Active suspension

An active suspension is a type of automotive suspension that uses an onboard control system to control the vertical movement of the vehicle's wheels and axles relative to the chassis or vehicle frame, rather than the conventional passive suspension that relies solely on large springs to maintain static support and dampen the vertical wheel movements caused by the road surface. Active suspensions are divided into two classes: true active suspensions, and adaptive or semi-active suspensions. While semi-adaptive suspensions only vary shock absorber firmness to match changing road or dynamic conditions, active suspensions use some type of actuator to raise and lower the chassis independently at each wheel. These technologies allow car manufacturers to achieve a greater degree of ride quality and car handling by keeping the tires consistently perpendicular to the road when turning corners, preventing unwanted contacts between the vehicle frame and the ground (especially when going over a depression), and allowing overall better traction and steering control. An onboard computer detects body movement from sensors throughout the vehicle and, using that data, controls the action of the active and semi-active suspensions. The system virtually eliminates body roll and pitch variation in many driving situations including cornering, accelerating and braking. When used on commercial vehicles such as buses, active suspension can also be used to temporarily lower the vehicle's floor, thus making it easier for passengers to board and exit the vehicle. Skyhook theory is that the ideal suspension would let the vehicle maintain a stable posture, unaffected by weight transfer or road surface irregularities, as if suspended from an imaginary hook in the sky continuing at a constant altitude above sea level, therefore remaining stable. Since an actual skyhook is obviously impractical, real active suspension systems are based on actuator operations. The imaginary line (of zero vertical acceleration) is calculated based on the value provided by an acceleration sensor installed on the body of the vehicle (see Figure 3).

Loopy Levy flights enhance tracer diffusion in active suspensions

Tomohiko Sano, Andrea Cairoli

A theoretical framework describing the hydrodynamic interactions between a passive particle and an active medium in out-of-equilibrium systems predicts long-range Levy flights for the diffusing particle driven by the density of the active component. Brownian motion is widely used as a model of diffusion in equilibrium media throughout the physical, chemical and biological sciences. However, many real-world systems are intrinsically out of equilibrium owing to energy-dissipating active processes underlying their mechanical and dynamical features(1). The diffusion process followed by a passive tracer in prototypical active media, such as suspensions of active colloids or swimming microorganisms(2), differs considerably from Brownian motion, as revealed by a greatly enhanced diffusion coefficient(3-10) and non-Gaussian statistics of the tracer displacements(6,9,10). Although these characteristic features have been extensively observed experimentally, there is so far no comprehensive theory explaining how they emerge from the microscopic dynamics of the system. Here we develop a theoretical framework to model the hydrodynamic interactions between the tracer and the active swimmers, which shows that the tracer follows a non-Markovian coloured Poisson process that accounts for all empirical observations. The theory predicts a long-lived Levy flight regime(11) of the loopy tracer motion with a non-monotonic crossover between two different power-law exponents. The duration of this regime can be tuned by the swimmer density, suggesting that the optimal foraging strategy of swimming microorganisms might depend crucially on their density in order to exploit the Levy flights of nutrients(12). Our framework can be applied to address important theoretical questions, such as the thermodynamics of active systems(13), and practical ones, such as the interaction of swimming microorganisms with nutrients and other small particles(14) (for example, degraded plastic) and the design of artificial nanoscale machines(15).

2020

Loopy Levy flights enhance tracer diffusion in active suspensions

Tomohiko Sano, Andrea Cairoli

2020

Loopy Levy flights enhance tracer diffusion in active suspensions

The kinetics of the ethanol dehydration under dynamic conditions - a reaction with stop-effect. In-situ surface and gas phase analysis

Spindle motor with active electromagnetic damping

Spindle motor with active electromagnetic damping

The kinetics of the ethanol dehydration under dynamic conditions - a reaction with stop-effect. In-situ surface and gas phase analysis

Loopy Levy flights enhance tracer diffusion in active suspensions