Control Engineering

Question 1

Describe proportional action?

Answer 1

Proportional action creates a corrective response which is directly proportional to the difference between the set point and actual value, thus a large error will cause a large response.

Question 2

Describe Integral action?

Answer 2

Integral action is creating a corrective response signal in proportion to the time integrated error, it continuously adjusts to eliminate any persistent steady state errors. It eliminates the offset created by the proportional action.

Question 3

Describe Derivative Action?

Answer 3

A derivative action creates a corrective reaction to the rate of change by calculating the slope of the reaction to anticipate the future errors. This minimizes overshoot by reducing the slope as the error gets smaller.

Question 4

What is Gain?

Answer 4

Gain is the proportional value that shows the relationship between the magnitude of the input to the magnitude of the output. seen as a ratio of output to input

Question 5

What is Two step control?

Answer 5

A two step controller is also known as an on - off controller, It has two states either inputting energy into the system (on) or not (off).

Question 6

Describe a cascade controller?

Answer 6

A cascade control system uses two controllers in one system, providing two feedback loops. The master is slow reacting and the slave id fast reacting this improves performance. The masters output is the slaves set point

Question 7

Describe a split range controller?

Answer 7

A split range controller uses one controller to control two more more control elements such as valves.

Question 8

What are Advantages of an electrical control circuit opposed to a pneumatic circuit?

Answer 8

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More accurate - pneumatic suffer from hystersis
Quick response time - travel along the wire is instant
Size of the control system and wires - air pipes are much larger than wires
Requires less maintenance - no compressor or drier to maintain

Question 9

Describe a Resistance temperature sensor?

Answer 9

A 4 wire PT100 is a highly accurate temperature sensor which uses the 4 wires to eliminate the error of the connecting wires. PT 100 uses a platinum wire which has a resistance of 100 ohms at 0 degrees, as the temperature increases, the resistance increases proportionally. This sensor is connected in a Wheatstone bridge circuit, Where when the resistance changes in the temperature sensing the circuit is out of balance and produces an output signal.

Question 10

Describe a Explosimeter?

Answer 10

An explosimeter uses 4 resistors in a Wheatstone bridge configuration. 3 resistors are of known resistance and the 4th one is a hot filament in a chamber. When a flammable gas enters the chamber it ignites thus increasing the temperature and therefore the resistance changes and the Wheatstone becomes unbalanced and an output current is produced, this is then converted into a concentration or PPM.

Question 11

Explain a Capacitance Level sensor?

Answer 11

A Capacitance level sensor uses a wien bridge oscillator circuit to give feed back of the level in the tank. A wien bridge oscillator uses 4 resistors and two capacitors, one of these capacitors is the level sensor. When the dielectric between the two plates changes so does the capacitance. The Wien bridge produces a sine wave output signal, the frequency is converted into a voltage output.

Question 12

How to test and calibrate the PT100?

Answer 12

To test the temperature sensor, you can place it in a heat source along with a thermometer and ensure the temperature reading from the sensor matches the thermometer.
To calibrate you must have a process loop calibrator and read the output signal at zero and span, the values can be adjusted as the output table.

Question 13

What are Disadvantages of an electrical control circuit opposed to a pneumatic circuit?

Answer 13

• High initial cost however due to maintenance long term its cheaper
  Not easy to trouble shoot, staff need training - pneumatic is easy to trace leaks
  Sensitive to temperature - resistance of wires can change with temperature
  Cannot be used in hazardous areas due to risk of fire - sparks, heat and arcs could cause a fire

Question 14

How to test and calibrate an explosimeter?

Answer 14

Using a calibration gas of a known concentration the explosimeter can be subjected to this gas and ensure the reading is the same as the concentration.
To calibrate a zero and span gas must be used to check the output at 0/100

Question 15

Why are valve positioners fitted?

Answer 15

Valves positioners are fitted to ensure accurate and responsive control valve operation by limiting hystersis from the friction of the valve and pressure changes in the system. They provide a feedback to the controller to adjustments can be made.