What do you do if you’re in the business of producing a wide variety of metal presses to meet special customer requirements? You choose your system components wisely, and flexibility of function plays a large part.
Multipress, Inc. of Columbus, Ohio, is a leader in design and production of hydraulic press technology based on flexible architectures, The company has been producing hydraulic presses since the 1920’s, for applications as diverse as high-speed stamping of self-locking nuts (3-4 nuts/second to a +/- 10 pound tolerance), metal extrusion, and forming of metal components. Over the years, Multipress has continually focused on evolving and enhancing the technologies employed in their presses.
Multipress’ two most recent presses will be used to form metal components for the automotive industry. These presses needed to move quickly, but also precisely. Multipress’ system designers met the challenge by setting up the press to operate at three different speeds. At the start of a press cycle, the hydraulic ram needs to move very quickly. Then, it needs to slow down as the business end of the press ram approaches the work piece.
The faster steps in the process use open-loop moves for maximum speed, and when the machine slows down near the end of the cycle, it does precise positioning by making minute, precision flow control adjustments via a servo valve. Though each press uses a 31-inch cylinder, closed-loop position control is only needed for the final 3 inches of motion. Under closed-loop control, final positioning of the axis is accomplished to within .001 inch of target values.
Flexibility the key
There are three key types of components in the typical press: mechanicals, hydraulics, and electrical/control elements. Correct decisions must be made in each area. If the mechanical design isn’t right, there may be no way for the hydraulics to make it work effectively. If the hydraulic components aren’t selected and sized correctly, an electronic motion controller may not be able to maintain precise control.
The hydraulics and electronic controls for the Multipress machines were designed in conjunction with Advanced Industrial Products, Inc. of Plain City, OH. Multipress strives for simplicity and flexibility in the designs they do and a track record of success proves that this approach works.
To control the hydraulics, Multipress needed a programmable electronic motion controller that was capable of smoothly transitioning between several different modes of operation. The company selected the RMC75, manufactured by Delta Computer Systems, Inc. of Vancouver, Washington (see Figure 1).
For maximum flexibility in the hydraulics, the Multipress machine (see Figure 2) uses an open loop regenerative circuit for fast advance and fast feed speeds. A small servo valve is used to do final press positioning on the main ram cylinder. The position of the ram is controlled using a Balluff transducer. The Delta controller enables this system to run in both open and closed-loop modes, allowing a cost savings and energy savings by not having to use large servo valves.
Multipress’ systems in the past have had pressure transducers (load cells) to indicate the completion of a cycle. The addition of the Delta RMC75 gives both the capability for pressure reversal and the added benefit of position reversal. The programming of the RMC75 allows the user to choose either mode (pressure or position) and is easily accessed through the HMI located on the operator screen.
According to Barney Raye, President of Multipress, “The beauty of the Delta RMC controller is that it is so adaptable. We can run a combination of open loop moves or closed loop moves. By using a servo valve, the RMC is capable of attaining a very high degree of precision, interfacing directly to position and pressure/ force sensors.
”Rick Snyder, Multipress Controls Engineer, wanted to ensure that any servo control system would work in concert with different types of PLCs. “We have many customers who have their own specifications for PLCs,” says Snyder. “We must maintain the ability to satisfy their specification requirements while providing the performance they demand. The digital I/O handshake we can do with the Delta RMC to any PLC I/O module makes interfacing the Delta RMC a very customer-friendly solution.”
Multipress has used PLCs for years. The new generations of systems use a marriage of PLCs and electronic motion controllers, with the motion controller and PLC coordinating elements of each other’s function. For example, the small Allen-Bradley PLC in the Multipress system performs supervisory control and safety functions such as controlling a light curtain. It also controls the sequential logic in the system, turning valves on or off based on inputs from the RMC.
Sometimes, using components with a high degree of flexibility can pose a challenge to system designers. The biggest challenge that Advanced Industrial Products’ Mike Joyce had in developing the latest Multipress control system was dealing with all the flexible control options that the RMC provides. “The online help menus that come with Delta’s RMCTools software are very self-explanatory,” says Joyce. “Whenever I had a question, I was able to get answers very quickly.”
One critical thing that Joyce needed to resolve was how to run the system open loop, even through there’s a servo valve in the system. Unlike two-position “bang-bang” valves, servo valves are controlled by a variable voltage or current, and setting this voltage or current level without feedback can cause unpredictable results. The electronic controller enabled them to use a much smaller servo valve than they would have otherwise needed.
Multipress and Advanced Industrial Products found tuning of the system for optimal operation to be relatively simple. To aid the tuning process, Delta’s software provides the ability to plot the system’s motion over time, displaying actual versus target values of parameters such as axis position, velocity, and acceleration. In the tuning process, control loop gain parameters are adjusted so that the actual and target curves are brought closer together. When they precisely overlap, the system is tuned correctly.
“The plotting feature provided by Delta’s software is worth its weight in gold,” says Joyce. “We use a counterbalance valve in the system to maintain pressure on the rod since it’s mounted vertically in this press,” he said, “and at the low initial gain setting, flow through the servo valve couldn’t catch up to enable the rod to move down smoothly. When we brought our gain up to the proper level, the target and actual position and velocity curves on the plot overlapped, signifying that the system was optimally tuned.”
For more information on Multipress, please visit www.multipress.com .