Digital Automatic Flight Control is the Answer - What is the Question?

Recently, the Plant Input Mapping (PIM) method was proposed as a digital-redesign (or an emulation) approach in digital control system design. This method guarantees stability of a redesigned digital control system for a wide range of sampling periods. In this paper the PIM method is applied to the design of a digital model-matching flight control system. In the model-matching control system design a plant must have no unstable zeros. However, unstable zeros appear for small sampling periods in a step invariance model of a continuous-time (CT) plant with the relative degree greater than two, even if the CT plant has no unstable zeros. Hence, in such a case, the direct-digital approach cannot be applied to the design of a digital model-matching control system. By contrast, the digital-redesign approach such as the Tustin's method or the PIM method is free from the problem. Simulation results for an unstable VTOL aircraft show that the proposed method gives better performance than the conventional design methods.

The insertion of communication networks in closed-loop of digital control systems makes the system synthesis and analysis complex. In this paper, we consider the combination of classical reliability, risk and safety evaluation with typical control system through digital networked control systems. The main purpose is to study the influence of transmission delays and packet dropouts on the reliability of digital networked control systems by using a modified statistical method-Monte Carlo simulation. The behaviors of two types of fault are modelled by different statistical distribution. The control strategies we apply for digital control systems are Proportion Integration Differentiation control strategies, generated by Genetic Algorithm. In terms of a realistic system model, we evaluate and compare the influences on the reliability of digital networked control systems with different control strategies from quantitative aspect. Numerical examples are presented to illustrate the effectiveness and correctness of the approach.

PurposeThe purpose of this paper is to describe the general idea, design, and implementation of control system for general aviation aircraft which reduces pilot workload.Design/methodology/approachProposed indirect flight control system framework is intended to simplify piloting, reduce pilot workload, and allow low‐end general aviation aircraft to operate under deteriorated meteorological conditions. Classical control theory is used for the design of the flight control laws. Although not inherently robust, controllers with classical control logic are made sufficiently stable using a correct and updated controller structure.FindingsDespite controversies between perception of a modern manned aerial vehicle and limitations imposed by legacy airworthiness codes it is shown that a pilot workload reducing system can be successfully implemented onboard of a low‐end general aviation aircraft.Research limitations/implicationsHi‐level control laws and optimization of handling qualities can lead to unfavourable and unpredictable forms of man‐machine interactions, e.g. pilot‐induced oscillations.Practical implicationsGeneral aviation aircraft are mostly flown by a single pilot, who could benefit from an intelligent system or “virtual copilot” assisting in or supervising the aircraft's safe operation under any conditions. Aircraft with this capability represents a next step in the evolution that might ultimately lead to trajectory‐based free‐flight concept of aircraft operations.Originality/valueThe paper introduces a safety enhanced digital flight control system on board small general aviation aircraft.

Over the past forty years, the number of fighter aircraft controlled flight into terrain (CFIT) accidents per flight hour has not improved despite the addition of multiple manual warning systems. The Air Force Research Laboratory (AFRL) has undertaken a program to develop an automatic system that will take over when the pilot can no longer recover the aircraft, perform an automatic recovery and quickly return control to the pilot. It is called the Automatic Ground Collision Avoidance System or Auto GCAS. This system has been successfully demonstrated on a Block 50 F-16 with a digital flight control system as shown in Figure 1 and is transitioning to operational Block 40/50 F-16s. Until now the technology could not be applied to early model F-16s with analog flight control systems because of the lack of a high authority autopilot needed for the automatic recovery and the absence of a way for the flight control system to ascertain system integrity to insure safe operation. AFRL is now focused on finding a technical solution that will allow the implementation of Auto GCAS on aircraft with analog flight control systems. The effort is called Analog Auto GCAS. The Analog Auto GCAS project has developed an innovative method of adding redundant digital processor modules to the existing analog flight control computers of pre-block 40 F-16's. This will not only allow Auto GCAS to be added to F-16s with analog flight control computer but demonstrates a technical path forward for incorporating Auto GCAS onto other older aircraft with analog flight control computers. This paper provides a description of Auto GCAS, discusses the changes needed to implement Auto GCAS on an analog aircraft, and covers the results to date as well as the future path forward for implementing Auto GCAS on the pre-block 40 F-16 fleet.

The availability of digital MSI and LSI has made digital flight control systems more cost effective than analog systems for all but the simplest systems. The organization of a digital flight control system is concerned not only with implementation of the control laws and associated logic but also with the management of the data used in these computations and the organization of the computation. The configuration taken by a digital flight control system is dominated by the integrity and data handling requirements. This paper considers the impact of these factors on the design and operation of digital flight control systems with particular emphasis on the use of fiber optics for transmission of data between lanes.

In airline service the reliability of a system is more than the confirmed failures for the system; it is the total removal experience of the system. Thus, the reliability of a system lies not only in the piece part failure rates, but also in the capability to verify and isolate failures. The application of digital technology brings several significant reliability improvements to automatic flight control systems (AFCS) when compared to contemporary analog systems. These advantages are demonstrated by the experience of the digital air data computer (DADC) of the DC-10 and the digital AFCS of the JA-37 Viggen. Experience with this equipment is reviewed, and the results are interpreted in terms of projections for airline DAFCS reliability. The digital system built-in test implemented by a stored program and the central processor gives a system integrity and dispatch reliability unequaled by analog systems. This high-integrity self-test reduces removal rates by giving line personnel a trustworthy tool and more complete automatic test processes for verifying maintenance actions. Digital circuit technology is directly suitable to largescale integrated circuits (LSIC) which reduce piece part counts and improve LRU reliability. Digital circuits are less subject to drift and the attendant difficulty to detect failures. These factors, coupled with the inherent high-integrity self-test, provide the basis for a significant improvement in reliability by the use of a digital automatic flight control system.

It is demonstrated that reconfigurable digital multiple model-following flight control systems incorporating fast-sampling error-actuated digital PID (proportional plus integral plus derivative) controllers can be easily designed using directly measurable input/output data. Thus, for example, it is shown in the case of the F-16 aircraft that desired flying qualities with respect to both pitch-rate and angle-of-attack response characteristics can be simultaneously achieved in the absence of sensor failures. In addition, in the case of failure of the angle-of-attack sensor, it is shown that desired flying qualities with respect to pitch-rate response characteristics and satisfactory flying qualities with respect to angle-of-attack response characteristics can still be achieved by reconfiguring the digital controller. >

A digital closed-loop control system with all digital phase locked loop (ADPLL) is designed and optimized, in order to improve the bias stability and transient response of the gyroscope. The nonlinear mathematical models for the closed-loop amplitude and phase control system are established. The linearization method is applied to analyze the nonlinear models. The control parameters of the drive loop are optimized. The experimental results show that the phase deviation between the demodulation reference signal of the drive loop and the sense signal is less than 0.25° in the temperature range from-40°C to 60°C, within 0.15° of the variation. At room temperature, the overshoot amount of the gyroscope control system is 4.5% with setting time of 0.12s. The bias stability is 1.45°h. Compared to the analog control scheme, the digital control system has advantages of short setting time, small overshoot, short phase locked time, large locked range, high phase precision, etc.

The article discusses an approach that allows you to move from a discrete model of a digital controller to its analog representation, which provides analysis and synthesis of digital systems in the continuous domain, taking into account the quantization period. In the proposed method for the control system, a transition is made from the discrete z-region to the Laplace region. This approach makes it possible to consider the quantization time as an additional parameter of the digital control system settings, evaluate the influence of the selected quantization period not only on the behavior of the set for discrete at the moments of quantization, but also in between, to evaluate the temporal characteristics of the control system at any time. The analog representation of the digital controller enables the calculation of transients using MatLAB. The article provides examples of the synthesis for a digital control system in the frequency domain based on the frequency response characteristic of an open-loop control system taking into account the quantization period. The method can be used by automation specialists to analyze and setting digital control systems

This paper discusses implications of digital flight control system design for rotorcraft and illustrates the analysis of the resulting handling qualities obtained with the Advanced Digital Optical Control System demonstrator in the context of the proposed new handling-qualities specification for rotorcraft. Topics covered are digital flight control design and analysis methods, flight testing techniques, handling-qualities evaluation results, and correlation of flight test results with analytical models and the proposed handling-qualities specification. The evaluation of the demonstrator system indicates desirable response characteristics based on equivalent damping and frequency, but undersirably large effective time delays (exceeding 240 ms in all axes). Piloted handling qualities are found to be desirable or adequate for all low, medium, and high pilot gain tasks, but handling qualities are inadequate for ultrahigh gain tasks such as slope and running landings. Correlation of these results with the proposed handling-qualities specification indicates good agreement for the bandwidth boundaries, but suggests the need for more stringent limits on allowable phase delay. Analytical models based on emulation (s-plane) techniques compare favorably with flight-extracted frequency-domain characteristics of the overall (end-to-end) system responses.

This paper focus on the digital control system for bearingless permanent magnet rotor motors. The bearingless permanent magnet synchronous motor (BPMSM) is an innovational type of motor, as bearingless motors have all advantages of magnetic bearings, they play an important role in solving difficult problems of special electric drives in life science, chemical processing industry, semiconductor manufacturing industry, foodstuff processing industry, and so on. In the paper, based on the basic working principle of BPMSM, the mathematics models of the radial forces and rotation part on BPMSM are deduced by adopting rotor magnetic field oriented control (FOC) strategy. A digital vector control system according to the demand of decoupling is designed and presented using DSP technique. An experimental platform of digital control system is developed to meet the need, The experiment results have shown that the steady suspension of the rotor is realized and the speed of the rotor can be continuously adjusted within the range of 0-3000 r/min.

Real-time programming has always been one of the most challenging programming disciplines. Real-time programming requires comprehensive command of sequential programming, concurrency, and, of course, time. Real-time programming has many application domains. In this chapter, however, we structure our discussion around digital control systems for the following two reasons. Firstly, digital control systems represent a large portion of real-time software in a diverse set of industries. Examples include automotive power train control systems, aircraft flight control systems, electrical drive systems in paper mills, or process control systems in power plants and refineries. Many of these systems distribute their components over networks with significant communication delays. Therefore, we also discuss networked real-time programming (Section 7). The second reason is pedagogical. Digital control is defined by a set of abstractions that are real-time programmable and mathematically tractable in the context of the dynamics of physio-chemical processes. The abstractions clearly define what application engineers (control engineers) expect of real-time programming. These abstractions therefore constitute a precise definition of the real-time programming problem (Section 2). Control engineers design in difference equations and often use modeling tools such as Simulink to simulate and validate their control models. The part of Simulink most commonly used to specify digital control, i.e., discrete-time with the discrete fixed-step solver and mode:auto, is well-explained by the so-called synchronous semantics [7, 38]. The key abstraction, we call the synchronous abstraction, is that any computation and component interaction happens instantaneously in zero time or with a delay that is exactly the same at each invocation. For example, if a controller computes every 20 msec, either the controller is assumed to read its inputs, compute its control, and write the corresponding outputs instantaneously at the beginning of each period in zero time, or to read its inputs instantaneously at the beginning of the period, compute during the period, and write the corresponding output exactly 20 msec (one period) later. In control parlance, these are the causal and strictly causal cases, respectively. Systems of difference equations can directly be expressed and structured in this model. The inclusion of strictly causal components (components with delays of a period or more) is typically required to break feedback loops and to account for the unavoidable computation or communication delays that will be present in the real control system. Real-time programmers design in interrupt handlers, device drivers, and schedulers, which are concepts on levels of abstraction that are obviously unrelated to difference equations. The result is a conceptual and technical disconnect between control model and real-time code that bedevils many control building projects. The control engineer specifies a Simulink design the real-time programmer finds unimplementable. The realtime programmer then fills in gaps to produce an implementation the control engineer finds uncontrollable. The process of interaction as the two iterate to produce a correct design is prolonged by the different backgrounds of control engineers and real-time programmers. This gap explains many of today’s problems in real-time software design for digital control such as high validation and maintainance overhead as well as limited potential for reusability and scalability.

Two variations of each of three types of digital optical transducers have been developed to provide the technology base for a digital optical flight control system for US Army helicopters. The transducer types are rotary position, linear position, and differential hydraulic pressure and all are electrically passive at the transducer location. The development and test of these six transducers has led to the selection of a reflective, time delay multiplexed concept using a single interconnecting fiber for use on the Advanced Digital Optical Control System Flight Demonstrator Program (ADOCS FDP).

A digital control system has been developed to control the frequency of an alternator operating at variable speeds. The power line frequency or a crystal controlled oscillator can be used as the reference frequency. The alternator output has a ±0.2 percent tolerance to the reference signal after the frequency control system was added. The frequency control of the alternator was accomplished by switching windings in the alternator field. The method is applicable to wind-powered alternators where the speed is variable and a constant 60 Hz output is desired.

The DC-DC converters at the Advanced Photo Source at Argonne National Laboratory are over 15 years old. An upgrade to digital control system is being evaluated to resolve the aging and obsolescence issues. These converters supply power to electromagnets that are used to guide high energy beams. Thus a highly accurate control system is required for the DC-DC converters. This paper describes the design and evaluation of a digital control system to regulate the output current. The digital control system is proposed to replace the current analog control system thereby resolving the aging and obsolescence issues. A digital controller is first designed based on the small signal model of the converter. Experimental data of the output current is analyzed. The complete control system is then simulated and results are evaluated.