While the advent of driver assists systems such as Traction Control, Stability Control, Park Assist, Lane Departure Warning and several others have made modern cars safer to operate than ever before, all of these systems depend on input data from the Steering Angle Sensor (SAS) if they are to operate as designed. To illustrate the critically important role the SAS fulfills in maintaining directional control of a modern vehicle, this article will briefly discuss why steering angle sensors are needed, and how they work, starting with this question-
Put simply, a steering angle sensor measures the position of the steering wheel relative to the “dead-ahead” position, as well as the rate at which steering inputs are made by the driver, and reports this information back to the ECU via the serial communication system.
In practice, the steering angle sensor is only one of many sensors that monitor parameters such as (among others) individual wheel speeds, yaw/roll rate, lateral acceleration, and vehicle speed, all of which is compared to the steering angle as indicated by the steering angle sensor. If, based on this information, the ECU concludes that a steering input will increase the yaw/roll rate, or lateral acceleration to beyond a safe limit given the vehicles’ road speed, it will initiate corrective measures via the Stability Control System to stabilise the vehicle.
As a practical matter, the Stability Control System is able to differentiate between the driver’s intentions based on steering and other control inputs, and the vehicle’s actual responses to those control inputs, based on an interrupted stream of live data from a network of related sensors. Based on the vehicle’s actual response to the driver’s intentions, the Stability Control system uses a complex, pre-programmed control algorithm that is based on a set of equations that can mathematically model the dynamics of the vehicle.
Depending on the calculated dynamical values, the Stability Control System may then initiate a corrective strategy that could include limiting (or even ignoring) throttle inputs, or applying selective braking forces to individual wheels via the ABS system. Other corrective measures may include limiting or preventing gearshifts on automatic transmissions, activating/deactivating AWD systems, and/or adjusting active suspension systems to improve vehicle stability and directional control.
NOTE: While the above might seem like a digression from the main topic, it is important to view the function of the steering angle sensor within the context of the broader network of sensors in which it operates. Typically, this network includes the following sensors-
Steering angle sensor
Determines both the intended change of direction, and the rate at which the change in direction is made. Steering angle sensors commonly contain AMR-elements (Anisotropic magnetoresistance), which describes a phenomenon in which the electrical resistance of a material depends on the angle between the direction of an electrical current and the direction in which magnetisation of that material occurs, and it is this property of some materials that makes some types of steering angle sensors work.
Yaw rate sensor
Measures the rate at which the vehicle is pivoting around a fixed point, i.e., how much and how fast the vehicle is actually turning. This data is compared to data from the steering angle sensor, and the result is used to calculate appropriate corrective strategies to stabilise the vehicle.
Lateral acceleration sensor
In most cases, this is a conventional accelerometer that is placed transversely across the vehicle’s thrust line to measure lateral accelerations. This information is used by the Stability Control System to predict and measure the forces that cause spinouts and slides.
Wheel speed sensors
Wheel speed sensors are used to measure the rates of rotation of individual wheels. This data is used by both the ABS and Stability Control Systems to stabilise the vehicle by means of applying selective braking forces to the wheels.
Depending on the application, other sensors could include a-
Longitudinal acceleration sensor
On some advanced Stability Control Systems, this sensor provides data on the vehicle’s motion and acceleration along the thrust line independently of the wheel speed sensors. The practical advantage of using a longitudinal acceleration sensor lays in the fact that the Stability Control System’s ability to predict dangerous situations is greatly improved.
Roll rate sensor
This sensor measures deviations from the horizontal plane perpendicular to the thrust line, which greatly improves the Stability Control System’s ability to calculate appropriate corrective strategies to stabilise the vehicle.
From the above, it should be obvious that in order for the Stability Control System to work as intended, it is critically important that the steering angle sensor, and all other implicated sensors be properly calibrated with respect to both the dead-ahead position and the vehicle’s thrust line if accurate input data is to be relayed to the ECU.
Steering angle sensor have evolved greatly over the past decade or so, but for the most part, only three designs predominate in the vehicles the average technician is likely to see on a regular basis. These include the following-
Generally, this type of sensor consists of a cluster of two (or sometimes more sensors), with each sensor in the cluster acting as a monitoring device for all the others. Typically, analogue steering angle sensors are provided with a 5-volt reference voltage by the ECU, a chassis ground circuit, and a dedicated signal circuit.
In practice, the sensors in the cluster each develop a signal voltage as the steering wheel is turned, and typically, the signals range between 0 and 5 volts as the steering wheel is rotated through a 360-degree revolution. Note though that in order to improve accuracy, the two sensors are out of phase, but the difference remains constant. For instance, on most vehicles with analogue steering angle sensors, one sensor will produce a signal of about 2.8 volts when the steering wheel is in the dead-ahead position, while the other sensor will produce a signal of about 0.4 volts.
In addition, on most vehicles with analogue steering angle sensors, both sensors will produce an increasing signal voltage when the steering wheel is rotated in one direction and a decreasing signal voltage when the steering wheel is rotated in the opposite direction.
Testing analogue steering angle sensors
Testing these sensors involves merely attaching a good quality digital multimeter to each of the sensors’ signal circuits, and observing the displayed values on the meters while rotating the steering wheel through a 360-degree revolution in both directions.
If both sensors are fully functional, their initial signal voltages with the steering wheel in the dead-ahead position will match the manufacturer’s specified value exactly (or to within a few percent) and the phase difference will remain constant throughout a full rotation of the steering wheel in opposite directions. Note though that the most common issues with analogue steering angle sensors are that the phase difference varies, or that all the sensors in the cluster display the same signal voltages. This is most commonly caused by short circuits, or abnormal resistances in the wiring between the sensors and implicated control modules, including the ECU.
Also known as “contactless sensors”, digital steering angle sensors use a LED (light emitting diode) as a light source, a perforated wheel that is connected to the steering shaft and acts as a sort of “shutter”, and an optical sensor that counts the number of times the light beam is interrupted as the steering wheel is rotated away from the dead-ahead position.
In practice, the light pulses are converted into a digital, square waveform whose frequency is directly proportional to the rate at which the steering wheel is rotated, but note that the amplitude of the waveform does not change; only the frequency of the waveform changes progressively as the steering wheel is rotated further away from the dead-ahead position.
Testing digital steering angle sensors
Testing these sensors requires the use of an oscilloscope, or a high-end scan tool that can function as an oscilloscope. Note though that the displayed waveform can only be correctly interpreted with the aid of manufacturer specific reference data.
These are the most advanced steering angle sensors in use today, and they can connect either directly to the Stability Control System or in some cases, these sensors are connected to the Stability Control System through a network of implicated sensors via the CAN (Controller Area Network) bus system. Note that all the implicated sensors (aka nodes), in this network have the ability to both create and interpret signals that are generated by all other nodes in the network.
Testing CAN Bus Module-based steering angle sensors
Note that while many high-end scan tools have the ability to access the steering angle network and to diagnose issues within the network, some scan tools may require additional, manufacturer specific software to access this part of the CAN system.
Note though that while it is possible to test this part of the CAN system manually with a good quality multimeter, the results are often misleading, confusing, or useless from a diagnostic perspective in the absence of manufacturer specific reference data.
Although modern steering angle sensors are designed to last for the life of the vehicle, nothing is ever that perfect and failures of these sensors do occur, albeit infrequently. Below are the most commonly occurring symptoms of steering angle sensor failures-
However, it should be noted that in the vast majority of cases where symptoms have manifested, the underlying cause involves a failure to re-calibrate the steering angle sensor; in fact, many, if not most manufacturers now insist on a re-calibration of the steering angle sensor being performed after any wheel alignment procedure. Therefore, failure to do this could mean the difference between your customers making it around the first corner after leaving your shop, or ending up in the proverbial ditch because the vehicle failed to respond appropriately to a sudden steering input, which brings us to-
The purpose of re-calibrating the steering angle sensor on a modern vehicle is to restore the steering angle geometry to OEM specifications to ensure that all driver assist systems perform as intended when they are required to. Below are some details on how to do this-
Automatic or self-calibration
On many new, late model vehicles, the re-calibration process is as simple as rotating the steering from lock-to-lock, then turning to the dead-ahead position, and cycling the ignition key in a prescribed pattern.
Re-calibrating with a scan tool
In some cases though, the re-calibration process can only be performed with a high-end scan tool, and in strict accordance with the manufacturers’ prescribed procedure. Refer to the relevant technical manual that applies to the affected application for details, but regardless of the procedure, it is critically important to only perform the re-calibration on a perfectly level surface, and while all the wheels are properly inflated to their prescribed inflation pressure.
Failure to observe the basic requirements of a flat surface and correctly inflated wheels will result in either a mis-calibration, or a failure to re-calibrate the steering angle sensor.
