09-05-2011, 11:53 AM
Open Real-time Robotics Control - PC Hardware, Windows/VxWorks Operating Systems and Communication
Abstract
In the modern robotics systems the users must combine hardware, operating
systems and network. Robot controller is a real-time operating system with multiple
tasks and processors, the hardest real-time requirements are in path control, there
coordinated movements of multible axes and other tasks must be controlled. The
robotics society is going to increase PC’s as part of the control platforms. Same time
modularised, distributed, open control system are more and more used, this increased
the importance of real-time communication. The article covers the recent
development on open robot control architecture based on PC, use of real-time
operating system VxWorks and Windows 95/NT in the same control system and
importance of internal and external communication in robot workcells.
1 Introduction
1.1 What is real-time?
The principal responsibility of a real-time (RT) operating system (RTOS) can be
summarized as that of producing correct results while meeting predefined deadlines
in doing so. Therefore, the computational correctness of the system depends on both
the logical correctness of the results it produces, and then timing correctness, i.e. the
ability to meet deadlines, of its computation.
Hard real-time (HRT) operating systems can be thought as a particular subclass
of RT systems, in which the lack of adherence to the above mentioned deadlines may
result in a catastrophic system failure. A RT application can be modeled as a set of
cooperating tasks. These tasks can be classified according to their timing
requirements, as hard real-time (HRT), and not real-time (NRT). A HRT task is a task
whose timely (and logically correct) execution is labeled as critical for the operation
of the whole system. The deadline associated to a HRT task is pronounced hard
deadline. Consequently, it is assumed that the missing of a hard deadline can result in
a tragic system failure. NRT tasks are those tasks which exhibits no real-time
requirements (e.g. system maintenance tasks that can run occasionally in the
background). [Brega and Honegger 1998]
The taxonomy of application tasks can de further expanded with the terms
periodic, aperiodic and sporadic. Periodic tasks are tasks which enter the execution
state at regular intervals of time, i.e. every T time units. These tasks are usually
associated with hard deadlines. Aperiodic tasks are tasks whose execution time cannot
AS 116.140L Juhani Heilala 2001 2
be anticipated, as their execution is determined by the occurrence of some internal or
external events. These tasks are usually NRT tasks. Finally, aperiodic tasks bound to
hard deadlines are termed sporadic tasks, e.g. tasks dealing with the occurrence of
system failures (exceptions), or prioritized responses to some event. With the above
classifications in mind, one can observe that the principal responsibility of a RT
operating system is to guarantee that each individual execution of each application
task can meet the timing requirements of that task. However, it is worth noting that, in
order to fulfill that responsibility, the objective of a RT operating system cannot be
stated just as that of minimizing the average response time of each application task;
rather the fundamental concern of a RT operating system is that of being predictable.
[Brega and Honegger 1998]
Two general paradigms for the design of predictable RTOSes can be found:
Event-Triggered (ET) and Time-Triggered (TT). In ET RTOSes any system activity is
initiated in response of the occurrence of a particular event, caused by the system
environment (mainly software or hardware interrupt vectors). In TT RTOSes system
activities are initiated as predefined instants of the globally synchronized clock recur
(scheduling, dispatching of events). [Brega and Honegger 1998]
In the robotic systems there are all the classes presented above. Path planning and
servo-loops are periodic (Time-Triggered), the sensor events are usually aperiodic,
emergency signals and some other sensor events are sporadic (Event-Triggered)
Download full report
http://www.automationit.hut.fi/old/julka...eilala.pdf
Abstract
In the modern robotics systems the users must combine hardware, operating
systems and network. Robot controller is a real-time operating system with multiple
tasks and processors, the hardest real-time requirements are in path control, there
coordinated movements of multible axes and other tasks must be controlled. The
robotics society is going to increase PC’s as part of the control platforms. Same time
modularised, distributed, open control system are more and more used, this increased
the importance of real-time communication. The article covers the recent
development on open robot control architecture based on PC, use of real-time
operating system VxWorks and Windows 95/NT in the same control system and
importance of internal and external communication in robot workcells.
1 Introduction
1.1 What is real-time?
The principal responsibility of a real-time (RT) operating system (RTOS) can be
summarized as that of producing correct results while meeting predefined deadlines
in doing so. Therefore, the computational correctness of the system depends on both
the logical correctness of the results it produces, and then timing correctness, i.e. the
ability to meet deadlines, of its computation.
Hard real-time (HRT) operating systems can be thought as a particular subclass
of RT systems, in which the lack of adherence to the above mentioned deadlines may
result in a catastrophic system failure. A RT application can be modeled as a set of
cooperating tasks. These tasks can be classified according to their timing
requirements, as hard real-time (HRT), and not real-time (NRT). A HRT task is a task
whose timely (and logically correct) execution is labeled as critical for the operation
of the whole system. The deadline associated to a HRT task is pronounced hard
deadline. Consequently, it is assumed that the missing of a hard deadline can result in
a tragic system failure. NRT tasks are those tasks which exhibits no real-time
requirements (e.g. system maintenance tasks that can run occasionally in the
background). [Brega and Honegger 1998]
The taxonomy of application tasks can de further expanded with the terms
periodic, aperiodic and sporadic. Periodic tasks are tasks which enter the execution
state at regular intervals of time, i.e. every T time units. These tasks are usually
associated with hard deadlines. Aperiodic tasks are tasks whose execution time cannot
AS 116.140L Juhani Heilala 2001 2
be anticipated, as their execution is determined by the occurrence of some internal or
external events. These tasks are usually NRT tasks. Finally, aperiodic tasks bound to
hard deadlines are termed sporadic tasks, e.g. tasks dealing with the occurrence of
system failures (exceptions), or prioritized responses to some event. With the above
classifications in mind, one can observe that the principal responsibility of a RT
operating system is to guarantee that each individual execution of each application
task can meet the timing requirements of that task. However, it is worth noting that, in
order to fulfill that responsibility, the objective of a RT operating system cannot be
stated just as that of minimizing the average response time of each application task;
rather the fundamental concern of a RT operating system is that of being predictable.
[Brega and Honegger 1998]
Two general paradigms for the design of predictable RTOSes can be found:
Event-Triggered (ET) and Time-Triggered (TT). In ET RTOSes any system activity is
initiated in response of the occurrence of a particular event, caused by the system
environment (mainly software or hardware interrupt vectors). In TT RTOSes system
activities are initiated as predefined instants of the globally synchronized clock recur
(scheduling, dispatching of events). [Brega and Honegger 1998]
In the robotic systems there are all the classes presented above. Path planning and
servo-loops are periodic (Time-Triggered), the sensor events are usually aperiodic,
emergency signals and some other sensor events are sporadic (Event-Triggered)
Download full report
http://www.automationit.hut.fi/old/julka...eilala.pdf