Steering you right?

Frank Massey continues his look at the thorny issue of autonomous control, focusing on steering and stability correction leading to autonomy

Published:  09 May, 2019

Picking up from my topic and opinions on autonomous vehicle control last month I think it reasonable to explore the very technology our safety is to be placed.

When considering the most challenging aspect of autonomous vehicle control, we must look to steering and stability correction. My references are limited to the Volkswagen Group, however most manufacturers now share similar drivetrain and chassis technology.

Steering assist
These systems have evolved over many years in what I term modular development. Steering assist is such a system. Steering assist is directly proportional to driver input force, the steering torque sensor g269 detects rotation, the steering angle sensor g85 provides angle and rotation acceleration.

Responding to this data the control module j500 calculates the required assistance from the power steering assist motor v187. When parking, a low or zero vehicle speed combined with a rapid steering input provides maximum assistance. During driving additional data relating to environmental conditions, urban or motorway, modifies the appropriate assistance.
One of the first problems to overcome was return to neutral or zero steering angle. This is activated with a reduction in force on the torsion bar, whereby the rate of return is also a function of environmental influences. The dual steering angle sensor is comprised of a LED and photo electric diode.

The steering torque sensor operates on the magnetic resistance principle. Failure results in a gradual reduction in assistance. The asynchronous brushless motor provides up to 4nm of assistance. Once again emphasis should be directed to programming and adaptive correction via e serial platform.

Stability & proximity
When introducing vehicle stability dynamics, even more data is required: An accelerometer as well as yaw and  pitch sensors will complement existing input requirements. Enhanced and shared functionality with ABS enables the braking system to support vehicle control through corners by applying a control force through the rear brakes.

We now need to consider the vehicle proximity control system; the system employs an ultrasonic sensor to monitor and determine the environment. However, this interim system has several critical shortcomings, especially due to its narrow detection field and inaccurate position calculation regarding other vehicles and obstacles. The next modular enhancement introduces side or blind spot monitoring or side assist. This system also has limitations with range and vector limitations. Although operating on a high speed can network, it operates on a master slave principle, for example; slave units only transmit data and diagnostics on demand from the master module.

It is of note that the vehicle now relies on no less than 13 control modules, with predictive position algorithms. Later evolution will include optical, video, ultrasound, infrared and laser. Optical lane assist which is mounted on the windscreen requires considerable coding and calibration, notwithstanding windshield replacement, so much for off-site repairers.

Calibration & correction
Calibration requires determination of the camera orientation, the exact installed location, the height at which the camera is installed and three orientation measurements. This is an electronic function as no mechanical adjustment is provided. Therefore any change in tyre, wheel diameter or suspension repair or modification will invalidate this system accuracy, including fault memory errors.

We now move into the era of de-coupling direct driver steering input. This system allows computer correction of steering angle. For example, with a loss of driver control, ESP can introduce a counter steer input to regain control. This system is intended to maintain the maximum static traction between the road surface and tyre. Should this be insufficient to maintain a safe curve radius, the ABS can be employed to help recover the vehicle attitude.

The system can carry out actual steering angle correction while the driver maintains a different steering wheel input, such as on snow, ice, or on flooded road surface conditions. In order to facilitate this function, a mechanical flexi-coupling is mounted in the upper steering column. The outer has 100 teeth, with the inner posessing 102. They can rotate together as one with direct driver input command or can rotate at a different angle disengaging driver direct input control.

In effect this system still complies with statutory requirements as having still a de-facto fail-safe mechanical connection between driver and steering mechanism. Therefore is still level 0 status, in terms of autonomy.

At this point we are a million miles from even level 2 or 3 autonomous control. Level 3 allows for the driver to release physical contact with vehicle controls yet remain available and alert in case of system failure. Please make your own mind up. However, I’m not for turning!



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