System Architecture for a CubeSat DTE Laser Communication System

Small satellites and CubeSats are playing an increasingly larger role in technology demonstrations, research, and obtaining data for commercial use from space. As these use cases and applications for CubeSats increase and expand the need to get fast, secure and reliable access to the data gathered by the payloads on these spacecraft also increases. To support these developments AAC Hyperion together with TNO has developed a down-to-earth laser communication module for small satellites and CubeSats with a downlink capability of 1 Gbit/s, namely CubeCAT. One of the challenges that needed to be solved to ensure success was how to deal with the pointing of the unit both for up-link and down-link purposes. Once this could be achieved the next challenge was the stability of the pointing mechanism since the pointing accuracy and stability of the laser link directly affect the link budget and achievable data rate. In other words rejection of mechanical vibrations and other disturbances. The goal of the development project was to develop such a system with a form factor and mass suitable for implementation on CubeSats. This meant that a maximum volume of 1U was set as a requirement and a maximum mass of \u3c2 kg

Sawtooth period pacing and locking by EC power control on TCV

Abstract #TP9.127 submitted for the DPP11 Meeting of The American Physical Society

Innovations in total knee replacement: new trends in operative treatment and changes in peri-operative management

The human knee joint can sustain damage due to injury, or more usually osteoarthritis, to one, two or all three of the knee compartments: the medial femorotibial, the lateral femorotibial and the patellofemoral compartments. When pain associated with this damage is unmanageable using nonsurgical techniques, knee replacement surgery might be the most appropriate course of action. This procedure aims to restore a pain-free, fully functional and durable knee joint. Total knee replacement is a well-established treatment modality, and more recently, partial knee replacement—more commonly known as bi- or unicompartmental knee replacement—has seen resurgence in interest and popularity. Combined with the use of minimally invasive surgery (MIS) techniques, gender-specific prosthetics and computer-assisted navigation systems, orthopaedic surgeons are now able to offer patients knee replacement procedures that are associated with (1) minimal risks during and after surgery by avoiding fat embolism, reducing blood loss and minimising soft tissue disruption; (2) smaller incisions; (3) faster and less painful rehabilitation; (4) reduced hospital stay and faster return to normal activities of daily living; (5) an improved range of motion; (6) less requirement for analgesics; and (7) a durable, well-aligned, highly functional knee. With the ongoing advancements in surgical technique, medical technology and prosthesis design, knee replacement surgery is constantly evolving. This review provides a personal account of the recent innovations that have been made, with a particular emphasis on the potential use of MIS techniques combined with computer-assisted navigation systems to treat younger, more physically active patients with resurfacing partial/total implant knee arthroplasty

Feedback control and injection locking of the sawtooth oscillation in fusion plasmas

In this thesis various different control strategies have been developed to control the period of the sawtooth oscillation inside a tokamak. This sawtooth instability arises at the q = 1 surface in nuclear fusion plasmas and manifests itself as a repetitive slow rise and sudden crash of the plasma core temperature and pressure. The mixing effect of the sawtooth crash can regulate the exhaust of helium ash from and the fuelling of deuterium and tritium to the plasma core. At the same time, the crash can trigger other instabilities, such as neoclassical tearing modes, which is generally undesirable. These processes are affected by the periodicity of the sawtooth oscillation, hence, control over the sawtooth period is essential to enable optimization of the plasma performance. Based on the dominant dynamics of the underlying magnetics of the sawtooth, a control-relevant sawtooth model has been developed and numerically implemented. This model is actuated via an electron cyclotron current drive (ECCD) term, characterized by two inputs: the amount of driven current and its deposition location. The output of the model is the sawtooth period. This sawtooth model mimics the static input–output behaviour observed on tokamaks, and has therefore been used as a casestudy for the controller designs. Two different control strategies have been investigated: feedback control (closed loop) and injection locking (open loop). Feedback control In the closed loop approach, first the sawtooth period dynamics has been determined. Via dedicated step response simulations and the application of approximate realization techniques, the dynamic behaviour around various operating points has been identified and represented in the frequency domain. Next, three different feedback control approaches have been based upon these identified systems: Low complexity - First, a standard linear controller has been designed, considering only the deposition location as a control variable. The parameters of the chosen controller structure (PII) have been optimized based on frequency domain specifications. The resulting closed loop is relatively robust, but its performance can sometimes be unsatisfactory whenever the ECCD mirror launcher, i.e. the actuator for the deposition location, is slow compared to the sawtooth period. High performance - In a second approach the closed-loop performance has been improved by the employment of gain-scheduling, feedforward and anti-windup techniques. Alternatively, the amount of driven current has been used as a control variable, which yields a closed-loop performance improvement as well. Moreover, a multivariable controller design has been proposed to combine fast settling times of the sawtooth period with power efficiency. These high-performance control designs are particularly interesting for future fusion reactors. High robustness - On experimental devices plasma uncertainties and disturbances are relatively large. For such applications a very robust feedback controller has been designed, based on extremum seeking. This adaptive controller makes online estimations of the sawtooth input–output behaviour via an external perturbation, based upon which the deposition location is adjusted to steer towards a desired sawtooth period. Various simulations have demonstrated the large robustness of the approach. Injection locking In addition, open-loop injection locking has been presented as an alternative sawtooth period control strategy. In this strategy, the deposition location is kept constant while the driven current (or gyrotron power) is modulated with a certain period and duty cycle. Extensive simulations have revealed that the sawtooth period can lock to the modulation period. All combinations of modulation period and duty cycle for which locking occurs define the locking range, which is particularly large for depositions close to the q = 1 surface. Similar sawtooth periods as with continuous wave ECCD can be achieved, while consuming time-averaged less power. Sawtooth period locking can be both fast and robust; simulations have demonstrated convergence speeds within only a handful of crashes, while locking is maintained even in the presence of plasma variations or disturbances. These predictions have been validated by experimental results; injection locking of the sawtooth period has been demonstrated on TCV plasmas. The experimentally obtained time responses and locking range show strong resemblances with the predictive simulations. Based on the identified locking range, an open-loop controller has been designed and implemented. Successful application to a TCV discharge has demonstrated that this controller can force a desired sawtooth period unto the plasma, even if this setpoint is slowly changing over time. The results in this thesis form a basis for further research on sawtooth control. Future work includes the application of the feedback control strategies in tokamak experiments, and further investigation of the locking phenomenon