Controllers advance micro-manufacturing

Thomas R. Kurfess, Georgia Institute of Technology -- Control Engineering, 8/1/2005

I recently spent several weeks abroad studying micro-manufacturing in a U.S.-government-sponsored technology review. The goal was to assess and understand what is happening globally in micro-manufacturing. The final report is being generated; initial findings are online at http://wtec.org/micromfg/—a site hosted by the World Technology Evaluation Center. I highly recommend this site, as it has several studies of great interest to the controls, manufacturing, and micro-manufacturing technology communities.

My role on the WTEC micro-manufacturing team was to evaluate advances in controls, metrology, and sensors for micro-machining. I learned quite a bit—some of which was surprising. My next few columns will impart what I learned. This month, I will discuss new controllers being used for micro-manufacturing.

In past articles, I mentioned that controls being used for ultra-high precision applications need to be fast to track small distance increments (on the order of nm) while moving relatively fast (10-100 mm/s). Also, I mentioned that large counters are critical, as ultra-high resolution feedback requires the ability to count many small increments over the stroke of the machine. These two capabilities are prevalent in all control systems I observed on micro-machine tools, in industry as well as in research and development efforts.

Two other capabilities were identified by all controller manufacturers and users as critical in micro-machine tool motion control. The first is spline interpolation, which is critical in generating smooth tool trajectories when cutting micro-component surfaces. Spline interpolation has been shown to generate significantly improved geometries and surface finishes over conventional G01, G02, and G03 linear and circular trajectory commands. The second capability is controlling higher order motion trajectories. Traditionally, these systems control position, velocity, and acceleration.

This new generation of controllers for the micro-manufacturing arena also control the time derivative of acceleration known as jerk, to smooth tool trajectory. In many cases, the machines we reviewed produced very small-scale optical components (for example, lenses for next-generation optical drives or molds for producing those lenses). The combination of spline interpolation and jerk control was extremely beneficial in successful implementation of these machines.

Most controllers we saw on a variety of micro-machine tools had the basic capabilities discussed here. In addition, many machines are reconfigurable and can therefore be used as a lathe, a mill, or a grinder, etc. However, controllers' openness varied substantially from system to system.

I believe these new controllers are going to be very impressive and offer significant capabilities. They will push forward the frontiers of micro-manufacturing, and enable cost-effective advances in manufacturing in general.

The next column will discuss trade-offs between open and closed architecture controllers. Following that, I will address controls designed for reconfigurable machines.