10171 Description and application of sensors

A list of sensors which can be used are given below; however, a limited combination of these sensors may only be installed depending on the sophistication of the suspension system adopted:

1 body height sensor

2 steering wheel sensor

3 longitudinal acceleration sensor

4 lateral acceleration sensor

5 brake pressure sensor

6 brake pedal sensor

7 acceleration pedal sensor

8 load sensor

9 vehicle speed sensor 10 mode selector

Height sensor (Fig. 10.121) The linear variable differential sensor is often used to monitor vertical height movement as there is no contact between moving parts; it therefore eliminates any problems likely to occur due to wear. It is basically a transformer having a central primary winding and two

Fig. 10.121 Height sensor (linear variable differential type)

secondary windings connected in series in opposition to each other. An alternative input supply voltage is applied to the primary winding; this produces a magnetic flux which cuts through the secondary winding thereby inducing an alternative voltage into the secondary winding. The difference between the voltage generated in each secondary winding therefore becomes the output signal voltage. With the non-ferromagnetic/soft iron armature bar in the central position each secondary winding will generate an identical output voltage so that the resultant output voltage becomes zero. However when the armature (attached to the lower suspension arm) moves up or down as the body height changes the misalignment of the soft iron/non-ferromagnetic armature causes the output voltage to increase in one winding and decrease in the other, the difference in voltage increasing in direct proportion to the armature displacement. This alternative voltage is then converted to a direct voltage before entering the electronic-control unit.

Steering sensor (Fig. 10.122) This sensor monitors the angular position of the steering wheel and the rate of change of the steering angle. The sensor comprises a slit disc attached to the steering column and rotates with the steering wheel and a fixed 'U' shaped detector block containing on one side three phototransistors and on the other side three corresponding light-emitting diodes. The disc rotates with the steering column and wheel and at the same time the disc moves between the light-emitting diode and the phototransistor block overhang. When the column is turned the rotating slotted disc alternatively exposes and blocks the light-emitting beams directed towards the phototransistors; this interruption of the light beams generates a train of logic pulses which are then processed by the microprocessor to detect the steering angle and the rate of turn. To distinguish which way the steering wheel is turned a left and right hand phototransistor is included, and a third phototransistor is located between the other two to establish the neutral straight ahead position. The difference in time between light beam interruptions enables the microprocessor to calculate the angular velocity of the driving wheel at any one instance in time. In some active suspension systems, when the angular velocity exceeds a pre-fixed threshold the electronic-control unit switches the suspension to a firm ride mode.

Acceleration sensor (Fig. 10.123) A pendulum strain gauge type acceleration sensor is commonly used for monitoring body acceleration in both longitudinal and lateral directions. It is comprised of a leaf spring rigidly supported at one end with a mass attached at its free end. A thin film strain gauge wired in the form of a wheatstone bridge circuit is bonded to the leaf spring on one side, two of the four resistors are passive whereas the other two are active. As the vehicle is accelerated the pendulum due to the inertia of the mass will reluctantly hold back thus causing the spring to deflect. The pair of active resistor arms therefore become strained (stretch) and hence alter their resistance, thus producing an imbalance to the wheat-stone bridge circuit resulting in an output voltage proportional to the magnitude of the acceleration. When using this type of sensor for monitoring

Phototransistor

Phototransistor interrupter (PTI) (right hand)

(neutral) PTI

(left hand)

Slit disc

Fixed detector block

Phototransistor interrupter (PTI) (right hand)

(neutral) PTI

(left hand)

Fixed detector block

Slit disc

Fig. 10.122 Steering sensor (photo interrupter type)

Steering column

Steering column

Light beam

Fixed detector block

Detecting slit

Phototransistor

Steering column

Light beam

Fixed detector block

Detecting slit

Light-emitting diode

Fig. 10.122 Steering sensor (photo interrupter type)

Terminals

Terminals

Support block

Strain gauge wheatstone bridge circuit

Thin film strain gauge strip bonded to spring

Tensile strain

Leaf spring deflecting

Pendulum mass

Fig. 10.123 Acceleration sensor (pendulum strain gauge type)

Support block

Strain gauge wheatstone bridge circuit

Thin film strain gauge strip bonded to spring

Vehicle body

Tensile strain

Leaf spring deflecting

Vehicle body

Acceleration

Drive wheels accelerating

Acceleration

Drive wheels accelerating

Pendulum mass

Fig. 10.123 Acceleration sensor (pendulum strain gauge type)

lateral acceleration, it should be installed either near the front or rear to enable it to sense the swing of the body when the car is cornering it, there is also a measure in the degree of body yaw.

Brake pedal/pressure sensor These sensors are used to indicate the driver's intentions to brake heavily by either monitoring the brake pedal movement or in the form of a pressure switch tapped in to the hydraulic brake circuit. With the pressure switch method the switch is set to open at some predetermined brake-line pressure (typically about 35 bar); this causes the input voltage to the electronic-control unit to rise. Once 5 volts is reached (usual setting) the electronic-control unit switches the suspension to 'firm' ride mode. When the braking pressure drops below 35 bar the pressure switch closes again; this grounds the input to the electronic-control unit and causes its output voltage to the solenoid control valves also to collapse, and at this point the suspension reverts to 'soft' ride mode.

Acceleration pedal sensor These sensors can be of the simple rotary potentiometer attached to the throttle linkage indicating the throttle opening position. A large downward movement or a sudden release of the accelerator pedal signals to the electronic-control unit that the driver intends to rapidly accelerate or decelerate, respectively. When accelerating hard the rapid change in the potentiometer resistance and hence input voltage signals the electronic-control unit to switch the suspension to firm ride mode.

Load sensor Load sensors are positioned on top of the strut actuator cylinder; its purpose is to monitor the body load acting down on each strut actuator.

Vehicle speed sensor Vehicle speed can be monitored by the speedometer or at the transmission end by an inductive pick-up or Hall effect detector which produces a series of pulses whose frequency is proportional to vehicle speed. Once the vehicle speed exceeds some predetermined value the electronic-control unit automatically switches the suspension to 'firm' ride mode. As vehicle speed decreases, a point will be reached when the input to the electronic-control unit switches the suspension back to 'soft' ride mode.

Mode selector This dashboard mounted control switches the suspension system via the electronic-control unit to either a comfort (soft) ride mode for normal driving conditions or to a sports (firm) ride mode. However, if the vehicle experiences severe driving conditions while in the comfort ride mode, the electronic-control unit overrides the mode selector and automatically switches the suspension to sports (firm) ride mode.

10.17.2 Active hydro/coil spring suspension system (Fig. 10.124(a-h))

A typical fully active hydraulic self-levelling suspension system utilizing strut actuators consisting of a cylinder, piston and ram-shaft installed between each spring and its body support. A hydraulic pump driven from the engine supplies high pressure fluid to the accumulator and to the individual strut actuators via a pressure regulating valve and a levelling valve. The purpose of the accumulator is to store fluid at maximum pressure so that it can instantly be discharged to the various strut actuator cylinders when commanded; this would not be possible without an excessive time delay since the pump could not generate and discharge sufficient quantity of high pressure fluid in the time-span required to maintain the car on a level keel. When the engine is running, high pressure fluid is supplied to each strut actuator to bring the body level up to its design height via the levelling control valve. Note some systems may have more than one body height setting.

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Responses

  • Harri
    Which sensor consist a photo transistors and light diode and rotates with the steering?
    2 years ago

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