Dynamic Constraint Models for Complex Production Environments

1
Dynamic Constraint Models forComplex Production
Environments

• Roman Barták
• Charles University, Prague
• bartak_at_kti.mff.cuni.cz

2
Talk Schedule

• Problem area
• Mixing Planning Scheduling
• Conceptual Models
• discrete event based time
• static vs. dynamic representation
• overview comparison of models
• mixing models
• Future research

3
Problem area

• complex production environments
• plastic, petrochemical, chemical, pharmaceutical
  industries
• several different resources
• producers, movers, stores
• batch/serial processing with time windows
• transition patterns (set-up times)
• by-products, co-products (re-cycling)
• non-ordered production (for store)
• alternatives
• processing routes, production formulas, raw
  material

4
Problem area (contd)

• complex production environment
• Task
• preparing a schedule for a given time period(not
  minimising the makespan)
• objective
• maximising the profit (minimising the cost)

silo
order
processor B1
silo
purchase
processor A
sacks warehouse
processor B2
order
5
Planning and Scheduling

• Planning
• finding a sequence of activities transferring the
  initial world into a required state
• high-level view of the factory
• low resolution
• longer time period
• what and how to produce?
• AI CP

• Scheduling
• allocating the activities to available resources
  over time respecting the constraints
• low-level view of the factory
• high resolution
• shorter time period
• how to produce in detail?
• OR CP

6
Mixing Planning and Scheduling

• Problems
• too tighten plans (impossible to schedule)
• too free plans (less profitable schedule)
• backtrack from the scheduler to the planner
• what if appearance of the activity depends on the
  allocation of other activities?
• transition patterns (set-ups), alternatives,
  non-ordered production
• Solution
• a scheduler with planning capabilities
• generating activities during scheduling

7
Conceptual models

• High-level declarative model of the problem
• data structures (composition of variables)
• composition of constraints
• resource constraints (compatibility, capacity)
• transitions between activities (set-ups)
• supplier/consumer dependencies
• Expressiveness
• What could be modelled? (problem area)
• What is easy/hard to express? (constraints)
• Efficiency
• constraint propagation

8
Conceptual models (contd)

• View of time
• discrete (time slices with the same duration)
• event-based (activities)
• Representation

9
Time-line model

• discretising the time line into time slices
• activities change at the edge between successive
  time slices
• duration the greatest common divisor of
  activities duration
• description of situation at each time point/slice
• Example the store - the stored quantity for each
  item
• good for both planning and scheduling
• activities for given time point/slice are chosen
  during scheduling
• a matrix representation (description x time)
• static / dynamic / semi-dynamic contents of the
  cells
• easy capture of initial future situations

10
Order-centric model

• a chain of activities per order
• assigning resources to activities
• description of the activity
• start, end (duration), resource
• representation
• production chain a list of (virtual) activities

slots

candidate activities
11
How to model? (in order-centric model)

• alternatives
• pre-processing (chosen by the planner)
• alternative activities in slots
• set-ups
• set-up slot is either empty or contains the
  set-up activity (depending on the allocation of
  the next activity)
• by-products (re-cycling)
• sharing activities between the production chains
• non-ordered production
• pre-processing (non-ordered production is planned
  in advance - before the scheduling)

12
Resource-centric model

• Activity based
• a sequence of activities per resource
• what the resource can process rather than how
  to satisfy the order
• description of the activity
• start, end (duration), quantities, state,
  suppliers, consumers
• representation
• a list of virtual activities
• transition constraints between successive
  activities

13
Comparison of models
14
Mixing the models

• Minimising drawbacks while preserving advantages
  of different models
• different task different model
• the time-line model for planning
• the order-centric model for scheduling
• different resource different model
• producer mover - activities (resource-centric)
• store - time-line

15
Whats next?

• constraint model
• a complete specification of the constraints
• implementation
• propagation (early detection of inconsistencies)
• labelling (incremental)
• heuristics (choice of alternatives)
• expressiveness
• secondary resources
• traceability
• agent based scheduling