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Pressure sensors Pressure measurement takes place either directly, by way of diaphragm deformation, or using a force sensor for the following applications in the motor vehicle (examples): • Intake-manifold or boost pressure (1 to 5 bar) for gasoline injection • Brake pressure (10 bar) on electropneumatic brakes • Air-spring pressure (16 bar) on pneumatic- suspension vehicles • Tire pressure (5 bar absolute) for tirepressure monitoring • Hydraulic reservoir pressure (approximately 200 bar) for ABS and powerassisted steering • Shock-absorber pressure (approximately 200 bar) for chassis and suspension control • Coolant pressure (35 bar) for air-conditioning systems • Modulation pressure (35 bar) for automatic transmissions • Brake pressure in master cylinder and wheel-brake cylinder (200 bar), and automatic yaw-moment compensation on the electronically-controlled brake • Overpressure/underpressure of the tank atmosphere (0.5 bar) • Combustion-chamber pressure (100 bar, dynamic) for detection of misfiring and knock detection • Element pressure on the diesel fuel injection pump (1,000 bar, dynamic) for electronic diesel control • Fuel pressure on the diesel common rail (up to 2,000 bar) • Fuel pressure on the gasoline direct injection system (up to 200 bar) Measuring principles Pressure as a measured variable is a nondirectional force acting in all directions which occurs in gases and liquids. It is propagated in liquids, and also very well in gellike substances and soft sealing compounds. There are static and dynamic measuring sensors for the measurement of these pressures. Diaphragm-type sensors The most common method used for the measurement of pressure (also in automotive applications) uses a thin diaphragm as a mechanical intermediate stage which is exposed on one side to the pressure to be measured and which deflects to a greater or lesser degree as a function of the pressure. Within a very wide range, its diameter and thickness can be adapted to the particular pressure range. Low-pressure measuring ranges lead to relatively large diaphragms which can easily deform in the range 1 to 0.1 mm. Higher pressures though demand thicker, small-diameter diaphragms which generally only deform by a few μm. Force and torque sensors Measured variables The following list underlines the wide variety of applications for force and torque sensors in automotive engineering: • In the commercial-vehicle sector, coupling force between the tractor vehicle and its trailer or semitrailer for the closed-loop controlled application of the brakes, whereby neither push nor pull forces are active at the drawbar • Damping force for use in electronic chassis and suspension control • Axle load for electronically controlled brakingforce distribution on commercial vehicles • Pedal force on electronically-controlled brake systems • Braking force on electrically actuated, electronically-controlled brake systems • Drive and brake torque • Steering and steering servo torque • Finger protection on power windows and electrically operated sliding sunroofs • Wheel forces • Weight of vehicle occupants (for occupant- protection systems) Strain gage principle (piezoresistive) Strain-gage measuring resistors (straingage strips) represent the most widespread and probably the most reliable and precise method for measuring force and torque (Fig. 4). Their principle is based on the fact that in the zone of the elasticmember material to which Hooke’s Law applies there is a proportional relationship between the mechanical strain σ in the member, caused by the introduction of force, and the resulting elongation ε. In this case, in accordance with Hooke’s law: whereby the proportionality constant E is the modulus of elasticity. Since it does not directly measure the strain resulting from the applied force, but rather the – locally – resulting elongation, the strain-gage method can be regarded as an indirect measuring method. Torque sensors A fundamental distinction is made in torque measurement, too, between methods using angle or strain measurement. In contrast to strainmeasurement methods (strain-gage resistors, magnetoelastic), angle-measurement methods (e.g. Eddy current) require a certain length l of the torsion shaft via which the torsion angle (approximately 0.4 to 4°) can be picked-off. The mechanical stress s proportional to the torque is aligned at an angle of less than 45° to the shaft axis. Flowmeters Measured variables The purpose of flow measurement in the motor vehicle is the detection of the intake air flow rate. This air flow rate must be known precisely so that the engine-management system – both in diesel and in gasoline engines – can set a defined airfuel mixture. This value can be determined by a flowmeter. The sensors which are used for measuring air flow rate or gas flows in general are also referred to as “anemometers”. As such, the often-used term “air quantity” is incorrect because it does not stipulate whether volume or mass is concerned. Since the chemical processes involved in fuel combustion are clearly based on mass relationships, the object of the measurement is the mass of intake air. On gasoline engines, the air-mass flow rate is the most important load parameter. In diesel engines, the exhaust-gas recirculation rate is regulated using the air-mass flow. Depending upon engine power, the average (over time) maximum air-mass flow rate to be measured is between 400 and 1,200 kg/h. Due to the low air requirements at engine idle in modern gasoline engines, the ratio of minimum to maximum flow is 1:50 to 1:100. Because of the higher idle air demand in diesel engines, these ratios must be assumed to be 1:20 to 1:40. The severe exhaust gas and fuel-consumption requirements dictate accuracies of 2 to 3 % of the measured value. Referred to the measuring range, this can easily correspond to a measuring accuracy of 2*10-4, which is unusually high for a motor vehicle. The air though, is not drawn in continuously by the engine, but rather in time with the opening of the intake valves. Particularly with the throttle valve wide open (WOT) in gasoline engines, this leads to considerable pulsation of the air-mass flow, also at the measuring point which is always in the intake tract between air filter and throttle valve, or between air filter and turbocharger. Intake-manifold resonance leads to the pulsation in the manifold sometimes being so pronounced that brief return flows can occur. This applies in particular to 4-cylinder engines in which there is no overlap of the air-intake phases. An accurate flowmeter must be capable of registering these return flows with the correct direction. Measuring principles Up to now, of the practically unlimited variety of flowmeters on the market, only those which operate according to the impact-pressure principle have come to the forefront for airquantity measurement in the vehicle. This principle still depends upon mechanically moving parts, and in principle correction measures are still needed to compensate for density fluctuations. Today, true air-mass meters applying thermal methods (hot-wire or hot-film air flowmeters) are used which can follow sudden flow changes without mechanically moving parts. Variable orifice plates (sensor plates) Hot-wire/hot-film anemometers Gas and concentration sensors Measured variables The concentration of a given material or medium defines the mass or volume percent of a given material in another given material or in a mixture or combination of other materials. With a concentration sensor (also known as a concentration probe, the important thing is that in the ideal case it is sensitive to only one medium, while at the same time practically “ignoring” all other mediums. Of course, in practice, every concentration sensor has its own cross sensitivity to other mediums even though, as is often the case, “temperature” and “pressure” are maintained constant. In the vehicle, the following parameters must be measured: • Oxygen content in the exhaust gas (closed-loop combustion control, catalytic-converter monitoring) • Carbon-monoxide and nitrogen-oxide content, as well as air humidity inside the vehicle (air quality, misting of vehicle windows) • Humidity in the compressed-air brake system (air-drier monitoring) • Dampness of the outside air (black ice warning) • Concentration of soot in diesel-engine exhaust gas. A still unsolved problem. In contrast to the above-mentioned gas concentrations, this is a particle concentration. The difficulties inherent in the measuring assignment are further aggravated by the possibility of the sensor being blocked by particles so that it no longer functions. The introduction of the fuel cell as an automotive drive means that further gas sensors will have to be developed, for instance for the detection of hydrogen. Measuring principles Measured mediums occur in gaseous, liquid, or solid state, so that in the course of time countless measuring methods have been developed. For automotive applications, until now only the gas-analysis area, and in particular the measurement of gaseous humidity, has been of any interest. Table presents an overview of the processes applied in general measurement techniques: Gas measurement in general Gas sensors are usually in direct unprotected contact with the monitored medium (in other words with foreign matter) so that the danger of irreversible damage exists. This form of damage is referred to as sensor “contamination”. For instance, the lead that may be contained in fuel or the exhaust gas can make the electrolytic oxygen concentration sensors (Lambda oxygen sensors) unusable. Moisture measurement In addition to the outstanding significance of the Lambda oxygen sensor in dealing with exhaust gases, moisture measurement also plays an important role. In the broader sense, moisture indicates the moisture content of gaseous, liquid, or solid substances. In the narrower sense, we are dealing here with the gaseous-water (water vapor) content in gaseous media – above all in the air. Temperature sensors Measured variables Temperature is defined as a nondirectional quantity which characterizes the energy state of a given medium, and which can be a function of time and location: T = T (x, y, z, t) where: x, y, z are the spatial coordinates, t is time, and T is measured according to the Celsius or Kelvin scale. Generally speaking, with monitored media which are in gaseous or liquid form, measurements can be taken at any point. In the case of solid bodies, measurement is usually restricted to the body’s surface. With the most commonly used temperature sensors, in order for it to assume the medium’s temperature as precisely as possible, the sensor must be directly in contact with the monitored medium (direct-contact thermo meter). In special cases though, proximity or non-contacting temperature sensors are in use which measure the medium’s temperature by means of its (infrared) thermal radiation (radiation thermometer = Pyrometer, thermal camera). Measuring principles for direct-contact sensors The fact that practically all physical processes are temperature-dependent means that there are almost just as many methods for making temperature measurements. The preferable methods though are those in which the temperature effect is very distinctive and dominant and as far as possible features a linear characteristic curve. Furthermore, the measuring elements should be suitable for inexpensive mass production, whereby they should be adequately reproducible and non-aging. Taking these considerations into account, the following sensor techniques have come to the forefront, some of which are also applied in automotive technology: Resistive sensors In the form of 2-pole elements, temperaturedependent electrical resistors are particularly suitable for temperature measurement, no matter whether in wire-wound, sintered ceramic, foil, thinfilm, thick-film, or monocrystalline form. Normally, in order to generate a voltage-analog signal they are combined with a fixed resistor RV to form a voltage divider, or load-independent current is applied. The voltage- divider circuit changes the original sensor characteristic R(T) to a slightly different characteristic U(T): Thermocouples Thermocouples are used in particular for measuring ranges ≥ 1,000 °C. They rely on the Seebeck effect, according to which there is a voltage across the ends of a metallic conductor when these are at different temperatures T1 and T2. This “thermoelectric voltage” Uth depends (independently of the development of this) exclusively on the temperature difference ΔT at the ends of the conductors. It is expressed by the equation: Since the instrument leads used to measure this voltage across the metallic conductor must themselves be equipped with terminals (for instance made of copper), these are also subject to the same temperature difference, so that unfortunately only the difference between the metallic conductor and the connecting cables is measured. Thermoelectric voltages are always listed based on Platinum as the reference material. Measuring principles for non-contacting temperature measurement The radiation emitted by a body is used for the noncontact measurement (pyrometry) of its temperature. This radiation is for the most part in the infrared (IR) range (wavelength: 5 to 20 μm). Strictly speaking, the product of the radiated power and the emission coefficient of the body is measured. The latter is a function of the material, but for materials which are technically of interest (including glass) it is usually around 1, although for reflective and IR-permeable materials (e.g. air, silicon) it is far less than 1. Imaging sensors (video) In particular, imaging sensors are beginning to gain a hold in the motor vehicle using visible light or infrared light. They can be used for passenger-compartment monitoring, but are principally aimed at observation outside the vehicle. All of these sensors have one objective in view, and that is the simulation of the superior capabilities of the human eye and its mental recognition capabilities (of course, only to a very modest degree at first). These were introduced in large numbers to industrial measurements some time ago – in particular on robot handling equipment. The costs of imaging sensors, and the associated very high-performance processors needed for the interpretation of a scene, are already of interest for applications in the automotive sector. Engine-speed sensors