Instrument & Control Flashcards
Draw and explain the theory behind the workings of a thermocouple
Two different wire materials connected via a junction. As the temperatures increase a current will flow around the circuit.
Use a diagram to explain how the theory is applied to produce a practical thermocouple temperature measuring device?
A third metal, copper is added to connect to a voltmeter and a junction where a set temperature can be referred to. The EMF is proportional to the difference between the hot junction and the reference of the cold junction.
How are thermocouples manufactured to cater for different temperature ranges?
By using different combinations of metals
Give two problems associated with thermocouples
Low accuracy
Prone to wiring faults
Explain the principle of a PT100?
The resistance will change with the change of temperature
What does PT100 stand for?
Platinum (PT)
Produces a resistance of 100 ohms at 0 degrees
Why do PT100s typically have 3 or 4 wires to connect them to the temperature indicating device?
The are connect in bridge Circuits as the device requires signal conditioning and the resistance of the leads need to be compensated. A 4th wire is used over long distances
With aid of a diagram, explain how a strain gauge based sensor can be used to measure differential pressure?
Strain gauges are glided to a diaphragm, as the diaphragm moves, distort Simon is translated into an increase in resistance. The resistance is then measured and a proportional signal is sent to the pressure controller.
State 3 examples of signal conditioning
Amplification
Analogue to digital conversion
Digital to analogue conversion
State the standard signal ranges used for electrical and pneumatic signals?
4mA to 20mA
0.2 to 1 bar
With signal ranges, explain why the ranges are termed ‘live zero’?
Both start with a non zero value to distinguish between 0% and an error
An actuator is used to control the flow of oil into a boiler burner. What would you consider when deciding whether to fit a fail safe or fail set actuator in this situation?
For a deep sea vessel I’d fit a fail set to shut of the fuel from filling the boiler with fuel and vapour.
Explain with an aid of a sketch how the diagram motor transducers a varying air pressure signal into a mechanical movement?
A diaphragm actuator has a rubber membrane supported by a metal disk in a pressure tight chamber. Operating air is entered above or below the diaphragm against a spring which moves the steam up or down.
Describe with an aid of a sketch how the process fluid pressures acting within the valve body are balanced in order to reduce the load on the diaphragm?
A single seat plug requires a large diaphragm force to open against the fluid. Whereas a double seat plug has the fluid trying to open and close causing an equal pressure. Therefore a smaller diaphragm force can be used.
Explain the gain characteristics of a linear control valve?
The valve travel and flow rate are linear related. Therefore an equal amount of opening equals an amount of flow.
Explain the gain characteristics of a equal percentage control valve?
When an equal amount of valve travel produces an equal amount of flow
Explain the gain characteristics of a Quick Opening control valve?
The control valve is fitted with a flat disk. It allows for maximum flow rate with minimum valve opening
Explain why double seated valves are used in control valves?
The double seated valve plug has the force of the fluid acting on both sides. This creates an equal pressure so a smaller diaphragm can be used and less operating force.
Outline why a valve positioner maybe used
Valve positioners are used when the stem has high friction or the controller is some distance away causing time lags.
State 4 circumstances why a valve positioner maybe used?
To minimise the effects of friction and hysteresis
Increase speed of response
To allow signal reversing
Allow valve characteristics to be changed by adjusting the rotating cam in the feedback system
State the principle used by a positioner in control technology?
The positioner compares the requested valve position with the actual valve position through the feedback linkage. They have there own air supply to remove error.
Outline the immediate action to be taken to restore automatic control upon failure of a positioner
Switch to manual control Regain control Replace parts Ensure system is steady state Switch back to automatic
Outline 4 advantages of pneumatic control valves
Robust
Cheap to repair
Fast acting
Can be used in hazardous areas
Outline 5 disadvantages of pneumatic control valves
Large Require positioners and boosters Require clean/dry air Long lengths of piping Requires complicated fail set
Sketch a block diagram of a closed loop control circuit and describe how it works?
A monitoring element measures the output. It sends the information back via the feedback to the comparing element. Here it compares with the desired value. An error is set to the controlling element where this decides how to correct. The correcting element then corrects the error as it heads back to the monitoring element. The process relates to keep the desired value.
Define the term measured value
This is the actual value of the process variable as measured by the relevant sensor
Define the term desired value
This is the value set by the operator
Define the term deviation
This is the difference between the set value and the actual valve. Also known as the error.
Describe with the aid of a sketch, the role of an electronic proportional controller for the operation of a control valve. Give a full answer as you can taking into account it’s operation and the type of control afforded
The regulate a process, such as, flow, temperature, pressure and level. A PID controller with Integral and Deviation turned off will create a proportional action.
The system will have a set point ( desired value). The monitoring element will measure the output and feedback the information to the controller where it is then compared to the set point. If there is a difference an error will be created which with then go to the controlling element which will creat a plan to fix. This is then transferred to the correcting element which eliminates the error.
The monitoring element then measures the output again and feedbacks the new information.
Explain the meaning of a two step controller
This is known as a on/off control. This works by the controller switching the energy in and out of the process. Allow energy to be transferred only when needed.
Explain the meaning of proportional control action
This produces an output that is directly proportional to the error between the set point and measured value
Explain the meaning of integral control action
Steadily increases the corrective action as long as an error exists.
State with reason, a typical ship board application for two step control action
For non critical processes where accuracy is not required. Such as air Compressors and hot water.
State with reason, a typical ship board application for proportional plus integral action
Engine jacket water cooling. The integral removes the offset but because reaction time isn’t important, differential control is not required
Use a diagram to explain how two step control can be used to regulate the temperature of a water heater
A two step is an on/off system, allowing energy only when needed. The monitoring element measures the temperature of the water and feedbacks the information to the controller which compares to the desired value. The difference between the actual value and the desired will decide if the the process is needed.
Show how a water heater system will respond to an increase in demand from the heater
The system will turn on and off to keep the water at the desired value with in the systems deadband. An increase in demand will cause a lot of new cold water enter then tank, dropping the temperature within the tank. The heater will now be on for a longer period to get back to the desired value.
Use a diagram to explain the term deadband and explain the results if the deadband is too wide or too narrow.
A deadband is when the temperature is not stable along the set point. Therefore a two step controller will turn on and off to allow it to fluctuate around the set point.
If the deadband is to narrow then the system will cut on and off regularly causing instability.
If the deadband is to wide the stability of the system increase but we lose out on accuracy of the system.
List the instrumentation necessary to convert a simple manually converted flow control system into an automatic closed loop system giving a brief overview of each item listed.
Measuring element - monitors the output of the process
Comparing element- compares the difference between the actual value and the desired value. Then creates an error to correct
Controlling element - receives and acts apron the error signal
Correcting element - corrects the error by the method approved by the controlling element
Sketch a basic pneumatic amplifier, label the main parts
Air supply Oriffice Output pressure Nozzle Flapper
State two examples where pneumatic amplifiers maybe used?
- sensor control systems where a small signal change from a sensor must be amplified to provide a proportional control pressure
- multiply a signal from one controller to operate several actuators in sequence
Sketch a pneumatic transmitter of the flapper/ nozzle type incorporating feedback, labelling all components
Air supply Nozzle Flapper Actual value Desired value Gain adjustments
Explain what PLC’s are, highlighting any of their advantages?
A Programmable Logic Controller PLC is a digital electronic device that uses programmable memory to store instructions and to implement specific functions, such as sequences and timings.
The program can be modified with features added and removed without requiring devices making it a flexible system.
Explain a cascade control system
When two parameters are monitored on the same system. This is also known as a master and slave controller
Explain a split range control system
Where there is one measurement but more than one manipulated variable
Explain the term distance velocity lag
The time delay due to the distance between the disturbance and the monitoring element
Explain the term Transfer lag
Time taken for energy to be transferred from one medium to another
For a control system fitted with a P + I controller:-
Describe how the proportional band and integral time can be adjusted, with the system in operation, to give slight under-damping and without offset.
- Turn off integral and derivative control.
- Increase proportional gain until the measured variable oscillates.
- Reduce the proportional gain until the measured variable stabilises.
- Adjust integral control until the measured variable just oscillates.
- Back off the proportional gain to stabilise the measured variable.
Describe how Ziegler & Nichols tuning could be applied to a P+I Controller
When the system is just oscillating, record the gain setting (KU) and measure the time period of the oscillation (TU) and use these values along with the appropriate Ziegler & Nichols table to obtain the required dial settings for KP and TI.
Draw a sketch of a Quarter Decay system response giving one advantage and one disadvantage with this type of response.
Removes error quickly
Poor stability
Describe why we need to tune a system
Every system is slightly different, therefore it is unique and requires to be able to tune different amounts of gain and integral action
Explain with the aid of sketches, the difference between under damped, over damped and critically damped system performance.
Underdamped will respond very quickly but create a large overshoot which will take a long time to settle out.
Critical damped, after change it will return to steady state in the quickest time possible without any overshoot.
Over damped will be slower than critical damped to return to steady state but again will not over shoot but takes a long time to get back to steady state
What test could you carry out to verify the performance curve of a P+I system?
While monitoring the system, stop a pump or closes a valve and monitor how quickly the system responds to the change
Explain how you would tune a P+I controller using: The Trial and Error Method.
- Turn off integral action (set to max) and derivative action (set to min).
- Decrease gain to a low value (0.5) and then increase in stages, until process becomes unstable.
- Reduce gain until process just stabilises.
- With the system stable, slowly increase the integral action until the offset is eliminated and the system just starts to become unstable.
- Decrease the gain slightly until stability is regained.
Explain how you would tune a P+I controller using: The Ziegler and Nichols Ultimate Cycle Tuning Method.
- Turn off integral action (set to max) and derivative action (set to min).
- Decrease gain to a low value (0.5) and then increase in stages, until process becomes unstable. Note down the gain setting (KU) and the oscillation cycle time (TU).
- Use appropriate Z & N chart along with the recorded values, to determine the optimum settings for P or P+I as required.
List 7 Diesel Engine process variables
Output Speed Load Torque Fuel Flow LO Pressure JW Temperature Bearing Temperature Cylinder Pressure
Define Steady State
The energy supplied to the system is equal to the energy used and output by that system
Define Process Stability
Ability of the process to achieve steady state after a change in operating conditions
Examples of Open Loop Control System
Washing Machine Cycle
Toaster
Microwave
Advantages and Disadvantages of an Open loop system
Simple and cheap to set up
highly inaccurate
Advantages and Disadvantages of an Closed loop system
Potentially unstable
Complicated and expensive to set up
Very accurate
Types of measuring sensors
Pressure Temperature Level Flow Displacement Speed Torque Viscosity
Draw and describe a measuring sensing system
A measuring system will include a sensor, transducer, conditioner, transmitter and a display. This senses the condition of the process and the information can be used or interpreted.
Define Discrete in measuring systems
A discrete system produces single on/off pulses or individual steps.
Define Analogue in measuring systems
An analogue system produces an infinitely variable signal between the limits of the working range.
Define Digital in measuring systems
A digital system produces a combination of on/off signals (binary code) that replicates a stepped analogue response
Methods of measurement sensing changes
Mechanical movement Elastic Distortion Expansion/ Contraction Change in thermal Properties Change in electrical properties
What calibration record would an instrument typically contain
Description of the instrument and identification number
Serial Number
Fitting date
Date of last calibration
Result of last calibration
Details of calibration procedure and acceptable error
Details of repairs and adjustments made
What two ways can pressure be measured
Absolute pressure relative to a perfect vacuum (Abs)
Pressure relative to atmospheric pressure (Gauge)
Draw 3 types of diaphragms
Simple flat diaphragms
Corrugated diaphragms
Corrugated bellows
Explain with a sketch a strain gauge
As the pressure changes across the diaphragm, it will deflect causing it to expand and contract. this will change the length of the sensors, which then changes the resistance properties.
Explain with a sketch a Piezo-Electric pressure sensor
It uses rock crystals which when they are compressed produce an electrical discharge. This is due to the crystals moving across each other. The larger pressure means a greater electrical discharge. The sensor can measure rapid changed in pressure.
Whats a PT100s temperature range
-200 to +600 degrees C
Draw and Explain, how an orifice plate measure flow?
It restricts the flow of liquid through a pipe. This changes the liquids pressure and velocity which is measured by sensors before and after.
What has to be thought of when fitting an orifice plate?
It must be 12 pipe diameters of pipe length down stream of obstructions and bends. Then it must have 5 pipe diameters of pipe length after the plate to reduce erosion.
Draw and explain how a turbine flow meter works?
The fluid spins the rotor. As this spins the magnet sends a signal to the detector. These frequencies are then related to a flow rate.
Draw and explain a Coriolis Flow meter
A fluid flows through the vibrating tube the liquid causes the tube to twist. The positions pickups can the measure the amount of twist which is relative to flow.
Draw and explain a Viscosity sensing device
A gear pump pushes the liquid through a capillary where it measures the pressure drop which relate to the viscosity via a D.P Cell. The D.P Cell sends a signal to the heater to adjust the viscosity.
Draw and explain hydrostatic pressure level sensor
It compares the hydrostatic pressure of the liquid at the bottom of the tank and the atmospheric pressure at the top of the tank.
Draw and explain an electro optical sensor
A light shines onto a disk with holes. As the light passes the holes it creates light pulses which are received by a photo transistor. This then senses the movement.
Sketch the 4 different actuator valves
Percentage V Port
Percentage Parabolic
Linear
Quick Opening
What is fail safe
This will fail fully open or fully shut depending on the system
What is fail set
This will fail in the position it was operating in when failed
Draw and table some of the ladder logic symbols
Relay Coil
Normally Open Contact
Normally Closed Contact
Describe timer blocks in PLC systems
Can be used to set time delays or time operation times
Describe counter blocks in PLC systems
Used to count the number of times a pump has started for maintenance logs
Describe specialised blocks in PLC systems
Carryout maths functions or PID Controls
Draw and Explain a boiler water level control system
As the level detector changes this informs the P+I controller which is also told the desired value. This control tells the I/P controller to open the feed water valve more to top the boiler water up. The level detects the change and the process is repeated.
Draw and Explain a ME Cooling water temperature control system
JCW leaves the ME while a P+I controller has a thermocouple temperature sensor on the inlet to ME. This tells the P+I controller to either cool the JCW via a three way valve to the SW cooler or bypass the cooler if still cool enough.
