During the daily use of an automobile, only 10–16% of the fuel energy is used to move the car to overcome the resistance from road friction and air drag (Zuoet al., 2010). Recent studies have shown that energy recovery through an active suspension system in a hydrogen-fuelled HEV may play a major part in improving efficiency and range extending capabilities (Rajamani and Hedrick, 1995; Thanapalanet al., 2012b).
Suspension energy recuperation can achieve both damping function and energy recovery by converting the suspension vibration produced by road roughness into useful electrical energy, which can stored in the battery system to be reused later.
6.3.1 Suspension systems of vehicles
When vehicles travel on a road, they are always subjected to excitation from road irregularities, acceleration forces, braking forces and inertial forces on a curved track, which leads to discomfort for the driver, and hence, influences overall manoeuvrability. Viscous shock absorbers, connected in parallel with suspension springs, have been widely used to suppress vibrations by dissipating the vibration energy into waste heat. To achieve better ride comfort and road handling, active suspensions have been explored by researchers (Sharp and Crolla, 1987; Okada and Harada, 1996; Nakano et al., 2003; Gysen et al., 2011; Gohrle et al., 2015).
However, active suspension requires a significant amount of energy, which limits its wide implementation. Hence, regenerative suspensions have been proposed, which have the potential to harvest vibration energy from suspension systems while reducing the vibration (Zhanget al., 2009; Zuoet al., 2010). Theoretical results revealed that a maximum of 10% fuel efficiency can be recovered from vehicle suspension system by realizing regenerative shock absorbers (Zhang, 2010).
Mechanical brake rear axle Mechanical brake
rear axle Mechanical brake
front axle
Regenerative brake front axle
Regenerative brake front axle Drag torque by regenerative braking Requested
braking torque
(a) (b)
Requested braking torque Mechanical brake
front axle
Developed braking torque Developed braking torque
Figure 6.18 Series regenerative braking system: (a) without drag torque and (b) with drag torque
Vehicle suspension is used to attenuate unwanted vibrations from various road conditions, such as road roughness. This system not only provides ride comfort but also provides safety by maintaining good road contact. Three types of suspension systems have been proposed and successfully realized: passive, active and semi- active (Rajamani and Hedrick, 1995; David and Bobrovsky, 2011; Ebrahimiet al., 2011; Liet al., 2012; Jayachandran and Krishnapillai, 2013; Benet al., 2014).
Passive suspension is established by an oil damper that provides a simple design and cost-effectiveness. Performance limitations are inevitable due to the damping force not being controllable (Seong et al., 2011). A conventional sus- pension system of this kind usually consists of a spring and a viscous shock absorber. The shock absorber dissipates vibration energy into wasted heat.
Semi-active suspension relies on an actuator’s ability to control the damping characteristics according to dynamic conditions. It can offer enhanced perfor- mance, generally in the active mode, without requiring a large power source or expensive component. Such a scheme is also energy absorbing. The damping profile is ‘active’ when both masses move in the same direction and ‘passive’ for the rest of the vibration cycle. The most common actuator developed recently is based on the flow of a magneto-rheological fluid having variable viscosity that can be controlled electromagnetically (Seonget al., 2011; Poussot-Vassalet al., 2012).
Fully active suspension is based on a variable damping principle similar to the semi-active schemes, and control can be applied over the damping characteristic throughout the entire vibration cycle. Hence, within some parts of the cycle, the element is energy consuming rather than energy absorbing. This can be provided by an external power source and continuous switching between consuming and absorbing operation modes. This system provides a near ideal and completely isolating damping profile (David and Bobrovsky, 2011).
6.3.2 System configuration of shock absorbers
An electromagnetic regenerative suspension system transforms shock energy into electrical energy, which is more convenient to store and reuse, and has better per- formance, increased efficiency and less space requirements.
According to its working principle, a regenerative suspension system can be classified into two categories: mechanical and electromagnetic regenerative sus- pensions. In this section, only the electromagnetic regenerative suspension will be discussed. According to the electric machine utilized for energy conversion, it can further be categorized into rotational and linear generators.
Energy recovery suspension can achieve both the damping function and energy recovery by converting the suspension input vibration produced by road irregula- rities into reusable electrical energy, thus recovering energy and achieving damping through the same system. The configuration of a suspension system is shown in Figure 6.19 (Thanapalan et al., 2012a). The control unit will detect the harvestable energy supply from the suspension subsystem through sensor circuits.
The recovered energy will then be stored in the EV’s main electrical energy storage (EES) system, such as a battery, to be utilized subsequently for range extension.
On-board electromagnetic energy regeneration for electric vehicles 169
6.3.3 Energy harvester based on rotational electric machine A PM machine is preferable in electromagnetic suspension (EMS), which provides an active force when operating in the actuator mode or a damping force when operating in the generator mode. The damping force can be changed simply by the tuning of shunt resistances. There are several types of electromagnetic regenerative suspensions classified by their structure and configuration, which are summarized in this section.
A ball screw is a traditional transmission device that converts linear motion into rotation. An electric shock absorber with a ball screw was developed to harvest vibration energy (Arsem, 1971). A ball screw electric damper was also proposed, and its damping force can be altered by controlling the shunt resistance (Murty, 1989). Figure 6.20 shows a ball screw–based harvester, and its dynamic and regenerative characteristics were also analysed (Sudaet al., 2000).
A rack-pinion can also convert linear motion into rotation. A rack-pinion damper incorporates a bevel gear to change the motor axis in parallel with the linear motion, as shown in Figure 6.21 (Zhang, 2010). A regenerative active sus- pension system combining a rack-pinion and rotary motor was also proposed (Suda and Shiiba, 1996). An electronically controlled active suspension system is a sys- tem, which utilizes the rack-pinion configuration. Experimental results revealed that the speed limit and manoeuvrability of vehicle were enhanced significantly with such a suspension system (Weekset al., 2000; Benoet al., 2002).
Controller
Control unit
Sensor Actuator Suspension
system Power
converter EES Motor
Figure 6.19 Schematic of vehicle suspension system
Screw thread
Nut Motor
Figure 6.20 Ball screw based energy harvester
A hydraulic electromagnetic shock absorber (HESA) was recently developed (Fanget al., 2013), which can not only isolate vibration but also recover part of the energy originally dissipated by heat. The HESA shown in Figure 6.22 comprises a hydraulic cylinder, hydraulic rectifier, hydraulic motor, accumulators and a gen- erator connected with pipelines. The operating principle of a hydraulic rectifier is analogous to a Wheatstone bridge. The piston inside the hydraulic cylinder is dri- ven to reciprocate under external stimulus, and the high-pressure oil therefore flows into the hydraulic rectifier. At the same time, oil flows out of the hydraulic rectifier from the specified export in the compression stroke or extension stroke and then flows through the accumulator for weakening the fluctuation, which drives the hydraulic motor to generate electricity.
6.3.4 Energy harvester based on linear electric machine
A traditional shock absorber is usually replaced by a linear PM machine in direct- drive EMS systems. A linear machine converts mechanical energy (the relative motion between the vehicle chassis and wheels) into electrical energy without any transmission devices. An active and regenerative vibration control suspension using a linear actuator was proposed (Okada et al., 2003), which can realize vibration isolation and energy regeneration simultaneously.
Motor
Pinion Rack
Figure 6.21 Rack-pinion-based energy harvester
Accumulator1 Accumulator2
HM EM
Electric machine Hydraulic
motor Hydraulic
rectifier Hydraulic
cylinder
Linear motion
Figure 6.22 Hydraulic-based energy harvester
On-board electromagnetic energy regeneration for electric vehicles 171
A self-powered active vibration control system with two linear motors for truck cabins was also proposed (Nakano and Suda, 2004). In this system, an electric generator installed in the suspension system of the chassis regenerates vibration energy and stores it in a condenser. An actuator in the cabin suspension provides active vibration control using the energy previously stored in the condenser. Since the mass of the chassis of a typical heavy-duty truck is much greater than the mass of the cabin, vibration energy in the suspension of the chassis is anticipated to be greater than that in the cab suspension. So, this system can be self-powered.
Furthermore, a self-powered active vibration control system utilizing a single electric actuator was proposed (Gysenet al., 2011). The proposed system shown in Figure 6.23 consists of a passive coil spring to support the sprung mass and a direct- drive brushless tubular PM actuator to deliver active forces. For safety reasons, the suspension system should provide damping even when there is a power failure;
hence, a passive damper should also be incorporated into the active suspension system. This condition can be achieved through an oil-filled damper connected in parallel. Since an electromagnetic actuator is considered, it is obtained from eddy currents that are induced in the conducting solid stator.
Generally, lower passive damping will increase the level of regenerated energy since the actuator has to provide the damping function; however, safety is com- promised because less passive damping is present when there is a power failure.
Therefore, during a design process, the amount of passive damping will be con- sidered as a variable initially, and the influence on performance and power con- sumption will then be analysed.
A regenerative shock absorber integrated with a linear generator (Zuoet al., 2010) is shown in Figure 6.24. The mechanism converts the kinetic energy from vibration between the wheel and a sprung mass into useful electrical energy. The shock absorber consists of an integrated magnet and a coil assembly. The integrated magnet is made with ring-shaped PMs and ring-shaped high permeability spacers stacked on a rod of high reluctance material. The magnets are inserted with like- poles of adjacent magnets facing each other to redirect the magnetic flux to a radial
Stator
Armature winding
Coil spring Mover PM
Figure 6.23 Linear machine–based energy harvester assisted by a passive coil spring
direction. A concentric outer cylinder made of a high permeability material is used to reduce the reluctance of the magnetic circuits, and hence, further enhances the magnetic flux density in the coils. The coil assembly is made of copper coils wound on a Delrin tube. As the copper coils traverse inside the magnetic field, a voltage will be generated and the coils will be connected to rectifier circuits.
6.3.5 Modelling of suspension systems
The vehicle suspension system can be established in three models, namely, the full vehicle model, half-vehicle model and quarter-vehicle model, as shown in Figure 6.25 (Chi et al., 2008; Montazeri-Gh and Soleymani, 2010; Marzbanrad et al., 2013). The full vehicle model is most comprehensive with seven DOF. These comprise three DOF for the vehicle body (pitch, bounce and roll) and a further vertical DOF at each of the four unsprung masses. The half-vehicle model has four DOF where the roll information is not accounted for. The quarter-vehicle model has two DOF where the pitch information is lost (Happian-Smith, 2002).
In the designing of shock absorbers and the establishing of suspension system models, full-vehicle and half-vehicle models are more comprehensive than the quarter-vehicle model. The quarter-vehicle model is acceptable and with less complexity in the mathematical formula. The suspension system is considered in this section because it is easier to regenerate energy. An active suspension control system based on the quarter-vehicle model is shown in Figure 6.26 (Okada and Harada, 1996).
Equation of motion of the suspension system is given by:
m€xðtị ỵk
xðtị x0ðtị
ẳfðtị (6.16)
where m is the sprung mass,k is the physical spring constant,xand x0are the displacement of sprung mass and unsprung mass, respectively, and f is the actuator force.
Armature
winding Stator Mover
Magnet Iron pole
Figure 6.24 Linear machine–based energy harvester
On-board electromagnetic energy regeneration for electric vehicles 173
Similar to the other damping systems, the energy can be regenerated only with high speed motion. For the low speed motion, active control will be applied (Okada and Harada, 1996). The actuator force is calculated by:
fðtị ẳ y2
R ðvv0ị ỵy
Reb; eb
y<ðvv0ị KpxKvv; eb
y <ðvv0ị<eb
y y2
R ðvv0ị ỵy
Reb; ðvv0ị<eb
y 8>
>>
>>
><
>>
>>
>>
:
(6.17)
Sprung mass
Unsprung mass (a)
(b)
(c)
Unsprung mass
Sprung mass
Unsprung mass
Sprung mass
Figure 6.25 Modelling of vehicle suspension systems: (a) full-vehicle model, (b) half-vehicle model and (c) quarter-vehicle model
whereyis the actuator constant,Ris the load resistance,vis the actuator velocity, v0is the boundary velocity for energy regeneration,ebis the battery voltage,Kpis the displacement gain andKvis the velocity gain.
The regenerative energy is calculated using the following equations (Okada and Harada, 1996):
W ẳ y R
yðvv0ị eb
ðvv0ị Ri2; eb
y<ðvv0ị Ri2 ðvv0ịðKpxỵKvvị; eb
y <ðvv0ị<eb
y y R
yðvv0ị ỵeb
ðvv0ị Ri2; ðvv0ị<eb
y 8>
>>
>>
><
>>
>>
>>
:
(6.18)
A quarter-vehicle model is used as shown in Figure 6.27 (Huanget al., 2011), with the assumption that all springs are linear within the maximum available working space and tire damping is negligible. The output force of the actuator comprises the electromagnetic vertical force, Fi, and the damping force, Fc. The latter comes from mechanical friction and is assumed to be linear and also pro- portional to the stroke speed of the actuator.
The electromagnetic vertical force,Fi, and the damping force,Fc, are expres- sed by:
Fiẳkiiẳ2Fcotj
r i (6.19)
FcẳCsvẳCsð_xwx_bị (6.20)
where F is the stator flux linkage, ki is the vertical force coefficient, r is the effective radius for force conversion,jis the screw lead angle,Csis the damping coefficient, xw is the unsprung mass displacement and xb is the sprung mass displacement.
m Sprung mass
Unsprung mass
Actuator x
x0
Gc(S)
Figure 6.26 Active suspension control system
On-board electromagnetic energy regeneration for electric vehicles 175
Thus, the model equations of motion can be written as follows:
mb€xbẳksðxwxbị ỵCsð_xwx_bị ỵkii (6.21) mw€xwẳ ksðxwxbị ỵktðxgxwị Csð_xwx_bị kii (6.22) wherembis the sprung mass,mwis the unsprung mass,ksis the spring stiffness and ktis the stiffness of a tire.
In conventional passive suspension systems, a significant amount of kinetic energy will be dissipated as heat, especially when excited by high-frequency input attenuation. Another important fact is that undesired high-frequency vibrations are usually orthogonal to the direction of a vehicle’s movement. Therefore, the men- tioned attenuation does not produce additional load to the drivetrain. The operating principle of a regenerative shock absorber is a magnetic element moving inside a coil that induces an electromotive force in the coil. The relative movement of the sprung and unsprung masses due to road roughness or inertial forces is the mechanical power input for the regeneration process. Moreover, the current of the coil induces a motion opposing mechanical force, producing a controlled variable attenuation of the undesired movements. The configuration not only attenuates vibrations but also transforms the kinetic energy into useful electrical energy, which is stored in the battery system for range extension.
Power regeneration is coupled to the damping level, based on the following model (David and Bobrovsky, 2011). The induced electromotive force, EInd, is
Fi mb
Actuator Fc
mw
xg
xw
xb
Figure 6.27 Quarter-vehicle model