199 A New Architecture for Absolute  Optical Encoders  

Timothy  Malcolm*, John Beasley * and Mike Jumper * 

Abstract    BEI Precision Systems & Space has developed an encoder technology, nanoSeries, that can calibrate itself in-situ and correct most of the common causes of error in typical encoders. The new nanoSeries ARA  design has detailed health and status readouts that can definitively indicate when a re- calibration is in order .  The re- calibration process can be done on- orbit if desired. The units can accommodate either full  revolutions or limited angle sweeps, and the principles are also applicable to linear encoders.  

Introduction  

Optical  encoders  manufactured  by BEI Precision   Systems & Space  (BEI)  have  been  used  in space since  the  earliest  days  of space flight.  The combination of  low we ight and high resolution  relative to electromagnetic   resolvers  made them  an obvious  choice  for many  applications.  Optical encoders have typically been of  only a  few types.  

The primary  type of optical encoder  selected for commercial  and industrial  applications  has been the   incremental  encoder.  This type of encoder  requires  a return to an index  or home pulse to index a counter,   which  then counts  the number  of ‘incremental’  pulses  or bits that pass  by. This is a very simple  and robust   concept  but it has some disadvantages  for space,  primarily  that if power  should go off or the counter  upset s,  even  for a very short  time,  the encoder  does  not ‘know’ where it is until the  system  moves  it back  to the home  position  and it re-acquires  the index  pulse.  A further  concern with  this type of encoder  in space is that Single  Event  Effects  can cause the number  of recorded pulses  to be in error,  or cause  a spurious  index  pulse  to  reset  the counter,  and the count will  not be corrected until the index  is reacquired.  Other  issues  with this type  of encoder  are that the light  source must  remain  turned on at all times,  increasing the power consumption and  limiting  the lifetime  of the source, and that the  data rates  for high resolution  devices  can be very high,  often  resulting in Electromagnetic Interference (EMI)  issues .   A second type of encoder  could be called  pseudo- absolute or chain code.  This type of encoder includes  a  set of tracks encoded with a non-repeating  digital  code,  similar  to an endless  bar code, plus  a set of  incremental  tracks  for timing.  This style of encoder  does  not need to return to zero to reset  itself;  instead  it  just needs  to read a certain number  of sequential  bits to know  where it is. This design mitigates  some  of the  issues  inherent  in the typical  increm ental  encoder.  However,  the Light -Emitting  Diode  (LED)  still must  remain   on all the time.  This sort of encoder  has not been used much in space to date.    A third type of optical  encoder  is the absolute encoder.  This style of encoder  actually  generates  a discrete   digital code for each of a number  of discrete  locations  encoded on a code disk or scale, and these bits are  read simultaneously. Thus, even if power flickers, the encoder recovers immediately  and there is no need   to return  to a home position.  Moreover  in this type of encoder  it is common to pulse  the LED to take  readings,  which has the effect of  maximizing the life of the encoder.  Of course it is still possible for readings   to be in error due to Single Event  Effects , but for this type of encoder,  the very next sample will correct  the  error.  The simplest  versions  of this device directly encode the digital  patterns  on the code disk, but higher   resolution types  derive the digital  data from sine and cosine patterns on the disk .                                                                 * BEI Precision Systems & Space Company, Maumelle, AR  

Proceedings of the 4 3rd Aerospace Mechanisms  Symposium,  NASA  Ames  Research Center,  May 4-6, 2016