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Automated Assembly Systems

Jun 02, 2018

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Chinmay Dalvi
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    INTRODUCTION

    Type III-A or I-A Manufacturing System

    ~ Designed to perform fixed sequence of assembly steps on a specific product

    ~ Used for high production of products that require limited number of parts to

    be assembled

    ~ Stations are integrated with MH system

    ~ These are examples of fixed automation

    Requirements

    High product demand

    Stable product design

    Limited number of components

    Product is designed for automated assembly

    Comparison wi th transfer lines

    Assembly operation vs processing operation

    Smaller work units are produced

    Less mechanical force is required and also the power

    For the same number of stations AAS tend to be smaller in physical size

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    Applications

    Typical Products made by Automated Assembly:

    ~ Alarm Clocks~ Ball Bearings

    ~ Ball Point Pens

    ~ Electrical Plugs and Sockets

    ~ Gear Boxes

    ~ Light Bulbs

    ~ Locks~ PCB Assemblies

    ~ Small electric motors

    ~ Spark plugs

    ~ Wrist Watches

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    Processes

    Typical Assembly processes/activities performed in

    Automated Assembly:

    ~ Adhesive bonding

    ~ Insertion of components

    ~ Placement of components

    ~ Riveting

    ~ Screw Fastening

    ~ Snap Fitting~ Soldering

    ~ Spot welding

    ~ Stapling

    ~ Stitching

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    INTRODUCTION

    Subsystems of AAS:

    ~ One or more assembly workstations

    ~ Parts feeding devices at individual workstation

    ~ Work handling system (Base part or partial assembly transfer)

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    System Configurations

    System Configurations:

    ~ In-line~ Dial type Assembly Machine

    ~ Carousel Assembly System

    ~ Single station assembly machine

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    System Configurations

    System Configurations:

    ~ In-line~ Dial type Assembly Machine

    ~ Carousel Assembly System

    ~ Single station assembly machine

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    System Configurations

    System Configurations:

    ~ In-line~ Dial type Assembly Machine

    ~ Carousel Assembly System

    ~ Single station assembly machine

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    System Configurations

    System Configurations:

    ~ In-line~ Dial type Assembly Machine

    ~ Carousel Assembly System

    ~ Single station assembly machine

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    Work part Transfer Systems

    Workpart Transfer Systems:

    ~ Synchronous~ Asynchronous

    Parts Delivery at Workstations:

    Elements are:

    ~ Hopper

    ~ Parts Feeder ~ Selector and/or Orientor

    ~ Feed track

    ~ Escapement and placement device

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    Work part Transfer Systems

    Workpart Transfer Systems:

    ~ Synchronous~ Asynchronous

    Parts Delivery at Workstations:

    Elements are:

    ~ Hopper

    ~ Parts Feeder ~ Selector and/or Orientor

    ~ Feed track

    ~ Escapement and placement device

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    Vibratory Bowl Feeder

    Most versatile of hopper feeders for small parts

    Consists of bowl and helical track

    Parts are poured into bowl

    Helical track moves part from bottom of bowl to outlet

    Vibration applied by electromagnetic base

    Oscillation of bowl is constrained so that parts climb upward along

    helical track

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    Selector and/or Orientor

    Purpose - to establish the proper

    orientation of the components for theassembly workhead

    Selector

    Acts as a filter

    Only parts in proper orientation

    are allowed to pass through to

    feed track Orientor

    Allows properly oriented parts to

    pass

    Reorients parts that are not

    properly oriented

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    Feed Track

    Moves parts from hopper to assembly workhead

    Categories:

    Gravity - hopper and feeder are located at higher elevation than workhead

    Powered - uses air or vibration to move parts toward workhead

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    Escapement and Placement Devices

    Escapement device

    Removes parts from feed track at time intervals that are consistent with the

    cycle time of the assembly workhead

    Placement device

    Physically places the parts in the correct location at the assembly workstation

    Escapement and placement devices are sometimes the same device,

    sometimes different devices

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    Escapement and Placement Devices

    (a) Horizontal and (b) vertical devices for placement of parts onto dial-indexing

    table

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    Escapement and Placement Devices

    Escapement of rivet-shaped parts actuated by work carriers

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    Escapement and Placement Devices

    Two types of pick-and-place mechanisms for transferring base parts from

    feeders to work carriers

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    Work part Transfer Systems

    Parts Delivery at Workstations:

    Elements are:

    ~ Hopper ~ Parts Feeder

    ~ Selector and/or Orientor

    ~ Feed track

    ~ Escapement and placement device

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    Quantitative Analysis of

    Assembly Systems~ Parts Delivery System at Workstations

    ~ Multi-station Automated Assembly Systems

    ~ Single Station Automated Assembly Systems

    ~ Partial Automation

    Parts Delivery System at Workstations:

    Parts feeder feed rate

    Probability of components passing through the selector

    Effective rate of delivery of components from the hopper into the feed trackDelivery rate > Cycle rate

    Low level sensor

    High level sensor

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    Quantitative Analysis of

    Assembly Systems~ Parts Delivery System at Workstations

    ~ Multi-station Automated Assembly Systems

    ~ Single Station Automated Assembly Systems

    ~ Partial Automation

    Parts Delivery System at Workstations:

    Example:

    The cycle time for a given assembly workhead = 6 sec. The parts feeder has a

    feed rate = 50 components per min. The probability that a given component fedby the feeder will pass through the selector is 0.25. The number of parts in the

    feed track corresponding to the low level sensor is 6. The capacity of the feed

    track is 18 parts. Determine:

    (a) How long it will take for the supply of parts in the feed track to go from high

    level to low level? and

    (b) How long it will take on average for the supply of parts to go from low levelto high level?

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    Quantitative Analysis of

    Assembly Systems~ Parts Delivery System at Workstations

    ~ Multi-station Automated Assembly Systems

    ~ Single Station Automated Assembly Systems

    ~ Partial Automation

    Parts Delivery System at Workstations:

    Example:

    A feeder-selector device at one of the stations of an automated assembly

    machine has a feed rate of 25 parts per minute and provides a throughput ofone part in four. The ideal cycle time of the assembly machine is 10 sec. The

    low level sensor on the feed track is set at 10 parts, and the high level sensor is

    set at 20 parts.

    (a) How long will it take for the supply of parts to be depleted from the high

    level sensor to the low level sensor once the feeder-selector device is turned

    off?(b) How long will it take for the parts to be resupplied from the low level sensor

    to the high level sensor, on average, after the feeder-selector device is turned

    on?

    (c) What proportion of the time that the assembly machine is operating will the

    feeder-selector device be turned on? Turned off?

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    Quantitative Analysis of

    Assembly Systems~ Parts Delivery System at Workstations:

    ~ Multi-station Automated Assembly Systems

    ~ Single Station Automated Assembly Systems~ Partial Automation

    Multi-station Automated Assembly Systems:

    ~ Synchronous transfer system

    ~ In-line, dial indexing, and carousel configuration

    ~ Constant element times, although the times are not necessarily equal at allstations

    ~ No internal storage

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    Automated Assembly Systems

    1)q-(1q)m-(1qmn

    1i

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    Quantitative Analysis of

    Assembly Systems Multi-station Automated Assembly Systems:

    qi probability that the component to be added during current cycle is

    defectivemi probability that a defect results in a jam at the station and consequent

    stoppage of the line

    Three events occur at a particular workstation:

    The component is defective and causes a station jam, pi = qi*miThe component is defective but does not cause a station jam = (1 mi)*qiThe component is not defective = (1 qi)

    The probabilities of the three possible events must sum to unity for any

    workstation

    mi qi + (1 mi) qi + (1 - qi) = 1

    For n station assembly line

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    Automated Assembly Systems

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    Quantitative Analysis of

    Assembly Systems Multi-station Automated Assembly Systems:

    Two of the three terms of earlier equation represent events in which a goodcomponent is added at the given station.

    miqi indicates that a station jam has occurred, and thus a defective component

    has not been added to the existing assembly.

    ( 1 - qi) means that a good component has been added at the station.

    The sum of these two terms represents the probability that a defective

    component is not added at station i.

    Multiplying these probabilities for all stations, we get the proportion of

    acceptable product coming off the line Pap.

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    Automated Assembly Systems

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    Quantitative Analysis of

    Assembly Systems Multi-station Automated Assembly Systems:

    The proportion of assemblies containing at least one defective component isPqp.

    Other performances are:

    machines production rate

    proportion of uptime and downtime

    F = Frequency of downtime occurrences per cycle

    If each station jam results in a machine downtime occurrence, F can be

    determined by taking the expected number of station jams per cycle

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    Example:

    A ten-station in-line assembly machine has an ideal cycle time of 6 sec. The base part is

    automatically loaded prior to the first station, and components are added at each of thestations. The fraction defect rate at each of the ten stations is q = 0.01, and the

    probability that a defect will jam is m = 0.5. When a jam occurs, the average downtime is

    2 min. Cost to operate the assembly machine is Rs. 2500 /hr. Other costs areignored.

    Determine:

    (a) Average production rate of all assemblies

    (b) Yield of good assemblies

    (c) Average production rate of good products(d) Uptime efficiency of the assembly machine, and

    (e) Cost per unit

    Quantitative Analysis of

    Assembly Systems

    q m Rp(pc/hr) Yield Rap(pc/hr) E Cpc($)

    0 0.5 600 1 600 100% 0.07

    0.01 0.5 300 0.951 285 50% 0.15

    0.02 0.5 200 0.904 181 33.3% 0.23

    0.01 0 600 0.904 543 100% 0.08

    0.01 0.5 300 0.951 285 50% 0.15

    0.01 1.0 200 1 200 33.3% 0.21

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    Keypoints:

    As fraction defect rate increases, meaning that component quality gets worse,all five measures of performance suffer.

    The effect of m is less obvious.

    At low values of m (m = 0) for the same component quality level q, production

    rate and machine efficiency are high but the yield of good product i.e.proportion of acceptable products, is low.

    Instead of interrupting the assembly machine operation and causing downtime,

    all defective components pass through the assembly process to become part of

    the final product.

    At m = 1, all defective components are removed before they become part of the

    product. Therefore yield i.e. proportion of acceptable products, is 100% but

    removing the defects takes time, adversely affecting production rate, efficiency

    and cost per unit.

    Quantitative Analysis of

    Assembly Systems

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    Instantaneous Control vs. Memory Control:

    Memory control (m = 0) requires sorting station and instantaneous control(m = 1) stops the machine when a defect occurs.

    Memory control has to consider cost of additional sensors, controls and

    sortation devices and will be more useful if sortation station is 100% effective.

    Instantaneous control has to consider the cost of lost production time due tostopping of the machines

    The selection of control may be justified using the cost per unit factor.

    Quantitative Analysis of

    Assembly Systems

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    Quantitative Analysis of

    Assembly Systems~ Parts Delivery System at Workstations:

    ~ Multi-station Automated Assembly Systems

    ~ Single Station Automated Assembly Systems~ Partial Automation

    Single-station Automated Assembly Systems:

    ~ A single workstation with several components feeding into the station to be

    assembled to a base part

    ~ Increasing the number of elements in the assembly machine cycle resultsin a higher cycle time, decreasing the production rate of the machine

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    Automated Assembly Systems

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    Single-station Automated Assembly Systems:

    ne = number of distinct assembly elements that are performed on the machine

    Tej = element timeTh = handling time (loading of base part and unloading of assembled product)

    Ideal cycle time Tc can be expressed as:

    Instead of component addition if certain activity is carried out (welding,

    fastening) then pj can be used as the probability of station failure duringelement j.

    Quantitative Analysis of

    Assembly Systems

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    Example:

    A single-station assembly machine performs five work elements to assemble four

    components to a base part. The elements are listed in the table below, together with thefraction defect rate (q) and probability of a station jam (m) for each of the components

    added (NA means Not Applicable).

    Time to load the base part is 3 sec and time to unload the completed assembly is 4 sec,

    giving a total load/unload time of Th = 7 sec. When a jam occurs, it takes an average of

    1.5 min. to clear the jam and restart the machine. Determine:

    (a) Production rate of all products

    (b) Yield of good product(c) Production rate of good products

    (d) Uptime efficiency of the assembly machine

    Quantitative Analysis of

    Assembly Systems

    Element Operation Time (sec) q m p

    1 Add gear 4 0.02 1

    2 Add spacer 3 0.01 0.63 Add gear 4 0.015 0.8

    4 Add gear and mesh 7 0.02 1

    5 Fasten 5 0 NA 0.012

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    Quantitative Analysis of

    Assembly Systems~ Parts Delivery System at Workstations:

    ~ Multi-station Automated Assembly Systems

    ~ Single Station Automated Assembly Systems~ Partial Automation

    Partial Automation:

    ~ Combination of automated and manual workstations

    ~ Reasons:

    Automation is introduced gradually on an existing manual lineCertain manual operations are too difficult or too costly to automate

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    Quantitative Analysis of

    Assembly Systems Partial Automation:

    Assumptions:

    ~ workstations perform either processing or assembly operations~ processing and assembly times at automated stations are constant,

    though not necessarily equal at all stations

    ~ synchronous transfer of parts

    ~ no internal buffer storage

    ~ upper-bound approach is applicable

    ~ station breakdowns occur only at automated stations~ human adaptability

    ~ Tc remains constant over time

    The ideal cycle time is determined with the bottleneck station (manual)

    Because of random variation in any repetitive human activity Tc will show

    certain degree of variation

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    Automated Assembly Systems

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    Quantitative Analysis of

    Assembly Systems Partial Automation:

    na = number of automated stations

    nw = number of stations operated by manual workersn = na + nw = total station count

    Casi = Cost to operate automatic workstation, i (Rs/min)

    Cwi = Cost to operate manual workstation, i (Rs/min)

    Cat = Cost to operate the automatic transfer mechanism (Rs/min)

    Co = total cost to operate the line