An electronic slip control system was designed and installed on MF285 tractor and its performance was evaluated under field conditions. In this system, a rotary encoder was used to measure the ground speed and a proximity sensor to measure the theoretical forward speed for calculation of drive wheel slip. The output signals from the sensors were transmitted to the programmable logic controller (PLC). After calculating the wheel slip and comparing it with the set value, the PLC computed the error value and send a command to a stepper motor, accordingly. The stepper motor rotated the control valve and then the three-point linkage system changed the depth of the plow in order to keep the wheel slip fixed in the allowable range.

The results of sensor calibration showed that the measurement error of slippage was about 2 percent in the field condition. Field experiments were conducted at the two levels of control system (electronic and mechanical), the four levels of ground speed (2.5, 3.5, 4.5 and 6.2 km/h), the three levels of the set values (slippage for electronic system including 1: 10%, 2: 15% and 3:20%, the position levels of draft control lever for mechanical system including: 1: low, 2: medium and 3: high) in a sandy–loam soil with a randomized complete block design. Results of statistical analyses showed that the mean of slippage in the electronic control system was decreased 35, 43 and 49 percent at three set levels of 1, 2 and 3 respectively compared with the mechanical system. The decrease in wheel slip in electronic control system caused reducing of fuel consumption 11, 31 and 36 percent at the set values of 1, 2 and 3 respectively. The results also showed that there was a significant difference between fuel consumption of both systems at the similar speeds.

Introduction

The main purpose of agricultural tractors is to perform traction work, which is defined based on pull and travel speed. Research shows that about 20 to 55 percent of the available tractor energy is wasted at the tire-soil contact surface, due to drive wheel slippage and rolling resistance. When using tillage implements, drive wheel slippage is necessary for the tractor to apply traction force. Many researchers have reported that in order to achieve maximum traction efficiency, in addition to adding appropriate weighting to the tractor, the wheel slippage should be controlled within the range of 8 to 15 percent.

Nowadays, most agricultural tractors, especially domestically manufactured tractors, have a hydro-mechanical draft control system. In this system, the compressive force in the upper arm or the draft force in the lower arms of the tractor is used as a control parameter. The response of hydro-mechanical systems to changes in soil conditions is weak (slow). The reason for this can be stated as follows: the spring experiences hysteresis (backlash) during the loading and unloading process, which leads to different rates of raising or lowering the tools in the soil. Also, there is no mechanism to adjust the lifting sensitivity of the hydro-mechanical system.Therefore an electronic slip control system with appropriate accuracy, low cost, and compatibility with domestically manufactured tractors was designed, developed, and installed on the MF285 tractor.

Material and Methods

An electronic slip control system, using a proportional control valve of the rotary type and with the ability to be controlled by a stepper motor, was designed and installed on a Massey Ferguson model 285 tractor. This system includes theoretical and actual tractor speed sensors, a controller unit, a display, a control valve, a stepper motor, and a stepper motor driver. The programmable logic controller (PLC) consists of a CPU unit model CP1L-J14D and an analog input unit model CP1W-AD041. A touch screen manufactured by Omron, model NB5Q-TW00B, was used to display the measured values and enter the set values. The control valve is installed at the outlet of the hydraulic pump, and its design is such that it provides three modes: liftting, neutral, and lowering. A Sanyo stepper motor with a torque of 13 kg/cm and a two-phase driver was used to rotate the control valve. The fifth wheel method was used to measure the forward speed. A 100-pulse encoder shaft sensor was used to measure the rotational speed of the fifth wheel. A magnetic pick-up sensor sensitive to iron was used to measure the rotational speed of the drive wheels.

Results and Discussion

The results of field experiments showed that there is a significant decrease between the averages of the slippage in the electronic system compared with the mechanical system in all similar set values, and this indicates that the mechanical control system has performed poorly in controlling the percentage of slip compared to the electronic control system. Because there is no supervision on the slippage of the drive wheels in the mechanical control system.

With increasing set values, regardless of the type of control system, the percentage of slip has increased, but the slope of the increase is different in the two systems. Given that increasing the set values causes the required traction force for the plow to increase, increasing the draft force increases the slippage of the drive wheels.

Comparing the average fuel consumption in different systems and speed treatments using the LSD test at the 5% level showed that the mechanical control system has the highest fuel consumption at the minimum travel speed, and the electronic control system has the lowest fuel consumption at the maximum forward speed. It is also clear from the graph that by increasing the speed in both control systems, the amount of fuel consumed has decreased. There is also a significant difference at the 5% level between the averages of two systems at all speeds, and using the electronic system has saved fuel consumption by 20%, 26%, 40%, and 32% at speeds of 2.5, 3.5, 4.5, and 6.2 km/h, respectively. There is a significant difference at the 5% probability level between the fuel consumption values of the two systems in all set values, so that the electronic system has reduced fuel consumption compared to the mechanical system. Also, by increasing the set values, regardless of the type of system, the amount of fuel consumed has increased.

Conclusions

1- The calibration results showed that the sensors of the system for measuring the percentage of slip have good accuracy. Also, there was a linear relationship with a high coefficient of determination between the percentage of slip measured by the system and calculated by the formula on the asphalt surface, and the measurement error in the field was approximately two percent.

2- The average percentage of slip when using the electronic system in all similar set values was significantly reduced compared to the mechanical system, and this led to a decrease in the average fuel consumption of the electronic system in all similar set values.

3- Due to the adjustable up-going and down-going sensitivity in the electronic system, the control system responded promptly to changes in soil conditions, and there was no need for the driver to intervene to control the vehicle. However, in the mechanical system, in some cases, the oprator had to intervene in controlling the plowing depth.