Gray Code for Rotary Incremental Encoders

June 17, 2014

Gray code was named for Frank Gray, a Bell labs researcher, who patented the procession in 1953, even though a form of it was used by Emile Baudot for telegraphy as early as 1878.

The most important thing to understand about Gray code is that only one bit changes from transition to transition. In binary it is possible for a number to go from all ones to all zeros, as is the case with 11111111 (255 decimal)  going back around to 00000000 (zero).

Notice how only one of the 0’s or 1’s of the Gray code change as the number increments? In binary there are times when all of the bits change, (0111  to 1000 (Seven to Eight ) and 1111 back to 0000 (Fifteen to Zero) ).

Error Checking

The advantage to only one bit changing in Gray code is that it gives you error-checking ability. If you sum the number of bits, the bit total will always change by only one.

You could also do some error checking knowing that the bit sum will always alternate between even and odd.

Gray Code in Incremental Encoders

The A & B channels of incremental encoders are in quadrature, which makes a two-bit gray code progression. Depending on direction, the incremental encoder bit progression with be a cyclical pattern of either 00 – 01 -11 – 10  or  00 – 10 – 11 – 01 – 00.

Only one bit changes from transition to transition.

The QD787 absolute encoder shown at the top of this post has an option for an eight bit Gray code output. Update: encoder model QD787 has been discontinued. Quantum Devices does not offer an absolute encoder at this time.

Links to More Information on Gray Code:

https://encyclopedia2.thefreedictionary.com/Grey+code

https://mathworld.wolfram.com/GrayCode.html

WIKI:

https://en.wikipedia.org/wiki/Gray_code#Constructing_an_n-bit_gray_code

For more information on encoders, Contact Quantum Devices today!