DRIVES & CONTROLS FEATURE


University team pull off high rankings at annual tractor design competition


When the Iowa State University (ISU) students planned its entry into the 2016 ASABE annual tractor design competition, they knew they were taking a risk by opting to use electronic throttle controls. However, with the help of Thomson Industries, the students discovered that their bold choice would set their tractor design apart from competitors and earn them points in design, innovation and performance


ach year since 1997, the American Society of Agricultural & Biological Engineers (ASABE) has sponsored a ¼-scale tractor design competition, pitting agriculture engineering students against each other to push the limits of tractor design. Each competing team receives a 31–horsepower Briggs & Stratton engine, which must be left stock, and a set of two Titan tires for the rear of the vehicle. How the power gets from the engine to the wheels is up to the students. The primary objective of the ISU team was to enable the tractor to deliver power to each wheel individually. This, they reasoned, would optimise traction by compensating for changing soil conditions, something that is not possible with traditional hydro-mechanical drive systems. The team started by analysing a number of drive train options, including hydraulic, mechanical and electronic solutions. They dismissed the option of using a hydraulic or mechanical system, concluding that the electronic drive train promised a simpler and more elegant solution.


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SYSTEM DESIGN The ISU design couples a DC electric generator to the engine. The generator powers a motor at each rear wheel, which accepts only a certain range of voltage to maintain optimal traction. Too little voltage and the tractor will not move; too much voltage and the wheels will slip and lose traction. The challenge was to figure out how to control engine speed so that the generator produces optimum voltage. The foot pedal controls engine speed, while the operator controls the propel speed with the joystick, which allows management of the speed at each of the rear wheels. Making this work requires a highly precise means of engine speed control to produce the optimal voltage for the electric drivetrain, which is essentially the function of an electro-mechanical actuator. Fortunately for the team, Thomson Industries was among the event sponsors and was more than happy to provide the team with advanced electro-mechanical actuation technology. Thomson provided the team with two sample actuators to evaluate. One was its rugged Max Jac linear actuator, a compact high-power density model designed specifically to operate and thrive


in some of the harshest environments typical to mobile off-highway equipment. The other, its Electrak Throttle linear actuator, is primarily found in applications requiring throttle control of a mechanical engine. The ISU team chose the Electrak Throttle because it provided the precise motion and force required for their application. In operation, position feedback devices on each


motor signal the travel speed and propel mode to the operator display via a CAN bus network. The display also shows engine speed and hours. Dynamic readouts in the display correlate motor speed with the joystick position and flash a warning when the positions do not match, indicating less than optimal traction. Based on this information, operators can adjust the joystick position, or the actuator will change engine speed to provide optimal power and desired traction.


operate at fewer RPMs, bringing their entry to between 78 and 80dB. The electric drivetrain implementation also drove them to the top rankings in overall innovation and safety. Contributing to ISU’s scores in


manufacturability was the fact that all components could be bought off-the-shelf, reducing the need for costly machined housings and other custom components. Eliminating the need for hydraulic tubing also enabled a modular construction strategy, so that a manufacturer could assemble front, back and operator stations in separate areas and join them together easily in a final step. This is not possible when tubing is extending across the length of the equipment. Removing tubing also won them serviceability points by not having to replace hydraulic fluid or deal with cleaning oily components. In addition, judges were impressed by the physical manifestation of the design, awarding top marks for how well the team described its process and design justification in writing, and second place in how they presented the solution. The ISU team scored in the top 10 in the


TAKING THE FIELD The 2016 ASABE competition took place at the Expo Gardens Fairgrounds in Peoria, Illinois. First, the judges confirm compliance with the rules and they check hitch height, brakes stand, overall size, wheel base, schematics, safety features and other design features. Although the rules committee had approved the use of an electric drivetrain in advance, it was not until the inspection that the rest of the competitors learned of ISU’s unique strategy.


Throughout the judging, the electric system proved to be a formidable competitor. Its first triumph was in the sound test. Rules restricted sound levels to less than 90dB. The precision with which the ISU team could hold the engine at optimal operating levels meant they could


/ DESIGNSOLUTIONS


maneuverability category, attributed mostly to the differential control of the back tires. The pulling competition measured the length to which the tractor can pull a weighted sled down a dirt track. Tractors usually produce 2,000 to 3,000lbs of force while pulling the sled. In last year’s competition, using a system with a hydraulic transmission, the team pulled the sled 197ft. This year, they exceeded that dramatically with a pull of 207ft. This compared well to the furthest pull of the day of 260ft. The ISU tractor’s core technology will ride again


in the next ASABE competition in June 2017. In the tradition of the annual event, each school will enter two tractors, an improved version of the previous year’s entry and a new design. Thomson has already agreed to provide electro-mechanical actuator technology to the next project, so when ISU once again fields the electronic throttle, it will come as no surprise. The only surprise may be how well the team builds on what is already an impressive foundation of success.


Thomson Industries www.thomsonlinear.com


DESIGN SOLUTIONS | JUNE 2017 27


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