SERVICE TECH Vol.491 07-813

"Sensor Fundamentals"6th Session: Inertia Sensor

This session introduces the construction and operation of the inertia sensor.

The inertia sensor possesses the function of a Gravity sensor (hereafter, G sensor) and yaw rate sensor,

and detects vehicle conditions within the Vehicle Stability Control (VSC) system. A G sensor detects the

acceleration added from the front, rear and sides of the vehicle. A yaw rate sensor detects the yaw angular

velocity*1 (hereafter, yaw rate).

*1: Yaw rate: The angular speed when a vehicle rotates about the vertical axis.

The VSC system maintains vehicle stability when turning. Normally, a vehicle follows steering operations to

maintain stability when turning. However, in unpredictable situations caused by road conditions, vehicle

speed, sudden turns, or external influences, the front and/or rear tires may slip severely in a horizontal

direction. In cases where tires experience a large degree of slip, the VSC-ECU controls engine output and

braking for each wheel to maintain vehicle stability and mitigate horizontal sliding.

1 OUTLINE

Vehicle Stability Control (VSC) system

Acceleration

(G)

Yaw Rate

Inertia SensorInertia Sensor

VSC System Configuration

Steering Angle SensorDetection of

Driver's Intention

Detection of

Vehicle Status

Target

Yaw Rate

Standard

Yaw Rate

Estimated

Yaw RateComparison

CAN

CAN

Throttle Position Sensor

Brake Pressure Sensor

Vehicle Speed Sensor

G Sensor

(Biaxial)

Yaw Rate

Sensor

Inertia

Sensor

VSC-ECU

Rear Wheel

Horizontal

Slip

Judgment

Front Wheel

Horizontal

Slip

Detection

Actuator

Actuation

Rear Wheel Horizontal

Slip Control

Front Wheel Horizontal

Slip Control

14

"Sensor Fundamentals"

SERVICE TECH Vol.491 07-8 (Bi-monthly publication)Date of publication : August 20, 2007

Editorial department : Service Department, DENSO

100% recycled papers are used in this magazine.

2007C

The inertia sensor consists of a case, cover and circuit board on which two G sensors and two yaw ratesensors are attached.

2-1 Yaw Rate Sensor

The yaw rate sensor internal oscillator undergoes symmetric vibration when an AC voltage is applied. Theoscillator is torsionally oscillated due to the Coriolis effect*1 generated when the vehicle turns. Yaw rate ismeasured by detecting the change in the amount of torsional oscillation.*1: Coriolis effect: A phenomenon in which objects are apparently deflected from a straight path if the objects are viewed

in a rotating coordinate system (from a rotating frame of reference). This phenomenon occursbecause, when an object moves, inertial force with a size proportional to the movement velocity willmove perpendicularly in relation to the direction of movement of the object. This inertial force is knownas the Coriolis effect. If the inertial force is replaced with a yaw rate sensor, the "rotating coordinatesystem" becomes the yaw rate of the vehicle, "movement" becomes the oscillation of the oscillator,and the "movement velocity" becomes the oscillation speed of the oscillator.

2-2 G Sensor

If a vehicle is accelerated, since the weight for the sensor chip inside the G sensor moves to the right (as inthe diagram below) to the spring effect, the movable electrode also changes position. As a result, thedistance between the movable electrode and fixed electrode changes due to the position change of themovable electrode, causing a change in capacitance between the electrodes. Vehicle acceleration isdetected using the change in capacitance.

2 CONSTRUCTION AND OPERATION

Yaw Rate Sensor G Sensor

Case

Cover

Assembly Construction

Yaw Rate Sensor

G SensorCircuit Board

Detection Signal

Oscillator

Yaw Rate

Speed

Coriolis Effect

AC Voltage

Cap

Base

Ceramic Package

IC

Oscillator

Lid

Construction Detection Principle

Sensor ChipMovable Electrode

Fixed Electrode

NeedleWeight

AccelerationMovable Electrode Changes Position

Construction Detection Principle

Sensor Chip

Ceramic PackageIC

Lid

Change in Capacitance Detected