Narrow bandwidth active suspensions are characterized by relatively low force-production bandwidth of up to few Hertz, which results from an architecture Fig. 42 Delft active suspension realized with a cone mechanism
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where the dominant compliance is placed in geometric series with the active force generator. Most of the NBAS implementations thus far have been of electro-hydraulic type. Some representative electro-hydraulic active suspension configurations are shown in Fig.43. Starting with a general structure shown in Fig.43a one can use the bond graph of Fig. 43b to derive an equivalent all-mechanical structure shown in Fig.43c. In the special case when theflow source QBis not present one ends up with the load-leveling-like configuration shown in Fig.43d where we assumed a very soft, possibly pneumatic or air spring compli- ance. Finally, if theflow source QA is not present then we end up with the con- figuration shown in Fig.43e. This is similar to some NBAS architectures—note in particular the serial arrangement between the dominant spring and the active force
Fig. 43 Electro-hydraulic suspension configurations:ageneral structure;bcorresponding bond graph;cequivalent mechanical system;dtypical structure withQB≡ 0;etypical structure with QA ≡ 0 (based on Karnopp1987)
generator represented by the electro-hydraulic actuator that includes the controlled flow source QB.
As suggested by Hrovat (1997) the above NBAS model could be further enhanced by inclusion of inertia effects due to hydraulic line dynamics, which can be especially relevant in case of relatively long and narrow lines or tubing. This is shown in Fig.44, where the hydraulic conduit connecting the twoflow sources,QA and QB, and suspension cylinder has been modeled as an inerter element repre- sented by a differential causality within the associated bond graph of Fig.44.
A corresponding all-mechanical configuration is also shown in thatfigure. At this point, it should be mentioned that more recently there were attempts to develop and commercialize the hydraulic inerter (Scarborough 2011) as an alternative to its more common mechanical inerter counterpart. This may have some advantages in terms of packaging and overall design/cost flexibility, depending on particular implementation situation.
Further extensions of the above electro-hydraulic structures are possible by including an additional compliance near the suspension cylinder. This is shown in Fig.45a along with an associated controlled damping mechanism. In the case that the latter is of an on/off type and neglecting all active sources (i.e. settingQAand QBto zero) one ends up with a semi-passive suspension shown in Fig. 45b. This is similar to Citroen hydro-pneumatic suspension (Carbonaro1990) where one uses the on-off valve to control the effective suspension stiffness. The bond graph for the generic configuration of Fig.45a is shown in Fig.45c. Note in particular, that the inerter element corresponding to fluid line inertia is now in an integral causality with corresponding increase in the number of system states. Based on this bond graph one can easily deduce the corresponding all-mechanical suspension structure shown in Fig.45d.
Fig. 44 Electro-hydraulic suspension model including hydraulic inertia (inerter-like) effects, equivalent bond graph, and all-mechanical counterpart
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A special case of the generic configuration of a typical electro-hydraulic (semi) active suspension from Fig.45 is shown in Fig.46. This is one of the first pro- duction implementations of the NBAS system developed by Nissan for their Infinity Q45a luxury vehicle (Akatsu et al. 1990). Note in particular the presence of an accumulator, which effectively acts in series with the actuator thus limiting the actuator bandwidth while at the same time filtering high-frequency road-induced disturbances. This corresponds to the accumulator with stiffness kA in Fig.45a.
While the Infinity Q45a system used pressure control valve (Fig.46a) another alternative would be to use theflow control valve shown in Fig.46b.
In late 1990s Mercedes introduced their Active Body Control—ABC advanced suspension control system illustrated in Fig.47 (Merker et al. 2002), which is structurally similar to the NBAS architecture generalized in Fig.45. However, there is an important practical difference. While Infinity Q45a system used an hydraulic Fig. 45 Electro-hydraulic suspension configurations including hydraulic inertia (inerter) effects, and additional compliance and controlled (on-off) damping:a generic structure;b Citroen-like hydro-pneumatic semi-passive equivalent with QA=QB= 0; c corresponding generic bond graph;dequivalent all-mechanical system
accumulator to implement the above mentioned in-series stiffness kA, the Mercedes ABC suspension uses a mechanical counterpart similar to the one shown in Fig.45d. For most cases the two would be equivalent except in the case when there is significant friction or even stiction within the actuator piston/cylinder combination in which case the mechanical implementation would lead to smaller road-induced disturbances resulting in better ride comfort. Through the years Mercedes has further developed and enhanced their system, which has recently included preview of the road based on stereo cameras. This system is now marketed under Magic Body Control (MBC) on their high-end luxury vehicles (Anonymous 2017a; Streiter 2008).
Fig. 46 Nissan infinity Q45a N-B active suspension
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