16-08-2012, 11:26 AM
Regenerative Braking on the Lotus Renewable Energy Vehicle
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Introduction
Overview
The burning of fossil fuels in transportation has a significantly adverse effect on the
environment and contributes to global warming. Additionally, world oil supply is limited and
is expected to run out by 2045 (Shuster 2008). In order to free the world from its dependence
on fossil fuels and to allow mankind to live in a sustainable manner, alternative transportation
technologies need to be developed. The REV project is developing electrically powered
Renewable Energy Vehicles to showcase the viability of alternative technologies. One of the
problems with electric vehicles is that they have a lower range per charge than conventional
Internal Combustion Engine (ICE) vehicles have per tank. To increase the range, electric
vehicles are able to take advantage of regenerative braking, whereby the motor is run as
generator during braking to convert kinetic energy into electrical energy to charge the battery.
The effectiveness of a regenerative braking system depends on the driving conditions as well
as the way the vehicle is driven. However, studies have shown that a regenerative braking
system can increase a vehicle's range by 10-15% (Dhameja, 2001).
Vehicle Stability
As can be seen from Figure 2.3, the actual curve will only intercept the ideal curve at one
point, and it is at this point only that the front and rear tires will lock up at the same time. This
point represents a specific road adhesive coefficient, μ. For all road adhesive coefficients less
than μ, the front wheels will lock up before the rear wheels, whereas for road adhesive
coefficients greater than μ, the rear wheels will lockup first. This has important implications
for the vehicle stability. If the rear wheels lock, the vehicle will lose directional stability as the
rear wheels lose the ability to resist lateral forces (Ehsani et al, 2009). If a slight lateral force
is initiated, such as that from wind or a centrifugal force, the car will spin out of control. In
contrast, if the front wheels lock, the vehicle will lose directional control, but not directional
stability (Ehsani et al, 2009). This means that the driver will lose the ability to steer the
vehicle left or right, but the vehicle will remain on a straight path. Loss of directional control
may be rectified by releasing the brakes slightly.
Regenerative Braking Systems
The fundamental principles of regenerative braking are based on the laws of
electromagnetism. When a conductor is placed in a changing magnetic field, a voltage will be
induced across the conductor. This voltage is known as the electromotive force (emf). In
accordance with Lenz’s law, the emf induced in an electric circuit always acts in such a
direction that the current it drives around the circuit opposes the change in magnetic flux
which produced the emf. In the context of generators, the emf is known as back emf and it
will produce an electromagnetic force in accordance with the Lorentz force law which applies
a negative torque on the rotor and causes the rotor to decellerate. If there is a circuit connected
to the terminals of the emf source, current will be driven through the circuit. In an Electric
Vehicle, a regenerative braking system works by running the electric motor as a generator.
The kinetic energy or mass potential energy provides the driving force for the generator. The
battery circuit is connected to the terminals of the motor and current is driven into the
batteries. At the same time, the motor slows down due to the negative torque acting on it and
this produces the braking effect.
Series Braking with Optimal Feel
Series Braking with optimal feel uses a braking controller to control braking forces on the
front and rear wheels. When the commanded deceleration (represented by the position of the
brake pedal) is low, only regenerative braking is used. For more aggressive deceleration, the
controller adjusts brake force such that the distribution between front and rear follows an
optimal curve. The regenerative braking will be used up to its maximum torque and friction
braking will be used to provide the rest of the necessary force. The maximum torque of the
motor is related to its speed. At lower speeds, maximum torque is constant but at higher
speeds, it decreases hyperbolically with speed. Therefore, the applied frictional brake force at
a given deceleration will vary with speed. As a result, active control of the friction brakes is
necessary. This adds a large degree of complexity to the system. It requires that there is an
electrical system in place to actuate both the friction and regenerative braking systems
simultaneously.
Conclusions and Future Work
From the literature review it was determined that regenerative braking is an effective way to
extend the range of an electric vehicle. The analysis showed that adding regenerative braking
in a parallel system would enhance the braking of the car and make it safer at the same time.
The design work was carried out effectively and the final system for both the brake and the
accelerator worked well. The objective to install a regenerative braking system on the REV
Elise was successfully attained.
The immediate future work is to fine tune the braking setup and test it with a multimeter to
ensure that it delivers the correct output. Once the REV Elise is completed, the system will
need to be tested while driving. It will then be possible to fine tune the parameters in the
software to get the optimum combination of braking feel and energy recovery. An analysis
should then be carried out to determine exactly how much energy is recovered and how far the
regenerative system is able to extend the vehicle’s range. Another small future task is to allow
the regenerative system to be turned on and off at the touch of a button, within the car. At the
moment, the regenerative braking can only be turned off by programming the motor controller
with external software and setting all maximum torque values to zero.