How Delta fits in the Motion Control WorldInjection Molding Case Study
HPM Hemscheidt: New Injection Molding Machine Control Improves Performance and Lowers Cost of Ownership
HPM Corporation is the second largest manufacturer of plastic injection molding machinery in the United States. An HPM subsidiary, HPM Hemscheidt of Schwerin, Germany produces hydraulic and hydro-mechanical presses for a wide range of applications, the largest segment served being the automotive industry. At the high end of HPM Hemscheidt's product line is the Next Wave™ family of rugged, two-platen clamp hydro-mechanical molding machines. Features of the Next Wave system include accurate high-speed clamp velocities, rapid tonnage buildup of 3,500-50,000 kN (380-5,500 US tons), flexible stroke and daylight specifications, and floor space requirements that are 30-40% less than other machines of the same type and class. The Hemscheidt machine employs retractable tie bars that facilitate easy removal of molded articles and fast mold exchange.
To offer their customers an increase in precision and maintainability, and to upgrade control system components to the newest-generation technologies, HPM Hemscheidt has just completed an overhaul of the control system that drives the Next Wave machine. One of the unique aspects of the new design is the use of a flexible "open system" bus to form the backbone of the control system architecture.
The chosen interface is PROFIBUS, an internationally accepted standard that is supported with over 1700 different system building blocks from more than 800 manufacturers, allowing HPM to use "best in class" components for the design and to enhance the machine's worldwide support ability. Reconfiguration and upgrading of the system is easy with PROFIBUS, a high performance (12 MBaud) RS485 serial link, because components can be added and subtracted by plugging them into the bus and programming their interfaces using standardized data communications structures. The standard "pluggable" system interconnect of the PROFIBUS also lessens the time to manufacture the new machines by reducing wiring time. This decrease in manufacturing cost helps HPM Hemscheidt price its machines more competitively in the marketplace.
The key control system components selected for the Next Wave system include the Siemens S7, a new programmable logic controller (PLC) from a leader in the PLC industry. The PLC acts like a functional supervisor for the machine. It issues motion commands and communicates with the operator interface (often called HMI for human machine interface). The PLC is connected via PROFIBUS to an RMC-101 series high-speed closed-loop servo motion controller from Delta Computer Systems, Inc. of Vancouver, WA. The Delta RMC controller has the ability to transition from hydraulic control based on position inputs to hydraulic control based on pressure, without stopping the motion. And the Delta module controls signals to a proportional valve, adding or subtracting hydraulic pressure in minute amounts. Because of the system's smooth operation, hydraulic pressure transients and pressure overshoot are reduced, eliminating flash and the potential for hydraulic leaks, and extending the life of the machine. The smooth injection also result in even density within the molded parts, which contributes to their strength. Some older-generation hydraulic control systems control via cylinder position sensing only, and use valving with only "open" and "closed" positions, which can result in imprecise controls and lower quality output as well as the need for more frequent system maintenance.
The selection of PROFIBUS and the Delta motion module provides other advantages to the HPM Hemscheidt system designers, which are directly linked to the performance advantages that the updated Next Wave control system offers for injection molding customers. One such advantage is support for higher "scan rates," the rates at which the controller tests its inputs, makes decisions and generates controlling outputs. The Delta module is capable of "closing the control loop" one thousand times per second. This is up to four times the rate of the previous control system in the Next Wave. Faster scans and closed loop motion control of the press clamp mean faster machine operation and higher productivity for the molder. To ensure the high sampling rate, all transducers and valves for the closed loop axes are direct connected to the Delta module.
The Siemens PLC programs the Delta RMC by writing sequences of motion commands called "steps" into the RMC, a process which is simple as loading numbers into a spreadsheet table. No complicated software programming is required to generate sophisticated motion profiles and PID loops as is usual with other motion controllers.
In the Next Wave machine, the Delta motion controller controls the following operations:
- Injection speed control, 10 step profile
- Holding pressure control, 10 step profile
- Back pressure control, 5 step profile
- Screw speed control, 5 step profile
- Mold motion control (5 steps opening, 5 steps closing)
Because of the simplified programming of the Delta RMC, HPM Hemscheidt was able to complete and deliver the redesigned control system in less than one year from design start to production. Since it typically takes two years to design and implement a sophisticated motion control application with most motion controllers, HPM Hemscheidt's customers benefit by getting quicker access to the new technology.
Delta's RMC motion controllers also support the ability to allow other computers or controllers to monitor its operation. Using this facility, the HMI computer can generate system diagnostic information, such as graphical displays of actual versus target motion profiles and logs of motion commands, which can decrease service times and allows for the machine to be serviced by less experienced maintenance people.
The temperature system consists of discrete closed-loop controllers with PROFIBUS slave interfaces. Each controller can control up to 32 temperature zones. The system is designed for 8 barrel zones on the machine and up to 56 zones for hotrunner control. 24 hotrunner zones are placed within the main cabinet. The other 32 hotrunner zones can be placed in a separate hotrunner cabinet.
The distributed I/O system (from Phoenix Contact of Blomberg, Germany) consists of one PROFIBUS node for all the I/Os within the cabinet (e.g., valves and limit switches that are used to activate the tie bar retract mechanism, and inputs that provide diagnostic information).
The new control system also includes a completely new HMI system. The HMI is used by the machine operator to set up the machine's operation and view diagnostic messages. A Siemens FI-15 industrial PC with custom keypad was chosen, with the Procon Win application software package that runs under Windows NT or Windows 98. The Procon package automates the process of developing operator interfaces by providing pre-programmed functions for controls and status displays, allowing HPM Hemscheidt to provide a state-of-the-art operator interface that can monitor the process in real time (the Procon application updates the operator screen approximately three times per second).
To the best of our knowledge, the new Next Wave system is the first injection molding machine to be based around PROFIBUS, using control elements assembled from different manufacturers. The result of this assemblage of leading edge technologies is a machine that offers a high level of precision, maintainability and reliability to the injection molding industry.