This drive optimizes hydraulic presses from a technical and energy efficiency perspective.
by Harald Poesch

When comparing the new technology with basic hydraulic presses that use conventional technology, the amount of energy saved is certainly less than with presses equipped with complex control systems.

The energy-saving effect is especially apparent with the drawing operation, during which the drawing pressure is provided by a pump equipped with a servomotor, with the motor of the die cushion pump operating as a generator.

Another advantage of the electrical system is that energy can actually be recovered when the hydraulic fluid decompresses. This occurs when the system is decompressed after the pressing operation and the main cylinder pressure must be reduced from 250 bar to approximately 10 bar before the filling valve to the tank can be released. This operation requires approximately 100 milliseconds. In the case of a compressibility of the hydraulic oil used of approximately 2.5 percent by volume, with a cylinder volume of approximately 800 liters of oil, the “spring energy” in the oil flows through the axial reciprocating pump and drives the servomotor. This effect is amplified when the press mechanical system is relieved. In this so-called generator or regenerative operation, the corresponding electrical energy is fed into the DC link of the drive during each cycle.

“However, in addition to the energy recovered, the other advantage is that there are no additional power losses in the form of heat, as would be the case with conventional concepts,” Barnickel explained.

Straightforward Optimization and Documentation
Although the controller has been specifically optimized to address motion control applications, it can also control the complete system. However, Lasco’s philosophy is to separate the different press tasks. Therefore, it uses a separate programmable logic controller (PLC) for the system control.

As a fail-safe control, this also addresses the safety technology requirements. The controller can be programmed in many ways, for instance, with high-level language, graphic programming, or a motion control chart as well as a classic logic control.

The trace function with eight-channel plotter supports commissioning engineers and ensures detailed documentation of the press operation. Control parameters are saved in the drive, which guarantees seamless documentation of the drive system.

Safety Technology Simply Integrated
In addition, the fail-safe PLC, which is used as the press system control, allows all safety technology to be implemented without any major additional expense.

The fail-safe signals are read via distributed stations from the same manufacturer and transferred to the control via Profisafe. This represents a simple yet complete solution for Barnickel, who said, “As a consequence, we obtain a clear machine structure.”

For instance, two valves are generally monitored, which keeps track of the reduced traversing speed of the plunger, when setting up. The new solution is far more precise, because the fail-safe drive also monitors the reduced speed (safely limited speed—SLS) as well as a safe stop (safe stop 1—SS1).

A Seamless Integrated System
The press experts at Lasco also view the integrated, seamless nature of the systems as an additional benefit. For example, the drive automatically identifies the servomotor and reads its parameters. Another feature of this servo solution is that the motors are available either as synchronous or asynchronous.

The application itself, or more specifically, the dynamic performance required, is the decisive element in selecting the appropriate motor type, according to Barnickel.

Both versions have the same mechanical mounting dimensions, the same wiring and the same encoder connection. This state-of-the-art technology has some significant advantages. For example, instead of a typical 17-core cable, a single cable with RJ45 connector is sufficient.

This new drive concept for hydraulic presses, based on  servo-electric pump control, results in a significant overall improvement.

“In addition to the energy savings, its extremely precise controllability is far superior to that of a classic valve control,” said Barnickel. He further explained that any defined motion profiles can be precisely implemented using this concept.

The motion control system has also proven to be valuable in certain steps of the pressing process. For instance, if the plunger moves to upper dead center, then the closed-loop position control in the motion control system ensures that any leakages are compensated.

“A far higher precision is achieved when using servo technology than when using pure valve technology.” Barnickel said.

Dynamic Performance of the Press
The development of new servo technology for large hydraulic presses, based on a motion control system, has already awakened significant interest among customers of Lasco. However, it is not only when building new machines that this company of 400 employees benefits from lower costs and improved quality. The drive concept described is also suitable for modernization and retrofit projects.

With the standard concept, comprising the servomotor, the drive platform and the motion control unit, Lasco has demonstrated that when hydraulic presses are equipped with servo-electric pump drives instead of pure valve technology, the dynamic performance, precision and especially energy consumption on the press are improved.

This results in tangible press improvements that are decisive in the marketplace.