GB2562308A

GB2562308A - Regenerative braking control system

Info

Publication number: GB2562308A
Application number: GB1707664.7A
Authority: GB
Inventors: Leszczynski Mirek
Original assignee: Arrival Ltd
Current assignee: Arrival UK Ltd
Priority date: 2017-05-12
Filing date: 2017-05-12
Publication date: 2018-11-14
Anticipated expiration: 2037-05-12
Also published as: GB201707664D0; GB2562308B

Abstract

A vehicle regenerative braking control system limits the maximum regenerative braking force which can be applied to each wheel of the vehicle. A memory 202 stores a tyre model 210, and a controller 202 receives data from at least one sensor (e.g. tyre pressure sensor 206, tyre temperature sensor 208, axle load sensor 212) associated with each wheel of the vehicle, and the data is input into the tyre model. The controller uses the tyre model to determine a maximum usable traction for each wheel and calculate the maximum regenerative braking force to be applied to each wheel, wherein the braking force is limited according to the calculation. The tyre model may comprise a maximum tolerated slip ratio, which may be set dynamically using sensor information, and which is used by the controller to limit an output of the tyre model when calculating the maximum regenerative braking force for each wheel. The controller may receive data relating to surface conditions under the wheels, which is input into the tyre model and used to determine the maximum usable traction for each wheel. The surface condition data may be received from one or more optical sensors (e.g. camera 216) that image the vehicles external environment.

Description

Regenerative Braking Control System

Technical Field

This specification relates to a regenerative braking control system in an electric or hybrid vehicle.

Background

In recent times, there has been significant interest in electric and hybrid vehicles which are regarded as more environmentally friendly than conventional petrol/diesel vehicles. Such vehicles include both passenger vehicles for personal use and commercial vehicles such as buses and trucks. Many electric vehicles use a regenerative braking system in which energy is recovered during braking and used to charge the onboard batteries. However, under certain low traction surface conditions, the amount of regenerative braking applied to the wheels may cause wheel slippage. Techniques to alleviate this problem are therefore required.

Summary A first aspect of the invention provides a regenerative braking control system comprising: a memory storing a tyre model; and a controller configured to: receive data from at least one sensor associated with each wheel of a vehicle; input the received data into the tyre model; use the tyre model to determine a maximum usable traction for each wheel of the vehicle; calculate the maximum regenerative braking force to be applied to each wheel of the vehicle; and limit the maximum regenerative braking force which can be applied to each wheel of the vehicle according to the calculation.

This control system prevents lock-up or excessive skidding of the wheels under braking, which would normally cause the regenerative braking system to be cut off completely. The control system therefore also improves the safety and drivability of the vehicle. A cut-off of the regenerative braking system also means that no energy is being recovered by the system for a time. The overall fuel economy of the vehicle can therefore also be improved by use of this control system since the regenerative braking force is limited, rather than being cut off. The control system results in a more refined control of the regenerative braking system since it takes into account the present tyre conditions and other factors which affect the amount of traction available to the tyres at any moment.

The at least one sensor may comprise a tyre pressure sensor. The at least one sensor may comprise an axle load sensor. Both tyre pressure sensors and axle load sensors maybe present.

The at least one sensor may comprises a tyre temperature sensor. The pressure in the tyre may be determined from or refined by the information from the tyre temperature sensor.

The tyre model may comprise a maximum slip ratio to be tolerated. The controller may be configured to calculate the maximum regenerative braking force to be applied to each wheel of the vehicle by limiting an output of the tyre model using the maximum slip ratio. The maximum slip ration to be tolerated maybe 10%. The maximum slip ration to be tolerated may be dynamically determined based on information from the at least one sensor. The controller may be configured to dynamically set the maximum slip ration to be tolerated based on information received from the at least one sensor.

The tyre model stored in the memory may comprise information on the make and model of the wheels of the vehicle. The tyre model stored in the memory may comprise information on the mileage of the wheel and the expected rate of degradation of the wheel.

Limiting the maximum regenerative braking force which can be applied to each wheel of the vehicle according to the calculation comprises distributing the regenerative braking force unequally between a rear axle and a front axle of the vehicle and/or distributing the regenerative braking force unequally between each of the wheels of the vehicle.

The controller may be further configured to: receive data relating to surface conditions under the wheels; input the data relating to surface conditions into the tyre model; and use the tyre model to determine a maximum usable traction for each wheel of the vehicle taking into account the surface conditions.

The data relating to surface conditions may be received from one or more optical sensors configured to image the external environment around the vehicle. The data relating to surface conditions may comprise road network information. The at least one sensor may comprise a tyre pressure sensor and/or a tyre temperature sensor and/or an axle load sensor. A second aspect of the invention provides a method of controlling a regenerative braking system, the method comprising: receiving data from at least one sensor associated with each wheel of a vehicle; inputting the received data into a tyre model; using the tyre model to determine a maximum usable traction for each wheel of the vehicle; calculating the maximum regenerative braking force to be applied to each wheel of the vehicle; and limiting the maximum regenerative braking force which can be applied to each wheel of the vehicle according to the calculation. A third aspect of the invention provides a computer program comprising computer executable instructions which, when executed by a processor, cause the method of the second aspect to be performed. A fourth aspect of the invention provides a computer readable medium containing computer readable instructions which, when executed by a processor, cause the method of the second aspect to be performed.

Brief description of the figures

For the purposes of example only, embodiments are described below with reference to the accompanying figures, in which:

Figure la is a schematic overview of an electrically powered vehicle, comprising first and second battery modules, first and second drivetrain modules and a range extender module;

Figure lb illustrates a chassis of an exemplary electrically powered vehicle;

Figure 2 shows a schematic of a regenerative braking control system according to some embodiments of the invention

Figure 3 is a flowchart illustrating exemplary operation of the regenerative braking control system of Figure 2; and

Figure 4 is a graph illustrating one part of the tyre model for a particular tyre.

Detailed description

Figure la is a schematic illustration of an electrically powered vehicle too. The vehicle may, for example, be a commercial vehicle such as a truck or a bus. Figure lb is a perspective illustration of an exemplary chassis of the same electrically powered vehicle too.

The vehicle too comprises a plurality of modules installed within a chassis 101 of the vehicle too. In the example of figures la and lb, the modules include first and second high-voltage battery modules 102,103 and a range extender module 104. The skilled person will appreciate that the nominal voltages of the first and second battery modules 102,103 will vary in dependence on the specific batteries chosen for the vehicle too. However, as an example, the first and second high-voltage battery modules 102,103 may each have a nominal voltage in a range between 250V and 750V.

The plurality of modules installed in the chassis 101 also include a first drivetrain module 105, located towards the front end of the vehicle too, and a second drivetrain module 106, located towards the rear end of the vehicle too. The first drivetrain module 105 drives the front wheels 107 of the vehicle too in order to cause forward/rearward motion. Likewise, the second drivetrain module 106 drives the rear wheels 108 of the vehicle too to cause corresponding forward/rearward motion. The first battery module 102, second battery module 103 and range extender module 104 may be mounted between the front and rear drivetrain modules 105,106, towards the middle of the chassis 101.

The first and second drivetrain modules 105,106 each comprise at least one electric motor-generator unit (not shown) and at least one driveshaft (not shown). Each driveshaft is mechanically coupled to at least one wheel and is caused to rotate by the electric motor-generator unit, for example via a gearbox, in order to cause corresponding rotation of the wheel. Each motor-generator unit may comprise a separate motor and generator or a combined motor-generator. The motor-generator units also form part of a regenerative braking system. Under braking, energy is recovered by switching the motor-generator units to generator mode. Mechanical energy from the wheels is converted to an electrical load which is used to recharge the battery modules 102,103.

The electric motors of the first and second drivetrain modules 105,106 receive electrical power from the battery modules 102,103, for example via a power inverter unit (not shown). It will be appreciated that this drains the electrical energy stored in the battery modules 102,103. The draining of energy from the battery modules 102, 103 can be counteracted by the range extender module 104, which is configured to supply electrical energy to the battery modules 102,103 for re-charging. In this example, the range extender module 104 comprises an internal combustion engine.

The internal combustion engine can be of any suitable type, powered by combustion of any suitable fuel e.g. petrol, diesel, LPG. A mechanical output of the combustion engine is connected to an electrical power-generating unit (not shown), which may include an alternator, in order to supply electrical energy to the battery modules 102,103. The range extender module 104 may also be configured to supply power directly to the drivetrain modules 105,106. A safety feature of some regenerative braking systems is that, if it is detected that a wheel is under-rotating i.e. if the wheel is slipping, then the regenerative braking system is cut off completely. This leads to both safety and drivability issues. When the regenerative braking system is cut while the electric vehicle is braking, this feels to the driver like the vehicle is accelerating because some braking force is immediately removed. This is potentially unsafe and leads to bad drivability from the user’s point of view. A driver may experience a loss of control of the vehicle or a feeling of the loss of control. A driver may also apply a greater breaking force than necessary when the regenerative braking system cuts out in response to the feeling of acceleration. In some cases this could then cause a further locking up of the wheels and loss of control. The cut off of the regenerative braking system also reduces the overall fuel economy of the vehicle since no energy is being recovered by the regenerative braking system for a time.

Thus a need for more refined control of regenerative braking systems is required which takes into account tyre conditions and other factors which affect the amount of traction available to the tyres at any moment.

Figure 2 shows a schematic of a regenerative braking control system 200 according to some embodiments. However, not all of the component illustrated need be included in every system, as will now be described.

The regenerative braking control system 200 comprises a controller 202, which maybe a processor, microprocessor, microcontroller or other suitable processing apparatus and a regenerative braking system 204. The regenerative braking control system 200 also comprises a memory 202, which may be a rewritable memory such as a flash drive. The controller 202 is configured to receive data from a number of sensors, to access data in the memory 202 and to send commands to the regenerative braking system 204. The controller 202 may comprise on chip volatile memory such as a RAM module, or may access an external RAM module in order to process data.

The memory 202 may store data and software modules which are executable by the controller 202. The memory 202 stores a tyre model 210 which may comprise both data for various different makes and models of tyres and also executable code.

The regenerative braking control system 200 comprises a number of tyre pressure sensors 206. For example each of the wheels of the vehicle 100 may have an internal tyre pressure sensor 206. The pressure of the tyre affects the amount of the tyre surface which contacts the road surface under a given load. In general, the lower the pressure, the greater the amount of surface contact between tyre and road. Alternatively, or in addition, the regenerative braking control system 200 may comprise a number of tyre temperature sensors 208. The controller 202 may use data from the tyre temperature sensors 208 to calculate a tyre pressure using the tyre model 210. Data from both the tyre pressure sensor and the tyre temperature sensor may be combined to calculate the true pressure in the tyre.

The regenerative braking control system 200 also comprises at least one axle load sensor 212. In some embodiments, the axle load sensor 212 is an air suspension sensor. For example, the air suspension sensor may measure the air pressure inside the suspension system of the axle and the controller may use this information to calculate the load on the axle and normal force (Fz) on the respective wheels. The normal force of the wheel affects the amount of the tyre surface which contacts the road surface for a given tyre pressure. Therefore information from both the tyre pressure sensors 206 and the axle load sensors 212 may be combined to determine the amount of tyre surface in contact with the road surface at any moment. The regenerative braking control system 200 may have two axle load sensors 212, one for each axle of the vehicle. Alternatively, each wheel of the vehicle may have an axle load sensor 212 (which therefore also maybe refereed to as a wheel load sensor 212).

The regenerative braking control system 200 may optionally comprise one or more of an external temperature sensor 214, an external camera 216, and a road network information module 218 stored in the memory 202. Data from the external temperature sensor 214 may be used to infer the likely road conditions, for example whether it is more likely to be icy. Images from the external camera 216 may also be used to infer the likely road conditions, for example by determining whether the ground conditions are wet or icy. To this end, the memory 202 may also store image analysis software (not shown). Information in the road network module 218 may indicate the type and quality of road surfaces and also the gradient of the road surface. This information may be combined with GPS data indicating the vehicles location to determine the likely ground conditions.

The regenerative braking control system 200 may further comprise a wireless transceiver 220 through which the controller 202 may send and receive data. The regenerative braking control system 200 may receive updates for the tyre model 210 and road network information module 218 and may send information about the performance of the system.

All of the components described above may be interconnected via a bus (not shown) or by other suitable interfaces.

In operation, the controller 202 regularly receives data from the tyre pressure sensor 206. The controller executes the tyre model software 210 and inputs the sensor data into the tyre model. As the parameters and behaviour of each model of tyre will differ, the tyre model software 210 is programmed with the make and model of the tyres currently being used by the vehicle too. In some embodiments, the controller also regularly receives data from one or more axle load sensors 212, such as an air suspension sensor associated with each wheel of the vehicle. From this data, the controller can calculate the normal force (Fz) currently being experienced by each tyre. This affects the amount of usable traction of the tyres. For example, the normal force experience by the front wheels increases up to a point when the vehicle is braking, but the normal force in the rear wheels may decrease under braking. The normal force also changes when the vehicle is cornering.

In some embodiments, the controller may additionally receive data from tyre temperature sensors 208, which may then be used to infer a tyre pressure, an external temperature sensor 214 and/or a camera 216. The tyre model 210 may also be programmed to receive data from these sensors, or information derived from the data output by these sensors.

The controller 202 causes the available data to be input into the tyre model 210. The tyre model may contain values for the friction coefficient of the road surface. The tyre model may adjust these values depending on data in the road network information module 218 and data or risk factors derived from the data received from the camera 216 and external temperature sensor 214. For example, the road network information module 218 may contain the locations of unpaved roads, such as gravel roads and dirt tracks, which are likely to have a lower coefficient of friction than a paved road. As a further example, images form the camera may be analysed to determine the albedo of the road surface and then to determine if that surface is wet, icy or snowy. The road network information module 218 may also contain information on the gradient of the road.

The tyre model 210 may also comprise information on the mileage of each wheel and the expected rate of degradation of the tyres under normal use. This information may be updated regularly as the vehicle too is used.

The output of the tyre model is a maximum usable traction for each wheel or each set of wheels of the vehicle. The controller 202 then sends the information on the maximum usable traction to a controller (not shown) of the regenerative braking system 204 or controls the operation of the regenerative braking system 204 directly. The controller 202 may control the operation for the regenerative braking system 204 such that the maximum regenerative braking force applied to a particular wheel by that wheel’s respective motor-generator unit does not exceed the calculated maximum usable traction for that wheel. In this way, the regenerative braking control system 200 prevents under-rotation of the wheels and skidding of the vehicle under braking.

Figure 3 is a flowchart illustrating exemplary operation of the regenerative braking control system 200.

At step 300, the controller 202 receives data from at least one sensor associated with each wheel of a vehicle. As previously discussed these sensors include tyre pressure sensors or tyre temperature sensors from which the tyre pressures can be calculated. These sensors may also include one or more axle load sensors such as air suspension sensors, external temperature sensors and cameras.

At step 302, the controller 202 inputs the received data into the tyre model 210 stored in the memory 204. The tyre model maybe programmed with a number of algorithms which express the relationship between tyre pressure, wheel load and traction for at least the tyres currently installed on the vehicle. At step 304 the tyre model is used to determine a maximum usable traction for each wheel of the vehicle. At step 306, the tyre model 210, or a separate algorithm, is used to calculate the maximum regenerative braking force to be applied to each wheel of the vehicle. In other words, the maximum amount of mechanical energy which should be converted to electrical load in the motor-generator unit of each wheel before the wheel will under-rotate (lock) is calculated.

At step 308, the regenerative braking system limits the maximum regenerative braking force which can be applied to each wheel of the vehicle according to the calculation in step 306. Limiting the maximum regenerative braking force which can be applied to each wheel of the vehicle may comprise distributing the regenerative braking force unequally between a rear axle and a front axle of the vehicle. For example, under braking when traction conditions are poor, the amount of usable traction in the rear wheels maybe less than the amount of usable traction available to the front wheels. Therefore, regenerative braking load may be moved from the rear wheels to the front wheels to prevent the load on the rear wheels from exceeding the maximum traction available.

Limiting the maximum regenerative braking force which can be applied to each wheel of the vehicle may comprise distributing the regenerative braking force unequally between the left and right sides of the vehicle. For example, when the vehicle is cornering, the load on the wheels on the outside of the corner is greater than the load on the inside wheels. In poor traction conditions and/or under heavy braking, the regenerative braking load may be moved from the inside wheels to the outside wheels to prevent the load on the inside wheels from exceeding the maximum traction available.

Steps 300 to 308 are repeated regularly or continuously, so that the regenerative braking control system 200 can quickly adjust to changes in road surface, tyre pressure and tyre load.

Figure 4 is a graph illustrating one part of the tyre model for a particular tyre. Figure 4 shows the maximum usable traction force (Longitudinal force) which can be applied via a tyre in relation to slip ratio at different downforces (Fz) for a particular tyre pressure. In the context of Figure 4, positive longitudinal force refers to acceleration, while negative longitudinal force refers to braking force.

The slip ratio may be defined as the ratio of the velocity of the tyre at the road contact point to the longitudinal velocity of the vehicle, minus 1. This may be expressed generally using the following equation:

Where VT is the modulus of the velocity of the tyre at the road contact point and Vv is the longitudinal velocity of the vehicle. Alternatively the slip ratio may be expressed as a percentage difference between the velocity of the tyre at the road contact point and the forward velocity of the vehicle. It can be appreciated from the above equation that if Vt =0 and Vv *0, then the wheels are locked while the vehicle is moving forwards and the slip ratio is -1 or -100%. Where Vv= Vt, then the wheels are free rolling and the slip ratio is 0. In general, whenever the vehicle tyres are undergoing accelerative or braking forces there will be some slippage or skidding of the tyres against the road surface. The slip ratio as define above will be negative when the tyres are applying a braking force and positive wen the tyres are applying an accelerative force.

The Graph of Figure 4 illustrates the behaviour of the tyre in both the accelerative and braking force scenarios. For the purposes of limiting the maximum amount of regenerative braking force which should be applied to a particular tyre, only the negative slip ratio area of Figure 4 is relevant. The graph of Figure 4 has three traces. The first trace 402 relates to a low downforce scenario, the second trace 404 relates to a medium downforce scenario and the third tract 406 relates to a high downforce scenario. As can be seen, the amount of longitudinal force (e.g. braking force) which

can be applied for a given slip ratio decreases as the amount of available downforce decreases. In other words, the downforce experience by a tyre is a variable in the tyre model. Data received from the axle load sensors 212 is therefore input into the tyre model to determine the maximum traction-slip ratio relationship accurately.

In using the relationship shown in Figure 4, the tyre model may set a maximum slip ratio which can be tolerated. This is illustrated by the dashed line 408 in Figure 4.

The maximum slip ratio maybe set depending on the driving experience required and the vehicle in question. The maximum slip ratio may also be dynamically determined based on current conditions such as the weather (as determined by the external temperature sensor 214 and/or camera 216). Where these sensors determine that the conditions may icy, the maximum slip ratio may for example be reduced. As one example, the tyre model may define a slip ratio of 10% as the maximum allowable slip ratio. The pressure of the tyre also has an effect on the longitudinal force-slip ratio relationship and is a variable in the tyre model. In general a lower tyre pressure results in more of the tyre surface being in contact with the road surface and an increase in the traction available. If the pressure in the tyre is too low however, then this may be detrimental not only to the handling of the vehicle and wear on the tyre but also to the traction force available through that tyre. Data received from the tyre pressure sensors 206 is therefore input into the tyre model to determine the braking force-slip ratio relationship accurately.

It will be appreciated that the graph of Figure 4 illustrates the longitudinal force-slip ratio relationship for a particular model of tyre in a particular condition. The composition, tread and wear condition of the tyre may all affect this relationship and maybe accounted for by variables in the tyre model.

The examples described above can be used singly or in combination. It will be appreciated that the above described embodiments are illustrative only. Other variants are possible and are within the scope of the claims appended hereto.

Claims

1. A regenerative braking control system comprising: a memory storing a tyre model; and a controller configured to: receive data from at least one sensor associated with each wheel of a vehicle; input the received data into the tyre model; use the tyre model to determine a maximum usable traction for each wheel of the vehicle; calculate the maximum regenerative braking force to be applied to each wheel of the vehicle; and limit the maximum regenerative braking force which can be applied to each wheel of the vehicle according to the calculation.

2. The regenerative braking control system of claim 1, wherein the at least one sensor comprises a tyre pressure sensor.

3. The regenerative braking control system of claim 1 or claim 2, wherein the at least one sensor comprises an axle load sensor.

4. The regenerative braking control system of any of claim 1 to 3, wherein the at least one sensor comprises a tyre temperature sensor.

5. The regenerative braking control system of any preceding claim, wherein the tyre model comprises a maximum slip ratio to be tolerated and wherein the controller is configured to calculate the maximum regenerative braking force to be applied to each wheel of the vehicle by limiting an output of the tyre model using the maximum slip ratio.

6. The regenerative braking control system of claim 5, wherein the controller is configured to dynamically set the maximum slip ration to be tolerated based on information received from the at least one sensor.

7- The regenerative braking control system of any preceding claim, wherein the tyre model stored in the memory comprises information on the make and model of the wheels of the vehicle.

8. The regenerative braking control system of any preceding claim, wherein the tyre model stored in the memory comprises information on the mileage of the wheel and the expected rate of degradation of the wheel.

9. The regenerative braking control system of any preceding claim, wherein limiting the maximum regenerative braking force which can be applied to each wheel of the vehicle according to the calculation comprises distributing the regenerative braking force unequally between a rear axle and a front axle of the vehicle and/or distributing the regenerative braking force unequally between each of the wheels of the vehicle.

10. The regenerative braking control system of any preceding claim, wherein the controller is further configured to: receive data relating to surface conditions under the wheels; input the data relating to surface conditions into the tyre model; and use the tyre model to determine a maximum usable traction for each wheel of the vehicle taking into account the surface conditions.

11. The regenerative braking control system of claim 10, wherein the data relating to surface conditions is received from one or more optical sensors configured to image the external environment around the vehicle.

12. The regenerative braking control system of claim 10 or claim 11, wherein the data relating to surface conditions comprises road network information.

13. The regenerative braking control system of any preceding claim, wherein the at least one sensor comprises a tyre pressure sensor and/or a tyre temperature sensor and/or an axle load sensor.

14. A method of controlling a regenerative braking system, the method comprising: receiving data from at least one sensor associated with each wheel of a vehicle; inputting the received data into a tyre model; using the tyre model to determine a maximum usable traction for each wheel of the vehicle; calculating the maximum regenerative braking force to be applied to each wheel of the vehicle; and limiting the maximum regenerative braking force which can be applied to each wheel of the vehicle according to the calculation.

15. A computer program comprising computer executable instructions which, when executed by a processor, cause the method of claim 14 to be performed.

16. A computer readable medium containing computer readable instructions which, when executed by a processor, cause the method of claim 14 to be performed.