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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/