Motion controller gives old presses new life
Case study: Retrofit controller on four hydraulic presses used a new programmable logic controller (PLC) to control the moving platen and the levelling cylinders would be controlled by a new electrohydraulic motion controller.
A recent retrofit project at a manufacturer and rebuilder of hydraulic presses involved the upgrade of the press controls on four older hydraulic presses before installing them at a truck component manufacturing plant in Monterrey, Mexico. The 2,340-ton presses, originally manufactured by Hoesch in Germany, use two main cylinders (1,170 tons each) for the moving platen and four leveling cylinders (125 tons each) to mold and in-mold coat large glass fiber body parts for trailer truck cabins (see Figure 1).
Before arriving at Macrodyne Technologies Inc. of Concord, Ontario, for refurbishing, the presses had been disassembled and all electrical controls were removed. Macrodyne could choose what control system to use in the press rebuild. The moving platen would be controlled through a new programmable logic controller (PLC), and the levelling cylinders would be controlled by a new electrohydraulic motion controller.
The press control upgrade included:
- Moving platen to bolster parallelism control during pressing and motion profiles (to allow uniform flow of fiberglass material).
- Leveling system force control to achieve precise, total resistance force. During the operation, the moving platen and bolster (visible in Figure 2) are kept parallel through velocity and force profiles and also through the injection of IMC for even part coating.
Selecting a motion controller
For the retrofit, Macrodyne selected motion controllers from a supplier used for more than 10 years (Figure 3). Easy integration was a goal, noted Jeremy Neil, executive vice president of Macrodyne. “You don’t need to be an expert in motion controls to use them,” and they worked well with two major PLC brands.
“We have experience with other motion controllers,” added Daniel Sion, Macrodyne electrical controls engineer. “Compared to other products we’ve tried,” the motion controllers selected are “easier to interface with PLCs and provide more functionality and flexibility.” They also are capable of more precise positioning and force control.
To provide position information, the four leveling cylinders were fitted with linear magnetostrictive displacement transducers that have synchronous serial interface (SSI) connections to the motion controller. To provide force information, each cylinder is instrumented with two pressure transducers, one connected to each cylinder port.
Virtual gearing, dual loop control
To keep the cylinders level throughout the cycle, the motion controller is programmed to perform virtual gearing. This is a command from the motion controller that causes each of the four leveling cylinders to precisely follow the motion of a master axis. While the position of each cylinder is being controlled, the force each cylinder applies is also being controlled. This arrangement supported by the motion controller is called dual loop position-force control.
Three sequential steps
The press operation consists of three sequential steps, with the motion controller driving the four leveling cylinders through three position and force control profile sequences. The three steps are:
- Velocity force: While being pushed down by the moving platen, the leveling cylinders hold parallelism and act to oppose the velocity of the moving platen until it closes on the glass fiber material.
- Resistance force: During the curing profiles, some of which also involves coating the glass fiber material, the leveling cylinders must hold parallelism and oppose the force being held by the moving platen to ensure parallelism control.
- Stripping force: At the end of all cure profiles, the moving platen will decompress and let its weight be carried by the leveling cylinders. The leveling cylinders will start a closed-loop position move while limiting the stripping force.
Programming the motion
Programming the motion is done using a software interface from the provider of the motion controller. It’s a “user friendly programming and commissioning tool,” Sion said. Most motion operations can be programmed by selecting functions from pull-down menus and inserting parameter values into fill-in boxes, but sometimes there’s a need for special function programming, math calculations and event triggers. The Macrodyne engineers took advantage of this “user program” feature to tailor the motion controller functionality to respond to PLC events.
“Tuning proportional-integral-derivative (PID) controllers can be difficult,” Sion said. “In particular, when multiple cylinders are connected to the same slide it creates tuning problems.”
To solve the problems, the Macrodyne engineers used tuning wizard software from the motion control vendor. The wizard develops a mathematical model of the system being controlled and uses the model to predict the correct gains for the control loop parameters. Some additional tweaking may be required, but the tuning wizard software can develop an optimal or close to optimal solution.
New life for old presses
Macrodyne’s experience designing and manufacturing new presses enables them to retrofit older machines. Older presses that are structurally sound can be given new life, and, in many cases, new levels of functionality and precision can be achieved with significant cost savings compared to buying new presses by upgrading the core hydraulics and the press control system.
Bruce Coons is regional applications manager, Delta Computer Systems Inc. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media, email@example.com.
KEYWORDS: Hydraulic retrofit, motion control
Large presses can be retrofit with new motion controls.
Wizard-based software helps with application development and optimization.
Retrofits save money compared to new press purchase.
What retrofits could increase performance among your applications?
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