SOFIE pointing control system

This paper presents designs of compact 21 and 30 m aperture telescopes with primary focal of f/0.7 and f/0.56. The 20 20 telescope moves on three axes; the elevation axis (which is below the primary vertex), the azimuth axis, and a tracking axis at the center of 100 m diameter tracks. The 30 m telescope has an elevation and azimuth axis. All of the axes move on hydrostatic bearings. A primary requirement for such large telescopes is stiffness against deformation by wind gusts. The mass and stiffness needed for the structure is substantially independent of the primary mirror mass, which can therefore be set by thermal and diffraction issues. For the 21 m design, whose primary has seven 8.4 m glass segments weighing 128 tons, the total moving mass is 905 tons, and the lowest resonant frequency 6.5 Hz. For the 30 m design, whose primary has, 13 whole and 6 half, glass segments 8.7 m, across the points, weighing 256 tons, the total moving mass is 3,460 tons, and the lowest resonant frequency 5.3 Hz. These practical designs offer two versatile telescopes with high performance.

Skylab's Apollo Telescope Mount (ATM) was this country's first full scale manned astronomical observatory in space. It was a predecessor to the currently proposed Spacelab experiment observational systems. The ATM experiment canister, which housed the eight principal solar telescopes, was three m long, over two m in diameter, and weighed approximately 11,200 kilograms. The system was designed to operate within a pointing accuracy of ± 2.5 arc-sec and within a stability accuracy of ± 2.5 arc-sec over a 15-minute observation period. Review of photographic data indicated that the Experiment Pointing Control System (EPCS), which controlled the ATM experiment canister, performed with a stability of better than 1.0 arc seconds, exceeding its design requirement by a factor of 2.5. The EPCS was a part of the Skylab Attitude and Pointing Control System (APCS) which utilized momentum exchange devices and cold gas thrusters for overall vehicle control. The EPCS operated in a nested configuration within the vehicle control system. It was designed to isolate the ATM experiments from vehicle disturbances in two axes, up/down and left/right. Roll positioning control was also available. A fine sun sensor (FSS) provided position information while rate information was obtained from canister mounted rate gyros. Torque motors on each of the dynamic axes responded to controller signals to position the experiment canister.

Substantial active nutation control of a dual-spin spacecraft is achieved through augmentation of the fundamental dynamic coupling between the payload azimuth pointing control system and the vehicle nutation dynamics. A significant increase in damping effectiveness over simple dynamic coupling is achieved through integration of a simple rotor-mounted nutation sensing accelerometer with the despun payload control system. This technique is shown to overcome limitations inherent in both passive mechanical damping systems and active nutation control through simple dynamic coupling. The system dynamics and the control laws of the composite nutation and pointing control systems are developed. A closed-form expression for the resulting damping strength is derived. Implementation and functional operation of the control system are discussed. Analysis of the characteristics of the interaction provides design tools for optimization of the system dynamics to achieve maximum effectiveness. A design example is presented for illustration, with simulation results that verify the analytical conclusions.

This paper presents a Hardware in the loop (HIL) simulation of a microcontroller using non-linear control implemented in PFC (Power Factor Correction) boost, buck and SEPIC converters. This is a non-conventional approach, where libraries are used to directly interface with the Texas's DSP 2812. Initially, the block diagrams that contain the control equations are constructed. Then, the codes are generated automatically. The advantages provided by the proposed method are: security, economy of development cycle time, easy understanding of the control process and programming, standardization and concurrent simulation. Three nonlinear control techniques - state feedback linearization (SFL), passivity based control (PBC) and interconnection and damping assignment passivity-based control (IDA-PBC) has been validated experimentally using HIL simulation for boost, buck and SEPIC converters.

This paper proposes a semi-active variable stiffness vibration isolation system based on electromagnetic spring for the low-frequency vibration isolation of mass-varying objects. It is achieved by four straight leaf springs in parallel to an electromagnetic spring system composed of a single electromagnet and a permanent magnet. The equivalent magnetic circuit method is used to compute electromagnetic force of the electromagnetic spring system, and mathematical model of the semi-active vibration isolation system is established according to Maxwell's equations. The nonlinear mathematical model is linearized at the equilibrium point by using the Taylor series expansion theorem to establish linear state-space representation of the system, and then using the traditional PID control method, a double closed-loop feedback control system of the inner current loop and outer location loop is designed. By controlling the current in the coil, the equivalent stiffness and electromagnetic force of the system are variable to achieve semi-active control. Furthermore, the control block diagram of the semi-active vibration isolation system is built based on Simulink software, then make a simulation analysis to the vibration isolation performance of the system and compare the effects of vibration isolation with inner current loop control and without inner current loop control, respectively. Finally, the experiments prove the correctness of the theory. It concludes that this semi-active vibration isolation system is a vibration isolation system with broad application prospects, which has fast current response, high vibration isolation efficiency, and an excellent vibration isolation effect for the low-frequency disturbance of mass-varying objects.

Tracking systems with stringent performance are required to take advantage of solar energy produced through the use of high concentration photovolatics and concentrating solar power technologies. Closed-loop tracking systems use measurements provided by a sun sensor, which picks up direct and diffuse sunlight. High levels of diffuse sunlight produce increased levels of measurement noise at the output of the sensor, which in turn limits the maximal achievable gains in the closed-loop controller and consequently the tracking performance deteriorates. The goal of this paper is to present the advantages of equipping a four-quadrant photodiode sun sensor with infrared and linear polarizing optical filters. Experimental results show that these filters reduce the effect of diffuse radiation and allow increasing the performance of sun tracking systems regardless of the type of controller used in the solar tracker. Tracking experiments are presented under indoor and outdoor conditions employing algorithms currently used in solar trackers, including proportional-integral (PI), and proportional-integral-derivative (PID) controllers, and a recently proposed cascade control law. In all the cases, the best performance is obtained with the sun sensor equipped with an infrared filter. Experiments show that an infrared optical filter reduces the filtered mean squared tracking error up to 75% under artificial light conditions and 90% under direct sunlight.

Midcourse Space Experiment attitude system performance for gyro disabled operations

Improving transients of voltage and current allows more effective utilization of the electric power delivery infrastructure and prevents violation of the operation limits. This paper develops an internal model control (IMC)-based cascade voltage control strategy, which consists of an inner current control feedback loop and an outer voltage control feedback loop, to improve the transient behavior of the three-phase voltages of a distributed generation (DG) unit in an islanded microgrid. The inner control feedback loop is responsible for fast compensation of input disturbances and phase delays caused by the RLC filter, and the outer control feedback loop is responsible for providing the reference for the inner current control feedback loop to maintain the desired voltage. The proposed IMC-based cascade voltage controller shows significantly improved dynamic behavior compared with the conventional PI-based voltage control approach. Simulation results in PSCAD/EMTDC environment confirm the desirable dynamic performance of the proposed approach.

Since the unmanned aerial vehicles (UAVs) bring forth many innovative applications in scientific, civilian, and military fields, the development of UAVs is rapidly growing every year. The on-board autopilot that reliably performs attitude and guidance control is a vital part for out-of-sight flights. However, the control law in autopilot is designed according to a simplified plant model in which the dynamics of real hardware are usually not taken into consideration. It is a necessity to develop a test-bed including real servos to make real-time control experiments for prototype autopilots, so called hardware-in-the-loop (HIL) simulation. In this paper on the basis of the graphical application software LabVIEW, the real-time HIL simulation system is realized efficiently by the virtual instrumentation approach. The proportional-integral-derivative (PID) controller in autopilot for the pitch angle control loop is experimentally determined by the classical Ziegler-Nichols tuning rule and exhibits good transient and steady-state response in real-time HIL simulation. From the results the differences between numerical simulation and real-time HIL simulation are also clearly presented. The effectiveness of HIL simulation for UAV autopilot design is definitely confirmed

Development, Design and Experimental Testing of Fuzzy-based Controllers for a Laboratory Scale Sun-tracking Heliostat

The implementation of a wide output range, programmable and efficient DC-to-DC power converter with 16-bit linearity is investigated. A medium power unit (/spl sim/200 W) and a high power one (2 kW) were built and fully characterized. These power converters are intended to be the power supply of the coils of a multipolar magnet for the generation of the magnetic field required for a magnetic resonance imaging system. A mixed switch-mode and linear technique, i.e. the combination of a switching preregulator and a linear final regulator, makes it possible to achieve high accuracy and good efficiency. The linear section is a high performance programmable current generator based on a MOS output stage in the feedback loop of an operational amplifier. The output of the switch-mode section (a conventional buck cell for the medium power version or a quadratic cell for the high power one) is regulated by an outer feedback loop that stabilizes the voltage drop across the MOS at the minimum value required for linear operation, thus allowing fair efficiency. Design criteria and experimental results are reported.

Vector control of the doubly fed induction generators (DFIGs) depends on accurate information from a stator voltage phase lock loop (PLL) and a rotor position encoder for the reference frame transformations required by theory. The effect of erroneous information due to noise picked up by commonly used hardware PLLs and incremental position encoders was analyzed in this paper. The analysis shows that erroneous information can give rise to destructively large currents due to: 1) the DFIG system wrongfully perceiving very large P and Q references; and 2) system instability. It is shown that instability does not arise from inside the inner current feedback loop, but from the outer feedback loop that regulates the complex power. Simulation and experimental test results validate the analysis. The experimental results are taken from a 1.5-MW DFIG intended for a wind turbine. A simple scheme to protect against the erroneous information has been tested successfully in a brief trial period.

This paper presents a new electro-hydraulic control system for screw conveyor device of shield tunnelling machine. The control model of the system is established, in which arotation and gate movement compound control approach is applied. It utilizes the motor speed feedback to design an outer loop and cylinder velocity feedback as an inner loop to realize flow rate disturbance compensation. Simulations are done and results are presented to verify the effectiveness and rationality of the proposed driving system and its control strategy.

This paper presents Hardware-in-the-Loop (HIL) simulation results of a novel configuration for guidance and tracking control laws for unmanned air vehicles (UAV). The proposed adaptive system is based on an extended non-linear dynamic inversion (NLDI) approach augmented with an artificial immune system mechanism that relies on a direct compensation inspired primarily by the biological immune system response. The implementation of the control laws is illustrated through HIL simulation using a mathematical model of an UAV research platform developed at Embry-Riddle Aeronautical University (ERAU) to support the design, testing and validation of bio-inspired fault tolerant adaptive control algorithms. The main objective of the control laws is to minimize forward, lateral, and vertical distances with respect to a desired trajectory, and maintain stability and adequate performance in the presence of sub-system failures. The performance of the control laws is evaluated during autonomous flight in terms of trajectory tracking errors, real-time execution on board the flight computer, and control activity at nominal and abnormal conditions. The results obtained with the ERAU-UAV HIL environment show that for all cases investigated the extended NLDI approach augmented with the immunity-based mechanism has desirable fault tolerant capabilities and is reliable for in-flight testing operation as a next step towards the validation and verification of this adaptive configuration.

A control method of adjusting the stiffness of suspension is proposed for magnetic suspension systems whose bias flux is provided by permanent magnets. In such magnetic suspension systems, zero-power control has often been applied. Since the zero-power magnetic suspension system behaves as if it has negative stiffness, it has been applied to vibration isolation system. However, the amplitude of negative stiffness cannot be adjusted. It is one of the obstacles to the industrialization of the vibration isolation system. In this paper, a proportional feedback is incorporated into the minor current feedback loop of the zero-power controller. It is shown that the stiffness can be adjusted by the gain of the added current feedback. The characteristics of the designed control system are studied both analytically and experimentally.