Measurement Computing USB-1616HS-BNC User Manual page 28

USB-1616HS-BNC User's Guide
The concentric pattern for B has the same number of window pairs as A—except that the entire pattern is
rotated by 1/4 of a window-pair. Thus the B signal is always 90 degrees out of phase from the A signal. The A
and B signals pulse 512 times (or 1024, 4096, etc.) per complete rotation of the encoder.
The concentric pattern for the Z signal has only one transparent window and therefore pulses only once per
complete rotation. Representative signals are shown in the following figure.
A
B
Z
Figure 17. Representation of quadrature encoder outputs: A, B, and Z
As the encoder rotates, the A (or B) signal indicates the distance the encoder has traveled. The frequency of A
(or B) indicates the velocity of rotation of the encoder. If the Z signal is used to zero a counter (that is clocked
by A) then that counter will give the number of pulses the encoder has rotated from its reference. The Z signal is
a reference marker for the encoder. It should be noted that when the encoder is rotating clockwise (as viewed
from the back), A will lead B and when the encoder is rotating counterclockwise, A will lag B. If the counter
direction control logic is such that the counter counts upward when A leads B and counts downward when A
lags B, then the counter will give direction control as well as distance from the reference.
Maximizing encoder accuracy
If there are 512 pulses on A, then the encoder position is accurate to within 360°/512.
You can get even greater accuracy by counting not only rising edges on A but also falling edges on A, giving
position accuracy to 360 degrees/1024.
You get maximum accuracy counting rising and falling edges on A and on B (since B also has 512 pulses.) This
gives a position accuracy of 360°/2048. These different modes are known as X1, X2, and X4.
Figure 16. Representation of rotary shaft quadrature encoder
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Functional Details