Chapter 5. Trajectory planning layer
2. Lateral motion
Lateral motion control without longitudinal motion control hardly exists, the rare example may be the assisted parking, where the lateral motion control is automated while the longitudinal motion control still remains at the driver.
The objective of assisted parking systems is to improve comfort and safety of driving during parking manoeuvres. Improve comfort by being able to park the vehicle without the driver steering and improve safety by calculating precisely the parking space size and collision-free motion trajectory avoiding any human failures that may occur during a parking manoeuvre. Series production assisted parking systems appeared on the market in the beginning of the 2000s. The advances in electronic technology enabled the development of precise ultrasonic sensors (Section 3.2) for nearby distance measurement and electronic power assisted steering (EPAS, Section 7.3) for driverless steering of the vehicle.
The task of trajectory planning starts with the determination of the proper parking space. By travelling low speed and continuously measuring the vehicle side distance, the parking gap can be calculated, thus proper parking space can be selected. As the first automated parking systems were capable of handling only parallel parking, the initial formulas for trajectory calculation were divided into the following segments:
1. straight backward path
2. full (right) steering backward path 3. straight backward path
4. full (left) steering backward path 5. (optional straight forward path)
Depending on the availability of the intelligent actuators in the vehicle the execution of the parking trajectory may also require assisted driver intervention. (Source: [65])
Figure 5.2. Illustration of the parallel parking trajectory segmentation (Source: Ford)
As there are different scenarios exist in everyday parking situations automated parking systems also try to assist the drivers in other situations then just parallel parking. Today‘s advanced parking assist systems can also cope with perpendicular or angle parking that require a slightly different approach in lateral motion control.
One topic of recent research in highly automated driving, which is especially challenging in urban environments is fully autonomous parking control. The challenge hereby arises from the narrow corridors, tight turns and unpredictable moving obstacles as well as multiple driving direction switching. The following figure shows common parking spots in urban environments that is subject of the research and development today. (Source:
[66])
Figure 5.3. Layout of common parking scenarios for automated parking systems (Source: TU Wien)
Besides parking another good example of low speed combined longitudinal and lateral control is the traffic jam assist system. At speeds between zero and 40 or 60 km/h (depending on OEMs), the traffic jam assist system keeps pace with the traffic flow and helps to steer the car within certain constraints. It also accelerates and brakes autonomously. The system is based on the functionality of the adaptive cruise control with stop & go, extended by adding the lateral control of steering and lane guidance. The function is based on the built-in radar sensors, a wide-angle video camera and the ultrasonic sensors of the parking system. As drivers spend a great amount of their time in heavy traffic, such systems could reduce the risk of rear-end collisions and protect the drivers mentally by relieving them from stressful driving. (Source: [67])
Figure 5.4. Traffic jam assistant system in action (Source: Audi)
Today‘s lane keeping assist (LKA) systems are the initial signs of higher speed lateral control of vehicles. Based on camera and radar information these systems are capable of sensing if the vehicle is deviating from its lane, then they help the vehicle stay inside the lane by an automated steering and/or braking intervention. An advanced extension of LKA is lane centring assist (LCA) when the vehicle not only stays inside the lane but lateral control algorithm keeps the vehicle on a path near the centre of the lane. The primary objective of the lane keeping assist and lane centring assist functions are to warn and assist the driver and these systems are definitely not designed to substitute the driver for steering the vehicle, although on technical level they would be able to do so. (Source: [68], [69])
Figure 5.5. The operation of today’s Lane Keeping Assist (LKA) system (Source:
Volkswagen)
Complex functions like highly automated driving with combined longitudinal and lateral control will definitely appear first on highways, since traffic is more predictable and relatively safe there (one-way traffic only, quality road with relative wide lanes, side protections, good visible lane markings, no pedestrians or cyclists, etc.). As highways are the best places to introduce hands-free driving at higher speeds, one could expect a production vehicle equipped with a temporary autopilot or in other words automated highway driving assist function as soon as the end of this decade.
Automated highway driving means the automated control of the complex driving tasks of highway driving, like driving at a safe speed selected by the driver, changing lanes or overtaking front vehicles depending on the traffic circumstances, automatically reducing speed as necessary or stopping the vehicle in the right most lane in case of an emergency. Japanese Toyota Motor have already demonstrated their advanced highway driving support system prototype in real traffic operation. The two vehicles (shown on Figure 83) communicate each other, keeping their lane and following the preceding vehicle to maintain a safety distance. (Source: [70])
Figure 5.6. Automated Highway Driving Assist system operation (Source: Toyota)
Nissan had also announced that they are also developing their highly automated cars, which is targeted to hit the road by 2020. Equipped with laser scanners, around view monitor cameras, and advanced artificial intelligence and actuators, is not fully autonomous as its systems are designed to allow the driver to manually take over control at any time. As Figure 84 illustrates the highly automated Leaf is being tested in a number of combined longitudinal and lateral control scenarios including automated highway exit, lane change, overtaking vehicles.
(Source: [71]