iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC19/36
The design tool shows the resulting speed/resolution
performance in a chart at columns M – T. The X
(Resolution) axis shows the range of output resolu-
tions that are possible using the selected input reso-
lution. The chart shows the resulting maximum
speed, maximum adaption speed, warning fault
speed and fatal fault speed at a given output resolu-
tion.
Filter Configuration
In the Filter Configuration tab, enter the desired fil-
ter mode for the AB signal path filter and then enter
one of the recommended filter instances. The design
tool confirms the actual filter mode and approximate
noise and jitter bandwidth for the selected filter in-
stance at the current crystal frequency. If “Not De-
fined” appears for the actual filter mode, the entered
instance is not one of the recommended ones. Non-
recommended instances may cause unstable filter
response. See Filter on page 6 for details on filter
modes and instances.
Finally, enter the desired hysteresis in equivalent
AB output edges to prevent output dithering at
standstill. The design tool calculates and displays
the closest possible (actual) hysteresis with the cur-
rent input and output resolution. Also displayed is
the equivalent hysteresis in AB output cycles, me-
chanical degrees (°m) and arc-minutes of rotation,
and input Sin/Cos cycle degrees. Desired hysteresis
can be less than 1 AB output edge. For best accura-
cy, choose the smallest value that produces stable
AB outputs with no sensor motion. Experimentation
may be required to determine the optimal value.
The following filter modes are available.
iC-TW8 Filter Modes
Mode
Description
P
Low-pass filter with lag.
Critically-damped low-pass filter with
no filter lag.
PI
Lag
Recovery
Under-damped low-pass filter with no
filter lag and reduced latency lag.
The complete range of output resolutions using the
selected input resolution is shown numerically in
cells B39 – B41. A complete list of output resolu-
tions and the corresponding interpolation factor re-
quired to achieve them is shown in the All Resolu-
tions tab for easy reference with respect to complet-
ed designs. For example, with an input resolution of
32 cycles per revolution, an output resolution of
16,384 CPR requires an interpolation factor of 512,
as shown previously. Using this same input resolu-
tion, the All Resolution tab shows that an output
resolution of 10,000 is also possible by choosing an
interpolation factor of 312.50 (row 1257).
P Mode is the simplest mode; position lag at con-
stant sensor input velocity is directly proportional to
the sensor velocity and the filter instance value.
There is no position or speed overshoot in P mode.
PI Mode eliminates filter lag at constant sensor in-
put velocity at the expense of critically damped fil-
ter response. In PI mode, filter lag at constant veloc-
ity is eliminated, but position and speed overshoot
are proportional to the filter instance value.
Lag Recovery Mode reduces latency-induced posi-
tion lag at constant sensor input velocity at the ex-
pense of under-damped filter response. In lag recov-
ery mode, position lag at constant velocity due to
interpolator latency is reduced, but filter response is
under-damped and position and speed overshoot are
proportional to the filter instance value.
In general, use the lowest instance value for the de-
sired filter type. This gives the fastest response,
minimum lag, and minimum position and speed
overshoot. Another instance may be used if addi-
tional noise and jitter filtering is required.
The design tool shows the encoder lag in AB edges
per kRPM and mechanical degrees (°m) of encoder
rotation at the chosen maximum speed for the se-
lected filter instance. For a given filter instance, lag
is directly proportional to encoder speed.
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