A three-point hitch dynamometer for load measurements between tillage implements and agricultural tractors during operation

Jordi Pijuan1*, Jordi Berga2, Martí Comellas1, Xavier Potau1, Joan Roca1

1 GREA Innovació Concurrent. Universitat de Lleida (UdL), Pere de Cabrera s/n, Lleida, 25001, Spain

2 Centre d'Innovació del Sector de la Maquinària Agrícola i dels Equipaments Industrials. MAQCENTRE, Edificio H3 Parc de Gardeny, Lleida, 25003, Spain

*Corresponding author. E-mail: jpijuan@diei.udl.cat

Abstract

In agriculture there are a lot of implements that are fitted to the tractor via the three point hitch for carrying out different tasks. In order to obtain a reliable design of such implements, an exhaustive analysis of the loads supported by the three point hitch between the tractor and the implement has to be done. Implement manufacturers usually have limited information as the measurement and analysis of the loads is a complex process. As a result, the design process of this kind of machinery mostly depends on the experience obtained when field-testing previous equipment, but with few quantified data.

In this work, a dynamometric device is presented in order to help manufacturers to optimise their designs. The device, which is placed between an agricultural tractor and its implement, measures all the loads necessary to fully analyse the three-point hitch operation. The loads are measured during the operation of tillage implements and, with the appropriate data processing, it is possible to determine all the loads in the three point hitch between the tractor and the implement. All this information can be used to analyse the functionality of the implement and the performance of the tractor, studying the power required or the weight distribution in the tractor’s wheels. Another application of the dynamometer is to obtain the loads during operation in the joints of the implement, data from which the structured design of the implement can be optimised.

The dynamometer has been designed to measure the longitudinal, vertical and lateral forces. Moreover, the resultant torques in the three directions that appear during tillage or transport of the implement are also obtained. The data come from 6 independent links with the corresponding load cells.

The developed device can be attached to category II and III agricultural tractors (tractor power from 48 to 185 kW). Based on that, a previous study determined that the maximum allowable loads are 80 kN in longitudinal, 80 kN in vertical and 45 kN in lateral direction. The dynamometer has been optimised to obtain a maximum reduction of weight and the longitudinal dimension, minimising the effect of the device on the measures. Also, this dynamometer has been built with adjustable pin joints that allow attaching it to a wide range of implements. Finally, the design allows using the tractor power take-off.

This work also presents the results of the first test carried out using the presented dynamometer with the objective of analysing the capabilities and the functionality of the data acquisition system. The results show that the new dynamometer can effectively measure the loads between an implement and a tractor, providing useful information in the design phase of such machinery.

Key words: three-point hitch, dynamometer, load, implement

1. Introduction

1.1. Interest and applications of three-point hitch dynamometers

In agriculture there is a lot of specialized machinery for different task, such plowing process or planting process. This machinery requires an exhaustive analysis of the loads applied, and this is difficult to achieve since the manufacturers have no enough information about these force values. Because of this, the design process of this machinery depends a lot of the previous experience.

Some researchers have developed devices and instrumentation systems to analyze the performance of agricultural tractors and its implement. Research on field performance of tractors and implements requires portable instrumentation to measure machine parameters while the equipment is operating under normal field conditions.

An example of the potential of combining different measuring systems and instrumentation can be noticed in McLaughlin (1993) were several variables of both tractor and implement are measured: three-point hitch forces, axle load and torque, fuel consumption, temperature, engine power, wheel and ground speed etc. With all this information it is possible to analyze the power flow during operation. The main objective in many studies is to analyze and optimize the performance of both tractor and implement, and also reduce to cost of farm operations and improve productivity.

An interesting work is developed by Yahya et.al. (2009). with similar information combined with a GPS system it is possible to obtain a map of soil characteristics, or mapping along the field the energy requirements and fuel consumption.

Another common application is the characterization of soil via the measure the necessary draft force to plow or cultivate it. An example of these studies is the measure of draft force and tractor speed done by Thomson (1989).

Other simpler studies perform fuel measurements combined with the draft force measures in order to improve productivity and efficiency in farm operations (Garner, 1988; Bowers, 1989). Other works are based on load measurement in joints between implement and tractor three-point hitch as information in the design process of this machinery.

1.2. Different types of dynamometers

Three-point hitch dynamometers can be classified depending on the conception and functionality. Most of them consist of an independent device that is attached between the tractor and the implement. Other designs rely on a modification of the three-point hitch mechanism of the tractor adding the force transducers.

The first type consists of a single frame with transducers that are attached between the tractor and the tillage. This system is the most used by the investigators. There are some existing different designs according to the intention of the developers. For example there are devices that are adjustable to any implement dimensions (Al-Jalil, 2001). Other example of designs consists of using a structural tube as main frame and the strain gauges located at this (Johnson, 1979). Another common design consists of using two sub-frames connected by the load cells. Then one part of the device is attached to the tractor and the other one attached to the implement (for example, Chaplin, 1987). Usually these devices allow implementing more measuring transducers and can be used in higher power tractors than a single frame dynamometer. The main advantage of these dynamometers is that they not require any modification on the three-point mechanism, and only attaching between the tractor and the implement is required. This allows realizing different test in different tractors with the same device.

The main disadvantage of a two frame conception is the extra-weight and the extra-length of whole tractor and implement assembly that cause differences between the measured forces and the real forces that would be without the device.

To avoid the use of additional frames, one possibility is the installation of transducers in the three-point hitch tractor mechanism, implementing this transducers in the linkage rods (Bentaher, 2008), or replacing part of the rods by transducers or other elements that allows to measure the element axial force (McLaughlin, 1993; Lang, 2002). These systems have the advantage that the weight of the transducers in the three-point hitch has no significant effect on the loads that would be measured in the original mechanism. The disadvantage is that it is necessary to modify the three-point hitch mechanism to implement the measuring system. Then, every different tractor model requires a particular mechanism modification.

2. Dynamometer characteristics and data processing

2.1. Objective

The dynamometer described in this work has been developed to measure all the loads between the tractor and the implement (figure 1). With the information acquired it is possible to study the performance of the implement and the tractor that would be interesting for agricultural machinery manufacturers.

One of the possibilities is to study and optimize the functional design of the implement in order to minimize the power required and improve its performance. Another possibility is to adjust the structural design of agricultural machinery using the information of the loads in the joints. With this study it could be possible to obtain a lighter design of the implement ensuring the mechanical resistance. Finally, this dynamometer allows studying the influence of the tractor configuration, for example, the load distribution of the weight in the front and rear wheels, or the geometry of the three-point hitch mechanism, and how these parameters affect in the implement performance.

FIGURE 1: CAD-3D model and picture of the three-point hitch dynamometer

2.2. Description of the dynamometer

The dynamometer consists of 2 frames with 6 independent links between them and with a load cell in every link. These 6 load cells are distributed thought out the device, 3 of them measure horizontal forces, 2 load cells measure vertical forces and one load cell measures lateral force. With the information of the loads cells it is possible to compute the longitudinal, vertical and lateral forces, and also the roll, pitch and yaw momentums that appear between the tractor and implement during the tillage work process o during the transport.

This device has been designed for category II and III agricultural tractors (tractor power from 48 to 185 kW). The total mass of the device is 220 kg. Because a lot of agricultural implements have no standard mounting dimensions, this dynamometer has been built with adjustable pin joins. The admissible distance range is from 650 mm to 960 mm at bottom joints and from 540 mm to 740 mm between upper joint and bottom joints axis.

2.3. Data processing

The mathematical model allows computing the resultant loads between the implement and the tractor. Using the six load cells in the dynamometer is possible to obtain the three forces and the three momentums between the tractor and the implement (figure 2).

Another computation consists of obtaining the loads at the implement joint. To do this, a rigid solid diagram of the implement part of the dynamometer has been analyzed, so it is possible to obtain a linear system of equations that allows calculate the loads at implement joints (Fu, Fbl, Fbr) from the data of the load cells (F1 to F6).

FIGURE 2: Rigid solid diagram of the implement part of the dynamometer

3. First test results

In this part some of the results obtained in a first field test with the dynamometer are presented. A chisel cultivator has been tested using an agricultural tractor of 100 kW power in a very variable soil resistance field (figure 3). The acquisition data period from the load cells is every 0.1 s and every 1 s is computed the value of the maximum, median and minimum resultants of forces and momentums.

In the results graphs it is shown the traction force obtained during plow process (figure 4) and the vertical force in the same test (figure 5).

FIGURE 3: Picture of the chisel cultivator test

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Tra

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Median Traction Force

Min. Traction Force

FIGURE 4: Graph of the traction force during a chisel cultivator test

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FIGURE 5: Graph of the vertical force during a chisel cultivator test

4. Conclusions and future work

The design of the dynamometer is light, compact and can be easily attached to different tractors and implements. This allows minimizing the effect of the dynamometer weight and dimensions in the measures. Regardless, the next step in this development is to analyze the influence of the device attached between the tractor and the implement, taking into account the effects of itself, such as the weight and loads redistributions.

In the first test field results it can be observed that the maximum forces are significantly higher than the median force in the acquisition interval. It is important to take into account these maximum loads during the structural design of the implement. Also the median results can be used to analyze the forces required in plough process, and with additional information such velocity, an energy balance can be realized.

Acknowledgments

The authors would like to thank Generalitat de Catalunya for partially funding this work (project 2009-SGR-534). This work was partially funded by the project UdL-Impuls 2011 (X10020). Martí Comellas and Jordi Pijuan would like to thank Universitat de Lleida for his research fellowship.

Reference list

Al-Jalil, H. F., Khdair, A., Mukahal, W. (2001). Design and performance of an adjustable three-point hitch dynamometer. Soil and Tillage Research, 62(3-4), 153-156

Bentaher, H., Hamza, E., Kantchev, G., Maalej, A., Arnold, W. (2008). Three-point hitch-mechanism instrumentation for tillage power optimization. Biosystems Engineering, 100, 24-30.

Bowers C. G. (1989). Tillage Draft and Energy Measurements for Twelve Southeastern Soil Series. American Society of Agricultural Engineers 0001-2351/89/3205-1492

Chaplin, J., Lueders, M., Zhao, Y. (1987). Three-Point Hitch Dynamometer Design and Calibration. American Society of Agricultural Engineers 0883-8542/87/0301-0010

Garner, T. H., Dodd, R. B., Wolf, D., Peiper, U. M. (1988). Force Analysis and Application of a Three-Point Hitch Dynamometer. American Society of Agricultural Engineers 0001-2351/88/3104-1047.

Johnson, C. E., Voorhees, W. B. (1979). A Force Dynamometer for Three-Point Hitches. Transactions of the ASAE-1979.

Lang, T., Harms, H. H. (2002). A new Concept for the Three-Point-Hitch as a Mechatronic System. Pp. 246-251 in Automation Technology for Off-Road Equipment, Proceedings of the July 26-27, 2002 Conference (Chicago, Illinois, USA).

McLaughlin, N. B., Heslop, L. C., Buckley, D. J., St. Amour, G. R., Compton, B. A., Jones, A. M., Van Bodegom, P. (1993). A General Purpose Tractor Instrumentation and Data Logging System. American Society of Agricultural Engineers 0001-2351 / 93 / 3602-0265.

Thomson, N. P., Shinners, K. J. (1989). A Portable Instrumentation System for Measuring Draft and Speed. American Society of Agricultural Engineers 0883-8542/89/0502-0133.

Yahya, A., Zohadie, M., Kheiralla, A.F., Giew, S.K., Boon, N.E. (2009). Mapping system for tractor-implement performance. Computers and Electronics in Agriculture 69, 2–11.