In a molding press that forms large body panels made of composite material, if the platen exerts uneven pressure across the panel as it’s molded, the result can be incomplete product forming (to the mold), uneven thickness, flashes at the edge of the molded panel, and other defects affecting the quality and consistency of the production units. Further, if the press platen is skewed or it does not apply the load evenly, it may also damage the mold, seriously affecting the service life of the equipment.
1. The 3,000-ton press manufactured by Wuxi Lanli Machine Tool Co. Ltd. of Jiangsu, China.
Wuxi Lanli Machine Tool Co. Ltd., Jiangsu, China, a hydraulic press manufacturer, was challenged with improving the operation of an older press design to produce body panels (Figs. 1 and 2) for a Chinese auto plant. With three main compression cylinders, the hydraulic press could easily handle the tonnage required for the new application, but there were issues with keeping the platen perfectly level as up to 3,000 tons of force were being applied.
2. Shown here is a typical automobile component produced by the Lanli press: a battery panel made of composite material.
To fix the potential platen skewing problem, the press manufacturer consulted with hydraulics distributor Wuxi Forever Automation Technology, also of Jiangsu. Wuxi Forever engineers recommended adding cushion cylinders at the four corners of the platen to ensure it remained level during a compression stroke (Fig. 3). To control the four cushion cylinders, the Wuxi Forever engineers recommended that Lanli use an RMC151 electrohydraulic motion controller (Fig. 4) from Delta Computer Systems.
3. Four leveling cylinders act to ensure that equal force is applied to all surfaces by the hydraulic press’ platen.
4. Delta Computer’s RMC151 is an eight-axis electrohydraulic motion controller used for controlling and synchronizing position and applied force of Lanli’s hydraulic presses.
The forming cycle is controlled by a PLC, which operates the three main compression rams, and provides a target position and maximum force value to the Delta motion controller for use in controlling the four 100-ton cushioning cylinders. The cushioning cylinders “catch” the platen as it descends, and apply a resistant force at each corner in order to keep the platen level during the forming operation.
“The press uses two separate systems: the three main cylinders on the top, controlled by an Omron PLC driving servo pumps, and the four cushion cylinders, controlled by the RMC driving Parker D30FP servovalves,” explains Henry Xie, Wuxi Forever sales manager. “With the Delta motion control system in place, Lanli faced two main challenges in the design of this press. First, how to make the upper platen and lower cushioning hydraulics work in concert; second, required accuracy of leveling position was very high (less than 0.05 mm).”
Cascaded Control Loops
The RMC151 performs its control functions using a “cascaded-loop” architecture. In this setup, an inner control loop works to level the platen while the outer loop simultaneously adjusts the target positions of the cushion cylinders to follow the motion of the platen. The outer control loop also monitors the force exerted by the cushioning cylinders to make sure that it remains under the limit provided by the PLC.
The current position information comes from data provided by a magnetostrictive linear displacement transducer mounted in each cylinder, and the force information comes from two pressure transducers mounted in each cylinder—one on each side of the piston (Fig. 5).
5. Each leveling cylinder is instrumented with a linear magnetostrictive linear-displacement transducer to provide position feedback, and pressure sensors on each side of the piston, whose values are differentiated to provide force feedback.
The cascaded control architecture is shown in Fig. 6. To level the platen, the motion controller computes the average position of the cushion cylinders, which it treats as a “virtual” master axis position and works to move each cylinder position to match this average. To calculate the force being applied by each cushion cylinder, the motion controller computes the difference between the hydraulic fluid pressure on each side of the piston. If the cumulative force is too high, the RMC will change the position of the platen to maintain the force within the set limit.
6. This cascaded control-loop diagram shows how the motion controller works to keep the platen level while it follows the motion of the platen and monitors the force exerted.
These mathematical computations done by the motion controller on the raw feedback from the position and pressure transducers are performed using a feature that Delta Computer Systems calls custom feedback. “By doing math on the raw transducer data before that feedback is provided to the control loop algorithm, the control algorithm itself can be much more straightforward and simple to implement and tune,” says Dennis Ritola, an applications engineer with the company.
The cascaded-loop architecture, custom feedback, and multi-axis synchronization functions used in this application are standard features supported by Delta motion controllers. These features are easy to set up using RMCTools, Delta’s development software package provided free by the company to its motion control customers. “A complex hydraulic control application such as this one can be accomplished with very few lines of code,” continues Ritola. “Programming a conventional motion controller to perform the nested control function would be extremely difficult.”
Tuning for Optimal Results
Once the motion steps are programmed, the system must be tuned for optimal operation. For this step in the development process, the RMCTools package provides tools for analyzing the motion and adjusting control-loop parameters. Motion analysis is simplified using Delta’s Plot Manager, which produces a graphical representation of actual versus target motion profiles, the goal being for the designer to adjust control-loop gain values to make the actual motion match the target profile.
Adjusting the gains is made easier using the Tuning Wizard, which Delta provides as part of the package. To help resolve any issues that Wuxi Forever engineers had during the design process, Ritola and the author were available for late night phone calls, as well as online exchanges of computer screens displaying tuning status, between China and Washington state.
“The completed press has met or exceeded all of the design goals for the project,” says Xie. “The development of the LANLI press was made easy using Delta Computer Systems motion controller and development tools, and Delta’s technical support was excellent.”
“The customer that ordered the press is very pleased with the results,” adds Ruhong Liu, technical director and an engineer at LANLI. “The press has been in production since July, running 24 hr./day for four months making auto body panels for a German auto maker’s Chinese plant. Because of the good performance of the press, we have gotten more orders from other foreign-China joint-venture auto makers. So we will build three more presses to fulfill increased demand.”
“The future prospects for the LANLI press design are excellent,” echoes Xie. “The trend in the automotive industry for ‘green’ lightweight vehicles and EVs has increased the demand for composite material automotive parts. This trend will likely continue for the foreseeable future.”
Gan Li, is director of business development-Asia Pacific Rim, Delta Computer Systems Inc., Battle Ground, Wash.