DELTA TAU CNC CONTROL SYSTEM DELIVERS NANO-MACHINING POSITIONING

The trend toward miniaturization in the biomedical industry is increasing the demand for nano machine tools that make precise nanometer-size moves. Delivering such high precision at high speeds requires a control that can process ultra high encoder counts at high speeds. For example, the U.S. Photonics 4-axis Femtostation laser, nano milling machines features Delta Tau UMAC controls because they respond to NC instructions and peripheral issues quickly, resulting in a very stiff servo system.

UMAC Control allows the use of multiple cards that slide in and out, depending on application requirements and needs.

Typically, nano milling machines use lasers for cutting 3D features as small 250-nanometers with repeatability of 10's of nanometers per meter and ultimately down to resolutions of 38 picometers. Using feedback from a very fast and ultrahigh count encoder, the UMAC is able to precisely position the laser beam. Laser optics, which create the beam, are considered similar to tooling, but unlike, for example an end mill with a deterministic shape, the column of light produced by the laser is not a straight line. The beam is shaped like an hourglass, with the focal point or "spot" at the narrowest point. The Femtostation allows a number of different optics and power settings that vary the spot size (equivalent of tool diameter), which may be as small as 200 nanometers.

The UMAC provides flexibility by allowing the servo loop update time to be adjusted very high. Additionally, feedforward, gain, acceleration and velocity can be adjusted to their maximum values for this application. Typically, the faster the update rate, the better. At higher update rates, disturbances become apparent sooner, and therefore compensation occurs sooner. Additionally, servo gain can be increased without introducing instability or jerk.

"The UMAC provides a very advanced PID servo loop that also processes Gcode instructions," said Jake Conner, president, U.S. Photonics. "The ability to process feedback from encoders with ultra-high resolution allows nanometer interpolation, which is crucial to our application. Because of the UMAC's open architecture, we can tune all PID loop adjustments, as well as feedforward, acceleration and velocity."

Although travel distances with the Femtostation are much shorter than most machine tools, it operates at velocities of up to 800mm/minute. The ability to process data from fast, ultra-high count encoders is important for precise positioning at such speeds. The UMAC reads ultra precise and fast rotary and linear encoders; Delta Tau's next generation machine will read interferometer encoders, allowing even higher servo update rates. With a large number of encoder counts and high-speed moves, a large number of counts must be processed in a very short time.

The Femtostation utilizes 20-micron sinusoidal scales. They are interpolated into 4,096 distinct states per sinusoidal cycle, which provides a theoretical resolution of 4.88 nanometers. "With the UMAC's look ahead and move segmentation abilities, acceleration and deceleration is excellent," said Conner. "We can move a large mass and stop in a few nanometers."

In combination with the UMAC, the Advantage 900 CNC user interface provides an operator console with the capability of interfacing up to 32 axes of distributed motion hardware. A CNC Autopilot setup utility simplifies integration by automatically creating the configuration files for a particular machine.

According to Conner, Delta Tau's NC Pro software allows the user to choose the power level and optics, just like a tool change with a conventional machining center. Mapping the changes and creating look-up tables compensate for changes in optics and power levels, which change the diameter of the spot. Additionally, cutter compensation is used to adjust for the spot size. For example, if the spot size measures 200 nanometers, NC Pro will offset it by 100 nanometers.

"We're able to machine channels in fused silica for a lab-on-a-chip, and we found in order to get the best results and surface finish, we have to have high feed rates," said Conner. "Since we're making devices with channels that are 10's of microns wide in 3D, we needed to have precisely coordinated motion in four axes at 800MM/minute with submicron resolution.

Lab-on-a-chip (LOC) technology integrates multiple laboratory processes on a single chip measuring a few mm2 to a few cm2. A LOC may use multiple sensors and detectors to determine blood sugar levels and potential for gestational diabetes.

In the LOC prototype machined by U.S. Photonics, the channels are curved. Sometimes reactor chambers are laser machined in the glass. These are machined below the surface of the silica. Using different settings and feedrates, wave guides can be etched into the glass. Instead of creating voids or plumbing inside the silica, fiber optic like networks can be created by changing the refractive index in the silica, allowing the creation of 3D optical wires for sensors inside the chip.

Conventional laser machining with microsecond and nanosecond lasers passes excess heat into the sample material causing slag, microcracking, and jagged features. To clean surfaces and smooth the features requires additional processing. Femtosecond laser pulses produce cleaner features than conven tional lasers without the additional postprocessing. The laser cuts leave the subsurface layer intact and do not create heat-damaged zones or rearrange crys tal structures.

"Programming nano parts doesn't require specialized CAM software," said Conner. "The UMAC has a very fast processor, which processes ultra high encoder counts quickly and reads G-code. Without the ability to process G-code instructions, we would have to have developed a proprietary post-processing program to convert from G-code to machine code. Setting aside the time and cost of developing such software, the additional processing time for a program would have negated the high performance benefits."

Tuning the servo parameters is a fairly straightforward process; Delta Tau provides software with extensive tuning utilities and features. This allows values for such parameters as feedforward to be adjusted appropriately for the application. For nanomachining, feedforward would be set to the maximum.

The feedforward capability anticipates the command needs of the system for programmed trajectory and infuses estimated values directly into the controller's command output without allowing for errors to build up. The feedforward estimate is not perfect, but a feedback algorithm can clean up errors and counteract disturbances.

Other sophisticated feedforward techniques are available, but a large number of applications are satisfied with velocity and acceleration. For laser machining applications, trajectory-based feedforward techniques are useful because the cutting loads are light.

"Currently, we use our machines in house but we are planning to apply this technology to industry specific tools in the very near future," said Conner. "The Delta Tau UMAC delivers the performance to keep up with today's encoder and laser interferometer resolutions. The UMAC allows ultra- precise motion and maintains high throughput speeds."