Transcript Pressure sensors

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