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MECH2200 Sensors > January Exam > Flashcards

January Exam Flashcards

1
Q

Two categories of bearing for the two types of mechanical motion
Three types of mechanical contact bearings
Three types of mechanical non-contact bearings

A

Linear, rotary
Sliding, flexing, rolling
Fluid film (hydrostatic)
Fulid film (hydrodynamic)

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2
Q

Two types of friction, equations for both

A 
Static friction, kinetic/dynamic friction
Fs = μ*F(N) 
μ = coefficient of static friction
Fk = μ*F(N) 
μ = coefficient of kinetic friction
F(N) = force downwards (m*g)
Fs/Fk = force required to move object
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3
Q

Slide bearings: used for linear or rotary?
Where are they placed between?
Pros, cons

A

Both
Between shafts and housings
Pros: low noise, cheap, small
Cons: high friction, can be damaged from lubricants, stringent lubricant requirements

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4
Q

Special slide bearings: jewel bearings
Properties
Where are they used?

A 
Low friction
Thermally stable
High hardness
Bearing is stronger than shaft
Watches, compasses, precision instruments
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5
Q

Sliding bearings in a linear application: pros, 5 of them

A 
Self lubricating
Low friction, noise
Dust and shit doesn't stick to bearing
Resists corrosion
Light weight
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6
Q

Sliding bearing: dovetail linear slide

Pros, cons

A

Pros: adjustable tension
Precision defined by adjustment
Cons: regular maintenance
Undefined static friction

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7
Q

Application loading: what is is

Two types, which direction on a shaft they come from

A

Direction of forces on the bearings
Axial/thrust load: goes through shaft
Radial load: perpendicular to shaft

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8
Q

Rolling bearings: two types
What two grouping types for each of these are there
Spherical roller thrust bearings, tapered roller bearings: pros and cons

A 
Ball or roller
Can be classified as either radial or thrust/axial
SRTB: Pros-help with misalignment
Cons-expensive to produce
TRB: Pros-cheaper to produce 
Cons-Doesn't help with misalignment
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9
Q

Needle radial bearings: applications and pros

A

Planetary gears
Universal joints
Constant mesh gears
Pros: low profile, lightweight, higher load capacity, cheap

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10
Q

Linear roller bearing-where sliding and rolling meet

Pros, cons

A

Pros: reduces risk of ‘Stick-slip’ (increased static friction due to angular forces)
Cons: expensive, bigger, more shaft damage

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11
Q

Slides: whats the ratio of length to width

Where should the F(pull/push) be on the slide?

A

1.6:1

As close to the centre line of mass as possible

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12
Q

Stepper motor equations: Whats pitch (definition)?
How to read a thread metric e.g. Tr, 12 x 3
Equation for number of motor steps?

A

P: distance between threads on a screw
Diameter of 12mm, pitch of 3mm
No. of steps = 306/( Step Angle) * Distance/Pitch

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13
Q

Stepper motor equations:
Relationship between travel time and step frequency
Equation for resolution

A

Travel Time = 1/Step Frequency

R = (Step angle*pitch)/360

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14
Q

Use for lead screw linear drives-think about stuff from lectures
For one revolution of a pulley, calculate distance travelled

A

Syringe driver, scissor action lift

Distance = Pitch * Number of teeth

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15
Q

Rack and Pinion: what is it, pros over belt

A

A rigid belt with a cog on an axel rotating along it

Pros: rigid, precise, easy to expand

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16
Q

Micro-stepping the stepper motor vs adding a gear box to the drive: pros and cons

A

MS: Pros-better resolution (256x)
Cons-torque, speed would decrease
GB: Pros-more torque, resolution would increase by gear box ratio
Cons-speed would decrease

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17
Q

Derailleur on a bike: what three tasks does it perform (the thing that sits on gears)

A

Controls chain/gear alignment
Limits range of lateral motion
Maintains a relatively constant tension on chain

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18
Q

Concentric and eccentric bearings: where is the shaft in both?
How to work out total offset using ‘e’ through 360°

A

CB: shaft at centre of the bearing
EB: shaft offset by value ‘e’, the eccentricity value
2*e

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19
Q

Eccentric bearings pros, cons

A

Pros: Can adjust for tension and remove gaps
Can adjust for wear and to counteract variance
Achieve higher motion accuracy
Cons: more expensive, set up required

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20
Q

Universal joint, cardan joint:
Use for both
What do they look like
What about velocity do you have to remember?

A

Change the axis angle of a rotational system-not for 90°
Pipe with a hinge in it
Pipe with two hinges in it
Velocity of output shaft isn’t same as input shaft unless they are aligned

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21
Q

Whats a Rzeppa joint used for

What effect does high and low belt tension have?

A

When an angle greater than 45° is needed
HBT: premature bearing wear
LBT: reduced accuracy and/or dislocation

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22
Q

Two ways to adjust a belt/pulley system

Adjustment distance for both

A 
Linear and rotational tension adjustment
L: slot distance
R: 2*R(p)*sin(θ/2)
R(p) = radius from pivot point
θ = angle to move
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23
Q

Tension measurement methods: two of them

A

Force/displacement measurement

Sonic measurement

24
Q

Rotary to linear equations-remember mechanics

A

Revise-lecture 5

25
Q

When does vibration occur?
Relate it back to bearings
Equation for centrafugal force F(CF)

A

When centre of mass does not coincide with the axis of rotation
I.e. when there is eccentricity ‘e’ betwwen CoM and AoR
F(CF) = meω^2
m = mass
e = eccentricity
ω = speed of rotation

26
Q

Effects of vibration-mostly negative but two positive

A

Cons: increased bearing wear, noise, heat
Material fatigue, decreased resolution
Pros: physical feedback, controlled agitation (like pile drivers)

27
Q

Absolute vs relative position on a computer system

Also works for angles

A

AP: measure each point from an absolute origin
RP: Make a point on the shape a temporary origin and measure from there

28
Q

Origin of drawing space in G code

A

Left and lower most location

29
Q

Mechanical switch overview:
Poles
Throws

A

Poles (P): how many switches are activated by the same mechanism
Single (S), Double (D), 3, 4 etc
Throws (T): How many contacts can it be switched to
SIngle(S)(1): One possible, one normally open contact (NO) OR one normally closed contact (NC)
Double(D)(2): two possible, one NO AND one NC
3, 4 etc throws: one NC, rest NO

30
Q

Mechanical switch overview: maintained, momentary

What do their circuit diagrams look like

A

Mech: throw position is maintained when switched (light switch)
Normal switch
Moment: throw position reverts to a static position when released (button)
Switch with a line pressing down into it with bar above

31
Q

What is a snap-action mechanism (long definition)

Properties

A

Mechanism where the moving contact quickly moves from one fixed contact to another fixed contact with minimal relation to the speed the switch operates at
-Little force need to activate them
-Repeatable, precise
Over 1 million operation cycles

32
Q

Other position detection devices: magnetic reed switch
What are they
Pros, cons

A

Reeds made from ferrous metals so that when magnetic field is applied, reeds are polarised and attract each other
Pros: sealed so can be used in specialist environments
Very small, cheap
Cons: not precisely predictable, depends on strength of magnetic field

33
Q

Given resolution of system, how many steps must X/Y motors take to get to a point P

A

X steps = P(X)/resolution

Y steps = P(Y)/resolution

34
Q

Three types of rotary incremental encoders (think about types of PC storage)
Three things they all do

A 
Optical, magnetic, capacitive
They:
-inform that a step has taken place
-inform about direction of travel
-inform when you have reached a full rotation
35
Q

Two types of optical encoders

Equation for angular distance, S(D), between ‘A’ and ‘B’ sensors

A

Transmissive, reflective
S(D) = (360/No. of windows) * (N + 0.25)
N = number of whole windows between A and B

36
Q

Optical encoding: 3 types of encoding to do with channel A and B (imagine a square voltage wave)

A

X1: Channel A OR B produce a pulse every 4° i.e. one +ve edge
X2: Channel A OR B produce two edges (+ve and -ve) every 4° i.e. an edge every 2°
X4: Channel A AND B produce four edges (+ve and -ve) every 4° i.e. an edge every 1°

37
Q

Whats PPR
Given PPR and X1/2/4, whats the resolution of the encoder?
Given PPR and period of pulse output, whats rpm

A 
Pulses Per Revolution
360/(PPR/X)
X = 1,2,4 depending on X1/2/4
Frequency = 1/Pulse period
rpm = 60*(Frequency/PPR)
38
Q

Pros of linear incremental encoders in linear drive systems-three points

A

If there any ‘backlash’, it won’t be detected by rotary encoders
Instant response to linear motion
Very high resolution up to 1 nm

39
Q

Absolute encoders: properties, pros, cons

A

Rather than a single track, they have multiple tracks with encoded data
Pros: needed for precision positioning
Can be optical, inductive, magnetic or capacitive
Cons: expensive

40
Q

Absolute encoders: whats gray code

Why is it like this?

A

Binary sequence where only 1 bit changes at a time

So there is no confusion during position transition

41
Q

Whats the definition of a servo?

What frequency and type of signal do servos generally need?

A

An automatic device that uses error-sensing, negative feedback to correct the action of a mechinism
Uses built-in encoder
50 Hz, PWM

42
Q

Servo: what is duty cycle

What four things make up a servo?

A

Percentage of time that signal is high relative to period of wave
DC motor, gear box, potentiometer, control circuit

43
Q

Analog vs digital servos:
What is the dead band?
Difference between analog and digital

A

Amount by which the pulse must vary before any change in position happens
Analog has large dead band
Digital has smaller dead band and better torque and response time
Analog cheap, digital expensive

44
Q

What is servo torque and speed dependent on?

What are they measured in?

A

Voltage applied

kg.cm, rpm OR stupid imperial

45
Q

Whats a solenoid?
Why are springs used with solenoids?
What component do you need in parallel with a solenoid and why

A

A cylindrical coil of wire acting as a magnet when carrying an electric current
To get the return action required
Diode, cos of back EMF

46
Q

Why use relays?
4 points
Why use them for the last application

A 
Circuit isolation
Power switching
Signal detection
Logic systems
Reliable with stuff like radiation
47
Q

What is the definition of:
hydraulics
pneumatics

A

Tech concerned with conveyance of liquids through pipes and channels
Tech concerned with mechanical properties of gases

48
Q

Hydraulics: wheres the fluid held

Pros, cons

A

In a closed system
Pros: Can exert big pressure as fluid can’t be squashed
Needs less energy
Self lubing
Cons: fluid is a contaminant so sometimes can’t be used

49
Q

Pneumatics: pros, cons

A

Pros: cleaner than hydraulics
Faster
Cons: needs a compressed source of gas
Air can contain water which can cause problems

50
Q

Hydraulic pump: why types of energy does it convert from and to
2 steps for how it works

A

Converts mechanical to hydraulic energy

  1. Mech action creates a vacuum
  2. Pressure forces liquid from reservoir into system
51
Q

Positive-displacement pump, whats it do?

How to make a hand pumped jack easier to use?

A

Displaces same amount of liquid for each pumping cycle

Use a longer handle

52
Q

What is cracking pressure in relation to valves?

What valve does every hydraulic system need?

A

Pressure at which the valve will open

Pressure relief valve

53
Q

Cylinder equations:
Pressure
Force: port A to port B
Port B to port A

A 
Pressure (psi) = Force(lb)/Surface area(in^2)
F(A) = Ps*π*(Cd/2)^2
Ps = pressure
F(B) = PS*π*(Cd^2-Rd^2)/4
Cd=cylinder diameter
Rd=rod diameter
54
Q

Given volume of fluid V in cylinder, how far does the piston travel?
Moving from port A to port B
Moving from port B to port A

A

P(TD) = 4V/π*Cd^2

R(TD) = 4V/π*(Cd^2-Rd^2)

55
Q

Pistons in parallel vs series: which order do they rise in when a weight is on one of them and by how much

A

Parallel: Non weight, then weight
Both rise by same amount
Series: Rise at same time
Non weight rises less

56
Q

What happens when air is compressed, what does this do

Two types of pneumatic components

A

Moisture condenses inside tank, must be removed or may corrode
Dry air, oil lubricated

MECH2200 Sensors (1 decks)

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