Interview question based on Industrial automation - part1

What is industrial automation?

Industrial automation is the use of control system such as the computer to control industrial machinery and processes, replacing human operator.

What are the most commonly used control systems?

  • DCS - Distributed Control System
  • PLC - Programmable Logic Control
  • SCADA - Supervisory Control And Data Acquisition

What are the main components in industrial automation?

  • Sensors
  • Transmitters
  • Control systems, such as PLC, DCS, SCADA
  • Output devices such as valves, actuators, motors

What are the different programming languages used in PLC?

  • Ladder logic
  • Structure Test Programming
  • Functional Block Programming
  • Instruction List
  • Sequential Function Chart

What is HART?

HART is Highway Addressable Remote Transducer, which is a communication protocol used in industrial automation. HART devices transfer information using HART commands. Each command holds information, such as the measured value of temperature, pressure, flow variables. HART commands are basically in three categories.

What are 4-20mA current signals?

Electrical analoge current signals are used to transmit measurement values in control logic so that to avoid noise in the transmission line. 4 - 20 mA is the common range of signals used in instrumentation.The signal is the current in the loop. This kind of signal can be transmitted for long distances over the wire without regard to voltage drops in the wire.

Why can’t we use 0-20mA signal instead of 4-20mA?

In 4-20mA signal 4 is the minimal value for the process parameter and the circuit OFF condition represents 0mA. With 0-20mA signal we cannot distinguish between signal connection break and OFF condition.

What is Seebeck effect and which transducer related to it?

Seebeck effect: Two dissimilar metal join to form a closed circuit and the junctions are at different temperature an Emf is developed. This emf is called is thermoelectric emf and this arrangement is called Thermocouple.

What are different types of thermocouple and their metal combinations?

Type T: Copper-Constantan Type J: Iron-Constantan Type E: Chromel-Constantan Type K: Chromel-Alumel

What is cold junction compensation?

The reference end of the thermocouple is maintained at 0°c this reference junction is called as cold junction. In most of the industries reference junction is kept at room temperature. The emf adjucment because of difference between the actual temperature and 0°c is refered as cold junction correction.

Temperature range for Thermocouple?

-270 - 2300°F

What are RTDs and PT100?

RTDs are Resistance Temperature Detectors which shows change in resistance when there is a change in temperature. With positive temperature coefficient, that means resistance increase as temperature increase.

PT100 is Platinum100 used in RTDs which exhibits 100ohm resistance at 0

Why to use compensation technique in RTD?

As RTD is a resistor, the ohm meter or the resistance measuring devices are connected parallel to the lead wires of the RTD. The lead wire resistance adds to the RTD resistance as the wires got resistance and this combined resistance will be shown as the RTD resistance incorrectly. So that lead compensation techniques are used in RTD connections.

There are 2 compensation techniques, 3 wire and 4 wire compensation techniques.

Temperature range of RTD?

-250 - 850°F