Main topic of this thesis is the question how to integrate positioning and navigation into LBS (draft standard DIN 9684 for an Agricultural Bus System) for local machine and process control.
Options for integration are derived based on fundamental system requirements for positioning and navigation in agriculture as well as on static and dynamic protocol definitions of the draft standard of LBS (DIN 9684).
Two contrasting approaches are conceivable. Either decentralized positioning or navigation is utilized, i.e. position data is sent on the bus, or the positioning and navigation is processed at a central system unit and only resulting application data is broad- or unicasted on the bus. The theoretical study results in following key messages:
• Coarse navigation: i.e. -+1 m accuracy
Possible by the means of central and decentralized positioning and navigation,
• Basic Navigation: -+0.1 m accuracy
Only possible by the means of central positioning and navigation,
• Precise Navigation: -+0.01 m accuracy
Not possible for a LBS with a data transmission rate of 50 kbit/s,
• The draft standard allows as a "LBS positioning source" either the task controller 1, a new LBS service "Positioning" or a redefinition of the position data as basic message 3 or 4 or as low-priority basic messages. Then, in principle, any LBS participant could act as "LBS positioning source".
• The definition of a new sub-identifier of process data matrix of LBS list 0 for the parameter "quality of position data" is requested. (The static protocol for the data content has to be specified, too).
Regardless of whether utilizing central or decentralized positioning and navigation in LBS, message transmission delays due to the shared transmission medium (bus structure !) cannot be neglected since this has a decisive impact on the real-time capability of machine or process controls.
The second factor being relevant for the performance of a LBS is the respective utilization of the maximum data transmission capacity. Typically this factor is represented by the parameter bus load.
Therefore it was required to implement a modular measurement platform based on a programmable CAN bus analyzer in order to enable the investigation of data transmission delays as a function of prevailing actual bus load. LBS is a field bus system based on CAN 2.0 A, so it has got a prioritized bus access. Thus, it is necessary to selectively determine bus load for each priority group in order to get a meaningful measure of reference for determining the data transmission delays of single message groups.
The required measurement platform was designed and implemented based on modeling of the LBS communication system according to the OSI reference model as well as on the basics of measurement and testing techniques for communication systems.
Documentation of the modeling and the CAN bus analyzer as well as of the implementation of all required measurement methods and tests is one of the key elements of this thesis.
For the first time, selective measurements of bus load and corresponding data transmission delays of a LBS prototype system were conducted using the implemented measurement system. All measurements obtained were analyzed and results are depicted in this thesis, in the case of selective bus load measurements even in a 3-D diagram format.
Key messages of the theoretical investigation (see above) are supported by the analysis of the practical measurements.
Outlook:
In the future, it is likely that a central approach will be pursued in which positioning and navigation will be assigned to and implemented as part of the LBS service task controller 1.
In case a LBS participant has a need for position data (X, Y, Z coordinate and quality information), this participient will directly request the data event-based or start a (measurement) program which sends out the data on a periodic base.
In doing so, cycle time should not exceed 250 ms. So, transmission delays of a position data packet are kept within limits (between 1/10 and 1/20 of cycle time) and the resulting load on the remaining system stays relatively low.
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