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?
Ian Perry Slide 344221: Information Systems: Systems Control Concepts
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
Ian Perry Slide 444221: Information Systems: Systems Control Concepts
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.
Ian Perry Slide 544221: Information Systems: Systems Control Concepts
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.
Ian Perry Slide 644221: Information Systems: Systems Control Concepts
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’.
Ian Perry Slide 744221: Information Systems: Systems Control Concepts
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
Ian Perry Slide 844221: Information Systems: Systems Control Concepts
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.
Ian Perry Slide 944221: Information Systems: Systems Control Concepts
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
Ian Perry Slide 1044221: Information Systems: Systems Control Concepts
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
Ian Perry Slide 1144221: Information Systems: Systems Control Concepts
Decision Making Make the necessary adjustments.
Harder to support with IT.
Do Something?
StockholdingProcess
Ian Perry Slide 1244221: Information Systems: Systems Control Concepts
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)
Ian Perry Slide 1344221: Information Systems: Systems Control Concepts
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.
Environmental Disturbances
Input(s) Actuator Sensor
Goal
Output(s)
Comparator
Feed-back
Process
Ian Perry Slide 1444221: Information Systems: Systems Control Concepts
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!
Ian Perry Slide 1544221: Information Systems: Systems Control Concepts
‘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.
Ian Perry Slide 1644221: Information Systems: Systems Control Concepts
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
Ian Perry Slide 1744221: Information Systems: Systems Control Concepts
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
Ian Perry Slide 1844221: Information Systems: Systems Control Concepts
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.
Environmental Disturbances
Input(s) Actuator Sensor
Goal
Output(s)
Comparator
Feed-back
Process
Cost B
Cost A
TotalCost
Optimum
Controllable Variable
Cost