Implement Coupling

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Copyright IFAC Intelligent Componenu md Instruments for Control Applications. Budapest, Hungary. 1994

COMPUTER AIDED TRACTOR/IMPLEMENT COUPLING M. GRAEF and G. JAIINS Institute of BiOlYstcms EDginecriD,. FcdcraI Apicultun1 Racarch CeIItre. 38116 BnuDlCbwei" Gcnnany

Abstract: Coupling tractors and implements cause severe accidents in agriculture. Automatic couplers are on the market. but they have a limited catching area. Because of restricted visibility. the operator often fails to meet this area. Therefore a system is proposed which is able to measure the distance and direction continuously, to calculate the required steering angle and to display it to the operator. Key words: Tractor, Implement, Coupling, Occupational Safety, Computer Simulation, Neural Network, Fuzzy Control, Ultrasonic Measuring System, On-Board Computer, Monitoring

1. INTRODUCTION

2. SOLUTION

Objectives are to improve occupational safety and driver comfort by technical means during coupling and uncoupling of agricultural tractors and implements.

To avoid the presence of the operator or any other person between tractor and implement during coupling; to avoid mounting or dismounting the tractor, and to avoid the operator twisting while driving backwards it is necessary: 1. to use automatic couplers for tractors and implements 2. to provide the right information to the operator, so that he can back the tractor up precisely into the catching area of the couplers even if he is not able to observe the implement.

The use of tractors and implements in agriculture is characterized by frequently changed implements, a task with the permanent risk of severe accidents [1,2]. To reduce this risk automatic couplers have been introduced. For proper operation the tractor has to approach the implement closely enough to bring the automatic couplers into action. If the operator fails to drive the tractor into the catching area where the automatic couplers come into action, then he or another person often steps into the space between tractor and implement to adjust the implement position and to connect the coupling elements manually.

To achieve this, a system continuously has to measure distance and direction between tractor and implement; to calculate the steering angle required to drive the tractor into the catching area and to display the required steering angle to the operator.

In the following, technical means are proposed to enable the operator to safely back up his tractor into the catching area of the couplers, even if the coupling area is not visible from his position. By these means working conditions are also improved.

The main parts of the project discussed here are shown in Figure 1. Distance and angle between tractor and implement can either be measured (left top) or generated by a model (left bottom). Three different ways to predict the required steering angle for successful coupling are indicated (middle), while at the right a user interface for manual control (top) or an automatic steering control is depicted. 283

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Fig. 2. Automatic couplers and threshhold values for angle of divergence (y) and lateral deviation (~yl{) limiting the catching area, beyond which the automatic couplers come into action

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2.1 . Automatic couplers

2.2.

Automatic couplers are already [3] on the market, especially for threepoint hitches. They are standardized or expected to be standardized in the near future [4]. Automatic couplers have a catching area. This is the area within which the automatic coupling mechanism comes into action. This area is defined by the design of the couplers. The Swedish Institute of Agricultural Engineering in Uppsala has investigated tractor/ implement hitchings and the effect of hitch design on the threshold values for the angle of divergence and the lateral deviation between tractor and implement. Because only beyond these values do the automatic couplers come into action [5, 6].

In order to provide the required information to the operator, it is necessary to measure angle and distance between tractor and implement, process these data and display the results in an ergonomic way. 2.2.1. Measurement of an~le and distance between tractor and implement For experimental purposes a laboratory test system has been set up. This system consists of two ultrasonic transmitters fixed to the implement and two receivers mounted on the tractor. From both transmitters an ultrasonic signal is transmitted alternatively. The beginning of each transmission is relayed by an infrared transmitter from the implement to the tractor, where an infrared receiver is mounted. Four distances between these two transmitters and two receivers are measured.

From left to right Figure 2 shows the A-frame coupler, the two-point coupler and the threepoint free-link hitch or link coupler. The latter is the most commonly used automatic coupler. This coupling system is standardized and can be used for nearly all purposes. It neither limits the necessary space nor shifts the center of gravity of the implement.

They are used to calculate the relative distance of tractor and implement. This system has been proved sufficiently for laboratory tests, but because active elements are used on the implement it is obviously not appropriate enough for agricultural practice.

The way a tracp tor approaches an implement depends on its ;I constructive maneuverability and the field III of vision of the rmin III operator. The maneuverabilII ity is set by dimensions of the tractor like (Figure 3) wheel base 1, wheel track s and radius of turn rmin . The distance of the Fig. 3. Tractor model instantaneous center of rotation P from the rear axle center with respect to the wheel base is the determining parameter of tractor maneuverability.

Therefore together with the Institut fUr zerstorungsfreie Pri1fverfahren (lfzP) of the Fraunhofer Gesellschaft (FhG) in Saarbrucken a system using only active elements on the tractor is under development. This system consists of an array of ultrasonic transmitters and one receiver mounted on the tractor. All elements on the implements are simple passive reflectors. The phased array principle well known for radar application is used to focus the ultrasonic beam and scan it across the area behind the tractor to detect distance and direction of the implement [8]. The system has no moving mechanical parts. For practical use a measuring range of about 5m and an angle of about 30° seems to be sufficient.

1/1

2.2.2. Data processin~ to predict the steering angle and distance. Assuming the tractor has been driven into the measuring range of the system, then distance and direction between tractor and implement will continuously be measured. From this, the desired steering angle to reach the catching area is calculated and displayed to the operator. Three different ways to predict the desired steering angle have been investigated: a kinematic model, a neural network and a fuzzy-controller.

The second condition affecting the coupling maneuver is the operator's field of vision. It is well understood and verified by investigations that the visibility severely affects the steering accuracy [7]. Adequate visibility is hindered by the increasing size of modem tractors, the operators' cab and its position.

285

driving curve according to the kinematic characteristics of the tractor steering system. For the calculation it is necessary to take into account the discrete control speed of the steering angle a parallel positioning of the tractor coupling elements relative to those of the implement at the end of the driving curve a geometrical compensation of different distances from the starting point to the coupling position.

To test and compare these alternatives, a simple tractor model according to Figure 3 was used. This is justified because driving backwards to couple an implement is only done at low speeds and angle and distance is measured frequently. Therefore it is not necessary to take slippage or other peculiarities into account. Kinematic model. The kinematic model is based upon the driving behaviour and the steering behaviour of tractors when backing up. The developed computer program calculates the feasible

Fig. 4. Kinematic design of driving curve segments, centrodes and line segments for compensation of distance to the coupling point ( point of inflection W 9)

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Figure 4 in particular shows the principle design of the driving curve with its centrodes and straight line segments for geometrical compensation of different distances from the starting point to the coupling position. The initial position is characterized by the tractor/implement distance Ko Ks ; by the appertaining angle ao between Ko Ks and the longitudinal axis of the implement; by the angle Bo between the xaxis and the longitudinal tractor axis. The latter is always the tangent to the driving curve. The driving curve is divided into circular arches, transition curves and straight lines to fulflll the above mentioned requirements. To get a smooth curve, transition conditions are part of the computer program. As mentioned, distance Ks Ko and angle measurement as well as calculation of the required steering ;angle are perfonned frequently while backing up.

The kinematic solution requires expert knowledge in kinematics to design the model. It offers precise solutions and additionally can predict if it is possible to reach the coupling point from the starting point. Neural network. A neural network is a model free estimator. It learns by examples. Therefore the data used to train the network have to be chosen carefully. To avoid expensive practical measurements to create these data the kinematic model according to Figure 3 has been used. A feed-forward network was used with four inputs for the four distances measured, eight hidden neurons and one output neuron for the desired steering angle. Different backpropagating learning algorithm were applied. The trained neural network was able to predict the required steering angle to guide the tractor into the de-

286

sired catching area of the automatic couplers. The achieved accuracy was within the demanded limits (Figure 2).

guage. The fuzzy--controller design has two linguistic variables, the tractor/implement distance Ko Ks and the appertaining angle aD (compare Figure 4). The output is the required steering angle. Minimal effort was needed to set up a rapid prototype by using a fuzzy development system which generated the C-Code of the controller. This prototype did go beyond all threshold values (Figure 2) for different starting positions. To achieve this a fine tuning was necessary.

Fuzzy--control. A fuzzy--controller may be an appropriate solution to avoid extensive efforts to model a system, or in cases where it is not possible to model a system. In order to establish a fuzzy--controller it is suffizient to articulate common or expert knowledge of a subject area. This holds true for driving a vehicle, because everybody able to drive a car or tractor will be able to describe the rules applied in common lan-

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Fig. S. Operator information displayed on a monitor

2.2.3. Operator infoonation Universal driver infoonation systems have been discussed and investigated for along time [9, 10]. Now the first are on the market including on-board computers and central graphic displays. Therefore, the following concentrates only on the manner in which the necessary infoonation should! could be displayed to the operator. The required steering angle is displayed on the top and the actual steering angle immediately below in Figure 5. Both are displayed in an analog manner, because this makes it easier to detect deviations and changes. In addition the actual values may be given (far left). The operator has to align both marks by acting on the steering wheel. Just below the steering angles, the distance between tractor and implement is displayed using a bar, and additionally, the ac-

tual values are indicated (left). For clarity a tractor symbol at the starting position, the path, and the target, the implement, are shown. Together with the system, the operator perfoons a control loop. It is mainly a question of expense to replace the operator by a controller. 3. Final remarks The discussed computer aided system for tractor/implement coupling has been proposed to improve occupational safety and operator comfort. It tackles the main problem during coupling: the restricted visibility. To avoid excessive costs the system is based on devices already on the market or expected in the near future, like automatic couplers and on-board computers

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with universal graphic displays. Only the required measuring system will cause additional hardware costs. The system under development should only have 'passive elements like reflectors on the implements, and the parts on the tractor should have no moving mechanical elements. Different principles to calculate the required steering angle from the measured distances and directions between tractor and implement have been investigated. The application of classic kinematic modelling to this problem is possible and has proved to be superior to neural network and fuzzy control application.

[1]

[2

[5] Aas, M., J. Bergstrom and O. Noren (1985). Lyftsnabbkopplingar for traktor och redskap. Jordbrugstekniska Institutet, Meddelande, No. 406, Uppsala.

[6] Carlson, B. and S. Persson (1982). Analysis of tractor-implement hitching and effeet of hitch design: a proposal. JTI-Rapport, No. 41, Uppsala. [7] Bottoms, D.J. (1983). The interaction of driving speed, steering difficulty and lateral tolerance with particular reference to agriculture. Ergonomics, 26, No. 2, pp. 123-239

4. References Heidt, H. and G. Groh (1984). Schwerpunkte landwirtschaftlicher Arbeitsunfcille und Moglichkeiten zu ihrer Verhiitung. Landtechnik, 39, No. 1, pp. 35-40.

[8] Gebhardt, W. et al. (1979). Ultraschallfeldsteuerung, Fehlerklassierung und Fehlerrekonstruktion mittels "Phased Arrays". Materialpriifung, 21, No. 12, pp. 437-443

Hammer, W. and G. Thaer (1990). Unfcille bei Bestellarbeiten. Landtechnik, 45, No. I, pp. 41-44.

[9] Jahns, G. and H. Speckmann (1985). Ein Bordcomputerkonzept ftir Scblepper und angekoppelte Gerate zur Optimierung landwirtschaftlicher Produktionsprozcsse. Grundlagen der Landtechnik, 35, No. 2, pp. 47-54

[3] Geisthoff, H. (1974). Ein automatisches Kuppelverfahren ftic landwirtschaftliche Gedite. Grundlagen der Landtechnik, 24, No. 3, pp. 87-89.

[10] Jahns, G. and H. Speckmann (1992). An open Agricultural BUS Standard. ASAEPaper, No. 92 3006

[4] ISOIDIS 11001 (1991). Agricultural wheeled tractors and implements - Threepoint hitch couplers. International Organization for Standardization.

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