1. What is Automation?
Automation is delegation of human control functions to technical equipment for increasing productivity, better quality, and reduced cost & increased in safety working conditions.
2. What are the different components used in automation?
The components of automation system include
Sensors for sensing the input parameters (RTD, Thermocouple, Pressure, Flow, Level; etc)
Transmitters for transmitting the raw signal in electrical form
Control system which includes PLC, DCS & PID controllers
Output devices / actuators like drives, control valves.
3. What are the different control systems used in Automation?
PID Controller based control system
PLC based control system
DCS based Control system
PC Based automation system
4. Explain PID based control system.
PID (Proportional Integral Derivative) is the algorithm widely used in closed loop control. The PID controller takes care of closed loop control in plant. A number of PID controller with single or multiple loop can be taken on network. PID Controllers are widely for independent loops. Although some logic can be implemented but not much of sequential logic can be implemented in PIDs.
5. Difference between PLC & Relay?
PLC can be programmed whereas a relay cannot.
PLC works for analog I/Os such as PID loops etc. whereas a relay cannot
PLC is much more advanced as compared to relay.
Modifications in relay base circuit is difficult compared to PLCs
6. Difference between PLC & DCS?
DCS: The system uses multiple processors, has a central database and the functionality is distributed. That is the controller sub system performs the control functions, the history node connects the data, the IMS node gives reports, the operator station gives a good HMI, the engineering station allows engineering changes to be made.
PLC: The system has processor & I/O’s and some functional units like basic modules, communication modules and so on. Uses a SCADA for visualization. Generally the SCADA does not use a central database.
7. What is PC based control system ?
In PC based control system, the CPU of computer acts as processor, the PCI based cards are used for connecting Input and Output. The RAM acts as memory. Hard disk is used as storage device. Currently this systems are very useful when the large data is to be proceed with very high speed. In many cases for greater accuracy we can use the real-time operating system.
8. What is Encoder?
A feedback device which converts mechanical motion into electronic signals. Usually an encoder is a rotary device that outputs digital pulses which correspond to incremental angular motion. The encoder consists of a glass or metal wheel with alternating clear and opaque stripes that are detected by optical sensors to produce the digital outputs.
9. Which are the leading PLC providers ?
The leading PLC providers include
Rockwell Automation : Allen Bradley (Micrologix, SLC, PLC, Control Logix)
Siemens ( S7 200, S7 300 , S7 400,S71200,S71500,LOGO)
Grouppe Schneider : Modicon ( Nano, Micro, Premium, Quantum)
GE Fanuc : Versa, Series 90-30, 90-70
Messung : Nextgen
12. What types of sensors are used for measuring different parameters?
Temperature sensors – RTD, Thermocouple, Thermister
Pressure Sensor – Borden Tube, Bellows, Strain gauge
Flow sensor – Pitot tube
Level, Conductivity, Density, Ph
13. What is transmitter?
A transmitter is an electronic device that is generally mounted in the field in close proximity to a sensor. The sensor (also known as a transducer) measures a physical variable such as temperature or pressure and outputs a very low level electronic signal. The basic function of the transmitter is to provide the correct electrical power to turn on (or excite) the sensor then to read the low level sensor signal, amplify it to a higher level electrical signal and send that signal a long distance to a control or read-out device.
Since low-level electrical signals do not transmit long distances with great accuracy, installing a transmitter generally gives a tremendous improvement in the accuracy of the information delivered to a larger control system. Typically the output form the transmitter is 4-20 mA or 0-10 V
56. What is PLC?
PLC means Programmable Logic Controller. It is a class of industrially hardened devices that provides hardware interface for input sensors and output control element. The field I/p include element like limit switches, sensors, push button and the final control elements like actuator, solenoid/control valves, drives, hooters etc
PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.
57. Applications of PLC
PLC can be used in almost all industrial application solutions right from small machine to large manufacturing plants. Even it caters applications of redundant systems at critical process plants.
58. Role of PLC in Automation ?
PLC plays most important role in automation. All the monitoring as well as the control actions are taken by PLCs. PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.
59. Role of CPU ?
This component act as a brain of the system.
CPU consist of Arithmetic Logic Unit, Program memory, Process image memory, Internal timers and counters, flags
It receives information from I/P device, makes decisions depending upon the information and logic written and sends information through the O/P devices.
The CPU’s are distinguished with following features
Memory capacity, Instruction set supported, communication option, time required to execute the control program.
60. Role of Power supply in PLC system ?
Power supply provides system power requirement to processor, I/O and communication modules. Typically the power supply has input voltage 120 V – 230 V AC or 24 V DC and back plane output current 2 A to 5 A at 5 V DC
61. Role of Rack or Chassis in PLC system ?
A hardware assembly, which houses the processor, communication and I/O modules. It does following functions.
Power distribution
Containment of I/O modules
Communication path between I/O module and CPU
The chassis are available in different slots in various PLC systems. Additional chassis can be connected using chassis interconnecting cable.
62. What is role of I/O modules?
Electronic plug in units used for interfacing the i/p and o/p device in the machine or process to be controlled.
I/P module receives data from i/p devices (Pushbutton, Switches, Transmitters) and send it to processor. The O/P module receives data from processor and send it to output device (Relay, Valves).
Digital/Discrete :- Sends and Receives On/Off signal
Analog :- Sends and receives variable input or output signals
65. Difference Between Fixed and Modular PLCs ?
In non modular PLCs the processor will have inbuilt power supply and I/Os in one unit.
The modular PLC, will have separate slots for components like Power supply, I/O modules. You can select the I/Os or power supply as per the need.
66. What are the Types of I/Os ?
Local – These are the I/Os placed in the PLC main rack containing CPU. These I/Os are connected to CPU through backplane.
Distributed - These are the I/O placed at remote location from the main rack containing the CPU. These I/O’s are to be connected on communication bus like control net, device net or FIP I/O.
67. What is meaning of resolution in I/O cards in PLCs ?
It is the minimum change in i/p parameter which can sensed by the i/p card. As far as Digital I/O is concerned it takes only one bit for operation. In case of analog input the resolution determines how much bits are used for input or output. For example a 12 bit resolution card means the input will come as 0 to 4095 count (2^12). For 16 bit data the counts will be from 0 – 65536(2^16). More the resolution the data will be more accurate
68. What is an Analog Input Module ?
An I/O module that contains circuits that convert analog input signals to digital values that can be manipulated by the processor. The signals for pressure, flow, level, temperature
transmitters are connected to this module. Typically the input signal is 4-20 mA, 0-10 V
69. What is Analog Output Module ?
An I/O module that contains circuits that output an analog dc signal proportional to a digital value transferred to the module from the processor. By implication, these analog outputs are usually direct (i.e., a data table value directly controls the analog signal value).
70. What is meaning of universal analog input card?
Normally there are different cards for different signals. But in universal input card the same channels can be configured for RTD, Thermocouple, Current or voltage input.
71. Give examples of I/P and Output connected to PLCs
Digital I/P (Pushbutton, Switches )
Analog I/P (Temp, Pressure, Flow, Level)
Digital O/P (Solenoids valves, Contactors)
Analog O/P (Control Valves, Speed)
72. Explain Source and Sink Concept?
Sinking:-Provides a Path to supply Common (-) or PNP type Input.
Sourcing:-Provides a path to supply source (+) or NPN type Input
73. What is forcing of I/O ?
Forcing the I/Os means making the desired status of I/O in PLCs irrespective of its status coming from the field.
In certain cases when there is problem in receiving field input /output, we can force the i/Os so that the logic takes desired state.
74. Scan cycle of PLC
PLC’s scan cycle follows following path Scan cycle of PLC
Input Image Updation
Process Logic Execution
And update result as per program.
75. What is meaning of scan time in PLC?
Scan time is the Time required to read the I/P, Process the logic and update the output in one cycle.
79. What is redundancy ?
The capacity to switch from primary equipment to standby equipment automatically without affecting the process under control.
Redundancy means provision for standby module. In case of failure of one module is running process, the standby module takes over. Hot redundancy means the changeover of control from active processor to standby processor in less than 1 scan time.
80. Need of redundancy
In critical processes, it is important to run the plant without failure. In such case it is important to have redundancy so that even if one system fails the redundant system can take care without affecting plant.
81. Types of redundancy
CPU redundancy: In case of CPU failure the standby CPU takes care of the plant
Power Supply redundancy: In case the power supply fails the standby power supply takes control of the situation.
Communication : Multiple communication channels are provided to take care of communication failure
I/O Redundancy : Multiple I/O channels are provided to take care of input or output failure.
101. Explain the System Bits available in S7-200 PLC Special Memory area ?
Special Memory Byte 0 (SM0.0 - SM0.7) provides eight bits that are updated by the S7-200 CPU at the end of each scan cycle. Your program can read the status of these bits and then make decisions based on a bit’s value.
Always_On SM0.0 This bit is always ON.
First_Scan_On SM0.1 This bit is ON for the first scan cycle only. One use is to call an initialization subroutine.
Retentive_Lost SM0.2 This bit is turned ON for one scan cycle if retentive data was lost. This bit can be used as either an error memory bit or as a mechanism to invoke a special startup sequence.
RUN_Power_Up SM0.3 This bit is turned ON for one scan cycle when RUN mode is entered from a power-up condition. This bit can be used to provide machine warm-up time before starting an operation.
Clock_60s SM0.4 This bit provides a clock pulse that is OFF for 30 seconds and ON for 30 seconds, for a cycle time of 1-minute. It provides an easy-to-use delay or a 1-minute clock
pulse.
Clock_1s SM0.5 This bit provides a clock pulse that is OFF for 0.5 seconds and then ON for 0.5 seconds for a cycle time of 1 second. It provides an easy-to-use delay or a 1-second clock pulse.
Clock_Scan SM0.6 This bit is a scan cycle clock that is ON for one scan and then OFF for the next scan. This bit can be used as a scan counter input.
Mode_Switch SM0.7This bit indicates the current position of the Mode switch (OFF=TERM position and ON=RUN position). You can use this bit to enable Freeport mode only when the switch is in the RUN position. Normal communication with the PC/programming device can be re-enabled by switching to the TERM position.
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