Rockwell Automation 999 IMC S Class Compact Motion Controller (Cat. No. 4100-999-122) - Manual (Page 33)

Technical Overview 3-3
Publication 999-122 - January 1997
Often a third output channel is available from the encoder. This
so-called "marker" output is also known as Channel Z, 0, or C. The
marker output from an encoder is a pulse that occurs at one specific
point on the encoder disc (rotary encoders) or slide (linear encoders).
Therefore, the marker may be used to establish a precise absolute
position reference. Note that for rotary encoders, the marker provides
a position reference within one revolution of the encoder. For multi-turn
applications, other means must be used in conjunction with the marker
pulse to determine the absolute position. The IMC-S/23x can interface
to encoders with marker pulse outputs of either active-high or
active-low polarity.
Encoder Counter The encoder counter circuitry is identical for all axes and consists of
optically-isolated AM26LS32 input buffers, programmable decode
logic, and 16-bit encoder counters contained within the CX2216.
Once buffered and isolated, the two TTL-level encoder signals
(Channel A and B) are connected to the CX2216 which decodes them
using 4X quadrature logic (4 counts per encoder line). After decoding,
the count signals are sent to the encoder counter in the CX2216, which
keeps a record of the number of counts and the direction of encoder
motion. The IMC-S/23x software extends the encoder counters to 32
bits giving a total position range of ±1,000,000,000 encoder counts.
A quadrature type encoder generates four counts for every line on the
encoder disc or slide. The sequence in which the counts are generated
is determined by the direction that the encoder is moving, and the
encoder counter increments or decrements accordingly.
The maximum encoder rate for any digital control is determined by
many factors, including the sample rate of the control loop, the size of
the encoder counters, and pulse rate limitations of the digital circuitry.
The maximum encoder count rate, or encoder bandwidth, for the
IMC-S/23x, is 4.0 Megahertz.
Every servo sample period, the microprocessor reads the encoder
counter for each axis and computes a count increment by subtracting
the previous counter value from the present counter value. This count
increment represents the distance the axis has traveled in the preceding
millisecond. This value is then used to update the 32-bit actual position.