What is steering-angle sensor?
The
steering-angle sensor is based on Giant Magnetoresistance technology (GMR
element) and provides an absolute steering-angle value over the complete
steering-angle range (multi-turn sensor). A unique feature of this
steering-angle sensor is the output of the correct angle within the measurement
range directly after ignition-on. Movement of the steering wheel or other
measures for initialization of the sensor (True Power-on) is not required even
after disconnection and re-establishment of the power supply.
No
standby current is necessary. The steering angle and steering-angle velocity is
made available via CAN interface. Sensor-internal plausibility tests and
special self-diagnostic functions allow the use of this sensor in safety
systems. The sensor concept also allows the integration of a second
microprocessor for safety-related applications.
Why do we need to Measure Steering
Angle?
The angle of the steering on its own can be used to
determine where the front wheels are pointed. When combined with other pieces
of information from the yaw, accelerometer and wheel speed sensors, it is
possible to measure the dynamics of the vehicle.
With the collected information, the stability
control system can be used to determine the driver’s intentions, how the
vehicle is reacting and what corrections can be made with the ABS hydraulic
control unit.
The steering-angle sensor is fixed onto the steering shaft, where
the hub gear wheel of the sensor transmits the rotary movement of the shaft to
two measuring gear wheels. In each measuring gear wheel a magnet is mounted,
whose field changes its direction in accordance with the rotational movement.
Below each magnet, a GMR sensor element is located to detect the angle position
of the magnet above. The analog values of the GMR elements are converted into
digital information directly on the circuit, which is then sent to the
microprocessor via serial interface.
The measuring gears have different numbers of teeth and therefore
change their turning position at different speeds. By means of mathematic
functions, it is possible to determine the absolute steering-wheel angle from
the position of the measuring gears. In addition, this function enables error
correction and a plausibility test of individual signals. This measuring
principle enables the measurement of the complete angle range of several
steering-wheel turns without the use of a rotation counter.
What is the Difference Between
Analog and Digital?
Analog SASs are similar to throttle position
sensors. SASs are wired with a 5-volt reference, chassis ground and signal
outputs, and can be tested via a port under the steering column. As the
steering wheel is turned, the SAS produces a signal that toggles between 0 and
5 volts as the wheel is turned 360º. It is possible to observe the 0- to 5-volt
signal with meters connected to the two SAS sensors. When the wheels are
straight, the sensors read 2.8V and 0.4V. If the readings are the same, the two
sensors could be shorted together. Most vehicles produce a positive voltage
turning right and a negative voltage turning left.
Digital steering angle sensors measure the angle and
turn it into information that can be shared on a serial data bus or discrete
connection with a module. Instead of changing voltage, these sensors produce a
signal in code that indicates the steering angle. This data PID can be used for
electric power steering, intelligent headlights and even stop/start systems.
Types of Steering Angle Sensors
Steering angle sensors come in two types… digital
sensors and analog sensors. Digital sensors utilize a small LED light to
calculate data about the rate of turn and angle positioning. Analog sensors
detect the differences in voltage to calculate this.
When digital sensors detect data related to the rate
of turn, the angle of the wheel, the direction of the turn and other important
information, it transmits it all to the main computer of the vehicle.
If the steering angle sensor notices an
inconsistency between the steering wheel’s position and the direction in which
the vehicle is going, then it will implement a stability action to help the
driver maintain control of the vehicle. If an understeer state is experienced
by your vehicle, the rear brake on the inside of the vehicle will be used to automatically
fix this problem.
Now, if the vehicle is experiencing an oversteer
state, the rear wheel on the outside will have a brake applied to it by the
stability system so that the vehicle will go in the right direction. If the
sensor ever malfunctions or stops working, then you should see a warning light
on your dashboard turn on to warn you. Once this happens, your stability system
is not functioning correctly.
Calibration and Diagnostics
When you don’t have
the steering position centered in certain Toyota car models, replacing or
disconnecting the car battery will result in symptoms of the car wanting to
drive itself. You may also notice the output of the engine dropping at certain
times.
If you experience any of these symptoms, you must
use a good quality automotive scan tool so you can get a reading on the faults.
Just remember that not all scanners will detect every kind of fault. If the
scan does not produce any engine codes to represent the reason for any power
loss, you may still notice that the SAS is reflecting a -560º reading as the
steering remains centered. Why would that be?
After you replaced the battery, the vehicle’s wheels
may have been turned completely to the left rather than remained in a central
position. The reason for this was due to the wheels being in a tight parking
area which meant that they needed to be pushed out to get to the battery.
Once the vehicle got its power back and the battery
was put back in and connected to the vehicle, the wheels continued to face left
while the steering angle sensor detected a zero reading.
In Toyota vehicles, they have a reset process that
is known as zero-point calibration. At the beginning of the process, you
disconnect the battery from the vehicle. Calibration will not be lost by the
vehicle’s stability control module when this happens.
However, there will be a recalibration within the
system for the newest position. It does not matter the direction in which the
wheels are facing either. That means zero will be given for this position now.
You can use a scan tool to diagnose the problem by
graphing the stability system sensors on it. Make sure you use leveled ground
to perform these diagnostics. If you cannot find ground that is leveled, then
an alignment rack may help.
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