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Exception Handling

The following examples demonstrate how sequences and their monitors are able to handle various scenarios.

Sequence Supervised by Monitor

The situation below shows a monitor firing during the runtime of a sequence.

Monitor checks condition and causes a switch to an exception sequence after which the original sequence is restarted

As soon as a monitor checking for a certain condition fires, an exception sequence is called. The exception must always be blocking. However, it would be possible to omit it altogether. After this the control goes back to the original sequence. How does this sequence now continue? Every monitor can be assigned one of the following properties:

  • resume: the sequence continues normally
  • abort: the sequence is aborted and control goes back to its calling sequence
  • restart: the sequence gets restarted, its timeout monitor is reset (that's the situation shown in the figure above)

The following table shows the behavior for various situations

Propertybehavior of sequence after monitor firesexception sequence if present
resumesequence continues normallyexception sequence will run for each further step
abortremaining steps abortedexception sequence will run once
restartremaining steps aborted, sequence restarts with first stepexception sequence will run once, after restarting will run again if monitor fires again

Sequence Supervised by more than one Monitor

A sequence can be checked by more than one monitor where each monitor checks for a certain condition to be met. While a first monitor could check for a timeout condition a second could supervise whether a payload of a robot didn't get lost.

Two monitors checking the same sequence. While monitor 1 causes the sequence to abort, the second restarts the sequence in case of a condition to be met.

Please note, that each monitor can have a different effect on its associated sequence. While one monitor can cause the sequence to abort further steps another can restart the sequence upon firing an exception. Further, one of the monitor can cause an exception sequence to run while another does not do so.

Monitor for Exception Sequence

Exception sequences are used to handle situations where regular sequences do not work due to certain conditions. Besides this, an exception sequence is a normal sequences with exactly the same features and has to be defined in the same way. Therefore, an exception sequence has the same built-in conditions and monitors as a regular sequence (timeout and abort conditions).

eeros_architecture/sequencer/exceptions.txt · Last modified: 2017/09/24 11:39 (external edit)