Using a Steering Shaping Function to Improve Human Performance in By-Wire Vehicles

Army Research Laboratory

Saturday, August 01 2009

A shaping function eliminates the need for a dynamic control system to monitor vehicle motion.

Currently, there is a performance issue regarding vehicle control at higher speeds for some indirect-vision, by-wire military vehicles; that is, those vehicles in which mechanical links between the driver and control devices are replaced by electronic or computerized signals. Work has been performed to assess the current state of knowledge regarding the shaping function. The overall goal was to identify design parameters critical to improving the current by-wire implementation for military tactical vehicles, and to ultimately optimize system (i.e., human-vehicle) performance for the execution of secure mobile operations.

Future military vehicles will be drive-by-wire (DBW) vehicles, which means that mechanical elements of the control system are replaced by operator-controlled input devices coupled with remote actuators via a central electronic control system. A DBW vehicle can be composed of several “x-by-wire” subsystems. Examples include steer-by-wire, brake-by-wire, and throttle-by-wire, each referring to the individual vehicle control subsystems for which direct mechanical linkages (such as the hydraulic brake line) have been replaced by electrical signals between the input device (e.g., brake pedal) and the actuators of the system (e.g., calipers). This work focuses on steer-by- wire subsystems.

Because the actuators in x-by-wire systems are controlled by electrical signals from a computer rather than a direct mechanical link to the human-machine interface (HMI), input from the driver can be supplemented with or modified by intelligent automation. A good example of such automation in civilian vehicles is adaptive cruise control (ACC). An ACC system is used in a manner similar to standard cruise control, except that after the driver sets the desired speed, an adaptive controller regulates spatial separation from other vehicles on the road as well as attempts to maintain the speed indicated by the driver.

For the purposes of this research, a shaping function is defined as a mathematical description of the scaling between the input and output of a given system; that is, how the input is “shaped” into output. This is similar to the concept of a transfer function, except that the transformation is not explicitly occurring in the frequency domain. For steer-by-wire systems, the shaping function maps the angular displacements of the HMI control input device (joystick, yoke, or steering wheel) to the system response in terms of the vehicle steering angle.

The examination of shaping functions is important for several reasons. First, the use of HMI devices with limited “throw,” or total angular range of displacement, will pose challenges to the operator. A specific consequence of limited throw is the magnification that the operator will perceive relative to his or her expectations, based on experience with standard vehicles with steering wheels. In other words, because of the smaller permissible angular range of alternate HMI, there will be a lower input position-to-wheel angle ratio (also known as the steering ratio), and therefore, smaller hand/arm motions will produce larger vehicle responses.

A second issue involves the need for differential steering sensitivity across various driving tasks. Consider, for example, the large angular range of steering motion required for parallel parking as compared with the relatively small motions needed for lane maintenance while one is driving on a highway. Now envision the variety of steering tasks that may be encountered on the battlefield or during off-road missions in military vehicles. Such task factors are further compounded by the third issue: vehicle speed. In particular, as the vehicle moves faster, the magnitude of lateral accelerations during steering increases, elevating the risk of oversteering, inducing excessive roll or loss of control leading to collision, spinout, or rollover.