8.1 Weeder tool

The specified requirements of weeding (chapter 4) are that the weeds as a minimum should be cut within a 40 cm radius circle area around each tree as close as possible to the ground. The operation scenarios that an ACW should cope with are specified in table 6.1, while the work tasks considered and the methods chosen are specified in table 6.2.

The simplest and most robust method for weed cutting is a rotary cutter, which is well-known from lawn mowers and agricultural forage mowers, which are available in many different designs and sizes. Figure 8.1 show some different cutter unit designs with the cutter blade shielded in a housing. This is a good safety precaution but will require rather frequent cutting to prevent weed plants from growing too large and vigorous to be bent beneath the cutter shielding.

8.2 Optimum operation pattern and vehicle positioning

Cutting of the minimum required circular areas around each tree may be performed by a loop type operation pattern where the ACW travels around each tree while cutting a band of 40 cm. Alternatively the ACW could travel parallel to the rows on either side (figure 8.2). Of these the parallel operation pattern is found to be the simplest and most favourable in terms of efficiency and costs (see chapter 9).

Figure 8.1.
Outline of different rotor cutter designs. The rotor may be mounted within a housing (a) to achieve maximum safety on the expense of ability to cut high robust weeds, or the rotor may be mounted just below the housing (b and c) to improve the ability to cut such weeds on the expense of safety. In all cases the shield will protect the trees, if the vehicle positioning should be insufficiently accurate. Type a and b are considered suitable for vehicle designs having the trailing wheels mounted close to the rotor, while type c, which has a skid-pan underneath the rotor, is suitable for rotor placement in larger distance from the trailing wheels.

The parallel operation pattern means that the vehicle travels along the row with the rotor cutter overlapping the row centre line by a few cm. When approaching a tree it may be designed to go about it by a quick sideward reciprocal movement. Alternatively the rotor cutter may be moved side wards in a similar way.

Figure 8.2.
Parallel (top) and loop type operation patterns.

8.3 Vehicle concepts

Of the vehicle concepts described in appendix A the "beetle type" and the portal type was selected for closer examination.

8.3.1 The beetle type

The "beetle type" is a small lawn mower type of machine (figure 8.3). It only travels within the cut swath and is small enough to move relatively close to the trees has a size suitable to move underneath branches and in between any two trees in a plantation. It would not have any restrictions from the size of the trees and would not cause problems of tree damage at headlands. In addition it would be suitable for weeding in a number of other tree and bush cultures, e.g. in horticulture.

It is probably also the simplest possible design with a minimum of moving parts as the wheels are used to do all positioning relative to the ground and to the trees. For the same reason 4-wheel drive and 4-wheel steering would be important.

The larger beetle type (figure 8.4) is somewhat more complicated but because of the floating rotor cutter suspension and skid-pad, it may have better abilities to work on uneven ground and to pass obstacles. The relative positioning of the rotor to the trees may in this case be done by one set of steering wheels, but it may be simpler to have the vehicle to travel on a strait line and to position the rotor cutter relatively to the vehicle by means of a simple control system.

This beetle version could even carry a rotor cutter on either side with individual near positioning systems, which would double the capacity, but also make turning more difficult. Another disadvantage would be the low clearance, which would affect the leftover weeds between the rows. Also, extra space would have to be provided between the field border and the first row to provide enough space for the wider machine.

Figure 8.3.
Outline of small "beetle type" ACW mounted with central placed rotor cutter.
    

Figure 8.4.
Outline of large beetle type weeder with front mounted rotor cutter. A wide design is proposed to place the wheel traffic in the cut lanes only.

8.3.2 The portal type

The portal type (figure 8.5) would be a two-rotor type machine travelling with a set of wheels on either side of a row.

Figure 8.5.
Outline of portal type weeder with two rotor cutters seen from the under side.

This type would have limitations regarding the size of the trees, and would also be more complicated to turn on headlands. Another negative point is, that it would be unsuited for operation in most other tree and bush cultures. However the methods of controlling the cutter units into the intra-row area would be similar to those describe above.

8.4 Vehicle operation procedure and transport

It is proposed, that the ACW should work autonomous only within the borders of plantations or a group of plantations having common borders. Before starting the work and eventually at times during the work the operator will input work instructions into the controller by suitable means (e.g. radio or chip card). As part of the work instruction map information (GIS data) for the field borders, occurring obstructions and the tree location has to be provided. The controller will also get information from vehicle-mounted sensors to be used for vehicle guidance, control of the weeding tool and for check up on its own status.

Having been placed in a plantation and given the necessary information the ACW should start to measure its own position and orientation in the field. Then it should work out the best overall operation strategy, i.e. the most efficient order of sub tasks. After that it should start the rotor cutter, move to the first tree and begin to work. Having finished the last tree the ACW should move to the point where it was started up, and report back to the supervisor.

As the production of Christmas trees mostly takes place in small plantations an ACW would in most cases have to serve several plantations to improve utilization and reduce costs. Therefore, the machine would often have to be moved between plantations. This calls for a small, light machine, which is easy to move. One person may move, service and operate many machines simultaneously in one or more plantations at the same time depending on locations.

8.5 System architecture

To achieve the above-described operation of an ACW it must have a controller, which can control machine movements and weeding tool functions. A proposal of a general type of system architecture suitable for this type of controller is provided in appendix B. The information needed for the controller would be a priory information on the positions of plantation borders and trees (GIS database), technical specifications of the vehicle (ACW database and tool database), real time sensor information on the ACW orientation and position absolute and relative to trees and other nearby items of relevance to be able to move and work relative to these, as well as sensor information on the vehicle itself (self awareness and check). Using this data would enable the ACW to work as described in section 8.4. To compensate for inaccuracies of sensor information and positioning of the ACW there seems to be need to sense tree positions relative to the vehicle.

8.6 Sub conclusion

Outlines were proposed of a weeder tool, various vehicle designs, operation patterns as well as vehicle operation procedure and system architecture. The weeder tool suggested is a shielded rotor cutter that can go close to the trees. For the same reason a small vehicle, being able to move beneath the branches, is considered best suited. An operation pattern of linear movement along tree rows was found most efficient. To navigate such a vehicle appropriately would require sensors for measurement of position, orientation and structure of surroundings and others, as well as information on the position of the trees. The system architecture should be adapted to these characteristics.

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