August 14, 2003
Mechanical presses are challenged to provide high-speed production with a greater number of hits per minute, smaller batch runs, and quicker die changes.
They no longer are dedicated to running one product for their entire life, and once dedicated lines now perform versatile tasks and provide flexibility from one job to the next.
Whether it's for setup of a servo roll feed, an in-die transfer system, or a complex tandem line, flexibility and changeover time are keys to maintaining a competitive edge.
A mechanical press historically has used a series of cams and gears driven by the press ram to move steel into the press (feeder), between the dies in the press (in-die transfer), between the presses (loaders and unloaders)—a design that works well when the press is dedicated to one product for life.
However, with the current U.S. economic situation, many stampers have lost those never-ending contracts to produce just one or two parts. To survive these times, stampers must take on smaller jobs. It is not uncommon, for example, for a press shop to run quantities of 100 of one part followed by 200 of another part. When runs are short, changeover time becomes crucial, and requiring different cams and gears for each part can reduce a stamper's ability to win a job.
Electrical servo automation systems can replace mechanical transmission elements such as gears, line shafts, and cams by synchronizing multiple servomotors to perform the motion.
The technology is designed to provide machine flexibility by simplifying job changeovers. Instead of physically changing out mechanical components between product runs, the operator selects the new "recipe" from the control, which reduces make-ready time.
Electric servo drives also can improve part quality because they do not wear over time. Mechanical cams and gears can produce inaccuracies in parts if their tolerances go out of specification.
With servo-driven press automation, each move is controlled individually by a servo drive and motor combination, and a motion controller synchronizes all operations to the press encoder. The automation follows the press because press speed is most often the limiting factor of the line. Each servo drive and motor combination can function like a mechanical cam or gear.
An electrical cam provides a position of the motor shaft for each position of the press encoder. For example, when the press resolver shows that the press is at top dead center, the motor should be at point X. When the press is at 90 degrees, the motor should be at point Y. When the press is at bottom dead center, the motor should be at point Z, and so on.
Typically, 1,024 motor positions are defined for a 360-degree rotation of the press—that's one for each 0.35 degree of a press stroke. When a new product recipe is chosen, a new electric cam table is loaded, and no mechanical changes are necessary.
Mechanical automation also has some benefits. Mechanical cams and gears are easy to troubleshoot because an operator usually can see or hear a mechanical problem. The problem often can be fixed with a simple tool such as a wrench or pliers—tools that all maintenance people have.
Electrical systems may show only an error code when the drive or motor goes down, and an operator's manual usually is necessary for troubleshooting. If a shop has many electric drives from different motor manufacturers, it is hard to keep track of all the manuals and be trained on all the systems.
Safety is always a priority when working with presses because of the power behind them. Most operators feel that if the press and automation are mechanically linked, everything is where it's supposed to be. While this is true, electric systems also can provide a similar level of die and automation safety through redundant press and motor encoder functionality.
A press's position or angle is first measured by an absolute encoder. The reason the encoder must provide absolute feedback instead of incremental feedback is that the system must know if the press is on the upstroke or the downstroke. With incremental feedback, both strokes are shown as the same position. But with an absolute encoder, 90 degrees and 270 degrees, for example, have unique positions even though they are the same position mechanically.
The absolute press encoder then is fed into the servo automation system and transmitted to each servo drive through a communications protocol. This must be a deterministic communication, meaning it must be updated to the automation system on a strict time cycle, not to be interrupted by other activities in the press system. If the press position is delayed, the automation also is delayed. If the automation is in the wrong place at the wrong time, the die can be destroyed.
SERCOS, which stands for SErial Realtime COmmunications System, is a nonproprietary standard that coordinates and synchronizes digital-based motion control products. SERCOS is a common choice for these communications between servo drives and controls because it is digital, high-speed, and deterministic. Many manufacturers of servo motion control products provide it, and they can be interoperable, which means that one manufacturer's motion controls can be used with another's drives.
But what if this press encoder, which electrically links the position of the press to the servo automation, fails? If the press encoder fails, some motion control systems can instantaneously switch to a backup, or redundant, press encoder, and the automation will follow the press to a controlled stop.
If the motor encoder fails, the motion control system can be configured to switch to a second encoder and ride the cycle to a controlled stop. This second encoder often is mounted directly to the automation mechanics. Motion controls with redundant encoder features can reduce the risk of the automation being crushed in the dies.
In the event of a three-phase power loss, the main AC press drive sends regenerative energy from the flywheel to the servo automation system. This provides energy for the transfer system to get out of the way of the press.
As stampers research mechanical press automation, they should evaluate the main press drive and motor technology. Many are eddy-current drives or DC motor-drive systems. Eddy-current drives are basic electromechanical systems and are easy to troubleshoot, while DC drives are inherently line regenerative.
In spite of the advantages of these drive types, press automation is moving to AC drives as the main press drive because they require minimal maintenance and have few mechanical components.
Mechanical presses use power from the main press drive on the downstroke and regenerate energy during the upstroke. In DC drive systems, the power is sent back to the power line in the form of AC. An AC drive and motor system uses regenerated energy to power the servo automation for the press (see Figure 1). The energy regenerated during the upstroke is turned back to DC power on the bus system of the drive. This raises the DC bus level of the drive.
The ability to store this energy can be beneficial. It reduces the need for extra power supplies and capacitors on servo systems, because additional energy can be supplied by the main press drive. Also, regenerated power can temporarily run servo automation during a power loss. For example, if three-phase power is gone, the only source that can supply the servo automation is energy off of the flywheel. This will not last long because the press will stop, but it can last long enough to remove the automation from the press bed.
Openness, or open architecture controls, refers to the ability to integrate new control systems with existing control systems. Mechanical presses can last up to 30 or 40 years, so a variety of control architectures and technologies are found on them. If a stamper's goal is to automate certain press components, such as a destacker or in-die transfer, it should look for a system that is open to communicate with existing press controls.
It should not matter which manufacturer, for example, designed the press controller or which manufacturer's cam box is used to fire the feed angles and programmable limit switches. It is not practical for a stamper to have to upgrade all systems of a mechanical press to improve one process.
A new open-motion control system will be able to communicate with existing fieldbuses like DeviceNet® or Profibus® and use open, standard protocols (SERCOS) wherever possible. These communications allow for real-time, detailed diagnostics, such as collision alarms and error logging. They also can help to simplify preventive maintenance.
Scalability is the other aspect of a good motion control system for mechanical presses. Automating a coil processing or blanking line with a press can be a costly and time-consuming proposition. Some stampers prefer to upgrade the motion of the roll feed process first, and then work their way back up the line to the straightener and uncoiler. In this case, it is important to choose a motion control system with the ability to add functionality and hardware (motors and drives) without changing the entire structure of the system. It also is important to be able to add intelligent digital drives and motors to the system so the controller doesn't get bogged down as additional motion axes are added. Power stage amplifiers do not offer this benefit.
As mechanical press automation moves forward, the press will become more of a system and less a series of components. This concept is being used on larger, more complex systems such as tandem press lines, press-synchronized coil lines (with or without loop pits), and crossbar transfer presses to help create productive, flexible stamping lines.