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By Julian Edgar The Revolution in Car Instruments New car instruments no longer use just electromechanical gauges and pointers C ARS HAVE PROGRESSED a long way from the time when a humble eddy-current electromechanical speedo was the only instrument in view. These days, many instrument panels feature LED, LCD and TFT displays – and even conventional-looking dials have stepper-motors driving the needles. Instrument clusters Fig.1: block diagram of a typical current-model instrument cluster. A microcontroller dominates and is used for signal acquisition, filtering, diagnostic functions and for driving warning lights and stepper motors. It also performs the calculations that allow the display of speed, service intervals and oil quantity. [Bosch] 8  Silicon Chip Rather than displaying just fuel level, coolant temperature, speed and engine RPM, instrument panels can now display literally hundreds of discrete parameters. These include trip computer information, GPS navigational information, time, outside temperature, selected radio station, cruise control action, gear position, date, door openings, service intervals, siliconchip.com.au oil level and quality, warning lights and text messages. Fig.1 shows the block diagram of a typical current model instrument cluster. As can be seen, a microcontroller dominates and is used for signal acquisition, filtering, diagnostic functions, and for driving warning lights and stepper motors. It also performs calculations that allow the display of speed, service intervals and oil quantity. At first glance, it would seem that a micro isn’t needed for some of these functions – but think again! Consider, for example, illuminating a low oilpressure light – surely that wouldn’t need to be controlled by a micro? But while the oil-pressure light in modern cars may look to have much the same function as in older cars, some oil-pressure lights are now intelligent in their operation. They monitor the relationship between oil pressure and engine speed, switching on the warning light only when the pressure is lower than it should be for those revs. Service interval indicators, which are used to show when the next service is due, use input data including throttle position, engine RPM and instantaneous fuel economy. From these inputs, a picture of how the car is being driven can be built up – for example, lots of short trips will result in a reduced indicated service interval. Some instrument panels are also being used to perform a hidden function – that of a communications gateway. Because so much information is needed by the instrument panel, it makes sense to position “bridges” between different bus systems (eg, between the engine CAN bus and the body CAN bus) at this location. The stepper motors used to drive needles allow a dramatic reduction in thickness over other electronic approaches. Stepper motor gear ratios of 60:1 and a power of about 100mW allow fast and accurate positioning of needles, with 720 steps available over a 300° needle sweep. Types of display The most common form of advanced display used today is TN-LCD – that is, twisted nematic liquid crystal display. The display can be used over a broad temperature range (typically -40°C to +85°C) and can be configured in either positive or negative contrast forms. Positive contrast means dark siliconchip.com.au The BMW 5 and 6 Series cars use a Head-Up Display system manufactured by Siemens VDO. The information is displayed in a 150 x 75mm field which is located within the driver’s line of sight, in line with the end of the bonnet. The display data is produced by LED light that is reflected across four mirrors positioned behind the instrument cluster. The windscreen is specially modified to reflect the display to the driver’s eyes. [Siemens VDO] characters on a light background, while negative contrast means light characters on a dark background. STN (Super Twisted Nematic) and DSTN (Double-layer STN) LCDs are also being used, with colour provided by the use of LED backlighting. However, TFT (Thin-Film Transistor) LCDs are making rapid inroads into the instrument panel market. TFTs can provide high-resolution colour with video capability. Display monitors that measure 10-18cm (diagonal measurement) are now being widely placed in the centre of dashboards and even larger devices (25-36cm) are expected to be implemented in the form of programmable instrument clusters. The first production car instrument The First Eddy Current Speedometer The eddy current speedometer was invented just over 100 years ago by Otto Schulze. Schulze used a flexible shaft to transmit the rotational speed of the wheel or transmission to the speedometer. Inside the speedo, a permanent magnet was rotated by the shaft and this induced eddy currents in a metal disc or cup located close by, causing it to be rotated against a spring. As the spinning magnet increased in speed, the disc rotated to a greater degree, thereby indicating the speed on the dial via an attached needle. March 2005  9 on Head-Up Displays (HUD). While these have been mooted for years (and one model of Nissan Bluebird was sold in Australia with a HUD projected into the lower corner of the windscreen), advances in technology are likely to lead to more widespread adoption. HUD basics The Mercedes Benz E-Class uses an instrument panel that incorporates many of the new technologies. So that the information display mounted in the middle of the speedo dial is unobstructed, the speedo needle is attached to a revolving ring which is fixed to a magnesium base. The base is gear-activated and driven by a small stepper motor. As a result, the ‘needle’ moves around the perimeter of the speedometer which is illuminated with electroluminescent foils. The central display incorporates both dot-matrix and segment displays. Segments are used around the inner periphery of the speedo to show the speed setting of the Adaptive Cruise Control. The dot matrix display is capable of over 240 warnings in seven languages. A 32-bit micro controls the gauges and displays. [Siemens VDO] cluster incorporating a TFT is the current Audi A8. The 320 x 240 pixel full-colour 125mm screen is located between the speedo and the tachometer and can display information from the on-board computer, navigation system, radio, telephone and adaptive cruise control. The navigation instructions are displayed in a pseudo 3D effect, with perspective, flowing colour changes and moving shadows all used. For the navigation displays alone, 1MB of data has been programmed in, with 35 different scenarios and turn-off instructions composed from over 300 bitmaps. In addition to this navigational information, the TFT screen can show several hundred pictograms and moving animations. Three hundred lines of text can also be displayed – in seven languages! The display is controlled by a dedicated 32-bit processor running 2MB of software. Much work is also being carried out Fig.2 shows the basics of a HUD. It uses an activated display to generate the image, a backlight, an optical imaging system and a “combiner” that reflects the image towards the driver. The windscreen can be used as the combiner. The most common displays used in HUDs are the cathode ray tube (CRT) and the vacuum fluorescent display (VFD), although LEDs can also be used. HUDs tend to display only simple information – eg, speed and navigation. This is to avoid overloading the driver with information that is always within his/her field of view. The advantage of a HUD is that the driver doesn’t need to refocus his/her eyes from infinity to 0.8-1.2 metres in order to read the instruments. This refocusing normally takes up to 0.5 seconds – that’s half a second when the driver cannot see what is happening on the road ahead. The recently released BMW 5 and 6 Series cars use a HUD system manufactured by Siemens VDO. In these cars, important information is displayed in a 150 x 75mm field which is located within the driver’s line of sight, appearing to the driver to be in line with the end of the bonnet. The amount of display data that is shown on the HUD can be configured by the driver or alternatively, the driver can switch it off. The display is produced by LED light that is re- A measure of the internal complexity of the BMW 7-Series instrument panel can be gained in this exploded schematic view. [Siemens VDO] The BMW 7-Series instrument panel uses stepper motor driven needles and back-lit negative liquid crystal displays. Note the navigation information shown on the face of the tachometer. [BMW] 10  Silicon Chip siliconchip.com.au (1) Initially, three conventional-looking round instruments appear on the display – from right to left: tachometer, speedometer and a combination gauge that shows hybrid power status, fuel level and battery voltage. [DaimlerChrysler] (2) The driver can elect – via a pushbutton – to change the instrument display to the one shown here – again from right to left: navigation, speedometer, trip computer (and other things we can’t read in German!). [DaimlerChrysler] Adaptive Instrument Display from Daimler Chrysler The DaimlerChrysler F 500 MIND concept vehicle takes instrument panel displays to the next step. Rather than have fixed instruments, a completely flexible display is used – different instruments can be displayed as the situation requires. (3) Alternatively, at night the driver can bring up the unit’s night-vision system, which uses infrared lasers integrated into the headlights to illuminate objects up to 150 metres away. [DaimlerChrysler] flected across four mirrors positioned behind the instrument cluster. The windscreen is specially modified to act as the combiner. Instrument lighting It is at night that modern instrument panels look most impressive – their display lighting is second to none in the mass-produced instrumentation world. Originally, instruments were frontlit, either by bulbs positioned in the cowl above and ahead of the instruments, or by edge illumination where light was reflected off individual instrument surrounds. Even relatively siliconchip.com.au The Visteon instrument cluster used in the 2003 Renault Megane features LED backlighting. [Visteon] March 2005  11 Fig.2: the main components of a of a Head-Up Display: (1) virtual image; (2) reflection in windscreen; (3) display generator; (4) optical system; (5) electronic control unit. [Bosch] The first production car instrument cluster incorporating a TFT is the current Audi A8. The 320 x 240 pixel full-colour 5-inch screen is located between the speedo and the tachometer and can display information from the on-board computer, navigation system, radio, telephone and adaptive cruise control. For the navigation displays alone, 1MB of data has been programmed in, with 35 different scenarios and turn-off instructions composed from over 300 bitmaps. In addition to this navigational information, the TFT screen can show several hundred pictograms and moving animations. Three hundred lines of text can also be displayed in seven languages! The display is controlled by a dedicated 32-bit processor running 2MB of software. [Siemens VDO] simple instrument lighting of this sort often used acrylic mouldings that acted as “light-pipes”, channelling illumination around the display. Incandescent filament lamps were universally used, with dimming by a current-reducing rheostat. These days, backlighting is becoming widely adopted. Light bulbs have been replaced by LEDs – their smaller size, lower power consumption, ruggedness and longer life having clear advantages over incandescent bulbs. Sources of illumination also now being widely used in instrument pan- els include electroluminescent film and cold-cathode lamps. Electroluminescent (EL) film features very uniform lighting distribution and is most appropriate for illuminating dial faces and displays. Typically, EL film requires 100V AC at a frequency of 400Hz. Cold Cathode Fluorescent Lights (CCFL) are mainly used for backlighting “black screen” instruments – those that appear black when deactivated. In Australia, Lexus has long used this approach. Because a heavily tinted cover (one source suggests the cover typically has a transmissibility of only 25%) is required, very bright lights are required. LCDs also require intense backlighting if they are to retain adequate contrast in daylight. CCFLs meets these requirements, with an efficacy of 25 lumens/watt – approximately 10 times that of the incandescent lamps used in instrument panels. CCFL lighting requires a power supply of 2kV AC at a frequency of 50-100kHz. Incandescent bulbs have a quoted life (based on a 3% probability of failure) of 4500 hours. However, the other three light sources have a minimum life that would usually equate to the life of the car – 10,000 hours or more. Conclusion As car systems become increasingly sophisticated, new techniques need to be found to communicate that information to the driver. The flexibility of electronic displays means that more and more will be found in car instruSC ment panels. The LED-illuminated instrument panels in the current Honda Accord and Accord Euro models feature 3-stage operation. At first, when the driver opens the door, the instrument panel lights with just the gauge markings, as shown in the photo at left. Then, as the driver inserts the key in the ignition, the display brightens further. Turning the key to start the motor brings up all the panel legends before the display settles down to show just the relevant information. 12  Silicon Chip siliconchip.com.au