Electrical power outlet in agricultural machinery

Study analyzes fuzzy control applied to an electrical energy generation system embedded in tractors to drive agricultural implements.

The agricultural sector's demand for greater efficiency and controllability of operations carried out by tractors and implements drives the development of alternative drives for these machines, replacing the mechanical and hydraulic systems widely spread on the market. Functions such as variable rate applications (ATV), georeferenced applications, monitoring and control of the operation through embedded devices, such as productivity sensors, and complex drives are examples of applications that feed this demand and are correlated with the advancement of agriculture. accuracy (AP).

In conventional tractors, power sources for the implements are the mechanical power take-off (TDP) and the hydraulic power take-off. With the proposal to use electric drives, there is a need to develop a new power source, the electrical power take-off (TDPE). Unlike TDP, a TDPE can eliminate the dependency relationship between the implement's drive and the angular speed of the diesel engine, traditionally used in agricultural tractors, also enabling the use of electronic control systems.

ELECTRIC POWER GENERATION IN TRACTORS

The generation of electrical energy in tractors is something that has been explored by manufacturers for decades. The first aspects date back to the beginning of the 1950s. In 1954, the company International Harvester launched the Farmall 400 Electrall agricultural tractor, which optionally had a three-phase electrical generator (208Vac; 10kW) with the purpose of supplying electrical energy to drive implements and small rural networks.

A modern example of the evolution of the use of electrical energy is the RigiTrac EWD 120 - Diesel Electric tractor, from the company Rigitrac Traktorenbau AG, developed together with the Technische Universität Dresden, in 2011. The tractor has a diesel electric propulsion system consisting of a 91kW diesel engine, an 85kW and 650Vdc electrical power generator and four electric motors, one on each driving wheel, with a nominal power of 33kW each.

There are currently three categories of tractors that carry electrical system technology: (a) electric diesel, with the generation system coupled directly to the engine and intended mainly for the use of electrical power in traction; (b) fully electric, powered by batteries or hydrogen cells, however, still off the market due to limitations arising from available technologies; and (c) the diesel tractor with integrated direct current generator, which has a low power TDPE, but not intended exclusively for use in implements.

ONBOARD ELECTRIC POWER GENERATION SYSTEM

Below we will present an on-board electrical power generation system driven by the tractor's PTO. The objective is to validate the operational viability of the controller, designed to regulate the voltage generated in the system. For its operation, the system must be capable of operating in the face of simultaneous variations in the load power demanded and the drive angular speed, maintaining the energy generated within the established limits. The proposed system and its main components are shown in Figure 1, being: TDP, mechanical power take-off of the tractor; GS, synchronous generator; w, GS rotor angular velocity; Frequency Inverter (where PWM is pulse width modulation); V_fd, voltage in the GS field circuit; V_dc, voltage on the DC bus of the frequency inverter circuit; P_o, load power.

It can be seen that the synchronous generator is driven by the tractor's mechanical power take-off shaft, while the three phases are connected to a frequency inverter circuit that supplies power to the load, which can be an electrical implement or other compatible load. . The GS parameters, necessary for the mathematical modeling of the system, were based on a three-phase synchronous generator model with salient poles, with nominal voltage values ​​of 380Vac, frequency of 60 Hz, power of 20kW and angular speed of 188,5rad /s (1.800rpm), from the manufacturer WEG, model GTA161AI22.

Construction and parameterization of the proposed system, as well as design and testing with the controller fuzzy, were carried out through computer simulations, using the Matlab/Simulink program.

Table 1 presents the nominal values ​​and the maximum and minimum values ​​assigned to the variables, based on the 20kW synchronous generator adopted.

GENERATION SYSTEM CONTROLLER DESIGN

For the proposed system, we chose to use a controller fuzzy. In the system, load power and engine angular speed are the independent variables of the system; voltage in the field circuit the variable acts from the DC bus voltage of the inverter circuit to the controlled variable, whose behavior determines the correct operation of the system. The controllers fuzzy are classified as intelligent. Intelligent controllers are capable of making operating decisions based on work variables and for this purpose they use instructions or rules. Unlike a proportional integral derivative (PID) controller, the controller fuzzy it is also capable of absorbing the complex dynamics of the system (nonlinearities) and acting over a wide operating range, characteristics necessary for the control of the proposed system.

Figure 2 illustrates the disturbances in angular velocity and load power applied in simulations of the generation system, to adjust the controllers.

In Figure 2, the disturbances adopted simulate the system starting from zero to a high load level, with the system operating below the nominal drive angular speed, followed by load relief and an increase in the drive angular speed. This sequence was adopted because there are simultaneous variations in angular velocity and load power in two critical scenarios for the system's operation.

To evaluate the controllers, the following performance indices were evaluated: absolute error integral (IAE) and time-weighted absolute error integral (Itae). The indices were calculated with the normalized values ​​of the V error_dc and time in the unit of seconds (s), recorded in the same simulation period.

CONTROLLER RESULTS AND DETERMINATION

Figure 3 illustrates the recorded voltage values ​​on the DC bus of the inverter circuit in the system simulations with the obtained controllers. Miso controllers have multiple input variables and one output variable, while Simo controllers have one input and multiple outputs. The curves corresponding to the controllers fuzzy PID Miso appear overlapping.

Table 2 presents the IAE and Itae performance indices of the controllers fuzzy and the number of rules used.

With the results obtained, it can be observed that both control architectures fuzzy PID Miso and fuzzy PI Simo maintained voltage on the DC bus of the inverter circuit at its nominal value, obtaining values ​​close to performance, while the others presented gross errors during duty and in the load power transition (instants of 2s). The selected controller is the fuzzy PI Simo, which kept the system operating within established limits, using a simpler architecture, with a single input variable and a reduced number of rules. Finally, the results demonstrated the system's operational capacity in the face of load and drive speed disturbances, maintaining the DC bus voltage within established limits.

Fabricio Theodoro Soares, Nelson Luis Cappelli, Angel Pontin Garcia and Claudio Kiyoshi Umezu, Feagri - Unicamp

Article published in issue 164 of Cultivar Máquinas.