Integrated control of agricultural tractors and implements: a review of potential opportunities relating to cultivation and crop establishment machinery

Computers and Electronics in Agriculture 30 (2001) 167– 191 www.elsevier.com/locate/compag

Integrated control of agricultural tractors and implements: a review of potential opportunities relating to cultivation and crop establishment machinery Andrew J. Scarlett Robotics and Automation Group, Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK

Abstract The quality of work and the output of a tractor-implement combination relies heavily upon the concentration and skill of the operator. Electronic systems are used increasingly to control tractor sub-systems, i.e. engine, transmission, implement hitch, external hydraulics, and driveline, and to monitor or control certain implements. However, current systems operate autonomously, relying entirely upon the operator for coordination. An integrated hierarchical control system could potentially monitor operating parameters pertinent to both the tractor and attached implements and use this information to control relevant tractor and implement sub-systems in a coordinated manner, thereby improving machine performance. Potential opportunities for the application of real-time, integrated, hierarchical control techniques to certain cultivation and crop establishment implements currently in use on European farms are reviewed. Specific implements (and their parameters) considered include primary cultivation machinery (control of working depth and working width), secondary cultivation machinery (control of working depth and seedbed quality) and crop establishment machinery (control of seeding depth and seed rate). Outline control strategies are proposed for these applications, and sensors and other hardware required to implement the control systems are identified. It is speculated that the agronomic and economic benefits which are likely to result from the implementation of the proposed technology, will enable economic justification of the proposed control systems in two to four operating seasons. These savings are likely to result from greater operational efficiency and more precise control of agronomic inputs. Copyright © 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Tractors; Implements; Electronics; Control; Cultivation; Drilling

0168-1699/01/$ - see front matter Copyright © 2001 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 6 9 9 ( 0 0 ) 0 0 1 6 3 - 0

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1. Introduction The agricultural industry is often perceived, albeit erroneously, as slow to embrace technology. Whilst until the mid-1960’s British agriculture was relatively labour intensive, despite increasing levels of mechanisation, during the 1970’s and early 1980’s greater agricultural production was achieved at a time when the farm workforce was diminishing, circumstances which necessitated advances in mechanisation and the utilisation of higher levels of technology. However, harsh and inherently variable operating conditions, coupled with a limited budget and user doubts regarding system reliability, have undoubtedly restricted the adoption of electronic-based technology on agricultural field machinery. Today, in 1999 agricultural profitability is under increasing pressure, necessitating a general reduction in inputs, such as labour, fuel/energy, seed, fertiliser and agrochemicals. Improved production efficiency is the main objective, achieved in practice by the optimal utilisation of every input. Such an approach requires technology, initially to determine current input levels and corresponding crop outputs, and subsequently to permit input control and/or optimisation. During the last 7 years, levels of electronic technology on agricultural tractors have increased substantially, in many instances utilising systems derived from the truck and bus industries (Stepper, 1993; Stone and Zachos, 1993). Also, potential customers/users have gained confidence in the reliability and potential abilities of this hardware, which in turn has increased their willingness to accept higher levels of technology in the future. Adequate evidence of economic payback and overall reliability are requirements of any modern system. This paper reviews the potential opportunities for the application of integrated control techniques to cultivation and crop establishment implements currently in use on European farms. It seeks to identify implements, and their specific parameters, which would particularly benefit from real-time, integrated, hierarchical control; suggest appropriate outline control strategies; identify additional mechanical and electronic hardware which may be required, and estimate the agronomic and economic benefits likely to result from implementation of the technology.

2. Control system integration The majority of agricultural field operations are performed by tractor and implement combinations, i.e. a mobile power source and attached tool(s). The quality of work and the output of a tractor-implement combination relies heavily upon the skill and concentration of the operator. Electronic systems are used increasingly to control tractor sub-systems, e.g. engines, transmissions, implement hitches, external hydraulics and traction drivelines (Holtmann, 1999), thereby improving vehicle sub-system response and reducing operator physical effort. However, to optimise tractor-implement performance or operational efficiency, it is

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necessary to coordinate the behaviour of the control systems (or rather the control sub-systems) present on both the tractor and implement; a task which currently falls to the operator. It is, therefore, questionable whether the operator is getting the most out of the increasingly complex control sub-systems he/she attempts to coordinate. A heavy decision-making load is unlikely to be sustained throughout a long working day, without a consequent loss of performance or concentration. Scarlett (1993) reviewed tractor control system developments and proposed a system for the integrated control of tractor engine, transmission and implement hitch control sub-systems, to address these perceived restrictions. It was speculated that a control system of this type could increase tractor operational efficiency by 15–20%, similar views are echoed by Tewes (1993), Jaufmann (1997). Tractor-implement control system integration attempts to automate coordination between tractor and implement control sub-systems, and thereby improve operational efficiency (either by performance or economy maximisation), improve the quality of work produced and reduce driver workload and fatigue (Scarlett, 1993). The prototype tractor-implement integrated control system proposed by the author (Fig. 1) embodies the following features, 1. dedicated microprocessor-based control sub-systems responsible for each tractor sub-system;  engine,  powershift (or continuously variable) transmission,  electro-hydraulic implement hitch,  external hydraulics,  traction driveline (four wheel drive, differential lock(s)), 2. microprocessor-based control sub-systems present on each attached implement; 3. central control unit and operator interface located in the tractor cab;

Fig. 1. Proposed structure of integrated control system (after Auernhammer).

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4. inter-communication/control network linking individual control sub-systems to the central control unit and operator interface. In operation the operator selects an appropriate system operating strategy and supervises integrated control system performance (when engaged). The central control unit exerts hierarchical control over the control sub-systems, according to the operating strategy, but manual control may be regained by the operator at any time. Such a system is necessary if agricultural implements are to benefit from integrated control technology, as many of their operational parameters are affected and/or controlled by the behaviour of tractor sub-systems. Consequently, for the full benefits of integrated control to be realised it will be necessary for implement control sub-systems to have the potential ability to influence the behaviour of their tractor-based counterparts, and vice versa. This degree of functionality will require both the modification of existing, and the procurement of new sensing, communication and control hardware upon both the tractor and attached implements. A number of methodologies have been proposed for tractor-implement communication and control systems (Schueller, 1988; Jahns and Speckmann, 1992; Auernhammer, 1993), but the successful adoption of a system necessitates both international standardisation and substantial take-up by both tractor and implement manufacturers. Following developments in on-road vehicle technology (Stepper, 1993) and substantial research and standardisation efforts in Germany (KTBL, 1993) and elsewhere, the controller area network (CAN) communication protocol (Bosch, 1991) has been selected for tractor-implement communication purposes. Further protocol development and standardisation is required before CAN systems may be fully utilised in agricultural applications, but these activities are being addressed by the International Standards Organisation (2000) within the 11783 draft standard, in conjunction with a number of tractor and implement manufacturers. A number of tractor manufacturers already utilise CAN communication systems within their vehicles, to a greater or lesser extent (Young et al., 1993; Holtmann, 1996, 1999; Roberts, 1999). System popularity and utilisation will almost certainly increase in the future, given the potential benefits of vehicle communication network implementation, which include, “ reduced wiring loom complexity; “ increased modularity, aiding system adaptation/modification; “ improved access to, and utilisation of, sub-system sensor data; “ simplification of system diagnostics; “ improvements in overall reliability. There are potential benefits in utilising techniques and/or standards developed for on-road (truck and bus) applications (Stone and Zachos, 1993), but the multiplicity of potential tractor-implement combinations, and their associated operational parameters, would necessitate substantial system adaptations. However, in specific areas where particular commonality exists e.g. vehicle powertrains, its is prudent and economically attractive to do so; a fact reflected by standardisation activities (ISO).

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3. Cultivation machinery

3.1. Primary culti6ation machinery 3.1.1. General The role of primary cultivation is to break the soil surface following crop harvest, producing a soil condition which, with further treatment, will be suitable for crop establishment. Primary cultivation can involve a number of tillage operations including soil loosening, surface drainage, soil inversion and crop residue incorporation. With the exception of the latter, the equipment used to perform these operations is relatively conventional, namely the subsoiler, mole plough and mouldboard plough, respectively. Mouldboard ploughs (and their derivatives) are also frequently used for crop residue incorporation, but heavy disc harrows and power take off (p.t.o.)-driven rotary cultivators may also perform this task effectively. The majority of primary cultivation machinery applies only a draught load to the tractor operating it. The operational efficiency of, say, a tractor and plough is, therefore, heavily dependent upon the tractive efficiency of the tractor in question. This has resulted in the widespread use of high-powered (70 –180 kW) four wheel drive tractors for primary tillage in European conditions, mainly in conjunction with mounted or semi-mounted implements. 3.1.2. Parameters requiring control The power requirement of primary tillage implements is dependent upon working depth, working width, soil strength and forward speed. As soil strength is inherently variable within any given field, tractor implement hitch control systems (Hobbs and Hesse, 1980) have been developed to minimise the draught force variations experienced by the tractor. This objective is achieved by automatically reducing or increasing implement working depth in stronger or weaker soil conditions, respectively. In use, an implement hitch or draught control system also transfers a proportion of implement mass onto the tractor rear axle, thereby reducing wheelslip and improving tractive efficiency without the requirement for additional tractor ballast. Modern electro-hydraulic implement hitch control systems (Hesse and Withington, 1992) incorporate wheelslip limitation facilities, whereby if slip exceeds an operator-selected level, the draught control system is momentarily overridden and implement working depth reduced. This momentary draught reduction, combined with additional weight transfer to the tractor rear axle, is often sufficient to reduce wheelslip and maintain productivity. At present the control responsibility for the majority of primary tillage implement parameters rests upon the tractor implement hitch control sub-system, whose main objective has been the maintenance of tractor-implement productivity, at the potential expense of tillage depth consistency. Unfortunately this can result in the production of soil conditions which do not encourage good crop establishment. Primary tillage is an expensive process, due to its high energy requirement and relatively low workrate. Mouldboard ploughing currently costs an ‘average’ farmer

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approximately £36 per ha (Nix, 1999), representing approximately 50% of the total cultivation costs for autumn cereal crop establishment. Poor quality control during primary cultivations can result in poor utilisation of expensive crop inputs, such as seed and fertilisers (Heege, 1993). It is obviously desirable to maintain machine productivity, for a substantial reduction in output could delay crop establishment and incur yield penalties, but if work quality standards are substantially compromised, productivity is of little value. The achievement of these objectives is the main role of integrated tractor-implement control in this context.

3.1.3. Control of working depth Improved control of primary tillage implement working depth has a number of immediate benefits. In purely economic terms, improved depth control will reduce energy wastage resulting from implement operation at greater than target depths. Greater system accuracy may also permit the selection of shallower cultivation depths with confidence. If a given crop requires a cultivation depth of approximately 150 mm, many operators will select a target depth of perhaps 200 mm, in the knowledge that existing draught control systems can produce working depth variations of 950 mm when operating in variable field conditions. As plough draught is proportional to working depth within the normal operating range, improved depth control may well result in energy savings or increased productivity of 10 – 15%. Uniform cultivation depth is also likely to encourage uniform plant root development and thereby improve crop utilisation of soil water and plant nutrients. Given that improved working depth control is desirable, its successful implementation necessitates reliable, accurate measurement of tillage depth. Researchers have used instrumented skids, instrumented wheels and non-contact sensors for this purpose, but it is likely that the end-user will favour non-contact methods for their apparent simplicity. Transducers of this type, which typically utilise ultrasonic pulse reflection principles, require refinement to fully suit the diverse nature of agricultural operating conditions (Rifal, 1992; Tischler and Moore, 1992; Yasin et al., 1992; Romes, 1994). Despite this current drawback, it is likely that this technology will eventually satisfy the requirement for a reliable and physically robust working depth transducer. Current hitch control systems vary implement working depth to reduce implement draught force variations due to changes in soil strength. If depth variation is to be minimised, a hitch control system whose principal control variable is implement draught, will require the addition of maximum and minimum working depth limits (McMullan, 1986). Alternatively, implement depth may be chosen as the principal control variable of the hitch system, with the provision of maximum draught and/or maximum wheelslip limitation features. In either case depth control accuracy should improve, but the tractor will be subjected to much greater variations in draught loading. Unless other tractor and implement control sub-systems are able to respond to these variations, by virtue of an integrated tractor-implement control system, productivity will be adversely affected. Instead of relying solely upon variation of working depth to minimise dynamic variations in draught

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Fig. 2. Potential integrated control system responses during draught operations.

loading, a combination of engine, transmission, implement hitch and implement control sub-system responses could be contemplated (Fig. 2). Recent developments in tractor engines and transmissions (Renius, 1994) and their associated control systems (Scarlett, 1993) have served to facilitate the adoption of integrated control technology. In addition to electronically-controlled diesel fuel injection systems, new forms of continuously variable transmissions (CVT’s) are now commercially available on certain agricultural tractors (Neunaber, 1997). Current systems utilise the ‘shunt’ operating principle, as discussed by Tinker (1991), in order to maximise transmission efficiency. Indeed, a recent evaluation (Neunaber, 1997) indicated the operating efficiency of this type of transmission to be in the region of 80%, broadly comparable with that expected of current stepped powershift transmission designs. Advanced ‘stepless’ transmissions benefit considerably from the application of intelligent electronic control systems, and arguably require such technology in order to be fully exploited when performing agricultural operations in typically variable conditions (Brunotte and Seeger, 1999). Intelligent engine-transmission management can undoubtedly improve tractor performance and the characteristics of a CVT enables maximisation of potential benefits (Bea, 1997; Vahlensieck, 1997). However, electronically-controlled multi-ratio full powershift transmission designs will probably offer competitive functionality, albeit with limitations, in the medium term (3–7 years). In operation, the central control unit (CCU) of a tractor-plough integrated control system (Fig. 3) will be programmed with an driver-selected operating strategy, and act in a supervisory role to monitor the performance of the control sub-systems present upon the tractor and implement. As changes in field conditions are encountered, the CCU will monitor the responses proposed by the individual control sub-systems and assess them in the light of the overall operating strategy. The optimum combination of responses (which comply with any imposed depth or wheelslip restrictions), will then be determined, and the expected improvement in tractor-implement performance calculated. If the predicted improvements are con-

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sidered worthwhile, the responses will be sanctioned for the individual control sub-systems to implement. The operating strategy selected will reflect the relative importance of performance (workrate) maximisation, fuel use minimisation and maintenance of work quality for the particular operation being conducted.

3.1.4. Control of working width Dynamic variation of implement working width may be a potential method of matching implement draught force requirement to tractor drawbar power availability, without affecting tillage depth. Certain mouldboard ploughs (Pearce, 1987) offer provision for on-the-move furrow width variation, by virtue of hydraulic actuators operating through mechanical linkages. It is debatable whether such an adjustment would have an adequate rate of response for implement draught control purposes, and the adjustment range has practical limitations (typically 300 –500 mm). The use of wider furrow widths can reduce implement specific draught (draught force required per unit of soil cross-sectional area (working depth× width)), due to the significant proportion of draught force which originates from the plough point/share, as opposed to the mouldboard assembly, consequently wider furrow widths can increase operational efficiency. However, on clay soil types, wide furrow widths can produce unacceptably rough surface finishes, requiring additional secondary cultivation efforts to make suitable seedbeds, negating any efficiency gains made during primary tillage. Also the resulting furrow deviations along a field, corresponding to soil strength variations, may not be acceptable to the end-user. However, in conjunction with other tractor and implement sub-system responses, as part of an integrated control system, the technique may have some merit (see Fig. 2). 3.1.5. Other control opportunities In addition to facilitating real-time implement control, parameter measurements acquired during primary cultivations could be used to influence the nature of

Fig. 3. Tractor and plough integrated control system.

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subsequent tillage or crop husbandry treatments. Paul (1992) demonstrated how implement draught force data, derived from the sensing elements of a conventional electro-hydraulic implement hitch control system, could be combined with in-field positioning information to permit spatial mapping of soil tillage resistance. Alihamsyah and Humphries (1991), Isensee and Lu¨th (1992) both developed horizontallyoperating penetrometers for soil mechanical impedance measurement, with potential for automatic control of soil-engaging implements. Stafford and Hendrick (1988) investigated the use of an instrumented slitter blade, set to run in undisturbed soil behind and beneath a subsoiler tine, to determine hard pan location in the soil profile. This information was subsequently used to adjust implement working depth to enable adequate soil loosening to be achieved without excessively deep cultivation. Alternatively, data from the above sources could be used to target soil loosening operations in subsequent seasons, on a spatial in-field basis.

3.2. Secondary culti6ation machinery 3.2.1. General Secondary cultivation or tillage operations attempt to produce seedbed conditions suitable for successful crop establishment and subsequent crop growth. Secondary tillage treatments usually comprises two components, namely aggregate size reduction and seedbed firming (compaction), but the severity of these actions must be tailored to suit the particular soil type, soil moisture status, cropping regime and climate in question. Also, the importance of timeliness of crop establishment, combined with the desire to reduce inputs such as fuel and labour, requires that secondary tillage, if performed at all, should be both a low cost and high output operation. Soil aggregate size reduction is costly both in terms of energy and time. Depending upon the severity of the soil conditions, this process may be performed by either pure-draught or power-driven implements. The former are cost-effective, because of their moderate energy consumption and relatively high workrate, but stronger soil types often require more harsh treatment, necessitating the use of power (p.t.o.)-driven implements (e.g. power harrow). Nix (1999) estimates the operating cost of a power harrow (p.t.o.-driven rotary cultivator) and ‘average’ (80 kW) tractor to be £24 per ha, more than twice that of pure-draught implements such as disc or spring tine harrows. Also, the power harrow in question would typically achieve a workrate of 1.12 ha/h (Nix, 1999), three-quarters that of the pure-draught implements. In trials on silt soil (Perdok, 1976), a power harrow exhibited a specific energy requirement of up to five or six times that of a pure-draught implement (see Table 1). Chamen et al. (1994) encountered similar relative differences in energy requirement during similar trials on clay soil types. However, a power harrow can often produce a seedbed in otherwise unfavourable conditions, frequently in only one pass, thereby reducing unwanted seedbed compaction and consequent energy losses. It can derive over 90% of its power requirement directly from the tractor p.t.o. (Heege, 1976), reducing energy losses due to wheelslip and rolling resistance. Additionally, a drill may be readily

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Table 1 Performance data for pure-draught and power-driven secondary tillage implements operating on silt soil (working width approximately 3 m, working depth approximately 0.05 m (after Perdok, 1976) Implement

Forward speed (m/s)

Spring tine harrow (pure-draught) Oscillatory harrow (power-driven) Rotary-horizontal axis (power-driven) Rotary-vertical axis (power-driven)

1.1–2.0

Power supplied by Specific energy tractor p.t.o. (%) consumption (J/dm3) 0





Crumbling effect % clodsB20mm 10

42–45

6.7–6.9

0.7–1.4

50–63

119–150

6.4–8.0

0.4–1.3

69–92

81–263

7.5–8.3

0.6–1.2

89–95

95–316

7.4–8.3

attached, providing a convenient one-pass tillage and crop establishment combination implement (Fig. 1). Consequently, many European farmers currently rely heavily upon power-driven secondary tillage machinery, despite poor performance in terms of operating costs and workrate. Hence, any improvement in operational efficiency which may be achieved, possibly through the utilisation of electronic control technology, would be both welcome and surprisingly cost-effective. A 10% reduction in machine operating costs, spread over an annual usage of 200 ha, would amount to a saving of £480 per year A £1000 electronic control system could, therefore, be paid for in 2–3 years, but a typical machine working life (assuming 200 ha annual usage) could potentially be in excess of 8 years (Nix, 1999). These figures are admittedly speculative; actual savings may be nearer 5% or the control system may cost more than estimated, however, the estimates are not entirely divorced from reality.

3.2.2. Parameters requiring control Power harrow workrate and operating costs are dependent upon a number of factors, including the depth of seedbed to be produced, the aggregate size reduction required and the strength of the oversize soil aggregates to be broken (largely a function of soil type and moisture content). Adequate seedbed depth and aggregate size reduction should be achieved to suit the following crop, but without excess (Estler and Scho¨nhammer, 1982). It is desirable to minimise tillage depth (and, therefore, energy requirement) within cultural limits, but the inherent in-field variability of soil aggregate strength and size distribution results in a varying tillage (and, therefore, energy) requirement across any given field. 3.2.3. Control of working depth Improved control of power harrow working depth, and consequent avoidance of inadvertent cultivation below the target operating depth, would substantially reduce

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energy wastage during p.t.o.-driven secondary tillage operations, given that power harrow p.t.o. power requirement is directly proportional to machine working depth (Bentley et al., 2000). Additionally, improved depth control would yield greater seedbed uniformity, thereby encouraging better crop establishment (Heege, 1993). To achieve this, some form of working depth sensor will be required and it is likely that non-contact ultrasonic transducers (Rifal, 1992; Tischler and Moore, 1992; Yasin et al., 1992; Hofmann, 1993; Romes, 1994) will satisfy this requirement (Fig. 4). However, the surfaces upon which these sensors will be required to operate may well be rough (e.g. ploughed), complicating the task of accurate depth sensing, but selective conditioning of the sensor output signal, perhaps in relation to the degree of surface roughness being experienced, will potentially alleviate this problem. Adjustment of working depth, once a set-point error had been established, would be the responsibility of either the tractor implement hitch control sub-system via the conventional three point (3 pt.) linkage, or the (tillage) implement control sub-system via external (hydraulic) actuators (Fig. 4). Many secondary tillage implements ‘float’ on the tractor implement hitch, their working depth being controlled by the vertical position of crumbler/packer rollers, relative to the soil-engaging tines/blades (Fig. 4). Hence an arrangement could be envisaged whereby a power harrow could modify its working depth by adjusting the vertical position of its packer roller, using on-board sensors, electro-hydraulic valves and actuators, but deriving hydraulic power from the tractor (Scarlett et al., 1997).

3.2.4. Control of seedbed quality It is desirable for secondary tillage implements to produce seedbeds of adequate and uniform quality at the highest workrates possible. Seedbed quality is a subjective term incorporating a number of parameters, of these soil aggregate size

Fig. 4. Tractor and power harrow integrated control system.

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distribution offers the greatest opportunity for integrated control. Ideally, the tillage effect of an implement would be tailored to suit local soil aggregate conditions, to produce a seedbed of uniform quality, despite soil type variations. This operating strategy would ensure that recalcitrant field areas receive tillage of adequate severity, whilst permitting workrate to be maximised in less demanding areas. This would improve energy utilisation and produce a higher quality seedbed for the following crop, the latter encouraging both a more uniform and higher overall level of seedling emergence (Estler and Scho¨nhammer, 1982). Practical implementation of a ‘seedbed quality control’ system would, however, require means of determining and subsequently modifying the quality of seedbed being produced by the tractor-tillage machine combination. Power (p.t.o.)-driven tillage implements offer the greatest opportunities for tillage effect control, but assessment of the seedbed quality level being produced by a moving field machine, is not a simple matter. Soil aggregate size distribution is important, but the distribution of aggregate sizes throughout the soil vertical profile may also be of consequence. The tillage effect required is largely dependent upon the desired reduction in soil aggregate size and the local soil strength. Consequently, it may be necessary not only to assess the seedbed quality (soil aggregate size distribution) being produced by the implement, but also possibly the strength of the uncultivated soil being encountered, and perhaps the aggregate size distribution entering the machine. A thorough approach of this type may be required in order to achieve ultimate seedbed uniformity, but the costs associated with multiple sensors may be prohibitive. An indication of soil strength may possibly be provided by some form of instrumented tine (Stafford and Hendrick, 1988) mounted in front of the tillage machine, assuming care is taken to ensure such a device does not run in the path of the tractor wheels or collect previously buried trash. Alternatively p.t.o. driveline torque will provide an instantaneous indication of soil strength, but will also be dependent upon machine working depth, forward speed and rotor speed (Bentley et al., 2000). Knowledge of these relationships may permit derivation of soil strength, given that the magnitudes of depth, forward speed and rotor speed are known. Assessment of soil aggregate size distribution is an equally difficult problem and requires further research effort: however, a number of approaches indicate promise. Stafford and Ambler (1990) utilised image analysis techniques to determine soil aggregate size distributions in seedbeds, and proposed that such technology could form the sensing component of an automatic control system for secondary tillage machinery. Vision techniques suffer from the fact that they can only assess the aggregate size distribution upon the seedbed surface, but performance of the system (Stafford and Ambler, 1990) compared favourably with seedbed assessment by a panel of experts. In the past, the utilisation of vision systems in harsh agricultural environments has required care, due to the conflicting requirements of performance, reliability and low-cost. Fortunately, product costs are reducing, inviting further exploitation of this technology. Other potential seedbed quality assessment techniques exist, but all currently require further development. Scarlett et al. (1997) demonstrated a correlation

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Fig. 5. Likely effects of rotor and forward speed upon p.t.o.-driven rotary cultivator performance.

between the signal produced by an ultrasonic working depth sensor, or rather the degree of scatter present upon the signal, and aggregate size distribution, by considering seedbed surface roughness to be directly related to aggregate size distribution. Alternatively, a representative signal may possibly be derived from the vibrational behaviour of a light spring tine in contact with the upper layers of the seedbed. Workers have used ultrasonic sensors, usually arranged in arrays, to successfully detect green crop windrows (Hofmann, 1993) and soil seedbed profiles (Robichaud and Molnau, 1990; Kitani et al., 1992). However, this approach may not necessarily be the most cost-effective means of acquiring aggregate size distribution data. Having assessed the seedbed quality being produced, the implement’s tillage effect must be adjusted to a suitable level. The tillage effect of p.t.o.-driven rotary cultivator, e.g. power harrow, is largely dependent upon the rotor/tine speed of the implement and the forward speed of the tractor (Fig. 5). The former affects the amount of energy available to fracture soil clods during their passage through the machine. Also, rotor speed, in combination with tractor forward speed, determines the effective ‘bite length’ of the implement, i.e. the distance on the soil surface (in the direction of travel of the tractor) between successive tine/soil contacts. In general, increasing the bite length reduces the severity of the tillage effect, causing less aggregate size reduction (Estler and Scho¨nhammer, 1982), and vice versa. This scenario creates an optimisation problem. A high forward speed is desirable in order to maximise workrate, but the higher rotor speeds which would be necessary to maintain bite length and tillage effect at acceptable levels can consume disproportionate amounts of energy (Fig. 5). A trade-off between workrate and energy consumption is required, in order to maintain adequate seedbed quality. Upon encountering a patch of stronger soil, a seedbed quality control system could

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cause tractor forward speed to be reduced (change transmission gear) or power harrow rotor speed to be increased (via a variable-ratio driveline), the latter assuming adequate engine power reserves exist to satisfy the additional energy demand (Fig. 5). Either course of action would reduce the implement bite length, increase the tillage effect and hopefully bring about the required aggregate size reduction. A seedbed quality control strategy could embody the following requirements: “ Maintain seedbed quality — vary implement tillage effect (within machine capabilities). “ Maximise workrate — maximise forward speed (within practical limits). “ Utilise engine power — maximise workrate (within tractor engine power limits).

3.2.5. Other control opportunities As with primary tillage, it is possible that data acquired during secondary tillage operations could be used to influence the nature of subsequent agronomic operations. Combining cultivation resistance or seedbed quality information with in-field position information (Stafford and Ambler, 1994) would enable spatial variation maps of these parameters to be produced. This might initially appear to be a great deal of information of limited value, but a number of potential uses may well develop. For instance, the effectiveness of pre-emergence herbicides is dependent upon seedbed quality, as large soil aggregates reduce pesticide efficacy. If reduced rates of pesticide are to be applied, as is environmentally desirable, in-field seedbed quality information could be used to identify field areas that require higher dose rates in order to maintain acceptable levels of weed control, whilst still reducing overall pesticide usage. A similar approach could be applied to the spatial variation of crop seeding rate (see Section 4.4), given the dependence of cereal crop percentage emergence upon seedbed quality (Heege, 1993, see Section 4.3).

4. Crop establishment machinery

4.1. General With the exception of transplanting machinery, whose purpose is to place growing plants in the soil, crop establishment machinery deals exclusively with dormant biological material such as seeds and tubers. The various types of machine used for crop establishment are usually defined according to the type of material they handle and their relative accuracy of planting depth and seed/tuber spacing. In Northern Europe general-purpose drills are used for sowing the majority of combinable crops (e.g. cereals, peas, beans, oilseed rape, linseed), typically in rows 100 – 175 mm apart. Historically, these machines have not attempted to control individual seed spacing in the row; rather the crop is established according to a desired ‘overall’ seed rate, defined in terms of seed mass per unit area (kg/ha). In actual fact a seed rate is usually selected for a given crop/field environment, to provide an established crop of suitable plant population or plant density (plants per

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m2) for optimum yield. Hence the seed rate chosen should reflect the seed bulk density, its germination potential and the local field and weather conditions. Single-seed or precision drills are used primarily for sowing root and field vegetable crops in circumstances where both the seeding depth and inter-seed spacing must be controlled. Again the overall objective is to obtain a established plant population suitable for optimum yield, but individual plant spacing has a significant effect upon crop uniformity and yield potential. Similarly, seeding depth variations can adversely effect plant establishment and subsequent yield.

4.2. Parameters requiring control General-purpose drills perform the majority of European crop establishment and have considerable potential for the application of control systems. Seed can represent a considerable proportion of variable costs associated with crop husbandry (seed, fertiliser and agrochemicals, Fig. 6), current estimates suggest seed accounts for 20% of winter wheat, 37% of linseed and 44% of winter field bean variable costs (Nix, 1999). Because of this, and also the potential consequences of poor crop development and subsequent yield, the crop establishment process must be performed to a high standard. For successful crop establishment the seed should be in intimate contact with the soil, sufficiently deep in the soil profile to provide access to moisture, whilst not so

Fig. 6. Combinable crops variable costs (after Nix, 1999).

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deep as to discourage seedling emergence. It should be adequately covered with sufficiently firm soil so as to discourage vermin and prevent potential damage from pre-emergence herbicides, whilst not restricting root development. Finally, the seed should be applied at an adequate spatial density to produce a suitable final plant population, on emergence, for optimum crop performance. All this must be achieved at a forward velocity of up to 3 m/s. Current general-purpose seed drills perform remarkably well in the light of these requirements, but perhaps electronic control systems may enhance their performance, specifically in the areas of seeding depth and seed rate control.

4.3. Control of seeding depth Target seeding depths are typically unique to the particular crop being established. If moisture is readily available throughout the seedbed profile, seeding depth usually reflects the physical size of the seed being sown-larger seeds being placed deeper. Heege (1993) comprehensively reviewed recent research relating to crop establishment techniques for cereals, oilseed rape and beans. He considered typical seeding depths for small grains (rape, cereals), in German conditions, to be in the range 25 – 45 mm. In good seedbed conditions, prepared by conventional ploughing and secondary cultivations, the standard deviation of seeding depth achieved by conventional general-purpose drills was in the range 6–11 mm, increasing to 12–17 mm in poorer conditions. Increasing seeding depth standard deviation from 6 to 16 mm reduced cereal crop field emergence from 82 to 60% (Heege, 1993). Consequently, with conventional crop establishment equipment, the importance of producing a good quality seedbed is evident. Alternative seed drill designs (e.g. Va¨derstad, Simba) attempt to reduce cultivation and crop establishment costs by incorporating cultivation, levelling and consolidation elements, in conjunction with heavy-duty coulter assemblies, into a single machine. This can enable drilling to be performed straight on to ploughed or minimally-cultivated land, whilst achieving good control of seeding depth. However, disadvantages of such equipment include high capital cost and very high power requirement, typically twice that of a conventional drill; limiting its economic justification to large farmers and contractors. Improved control of seeding depth, to reduce variation, leads to more uniform germination of a higher percentage of the seed sown. This in turn produces a more uniform crop, with less variation in growth, which is subsequently easier to target with chemical applications (Heege, 1993). The effectiveness of an increasing proportion of chemical treatments, especially at cost-saving reduced application rates, is highly dependent upon the growth stage of the crop in question. Hence, improved quality control of the crop establishment process offers potential cost savings, not only in terms of the quantity of seed required to obtain the desired final plant population, but also the amount of chemical active ingredient required to treat the established crop during the growing season. The potential techniques for seeding depth control are heavily dependent upon drill design. European general-purpose drills are usually supported by their own

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Fig. 7. Operating arrangement of a conventional general-purpose drill coulter assembly. Coulter pressure requires adjustment to maintain a consistent seeding depth in different soil conditions (a) soft soil, (b) hard soil.

wheels, whether they are trailed behind the tractor or mounted on the tractor implement hitch. In either case the drill coulters or openers, which cut grooves in the soil into which the seed is dropped, usually comprise trailing arm assemblies, attached to the drill frame so they can move in the vertical plane. Individual spring assemblies are typically set to operate vertically, at some point along the coulter arm, thereby forcing the coulter into the soil when pressurised by means of an actuator and linkage (Fig. 7). Consequently, vertical pressure on the coulter-arm can be varied and the seeding depth obtained is a function of local soil resistance (Fig. 7). This works adequately in uniform seedbed conditions, but in variable soil conditions seeding depth variation can be considerable (Heege, 1989). Precision (single-seed) drills and certain general-purpose drills use depth limiting wheels or skids to prevent excessive coulter penetration. This approach works well in the higher quality seedbeds typically experienced by precision drills, but it can encounter difficulties in the cloddy conditions often found in cereal crop seedbeds (Heege, 1989). Also, additional weight of the depth limiting wheels/skids can add significantly to the mass of a general-purpose drill.

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Given that variations in seedbed soil resistance will be encountered during crop establishment operations, no matter how much care is taken during seedbed preparation, the implementation of electronic sensors and control systems can indicate variations in drill coulter operating depth to the operator and, if desired, take remedial action. Heege (1989) describes an electronic system, devised to measure and indicate drill coulter operating depth (Fig. 8a). An ultrasonic non-contact distance sensor is mounted on one (or more) of the drill coulter arms, ideally only a few centimetres in front of the coulter, and measures the height of the coulter arm above the soil surface. Knowledge of the coulter arm geometry enables calculation of the coulter working depth, which is subsequently displayed to the operator via an in-cab console. Upon noting an undesirable depth variation, the operator can modify the drill coulter pressure setting, via a hydraulic actuator, and thereby hopefully re-attain the desired seeding depth. The next logical step in the development of this control system (Auernhammer, 1989) is to include an electronic control unit (ECU) and electro-hydraulic valve to automatically adjust drill coulter pressure in response to sensed coulter depth variations, in order to achieve a constant seeding depth despite soil strength variations (Fig. 8b). Dyck et al. (1985) developed a similar system for the control of tillage/seeding depth of trailed cultivators and seeders used in prairie conditions. A number of ultrasonic sensors were fitted to a cultivator or seeder framework, to indicate its relative height above ground. This information was relayed to an electronic control unit which, in

Fig. 8. Methods of seeding depth control (a) seeding depth indication system-requires manual error correction, (b) automatic seeding depth control system (after Auernhammer, 1989).

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conjunction with ‘desired working depth’ data supplied from an operator’s control panel, adjusted the implement’s working depth by means of electro-hydraulic valves connected to hydraulically-actuated depth wheels. No attempt was made to control individual tine or coulter working depth.

4.4. Control of seed rate The control of seed rate may be addressed in numerous ways. The majority of current general-purpose drills utilise seed metering mechanisms driven by groundengaging wheels. Seed is delivered on a volumetric basis, the delivery rate being dependent upon the rotational speed of the metering mechanism and the area of the mechanism in contact with the seed flow. No attempt is made to control seed spacing in the row, the aim being to apply a consistent seed mass per unit area of the seedbed (kg/ha). The accuracy of this process can be affected by a number of factors including seed size and bulk density (Heege, 1993), but within a given batch of seed, variation is unlikely to be large. Variation in slip of the drill mechanism driving wheel(s) is a much greater source of error within a given field, leading to variations in seed delivery rate. This problem has been solved by a number of manufacturers by driving the seed metering mechanism independently, via variable-speed electric or hydraulic motors. Tractor-drill true forward speed data, typically derived from a Doppler radar-type sensor, is supplied to a combined electronic control unit (ECU) and operator interface; the latter permitting selection of the desired seed rate, plus entry of seed bulk density data and other relevant factors. The metering mechanism target speed is subsequently calculated and the motor speed adjusted accordingly. Independent drive of the drill metering mechanism can also enable seed rate to be adjusted in response to other parameters, which may include seedbed quality (see Sections 3.2.4 and 3.2.5) or yield potential of a particular in-field location. Until recently the commercial implementation of this feature had been limited to a facility whereby the operator could increase or decrease the seed rate by a pre-determined margin (usually 10%) via buttons on the operator interface. In this form, the operator must determine which areas of the field require such differential treatment and remember to apply it, an operating philosophy which increases driver workload and fatigue (Scarlett, 1993). A superior technique (Fig. 9) would incorporate a dynamic means of in-field position determination (Stafford and Ambler, 1994) and a pre-prepared treatment map, derived from former season’s crop yield map data and other relevant agronomic or treatment information. Seed rate would be automatically varied as the field is drilled, without the necessity for operator intervention, although no doubt a manual override facility would be provided. Systems utilising these basic principles have been developed and evaluated by Neuhaus and Searcy (1993), Anderson and Smette (1994). The nature of the operating strategies for such systems is the subject of much current research, but a potential operational technique may be to increase seed rate in areas of poor seedbed quality (and consequent poor crop establishment) and also in areas of high yield potential. Seed rate could then be reduced in areas of good seed bed quality

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Fig. 9. Tractor and drill integrated control system, featuring automatic control of seed rate with respect to in-field position.

(good crop establishment) and also in areas of poor yield potential. Whatever strategy is selected, the overall objectives remain the same, namely to maximise crop yield potential whilst minimising inputs, in order to maximise overall profitability. Commercial crop establishment systems embodying the necessary control hardware and software, usually as an integral part of a ‘precision farming’ system, are now becoming available from a number of manufacturers, but as yet their use is not widespread. Nonetheless, at current costs (Nix, 1999), a 10% reduction in seed usage for 200 ha of winter wheat would return a saving £960 per year (see Table 2), at these levels control system investment could probably be justified in 2 – 4 years of operation. As mentioned earlier (see Section 4.1), seed rate is selected for a given crop/field situation to provide an established crop of suitable plant population or plant density (plants per m2) for optimum yield. The seed rate chosen should, therefore, reflect the seed bulk density, its germination potential and the likelihood of plant losses. Variations in seed size and density between different crop types, and even between different varieties of the same crop, necessitates frequent calibration of Table 2 Financial savings which may result from the application of integrated control system(s) to certain implements on a UK farm producing 200 ha of winter wheat (after Nix, 1999) Operation

Potential cost saving (£/ha)

Primary cultivation Secondary cultivation (one pass only) Crop establishment Total annual cost saving

3.6 2.4 4.8

Annual cost saving (£) 720 480 960 2160

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seed metering mechanisms; a time consuming activity which reduces output during a hectic seasonal period. Heege (1989, 1993) suggested that it would be preferable, both from an agronomic and operational viewpoint, to control seed rates in terms of seed numbers per unit area. This would require a sensor system capable of determining the output of the drill metering mechanism(s) in real-time, in terms of number of seeds supplied to an individual coulter unit. From this, the number of seeds per unit area could be calculated and controlled (via an independent metering mechanism drive), irrespective of seed size and density. Heege (1989) states that despite the use of photoelectric techniques to indicate seed flow in precision (single-seed) drills (Holtmann, 1997), reliable methods have yet to be developed for the larger seed flows encountered in general-purpose drills. A technique under development involves the installation of a piezoelectric sensor in certain of the drill seed delivery tubes. Seeds falling from the metering mechanism collide with the sensor, which indicates their passage to the coulter unit, this information may be used to adjust seed delivery rate, via the drill control system, to obtain the target seed population in a given in-field location. The accuracy of such a system would undoubtedly be highly dependent upon the quality of sensor information received, but research developments in this area show promise (Heege and Feldhaus, 1997).

4.5. Other control opportunities The ability to control seeding depth by means of an electronic logic-based control system can open other potential control avenues. Price et al. (1991), Carter and Chesson (1993) explored the technique of adjusting seeding depth in response to variations in the seedbed soil moisture profile, to ensure that seeds are placed in sufficiently moist soil in order to encourage germination. The workers used sensors operating on the principles of variations in soil bulk electrical resistance and near-infrared (NIR) reflectance in response to changes in soil moisture content, respectively. Excessive seed placement depth can arise from an operating strategy of planting deeper in dry conditions, in search of moisture, thereby potentially reducing seedling emergence. Carter and Chesson (1993) overcame this problem by utilising a narrow blade to remove a layer of soil immediately in front of the coulter unit, exposing a new soil surface below which seeding depth was gauged in a conventional manner. The depth of soil removed by the blade was in proportion to the depth in the seedbed profile of adequate soil moisture for seed germination. Whilst this approach ensures a constant seeding depth below the immediate soil surface, and may locate adequate soil moisture for seed germination, it would not be suitable for crops grown in narrowly-spaced rows e.g. cereals, oilseed rape and pulses. Practical restrictions relating to the soil moving hardware (and the volume of soil to be moved) would be excessive, but this would not be the case for crops such as corn (maize) and cotton, grown on wide-spaced (approximately 0.75 m) rows. Whalley and Stafford (1992) reviewed potential techniques for real-time sensing of soil moisture content from mobile machinery. Subsequently, ground-engaging sensors utilising microwave attenuation (Whalley, 1991) and capacitive (Whalley et

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al., 1992) principles of operation were field tested, each being designed to be incorporated within a narrow cultivator tine. Unfortunately, these methods of soil moisture content measurement were found to be affected to an extent by soil structure and bulk density. Despite this apparent drawback, future crop establishment equipment may well incorporate sensors for soil moisture content and other parameters considered relevant.

5. Summary and conclusions In a field crop production context, control system integration attempts to automate coordination between tractor and implement control sub-systems, and thereby improve tractor-implement operational efficiency (by performance or economy maximisation), improve the quality of work produced and reduce operator workload and fatigue. Potential opportunities associated with the application of integrated control techniques to certain types of European agricultural implements have been reviewed. Specific implement types, and their parameters which have been considered include primary cultivation machinery (control of working depth and working width), secondary cultivation machinery (control of working depth and seedbed quality) and crop establishment machinery (control of seeding depth and seed rate). Outline control strategies have been proposed for these applications, and sensors and other hardware required to implement the control systems have been identified. Finally, speculation has ensued regarding the agronomic and economic benefits which may result from the implementation of the proposed technology. Regarding economic benefits, a potential cost saving of 10% has been suggested in these applications. There is little or no evidence to support this speculation, as few of the control systems proposed exist in the form described, as yet. It is unlikely that operational cost savings will exceed 10%; they may well be only half of this figure. However, for indicative purposes only, if the cropping regime of a fictitious farm comprised 200 ha of winter wheat and 10% cost savings per operation were achieved, at current (1999) costs, the annual financial savings would be as indicated in Table 2. This is admittedly a simplistic representation, but it raises some important points. “ The farm considered is not large by UK standards; a larger enterprise would achieve greater (pro rata) savings, whereas the cost of the control system(s) would be similar. “ The high degree of hardware commonality which exists between the integrated control system(s) proposed for these different agronomic applications, spreads the overall cost, making the system(s) easier to justify. “ The savings outlined in Table 2 are not insignificant and would probably exceed the additional cost of the control system(s) in two or three seasons. “ Even if the actual cost savings are only half of those speculated, it may still be possible to justify integrated control technology, albeit over a larger farm area or longer machine working life; both of which are increasingly common characteristics of UK arable crop production.

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It is suggested that integrated control technology could increase tractor-implement performance and/or reduce operating costs in the applications considered. Of these, secondary cultivation machinery would probably benefit most from the proposed control system(s) for, contrary to Table 2, in many conditions more than one pass is required to produce a suitable seedbed. A reduction in the number of passes required, thereby potentially doubling a particular machine’s effective annual cultivation capability, will increase the potential economic benefit associated with control system implementation in this application.

Acknowledgements The author gratefully acknowledges the financial support provided by the Ministry of Agriculture, Fisheries and Food (MAFF).

References Alihamsyah, T., Humphries, E.G., 1991. On-the-go soil mechanical impedance measurements. In: Proceedings of the Conference on ‘Automated Agriculture for the 21st Century’, Chicago, IL, 16 – 17 December 1991. ASAE, St. Joseph, MI. Anderson, N.W., Smette, M.W., 1994. Automation of an air drill system. In: Watson, D.G., Zazueta, F.S., Harrison, T.V. (Eds.), Proceedings of the Fifth International Conference on ‘Computers in Agriculture 1994’, 6–9 February 1994, Orlando, FL. ASAE, St. Joseph, MI, pp. 71 – 75. Auernhammer, H., 1989. Electronik in Traktoren und Maschinen (Electronics in Tractors and Machines). BLV Verlagsgesellschaft mbH, Munich, p. 188. Auernhammer, H., 1993. Requirements for a standard for tractor-implement communications. ASAE Paper No. 93-1532. Presented, Chicago, IL, December 1993. Bea, S., 1997. Nutzwertanalyse eines neuen stufenlosen leistungsverzweigten Traktorgetriebes (benefits of a new continuously variable power-split tractor transmission). Agrartechnische Forschung 3 (1), 28–33. Bentley, D., Scarlett, A.J., Godwin, R.J., 2000. Characterising the operational behaviour of a powerdriven rotary harrow. In: Proceedings of the Fourth International Conference on Soil Dynamics, 26–30 March 2000, Adelaide, South Australia. Bosch, R., 1991. Controller area network, version 2.0. Robert Bosch GmbH, Postfach 30 02 40, D-70442, Stuttgart, Germany. Brunotte, D., Seeger, J., 1999. Kommunikation von motor und getriebe u¨ber CAN-bus (communication between engine and transmission via CAN-bus). Agrartechnische Forschung 5 (1), 54 – 67. Carter, L.M., Chesson, J.H., 1993. A soil moisture seeking planter control. ASAE Paper No. 93-1533. Presented, Chicago, IL, December 1993. Chamen, W.C.T., Cope, R.E., Longstaff, D.J., Richardson, C.D., 1994. The energy efficiency of seedbed production following mouldboard ploughing. In: Jensen, H.E., Schjønning, P., Mikkelsen, S.A., Madsen, K.B. (Eds.), Proceedings of the 13th International Conference of International Soil Tillage Research Organization, 24–29 July 1994, Aalborg, Denmark, pp. 1227– 1232. Dyck, F.B., Wu, W.K., Lesko, R., 1985. Automatic depth control for cultivators and air seeders developed under the AERD program. In: Proceedings of the Agri-Mation 1 Conference and Expo., 25–28 February 1985, Chicago, IL. ASAE, St. Joseph, MI, pp. 265– 276. Estler, M., Scho¨nhammer, J., 1982. Working effect of soil preparing equipment with p.t.o.-driven tools and its influence on plant emergence. In: Proceedings of the Ninth Conference of International Soil Tillage Research Organization, Osijek, Yugoslavia, pp. 609– 614.

190

A.J. Scarlett / Computers and Electronics in Agriculture 30 (2001) 167–191

Heege, H.J., 1976. Comparison of implements for secondary cultivation. In: Proceedings of the Seventh Conference of International Soil Tillage Research Organization, Sweden. Heege, H.J., 1989. Saattiefe und Saatmenge Steuern (the control of sowing depth and seed rate). DLG-Mitteilungen 104 (21–22), 1108– 1109. Heege, H.J., 1993. Seeding methods performance for cereals, rape and beans. Trans. ASAE 36 (3), 653–661. Heege, H.J., Feldhaus, B., 1997. On the go control of seeds per unit area for grain drills. ASAE Paper No. 97-3061. Presented, Minneapolis, MI, USA, August 1997. Hesse, H., Withington, G., 1992. Digital electronic hitch controls for tractors. In: Proceedings of the IMechE/Silsoe Link Conference ‘Fluid Power in Agricultural Machinery’, 4 November 1992, Silsoe College, Bedford, UK, IMechE. Hobbs, J., Hesse, H., 1980. Electronic/hydraulic hitch control for agricultural tractors. SAE Paper 801018. Presented, International Off-Highway Meeting and Exposition, Milwaukee, WI, 8 – 11 September 1980. Society of Automotive Engineers, Warrendale, PA. Hofmann, L., 1993. Schwadabtastung mit Ultraschall: Mo¨glichkeiten und Grenzen (window scanning with ultrasonics: possibilities and limitations). Landtechnik 48 (5), 266– 268. Holtmann, W., 1996. Electronics on Fendt tractors: a voyage through the tractor via computer. Profi Int. 96 (11), 56–59. Holtmann, W., 1997. Kleine Synchrodrive: precision electronics for precision drills. Profi Int. 97 (8), 58–60. Holtmann, W., 1999. Same SDS sequential drive system: one button-eight operations. Profi Int. 99 (10), 46–48. International Standards Organisation, 2000. ISO 11783 (Draft) Tractors, machinery for agriculture and forestry-Serial control and communications data network. In: Proceedings of the Working Group ISO/TC23/SC19/WG1. International Organisation for Standardisation, Case Postale 56, CH1211, Geneve 20, Switzerland. Isensee, E., Lu¨th, H.-G., 1992. Kontinuierliche Messung der Bodendichte (continuous recording of soil density). Landtechnik 47 (9), 449–452. Jahns, G., Speckmann, H., 1992. An open agricultural bus standard. ASAE Paper No. 92-3006. Presented, Charlotte, NC, June 1992. Jaufmann, A., 1997. Potential eines traktormanagementsystems (potential of a tractor management system). Landtechnik 52 (6), 290–291. Kitani, O., Kashiwazaki, T., Okamoto, T., Torii, T., 1992. Ultrasonic sensing system to measure fields. ASAE Paper No. 92–3007. Presented, Charlotte, NC, June 1992. KTBL (Kuratorium fu¨r Technik und Bauwesen in der Landwirtschaft e.V.), 1993. Landwirtschaftliches Bus-System-LBS: Normung und Stand der Realisierung (mobile agricultural BUS-system-LBS: standardization and state of the art). In: Proceedings of the Conference, 30 November 1993, Frankfurt/Main. Arbeitspapier 196, ISBN 3-7843-1841-X. KTBL, Bartningstraße 49, 64289 Darmstadt, p. 200. McMullan, T.A.G., 1986. Implement coupling and control research at the NIAE: 1979– 1985. (confidential) ITD Report 13, BSRAE, National Institute of Agricultural Engineering, Silsoe, UK. Neuhaus, P.E., Searcy, S.W., 1993. Variable planting density and fertilizer application system. ASAE Paper No. 93-1554. Presented, Chicago, IL, 14 – 17 December 1993. Neunaber, M., 1997. Simply stepless: Profi test of the Fendt Favorit 926 Vario tractor. Profi Int. 97 (1), 10–16. Nix, J., 1999. Farm Management Pocketbook, 30th ed. Wye College, University of London, UK, p. 244. Paul, W., 1992. Zugkraftmessungen zur teilschlagkartierung (tractive force measurements for partial plot mapping). Landtechnik 47 (10), 490– 491. Pearce, A., 1987. Furrowing the brow’-plough developments. Power Farming 67 (9), 28 – 31. Perdok, U.D., 1976. Multi-powered soil tillage implements. In: Proceedings of the Seventh Conference of International Soil Tillage Research Organization, Sweden. Price, R.R., Huang, X., Gaultney, L.D., 1991. Development of a soil moisture meter to predict corn seed planting depth. ASAE Paper No. 91-3525. Presented, Chicago, IL, 17 – 21 December 1991.

A.J. Scarlett / Computers and Electronics in Agriculture 30 (2001) 167–191

191

Renius, K.T., 1994. Trends in tractor design with particular reference to Europe. J. Agric. Eng. Res. 57, 3–22. Rifal, M.N., 1992. Evaluation of an automatic depth controller. Agric. Mechanisation Asia, Africa Latin America, 23(3) 28–30, 33. Roberts, M., 1999. MF 8200 tractors: take a CAN bus trip around the big new Masseys. Profi Int. 99 (9), 47–49. Robichaud, P.R., Molnau, M., 1990. Measuring soil roughness changes with an ultrasonic profiler. Trans. ASAE 33 (6), 1851–1858. Romes, R., 1994. Electro-hydraulic header control for combine harvesters. SAE Paper 941799. Presented at International Off-Highway and Powerplant Congress and Exposition, Milwaukee, WI, 12 – 14 September 1994. Society of Automotive Engineers, Warrendale, PA. Scarlett, A.J., 1993. Integration of tractor engine, transmission and implement depth controls: part 2, control systems. J. Agric. Eng. Res. 54, 89 – 112. Scarlett, A.J., Lowe, J.C., Semple, D.A., 1997. Precision tillage: in-field, real-time control of seedbed quality. In: Proceedings of the First European Conference on Precision Agriculture, Warwick, UK, pp. 503–510. Schueller, J.K., 1988. A proposed scheme for integrated tractor-implement communication and control. ASAE Paper No. 88–1533. Presented, Chicago, IL, 13 – 16 December 1988. Stafford, J.V., Hendrick, J.G., 1988. Dynamic sensing of soil pans. Trans. ASAE 31 (1), 9 – 13. Stafford, J.V., Ambler, B., 1990. Computer vision as a sensing system for soil cultivator control. In: Proceedings of the Institution of Mechanical Engineers C419/041, pp. 123– 129 Stafford, J.V., Ambler, B., 1994. In-field location using GPS for spatially variable field operations. Comput. Electron. Agric. 11, 23–36. Stepper, M.R., 1993. J1939 high speed serial communications, the next generation network for heavy-duty vehicles. ASAE Paper No. 93-1529. Presented, Chicago, IL, 14 – 17 December 1993. Stone, M.L., Zachos, M., 1993. Application of J1939 networks in agricultural equipment. ASAE Paper No. 93-1530. Presented, Chicago, IL, 14 – 17 December 1993. Tewes, G., 1993. Tracktormanagement: Optimierung durch konsequente Anwendung vorhandener Technik (tractor management: optimising by consistent use of existing technology). Landtechnik 48 (10), 510–513. Tinker, D.B., 1991. Integration of tractor engine, transmission and implement depth controls — a literature review: part 1, transmissions. Divisional Note DN 1602. Silsoe Research Institute, Silsoe, UK, p. 48. Tischler, C.R., Moore, G.A., 1992. Evaluation of ultrasonic depth controllers. Agric. Eng. Aust. 21, 1–6. Vahlensieck, B., 1997. Steigende motorauslastung durch geregelte stufenlose getriebe (increasing engine utilisation rates with controlled continuously variable transmissions). Landtechnik 52 (5), 234– 235. Whalley, W.R., 1991. Development and evaluation of a microwave soil moisture sensor for incorporation in a narrow cultivator tine. J. Agric. Eng. Res. 50, 25 – 33. Whalley, W.R., Stafford, J.V., 1992. Real-time sensing of soil water content from mobile machinery: options for sensor design. Comput. Electron. Agric. 7, 269– 284. Whalley, W.R., Dean, T.J., Izzard, P., 1992. Evaluation of the capacitance technique as a method for dynamically measuring soil water content. J. Agric. Eng. Res. 52, 147– 155. Yasin, M., Grisso, R.D., Lackas, G.M., 1992. Non-contact system for measuring tillage depth. Comput. Electron. Agric. 7, 133–147. Young, S.C., Sokol, D.G., Strosser, R.P., 1993. Utilization of CAN technology in a distributed control system. ASAE Paper No. 93-1535. Presented, Chicago, IL, 14 – 17 December 1993.

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