7. semester
Projekttitel: |
Developing a Hard Real-time Protocol for The Autonomous Platform |
Abstract: |
Introduction:
The necessity of reducing the use of herbicides in agriculture is
causing an increasing demand for intelligent automated devices. The
Autonome Platform and Information System (API) is such a device
[1]. However, the efficiency of an automated vehicle demands that data
is transferred between controlling units precisely at the scheduled
times. The aim of this project is to find out whether or not it is
possible to develop a Hard Real-Time protocol for the CANbus system of
the API. The system consists of one master computer and four node
computers, each controlling one wheel of the platform. The protocol
has to provide means for transfer of controller references from master
to nodes, and feedback data from nodes to master.
Methods and Material:
A model of the API was built using five standard PC's, each equipped
with a CAN interface card. The Real-Time performance of the masters
original operating system (RedHat 6.2) was tested, by measuring the
time needed to handle interrupts, and found unable to fullfill Hard
Real-Time demands. Therefore, the masters OS was patched with
Real-Time Application Interface [2]. Based on an analysis of the
service requirements of the API, a protocol was designed using parts
of the OSI model. A model of the time aspects of the network was
developed. Linux Trace Toolkit [3] was used to verify model
parameters. Based on the model, the minimum time between the node
controllers sending feedback and the master controller needing
feedback was found. This ensured that the feedback data were as
current as possible. The protocol was designed to perform
synchronization between the master and the nodes, ensuring that tasks
were carried out at pre-scheduled times. Tests were made by letting
the protocol generate signals on the parallel port, and measuring the
time between these signals with an oscilloscope.
Results:
The designed protocol was implemented with the functionality required
by the API. Time synchronization was obtained with an upper deviation
boundary of 20 us.
Discussion:
The test results confirm that it is possible to design and implement a
Hard Real-Time protocol for the CANbus system of the API. The
developed protocol is able to synchronize the CANbus system and
provide the services required. With the found synchronization
deviation of 20 us a maximum controller frequency of 5 KHz can be
guaranteed, taken that a maximum deviation of 10% is accepted.
References:
[1]The API Project Homepage, Aalborg University, 2002, http://www.cs.auc.dk/~api/
[2] Real Time Application Interface, Dipartimento di Ingegneria
Aerospaziale Politecnico di Milano, 2002, http://www.rtai.org/
[3] Homepage of Linux Trace Toolkit, Opersys Inc., http://www.opersys.com/LTT/
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