Open Questions Industrial Technologies

It can be defined as total work content or, equivalently, total time

3. Calculate the cycle time CT

CT = available time / demand

4. Calculate the minimum number of stations K*:

K* = T/(CT ∙ alfa)

5. Assign tasks to stations, respecting the constraints

(CT, precedence relationships, etc.)

If there is more than one task available to be assigned, use a rule to prioritise tasks

Probability of no-completion

This method has some analogies with the method of the maximum value of utilization

rate of operators imposed as design criterion.

15. Comment and motivate the main strengths and weaknesses of the different types of

assembly systems. Be schematic and provide quick definitions of each type of assembly

system.

Fixed position: in a fixed position assembly, the product is assembled in a single site, rather

than being moved through a set of assembly stations. Materials (i.e. components),

equipment, tools are brought to the site.

-Strenghs: High flexibility, Low investment, Job enlargement, enrichment and rotation

for the employee

-Weaknesses: Investment depends on the level of automation of the system, it might be

difficult to manage the flows of products and components, the complexity of production

planning and control can cause bottlenecks and idle-times

Assembly shop: an assembly shop consists of a series of stations and each station

(generally, more than one station) is assigned a phase of the assembly process of a

product type.

-Strengths: The stations (phases) are decoupled by buffers, flexibility is high

-Weaknesses: potentials for intertwining of material flows, high WIP, large space

requirement, labour training might be difficult and time-consuming, high cost for

workforce

Assembly line: each assembly line consists of a series of stations where the product is

progressively assembled.

-Strenghs: Rationalization of material flows, low WIP, limited space requirement, labour

training might be easy, low cost for workforce

-Weaknesses: low flexibility, long time required to start new productions, repetitive work,

line balancing might be difficult

16. Briefly introduce the main configurations of assembly lines (paced vs un-paced) underlying

the main differences. Highlight the pros and cons of the possible alternatives. Be

schematic!

Paced line

In a paced assembly production system, a common cycle time is typically given, which

limits process times at all stations. The pace is maintained by either intermittent transport,

in which the workpiece comes to a complete stop at each station but is automatically

transferred after a predetermined time span, or by a continuously advancing material

handling device, such as a conveyor belt, which forces operators to complete their

operations before the workpiece reaches the end of the respective station.

Machine-paced:

-PRO-> Cycle time perfectly controlled

-CONS-> Probability of no completion

Operator-paced:

-PRO -> No problem of unfinished pieces

-CONS -> Cycle time is not perfectly controlled

Continuous flow paced:

- If the operator cannot stop the line:

PRO -> Cycle time perfectly controlled

-CONS -> Probability of no completion

- If the operator can stop the line:

-PRO -> No problem of unfinished pieces

-CONS -> Cycle time is not perfectly controlled

Unpaced line

Workpieces are transferred in unpaced lines whenever the required operations are

completed, rather than at a set time. It can also be distinguished whether all stations

transfer their workpieces at the same time (synchronous) or whether each station decides

on transference separately (asynchronous). Under synchronous workpiece movement, all

stations wait for the slowest station to finish all operations before transferring workpieces at

the same time.

PRO -> No problem of unfinished pieces

CONS -> Cycle time is not perfectly controlled

17. Considering the assembly un-paced lines, you are asked to define the blocking and

starvation situations and to explain the role of buffers to limit the impact of blocking and

starvations on the production capacity

Since buffers have finite capacity, two kinds of problems could happen:

problem of blocking: when a buffer is full the upstream station can’t drop the piece

(having finished its assigned task on the current piece);

problem of starvation: when a buffer is empty the downstream station can’t take a new piece

(having finished its assigned task on the current piece)

If these events are frequent, there is a risk for the reduction of the line production capacity.

The frequency (of such events) depends on:

- the sizing of buffers -> for this reason it is important –during the design of the line-

to size buffers correctly (to reduce probability of blocking/starvation);

- the balancing of the line (the presence of stations overloaded with work increases

the frequency of these events directly down and upstream).

Effect of blocking and starvation -> loss of time due to these phenomena at each station +

loss of production capacity at the end of the line (over a given period).

18. Considering the un-paced assembly lines, you are asked to describe i) the basic criteria for

allocating assembly operations and the resulting cycle time, ii) the importance to limit

blocking and starvation as phenomena along the line, iii) the impact of inventory buffering

the production capacity.

i) The Unpaced lines allow to decouple each station through the presence of buffers. In this

case, buffers allow operations to exceed expected/given cycle time sometimes (a

systematic violation would cause blocking or starvation phenomena). With this

configuration, therefore, we have no problem of no completion but at the same time low

control of CT and PC since each station has its own CT and is independent from the others.

It is necessary to identify/define the balancing constraints. Such constraints are: - Cycle

time - Precedence relationships among operations - Incompatibility between operations that

cannot be assigned to the same station - Opportunity or necessity to assign some

operations to the same station - Constraints related to space

- Constraints related to workers

- Constraints related to the material feeding

One of the most used method to define the precedence relationships among operations is

drawing an Assembly Graph. Such graph determines the constraints to be taken into

account when balancing the line: precedence relationships (i.e. arcs) b/w operations (i.e.

nodes) have to be considered both within the stations and between successive stations.

ii) Since buffers have finite capacity, two kinds of problems could happen: - problem of

blocking: when a buffer is full the upstream station can’t drop the piece (having finished its

assigned task on the current piece); - problem of starvation: when a buffer is empty the

downstream station can’t take a new piece (having finished its assigned task on the current

piece).

iii) If these events are frequent, there is a risk for the reduction of the line production

capacity. The frequency (of such events) depends on:

- the sizing of buffers -> for this reason it is important –during the design of the line- to size

buffers correctly (to reduce probability of blocking/starvation);

- the balancing of the line (the presence of stations overloaded with work increases the

frequency of these events directly down and upstream). Effect of blocking and starvation ->

loss of time due to these phenomena at each station + loss of production capacity at the

end of the line (over a given period).

Module 4 - Modelling and Simulation of Production Systems

19. How can you define a process plant? Please, describe the general features of such types of

production systems.

A process plant is formed by a series of production equipment used to make non reversible

chemical-physical transformation of materials through a fixed technological routing.

Examples of sectors in the process industries are: Petrochemical/oil refineries, cement,

glass, rubber, paper production plastics, metals etc.

Process plants – general features

The production flow is serial (simple transformation of raw material to product), analytical or

synthetic (from one material to many products or from more materials to one product)

Process plants are highly automated

Plants are designed to operate a continuous flow production process (materials subject to

transformations are moved continuously through the production equipment of the plant) or a

batch production process (materials are processed as batches)

In general, the production logistics and production management can be considered simple

High plant utilization and equipment efficiency are mandatory characteristics to achieve an

economically-reasonably investment

Low need of work force (high automation)

Qualitative characteristics of products are stable (when process conditions are kept stable)

Low flexibility is clearly a characteristic of this kind of plants concerning different flexibility

dimensions (limited mix flexibility, fixed plant structures, limited expansion flexibility, limited

volume flexibility)

High investments needed in order to achieve high economies of scale

High risk of obsolescence

Significant impact of failures

Importance of variations in process conditions

20. Explain Little’s Law: define the main formulation and explain it through a numerical example

(if you deem appropriate use the Penny Fab example or a similar one).

Little’s law allow to understand relationships among WIP, Throughput TH and Lead Time LT

of clients (i.e. parts) flowing in a generic system for discrete production, given the input rate

of clients (i.e. parts) is constant (steady status).

TH=WIP/LT

Little’s law is independent of the configuration of the system, of the type of distribution of

processing times, of routing and of the distribution of inter-arrival times. If we increase the

input rate of parts to the system, then WIP will increase progressively and TH will increase

linearly, while LT will remain constant, but ..

When we reach the critical WIP, TH will achieve its maximum and will never increase more,

while LT will start increasing. (Critical WIP: the WIP level in which a line having no

congestion would achieve maximum throughput with minimum throughput time.

If we want to decrease LT, while maintaining TH constant, we have to reduce the WIP

21. What is the so-called Little’s Law? Why this law could be useful for understanding a

production system? Which is its analytical definition? Normally, how it should be the

expected outcome of a system in Little’s Law (you can design a graph)?

The Little's law is derived from queuing theory and is useful for understanding the

relationships between WIP, lead time, and throughput flowing in a discrete production (no

process industries) with a fixed input rate of clients (i.e. the parts) or in a steady state. The

law is quite useful because it is independent of system configuration, the type of distribution

of pro