1

Inertial SensorsInertial Sensors

Inertial Sensors?

• Inertial sensors in inertial navigation : big & expensive

• MEMS(Micro-Electro-Mechanical Systems) Technology

Accelerometer Types

• pendulous accelerometer

• vibrating beam accelerometers

Gyroscopes Types

• mechanical gyroscope

• Dynamically Tuned gyroscope

• vibrating gyroscope

• optical gyroscope (Fiber-optic gyro, ring laser gyro)

2

Inertial NavigationInertial Navigation

Inertial Navigation

• A typical inertial navigation system integrates the information gather from a combination of gyroscopes and accelerometers in order to determine the current state of the system.

Western Inertial Markets

• ICBM Trident D5 accuracy : CEP 111 m at 7400 Km (about Korea – Hawaii)

3

Accelerometer PrincipleAccelerometer Principle

mass-spring type accelerometer

• To increase accelerometer sensitivity : m large or K small

4

ADXL 202 Dual Axis AccelerometersADXL 202 Dual Axis Accelerometers

Analog Device ADXL202

• surface-micromachined accelerometer

Sensor Principle

• To increase accelerometer sensitivity :

- large m, small K, large A

measure capacitance, which is inversely proportional to the gap

5

ADXL 202 Dual Axis AccelerometersADXL 202 Dual Axis Accelerometers

To increase the mass, common beam mass is used

x

y

x direction acceleration is detected here

spring

6

ADXL202 SpecADXL202 Spec

ADXL202 Diagram & Features

7

ADXL203 SpecADXL203 Spec

8

Accelerometer resolutionAccelerometer resolution

10 bit A/D converter : 1024 division

• A/D converter : 1024 div : 5v 4.88 mv/div about 5mv/div

• 1 mg/mv

• resolution : 5mg 12 bit A/D converter : 4096 division

• A/D converter : 4096 div : 5V 1.22mv/div

• resolution : 1.22 mg / div accelerometer noise level

• 0.11*0.11 mg^2 / Hz * 60 Hz = 0.726 mg^2 square root 0.85 mg? 12 bit A/D converter in practice

• A/D converter noise 2.5 mg (2div) + accelerometer noise 0.85 mg

• about 4-5 mg?

Bias & scale factor!

9

Gyroscope Gyroscope

Gyroscopic Precession

• What will happen if there is rotation around the rotation axis

10

Precession of a gyroscopePrecession of a gyroscope

11

Gyroscope based on gyroscopic precessionGyroscope based on gyroscopic precession

Single-axis Gyroscope

12

Coriolis AccelerationCoriolis Acceleration

Coriolis acceleration

• A person moving northward toward the outer edge of a rotating platform must increase the westward speed component (blue arrows) to maintain a northbound course. The acceleration required is the Coriolis acceleration.

13

Coriolis Acceleration Coriolis Acceleration

Constrained motion means force is appliedturning fork gyroscope

ADXRS150

14

Gyroscope using Coriolis effectGyroscope using Coriolis effect

Schematic of the gyro’s mechanical structure

The displacement is proportional to the rotation speed

15

ADXRS 150 Single Axis GyroscopeADXRS 150 Single Axis Gyroscope

ADXRS150 gyroscope structure

opposite drive direction

(robust to shock & vibration)

16

ADXRS specADXRS spec

ADXRS150 Diagram & Features

17

ADXRS150 Spec.ADXRS150 Spec.

18

Gyroscope resolutionGyroscope resolution

10 bit A/D converter

• about 5mv/div

• 0.08 deg/sec / mv

• resolution : 0.4 deg/sec

12 bit A/D converter

• resolution : 0.1 deg/sec

gyroscope noise level

• 0.05*0.05 deg/sec / Hz * 60 Hz = 0.15 (deg/sec)^2 square root 0.38 deg/sec

gyroscope resolution

• 0.5 deg/sec ???