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ADXRS450_11 Datasheet(Arkusz danych) 9 Page - Analog Devices

Numer części ADXRS450_11
Szczegółowy opis  High Performance, Digital Output Gyroscope
Pobierz  28 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXRS450
Rev. 0 | Page 9 of 28
THEORY OF OPERATION
The ADXRS450 operates on the principle of a resonator gyro-
scope. A simplified version of one of four polysilicon sensing
structures is shown in Figure 18. Each sensing structure contains
a dither frame that is electrostatically driven to resonance. This
produces the necessary velocity element to produce a Coriolis
force when experiencing angular rate. In the SOIC_CAV package,
the ADXRS450 is designed to sense a z-axis (yaw) angular rate;
whereas the vertical mount package (LCC_V) orients the device
such that it can sense pitch or roll angular rate on the same PCB.
When the sensing structure is exposed to angular rate, the
resulting Coriolis force couples into an outer sense frame,
which contains movable fingers that are placed between fixed
pickoff fingers. This forms a capacitive pickoff structure that
senses Coriolis motion. The resulting signal is fed to a series of
gain and demodulation stages that produce the electrical rate
signal output. The quad sensor design rejects linear and angular
acceleration, including external g-forces and vibration. This is
achieved by mechanically coupling the four sensing structures
such that external g-forces appear as common-mode signals
that can be removed by the fully differential architecture
implemented in the ADXRS450.
X
Y
Z
Figure 18. Simplified Gyroscope Sensing Structure
The resonator requires 22.5 V (typical) for operation. Because
only 5 V is typically available in most applications, a switching
regulator is included on chip.
CONTINUOUS SELF-TEST
The ADXRS450 gyroscope uses a complete electromechanical
self-test . An electrostatic force is applied to the gyroscope frame,
resulting in a deflection of the capacitive sense fingers. This
deflection is exactly equivalent to deflection that occurs as a
result of external rate input. The output from the beam structure is
processed by the same signal chain as a true rate output signal,
providing complete coverage of the electrical and mechanical
components.
The electromechanical self-test is performed continuously during
operation at a rate higher than the output bandwidth of the
device. The self-test routine generates equivalent positive and
negative rate deflections. This information can then be filtered
with no overall effect on the demodulated rate output.
RATE SIGNAL WITH
CONTINUOUS SELF TEST SIGNAL.
SELF TEST AMPLITUDE. INTERNALLY
COMPARED TO THE SPECIFICATION
TABLE LIMITS.
LOW FREQUENCY RATE INFORMATION.
Figure 19. Continuous Self-Test Demodulation
The difference amplitude between the positive and negative
self-test deflections is filtered to 2 Hz, and it is continuously
monitored and compared to hardcoded self-test limits. If the
measured amplitude exceeds these limits (listed in Table 1), one
of two error conditions asserts depending on the magnitude of
self-test error. For less severe self-test error magnitudes, the CST
bit of the fault register is asserted; however, the status bits (ST[1:0])
in the sensor data response remain set to 0b01 for valid sensor
data. For more severe self-test errors, the CST bit of the fault reg-
ister is asserted, and the status bits (ST[1:0]) in the sensor data
response are set to 0b00 for invalid sensor data. Table 1 lists the
thresholds for both of these failure conditions. If desired, the user
can access the self-test information by issuing a read command to
the self-test memory register (Address 0x04). For more infor-
mation about error reporting, see the SPI Communication Protocol
section.




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