IC-TW8_16 Datasheet, PDF (29/67 Pages) IC-Haus GmbH – 16-BIT SIN/COS INTERPOLATOR

iC-TW8 16-BIT SIN/COS INTERPOLATOR
Pin Configuration Mode
Aug 3, 2016 Page PC13/15
encoder (without bearings) or to use the full speed
capability of the iC-TW8, enter an arbitrarily large
value (such as 1,000,000 RPM). Note that he calcu-
lated actual speed limit is affected by the chosen
crystal frequency and the interpolation factor used.
The graphs below show typical encoder lag and mo-
tion profile performance of the iC-TW8 using nor-
mal filtering and lag recovery at 15,261 radians per
second² maximum acceleration and a resolution of
16, 384 AB cycles per revolution.
Next, enter the desired level of filtering for the AB
outputs and whether lag recovery is desired. Per-
formance graphs of encoder lag in AB edges and
response to a trapezoidal motion profile with con-
stant acceleration, velocity, and deceleration are
displayed to allow observing the effect of the differ-
ent filter settings. Scroll to the right on small com-
puter screens to see the performance graphs.
A single parameter (maximum acceleration) is used
to generate the filter response graphs. Enter a value
in radians per second² that represents the maximum
expected acceleration of the encoder. The design
tool calculates the equivalent acceleration in units of
kRPM (1000 RPM) per second and time to reach the
chosen maximum speed, which may be more easily
understood. The acceleration value is used only to
generate the performance graphs, it does not affect
the configuration of the iC-TW8. The motion profile
is generated by accelerating for 1000 ADC samples
at the entered acceleration, running at the attained
speed for 1000 ADC samples, and decelerating at
the entered acceleration for 1000 ADC samples.
The maximum acceleration ( α_m ) of a motor in radi-
ans per second² may be calculated knowing its stall
torque ( Τ_s ) in Newton meters and its rotor inertia
( J_r ) in kilogram meters² using the formula
α_m = Τ_s J_r . For example, the Maxon EC 90 flat
90W motor with the 24V winding (order number
323772) has a rotor inertia of 3,060 gcm² and a stall
torque of 4,670 mNm. Thus its no-load maximum
acceleration in radians per second² is
α_m = Τ_s
J_r
=
4670
3060
⋅10,000
=
15, 261
Using this value for the maximum acceleration pa-
rameter in the design tool will then show the per-
formance of the iC-TW8 when used in an encoder
for feedback with this motor.
Speed Profile
350
300
Input
Output
250
200
150
100
50
0
-50
0
2
4
6
8
10
12
14
16
18
Time (ms)
Encoder Lag
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
0
2
4
6
8
10
12
14
16
18
Time (ms)
Figure 12: Typical Encoder Performance Graphs
The design tool also calculates the encoder lag in
AB edges per kRPM and mechanical degrees (°m)
of encoder rotation at the chosen maximum speed
for the selected filter configuration. For a given fil-
ter and lag recovery setting, encoder lag is directly
proportional to encoder speed.
The design tool also calculates the position and
speed overshoot due to the AB output filter. Position
overshoot (shown in AB edges and mechanical de-
grees of encoder rotation) is proportional to the en-
tered maximum acceleration and is typically noticed
on stopping (deceleration). At low acceleration val-
ues and/or low encoder resolutions, position over-
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