ASAE Conference Proceeding
This is not a peer-reviewed article.
A new Concept for the Three-Point-Hitch as a Mechatronic System
T. Lang, H.-H. Harms
Pp. 246-251 in Automation Technology for Off-Road Equipment (ATOE) Proceedings of the 26-27 July 2002 Conference (Chicago, Illinois, USA), ed. Qin Zhang ,Pub. date 6 July 2002 . ASAE Pub #701P0509
In order to guarantee an efficient and economic working procedure, modern agricultural machines are generally equipped with extensive electronic control loops. Thereby some working steps can be automated and accelerated substantially. The tractor as a universally applicable machine is still one of the most important agricultural machines. A large amount of attachments can be operated by the tractor using the three-point-hitch. In order to arrange the three-point-hitch more flexibly, a system was developed which used three independently adjustable, position controlled hydraulic cylinders. By this modification the degree of freedom can be substantially extended and the possibilities of automation can be enhanced.Mobile Hydraulic Application, Mechatronic Systems, Automation
It is well known that the objective of agricultural engineering is to meet the constantly growing requirements of food production. Additionally, in the industrialized countries the farmers are exposed to substantial economic pressure. These requirements can only be met if modern and well-equipped machines are used, which are operating fast, efficiently and with high quality and are simultaneously reducing personnel deployment. Therefore, modern agricultural machines are usually equipped with extensive electronic control loops. The drivers are assisted by automatic steering systems or systems for managing complex work procedures or the whole vehicle respectively. The automated operation of recurring working steps - after programming by a teach-in procedure - is state of the art. The tractor still ranks among the most important agricultural machines, since it is universally applicable in combination with various attachments. For coupling these attachments, the three-point-hitch (or so called power lift) is usually used. In this proceeding, a system is presented, which substantially improves both the mechanical degree of freedom and the possibilities of automation of the three-point-hitch.
State of the art
The most important development steps of the three-point-hitch since its invention in 1925 were formed by standardization, the introduction of quick-couplers and electronic controllers as well as the rearrangement of the integrated cylinders to a modular construction . The common setup of today's three-point-hitches is represented in Figure 1. The lower links are connected to the lifting arms by lift rods. The lifting arms are rigidly coupled by the rock shaft. The lifting cylinders are driven by one common valve. Generally the controlling of this valve is managed by the "Electronic Hitch Control". The traction forces, the position of the hitch and (by comparison of theoretical and actual driving speed) the driving slip are measured by appropriate sensors and then processed by the controller. The driver is able to adjust the position limits, the lowering speed and the influence of the position, the traction forces and the driving slip within the controller algorithm. Furthermore, today the active vehicle oscillation damping is an important feature for high-speed transportation driving.
Figure 1. The conventional three-point-hitch of agricultural tractors.
Usually the lengths of the upper link and the lift rods are adjusted once and then kept the same for each particular application. The kinematical behavior of the attachment is determined by this length in combination with the positions of its cross points and thus, an adjustment can be made only for special operating points. A more comfortable version of this system can contain a manually adjustable hydraulic upper link, hydraulic lift rods or mechanical automatic implement stabilizers. Because the hydraulic components are only manually adjustable the regaining of earlier used positions is executed visually and thus inaccurate.
The position controlled upper link
If special functions are desired, e.g. parallel lift or steep lift (figure 2), a dynamic length control of the upper link is necessary. Figure 3 shows a solution with an integrated position sensor which has an integrated electronic system.
Figure 2. Conceivable operation-modes with the power lift.
Figure 3. The modified upper link and the used sensor.
By this setup, the best possible protection of the sensor against mechanical damage is achieved. The sensor is replaceable. The version which is used in this case operates according to the principle of a contactless potentiometer. Besides, several sensors with different technical properties and costs are available on the market. With this sensor in combination with a suitable proportional valve, the adjusted position is attainable with an accuracy of up to tenths of a millimeter. Apart from storing reproducible absolute position value, it is possible to adjust the length automatically depending on the variable lower link position for realization of e.g. parallel lift. The driver can influence the kinematical dependency by teaching-in some sampling points of a trajectory.
The position controlled upper link
An obvious way of increasing the flexibility of the three-point-hitch even more is the use of two additional position controlled cylinders, which adjust one lower link respectively. On the one hand thereby the rigid coupling of the lift rods to the rock shaft can be omitted which makes the rear of the tractor less complex (figure 4). On the other hand the possibilities for the control of implements are substantially improved by the additional degree of freedom, because three-dimensional movements are now available. Coupling and uncoupling of the attachments is made easier by omitting undesirable bracings between tractor and implement. By using double acting cylinders a defined pushing is possible. For example, an improvement of the insertion behavior of cultivation ploughs is conceivable by defined movements.
Figure 4. The modified Power Lift .
Theoretical and practical investigations
For the practical testing of the system a tractor was used. This tractor (Figure 5) is equipped with extensive experimental electronics and electrohydraulic proportional valves of different manufacturers .
Figure 5. The experimental tractor.
The programming is realized by using the program package MATLAB/SIMULINK and hardware of the company dSpace, Germany. A 4-furrow-plough with an approx. weight of 1500 kg was used as an attachment. In order to gain practical experience, field tests with the plough were executed and extensive measurements of hydraulic pressures, cylinder positions etc. were carried out (Figure 6).
Figure 6. Field test with ploughing.
For the theoretical examination of the kinematical behavior of the three-point-hitch two comparative 2-dimensional simulations were executed. Analytically based mathematical equations were implemented within the graphical simulation language SIMULINK. The practical advantage of this procedure is that this simulation can be compiled together with the basic control software. Afterwards it will be operated on the common hardware platform simultaneously in real time, next to the real operation of the tractor. For this, the simulation is fed with real input values of the sensors. The lower part of figure 7 shows the measured position of the lift rods and the upper link. Two operation modes are displayed. At the beginning (for the first 20 sec.), the upper link was displaced in manual mode, but then switched to automatic mode for displacing the upper link with the goal to keep the attachment angle at zero degrees. In both cases, only the lower links were manually adjusted, so the upper link stood still in the first period, but the attachment angle was changing. In the upper part of the figure, the comparison of the measured and simulated angle of the implement (the inclination of the coupling-triangle) is represented. It is obvious that the simulation matches the real movement very well. However since an absolute inclination sensor was used for verification, the tractor had to stand still during the measurements.
Figure 7. Comparison of simulation and measurement by the example of the attachment angle.
For the basic verification of the implemented mathematical equations a second geometrically based simulation with the CAD orientated multi-body simulation software ADAMS was implemented (Figure 8). Both simulations are practically identical in their results, whereby the accuracy of the SIMULINK model was confirmed.
Figure 8. Simulation of the tree-point-hitch.
Aspects for control engineering
For control engineering the tuning and harmonization of the movements of both hydraulic lift rods is an important aspect. Since a simple position control naturally has no influence on the cylinder speed, one of the cylinders will always move faster than the other without suitable measures, particularly with extremely asymmetrical and pulling loads. Figure 9 shows measurements of a simple position control system of both cylinders (measurement A) and with a method to limit the length deviation between both cylinders (measurement B). The asymmetrical load was provided by a 1500kg-plough. With this method the undesired length deviation of the cylinders was overlaid onto the pure position control. The temporary length difference could be reduced clearly.
Figure 9. Step response of the lift rod length and length of deviation of left hand lift rod compared to right hand lift rod.
Another strategy consists of the implementation of a velocity control. In this case the actual velocity command value is determined by the actual position of the cylinders and it reaches zero at the position set point. With this profile (the relation between position and speed) it is possible to definitely affect the acceleration and deceleration of the cylinders. However, the main problem is the generation of an accurate speed signal from the DC voltage output of the position sensor. Basically two methods to achieve this goal are conceivable, the analogous and the digital differentiation. Tests with several combinations of analogous and digital differentiators and filters were executed.
Figure 10. Velocity of the lift rods with analog and digital differentation.
Figure 10 shows results with an active analogous electronic circuit and a common digital derivation algorithm during a forward and a return stroke of one cylinder. By using an overlaid position and speed control in combination with analogous differentiators, generally the length deviation of the lift rod cylinders at the three-point-hitch could be reduced to a few millimeters.
The presented concept of the modified three-point-hitch provides extended possibilities of functionality, with a simultaneous achievement of less constructional complexity. This can be a significant step forward, particularly with respect to the general trend towards automated operation and tractor-implement management systems. However, there is more optimization potential for the improvement of the dynamics. This concerns especially the interaction of the cylinders with low-priced proportional valves, which are normally used in mobile machines.
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