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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
When does vibration occur? Relate it back to bearings Equation for centrafugal force F(CF)
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) = m*e*ω^2 m = mass e = eccentricity ω = speed of rotation
26
Effects of vibration-mostly negative but two positive
Cons: increased bearing wear, noise, heat Material fatigue, decreased resolution Pros: physical feedback, controlled agitation (like pile drivers)
27
Absolute vs relative position on a computer system | Also works for angles
AP: measure each point from an absolute origin RP: Make a point on the shape a temporary origin and measure from there
28
Origin of drawing space in G code
Left and lower most location
29
Mechanical switch overview: Poles Throws
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
Mechanical switch overview: maintained, momentary | What do their circuit diagrams look like
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
What is a snap-action mechanism (long definition) | Properties
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
Other position detection devices: magnetic reed switch What are they Pros, cons
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
Given resolution of system, how many steps must X/Y motors take to get to a point P
X steps = P(X)/resolution | Y steps = P(Y)/resolution
34
Three types of rotary incremental encoders (think about types of PC storage) Three things they all do
``` 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
Two types of optical encoders | Equation for angular distance, S(D), between 'A' and 'B' sensors
Transmissive, reflective S(D) = (360/No. of windows) * (N + 0.25) N = number of whole windows between A and B
36
Optical encoding: 3 types of encoding to do with channel A and B (imagine a square voltage wave)
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
Whats PPR Given PPR and X1/2/4, whats the resolution of the encoder? Given PPR and period of pulse output, whats rpm
``` Pulses Per Revolution 360/(PPR/X) X = 1,2,4 depending on X1/2/4 Frequency = 1/Pulse period rpm = 60*(Frequency/PPR) ```
38
Pros of linear incremental encoders in linear drive systems-three points
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
Absolute encoders: properties, pros, cons
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
Absolute encoders: whats gray code | Why is it like this?
Binary sequence where only 1 bit changes at a time | So there is no confusion during position transition
41
Whats the definition of a servo? | What frequency and type of signal do servos generally need?
An automatic device that uses error-sensing, negative feedback to correct the action of a mechinism Uses built-in encoder 50 Hz, PWM
42
Servo: what is duty cycle | What four things make up a servo?
Percentage of time that signal is high relative to period of wave DC motor, gear box, potentiometer, control circuit
43
Analog vs digital servos: What is the dead band? Difference between analog and digital
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
What is servo torque and speed dependent on? | What are they measured in?
Voltage applied | kg.cm, rpm OR stupid imperial
45
Whats a solenoid? Why are springs used with solenoids? What component do you need in parallel with a solenoid and why
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
Why use relays? 4 points Why use them for the last application
``` Circuit isolation Power switching Signal detection Logic systems Reliable with stuff like radiation ```
47
What is the definition of: hydraulics pneumatics
Tech concerned with conveyance of liquids through pipes and channels Tech concerned with mechanical properties of gases
48
Hydraulics: wheres the fluid held | Pros, cons
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
Pneumatics: pros, cons
Pros: cleaner than hydraulics Faster Cons: needs a compressed source of gas Air can contain water which can cause problems
50
Hydraulic pump: why types of energy does it convert from and to 2 steps for how it works
Converts mechanical to hydraulic energy 1. Mech action creates a vacuum 2. Pressure forces liquid from reservoir into system
51
Positive-displacement pump, whats it do? | How to make a hand pumped jack easier to use?
Displaces same amount of liquid for each pumping cycle | Use a longer handle
52
What is cracking pressure in relation to valves? | What valve does every hydraulic system need?
Pressure at which the valve will open | Pressure relief valve
53
Cylinder equations: Pressure Force: port A to port B Port B to port 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
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
P(TD) = 4V/π*Cd^2 R(TD) = 4V/π*(Cd^2-Rd^2)
55
Pistons in parallel vs series: which order do they rise in when a weight is on one of them and by how much
Parallel: Non weight, then weight Both rise by same amount Series: Rise at same time Non weight rises less
56
What happens when air is compressed, what does this do | Two types of pneumatic components
Moisture condenses inside tank, must be removed or may corrode Dry air, oil lubricated

MECH2200 Sensors (1 decks)

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