User`s guide

The encoder variables contained in this final equation are described in later sections.
The Belt Encoder
Six belt encoders are supported by the conveyor tracking feature.
Each belt encoder generates pulses that indicate both the distance that the belt has moved
and the direction of travel. The pulses are counted by the belt interface, and the count is
stored as a signed 24-bit number. Therefore, the value of an encoder counter can range from
2
23
-1 (8,388,607) to -2
23
(-8,388,608). For example, if a single count of the encoder
corresponds to 0.02 millimeters (0.00008 inch) of belt motion, then the full range of the
counter would represent motion of the belt from approximately -167 meters (-550 feet) to
+167 meters (+550 feet).
After a counter reaches its maximum positive or negative value, its value will roll over to the
maximum negative or positive value, respectively. This means that if the encoder value is
increasing and a rollover occurs, the sequence of encoder counter values will be ... ;
8,388,606; 8,388,607; -8,388,608; -8,388,607; ... As long as the distance between the
workspace of the robot and the nominal transformation of the belt is within the distance that
can be represented by the maximum encoder value, V+ application programs normally do not
have to take into account the fact that the counter will periodically roll over. The belt_
distance equation described above is based upon a relative encoder value:
encoder_count - encoder_offset
and V+ automatically adjusts this calculation for any belt rollover that may occur.
Care must be exercised, however, if an application processes encoder values in any way. For
example, a program may save encoder values associated with individual parts on the
conveyor, and then later use the values to determine which parts should be processed by the
robot. In such situations the application program may need to consider the possibility of
rollover of the encoder value.
NOTE:While the encoder counter value is stored as a 24-bit number, the rate of change of
the belt encoder (the speed of the belt) is maintained only as a 16-bit number. The belt
speed is used internally by V+ to predict future positions on the belt. Therefore, the rate of
change of the belt encoder should not exceed 32,768 counts per 16 milliseconds. The
Adept application program for belt calibration includes a test for this condition and prints a
warning if this restriction will be violated. This requirement will be a limitation only for very
high-speed conveyors with very high-resolution encoders.
The Encoder Scaling Factor
For any given conveyor/encoder installation, the encoder scaling factor is a constant number
that represents the amount the encoder counter changes during a change in belt position.
The units of the scaling factor are millimeters/count.
Basic Programming Concepts
V+Language User's Guide, v17.0
Page 312