In an ambitious pilot installation, press manufacturer Schuler Hydrap uses Ethernet for super-fast communication between its PC-based control system and the distributed I/O in its Profiline® hydraulic presses. The new line runs on a network that is up to 500% faster and provides a very attractive price point to Schuler''s customers, thanks to open technologies such as Beckhoff''s EtherCAT® and TwinCAT® PC-based control.
Schuler Hydrap, located in Pluderhausen, Germany, specializes in the manufacture of hydraulic presses, such as the Profiline, designed to meet the demands of electric appliance manufacturers and automotive suppliers. It was the perfect time to implement a new PC-based control system on their new 160 and 1,600 ton product lines.
Schuler previously used conventional PLC controls such as a Simatic S7, and proprietary hydraulic controllers for the hydraulic control tasks. Both controls communicated to peripheral via the Profibus fieldbus system. The hydraulic control is the core competence of a press manufacturer, and is usually not disclosed to third parties. Schuler was interested in replacing the traditional PLC control and hydraulic controller.
By switching to high-performance PC-based control, Schuler gained the ability to integrate hydraulic control into PC-based logic, where it could be easily modified as well as protected. Also of importance to Schuler was to have a uniform control platform across all products and to be IEC based, covering all demands for control tasks and offering platform independence.
"A control like the PLC -- with absolute-based addressing and no local data encapsulation -- just does not belong with this class of controllers," says Peters.
Following a thorough market study, Schuler decided to implement Beckhoff''s TwinCAT Automation software as the preferred control system. All control tasks, from sequence to hydraulic to drive control, could be implemented in one uniform platform. In addition to this, it was possible to operate Schuler''s in-house HMI software, called Basic-View, which is based on a Siemens protocol and provides operator interaction with the machine. The HMI and TwinCAT control run on a single industrial PC, thus replacing three controllers, including the HMI, PLC, and hydraulic controller, with one industrial PC.
"The integration of the hydraulic control into the PLC software already allowed us to run an improved process and thereby improve the quality of the manufactured parts," says Ralf Sohr, manager, Electric Design at Schuler Hydrap.
Despite this encouraging progress, the interface in the process -- the bus system -- was still seen as a bottleneck. Clement Peters, corporate manager, Controls & Drives Design for Schuler, explains that "by using NC Tasks in TwinCAT, we are able to model transfer applications with high quality motion control, utilizing 1 to 2 ms cycle time. It was critical to make use of a uniform platform -- using TwinCAT in the hydraulic control -- the bus system, which would fulfill the requirements of the hydraulic control. This means that fast (significantly less than 1 ms) reading, computation, and writing of analog signals and encoder positions is mandatory for the system."
With the existing system -- a conventional PLC and proprietary hydraulic control modules with Profibus -- Schuler was unable to reduce the cycle time to lower than 5 ms, because of the relatively high overhead of the communication system. By moving to a PC-based system, the CPU was able to provide the required cycle times; however, "as long as Profibus was the communication network, the TwinCAT system consumed only about 10% of the total cycle time," says Sohr. "The remaining 90% was mainly consumed by the communication with the I/O system." With this in mind, Schuler decided that further enhancements to the control system would be too limited by the bus speed.
Ethernet Pushes the Boundaries
The network bottleneck situation improved significantly with the introduction of Beckhoff''s Ethernet-based EtherCAT® system. Schuler decided to integrate EtherCAT into a pilot project for the new Profiline control.
"The speed of EtherCAT and its small overhead gives us significant advantages in process velocity. This means that we are able to execute fast motion and hydraulic control for all applications," says Sohr. "And, most importantly, the extremely high performance of EtherCAT gives us a greater ability to adapt to future developments in technology and handle more demanding control tasks without any restrictions in process speed."
Schuler did not make the decision based solely on the performance features of EtherCAT.
"The EtherCAT system''s high granularity to interface with machine modules also contributed to our decision," Sohr says. "During the implementation we recognized that there is a significant difference in cost between the Profibus and Ethernet solutions. A Profibus Master card costs approximately eight times more than an Ethernet card. And for the slave modules, you can buy about three EtherCAT modules for the cost of just one Profibus module."
Reduced Hardware Cost, Increased Simplicity
Despite all advantages in technology and the considerable cost-effectiveness of EtherCAT, Sohr clearly sees further potential as technology providers improve Ethernet connector technology. For example, the RJ45 connector requires no shield and is simply and easily assembled with the help of a screwdriver.
Although the priority of using EtherCAT at Schuler is on hydraulic applications, Clement Peters plans to expand EtherCAT applications into other areas. "We see the potential for application in the press transfer system," he says. "We expect to guide the transfer system with centralized command values for a virtual main shaft and synchronize modules and components such as transfers, conveyors, feeders, or palletizers. This means that we would leave consecutively reacting subsystems behind and can, therefore, achieve a higher output from the press line."
Today, the Profiline control -- despite the EtherCAT network -- needs additional bus systems, but Schuler plans to eliminate those through the expanded use of EtherCAT. The reason is obvious: multiple bus systems require extra effort in planning, configuration, and commissioning. Peters explains, "It is mandatory that our key suppliers are able to deliver EtherCAT-enabled components."
Currently, the drive manufacturer for Schuler does not offer an EtherCAT interfaced drive. Therefore, SERCOS is still used as interface between axes and the TwinCAT PC control system, also provided by Beckhoff. For similar reasons, the interface between TwinCAT and the Profiline safety system of the press is Profibus. This situation is one of the reasons why Schuler joined the EtherCAT Technology Group (www.ethercat.org). "We wanted to send a signal to suppliers by joining the EtherCAT Technology Group to encourage them to support EtherCAT."
The Ultimate Goal: Ethernet As Uniform Control Standard
As far as Schuler is concerned, support from component suppliers is only half of the deal. The $20,000 question is whether the end user of the machine will accept an Ethernet-based control system. The controls expert from Schuler is optimistic: "Beckhoff controls will be mainly used in the Profiline presses, with its main focus on the supplier market. Because we defined this control as our standard, we will have to deal with less customization. For the end customer, this will result in a very attractive price."
The future goal for Schuler as a system provider in the metal forming business is to use a uniform control platform throughout the Schuler group. After successfully implementing common practices in motion control applications and programming standards for the main control platforms, now the components are being uniformly selected. With these activities, Schuler is creating a common look and feel for its machines and machining lines.
Along the work of standardizing PLC and motion control applications within Schuler, a library for hydraulic control is in the works. In practice this means that, in the future, a mechanical cross bar transfer press from Schuler will be based on the same function blocks as a hydraulic press from Schuler SMG or Schuler Hydrap.