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Control & Dynamic Systems > Motion > Flashcards

Motion Flashcards

1
Q

What is motion?

A

the change in the physical position of an object

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2
Q

What are linear and angular displacement devices are capable of?

A
  • measuring displacement against time, and therefore velocity and acceleration (i.e position of gear wheel with respect to time tells you angular velocity and acceleration)
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3
Q

What is displacement?

A
  • the distance measured in a stated direction from a reference point
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4
Q

What is linear displacement and how is it measured?

A

the distance measured IN A STRAIGHT LINE in a stated direction from a reference
- measured in terms of an angle of rotation about an axis it is called angular displacement

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5
Q

How are large displacements measured?

A
  • requiring highly accurate measurements

- measured by radar, sonar, or sophisticated optical systems.

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6
Q

How is displacement restricted for the purpose of designing simple control systems

A

restrict the measurement of displacement to less than one metre

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7
Q

Name simple devices used for measuring linear displacement and the advantages and disadvantages

A

rulers
slip gauges micrometers and vernier gauges
- these are quick and easy to use but their accuracy is heavily dependent on the skill of the operator and cannot be used for remote measurement A mechanical linear displacement device which is generally more accurate is the dial test indicator (DTI).

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8
Q

How can linear displacement be more accurately measured?

A

using a dial test indicator (DTI)

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9
Q

What is a potentiometer, what does it consist of, what materials are used and why?

A
  • an electrical device which is a form of variable resistor, consisting of a sliding contact which moves over the length of a resistance element
  • the sliding contacts are often made of copper alloys which have good electrical conductivity and have good elastic properties which maintain good electrical contact with the element
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10
Q

See lecture notes for example and solution

A

See lecture notes for example and solution

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11
Q

What is the capacitance C of a parallel plate capacitor given by

A

C = (ε0 εrA)/d

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12
Q

What is εr?

A

the relative permittivity of the dielectric between the capacitor plates

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13
Q

What is ε0?

A

the permittivity of free space (8.85 x 10-12 F/m)

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14
Q

What is A?

A

the area of overlap between the two plates

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15
Q

What is d?

A

the plate separation

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16
Q

What does the capacitance of a parallel plate capacitor depend on?

A

the plate separation

- so a change in this separation produces a change in capacitance

17
Q

If the separation increases by x (as shown in lecture notes), what does the capacitance become?

A

C = (ε0 εrA)/(d + x)

18
Q

What does LVDT stand for?

A

Linear Variable Differential Transformer

19
Q

Up to what displacement can the LVDT measure?

A

300mm

20
Q

Describe how the LVDT works? (relates to name)

A
  • firstly it is a transformer obeying the principles of electromagnetic induction
  • It has one primary and two secondary windings connected to provide the difference in respective voltage levels (differential)
  • It is variable since the magnetic coupling between the primary and each of the secondary coils can be varied to affect the magnitude of induced emfs
  • Finally the whole assembly is designed to give a linear response in variation
21
Q

Describe operation of LVDT

A
  • The secondary voltages are in phase and should the winding be connected together they will cancel giving zero voltage
  • If the soft iron core is now moved in either direction the flux linkage to one secondary winding will increase while the other will be reduced.
  • Eventually the core is wholly within one secondary winding and not the other so the output voltage is at a maximum and unable to increase any further (saturated).
  • This limits the effective working range of such a device.
22
Q

What do bonded strain gauges measure?

A
  • the size of a solid object, due to the object being strained (stretched or compressed)
23
Q

Why are they bonded to the object?

A

so that as it changes size so do they

- when they stretch or compress, the resistance changes and this can be analysed in terms of displacement

24
Q

What is the range of size in a strain gauge?

A

from fractions of a millimetre to about fifty millimetres

25
Q

How are they fixed to an object?

A

by special insulating adhesives but may also be embedded or welded

26
Q

What expression illustrates - when the strain gauge is bonded to an object, and the object changes in size the strain gauge resistance R changes

A

R = pl/A

where p is resistivity of the material, l is length and A is correctional area

27
Q

What does stretching the filament cause?

A

causes its length to increase and the area to decrease thus changing the gauge resistance

28
Q

What is strain?

A

the ratio of the change in dimension of an object to its original dimension and thus has no units

29
Q

Equation for electrical strain

A

Electrical Strain = Change in Resistance/Original Resistance = deltaR/R

30
Q

What is the Gauge Factor (G)?

A

defined as the ratio of fractional resistance change to the applied strain

31
Q

See notes for equation

A

See notes for equation

32
Q

What range does the gauge factor usually lie between and what resistances are commonly available?

A

1.8-2.2

Gauge resistances of 50-2000

33
Q

What is the purpose of a Wheatstone Bridge?

A

to convert the change in resistance (passive signal) into an electrical signal (active signal)

34
Q

What happens when you change R1 to a strain gauge?

A

Normally this circuit is balance (Vo = 0) but if we replace R1 with a strain gauge any change in gauge resistance will result in the bridge becoming unbalanced and a resultant change in output voltage Vo

35
Q

See notes for equation

A

See notes for equation

Control & Dynamic Systems (9 decks)

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