A hydraulic pump directly coupled with a servomotor is an energy-efficient solution that delivers highly dynamic performance with double-digit energy savings typical. The press manufacturer Lasco Umformtechnik GmbH, a machine tool builder in Coburg, Germany, uses this servo-electric pump drive for solid and sheet metal forming presses.
“This drive system provides many performance advantages,” said Harald Barnickel, head of the Electrical Engineering Department at Lasco. “It provides substantial energy savings for companies operating our presses,” said Barnickel in describing the advantages of this forming press technology.
This manufacturer of systems for solid forming and sheet metal forming produces screw presses, forging hammers, forging and cross wedge rolls and complete plants that are used by the building materials industry to produce sand-lime products.
When Lasco was founded in 1863, the main focus was on the iron foundry. However, just a few years later, it began producing water turbines, followed by forging machines in 1880. Since then, the company has concentrated on producing machines for forming technology and has been active worldwide for many years.
“As a solution provider, we develop customized systems for optimized work piece transport within the press as well as for feeding and removing work pieces,” Barnickel said.
The company has a strong focus on continuous improvement in its processes and technology. The servo technology is a technological milestone, as demonstrated through Lasco’s implementation in a recently delivered deep drawing press with a press force of 800 tons for up to 40 strokes per minute.
The servo-electric pump control for presses, developed by Lasco Umformtechnik GmbH in Coburg, uses far less energy than conventional valve control systems, often well into the double-digit range, according to this press builder.
A Fast Controller
At the core of this solution is a servomotor, which directly drives the hydraulic pump. This drive is controlled by a drive platform.
A motion control system handles the complete path, velocity and position control of the axes—the controller and drive are both from the same manufacturer.
With a response time of 250 microseconds, up to 256 axes can be rapidly synchronized and precise axis motion and curve profiles are executed.
“The initial applications have demonstrated that this type of servo technology provides outstanding results,” said Barnickel. This applies to the closed-loop control performance, its energy efficiency and the application as a whole.
The challenge on the deep drawing press was to control the press force and press speed, based on a motion profile that could be set individually. The hydraulic force and, therefore, the press force can be adjusted at the servomotor by means of the torque. The plunger speed is controlled by the motor speed and by the volumetric flow rate of the pump. For the project described, axial reciprocating pumps with a fixed displacement per revolution were used.
Pump System Scalability
Four axial reciprocating pump systems were connected simultaneously for a maximum pressure of 250 bar supply to the pressure line for the press stroke of the plunger. Three additional pump systems are responsible for the return stroke of the press plunger.
“The possibility of scaling or adapting this kind of standard servo solution to the actual requirements is another advantage of this new technology that certainly cannot be underestimated,” Barnickel said.
The hydraulic oil tank of the deep drawing press holds about 2,377 gallons (9,000 liters). Because of a high flow rate of 16,000 liters per minute, filling valves are used for the fast downward motion of the plunger. The servo pumps provide the required flow rate for the actual pressing operation, with a speed of up to 100 millimeters per second.
In the past, high-precision control valves with zero overlap were required to obtain the precise traversing profiles. In this arrangement, this function is handled by the motion control system in conjunction with the servo pumps. Because of this new technology, the valve systems in the press can be reduced by up to 40 percent. The remaining valve technology is mainly needed to comply with machine safety specifications.
The drive unit for the pump control consists of servomotors, drives and motion control devices.
Energy Milestone
The new technology allows expensive, traditional valve systems to be eliminated, and energy efficiency has been significantly improved.
“In the past, the large cooling systems for the hydraulic oil had to be installed before the first test run at Lasco. Now, with the servo technology, significantly smaller units can be used, and they are not required until the press is actually operational at the customer’s facility,” said Barnickel.
One reason for this is that the oil is no longer forced through the narrow gaps at the control edges of the control valves, which means that these types of technology-related losses no longer occur. The press manufacturer sees efficiency improvements well into the double-digit range depending on the application, significantly reducing the end users’ electricity costs.
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.