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Steering angle sensors

Connecting to the ESC via CAN

The steering angle sensor (SAS) is a critical part of any Electronic Stability Control (ESC) system that measures steering wheel position angle and rate of turn. SAS clusters can connect to the ESC via CAN.

Components of the Electronic Stability Program ESP from Bosch

Components of one of Bosch’s Electronic Stability Control
(Photo: AASA)
  1. ESP-Hydraulic unit with integrated ECU
  2. Wheel speed sensors
  3. Steering angle sensor
  4. Yaw rate sensor with integrated accekeration sensor
  5. Engine-management ECU for communication

A scan tool can be used to obtain the data on the steering wheel position and rate of turn in degrees. The SAS is located in a sensor cluster in the steering column. The cluster should have more than one steering position sensor for redundancy and to confirm data. The ESC module must receive two signals to confirm the steering wheel position. These signals are often out of phase with each other.

SAS clusters connect either directly or through other modules to the ESC via the CAN network. Nodes on the CAN network have the ability to interpret and create signals that can be understood by other nodes on the bus. The best way to test modules on a high speed CAN network is with a scan tool. Most tools can look at the data directly, but some need additional software. If a scan tool needs software, the user can still use a meter connected to the high speed CAN network wires to confirm that the SAS is communicating with the rest of the network.

Analog SASs are similar to throttle position sensors. SASs are wired with a 5-V reference, chassis ground and signal output 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 V and 5 V as the wheel is turned 360º. It is possible to observe the 0 V to 5 V signal with meters connected to the two SAS sensors. 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.

A digital SAS is often called a “contactless sensor.” This type of sensor uses LED light, a wheel that acts as a shutter and an optic sensor that measures interruption in the light. These sensors produce a digital square wave signal with the frequency depending on the speed the wheel is turning. With the wheel straight, the voltage is low, but it increases as the wheel is turned away from center.

An example of a CAN transmit message layout (Photo: Bourns)

One such steering angle sensor is Bourns’ Type 6002 non-contacting sensor. It is based on two magneto-resistive (AMR) sensor chips. Each of them converts an angle position of a permanent magnet into two analog signals (one sine and one cosine signal). An algorithm allows for estimating the absolute angular position of a drive shaft that is connected to the device. The device sends a CAN message with the measurement data every 10 msec. OEM-specific CAN handler is optional. The device checks the angular speed value, which is limited to 1016 degrees per second. If this limit is exceeded, the device sends an error message according to the CAN Transmit Message.

An example of a CAN transmit message layout (Photo: Bourns)

Hoffmann + Krippner has developed ESCs even further: In cooperation with Guttersberg Consulting and Freescale Deutschland, the company has developed an alternative resistive hand recognition system for a variety of vehicle applications. The EP 2611642 As patented detection consists of spaced out films, which convert the deformation during contact into a usable switching signal. These films are located under the leather jacket, covering about 65 % of the steering wheel rim.

Part of the steering wheel is also an electronic evaluation system. To compensate for tolerances, the electronics in the end-of-line tester sets the specific level of the respective steering wheel to zero. Deviations caused for example by temperature differences are compensated by evaluating resistance values along several areas of the steering wheel. The sensor consists of two films on top of each other separated by printed spacers. A resistor paste is applied to the films during screen printing. An electric signal is created when the films touch. This signal is evaluated and can trigger appropriate responses using intelligent software.