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ADW71205WSTZ Datasheet(Arkusz danych) 11 Page - Analog Devices
AD [Analog Devices]
Rev. A | Page 10 of 20
SIGNAL DEGRADATION DETECTION
Degradation of signal (DOS) is detected when either resolver input
(Sin or Cos) exceeds the specified DOS Sin/Cos threshold. The
AD2S1205 detects this by comparing the monitor signal to a
fixed maximum value. In addition, DOS is detected when the
amplitudes of the Sin and Cos input signals are mismatched
by more than the specified DOS Sin/Cos mismatch. This is
identified because the AD2S1205 continuously stores the
minimum and maximum magnitude of the monitor signal in
internal registers and calculates the difference between these
values. DOS is indicated by a logic low on the DOS pin and is
not latched when the input signals exceed the maximum input
level. When DOS is indicated due to mismatched signals, the
output is latched low until a rising edge of SAMPLE resets the
stored minimum and maximum values. The DOS condition has
priority over the LOT condition, as shown in
. DOS is
indicated within 33° of the angular output error (worst case).
LOSS OF POSITION TRACKING DETECTION
Loss of tracking (LOT) is detected when
The internal error signal of the AD2S1205 exceeds 5°.
The input signal exceeds the maximum tracking rate.
The internal position (at the position integrator) differs
from the external position (at the position register) by
more than 5°.
LOT is indicated by a logic low on the LOT pin and is not
latched. LOT has a 4° hysteresis and is not cleared until the
internal error signal or internal/external position mismatch
is less than 1°. When the maximum tracking rate is exceeded,
LOT is cleared only if the velocity is less than the maximum
tracking rate and the internal/external position mismatch is
less than 1°. LOT can be indicated for step changes in position
(such as after a RESET signal is applied to the AD2S1205), or
for accelerations of >~65,000 rps
. It is also useful as a built-in
test to indicate that the tracking converter is functioning
properly. The LOT condition has lower priority than both the
DOS and LOS conditions, as shown in
. The LOT and
DOS conditions cannot be indicated at the same time.
Table 4. Fault Detection Decoding
Loss of Signal (LOS)
Degradation of Signal (DOS)
Loss of Tracking (LOT)
RESPONDING TO A FAULT CONDITION
If a fault condition (LOS, DOS, or LOT) is indicated by the
AD2S1205, the output data is presumed to be invalid. Even
if a RESET or SAMPLE pulse releases the fault condition and
is not immediately followed by another fault, the output data
may be corrupted. As discussed previously, there are some fault
conditions with inherent latency. If the device fault is cleared,
there may be some latency in the resolver’s mechanical position
before the fault condition is reindicated.
When a fault is indicated, all output pins still provide data, although
the data may or may not be valid. The fault condition does not
force the parallel, serial, or encoder outputs to a known state.
Response to specific fault conditions is a system-level requirement.
The fault outputs of the AD2S1205 indicate that the device has
sensed a potential problem with either the internal or external
signals of the AD2S1205. It is the responsibility of the system
designer to implement the appropriate fault-handling schemes
within the control hardware and/or algorithm of a given appli-
cation based on the indicated fault(s) and the velocity or position
data provided by the AD2S1205.
FALSE NULL CONDITION
Resolver-to-digital converters that employ Type II tracking loops
based on the previously stated error equation (see Equation 4
in the Theory of Operation section) can suffer from a condition
known as a false null. This condition is caused by a metastable
solution to the error equation when θ − ϕ = 180°. The AD2S1205
is not susceptible to this condition because its hysteresis is
implemented external to the tracking loop. As a result of the
loop architecture chosen for the AD2S1205, the internal error
signal constantly has some movement (1 LSB per clock cycle);
therefore, in a metastable state, the converter moves to an
unstable condition within one clock cycle. This causes the tracking
loop to respond to the false null condition as if it were a 180°
step change in input position (the response time is the same, as
specified in the Dynamic Performance section of Table 1).
Therefore, it is impossible to enter the metastable condition
after the start-up sequence if the resolver signals are valid.
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