NASA Goddard Space Flight Center invites companies to license new linear and rotary encoders. These technologies use an area array image sensor to measure the relative and absolute position of an object by imaging a microlithographic scale on the object. Optical encoders perform precision measurements of angular and linear position and speed for a variety of applications. This invention won the NASA Government Invention of the Year Award for 1999 and has since undergone several important design improvements, including temperature-resistant engineering, high-efficiency vertical binning technology, and encoding speed improvements.

Linear encoder on a baseline mirror stage in an imaging interferometer testbed at NASA GSFC. The range of encoded travel of this stage is 250 mm, and the encoder's resolution is about 40 nm.

Rotary encoder used to measure the angles of light beams in a cryogenic prism refractometer for studying the index of refraction of infrared optical materials for the James Webb Space Telescope at NASA GSFC. Light from the red LED passes through the rotary scale. An image of the scale is transmitted to an image sensor outside the cryostat. This version of the rotary encoder has an angular resolution below 0.01 arcseconds.

+ See also, Absolute Cartesian Encoder technologies

• Highly sensitive: Goddard’s linear encoder can resolve changes in position as small as 1 nm, while the rotary encoder can resolve angular changes to 0.02 arcsecond or smaller, depending on scale diameter. The absolute linear encoder can measure travel in excess of 3 m with 0.1 µm sensitivity.
  
  
• Fast: Vertical binning capabilities when using a charge-coupled device (CCD) array image sensor in the linear encoders help reduce exposure, image readout, and image processing times, yielding conversion rates exceeding 20 kHz while allowing for higher motion speeds.
  
  
• Cryogenic tolerant: Goddard’s encoders can also operate in vacuum and at technologically important temperatures, ranging from as low as absolute zero to well over 100 ºC.
  
  
• Lower cost: Goddard’s encoders are inexpensive to manufacture.
  
  
• Damage tolerant: The design of this encoding system is far less susceptible to scale damage or contamination than conventional absolute encoders.
  
  
• Simple: The encoder design is very compact and is simple to assemble, install, and align.
  
  
• Versatile: Goddard’s encoders are suitable for a wide variety of travel and resolution requirements.

• Aerospace and aviation
• Computer-aided machining
• Semiconductor manufacturing
• Inspection equipment
• Linear positioning mechanisms
• Machine tools and robotics
• Medical imaging
• Profilometers and other instruments
• Surveying and telescopes
  
  

This solid model (above) of a 2-inch long, high-precision, encoded linear stage illustrates how the encoder’s components can be packaged into the stage without consuming a great deal of real estate. The encoder’s patterned linear scale (clear blue-1) travels with the stage’s carriage (dark lavender-2). The scale passes between an LED on the opposite side of the stage’s base (green-3) and an image sensor (grey with gold pins-4) embedded in the side of the base. This configuration achieves a resolution of 40 nm or less, depending on the choice of image sensor.  This end view (below) of the stage shows the light path through the encoder’s components.

Optical encoders measure the linear or angular position of an object by optically detecting marks on a scale affixed to the object. Incremental encoders simply detect the relative motion of the object, rather than its absolute position by counting these marks. Although absolute linear optical encoders are available, they do not offer very high resolution (i.e., sensitivity). Furthermore, with conventional absolute linear encoders, the moving object is limited to 4 mm of travel at the highest practical resolution. Also, if the scale on the object is damaged, most optical encoders yield “dead spots,” no longer providing complete and accurate information.

Goddard’s new absolute and incremental linear and rotary encoders address many of the limitations of current encoder technologies. Goddard’s linear encoder uses a microlithographically patterned scale and an image sensor. A light source projects the scale’s pattern onto the image sensor, and the image information is digitized and analyzed by an image processor. In some implementations, encoding speed is dramatically enhanced using a technique known as vertical binning of image features. Pattern recognition algorithms are then used to determine the relative and absolute position of the object.

Why it is better

Goddard’s optical encoders offer many advantages over other absolute encoders. Notably, the encoders offer conversion rates exceeding 20 kHz, making them ideal for many commercial applications. Previously, conversion rates were stalled due to the fact that images had to be exposed for relatively long periods of time to be read in their entirety, pixel by pixel. Through the technology’s vertical binning capability, exposure time, image readout time, and image processing time are reduced—shortening the overall conversion time. Further, vertical binning makes precise alignment of the image sensor to the scale in the direction perpendicular to travel less critical because of the reduced vertical information in the new scale pattern, making it simpler to design, assemble, install, and align encoders based on the new pattern. 

Unlike conventional encoders, Goddard’s new encoders have also proven to be operable in cryogenic environments. Certain CCD image sensors have been qualified, for cryostatic applications operating as low as 70 K. For these applications, an encoder scale is attached to a mechanism inside the cryostat, and source and image light are guided into and back out of the cryostat to and from the moving scale through a fiber optic light guide and an image conduit, respectively, each having low thermal conductance. The light-emitting diode (LED) and image sensor with magnifying optics are located outside the cryostat where they work well and where their heat dissipation does not shorten cryogen life.

This solid model of a 4-inch diameter, high-precision, encoded air bearing spindle illustrates the compact packaging of the rotary encoder’s components. This assembly includes four read heads, but there could be more or fewer. The LED light sources (red-1) are held in a top cover through which the bearing’s work table (white-2) is exposed.  Each tubular read head (gold-3), comprising a short focal length aspheric lens (yellow-4) and an image sensor at opposite ends of the 18-mm long tube, is held at quadrants into the periphery of the bearing’s shell (clear green-5) such that it focuses on the patterned surface of the encoder’s rotary scale (lavender-6). The configuration shown here achieves an angular resolution of 0.01 arcseconds.

Goddard’s absolute linear encoder has a resolution of 1 nm using a modest scale magnification of 10X and is capable of encoding motion over 400 mm. Resolution of 40 nm is readily achieved in designs using no optics for magnification of the scale (shadow mode) and range of motion can be up to several meters.  The rotary encoder’s resolution is 0.02 arc second for a 125-mm diameter code disk. The manufacturing costs for Goddard’s encoders are also significantly lower than for conventional encoders, and the encoders are smaller in size than others with comparable resolution. In addition, because of the novel encoding method, the microlithographic scale pattern is far less susceptible to damage and contamination than are conventional encoder scale patterns. Scale features are coarse and defects are inconsequential, so yield is naturally high.

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 incremental and absolute optical encoding technologies (GSC-13703, GSC-14633, GSC-14766) 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: linear-rotary-encoders@gsfc.nasa.gov