How Delta fits in the Motion Control WorldPowder-Compacting Press Case Study
Best Press: New Generation Powder-Compacting Press Employs the Latest in Hydraulics Control Technologies
New hydraulic presses can produce complex parts better and less expensively than even the best mechanical presses. One suchmachine is a new generation powder-compacting press from Best Press Corporation of Castle Hayne, NC. The press is capable of forming complex-shaped molded parts with uniform density throughout a varying cross-section and can handle a range of different powder materials.
The application integrates a variety of off-the-shelf products and employs the latest in PC-based control software. It is unique from a control system standpoint in its use of a smooth transition from position-based servo control to pressure-based control, with up to 6 axes moving simultaneously. The Best Press design incorporates hydraulics control technologies from Delta Computer Systems of Vancouver, WA and Tri-Tech Engineering of Saginaw, MI, and control software by Steeplechase Software of Ann Arbor, MI.
Hydraulic press offers better control for complex parts production
Uniform material transfer is key to powder press design. Multiple concentric cylinders and pistons form a cavity. Powder is pushed into the cavity and then is compressed by the cylinders into the shape of the part being manufactured. The powder has binders in it that, along with the pressure, hold the pressed part together. After the compression step, the part is baked in a process known as sintering to achieve the hardness of the final product. The process of manufacturing formed products in this manner allows the manufacturer to avoid the expensive machining step that was traditionally used in parts manufacture.
The key to pressing multiple-level carbon parts and achieving uniform material density is to make sure that each segment of the manufactured part is compacted according to its own pressure. This level of control can't be obtained with a mechanical press; mechanical presses change the material density as the part profile changes. With a hydraulic press, on the other hand, the control authority and flexibility exist to allow the press to be adapted to fit the material rather than the other way around. Using the new hydraulic press, a compacted-powder part can be made for less than half the cost of a machined part.
The machine goes through multiple steps in the molding process. Each step is coordinated by a closed-loop control algorithm. The steps are:
- Go to "fill" position. During this step, the cylinders come to initial position for filling. The lower main cylinder drops down and the top cylinders move up.
- Material transfer phase. The feed shuttle is extended to fill the powder cavity with a single operation (Best Press has a patented rotating blade and auger system for uniform displacement of material). Then the shuttle is retracted as the upper cylinders are brought down to close the cavity. All the cylinders move simultaneously into press position. As the upper cylinders come down, their velocity is profiled (i.e., tapered down) to make sure they don't force (blow) material out of the cavity. As this step in the process continues, the upper and lower cylinders are brought in contact with the material. The cylinders must all be moved simultaneously to displace the material equally and to fill up all the spaces in the mold cavity
- 3. Compression phase. Once the system is at the press position, the control algorithm seamlessly switches from closed-loop positional control to closed-loop pressure control for all the axes. During the compression (or "tonnage") phase, each cylinder is brought to its peak pressure in a coordinated manner so that the peak pressures all happen at the same time. That allows the press to produce a part that has uniform density throughout.
- 4. Eject phase. Once the compression phase has been completed, all the axes move up simultaneously in order to eject the part. Care must be taken to make sure that the part receives no further compression. The part is pushed up out of the die cavity by the lower cylinders. The feed shuttle then comes forward as the upper cylinders retract to get out of its way, and displaces the part onto the table.
New hydraulic press incorporates off-the-shelf components
A major factor in the new system design was our need to ensure that the hardware be easy and quick to configure and reconfigure to support different press models and different types of powder stock. In the interest of flexibility and ease of programming we chose an industrial PC as the control and human-machine interface computer, and Steeplechase's Visual Logic Controllerâ software to supervise and coordinate the process steps.
To connect the industrial PC to the rest of the system, Best Press wanted an industry standard data interface that was already supported by a PC plug-in card and also well-supported by the control modules they would be using in the rest of the system. We selected a PROFIBUS interface card from SST of Waterloo, Ontario. PROFIBUS, the popular serial fieldbus, gives this standardized system expandability and high-speed (12 Megabaud) communications.
We chose the Delta RMC100 motion controller as the heart of the control system due to its high-resolution position/pressure control, fast loop times and ease of programming/monitoring. Each RMC is capable of controlling up to eight axes of position-based control or four axes of motion based on inputs of both pressure and position information. We needed to use two RMCs in the Best Press design because 6 total control axes of position/pressure control were required. Each RMC controller is a standalone module in a DIN rail-mountable box containing a PROFIBUS interface, a CPU, and 16-bit analog input and position transducer interfaces.
When it came time to select the hydraulic control valves for the system, we needed valves that can be controlled by proportional inputs to provide high-speed responsiveness and very precise operation. These requirements were met by zero-overlap linear spool valves from Moog Hydraulics of Aurora, NY. These valves are often used in the aircraft industry, and can respond to high-frequency (up to 1KHz) inputs. The valves are called "zero overlap" because there's no "dead zone" between active control ranges (ranges that increase or decrease hydraulic pressure). Valves with overlap may be advantageous for manually-controlled systems, but not for high performance, high precision position/pressure systems such as the Best Press Design.
The next step in the design was the selection of position sensors to match the choices of motion controller and hydraulics. We selected Temposonics® magnetostrictive displacement transducers (MDTs) from MTS Systems Corp. of Cary, NC. A big advantage of using an MDT to measure cylinder position is that it knows exactly where the hydraulic cylinder is at all times. There's no need for performing homing functions at power-up or system reset. MDTs have the additional advantage of providing non-contacting measurement. Because of this, there's very little to wear out in contrast to measurement systems that involve resistive measurements or limit switches. Also, the repeatability of measurements taken by MDTs is very high.
To simplify the design of the system electronics, the Delta RMC motion controller comes with a standard MDT interface. Two twisted wire pairs connect directly from the RMC to each position transducer.
We chose Phoenix Contact interfaces for digital I/O functions including operator pushbuttons, discrete solenoid valve control (e.g., for hydraulic loading/unloading and control of functions requiring only on/off control). We used Phoenix analog outputs to control the open loop pressure relief valves. Phoenix Contact (Blomberg, Germany) was chosen because their controls were easy to use and because they could provide a 100% deterministic scan rate.
The application development process
The first step we went through in the application development process was to prove out the communication between the hardware modules in the system. Next, we programmed a process sequence into the Steeplechase Visual Logic Controller software to handle communications between the PC and the Delta motion controllers. As a hydraulic motion profile is set up, the PC downloads motion sequence instructions into the Delta RMC. For example, the profile for bringing the upper ram cylinders down to initiate the compression phase involves multiple steps that we loaded into the RMC all at the same time.
The RMC is programmed via tables of information. In the table, each axis has 6 words or command fields that describe how that axis is to move. For example, when an RMC is in position control mode, we put the following information into the table to give a command to move a hydraulic cylinder to a particular position: a mode word (information on how to interpret the other fields and what inputs to monitor), the rate of acceleration at start of the motion, the rate of deceleration at end of motion, the maximum speed to accelerate to in the middle of the motion, the final position we want the hydraulic cylinder to go to, and the command to "go." All this allows for smooth transition from position to pressure control using the same servo valve.
In pressure mode, i.e., when the motion controller is getting its inputs from pressure transducers, the commands which we program into the controller's function tables include information such as a pressure set point which specifies when the controller is to go into pressure set mode, the pressure at which to go out of pressure mode, and the pressure ramp rate.
By changing the numbers stored in the table entries, we can tune the speed and duration of the hydraulic cylinders' motion and the pressure profiles of the compression phase. The RMC provides the added advantage of allowing an attached computer to interrogate its internal function table and sensor inputs, for the purpose of comparing actual and desired motion results.
Each command in the motion controller's function table also has a link to the next command to be performed, so the motion controller can execute a sequence of operations without further intervention from the controlling computer. Links between active motion commands can introduce delays or additional testing or setpoint criteria. Because the RMC has the ability to execute multiple motion steps in sequence and to test for conditions, the PLC or PC that controls the RMC can be offloaded of this work.
The RMC continuously monitors its input conditions as fast as once every millisecond, and uses "smart" PID loops to maintain pressure by "dithering" the hydraulic valves. Dithering is a process whereby the hydraulic valves are controlled to add and release fluid pressure in minute amounts very quickly. The servo valves accept voltage or current inputs that vary proportionately with the rate of fluid to add or decrease from the cylinders. The servo-controlled loop that is maintained by the system can be tuned very precisely, allowing for very little hysteresis effect.
Because the motion sequence information is software programmed, it is easy to reconfigure the press to produce different manufactured parts or to use different powder materials (e.g., ceramics, carbon, powdered metal). Every time changes are required, the PC sends new profile information over the PROFIBUS to the Delta RMCs. As a safety check, the contents of the Delta motion table are read back by the PC to make sure that the correct information was loaded. Changing setpoints can be done on the fly to adapt the system to changing environmental conditions (e.g., temperature and humidity of the press room).
The position-sensing operation involving an MDT works as follows: The MDT provides a pulse in response to an interrogation pulse, with the time delay between pulses interpreted and translated into a distance measurement by the Delta RMC (the delay/distance conversion factor happens to be 9 microseconds/inch). The RMC is capable of translating the position information to within 1/1000" resolution.
Best Press chose Wika pressure sensors (Wika Instrument Co. of Lawrenceville, GA) because the company had previous experience with them. The pressure sensors are positioned in the system as close to the center of each cylinder as possible to increase the accuracy of pressure measurement as much as possible. The pressure control algorithm is provided by Delta as a function contained in their firmware. When the press is being programmed, the operator enters a tonnage setpoint, which is loaded into the function table for the RMC motion controller. When in operation, the RMC controls pressure to this setpoint.
Overcoming design challenges
One of our main challenges in implementing the Best Press design was the coordination of the six independent position/pressure control axes of the machine. As mentioned before, the control task was partitioned between two RMCs to handle the six motion axes. Separate sets of program sequence information and commands are flowing over the PROFIBUS between the PC and each motion controller, yet the two motion controllers need to be synchronized with each other. We implemented this using the parallel branch and merge capabilities of the Steeplechase control software to simultaneously initiate the execution of two different sets of motion control commands over PROFIBUS. To further ensure that their activities are coordinated, we set up the RMCs to signal one another using their onboard digital I/O.
During the development process, we tuned the operation of the machine using the RMCWin setup and tuning software provided by Delta. This software continuously reads back and graphs the actual position/pressure data from the RMCs. The raw data from the sensors is scaled and translated automatically by each RMC, which allowed us to work with sensor readings in human-meaningful terms. For example, we have the ability to obtain data from the RMC in inches of position or pressure in PSI instead of the raw readings from the transducers. This data can also be used for statistical process control.
The flexibility and open modularity of the Best Press system is state of the art. With its smooth transition from position to pressure-based control, the system offers tighter control and more flexibility than mechanical presses, with higher-resolution linear positioning in the control of hydraulic cylinders than older-generation hydraulic control with discrete "bang-bang" valving. The press is programmed electronically to dramatically shorten setup time and to adapt to the materials being formed. For the powder-compacting press industry, hydraulic is the way to go.