• Based on mature technologies: Goddard’s technology uses a highly accurate microlithographic scale, electronic image sensor, and simple image processing.
  
  
• Long travel reading: The encoders can read travel lengths as long as the scale.
  
  
• Ultra-fine resolution: Resolution of 50 nm has been demonstrated for a coarse scale; resolution below 10 nm is obtained using a finer scale.
  
  
• Accurate readings: Accurate X-Y readings can be obtained regardless of assembled orthogonality or straightness of travel of stages, and absolute X-Y accuracy is limited only by the accuracy of the microlithography.
  
  
• Damage tolerant: The encoder design is far less susceptible to scale damage or contamination than conventional optical encoders.
  
  
• Small: The encoder design is very compact for the level of resolution provided.
  
  
• Scalable: The encoder design can easily be upgraded with additional software.

The absolute Cartesian encoder has been deployed in several applications at NASA, including optical metrology, radiometric calibration, and cryogenic environments. The technology is also valuable for the following applications:

• Industrial applications
  • Inspection
  • Manufacturing
  • Assembly
• R&D
  • Metrology
  • Astronomy
  • Cryogenic environments

Solid model of high-accuracy absolute Cartesian encoder test setup: Cartesian encoder scale is attached to an ultra-square gauge block, which is in turn attached to an X-Y cross-slide composed of linear air bearings with super straight travel. X-Y motion of the gauge block is simultaneously measured by the Cartesian encoder and a pair of laser interferometers, which look at orthogonal faces of the gauge block. The encoder’s read station comprises nothing more than an LED and an image sensor in close proximity to the scale. (Mounting hardware not shown for clarity.)

Close-up of encoder’s read station. Light from the LED travels inside the yellow block and is folded upward off of a flat mirror attached to the block through the scale. The light is essentially collimated by the time it reaches the scale. A shadow of the pattern on the scale is cast on the image sensor. The image of the shadow contains information about the exact position of the scale and the gauge block in two dimensions.

How It Works

NASA Goddard’s design uses a backlit, microlithographically patterned scale that is attached to a moving object. The scale carries X-Y information that uniquely identifies the horizontal and vertical location of the scale image as seen by a fixed electronic image sensor, thereby allowing determination of the absolute Cartesian position of the object.

Why It Is Better

Traditional X-Y positioning systems with closed-loop feedback encoders consist of two, usually stacked, linear translation stages. Because two encoders must be deployed to encode X and Y accurately, cost and complexity are increased, limiting viable applications. Linear encoders offer accurate coordinate determination but problems with this approach include imperfect alignment of the encoder to the linear directions of travel, the inability to account for lack of straightness of travel of the mechanical axes, and the lack of orthogonality of the directions of travel. Meanwhile, laser interferometers with plane mirrors offer high accuracy and resolution but are very expensive.

Unlike these other encoder options, NASA Goddard’s design uses a microlithographically patterned absolute encoder scale containing all of the information necessary to encode any position within the range of X-Y travel. Because the information on the scale is strictly Cartesian, many problems associated with other encoding methods are eliminated. In addition, because one encoder serves both axes, cost and complexity are reduced. Nonstraightness of travel and lack of orthogonal mounting of stages are accounted for, and accuracy is limited only by that of the scale lithography.  

NASA has demonstrated a 15-cm x 15-cm (6-inch x 6-inch) absolute Cartesian encoder with coarse feature separation. Resolution of around 50 nm has been measured. Travel is not limited to that size. Even greater resolution, below 10 nm (competitive with many laser interferometers), is obtained using a more finely patterned scale, which still does not challenge the state-of-the-art in microlithography.

Measurement speed or conversion rate is a function of the image sensor frame rate and the image processing software. NASA has demonstrated a 20-Hz measurement rate using a very inexpensive CCD-based camera. Using more advanced modern digital image sensors in conjunction with inexpensive digital signal processors (DSPs) has improved measurement speed to well in excess of 1 kHz.

Benefits of the Cartesian Electronic Absolute Autocollimator

• Sensitive: The only instrument of its kind, the technology has unsurpassed sensitivity and stability for its size. Higher angular sensitivity per focal length enables smaller, cheaper packaging for the same angular sensitivity or far higher sensitivity in the same package.
  
  
• Large field of view: An exceptional field of view (range of regard) compared to conventional autocollimators, limited only by the off-axis imaging fidelity of the projecting optics for a given target range.
  
  
• Stable linear output: By processing digital images from its area array image sensor, output is inherently linear and stable, while the absolute coding of each individual gridline in the target and image assures unambiguous position readout.
  
  
• High accuracy: Intrinsic orthogonality of the gridlines in the target is limited only by the art of microlithography, helping to ensure the highest degree of orthogonality or azimuth and elevation readouts over the field of view.
  
  
• High-quality images: Low signals on the image sensor may reduce sensitivity but do not encourage drift, helping to maintain image quality. The technology also controls optical signal level in a servo loop, helping to optimize image brightness.

Applications for the Cartesian Electronic Absolute Autocollimator

Used to support development of the James Webb Space Telescope’s Near Infrared Camera (NIRCam) as well as for other NASA infrared space instruments, the Cartesian electronic absolute autocollimator is also valuable for use in the following applications:

• Space-flight applications
  • Remote determination of payload pointing for interferometry missions
  • Ultra-high resolution feedback for wavefront control in adaptive optic systems
  • Fine guidance sensing
• Cryogenic environments
  • Infrared and X-ray mission applications
• Machine making
• Optical metrology
• Surveying
• Aerospace and defense
  • Instrument building
  • Spacecraft construction
  • Aircraft assembly
• Other
  • Augment theodolite capabilities

Cartesian Electronic Absolute Autocollimator Technology Details

Compact Cartesian autocollimator measuring azimuth and elevation angles of a flat mirror on a small optical mount having two axes of rotation. This autocollimator is only 75-mm long yet measures angles with a resolution of less than 0.1 arcseconds.

Autocollimators are optical metrology instruments that measure the compound angular deviation of a flat datum mirror with respect to the pointing direction of the optical axis of the autocollimator itself. Conventional autocollimators consist of an illuminated target, a beamsplitter, an objective or collimating lens, and a viewing reticle or screen. Typically, a pinhole target is projected, and the viewing screen is replaced with an analog position-sensitive detector (PSD), providing bi-axial indication of the flat datum mirror’s pointing direction.

How it works

NASA Goddard’s technology replaces the conventional target with a Cartesian encoder scale, consisting of coded X and Y gridlines. An area array image sensor replaces the typical PSD, and analog signal-processing circuitry is usurped by computational image processing. A captured image is surveyed to find row and column indices of the intersections of the gridlines, and binary coded identities are derived by examining the code bits for each gridline. The two-dimensional angular relationship of the flat mirror is computed by determining how far off the center row and column of the image sensor each gridline is with respect to an arbitrary reference angle with extreme sensitivity.

Why it is better

NASA Goddard’s design eradicates many of the problems faced by conventional electronic autocollimators. Notably, traditional designs suffer from readout instability due to drifts in their analog circuit elements and to temperature effects on the PSD. This readout circuitry includes several stages of amplifiers for raw signal gain, X and Y signal sums and differences, and ratios of amplified sum and difference signals in X and Y. Even in best-case-scenario conditions, each of the five to seven amplifiers in the chain will suffer its own inherent gain and bias drifts. NASA Goddard’s autocollimator processes digital images from its area array image sensor rather than from a conglomeration of drift-prone analog amplifiers. Therefore, the technology provides highly stable linear output that does not change due to the temperature of the detector or its processing electronics. NASA has demonstrated stable performance of its full-size autocollimator (350-mm focal length) at an impressive 0.08 arc seconds peak-to-peak and 0.01 arc seconds root-mean-square (rms).

Higher angular sensitivity compared with conventional designs is also achieved because the absolute Cartesian encoder technology enables centroid location determination to a finer dimensional scale than allowed by the traditional PSD. Further, absolute encoding of each individual gridline in the target and image ensures unambiguous position readout, and the orthogonality of the target gridlines assure the highest degree of orthogonality of azimuth and elevation readouts over the field of view. The field of view itself is also larger than that of a conventional autocollimator because the design projects a multiplet of gridlines, making the edges of the Cartesian scale visible for a greater distance to the right and left of the field of view. NASA has demonstrated fields of view between 0.42 and 2.3 degrees for its autocollimators, compared with a smaller 0.33-degree field of view for a commercially available autocollimator.

These technologies are part of NASA’s Innovative Partnerships Program Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing the absolute Cartesian encoder (GSC-14330-1) technology and/or the Cartesian electronic absolute autocollimator (GSC-14718-1) technology for commercial applications.

For information and forms related to the technology licensing and partnering process, please visit the Licensing and Partnering page. (Link opens new browser window)

If you are interested in more information or want to pursue transfer of this technology, please contact:

Innovative Partnerships Program Office
NASA Goddard Space Flight Center
E-mail: cartesian-encoder-techs@gsfc.nasa.gov