Spatial RAM for high precision data acquisition systems

Details Spatial RAM for high precision data acquisition systems Spatial RAM for high precision data acquisition systems

1997-05-09

2001-07-24

Grant, William (Department: 2786)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Specific application of positional responsive control system

C700S012000

Reexamination Certificate

active

06266581

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to high precision data acquisition systems, and more particularly to a system and method for acquiring data based on data clocking instead of temporal clocking.
BACKGROUND
In general, servo control systems utilize the concept of fixed-rate sampling, or temporal clocking, to obtain feedback sensor readings and maintain smooth control. A traditional approach to data acquisition in any high precision instrument such as a scanning tunnelling microscope (STM) also uses temporal clocking. In a STM, a high frequency clock is used to latch data as the data sensor moves over the surface of interest. The spacing of the data across the surface is a function of the clock frequency and the speed of the data sensor. The accuracy of the data spacing is a function of the ability of the servo control system to maintain a precise speed over the surface. This places extreme burdens on the servo control system as the desired precision of the instrument approaches atomic dimensions.
Using a single bit from a positional sensor (an interferometer in this case) to act as the data clock for a microtopographer instrument is disclosed in Teague et al., “Para-flex stage for microtopographic mapping,” Rev. Sci. Instrum. 59, 67 (January 1988).
SUMMARY
The need for a high precision data acquisition system has been recognized, inter alia, in connection with the Molecular Measuring Machine (M-Cubed) project at NIST (National Institute of Standards and Technology). The basic concept of the M-Cubed was the creation of a metrology reference frame around a scanning tunnelling microscope (STM) or atomic force microscope (AFM). Most, if not all, STMs and AFMs use stepper motors to drive the X and Y stages and simply temporally clocked in the data at each step. Even those microscopes with servo motors did not have a separate positional sensor to monitor the data sensor motion, but trusted that the data sensor moved with a known velocity over the surface. Both the resolution of the measurement and the spacing accuracy of the data relied on the division of the data sensor speed by a clock frequency, or what is referred to hereinafter as temporal clocking.
The metrology reference frame allowed NIST researchers to tie atomic scale measurements to a high precision interferometer positional sensor. Although the use of an interferometer yielded a higher resolution than the traditional approach, it could not, in and of itself, increase the spacing accuracy of those measurements. In other words, each data point was known to a high degree of precision, but the data was still being clocked in temporally. Any system utilizing temporal clocking cannot guarantee precise data spacing unless the servo control is perfect.
Another requirement of the M-Cubed was that of error correction. The traditional approach characterizes the physical motion of the device and then applies the error correction as an additive term in the final servo calculations. This allows for greater accuracy for each data measurement, but at the expense of a lower servo control bandwidth.
Another problem was that the servo system could not be designed to the same degree of accuracy as the interferometer, and that the error correction requirements made it worse. The question of fixed data spacing was not addressed by this approach, and any desire to see the surface at a fixed spacing would require interpolation of the data, something that was not acceptable.
The spatial RAM concept of the present invention expands greatly on the work of Dr. Teague et al. mentioned above, and incorporates several features to aid in the error correction and practical use in the M-Cubed. Generally speaking, the spatial RAM concept replaces the temporal clocking previously used with spatial (data) clocking. The use of spatial clocking effectively decouples the task of data acquisition from that of servo control. This is done by matching data sensor readings along the scanning path with values preloaded into a simple RAM circuit. The spatial RAM generates a latching pulse to the data acquisition system as each value is reached and directly replaces the high frequency clock. Since the spatial RAM allows for any data value to be loaded and matched, a researcher or other user can create non-uniform data spacing over the sample and/or use values to compensate for known errors in the motion.
The spatial RAM eliminates the need for a high-precision servo control system to maintain a precise speed of the sensor and thus reduces the cost of the overall data acquisition system. The ability to preload error compensation values into the RAM eliminates this calculation from the servo loop and allows for greater stiffness and increased accuracy.
The spatial RAM system also enables the use of space-based sampling for data acquisition instead of time-based sampling. The system uses RAM to hold the sensor vales to be sampled, and a comparator to provide matching between the sensor value and the desired location in space. All of this allows the spatial RAM system to perform nonlinear space sampling to aid in error correction techniques, and/or perform non-uniform sampling of the data sensor, such as logarithmic or trapezoidal sampling.
The spatial RAM system is, in general, applicable to any data acquisition system, where an input signal is received, using at least a single sensor and temporal clocking. Some specific examples include any system where a probe is moved over a surface (such as STM or coordinate measuring machines). The spatial RAM system is also applicable to any system where a sensor is swept through space to acquire an image by outputting/emitting a signal and then receiving a return signal (such as radar or laser imaging systems).
In accordance with a preferred embodiment of the invention, a spatial RAM system is provided comprising: means for activating a function based on the position of said system relating to an object of interest; means for sensing the actual position of said function activating means and for producing, responsive to the actual position sensed, a plurality of actual position data bits; memory means for storing a plurality of target positions of said function activating means, each target position being represented by a plurality of bits; means for comparing the plurality of actual position bits and a first of the plurality of the target position bits; and a state machine, including a flip-flop, having a plurality of states for operating the system such that a first state causes both a first set of the first target position bits and a corresponding first set of the plurality of the actual position bits to be read into the comparing means, a second state causes the comparing means to compare the first set of bits with the corresponding first portion of bits, and to latch the flip-flop upon a successful match that sends a clock pulse to the data acquiring means in order to acquire data from the object. Preferably the function activated acquiring data using a data sensor, but alternatively, the function can cause a signal to be emitted and/or received.
In accordance with a further aspect of the invention, a method is provided for activating a function of a system having a component, a state machine, a computer, and a memory, wherein said function is activated based on the position of said component relative to an object of interest, said method comprising the steps of reading a memory address containing a first plurality of target bits representing a first target position of said component; determining a first actual position of said component in space relative to said object; representing said actual position by a second plurality of bits; comparing at least one of said first plurality of bits with a corresponding set of said second plurality of bits; and if said compared bits are equivalent, activating said function and then proceeding again with step (a) using a further plurality of target bits, or if said compared bits are not equivalent, moving said component through space to a further actual position and pro

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