MicroE 3500Si Mercury - Manual (Page 19)

Index Processing
A unique physical position is referenced on all gratings and is called an index position. The value of this position is determined
during an index capture routine initiated by a button press or the SmartPrecision Software and is permanently stored for use
after power cycling. The index value has the same resolution as the interpolated position.
The SS-350cSi has four modes of operation that use the index position to generate a physical reference position. The position
word calculated by the SS-350cSi electronics is a 30-bit number, which includes 18 fringe counter bits and 12 sub-fringe
interpolation bits. The fringe counter keeps track of the number of electrical cycles encountered caused by a grating moving
past a sensor and can be reset. The sub-fringe position is absolute because the voltage relationship between sine and cosine
are fixed electrically and therefore cannot be reset or cleared.
A physical mark on the grating called an index window is used to generate an accurate index position. The index window
is approximately one fringe wide. By monitoring the edges of the window with respect to the absolute sub-fringe position
during the index capture mode, an accurate index position is calculated and stored.
At power up the encoder is in an undefined position relative to the outside world. By traveling past the index mark on the scale
and knowing where the index is relative to the outside world the encoder position becomes defined. The M3500Si supports the
following index processing modes:
No Index: No changes are made to the position word at the index mark.
Mode 1: Zeros the fringe counter at the first encounter with index mark after power up.
Mode 2: Zeros the fringe counter at every encounter with index mark.
Mode 3: Zeros the fringe counter at the first encounter with index mark after power up and
subtracts the index position from the calculated position making the index mark the
zero position of the encoder.
Mode 4: Zeros the fringe counter at every encounter with index mark after power up and
subtracts the index position from the calculated position making the index mark the
zero position of the encoder.
The Index mode can be factory set or selected by the customer using the optional SmartPrecision software.
Serial Output Specification
Page 17