Theory of Operation

A GPI Virtual Absolute encoder system can be in any one of 3 distinct operating modes: Wait mode: Aware of an interruption of power or some other interference to normal operation which it interpreted as an invalid tag, it waits until a (re)initialization traverse has been executed to refresh the code tag in the decoder. The Model VB reports a fault condition while it is waiting for a full tag to be acquired. Search mode: Having acquired a complete new tag from an initialization sweep, it may be busy pinpointing the absolute position of that tag in the overall code sequence. How long this tag search will take depends on design characteristics of the decoder including its clock speed, how far away the read station happens to be from the starting point of the search, and whether the decoder is "chasing" the shaft motion from behind as it moves during the search. Typical tag search times are sub-millisecond and are not challenged by velocity. Track mode: It may be decoding all new indexing code tags obtained from the read station in satisfactory fashion, validating the code bit-wise as it's detected, and providing reliable absolute position information, i.e., tracking the motion input in servo fashion.If electrical noise or disc contamination should invalidate the two quadrature signals, the index signal, or even the proper operation of the decoder itself, this will result in a reported fault. The decoder is capable of quickly detecting error in several ways: Proper quadrature sequence is verified at all times. Since this can corrupt the tag bit due to improper sample timing, the decoder resets and awaits a fresh tag. Successors to a false tag will survive very little following motion of the disc before faulting due to the non-repeating nature of the code sequence. measured locally .If the tag decoder detects a fault condition and is waiting for an initial tag sweep or is industrial still engaged in a tag search, the fault bit will be set, signifying the current output value is invalid and position is unknown. Any fault condition may be cleared by re- initializing. In the case of disc damage or fouling, the host system may measure the affected region by approaching it from alternate directions and observing the behavior of the Status bit reporting the validity of the absolute position output. The standard design configuration for a Virtual Absolute decoder locates the Status bit at the MSB position to offer a simple mechanism for a microprocessor-based host system to quickly assess data validity. Most processors contain instructions in their instruction sets designed to test the most significant bit of a data word, and branch program execution based on the result. Other mask, shift, rotate, and arithmetic operations may then be used to reformat data to BCD, add a position offset adjusting the "zero" position of the axis to a more convenient location, or complement the binary data word to reverse the perceived direction of ascending count. A supply voltage supervisor and power-up reset IC is included to reset the decoder if the power supply dips below +4.70 VDC,
Gurley Precision Instruments514 Fulton StreetTroy, NY 12180 U.S.A.(800) 759-1844, (518) 272-6300, fax (518) 274-0336, Online at www.gurley.com, e-mail: info@encoders.com

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VB

V3.1

GPIGPI

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