Measurement Flashcards
What are the 3 main purposes measuring systems are used for?
(1) to obtain data about some event
(2) for inspecting or testing
(3) as an element in a control system
What are the three basic constituent elements of a measurement system?
(1) Transducer/sensing element
(2) Signal conditioner
(3) Display
What is the first element in the measurement system?
Sensing element/transducer
What is the purpose of a does a transducer /sensing element?
- Produces a signal which is related to the quantity being measured
- Take information about the thing being measured and changes it into some form which enable the rest of the measurement system to give a value to it
What is the second element in the measurement system?
Signal conditioner/signal converter/signal processor
What is the purpose of a signal conditioner/signal converter/signal processor?
- takes the signal from the sensing element and converts it into a condition which is suitable for the display part of the measurement system, or in the case of a control system for combining with the reference signal
What is an example of this?
An amplifier which takes a small signal from the sensing element and makes it big enough to activate the display
Where is the output from the sensing element displaced?
Display or indicating element
What is the purpose of a display?
the display element takes the information from the signal converter and presents it in a form which enables an observer to recognise it
What is an example of a display or indicating element?
A pointer moving across a scale
See lecture notes example
See lecture notes example
What is the basic function of a sensor?
To detect a signal or stimulus and from this produce a measurable output
Of what common physical parameters require measurement? (9)
displacement speed acceleration fluid flow liquid level force pressure proximity and temperature
Give three other parameters
chemical
sound
radiation
What factors do the choice of sensor depend on? (6)
(1) the nature of the parameters being measured
(2) cost
(3) reliability
(4) quality of information needed
(5) suitability of the sensor design to the environment in which they are used
(6) whether the information is needed immediately or for later analysis
What is the main difference between a temperature sensor used in the home and a temperature sensor used in a chemical plant
one used in a chemical plant which may be inaccessible, subjected to high temperatures and pressures or be in a corrosive atmosphere.
What are the two distinct areas where sensor technology is used?
(1) for gathering information
(2) for controlling systems
What may sensors be?
Mechanical, electrical or a combination of both
What is the difference between a sensor and a transducers?
- a transducer is any device that transforms one form of energy to another
- a sensor is usually a transducer, but not all transducers are strictly sensors
Give an example of this
- the domestic light bulb is a transducer that converts electrical energy into light (and heat)
- the purpose of the light bulb is (normally) to illuminate the room, not to indicate the presence of electricity
- If the light bulb is used to test an electric circuit, i.e to illuminate when electricity flows through it, it could then be considered a sensor
What do both open- and closed-loop control systems try to do?
Maintain a variable at a predetermined value
What do these control systems contain?
Measurement systems
How do they differ from pure measurement systems?
the output from a control system regulates a parameter whose value is not necessarily displayed to the user
Describe an open-loop system
- to control the signal which is at a preset value
- this preset value will not change even if other factors do which may render the system output incorrect
Give an example of a open-loop control system
- switching on and off street lamps along a road
- the control required is that when darkness falls, the lights turn on, and when it gets light, the lights turn off
- the control signal could be set using set times in the evening and the morning but as the hours of darkness change over the year the preset signal would soon become incorrect and the lights would be going on and off too early or too late
What are the problems of open-loop systems?
- there is no input to the open-loop system which detects what is actually happening (whether it is getting dark or light)
- as an open-loop system someone has to estimate and set the timer control and have to alter these as the hours of darkness change
- this also does not take into account evenings which are dull or cloudy compared to those which are bright and sunny
Give 2 advantages and 1 disadvantage of an open-loop system?
- relatively simple in design
- inexpensive
- do require frequent operator intervention due to preset value becoming incorrect
How do closed-loop systems differ from open loop systems?
- in a closed-loop system, the state of the output directly affects the input and so can automatically change the input accordingly
More disadvantages of open-loop systems!
- effected by disturbance
- possible disturbance will corrupt the output
- system cannot correct for the disturbance (no feedback)
Example of this..
Toaster
Input = Desired colour of toast
Output = Actual colour of toast
- Often get ‘Burnt Toast’
- System does not measure colour of bread
- Cannot correct for different thickness and type of bread
How does a closed-loop system operate?
- the desired value/reference signal/set point is compared to the feedback signal (the actual value of the parameter being controlled)
- the difference between the two is known as the error
- this error signal is then modified so that it can adjust the performance of the system and is then known as the control signal
Give a closed-loop system example
Tank in a chemical plant containing a hazardous liquid
- the tank is filled with liquid by a pump and when the liquid is required for further chemical processing, another system opens the discharge valve to take the amount of liquid needed
- for chemical plant to work effectively the tank must be maintained at optimum level (must not be overfilled (spilling hazardous material), or run dry (stopping plant operation))
- a closed loop system is designed which uses a liquid level sensor to determine the level in the tank
See lecture notes diagram
See lecture notes diagram
Advantages of closed loop feedback system
- compensates for disturbance
- this system can correct for the disturbance (feedback)
Example of this..
‘Smart’ Toaster Input = Desired colour of toast Output = Actual colour of toast Light Sensor: Measure colour of toast - Feedback from sensor allows input to be adjusted - Heat from elements is auto-adjusted = Good toast!
See lecture notes for Dynamic Responses
See lecture notes for Dynamic Response
