Automotive Sensor Technology.

Penny + Giles provides an insight into the sensor technologies
used in the motorsport and heavy-duty automotive applications
- including potentiometer, inductive and Hall Effect.

Displacement sensors are used throughout the automotive industry to
monitor the position of moving parts on a vehicle, providing continual
feedback on the operating conditions. As a result, they allow designers
and engineers to maximise the operation of the vehicle and improve
operator safety.

Automotive applications are typically hostile environments and sensors
have to endure extreme shock, vibration and high temperatures. With
current demands in the motorsport market requiring ultimate reliability
to achieve peak performance, and demands in the heavy duty, off-road
vehicle market for more reliable and long-lasting sensors, it is clear to
see why non-contacting technologies are fast becoming the preferred
choice amongst vehicle designers and OEMS.

Sensor technologies are chosen to provide optimum performance and
reliability in specific applications.

Potentiometer technology is well established and has provided reliable
position measurement in a wide variety of applications for over 40 years.
Two of the key advantages of this technology are its linear output and the
versatility it derives from its simplicity. It consists of a printed carbon-based
track (or 'hybrid' conductive plastic-on-wire track) with no complex circuitry
or electronics. As such it is not susceptible to electromagnetic interference
or magnetic fields. As a contacting device, it can be susceptible to the
effects of shock and dither vibration and is not always the most appropriate
choice for high intensity use applications, or high dither or vibration induced

The majority of suspension position monitoring on saloon cars, single seat
race cars and motorbikes use hybrid linear potentiometers. They are simple
voltage dividers and don't need any special conditioning electronics - they
simply work off a dc supply and supply a dc signal. In addition, they monitor
in real-time, without any signal lag from electronics, to give an accurate
representation of what the suspension is doing.

In heavy duty applications, linear sensors such as Penny + Giles' SLS320
are used on refuse trucks for sensing the position of the hydraulic rams
that move the lifting arms on front loader and side loader vehicles. Again,
potentiometer technology is used because it is a simple solution without
the need for complex electronics. Rotary sensors are used to monitor the
movement of vehicle pivot points.

Inductive sensors used in the automotive industry are constructed from
one primary and two secondary coils forming a transformer. These work
on the principle of mutual inductance, producing an electrical signal
proportional to a separate moving core. Also known as linear or rotary
variable differential transformers (LVDTs or RVDTs), they are non-contacting
and provide a long operational life which is not affected by vibration.

Inductive sensors, however, require sophisticated signal conditioning electronics
to condition and linearise the coil signal. This complexity is the main reason
why they have been superseded with other non-contact sensors such as those
based on the Hall Effect which generally cost less and are simpler to use.

In the motorsport market they can be used to provide measurement and feedback
for brake pad wear or steering angle. These applications are not critical to the
control of the vehicle, but give data to assist engineers in performance improvements. Developments in GT and Fl brake caliper designs have given the option of monitoring
the wear of the brake pads and discs during a race - the movement of the calliper
position is sensed by a very small LVDT embedded in the caliper body. The sensors must withstand extremely high temperatures of up to 2000G. Two LVDTs are fitted
to each caliper and the signals are fed to the car's data acquisition system and
alert engineers to the condition of the brake pad and disc wear.

The sensors are also used in control functions such as throttle pedal position
and clutch actuator position, which are fundamental to the operation of the car.
Failure of displacement sensors in these applications would have an immediate
effect on the car. Engine throttle uses a closed loop electro-hydraulic system
that requires a displacement sensor on the driver's foot pedal and another on
the actuating mechanism mounted on the engine. An on-board computer monitors
the demand from the throttle pedal and the feedback signal from the engine-mounted sensor and the closed loop system works to match the signals. This arrangement
allows faster acceleration and can preserve engine life by restricting the driver from
going beyond the maximum safe rev limit for the engine.

In heavy-duty, off-road vehicles, single coil inductive linear sensors are fitted inside
the hydraulic actuators to monitor and operate the lifting mechanisms of buckets. Traditionally this movement is measured using linear, contacting potentiometers
to give the operator feedback about the position of the arm. However, the conditions inside the hydraulic actuators are harsh and problems can often occur because
of a lack of, or uncontrolled, maintenance. The hydraulics systems can become contaminated and this can affect the contacts on potentiometer sensors. Designers looking for increased reliability are turning towards inductive sensors because
they are non-contacting and are unaffected by the condition of the hydraulic fluid.

Hall Effect
Hall Effect sensors are based on the small electrical potential created when a
stationary magnetic field is placed perpendicular to a current-carrying conductor.
They produce a digital or analogue output proportional to the magnetic field
strength which is amplified to enable different voltage outputs. Hall Effect
sensors are fast becoming an attractive alternative to inductive sensors,
offering the same reliable, virtually unlimited life as a non-contacting technology,
but at lower cost as they do not require sophisticated electronics. The main
disadvantage is that they are sensitive to electromagnetic interference but
this can be overcome with the use of appropriate shielding and good internal
circuit design for all but the most demanding environments.

These are particularly suitable for designers in motorsport and heavy-duty
applications who are looking for the long-life of a non-contacting sensor, but
without the complex signalling that inductive sensors require. In most cases,
wherever a rotary potentiometer is being used, it could (in theory) be replaced
by a Hall Effect device depending on the surrounding environment.

For example, Penny + Giles engineers are designing a rotary Hall Effect
sensor, SRH2BO, which could be suitable for existing potentiometer applications
such as those used in sequential gearbox indicators. These indicators combine
a set of electronics and a highly accurate rotary sensor fitted to the selector
barrel. The action of the gearstick rotates this barrel to set a position
corresponding to the selected gear and the rotary Hall Effect sensor measures
this position precisely and allows the electronic control unit to calculate and
display which gear is selected. This method of sensing the selected gear
offers highly reliable and repeatable results, compared to microswitches that
count the number of times the gearstick has been knocked forwards or backwards
by the driver.

Automotive demands
Traditionally, designers in motorsport and heavy-duty applications favoured the
use of simple potentiometers for position monitoring. This is because they can
be connected directly to the data acquisition system without the need for the
complex signal conditioning. Demands within the automotive industry for ultimate reliability, however, mean designers are switching to inductive displacement

Rotary and linear position sensors are used extensively throughout the automotive industry, especially in motorsport cars and off-highway vehicles. Developments
in sensor technology from potentiometer to Hall effect technology provide designers
with a range of long-life sensing elements which provide a highly reliable solution
for all position sensing problems.

For more information contact:-

Penny & Giles Controls Ltd.
Tel: +44(0) 1202 409409

October 2005

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