Motion Control Resources
Motion Control Supports Industrial-Scale Additive Manufacturing
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 04/29/2020
Back in the day, developing new products meant spending precious time and money prototyping specialty parts, structural members, housing, and packaging. Job shops would have to fabricate metal parts from scratch. Conformal parts required the development of custom molds, which would pay for themselves in sufficient volume, but required significant front-end loading of time and cost. Worse, they were unforgiving of the kind of changes that might need to be made throughout the development and testing process, particularly when digital design was less sophisticated.
Subtractive manufacturing enabled some types of parts to be fabricated in CNC machines using blanks and G-code files, but they still had limitations. Enter 3D printing, which involves building complex three-dimensional shapes layer by layer, from a variety of materials. Commercially available systems can deposit microscale or even nanoscale features. Overall frame sizes range from millimeters to multiple meters. The technology is flexible, robust, reliable, and the market is surging. Indeed, the Global Additive Manufacturing Market report expects the market for additive manufacturing equipment to exceed US$ 6.50 billion by 2024, for a CAGR of more than 13% in the given forecast period.
|Figure 1: Image shows selective laser melting, a type of powder-bed fusion in which material is deposited, then fused by laser beam. (Courtesy of Siemens Industry)|
3D printing was a game changer. Suddenly, even highly complex custom parts of various types of metals, polymers, glass, and could be printed in a matter of days, or potentially even hours. The technology has matured to deliver the reliability, speed, and performance necessary to produce parts ranging from aircraft wing spars to chip-level radio antennae. And it rests on a foundation of motion control.
The Basics of Additive Manufacturing
Early 3D printers were novelty desktop models capable of only limited accuracy and quality, not to mention part size. Developed in research labs, the systems soon became the darlings of home hobbyists and provided important new support for the maker movement. They were in no way suitable for industrial applications. However, that was a major market gap. In recent years, OEM machine builders have stepped into the gap to produce industrial grade systems fast enough and reliable enough to be used not just for prototyping but for production (see Figure 1).
Additive manufacturing can be roughly grouped into seven classes (see Figure 2):
- Vat photopolymerization: Liquid photosensitive resin is selectively hardened in layers by laser curing. The process takes place on a build platform, which is lowered into the vat by a very small amount after each layer is hardened.
- Powder bed fusion: Powdered material is melted or sintered by a focused energy source such as laser beam or electron beam. Here too, the build platform simply needs to be lowered as the build progresses.
- Binder jetting: Liquid binder is applied to powdered material to build up 3D forms. A thin layer of powder is rolled onto the build platform, then nozzles mounted on a moving print head apply the binder.
- Material jetting: In a process similar to ink-jet printing, nozzles release jets of liquid material that is subsequently hardened, either by photo curing or by simply cooling into its solid form.
- Sheet lamination: Individual layers are laminated together by ultrasonic welding (metal sheets) or adhesives (paper sheets). Motion control is used to drive the web of laminating material.
- Material extrusion: Part is built up by material extruded from an overhead nozzle. Motion control is required to move either extrusion head or build platform, as well as to advance the coil in the extrusion head.
- Directed energy deposition: Raw material, typically a wire, is heated by focused energy source to create small pool of melt at the point of deposition. Motion is used to both advanced coil in the deposition had and position the head itself.
Figure 2: Courtesy of Hybrid Manufacturing Technology
Additive manufacturing presents a number of challenges for motion control:
High speeds and rapid reversals: To speed production, moves take place at high speeds. Depending on the part and the modality, this can require as much as 5G of acceleration.
High accuracy and repeatability: The layered nature of the process makes it absolutely essential that one layer is deposited directly on top of the last. Meanwhile, applications such as nano printing require nano-scale resolutions.
High duty cycles: Depending on the size and complexity of the parts, a build might last for hours, or even multiple days. Most shops that invest in these systems have high usage expectations. As a result, systems must survive high duty cycles of the types of demanding motion profiles discussed above.
Long lifetimes: Industrial-grade additive manufacturing equipment can range in price from hundreds of thousands of dollars to multiple millions of dollars. Customers expect them to last for 15 or 20 years of potentially 24/7 operation.
Contamination: Additive manufacturing creates a harsh environment for sensitive motion components. Depending on the modality used, equipment can be exposed to powdered metal or vaporized metals or polymers. Motion equipment typically needs to be IP rated as appropriate to conditions.
Thermal drift: Most of the modalities involve the application of heat. Over the course of a 30-hour build, for example, that can cause drift in different parts of the system ranging from encoders to the galvo scanners that steer the laser beam, to the build platform itself. “Long-term stability and drift are probably the biggest accuracy challenges,” says William Land, business development manager at Aerotech (Pittsburgh, Pennsylvania). “It is inherently a system that is not at a steady state temperature. You are melting metal inside the machine. Some layers have a larger cross-section than others, so you’re constantly putting more laser energy or less laser energy into the metal.” Over that long build, serve system temperature might rise 20° C or more but positioning performance needs to remain consistent.
Communications speed: One key requirement that can be easily overlooked is to ensure that communications among the different elements of the machine is fast enough to support the required motion profiles. The axes controlling the laser curing head on a vat photopolymerization system must be tightly coordinated with the system that lowers the build platform.
|Figure 3: Common architectures for additive manufacturing include the laser printhead mounted on an XY gantry over a fixed build platform (top) and a bridge in which the build platform moves on an XY stage beneath a fixed printhead that positions only along the Z axis (bottom). (Courtesy of PI)|
Motion Control Design for Each Application
The motion-system architectures used in additive manufacturing varies from modality to modality and system to system. Vat photopolymerization just requires a high resolution, high accuracy Z axis positioner to lower the build platform. Binder and material jetting can be served by XYZ designs. Elsewhere, the extrusion head of a material extrusion system might be mounted on a six-axis robot while the build platform has tilt capabilities that can make it possible to deposit a layer in a way that minimizes the amount of material used.
Even Z-positioning has its complexities. In powder-bed fusion, for example, the build platform starts out unloaded. It needs to move downward by a precise amount after each layer in order to accurately build the part. Over the course of the build, however, the load on the platform increases as a result of not just sintered powder but the raw powder that doesn’t get processed. A seemingly simple axis becomes a dynamic load that must be properly managed.
“If you’re building up 10,000 layers, the consistency of the layer thickness can really determine whether the part winds up the right size,” says Land. “Some of these aerospace components might involve a 20 inch high build. The platform starts with zero load, but by the end of the day, it is supporting several hundred kilograms of metal powder.” The load is always increasing and yet the stage needs to make 100 µm steps between layers with very high consistency and precision. It’s a challenge,” he adds.
Two common architectures are the gantry, in which a mobile print head positions over a stationary build platform, and a bridge, in which the build platform moves in XY while the printhead is fixed to the bridge laterally and only positions on the c-axis (see Figure 3).
As another example, consider the plasma-metal deposition machine shown below. It requires highly coordinated motion in the form of a pair of horizontal XY axes operating in a master-slave architecture to position the deposition head, and a rotary axis to turn the build platform. Although OEMs are increasingly exploring alternative control architectures such as skipping a dedicated motion controller in favor of daisychained smart drives, this type of application is complex enough to require the computing power and flexibility of a standalone controller.
“You not only have motion, you have multiple processes that need to be coordinated,” says RoseMary Burns, market segment manager at Yaskawa. “There are four axes in that particular path, along with synchronizing wire feeds, which could be metal or plastic. There could be heater bands in there. If it’s a plasma process, it could involve mixing gases.” In the 90s, this type of design would have required a PLC to handle a machine logic and emotion controller for the motion. Modern motion controllers increasingly feature machine control capabilities. “The motion controller handles most of the motion and the mathematical calculations and process, then we hand off just the discrete I/O to a remote I/O blocks basically.”
The Evolution of Additive Manufacturing
The early lines of desktop printers were designed to deliver cost-effective operation. Performance tolerances were more forgiving. As a result, those systems typically used stepper motors running open loop. Equipment designed for industrial applications requires better performance. Stepper motors are no longer equal to the task. “OEMs are looking for industrial equipment to apply to their machines, so they're going from stepper controls to servo controls,” says Burns. “When you go from stepper control to servo control, you get higher speeds and higher precision.” Pair that stepper motor with a drive capable of a several kilohertz frequency response and performance improves dramatically. “You’re getting closed-loop feedback, which can decrease settling time and decrease overshoot significantly.”
Switching from stepper motors to servomotors can enable a system to go from providing no more than 0.25G of acceleration to supplying 5G, making them good fits for highly dynamic applications. Ultimately, it comes down to the needs of the application. “what is their end goal really?” Burns asks rhetorically. “In some instances, steppers are absolutely fine. When the OEM wants to go faster, bigger, more precise, that's when they should start thinking about going from stepper motor to servo motor.”
The foregoing presents just a few of the challenges and solutions available with additive manufacturing. By working closely with vendors, OEMs can focus on the important process aspects of their equipment while ensuring that the motion aspects of the system perform as necessary for success.
Thanks go to Zachary Gray of Siemens Industry and Matt Price and Stefan Vorndran of PI for useful conversations.