentation. National Instruments, the pioneer in Virtual Instrumentation has come a long way in the Test and Measurement domain. Yet, the need for test has never been greater. As the pace of innovation has increased, so too has the pressure to get new, differentiated products to market quickly.

       These conditions drive new validation, verification, and manufacturing test needs. A test platform that can keep pace with this innovation is not optional, it is essential. The platform must include rapid test development tools adaptable enough to be used throughout the product development flow, high-throughput test capabilities and precise, synchronized measurement abilities.

       Virtual instrumentation is an innovative solution to these challenges. It combines rapid development software and modular, flexible hardware to create user-defined test systems.

Virtual Instrumentation for Design

        The same design engineers that use a wide variety of software design tools must use hardware to test  prototypes. Commonly, there is no good interface between the design phase and testing/validation phase, which  means that, often the issues discovered in the testing phase require a design-phase reiteration.
Test plays a critical role in the design and manufacture of today's electronic devices.

        In reality, the development process has two very distinct and separate stages – design and test are two individual entities. On the design side, EDA tool vendors undergo tremendous pressure to interoperate from the increasing semiconductor design and manufacturing group complexity requirements. Engineers and scientists are demanding the capability to reuse designs from one tool in other tools as products go from schematic design to simulation to physical layout. Similarly, test system development is evolving toward a modular approach. The gap between these two worlds has traditionally been neglected, first noticeable in the new product prototype stage.

        Systems with intrinsic-integration properties are easily extensible and adapt to increasing product functionality. When new tests are required, engineers simply add new modules to the platform to make the measurements. Virtual instrumentation software flexibility and virtual instrumentation hardware modularity make virtual instruments a necessity to accelerate the development cycle.

A Future with Virtual Instrumentation

         Today, to meet the ever-increasing demand to innovate and deliver ideas and products faster, scientists and engineers are turning to advanced electronics, processors, and software. Consider a modern cell phone. Most contain the latest features of the last generation, including audio, a phone book, and text messaging capabilities. New versions include a camera, MP3 player, and Bluetooth networking and Internet browsing.

          The increased functionality of advanced electronics is possible because devices have become more software  centric. However, this increase in functionality comes with a price. Upgraded functionality introduces the  possibility of unforeseen interaction or error. So, just as device-level software helps rapidly develop and extend  functionality, design and test instrumentation also must adapt to verify the improvements.

Visual: Virtual instrumentation has been widely adopted in test and measurement areas and is rapidly making headway in control and design areas.

           The only way to meet these demands is to use test and control architectures that are also software centric.  Because virtual instrumentation uses highly productive software like NI LabVIEW, modular I/O, and commercial  platforms, it is uniquely positioned to keep pace with the required new idea and product development rate.

           Virtual instrumentation has thus been widely adopted in test and measurement areas and is rapidly making  headway in control and design areas. The benefits that have accelerated test development are beginning to accelerate control and design. Engineers and scientists who are increasing demands for virtual instrumentation in hopes of efficiently addressing worldwide demand are the driving force behind this acceleration.

Article Source


Basics of Instrumentation & Control

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Analytical Instrumentation

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Hai friends…welcome to my blog. This blog is exclusively for instrumentation engineering students which will provide sources for their reference and studies. As you all know Instrumentation is now a fast emerging and developing field in Engineering. This blog has different categories like PLC, SCADA, DCS, Sensors and Transducers, Computer control of process, Industrial Instrumentation, etc.

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