iC-TW8 16-BIT SIN/COS INTERPOLATOR
Pin Configuration Mode
Jan 9, 2013 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.
Speed Profile
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.
350
Input
300
250
200
150
100
50
Output
0
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.
-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
The maximum acceleration ( m ) of a motor in radi-
ans per second2 may be calculated knowing its stall
-1,0
0
2
4
6
8
10
12
14
16
18
J
torque ( s) in Newtons and its rotor inertia ( r ) in
Time (ms)
kilogram seconds2 using the formula
.
r
J
m
s
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
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.
4670
m s Jr
10,000 15,261
3060
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.
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-
shoot may be less than one AB edge and thus totally
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