GB2483477A

GB2483477A - Control of regenerative and friction braking

Info

Publication number: GB2483477A
Application number: GB1014970.6A
Authority: GB
Inventors: Timothy Peter Martin; Arthur Thomas Wolstonholme; John William Anderson Paterson
Original assignee: LIGHTNING CAR Co Ltd
Current assignee: LIGHTNING CAR Co Ltd
Priority date: 2010-09-09
Filing date: 2010-09-09
Publication date: 2012-03-14
Anticipated expiration: 2030-09-09
Also published as: GB2483477B; GB201014970D0

Abstract

A method of reducing the speed of an electric motor vehicle through a braking process, comprising the step of receiving an indication to apply braking to the vehicle. The method further identifies a braking condition as being in accordance with a first mode of braking or in accordance with a second mode of braking. The method then applies regenerative braking only upon identifying said first mode of braking and applies mechanical braking in addition to or instead of regenerative braking upon identifying the second mode of braking. The first and second modes may be based on distance of travel of the brake pedal, speed of the vehicle, adaptive cruise control system, detection of a fault, parking or the vehicle being stationary.

Description

I

Braking

CROSS REFERENCE TO RELATED APPLICATIONS

This application represents the first application for a patent directed towards the invention and the subject matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of reducing the speed of an electric vehicle through a braking process.

2. Description of the Related Art

It has been known for some time that it is possible to effect braking of an electric vehicle by effectively reversing the functionality of the electric motor such that instead of drawing electrical power and driving the wheels of the vehicle, it is possible for mechanical energy received from the wheels to drive the motor (which then becomes a generator) which in turn may be used to re-energise the battery. Thus, such an approach reduces wear because the braking process is not relying upon friction. Furthermore, power is returned to the battery thereby prolonging range and improving power consumption.

This process of regeneration is often seen as highly desirable but in practice it can be difficult to implement. In theory, it would be possible to construct an electric vehicle that relies exclusively upon regenerative braking, except possibly for the provision of a parking brake. Thus, all braking would be achieved by regenerative methods while the vehicle was in motion. Two problems exist with this exclusive approach in that a reluctance exists in terms of relying upon systems of this type, although this may change over a period of time. Furthermore, it is difficult to introduce failsafe procedures for regenerative braking therefore, currently, this application is limited.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a method of reducing the speed of an electric motor vehicle through a braking process, comprising the steps of receiving an indication to apply braking to the vehicle; identifying a braking condition as being in accordance with a first mode of braking or in accordance with a second mode of braking; applying regenerative braking only upon identifying said first mode of braking; and applying mechanical braking in addition to or instead of said regenerative braking upon identifying said second mode of braking.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figure 1 shows a system for reducing the speed of an electric motor vehicle through a braking process; Figure 2 shows the electric vehicle provided with a foot operated accelerator and a foot operated braking pedal; Figure 3 shows an example of regenerative braking; and Figure 4 shows diagrammatic representations of the percentage travel of a braking command.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 A system for reducing the speed of an electric motor vehicle through a braking process is shown in Figure 1. A processing device 101 receives an indication to apply braking from a brake application device 102. Upon receiving this command to apply braking, a processor 101 identifies the braking condition as being in accordance with a first mode of braking or in accordance with a second mode of braking. The processor 101 provides an output signal to a regenerative braking control subsystem 103 which will cause the vehicle to brake by the application of regenerative braking, detailed in Figure 3. The processor 101 may also apply control signals to a hydraulic braking subsystem 104, configured to apply a mechanical brake so as to reduce velocity or bring the vehicle to a stop in a more conventional manner.

The processor 101 applies regenerative braking only upon identifying the first mode of braking. Upon identifying the second mode of braking, the processing device applies mechanical braking either in addition to the regenerative braking or as an alternative to the regenerative braking.

In order for the processor 101 to make a decision as to whether the first mode of braking should be applied or the second mode of braking should be applied, an embodiment provides additional information to the processor 101 concerning the condition of the vehicle. In the example of Figure 1, processor 101 receives an indication of speed from a velocity measuring device 105 and receives an indication as to whether cruise control is being used from a cruise control indicator 106. In an embodiment, the system also includes a fault detection subsystem 107 associated with the regenerative braking subsystem 103. Thus, information is provided to processor 101 upon detection of a fault, Is which again may have an influence upon whether the first mode of operation or the second mode of operation is selected.

Figure 2 Within a vehicle operating substantially under electronic control, it is possible for many types of interface devices to be deployed in order to cause an activation of the braking system. However, in an embodiment, the electric vehicle is designed to operate in a way that is familiar to drivers of conventional hydrocarbon powered vehicles.

In an embodiment, the electric vehicle is provided with a foot operated accelerator 201 and a foot operated braking pedal 202. Thus, the brake application subsystem 102 includes pedal 202 and the subsystem 102 is configured to determine the extent to which the pedal 202 has been depressed, or the extent to which force has been applied to the pedal 202.

Conventionally, the application of a brake pedal results in pressure being applied to a hydraulic system in which the pedal returns to its original condition when pressure is removed. This in turn gives the pedal a particular mechanical feel which provides feedback to the driver and ensures that the pedal is depressed by the requisite amount in order to obtain the desired level of braking. Many modern vehicles have moved away from this system and towards a "drive by wire" approach in which the pedal is effectively mechanically detached from the braking system itself and electric sensors are used to determine the extent of braking required. It is also known for braking systems to include varying degrees of servo assistance so again feedback mechanisms to the driver to achieve the required level of brake activity are somewhat artificial. In the present embodiment, pedal 202 is therefore provided with appropriate springing mechanisms in order to give the driver a recognisable feel but data relating to the level of pedal activity is, in the embodiment, related to processor 101 as a digital signal. This digital signal identifies the degree of movement of the pedal or the extent to which force has been applied to the pedal.

Thus, in an embodiment, the first mode of braking is identified in response to the brake pedal 202 being depressed by a predetermined degree (distance or force) and the second mode of braking is identified in response to the brake pedal being depressed beyond said predetermined degree. Thus, in operation, a first movement of brake pedal 202 will result in the first braking mode being selected and the braking process being achieved exclusively by regenerative braking. However, should an emergency condition arise for example, the brake pedal 202 is pressed further, beyond the predetermined limit, resulting in the hydraulic braking system 104 being activated. Thus, in the embodiment, mechanical braking is used in combination with the regenerative braking.

In many systems using regenerative braking, a problem exists in that the braking may be applied automatically whenever the accelerator pedal is released. This is disconcerting for many drivers as the car starts to brake before the driver has taken action to invoke the braking systems. In an alternative embodiment, it is possible for a first proportion of the braking degree to be provided by mechanical braking so as to provide a familiar feel.

Once the braking degree reaches a predetermined level, say, above ten percent, the mechanical braking is replaced by regenerative braking.

Thereafter, in accordance with the present invention, mechanical braking is reintroduced should the degree of braking exceed a second threshold of, say, seventy percent. Thus, it is possible to have a sophisticated system in which the braking is initiated by mechanical braking, subsequently replaced by regenerative braking and then again reintroducing mechanical braking. Such a procedure introduces a feel to the driver in an electric performance car that is substantially similar to that experienced in high performance petrol cars.

Figure 3 An example of regenerative braking is illustrated in Figure 3. Under normal operation, a battery 301 supplies electrical energy to a motor 302. To reduce weight and improve the efficiency of motor 302, the motor is a synchronous AC motor and as such requires an alternating current. An output from battery 301 is supplied to an inverter 303 which then supplies alternating current to the motor 302 via a switching system 304. The flow of power to the motor 302 is indicated by arrow 305 and arrow 306. Arrow 305 and arrow 306 do not represent current flow, however, due to the alternating nature of the current.

In response to a command for regenerative braking, switching system 304 stops providing power to the motor 302 and now receives power from the motor 302, as represented by arrow 307 and arrow 308. The alternating regenerated current generated by motor 302 is returned to the battery 301 through a rectifying system 309 such that, by the regenerative process, current flow through the motor 302 resists mechanical rotation (thereby achieving braking) while the power generated is returned to battery 301. However, it is appreciated that sophisticated electronics are required within rectifying circuit 309 in order to return regenerated power to the battery 301 in a form that allows the battery to be recharged without causing damage to the battery, as is known to those skilled in the art.

Figure 4 In an embodiment, the motor vehicle includes an adaptive cruise system 106. The adaptive cruise system operates in a manner similar to a conventional cruise control, controlling the vehicle such that it maintains a selected speed. However, an adaptive cruise control system is provided with radar detectors which will identify the presence of slower moving vehicles ahead and automatically compensate so as to maintain a predetermined distance between the vehicle and the vehicle in front. In an embodiment, the first mode of braking is selected if the command for braking originates from the cruise control. A diagrammatic representation of this approach is shown in Figure 4. An arc 401 illustrates the travel of a braking command which, for adaptive cruise, will originate form the cruise control system and not from the application of the driver's foot. The example shows that when a call for braking is made under adaptive cruise control, one hundred percent of the braking is achieved using regenerative braking and the automatic system does not make any use of the mechanical hydraulic system.

An alternative mode of operation is illustrated at 402. On this occasion, the vehicle is travelling at high speed and it is accepted that at high speed regenerative braking is more effective. Consequently, when travelling at high speed the first seventy percent of the demand for braking will be met by regenerative braking. Thus, brake pedal 202 may be depressed by a full seventy percent of its travel before the mechanical braking system will be activated. Thus, the second mode of braking is only achieved after seventy percent of the call for braking has been deployed.

An alternative arrangement is shown at 403 in which the vehicle is travelling at low speed. On this occasion, it is appreciated that regenerative braking is less effective and therefore the first mode of operation is only present for the first thirty percent of the call for braking. Thus, after foot pedal 202 has travelled by only thirty percent of its full distance, further activation will introduce mechanical braking, with less reliance overall being made of the regenerative system.

Arc 404 illustrates a situation which occurs when the vehicle is stationary. When the vehicle is switched off, parking brakes may be applied, possibly automatically. However, alternative braking systems are required if the vehicle stops temporarily, in traffic or in response to traffic control measures. Under these circumstances, it would be possible for the vehicle to be held stationary using the regenerative process or at least partially by activating the electric motor in reverse. However, in the embodiment this is seen as an unacceptable waste of power therefore it would be preferable for mechanical braking to be deployed. Thus, as soon as a detection is made to the effect that the vehicle is stationary, no braking whatsoever is provided by the regenerative system and one hundred percent of braking is provided by the mechanical system. Thus, when the vehicle is stationary, a slight activation of foot brake 202 will cause the mechanical braking system to be deployed.

It is also possible for an embodiment to be provided with parking is sensors and the sensors may in turn automatically generate a demand for braking. In an embodiment, a second mode of braking would be identified in response to a command for braking being generated by a parking sensor.

As previously stated, a subsystem 107 is provided for the identification of faults. In the embodiment, the second mode of braking is identified in response to entering a modified driving condition following the detection of a fault. Thus, a fault may be encountered and a driver may be informed that the vehicle requires attention although some further driving is allowed in order for the driver to reach a safe parking location or to reach a location where the vehicle may be serviced. Under these situations, operation of the vehicle is modified (possibly restricting speed) until the repairs have been performed. In an embodiment, part of this modification may involve disabling the regenerative braking system and relying exclusively upon mechanical braking.

Claims

BClaims 1. A method of reducing the speed of an electric motor vehicle through a braking process, comprising the steps of: receiving an indication to apply braking to the vehicle; identifying a braking condition as being in accordance with a first mode of braking or in accordance with a second mode of braking; applying regenerative braking only upon identifying said first mode of braking; and applying mechanical braking in addition to or instead of said regenerative braking upon identifying said second mode of braking.
2. The method of claim 1, wherein: said first mode of braking is identified in response to a brake pedal being depressed by a predetermined distance; and said second mode of braking is identified in response to said brake pedal being depressed beyond said predetermined distance.
3. The method of claim 2, wherein said predetermined distance increases as the speed of the vehicle increases.
4. The method of claim 1, wherein said second mode of braking is identified if the speed of the vehicle is below a specified speed.
5. The method of claim 4, wherein only mechanical braking is available below said specified speed.
6. The method of claim 4, wherein only mechanical braking is available when the vehicle is stationary.
7. The method of claim 1, wherein said first mode of braking is identified in response to receiving an indication to apply braking from an adaptive cruise control system.
8. The method of claim 1, wherein said first mode of braking is identified in response to receiving an indication to apply braking from a speed monitoring system.
9. The method of claim 1, wherein said second mode of braking is identified in response to receiving an indication to apply braking from a parking sensor.
10. The method of claim 1, wherein said second mode of braking is identified in response to entering a modified driving condition following the detection of a fault.
11. An apparatus for reducing the speed of an electric motor vehicle through a braking process, comprising: a processing device configured to receive an indication to apply braking to the vehicle and identify a braking condition as being in accordance with a first mode of braking or in accordance with a second mode of braking; wherein said processing device applies regenerative braking only upon identifying said first mode of braking and applies mechanical braking in addition to or instead of said regenerative braking upon identifying said second mode of braking.
12. The apparatus of claim 11, wherein: said first mode of braking is identified in response to a brake pedal being depressed by a predetermined distance; and said second mode of braking is identified in response to said brake pedal being depressed beyond said predetermined distance.
13. The apparatus of claim 12, wherein said predetermined distance increases as the speed of the vehicle increases.
14. The apparatus of claim 11, wherein said second mode of braking is identified if the speed of the vehicle is below a specified speed.
15. The apparatus of claim 14, wherein only mechanical braking is available below said specified speed.
16. The apparatus of claim 14, wherein only mechanical braking is available when the vehicle is stationary.
17. The apparatus of claim 11, wherein said first mode of braking is identified in response to receiving an indication to apply braking from an adaptive cruise control system.
18. The apparatus of claim 11, wherein said first mode of braking is identified in response to receiving an indication to apply braking from a speed monitoring system.
19. The apparatus of claim 11, wherein said second mode of braking is identified in response to receiving an indication to apply braking from a parking sensor.
20. The apparatus of claim 11, wherein said second mode of braking is identified in response to entering a modified driving condition following the detection of a fault.
21. A method of reducing the speed of an electric motor vehicle through a braking process, substantially as herein described with reference to the accompanying Figures.
22. An apparatus for reducing the speed of an electric motor vehicle through a braking process, substantially as herein described with reference to the accompanying Figures.