Advanced simulation of dynamic loads
What made this project unique was the stringent specification on accuracy for an application with huge dynamic load changes. The control system has to react fast enough to maintain a very tight cylinder stroke tolerance across four differential hydraulic cylinders.
The Woodings hydraulic distributor employs hydraulic cylinders, directly operated using high-response, directional, proportional valves with electrical position feedback. The closed-loop cylinder position control system employs load pressure feedback provided by electronic pressure transducers.
A short nine-month time window ruled out building a physical prototype, so the project used several simulation tools to model the system. Bosch Rexroth’s proprietary simulation tool, called MOSIHS (Modular Simulation of Hydraulic System), allows users to model and simulate an overall hydraulic system. To verify that the targeted accuracy and repeatability of the hydraulics system would be achieved, the engineering team involved Bosch Rexroth simulation groups both in the US and at its headquarters in Germany.
With a library of standard hydraulic components and Bosch Rexroth valves, the engineer can design a system of any size and complexity and determine how it will behave. Each component can be parameterized to match the design requirements. An extensive array of electronic control and mechanical components is available in MOSIHS to replicate real-world applications.
As in the real world, mechanical structures, such as the distributor equipment, interact with the hydraulic drive. Sub-models of the hydraulic system, mechanical systems, and controls can be coupled with each other and simulated together. This allows the designer to investigate dynamic behavior of the overall system, including resonance modes of hydraulic and mechanical systems and the interaction of the controller, actuators, and mechanical structure.
The hydraulic circuit and controls for the distributor comprised input data for the MOSIHS simulation program. CAD drawings of the distributor assembly, including 3D models of Woodings-sourced components such as the chute and the rotating distributor head, were imported into a commercially available multi-body simulation program coupled with MOSIHS.
The simulations verified the hydraulic circuit design and that the selected Rexroth components were suitable to meet flow, pressure, and other specifications. It also demonstrated that the required static, dynamic, and response time behavior would reliably deliver the hydraulic distributor endpoint accuracy. In addition, the MOSIHS simulation showed that using a specialized spool-flow characteristic control valve would increase control performance.
Virtual commissioning and 3D animation
One of the most significant issues studied using the simulation was evaluating the hydraulic circuit’s motion control scheme. Simulation demonstrated that a large moved mass and long hydraulic lines result in relatively low natural frequency of the hydraulic distributor system.
Specialized control algorithms, using dynamic pressure feedback, actively dampen system oscillations and provide optimal performance. In the course of evaluating the closed-loop control and fine-tuning the system feedbacks, the Rexroth engineers developed a better understanding of how the forces moving through the chute can affect accuracy as the distributor rotated.
As it turns out, the cylinder load changes with the chute angle. The drive and controls have to quickly react to load variations as the chute working point changes, as well as centrifugal forces and dynamic loads. The simulation verified the closed-loop control strategy and demonstrated that all four cylinders remained synchronized as required. The simulation also provided Woodings with an additional valuable tool: an animation that was used to prove to the Chinese steel manufacturer that the hydraulics solution proposed delivered the targeted endpoint accuracy.