If we talk about building automated vehicle guidance systems in aircrafts and control to people who have their own experience in being often directly involved in such a process like you and us as car drivers or like airplane pilots, they may at first think about what have been the challenges and problems when driving a car or flying an aircraft, and how they mastered the demanding situations. If we talk about vehicle guidance and control with control engineers, they may look at the same thing from a different perspective. At first they may think about technical systems which are or can be installed in the vehicle in order to support the vehicle operator, usually to make his job an easier one. Either view does not cover the whole issue, but both are important ones on their own right. Before we put both these views together into a single comprehensive one, this article will deal just with the engineer's view with the focus on operational systems in aircraft and automotive vehicles.
Although even today aircraft, e.g. in general aviation, aerobatics or gliding are still operated in pure manual control mode, the pilots' flight guidance tasks are in general dominated by the use of automated systems of various kinds. Major reasons for the early and ongoing introduction of automation in aircraft cockpits were limitations of human
• sensing capabilities for aircraft state parameters and the aircraft environment,
• bandwidth in manual control tasks, and
• endurance in continuous and fatiguing tasks.
In addition to this list, the demand for information and communication may be added, especially in networked scenarios. Billings classifies the various types of automation serving the various requirements into
1. control automation
2. information automation
3. management automation.
Concerning sensing capabilities humans perform poorly regarding the direct perception of e.g. the flight altitude, velocity or heading. Being essential parameters for flight guidance, these have to be measured and displayed to the pilot by technical means. Human bandwidth limitations have to be taken into account when it is demanded to stabilise highly agile aircraft, where the given dynamic frequencies are beyond human cut-off frequency and the given stability margin of the aircraft is not sufficient. In these cases usually stability augmentation systems ranging from simple damping feedback contol loops to highly complex flight control automation will be used. As a very simple example, the pitch damper, which mainly improves the stability of the short-period mode of the longitudinal aircraft dynamics might be mentioned. Furthermore, autopilot and automated flight management functions relieve the pilot from tedious manual control tasks associated with the maintenance or acquisition of certain flight parameters such as a desired altitude, speed or heading. Thinking of an intercontinental airline flight there might be the need to keep these parameters within certain narrow limits over hours with only very infrequent adjustments of set points. By use of modern Flight Management Systems (FMS) even these flight guidance tasks may be automated. As a result, at least in civil air transport, it is possible to perform a full flight mission from take off to landing concerning its navigational aspects in a more or less fully automated manner. About the only input needed is a flight plan containing the geographical and some additional information about the desired flight, e.g. the vertical profile. On the first level of interaction the pilot directly operates on the control inputs of the aircraft, i.e. aerodynamic surfaces and engine throttle by use of dedicated primary flight control devices such as a control stick or wheel. Characteristic to this interaction level is the continuous control of the aircraft in 3D space and time.
Thus, the pilot continuously has to compute the control law in order to achieve the desired aircraft movements as according to the given aircraft dynamics. The pilot observes the aircraft movement by use of primary flight instruments, above all the artificial horizon. Related sensors are gyro instruments and atmospheric probes. In flight mechanics the aircraft dynamics commonly is modelled by use of a set of differential equations describing the development of the aircraft state variables over time as effect of the external forces and moments. The movement of the aircraft seen as rigid body can be described sufficiently accurate for the most principle considerations by twelve degrees of freedom, i.e. the three rotational attitude angles (yaw, pitch, roll), the related angular velocities, the three spatial positions (longitude, latitude, altitude) and the related translational velocities. While the three position parameters and the rotation angle against north in the lateral plane characterise the navigational movement of the aircraft against the geographic frame of reference, the remaining eight degrees of freedom allow describing the dynamic characteristics of the aircraft movement including stability and control issues. These are the important ones for the first and second interaction level of the pilot with the aircraft, since the dynamic characteristics have to satisfy certain standardised requirements. It might be interesting to mention that these handling quality specifications belong to about the earliest human factors related requirements in aeronautical engineering. The natural flight characteristics of a given aircraft can be modified to some very limited extent by controllers feeding back angular rate information to the aerodynamic control surfaces, thereby, generating damping moments. When there is need for some more radical changes of the dynamic behaviour, either for the sake of stability or for any other control requirement coming out of the application domain of the aircraft, further automation functions are necessary. In this case automated flight control systems (AFCS) are put into place. Although still being operated by the pilot in continuous manual control mode through the conventional primary flight control elements, i.e. control stick or control wheel, there is at least one fundamental difference to the first interaction level. The pilot no longer determines the deflection of aerodynamic surfaces, but commands desired demand values for certain aircraft state parameters, such as accelerations or angular rates. A very common example for level 2 interaction is the so-called rate command / attitude hold mode. Here the angular attitude of the aircraft will be kept constant, while there is no pilot input on the stick.
A pilot's pitch input to the control stick will be interpreted by the AFCS as a proportional pitch rate command to the aircraft and the actual pitch rate will be adjusted accordingly by the automatic control. In order to technically achieve this, the classical principle of a mechanical connection between the control stick and the aerodynamic surfaces had to be given up. Because of its technical solution mainly based upon electrical signals, this automation technology is often referred to as fly-by-wire (FBW). On the third level of interaction the pilot now is relieved from the task of continuous input of demand values for kinematic parameters such as accelerations or angular rates. On this level the pilot has to enter certain flight trajectory related set points whenever a change is demanded. This may occur rather infrequently. In between these adjustments there is no further control operation required by the pilot, though, except for the monitoring of the proper execution of the function. Therefore, operating on the third interaction level can be seen as a typical supervisory control task. The family of very diverse systems being in use on this interaction level can be summarised under the term autopilot. Simple autopilots are capable of functions such as ALTITUDE (or SPEED, or HEADING) HOLD, i.e. keeping a desired barometric flight altitude (or airspeed, or aircraft heading) at a constant value. In fact, realised autopilot systems, e.g. in airliners are usually some more complex concerning modes and operation. Operation of an autopilot of transport aircraft is done via a dedicated Flight Control Unit (FCU).
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