Numerous complementary lighting systems are fitted inside the vehicle. There is no specific legislation for these and hence manufacturers are free to equip vehicles in any way they choose. Good interior light includes switchgear and appropriate illumination of the driver controls and is important to provide a safe, relaxing environment for the driver.
Interior lamps
Also known as dome lamps, these illuminate on entry to the vehicle, either via door switches or via integration with the vehicle security system. Often the interior lamps are fitted with a time delay unit to allow the occupants to find and fasten seat belts etc. on entry. In addition, these lamps can be activated automatically on ignition key ‘off’, so that the occupants have light to exit the vehicle safely. These functions are often implemented via a central body electronic module which controls the operation of interior lights, in addition to other accessories, implementing logical features as described. Map lights are also fitted to illuminate specifically the area in front of the occupants for reading purposes. These lamps are designed to provide a light source which does not distract the driver. Additional interior lighting is fitted to many vehicles to illuminate storage areas such as glove compartments or the boot loading area.
Lights 183
Instrumentation
Controls for the vehicle accessories and equipment must be clearly visible at night in order to maximise ease of use for the driver. In addition, it is common that the display-instrument lighting intensity can be adjusted manually or automatically (via a rheostat or electronic voltage controller). Indicator lamps are used to monitor various operating conditions and some of the colours for these are specified in a uniform European code. Examples are shown in Figure 8.29.
Comfort and convenience controls must be gently backlit so that they can be seen by the occupants but without causing glare or distraction for the driver. Switch illumination is necessary to allow the driver to identify specific functions and to assist orientation.
Light output is measured in candela which is a unit derived from candle power. The power of light falling on/reflected by a surface is measured in lux. When the distance is increased between the light source and the reflective surface, the intensity of the illumination decreases significantly (i.e. by the square of the distance)
For headlamp applications, traditional incandescent bulbs have been replaced by halogen types due to their superior light output. Note that these are now being replaced by new technology such as gas-discharge light sources, and LED technology will be used in the future
Gas-discharge lamps use a high-voltage arc through gas to generate the high-intensity light source. The arc is controlled by an electronic ballast unit to control the light source. Note that this lamp technology is always used in conjunction with automatic levelling and cleaning technology to prevent glare to other drivers
Modern vehicle lamp assemblies are designed using computer-aided design and simulation techniques. Complex reflector shapes can generate the required beam pattern without the need for a sophisticated lens. Often, headlamps of this type are fitted with a completely clear plastic lens Headlamp levelling provides active compensation to headlamp beam deviation caused by vehicle loading. More sophisticated systems can compensate for acceleration and braking effects. Active front lighting is a development of this technology. In addition to levelling compensation this system can direct the headlamp beam in the direction of intended travel. This improves road illumination and safety
Developments in rear lighting technology are mainly due to the ability to use different light sources other than incandescent bulbs. LED technology is to the forefront here and allows compact lamp units to be created with advanced features like substitution of lamp types (i.e. a stop lamp fails so a tail lamp can be used in its place as a temporary measure). Also, one lamp can execute more than one function due to multicolour LED light sources. In addition, LEDs have a very fast response compared to an incandescent bulb lighting up time and this provides a valuable safety benefit, particularly with respect to brake lights
K ey P oints K ey P oints
1 Braking system defect 2 Front fog lamp 3 Hazard-warning flashers 4 Instrument-panel illumination 5 High beam
6 Fog warning lamps 7 Windshield wiper and washer 8 Rear-window washer 9 Main headlamp switch 10 Rear-windown defroster
11 Headlamp wipe/wash 12 Spot lamp
13 Windshield washer system 14 Interior lighting 15 Windshield defroster 16 Floodlamp 17 Windshield wipers 18 Ventilation/heater fan 19 Heated mirror 20 Rotating beacon
184 Signalling and vision Fundamentals of Motor Vehicle Technology: Book 3
8.2.1 Cleaning
Front wiper
A front wiper system is essential on all vehicles to ensure good visibility for the driver under all weather conditions. This fact makes an efficient screen cleaning system an absolute essential contribution to road safety. The fundamental requirements are as follows:
● the system must clear the windscreen of snow, ice, dirt and rain
● the wipe area must be a specific size in accordance
with regulations
● it must be a practically noiseless operation
● it must operate over a wide temperature range
● it must be capable of handling overload (e.g. blades
frozen on the screen)
● it must be corrosion proof.
The force required to drive the wiper system can be considerable in modern vehicles due to the complex curvature of the windscreens which require long wiper blades. The drive motor must generate the required
power. In the past, shunt-type motors were used, but for modern vehicle applications powerful permanent magnet motors are used. Figure 8.30 shows the typical layout of a wiper system. The motor power is transformed to low speed/high torque via a worm gear reduction drive. This drives a crank mechanism which converts the rotary motion into a reciprocating linear motion.
The design of the wiper system is subject to legislation that defines the required wiped area. A number of configurations are available to provide the required patterns and these are shown in Figure 8.31.
The wiper system is generally controlled via a driver lever or stalk near the steering column. Most systems have basic bi-speed operation plus an intermittent wipe function. There are also a number of enhancements available to improve the convenience of the system.
8.2 SCREENS
Two-speed operation
All front wipers have a least two-speed operation. Low or normal speed gives approximately 50 wiping cycles per minute (a wiping cycle is one full back-and-forth motion of the blades). High speed gives approximately 70 wiping cycles per minute. The actual value of these speeds is defined in country-specific legislation.
The method of achieving the two speeds with a simple permanent magnet motor involves the inclusion of a second brush on one side of the commutator giving three in all. This is shown in Figure 8.32.
When the switch supplies current to brush ‘B’, low-speed operation is obtained. If high-speed wipe is selected then current is supplied via brush ‘C’. This moves the armature magnetic field out of the magnetic circuit of the yoke, thus having a field weakening effect. Weakening the field increases the motor speed but reduces the torque and this setting should not be
used under high load conditions, e.g. snow build up on the screen.
Self-switching and self-parking
When the wipers are switched off it is essential that they rest in a position that does not impair the vision of the driver. This is difficult to achieve manually so all wiper systems are fitted with either a self-switching or self-parking function. When the driver switches the wipers off, the wiper motor continues to run until the wiper blades are at the end of the stroke, off the screen (known as self-switching). In some cases, the wiper blades move to a position beyond the normal wiping arc so that they are completely off the screen. This is known as self-parking. The switching assembly that allows this function is normally mounted at the motor gearbox. A simple arrangement which demonstrates the principle is shown in Figure 8.33.
Figure 8.31 Wiper system – wiped area patterns
1 Tandem
2 Opposed-pattern system 3 Single-lever system, not controlled 4 Single-lever system with lift control … Legally prescribed visibility area (USA)
Figure 8.32 Two-speed wiper operation
186 Signalling and vision Fundamentals of Motor Vehicle Technology: Book 3
The limit switch is in parallel with the driver switch so that both switches must be open before the motor stops, i.e. the driver switch must be off and the blades in the park position. However, this simple arrangement is not sufficient as, due to the motor inertia, it is possible for the motor to ‘run on’, passing through the limit switch open position, and thus the wiper will not park. The solution is to brake the armature in addition to cutting the supply and this is implemented electrically using the principle of regenerative braking. Figure 8.34 shows a typical switching arrangement.
The limit switch contacts are changeover instead of normally closed. When the limit position is reached, the supply to the motor is cut and the motor armature is short-circuited via an earth connection on each side. Any residual energy in the armature is then dissipated via current flowing through this circuit and the motor stops abruptly thus preventing any run on.
Self-parking is an extension to self-switching and allows the wiper arms to be positioned off the screen when not in use. This improves the vehicle dynamics and wind noise and provides better visibility for the driver. In order to achieve this a complex, mechanical linkage system allows an extended wiping arc when the motor is reversed. Thus, when the self-parking mechanism is activated (via switch off of the driver switch) the wiper motor stops and reverses in sequence to achieve the extended stroke. In modern vehicles this action is controlled via a wiper ECU or alternatively a body electronic ECU (in conjunction with intermittent functions).
Intermittent wipe
Light rain or spray conditions need regular but infrequent use of the wiper system. This is achieved via an intermittent wiping function fitted to most vehicles. When selected this mode operates the wipers for a single sweep every few seconds. An extension to the basic function allows the driver to select the time delay between wipes according to the conditions. Due to the regenerative braking system used, the intermittent wipe function is provided by a timer relay that is integrated into the self-switching circuit. A typical circuit diagram is shown in Figure 8.35.
The circuit that activates the intermittent relay is a pulse generator which generates a pulse of sufficient time to allow the motor to move the limit switch beyond the ‘park’ position. The time difference between the pulses is controlled by an electronic circuit using an R–C network or timer microprocessor. Typically the time difference can be adjusted by a suitable driver control between 2 and 40 seconds.
Figure 8.33 Limit switch to give self-switching action
Electronic wiper motors
Many vehicles are being fitted with wiper motors that incorporate integrated control electronics. The advantage is that the motor can be continuously reversed by the electronic controller and hence produce the required reciprocating motion directly. This means that the installation space required for the wiper linkage is less and provides more flexibility to allow the maximum wiped area (see Figure 8.36).
Sensors detect wiper blade position and motor speed and torque, thus the electronic controller can ensure the correct operation of the wiper arm irrespective of friction or wind speed. In addition, when used in conjunction with a rain sensor, the motor speed can be matched exactly to the conditions. An interesting
point is that it is possible to fit a separate motor for each blade with no mechanical linkage. An electronic controller synchronises the operation of the blades; communication between the motor controllers is via CAN or LIN bus. The advantage of such a system is that it requires less installation space and is lighter.
Additional features
Many wiper systems feature additional logical features to enhance the basic operation of the system. This is achievable due to the high level of integration of the electronic control systems of modern vehicles:
● Intermittent time as a function of road speed: the wiper control systems account for vehicle speed and reduce the delay time as road speed increases.
Figure 8.35 Intermittent wipe control
a Electronic wiping-angle control
(reduces the installation space A)
b Extended wiping area 1 Upper/lower reversing point 2 'Hidden' (extended) parking
position
Figure 8.36 Electronically controlled reversing wiper motor system
188 Signalling and vision Fundamentals of Motor Vehicle Technology: Book 3
● Automatic operation of wipers: the wiper system is fitted with a rain sensor (see Chapter 2) that can adjust intermittent time and full operation of the wipers automatically according to the intensity of the rain on the windscreen as measured by the sensor.
● Automatic wash/wipe sequence: when the screen
washers are activated, the wipers automatically operate for a number of sweeps to wash and clear the screen without the driver having to switch the wipers on/off.
Rear wiper
Rear wiper systems are fitted mainly to hatchback or estate cars. The technology is similar to front wiper systems in respect of:
● motor technology and self-switching arrangements
● intermittent and continuous wipe operation ● wash/wipe operation sequence.
Two-speed operation is not required. Also, generally, only one wiper arm is fitted, therefore complex linkages are not needed. A general trend is towards electronically reversing motors as the need for a mechanism to convert rotary to linear motion is not then required. This also allows a wider wiping arc of up to 180º if necessary, and the weight/complexity of the motor mechanics is reduced. Typically the service life of a rear wiper is much shorter as its operation is less critical to safety than the front wiper system. Typical wiper patterns for a rear wiper are shown in Figure 8.37.
Washers
Screen washers are fitted to complement the wiper operation (front, rear and headlamp). Normally they are electrically operated from a driver stalk and employ simple centrifugal pumps powered by small DC motors (see Figure 8.39). These generate fluid pressures of approximately 0.7 bar and this is sufficient to provide a jet of fluid on the windscreen via two or four jets mounted on the bonnet or integrated in the wiper blade itself. The cleaning fluid (water mixed with additive) is held in a separate container of approximately 2 litres (more if the same reservoir supplies headlamp washers >5 litres). The screen-wash additive helps the cleaning action but also lowers the freezing point of the fluid. Often the washer jets are heated to improve performance in freezing conditions. A typical pump design is shown in Figure 8.38.
Figure 8.37 Typical wiper patterns for a rear wiper
a Rear-window wiped area (the shaded areas indicate impaired view of passing vehicles)
b Rear-window wiper arm (plastic)
Figure 8.38 Washer pump design
1 Intake fitting 2 Impeller 3 Pump housing 4 Pressure fitting 5 DC motor
Note that often a single pump supplies front and rear screen wash (if both are fitted). This is achieved via bi- direction operation of the pump which has two outlets (to the front and rear screens). The pump is switched to rotate in the appropriate direction according to whether rear or front screen wash is required. Of course it is also possible for an individual pump to be fitted, one for the front and one for the rear. Often screen-wash operation activates the wipers to execute a number of sweeps automatically to clean the screen (as mentioned above).
8.2.2 Heating
Rear screen
Most vehicles are fitted with an electrically heated rear screen. This helps to clear and prevent condensation which can impair the driver’s rear view. The heating element consists of a strip or wire element bonded to the glass surface. When supplied with current the element has a heating effect just sufficient to evaporate the condensation. Note that the element consumes a considerable amount of power as it has to heat a large area. Therefore the current supply to the heated rear window is nearly always via a relay to minimise power loss due to voltage drop in the cable. Operation of the heated rear window is normally only possible with the ignition on, and often only when the engine is running. In addition, timer units can be fitted so that it is not possible to leave the unit switched on for a long time as this could put excessive load on the charging system. A circuit diagram for a simple heated rear window system is shown in Figure 8.40.
Front screen
Many vehicles are now fitted with heated front screens. These help with fast defrosting of the screen in harsh weather. The screen heating elements must be much more discreet in order that they do not impede the driver’s vision. This is achieved by bonding micro-thin elements within the glass which generate sufficient heat to melt snow and ice. Due to that large surface area the current draw is considerable, and for this reason heated front screens are equipped with timer controller to prevent excessively long periods of use. In addition, they normally only operate when the engine is running.
Often the operation of heated screens is monitored when a central electronic control unit is fitted to manage the body electronics. If the battery condition or charging system operation is compromised by these significant consumers they can be switched off by the control unit in order to conserve power in a critical situation.
Most front wiper systems have two continuous speeds – low and high. The high speed is achieved with a second brush supplying current to the armature with a reduced field flux. This increases the speed but reduces the motor torque
In addition, most vehicles have an intermittent wipe function. This activates the wipers for a single sweep every few seconds. An intermittent wipe relay performs this task. It is activated by an electronic timer circuit and is connected into the wiper self-switching (parking) circuit
All wiper systems must be able to continue after switch-off until the blades reach a suitable position off the screen. This is known as self-switching. Some wiper systems actually move the blades off the screen when switched off. This is known as off-screen self-parking
Electronic wiper motors are being fitted to many vehicles. These do not need an external crank mechanism as the motor is reversed by the electronics. This saves installation space. If two motors are fitted they can be synchronised electronically so that there is no mechanical linkage between them at all
Rear-screen heaters are very common. Front- screen heaters are also being seen fitted to vehicles. Note that screen heaters draw significant power from the electrical system and their operation is often controlled by a timer and/or monitored