Adjustable Speed Control and Damping Analysis of Torsional Vibrations in VSD Compressor Systems

This paper proposes a model-based two-degree-of-freedom (2DOF) speed control for a medium voltage (MV) variable speed drive (VSD) connected to a centrifugal compressor (CC) train. Torsional mode excitations in the drive shaft due to converter switching behaviour are considered. An effective description of the harmonics transfer is proposed. The tuning strategy aims to optimize the tracking behaviour of the step and ramp command, taking care of critical speed excitations. The stability of the closed-loop dynamics against time delay and drive parameter variations are studied by means of Nyquist diagrams and time-domain simulations. A descriptive method for the process damping behaviour is proposed. The control strategy is evaluated through simulations as well as an experimental setup, based on a hardware in the loop (HIL) in a master–slave configuration.

Insights into the Importance of WPD-Loop Sequence for Activity and Structure in Protein Tyrosine Phosphatases

Protein tyrosine phosphatases (PTPs) possess a mobile, conserved catalytic loop, the WPD-loop, which brings an aspartic acid into the active site where it acts as an acid/base catalyst. Prior experimental and computational studies, focused on the human enzyme PTP1B and the PTP from Yersinia pestis, YopH, suggested that loop conformational dynamics are important in regulating both catalysis and evolvability. Also, work on Chimeras of YopH bearing parts of the WPD-loop sequence from PTP1B demonstrated unusual structural perturbations and reduced activity. In the present study, we have generated a chimeric protein in which the WPD-loop of YopH is transposed into PTP1B, and eight chimeras that systematically restored the loop sequence back to native PTP1B. Of these, four chimeras were soluble and were subjected to detailed biochemical and structural characterization, and a computational analysis of their WPD-loop dynamics in catalysis. These chimeras maintain backbone structural integrity, with somewhat slower rates than either wild-type parent, despite unaltered chemical mechanisms and transition states. The chimeric proteins’ WPD-loops differ significantly in their relative stability and rigidity. In particular, the open WPD-loops sample multiple metastable and interconverting conformations. The time required for interconversion, coupled with electrostatic effects revealed by simulations, likely accounts for the activity differences between chimeras, and relative to the native enzymes. These differences in loop dynamics affect both the pH dependency of catalysis and turnover rate. Our results further the understanding of connections between enzyme activity and the dynamics of catalytically important groups, particularly the effects of non-catalytic residues on key conformational equilibria.

Essential Loop Dynamics Modulates Catalytic Activity in α-Chymotrypsin

Conformational dynamics of macromolecules including enzymes are essential for their function. The present work reports the role of essential dynamics in alpha-chymotrypsin (CHT) which correlates with its catalytic activity. Detailed optical spectroscopy and classical molecular dynamics (MD) simulation were used to study thermal stability, catalytic activity and dynamical flexibility of the enzyme. The study of the enzyme kinetics reveals an optimum catalytic efficiency at 308K. Polarization gated fluorescence anisotropy with 8-anilino-1-napthelene sulfonate (ANS) have indicated increasing flexibility of the enzyme with an increase in temperature. Examination of the structure of CHT reveal the presence of five loop regions (LRs) around the catalytic S1 pocket. MD simulations have indicated that flexibility increases concurrently with temperature which decreases beyond optimum temperature. Principal component analysis (PCA) of the eigenvectors manifests essential dynamics and gatekeeping role of the five LRs surrounding the catalytic pocket which controls the enzyme activity.

Interpersonal synchronization of movement intermittency

Humans manifest remarkable sensorimotor coordination abilities as showcased in the skilful performance expressed by orchestras and dance ensembles. In multi-agent interactions, sensorimotor loops that are normally involved in the control of one's own movement must accommodate also for sensory data (e.g., visual feedback) informing about others' movement to adjust performance and ultimately co-adapt to each other. Yet, a mechanistic understanding of how sensorimotor control comes into place to enable interpersonal coordination is still lacking. By examining movement intermittency, we here open a window into the dynamics of visuomotor loop control during interpersonal coordination. Specifically, we analysed submovements, i.e., recurrent (2-3 Hz) force pulses that are naturally engraved in our kinematics and deemed to reflect intrinsic intermittency in (visual-based) motor control. Participants were asked to synchronize rhythmic (0.25 Hz) finger flexion-extension movements. Besides synchronization at the common movement pace, finger velocity shows 2-3 Hz discontinuities that are consistently phase-locked between the two interacting partners. Notably, submovements alternate in a seemingly counterphase pattern, showing highest probability ~200ms before as well as after submovements generated by one's partner. Further, when the real partner is replaced by an unresponsive partner - a dot moving according to a pre-recorded human kinematics - submovements systematically follow the dot submovements, indicating that movement intermittency is causally linked between partners. These results show that submovements are actively adjusted (inter-locked) during interpersonal coordination. Visuo-motor loop dynamics of interacting individuals can thus couple to optimize synchronization of the sense-and-correct process that is required for behavioural coordination.

The Development of Turboexpander-Generators for Gas Pressure Letdown Part I: Design and Analysis

This paper describes the design and development of an innovative 280 kW and a 125 kW Turboexpander Generator (TEG) for natural gas pressure letdown (PLD) applications. The flange-to-flange TEG is supported by active magnetic bearings (AMB) and uses an advanced thrust balancing scheme to minimize the net load on the thrust bearing. The machine designs for the two TEG frame sizes are very similar to maintain commonality between parts. A review of the high-speed generator (HSG) and AMB design is provided. A complete AMB closed-loop dynamics study is presented, including a comprehensive rotordynamics and controls analysis. The touchdown bearing design is shown and discussed, and design details of the touchdown bearing resilient mount are presented. The touchdown bearings are given resilience with a tolerance ring. A detailed simulation of a rotor touchdown event at full speed is shown. The magnetic bearing controller (MBC) and variable speed drive (VSD) are located approximately 35 m from the TEG, exposed to the outside environment, and are not required to be explosion-proof. The prototype TEGs are intended to be manufactured and tested in Q1 2021. They will be commissioned, and field tested in Q2 2021. A follow-up paper detailing the mechanical testing and field testing of the units will follow in 2022.

A Single Residue on the WPD-Loop Affects the pH Dependency of Catalysis in Protein Tyrosine Phosphatases

<p>Catalysis by protein tyrosine phosphatases (PTPs) relies on the motion of a flexible protein loop (the WPD-loop) that carries a residue acting as a general acid/base catalyst during the PTP-catalyzed reaction. The orthogonal substitutions of a non-catalytic residue in the WPD-loops of YopH and PTP1B results in shifted pH-rate profiles, from an altered kinetic p<i>K</i>_a of the nucleophilic cysteine. Compared to WT, the G352T YopH variant has a broadened pH-rate profile, similar activity at optimal pH, but significantly higher activity at low pH. Changes in the corresponding PTP1B T177G variant are more modest and in the opposite direction, with a narrowed pH profile and less activity in the most acidic range. Crystal structures of the variants show no structural perturbations, but suggest an increased preference for the WPD-loop closed conformation. Computational analysis confirms a shift in loop conformational equilibrium in favor of the closed conformation, arising from a combination of increased stability of the closed state and destabilization of the loop-open state. Simulations identify the origins of this population shift, revealing differences in the flexibility of the WPD-loop and neighboring regions. Our results demonstrate that changes to the pH dependency of catalysis by PTPs can result from small changes in amino acid composition in their WPD-loops affecting only loop dynamics and conformational equilibrium. The perturbation of kinetic p<i>K</i>_a values of catalytic residues by non-chemical processes affords a means for nature to alter an enzyme’s pH dependency by a less disruptive path than altering electrostatic networks around catalytic residues themselves. </p>