Optimum-AIV, A Planning Tool for Spacecraft AIV
Optimum-AIV, A Planning Tool for Spacecraft AIV
Authors: Y.Parrod, Matra-Marconi Space (F) and S.Valera, Simulation and Electrical
Facilities Division, ESTEC
CRI (DK), Matra-Marconi (F), AIAI (UK)
ESTEC Basic Technology Research Programme
Planning is a key issue in the management of the assembly, integration
and verification (AIV) activities of a space project. Not only
technical achievements are important for the success of a project;
cost and planning aspects are also important. As many participants
(agencies, contractors, launcher authorities, users) and costly
facilities are involved in the fulfillment of a project, a delay caused
by one of the participants normally leads to serious problems for the
At all levels of a space project, managers are concerned with
planning, and to this end, they control closely the progress of the
work; however it is difficult to find a planning tool tailored to
their specific needs. General purpose commercial tools are either too
simple to represent their problems correctly or else they are too
complex to be used interactively.
Due to the special characteristics of space projects (mainly their
size and duration), well adapted computerised planning and scheduling
systems are needed, and software tools are also needed to help solve
some of the problems arising when AIV plans, initially managed by one
team, are taken over by another team during the life of the project.
Planning tools should be able to keep track of planning decisions and
to explain the rationale of the plan, which governs the links between
activities. Planning tools should also support, correctly and easily,
the management of the specific constraints and solutions which are
typical of AIV activities, for instance changing from single-shift to
double-shift working in order to meet a deadline.
Frequently, because of the inadequacy of conventional planning tools, planning
is simplified to a network problem of seeking the critical path, in which only
temporal constraints are managed. To avoid managing other resources, the plan
is over constrained by adding hard-precedence-links between activities.
Parallel activities are rescheduled as sequential activities and alternatives
which potentially existed in the process are lost. Good planning tools should
be efficient enough to discourage such bad practices.
The object of Optimum-AIV is to meet specific planning requirements of
AIV activities and to demonstrate the capabilities of artificial
intelligence techniques to solve the problems which arise in AIV
planning. Optimum-AIV is a tool designed to provide AIV team leaders
with straightforward access to planning management techniques.
Optimum-AIV covers all of the basic scheduling functions. It can, for
instance, define an `activities network' associate minimum and maximum
delay constraints between activities, allocate shared or consumable
resources to activities, and compute a schedule which meets these
temporal and other resource constraints. In addition, Optimum-AIV
provides some advanced planning and scheduling functions.
Structured Management of Projects
The structure of large projects can be broken down into a tree of
hierarchical activities in which macro-activities are split into
sub-activities. Temporal, resource and precedence constraints are
propagated through this hierarchical structure. It is, for instance,
possible to allocate resources not only during the execution of an
activity, but also between concurrent activities.
Activities can be modelled as operators containing preconditions and
effects. Let us suppose that the outcome of an integration activity
may be expressed as `equipment A integrated on Platform'.
Preconditions must be met before the activity can start, and the
activity produces effects which change the process configuration.
Optimum-AIV checks the preconditions for an activity using the current
process configuration, and computes the new process configuration from
the effects of that activity.
This feature allows one to specify explicitly the logic which lies
behind precedence links, and to check automatically the logical
consistency of the plan.
Optimum-AIV offers a resource-driven scheduling mechanism to
facilitate the specification of different scenarios for working hours,
which can be easily modified to observe the direct impact on the
duration of the activity to which those resources have been allocated.
The working hours can also be dependent on the activity to which they
are allocated. For instance, double-shift working can be restricted
to a single work package on which additional effort is needed.
Optimum-AIV also has an automatic mode to compute the schedule and
to resolve conflicts of demand for resources. If a conflict is
detected, activities affected by the conflicts are shifted along the
time axis according to the scheduling strategies defined for those
activities by the user. An important point is that the development of
a solution can be monitored while alternately using automatic mode and
manual mode. When the schedule computed by the system in automatic
mode does not correspond to the user's wishes, he may restart the
scheduling process in manual mode at the scheduling point where the
automatic solution diverges from the desired solution.
Starting with global constraints, it is possible to define constraints
on activities, which are dependent on the characteristics of other
activities. The syntax of global constraints follows the production
rule: `IF conditions THEN conclusions'.
The conditions involve planning `objects' such as activities and/or
resources. The conclusions state facts or temporal relations between
activities. Using the conditions as variables, it is possible to
determine relations or constraints between sets of activities which
share the same characteristics. For example:
IF activity A is of type TEST of an element E and activity B is of
type MECHANICAL INTEGRATION of an element E
THEN activity B must be performed before activity A.
The global constraints are applied after completion of the scheduling
process, in which the period allocated to each activity is computed.
Assistance in resolving conflicts
Optimum-AIV notes the various conflicts of demand for resources than
cannot be solved automatically during the planning and scheduling
process and supports the user in solving them.
Monitoring and Re-Planning
Optimum-AIV also provides facilities for monitoring the execution of
the plan and for implementing any consequential changes in the
planning which may be necessary. Last but not least, these planning
functions are supported by a clear and user-friendly man-machine
interface which facilitates access to information.
Optimum-AIV is now used by Matra-Marconi Space for planning the
production of the vehicle equipment bays (VEB) for the Ariane-4
launcher. Here, scheduling is characterised by the fact that, at a
given time, several VEBs may be concurrently in production and
competing for available resources of manpower and materials.
The main criteria which have led the Ariane-4 project management to
the choice of this planning tool are the following:
A module for automatically allocating available equipment amongst VEBs
has been developed and works in conjunction with the scheduling
module based on Optimum-AIV.
- the wealth of information which can be provided and used by the
tool to describe the constraints inherent in the AIV activity
- the quality of support provided by the tool in scheduling
production in resolving conflicts of demand for resources
- the clear representation of information and the interactive
capability which enables engineering management to assess several
planning scenarios online.
- the fact that Optimum-AIV provides a single solution to problems
of managing both schedule and the allocation of component equipment
modules amongst competing VEBs. Equipment allocation has a direct
impact on the schedule because most of planning events are driven by
the delivery of equipment.
Optimum-AIV provides unique features for the presentation and editing
of AIV planning and scheduling information. This information is used
to check the logic of the planning and to specify global constraints
on the project.
Optimum-AIV can also model a large set of scheduling constraints, and
it provides the facilities to manage and solve problems within these
constraints. It is thus possible to achieve an exact representation
of the constraints acting on a project and to get a more reliable
Optimum-AIV is now in use on pilot projects at ESA and also at
Matra-Marconi Space. The objective of these projects is to obtain
feedback from user experience in applying the tool to practical
problems, with a view to optimising it for planning AIV activities in
an operational environment.