Design of a rotary fast tool servo for ophthalmic lens fabrication

We have developed a novel fast tool servo and associated prototype diamond- turning machine for the production of plastic spectacle lenses. Our fast tool servo carries the cutting tool on a rotary arm and, thus, on a circular path, as opposed to straight line paths in conventional designs. The actuator, sensors, and bearings are standard elements that together allow experimentally demonstrated 500 m/s² instantaneous accelerations at the tool tip over a 3-cm range of cutting depth. We also describe in this paper new approaches we have developed for toolpath generation and calibration. The paper also presents associated control algorithms, because the controller must supply very high dynamic stiffness to the tool servo axis at multiples of the spindle frequency. This stiffness is achieved by means of repetitive control techniques. The new fast tool servo is shown to have great promise for machining asymmetric surfaces with large amplitude asymmetries.     

A loop shaping perspective for tuning controllers with adaptive feedforward cancellation

This paper presents a loop-shaping perspective on the tuning of systems using adaptive feedforward cancellation (AFC). The AFC technique is very useful for reducing/eliminating errors in a controlled system at a selected set of harmonics. Such a technique has applicability for hard disk and optical disk track following, vibration rejection in spindle systems and helicopters, camshaft and piston machining, as well as in fast tool servos for diamond turning. This last application provides the motivation for the work described herein. The paper shows how an AFC loop with N resonators results in 2N parameters, a gain and phase advance parameter for each resonator, which must be adjusted to maximize closed-loop performance. This paper details a frequency shaping method of selecting the AFC phase advance parameters and resonator gains in multiple AFC resonator systems. We have applied these techniques to our prototype diamond turning machine with a fast tool servo for cutting ophthalmic lenses. Experimental results comparing the performance of this machine with and without AFC control are presented.     

Magnetic/fluid-bearing stage for atomic-scale motion control (the angstrom stage)

This paper describes a magnetically suspended six-degree-of-freedom precision motion control stage with subnanometer positioning noise inside a 100-μm cube of travel. This stage utilizes multiple electromagnetic actuators and capacitance probes to provide control forces and position feedback, respectively. The suspended platen (3-kg mass) is floated in oil to enhance the performance of the magnetic bearings. The stage has been designed for use as a sample positioning stage for scanning tunneling microscopy (STM). Images obtained by STM show that the stage has a positioning noise of less than 0.2 nm peak-to-peak over 5 seconds, while the capacitance probes (used for position feedback) indicate a positioning noise less than 0.1 nm peak-to-peak We present several STM images and the step-following response for a 0.1 nm step input in order to demonstrate the utility of this stage as a new concept for precision motion control.     

Design and control for an electromagnetically driven X–Y–θ stage

In this paper, the design and control of a novel precision motion stage is presented. It combines a flexural stage, electromagnetic actuation, capacitance position measurement, and nonlinear digital feedback control. This system has high open loop stiffness, and such a spring-dominated regime design provides a sufficient phase margin for applying phase lag-based control to reduce environmental effects such as base vibration and high frequency noises. In addition, the incorporation of the rotational motion not only increases the degree of freedom, but also provides capability for correcting possible undesired coupling between major axes. As a result, there would be fewer requirements in degree of precision for stage machining and assembly. With nonlinear control, the performance of this stage would be more consistent than those with controller based on linearizing system dynamics. Experiments show that the system has a bandwidth of 85 Hz with a minimum resolution of 50 nm, which is dominated by the quantization of data acquisition devices. This implies that the present design can be readily modified to achieve higher performance. However, it is also found that a major difficulty in the present design is the insufficient damping of the rotational mode, which limits the applicable control gain and achievable performance and should be corrected by proper mechanical redesign in the future. Nevertheless, this design still shows its potential advantages. Finally, the stage is integrated with a stylus to demonstrate the possible application in surface morphology measurement. Possible applications also include the fine motion control and scientific instrumentation.     

Probenecid protects against In vivo acetaminophen-induced nephrotoxicity in male Wistar rats

Renal effects of acetaminophen (APAP) were studied in rats pretreated with probenecid to analyze whether acute APAP-induced nephrotoxicity could be related to a probenecid-sensitive transport system for APAP or its S-derived conjugates. The administration of probenecid (200 mg/kg b.wt. i.p.) 30 min before APAP administration (1000 mg/kg b.wt. i.p.) improved urine flow rate and protected against the alterations on glomerular filtration rate and urea and creatinine plasma levels induced by APAP. Fewer epithelial cells and granular casts and a decrease in the urinary excretion of protein and glucose were observed in rats pretreated with probenecid. Probenecid pretreatment promoted an elevation in the urinary 16-hr excretion of APAP and a diminution in the plasma levels attained by APAP. These results suggest that protection afforded by probenecid in vivo could be a consequence of the inhibition of APAP S-conjugate renal uptake and/or an increase in APAP renal clearance. The effects of APAP in presence of probenecid were studied with the isolated perfused kidney model. Perfusion with probenecid (0.1 mM) before APAP (10 mM) did not change APAP direct renal effects, APAP urinary excretion, or APAP renal clearance relative to glomerular filtration rate. Our results suggest that protection afforded by probenecid in vivo could be the result of the inhibition of the uptake of nephrotoxic APAP metabolites and/or a diuresis-induced enhanced APAP renal excretion.     

Spindle rotary position estimation for fast tool servo trajectory generation

In diamond turning of non-rotationally symmetric surfaces, fast tool servo (FTS) trajectory is generated in real time based on the spindle rotation angle. Essentially, a digital rotary encoder updates its output (samples the spindle angle) at fixed rotary angle interval. In time domain, the encoder sampling frequency is different from that of the FTS control computer. Without proper spindle position estimation, this asynchronous nature between the encoder spatial sampling and the computer temporal sampling can cause significant amount of aliasing noise in the generated trajectory. In this paper, we first analyze this aliasing phenomenon using discrete-time signal processing theories. Based on this analysis, a spindle rotary position estimator is designed to improve both accuracy and resolution of the spindle angle feedback. Its effectiveness and the associated trajectory generation are experimentally verified in diamond turning of sinusoidal surfaces with ultra-FTS.