44221: information systems lecture 3 (week 4) systems control concepts by ian perry

44221: Information Systems

Lecture 3 (Week 4)

Systems Control Concepts

By Ian Perry

http://itsy.co.uk/ac/0506/Sem1/44221_IS/

Ian Perry Slide 244221: Information Systems: Systems Control Concepts

Remember This?

How can a systems approach help us apply IT in order to control the above process?

Money Disappears Costs Rise

Goods are Damaged

Budgeting

Unspent(unallocated)funds

Spent(allocated)funds

FundsRemaining?


The ‘Feed-back Control Loop’ The basic unit of Control for all Processes.

Environmental Disturbances

Input(s) Actuator Sensor

Goal

Output(s)

Comparator

Feed-back

Process


Elements of Control Sensor

Monitor/Measure the Output(s) of the Process. Feed-back

Communicate the Sensor Measurement to someone/something.

Comparator Compare the Sensor Measurement to the Goal

(i.e. the desired state of this Process). Actuator

Some means of ‘doing something’ in order to effect a change in the inputs to the Process.


Negative Feed-back is best! Positive feed-back leads to instability.

E.g. Cold War US built/deployed nuclear missiles. USSR built/deployed more nuclear missiles. US built/deployed more nuclear missiles. Etc.

Negative feed-back leads to stability. E.g. Heating System

Temperature rises above thermostat setting. Heating System shuts down. Temperature falls below thermostat setting. Heating Systems fires up again.


E.g. Stock Control The Goal of the Stockholding Process is

to keep the 'correct' amount of any item in stock. enough to satisfy any customer demand. not so many that we might be left holding

stock, which: costs us money. we might not be able to sell if customer

demand changes. we might end up throwing away if it

‘spoils’.


The Stockholding Process

Do Something?

Compare with Goal?

Communicate Stock Level

Customer Demand

INPUT(s) OUTPUT(s)

Theft

StockholdingProcess

New Stock Stock Level

MeasureStockLevel

Goal = 20


MUST Make a Distinction Between: Measurement

extracting data that represents what has happened.

Comparison comparing the new measurement to our

desired goal(s). Decision Making

deciding what to do about it.


Measurement Count the stock to determine the

current Stock Level. Very easy to support with IT.

Customer Demand Theft

StockholdingProcess

New Stock Stock Level

MeasureStockLevel


Comparison Check to see how well the process is going.

Relatively easy to support with IT.

Communicate Stock Level

StockholdingProcess

Compare with Goal?

Goal = 20


Decision Making Make the necessary adjustments.

Harder to support with IT.

Do Something?

StockholdingProcess


All Control Systems suffer from Lag

It takes time to ‘notice’ a change in the real world, ‘extract’ the data that represents this change, and ‘process’ this data in order to update our model.

EntityModifiedEntityEventreal

world

ModifiedModel

Model

Entityof

ProcessingSystemmodel

our

Data Extracted(Transaction)


Lags in the Feed-back Loop

It takes time: for the ‘Process’ to transform ‘Inputs’ into ‘Outputs’. for the ‘Sensor’, ‘Comparator’ and ‘Actuator’ to notice

any variation from the ‘Goal’ and effect a change.



Goal

Output(s)

Comparator

Feed-back

Process


E.g. In a Supermarket It takes time:

to accept new deliveries of products. place products onto shelves. modify stock levels when customers buy

products. compare the actual level of stock for

individual products with the desired level of stock.

re-order those products that have fallen below their desired level.

then, wait for another delivery!


‘Accommodating’ the Environment Systems should be designed to exhibit

the ‘optimum’ (i.e. best) response to disturbances from the environment.

Two new concepts: Buffers

E.g. the amount of Product A we must hold in order that we never run out, given that both supply and demand are outside of our control.

Control Frequency E.g. the frequency with which we check the

stock level of Product A against our desired stock level.


The ‘optimum’ Stock Level? Holding stock is not automatically undesirable:

but, what we don’t want is ‘too much’ or ‘too little’.

Cost ofStock-outs

Cost ofHolding Stock

TotalCost

OptimumRange

Stock Level Lots!0

Cost

Lots!

0


The ‘optimum’ Control Frequency? How often should we check on a process?

i.e. measure/compare/decide.

Cost ofNot Checking

Cost ofChecking

TotalCost

OptimumRange

Control Frequency ContinuousNever

Cost

Lots!

0


Summary Basic unit of control for all

processes: the Feed-back Control Loop.

Negative Feed-back is best: leads towards stability.

Information Technology might help: by speeding-up/automating the

measure/compare/decide part of the feed-back loop.

Can’t control the Environment: need to ‘accommodate’ disturbances.

New concepts: Buffers. Control Frequency.

MUST ‘optimise’ both: in order to ‘accommodate’

environmental disturbances.



Goal

Output(s)

Comparator

Feed-back

Process

Cost B

Cost A

TotalCost

Optimum

Controllable Variable

Cost

44221: information systems lecture 3 (week 4) systems control concepts by ian perry

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