PLEXIL (Plan Execution Interchange Language) is an

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technology for

automation Automation describes a wide range of technologies that reduce human intervention in processes, mainly by predetermining decision criteria, subprocess relationships, and related actions, as well as embodying those predeterminations in machine ...

, created and currently in development by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

Overview

PLEXIL is a

programming language A programming language is a system of notation for writing computer programs. Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...

for representing plans for

. PLEXIL is used in automation technologies such as the NASA K10 rover, Mars ''Curiosity'' rover's percussion drill, Deep Space Habitat and Habitat Demonstration Unit, Edison Demonstration of Smallsat Networks,

LADEE The Lunar Atmosphere and Dust Environment Explorer (LADEE; ) was a NASA lunar exploration and technology demonstration mission. It was launched on a Minotaur V rocket from the Mid-Atlantic Regional Spaceport on September 7, 2013. During its s ...

, Autonomy Operating System (AOS) and procedure automation for the

International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...

. The PLEXIL Executive is an

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engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...

that implements PLEXIL and can be interfaced (using a provided software framework) with external systems to be controlled and/or queried. PLEXIL has been used to demonstrate automation technologies targeted at future NASA space missions. The binaries and documentation are widely available as

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d open source from

GitHub GitHub () is a Proprietary software, proprietary developer platform that allows developers to create, store, manage, and share their code. It uses Git to provide distributed version control and GitHub itself provides access control, bug trackin ...

Nodes

The fundamental programming unit of PLEXIL is the Node. A node is a

data structure In computer science, a data structure is a data organization and storage format that is usually chosen for Efficiency, efficient Data access, access to data. More precisely, a data structure is a collection of data values, the relationships amo ...

formed of two primary components: a set of conditions that drive the execution of the node and another set which specifies what the node accomplishes after execution. A hierarchical composition of nodes is called a plan. A plan is a tree divided in nodes close to the root (high level nodes) and leaf nodes that represent primitive actions such as variable assignments or the sending of commands to the external system. Node Types: As of September 2008 NASA has implemented seven types of nodes. *''List nodes'': List nodes are the internal nodes in a plan. These nodes have child nodes that can be of any type. *''Command nodes'': These nodes issue commands that drive the system. *''Assignment nodes'': Performs a local operation and assigns a value to a variable. *''Function call nodes'': Accesses external functions that perform computations, but do not alter the state of the system. *''Update nodes'': Provides information to the planning and decision support interface. *''Library call'' nodes: These nodes invoke nodes in an external library. *''Empty nodes'': Nodes that contain attributes and do not perform any actions. Node states: Each node can be in only one state. They are: * Inactive * Waiting * Executing * Finishing * Iteration_Ended * Failing * Finished Nodes transitions: *''SkipCondition T'' : The skip condition changes from unknown or false to true. *''StartCondition T'' : The start condition changes from unknown or false to true. *''InvariantCondition F/U'' : Invariant condition changes from true to false or unknown. *''EndCondition T'' : End condition changes to true. *''Ancestor_inv_condition F/U'' : The invariant condition of any ancestor changes to false or unknown. *''Ancestor_end_condition T'' : The end condition of any ancestor changes to true. *''All_children_waiting_or_finished T'' : This is true when all child nodes are either in node state waiting or finished. *''Command_abort_complete T'' : When the abort for a command action is completed. *''Function_abort_complete T'' : The abort of a function call is completed. *''Parent_waiting T'' : The (single) parent of the node transitions to node state waiting. *''Parent_executing T'' : The (single) parent of the node transitions to node state executing. *''RepeatCondition T/F'' : the repeat condition changes from unknown to either true or false.

References

{{Reflist

External links

PLEXIL at NASA

PLEXIL at GitHub

Overview

Nodes

References

External links

See also