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Computers and Electronics in Agriculture 25 (2000) 87–106 Providing measured position data for agricultural machinery Hermann Speckmann Federal Agricultural Research Centre Braunschweig (FAL), Institute for Biosystems Engineering, Bundesallee 50, D-38116 Braunschweig, Germany Abstract Agricultural machinery and vehicles require position data for guidance and to control implements for optimal working positions. Position data are also needed for such applica- tions as precision farming. The necessary accuracy, resolution and frequency of position data vary according to the specific application. Only one system, installed at a central vehicle (e.g. the tractor), should provide position data for each task. The basic concept for the proposed central system is that position data are calculated in accordance with the application and transferred directly to the point at which they will be used. The paper describes the fundamentals of measurement and calculation of position data, and gives a short introduc- tion to the existing agricultural networks to transfer these data. It concentrates on a proposal for a network service to provide and transfer position data. The solution discussed is based on the agricultural BUS (DIN 9684, ISO 11783). ? 2000 Elsevier Science B.V. All rights reserved. Keywords: Local area network; Controller area network; Agricultural BUS system; LBS; Calculation of position; Calculation of direction; LBS service :locate:compag 1. Introduction The purpose of position guidance is to bring the means of production to the plants, which grow at a fixed location on the field. The plants, or rather their location on the field surface, are the reference for guidance. Position data are needed to guide agricultural vehicles, to control implements and to support precision farming. Accuracy, resolution and frequency depend on their application. E-mail address: hermann.speckmann@fal.de (H. Speckmann) 0168-1699:00:$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S0168-1699(99)00057-5 H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–10688 It must be emphasized that this paper does not address the problem of suitable sensors to generate the data. Rather, the problem studied here is that a position signal is generated with reference to a certain location on the mobile unit, but this position is not identical with the location where the position data are needed. Moreover, position information may be needed for several purposes at the same time, and the configuration of the vehicle–implement combination may change frequently. As mentioned by Freyberger and Jahns (1999), Wilson (1999), the measuring system can either be an absolute position system, such as the satellite system described by Bell (1999), or a relative system, such as the machine vision systems described by Debain et al. (1999), Hague et al. (1999). It may also include auxiliary sensors. Sensor systems measure position only in reference to a specific location, such as the mounting point of the camera or the foot of the aerial. In the following presentation, this location is called the measuring point. For various reasons, the location of this measuring point is predetermined, meaning the satellite antenna will be mounted as high as possible on the roof of the tractor cab to minimize shading. A camera will be mounted where optimal view is guaranteed. Movement caused by rough or sloping field surfaces may cause the measured position and the position on the field surface to differ widely. For example, for a vehicle with a satellite antenna mounted on top of the cab, at about 3.5 m, driving on a sloping surface of 10°, the difference in direction of the inclination will be about 60 cm. Fig. 1 illustrates this scenario for one dimension. In this example, it may be appropriate to calculate the position of a reference point. Bell (1999) proposes the middle rear axes of the Fig. 1. Difference in position for two locations due to sloping terrain. H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–106 89 tractor as a reference point. A point in the field surface, for example, vertical under the middle of the rear axis seems more appropriate for some applications. For certain applications, such as the control of implements, the position of a certain point of the implement may be of final importance. This point will be called the target point. In cases where position data are needed for different purposes, it is not very efficient to measure the position for each purpose separately with an independent measuring system. Multiple hardware can be avoided when the position is measured only once, and the positions of the other points on the vehicle or implements are calculated. This is possible if position and attitude are measured, and the spatial vector between the measuring point and the point to be calculated is known. If both points are rigidly coupled, meaning that both points are on the tractor, the vector between these points is constant, and a simple matrix calculation yields the result. If these points are not rigidly coupled, meaning, for example, that one point is on the tractor and the other is on an attached implement, the vector is variable. Additional measurements become necessary to establish the vector between these two points or other principles to calculate the position of the target point must be applied. 2. Data processing and data transfer Position data of any point on the vehicle or implement can be calculated from the position and attitude measured at a measuring point. This calculation can be made by the measuring system (central data processing) or by each system requesting target position data (distributed data processing). 2.1. Distributed data processing The measuring system serves only as an intelligent sensor in the case of distributed data. It measures position and attitude on request, and provides these data without any processing. Characteristics such as frequency and accuracy are determined by the requesting unit. This unit performs all processing to calculate the position. The unit must know the position of the measuring point and all relevant parameters to do this. The advantage of this procedure is that the measuring device can be relatively simple. On the other hand, each requesting unit needs the full capacity to perform this calculation. 2.2. Central data processing The measuring unit is extended by components to calculate the position of target points for any user. This measuring and processing system forms one unit of a so-called position and navigation service (PNS), which provides final position data of any target point. In this case, only one measuring and processing system is necessary even when position data are requested by more than one user. To do so, only the PNS must know all of the relevant parameters for the calculation. H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–10690 2.3. Data transfer A data transfer is necessary no matter where the data are processed. For such a data transfer, a standardized network is appropriate. For agricultural purposes, a BUS for data transfer between mobile units and stationary farm computers exists. The agricultural BUS system (LBS) has been standardized to exchange information between the electronic units (LBS participants or BUS nodes) in a network. The standard defines the physical layer of the network, network protocol, system management, data objects and central services for common tasks (Speckmann and Jahns, 1999). The LBS has been standardized as DIN 9684 (DIN, 1989–1998). Currently, efforts are being made to establish an international standard (Nienhaus, 1993), ISO 11783, for such purposes. Like LBS, ISO 11783 will also define an agri- cultural BUS as an open system to exchange data on agricultural machinery, particularly on tractor–implement combinations and from the mobile units to the stationary farm computer. The standards are based on the controller area network data protocol (CAN; BOSCH, 1991). Corresponding hardware is on the market. In the LBS, data objects are defined for the transmission of general position data (geographical positions: longitude, latitude, altitude, or position in a tramline). The standard allows definition of additional data objects such as multidimensional distances, directions and speeds. No data objects exist presently in the LBS for geometric implement parameters. ISO 11783 provides, in Part 7 (Implement Mes- sages Application Layer), the first definitions of implement navigational offsets. Current standards do not define where which data are processed. Therefore, it is immaterial on which unit the BUS calculates the data for the target point, and which unit or units measure the data. The LBS provides so-called LBS services to execute common tasks. LBS services are functional units, which perform frequently recurring tasks for LBS participants. Such a service is the LBS user station. This is a central interface to the user (operator) for input and output of data which is at the disposal of any node (LBS participant) on the BUS. Another service provides the data exchange between the mobile unit and the stationary computer, the farm computer. Some more services are named in the LBS but not yet stan- dardized in detail, such as for diagnosis services or the service ‘Ortung und Navigation’ (position and navigation), which will be discussed in the following as PNS. In Fig. 2, an exemplary simplified scheme of an agricultural network is shown for a tractor–sprayer combination. This scheme includes the physical BUS line, which is the backbone of the network. At this BUS, participants such as electronic control units (ECUs) of the tractor and sprayer are coupled. Additionally, two LBS services are connected on the BUS. One of these services represents the LBS user station. The other is the LBS service ‘position and navigation’, with the measuring and processing system for position data. H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–106 91 Fig. 2. Scheme of an agricultural network in a tractor–sprayer combination. 2.4. Comparison of distributed and central data processing For a distributed data processing, the agricultural BUS, according to DIN 9684 or ISO 11783, defines the necessary data exchange between the measuring system and any participant; respectively, any ECU. The question how each ECU gets geometric and kinematic parameters that are necessary to compute position data remains open. Each ECU knows its own parameter from its coupling point to the target point, but it does not know the parameter from the coupling point to the measuring point. These parameters must be provided from other ECUs. None of the standards define corresponding data objects or procedures requesting the data. For distributed data processing, these definitions have to be supplemented. Also, for central data processing, all kinematic parameters between the measur- ing point and the target point must be known. In addition, methods are to be defined for the use of the central service with regard to the calculation of position data of target points. A position and navigation service requires an extension of the standards, but the following advantages in practical use are essential: To determine the position data of a target point, the corresponding ECU has only one dialogue partner in the network. It works independently from the respective network configuration, delivers only its own parameters and receives only its specific position data. The PNS receives parameters from all ECUs. It knows all geometric conditions and kinematic parameters of the vehicle–implement combination. Thereby, an unambiguous determination of the position of any target point is possible. H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–10692 The standard defines the procedures to calculate and present the position data of a target point unambiguously. The computing performance to calculate the position data is provided solely by the PNS. No computing capacity is needed for this purpose from the ECUs. As mentioned in the previous section, a service to provide position and naviga- tion data is already planned in the LBS. In the following, a sample solution of a PNS is presented. 3. Proposal for a positioning and navigation service At this time, it should be mentioned that the following description of a PNS is a proposal. It provides a platform for discussion, which may lead to the standard- ization of such a service. 3.1. Main features of a PNS The features of a PNS depend, first of all, on the purpose for which it will be used. From the foregoing, it is clear that position data are measured at one location and used at different locations. The following requirements must be fulfilled to provide the data needed to guide a vehicle, to control positions of implements and to assist any kind of precision farming: The PNS provides data related to the measurement point(s). The PNS provides data related to the reference point(s). The PNS provides data related to the target point(s). The characteristics of such a service are as follows: 1. The way the data are requested and transmitted is already standardized and defined by the LBS (DIN 9684) and will be standardized by ISO 11783. Therefore, it will not be discussed here. In the following, LBS will be used as a standardized agricultural BUS system. 2. The volume, accuracy, frequency and range of the data are determined by the purpose of the data. 3. The hardware and software to fulfil these demands should not be standardized, but be determined by the manufacturers. 3.2. Influence of the standard on measuring and calculation methods for position data The kinds of measuring systems and methods used to determine position data by the PNS is not in the scope of the standard. Systems based on satellites, machine vision, inertial navigation, geomagnetics or a combination of these may be applied. As a consequence, the manufacturer may determine how to generate the position data as long as he meets the stated requirements and accuracy. H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–106 93 3.3. Integration of the PNS into an agricultural BUS system There are some benefits of integrating the positioning and navigation service into the LBS, because many characteristics are already defined. The LBS already includes the option of a PNS as part of the standard. It allows the realization of a service either as an independent physical unit or as a logical unit inside of another physical unit. The physical properties of the BUS interface and the BUS protocol (DIN 9684, part 2) are defined by the standard. For integration of the service into the LBS, the definitions of the system functions are decisive (DIN 9684, part 3). They define the performance of the nodes at the LBS. Part 3 also gives the general definitions of LBS services. An LBS service forms a point-to-point link with LBS participants. The use of a service by an LBS participant can neither be influenced by other users, nor can an LBS participant influence links between the service and other participants. All further definitions of the PNS are not yet standardized. 3.4. General mode of operation of the PNS For the design of the PNS, the following basic assumptions apply: 1. Each ECU knows only its parameters, meaning coordinates and numbers of reference points, target points, positions of couplings, vehicle types or wheelbases. 2. Only the ECU can define necessary time intervals, accuracy and resolution for position data, depending on the working conditions. 3. Each ECU can get different position data at arbitrary times. 4. Parameters and the way of calculating and providing position data will be defined before the working processes of the field machinery are started. 5. The PNS provides a library of procedures to calculate position data for standard implement and vehicle types. 6. Position data are provided automatically (cyclically) or on demand. To meet these requirements, the service provides the tools, and the ECUs determine how and which tools are used. This means they define one or several task(s). Such a task basically represents a list that includes commands to activate the specific tools. These tasks are sent to the PNS, which subsequently performs these tasks. Different tasks of one ECU are executed independently of each other. Fig. 3 illustrates the data transfer between the PNS and one ECU. It also shows the main parts of the PNS. The tools of the PNS include the system for measuring the position and attitude data of the measuring point, and a library of methods to process these data. Methods exist: to calculate position data (position methods); to calculate mean, maximum, minimum and integral values of position data (arithmetic methods); to export and import data (transport methods); to send data to the ECU (transmission methods); and to control the data processing (data control methods). H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–10694 For some of these methods, the ECU has to define corresponding parameters. It also defines data objects for position data. The central tool of the PNS is the program system to execute the tasks defined by the ECU. Simplified, the program system interprets the instructions of the task, calls the corresponding methods, calculates the demanded position and sends the data to the ECU. For the definition of a task, the ECU generates a task resource. A task resource is mainly a list of instructions to call methods of the PNS or to call nested task resources. Parameters are defined by the ECU and placed in parameter resources. To store calculated position data, the ECU has to define data resources. The resources have to be transmitted from the ECU via the BUS to the PNS before activating corresponding tasks. Fig. 3. Strcture of a PNS and its data exchange with one ECU. H. Speckmann : Computers and Electronics in Agriculture 25 (2000) 87–106 95 Fig. 4. Example of the use of a position method in the course of a task resource. 3.5. Predefined methods of the PNS Predefined methods of the PNS are procedures to process position data or to control this data processing. Methods exist to perform different functions. The different methods are distinguished by a unique designator. They are called ‘within tasks’ (task resources). It will be a part of the standard to define the designators, function specifications and calling specifications of the methods. 3.5.1. Position methods Position methods (methods to calculate position data) are the basis for calculat- ing position data of target points. These methods calculate from an initial position (input position data, data of a reference point or previously computed data) the position of a new point (output position data, data of a target point or as an interim result). Position methods exist for different configurations (one-, two- or three-dimensional model considerations, rigidly coupled points, non-rigidly coupled points for several basic types of vehicles, implements and vehicle–implement combinations). These methods get their actual parameters (coordinates of the target point, vehicle length, width, height, type or wheelbases) from parameter resources which are defined by the concerned implement ECU. Fig. 4 shows a section of a task resource using a position method. The program system of the PNS executes this task resource. At a certain part of the task resource, it finds a calling instruction for a position method. This calling instruction includes the designator of the specific method and a reference to a relevant parameter resource. At this moment, the program system owns actual position data, which result from previous operations. Now it uses these actual data as input data, and the parameter resource reference for the position method. Then, it executes the specified me