ADXRS453 Analog Devices, ADXRS453 Datasheet - Page 9

• Manufacturer Part Number: ADXRS453
• Description: High Performance / Digital Output Gyroscope
• Manufacturer: Analog Devices

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THEORY OF OPERATION
The ADXRS453 operates on the principle of a resonator gyroscope.
Figure 18 shows a simplified version of one of four polysilicon
sensing structures. 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 the
device experiences angular rate. In the SOIC_CAV package, the
ADXRS453 is designed to sense a z-axis (yaw) angular rate; the
LCC_V vertical mount package 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 ADXRS453.
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.
Z
X
Y
Figure 18. Simplified Gyroscope Sensing Structure
Rev. 0 | Page 9 of 32
CONTINUOUS SELF-TEST
The ADXRS453 gyroscope implements a complete electro-
mechanical 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 both 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.
The difference amplitude between the positive and negative
self-test deflections is filtered to f
continuously monitored and compared to hard-coded self-test
limits. If the measured amplitude exceeds these limits (listed in
Table 1), one of two error conditions is asserted, depending on
the magnitude of the self-test error.
•
•
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).
See the SPI Communication Protocol section for more informa-
tion about error reporting.
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 01 for valid
sensor data.
For more severe self-test errors, the CST bit of the fault
register is asserted and the status bits (ST[1:0]) in the
sensor data response are set to 00 for invalid sensor data.
INTERNALLY COMPARED
SELF-TEST AMPLITUDE.
TO THE SPECIFICATION
TABLE LIMITS.
Figure 19. Continuous Self-Test Demodulation
CONTINUOUS SELF-TEST SIGNAL.
RATE SIGNAL WITH
0
/8000 (~1.95 Hz) and is
LOW FREQUENCY RATE
INFORMATION.
ADXRS453