Measurement

Question 1

What are the 3 main purposes measuring systems are used for?

Answer 1

(1) to obtain data about some event
(2) for inspecting or testing
(3) as an element in a control system

Question 2

What are the three basic constituent elements of a measurement system?

Answer 2

(1) Transducer/sensing element
(2) Signal conditioner
(3) Display

Question 3

What is the first element in the measurement system?

Answer 3

Sensing element/transducer

Question 4

What is the purpose of a does a transducer /sensing element?

Answer 4

• 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

Question 5

What is the second element in the measurement system?

Answer 5

Signal conditioner/signal converter/signal processor

Question 6

What is the purpose of a signal conditioner/signal converter/signal processor?

Answer 6

• 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

Question 7

What is an example of this?

Answer 7

An amplifier which takes a small signal from the sensing element and makes it big enough to activate the display

Question 8

Where is the output from the sensing element displaced?

Answer 8

Display or indicating element

Question 9

What is the purpose of a display?

Answer 9

the display element takes the information from the signal converter and presents it in a form which enables an observer to recognise it

Question 10

What is an example of a display or indicating element?

Answer 10

A pointer moving across a scale

Question 11

See lecture notes example

Answer 11

See lecture notes example

Question 12

What is the basic function of a sensor?

Answer 12

To detect a signal or stimulus and from this produce a measurable output

Question 13

Of what common physical parameters require measurement? (9)

Answer 13

displacement
speed
acceleration
fluid flow
liquid level
force
pressure
proximity and temperature

Question 14

Give three other parameters

Answer 14

chemical
sound
radiation

Question 15

What factors do the choice of sensor depend on? (6)

Answer 15

(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

Question 16

What is the main difference between a temperature sensor used in the home and a temperature sensor used in a chemical plant

Answer 16

one used in a chemical plant which may be inaccessible, subjected to high temperatures and pressures or be in a corrosive atmosphere.

Question 17

What are the two distinct areas where sensor technology is used?

Answer 17

(1) for gathering information

(2) for controlling systems

Question 18

What may sensors be?

Answer 18

Mechanical, electrical or a combination of both

Question 19

What is the difference between a sensor and a transducers?

Answer 19

• 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

Question 20

Give an example of this

Answer 20

• 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

Question 21

What do both open- and closed-loop control systems try to do?

Answer 21

Maintain a variable at a predetermined value

Question 22

What do these control systems contain?

Answer 22

Measurement systems

Question 23

How do they differ from pure measurement systems?

Answer 23

the output from a control system regulates a parameter whose value is not necessarily displayed to the user

Question 24

Describe an open-loop system

Answer 24

• 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

Question 25

Give an example of a open-loop control system

Answer 25

• 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

Question 26

What are the problems of open-loop systems?

Answer 26

• 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

Question 27

Give 2 advantages and 1 disadvantage of an open-loop system?

Answer 27

• relatively simple in design
• inexpensive
• do require frequent operator intervention due to preset value becoming incorrect

Question 28

How do closed-loop systems differ from open loop systems?

Answer 28

• in a closed-loop system, the state of the output directly affects the input and so can automatically change the input accordingly

Question 29

More disadvantages of open-loop systems!

Answer 29

• effected by disturbance
• possible disturbance will corrupt the output
• system cannot correct for the disturbance (no feedback)

Question 30

Example of this..

Answer 30

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

Question 31

How does a closed-loop system operate?

Answer 31

• 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

Question 32

Give a closed-loop system example

Answer 32

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

Question 33

See lecture notes diagram

Answer 33

See lecture notes diagram

Question 34

Advantages of closed loop feedback system

Answer 34

• compensates for disturbance

- this system can correct for the disturbance (feedback)

Question 35

Example of this..

Answer 35

‘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!

Question 36

See lecture notes for Dynamic Responses

Answer 36

See lecture notes for Dynamic Response