Bond graph modeling of hydraulic circuits on a sweet sorghum harvester

A whole-stalk harvester was developed as part of a sweet sorghum-for-ethanol production system. Gathering chains grasped the stalks as they were cut at the base with a disk-cutter. These stalks were flipped onto a cross conveyor and deposited into an accumulator. Periodically the machine stopped and the accumulator was dumped. All the components on the harvester are powered hydraulically. Five pumps on the harvester supply flow to seven actuator circuits. Power is delivered to the pumps from the tractor PTO via a universal joint driveline. Each of the six existing circuits and one proposed circuit were modeled with bond graphs and implemented for computer analysis using TUTSIM. Model validation was done by comparing simulated and measured driveline torque, line pressure, and return line flow rate in each of the six existing circuits.

Data collected on the gathering chains circuit was used to analyze the effect of driveline joint angles on transmitted torque and pump output. Torque measurements at three driveline angles showed a torsional vibration with a primary harmonic at driveline rpm and a secondary at twice driveline rpm. A combination of Cardan joint characteristics, mass unbalance, the secondary couple, and non-linear driveline and V-belt stiffness was used to model the driveline. Resulting simulated torque emulated the experimental very well. Measured pressure in the gathering chains circuit showed relatively low fluctuations at the highest amplitude torsional vibration (highest driveline joint angles). It was concluded that driveline vibration would not significantly affect the gathering chains circuit performance.

The cross-conveyor motor circuit simulation showed close agreement to experimental results. Mean predicted flow, pressure, and torque were within 8.9, 7.3, and 7.7 percent of mean measured values. A simulation with a stalk load on the conveyor showed that power requirement increased only 8.0 percent.

The accumulator dump circuit was analyzed to determine if the load on the motor would become over-running and cavitate the pump or motor as the stalks were being dumped. Simulation showed that a bundle up to 300 kg could be dumped without over-running the motor, and since this was a larger bundle than the bin could hold, a design modification was not necessary.

The disk-cutter circuit was designed based on simulation results for several combinations of motor, pump, and sheave ratio. A 7.3 cm³/rad motor, 2.53 cm³/rad pump , and 2:1 sheave ratio produced the correct disk-cutter speed, and low torsional vibration when cutting the stalks, consequently this combination was selected for the design.