System and method for controlling a four wheel drive vehicle

The invention relates to a four wheel drive hybrid vehicle provided with at least one power train on each wheel set, a first power train (1) including at least one heat engine, a second power train (2) including at least one electric machine, the vehicle also being provided with a friction braking system on each drive wheel and sensor (7). The control system includes: a means (9) for distributing a braking request between the friction braking system and at least one electric machine from a power train, said electric machine being capable of producing a resisting torque; a torque instruction modulation means (10) for modulating torque instructions to braking systems and power trains based on signals coming from the sensors; and a power train control means (8); the distribution means (9), the torque instruction modulation means (10), and the power train control means (8) being capable of dynamically interacting so as to output torque commands to the power trains and to the friction braking systems with a view to promoting the stability of the vehicle.

BACKGROUND

The present invention relates to the field of motor vehicle control systems and, more particularly, to control systems for powertrains and braking devices for hybrid motor vehicles.

Vehicles incorporating electrical machines to propel them are increasingly prized for their quietness and the fuel savings they have to offer.

However, co-ordinating these electrical machines with one another or with other propulsion systems entails advanced control electronics. Moreover, as the braking functions can be provided partly by operating these electrical machines as generators, it is important also to control the braking aspect.

BRIEF SUMMARY

Hence, there is a need for a control system capable of managing the integration of the electrical machines into the propulsion and braking functions of a motor vehicle.

The subject of the present invention is a system and a method for controlling the electrical machines of a four-wheel drive vehicle.

Another subject of the invention is a system and a method for controlling the electrical machines of a four-wheel drive vehicle used as a braking system.

One aspect of the invention defines a system for controlling a motor vehicle of the four-wheel drive hybrid propulsion type equipped with at least one powertrain on each wheelset, a first powertrain comprising at least one combustion engine, a second powertrain comprising at least one electrical machine, the vehicle also being equipped with a friction braking system on each of the driven wheels and with sensors.

The control system comprises a distributing means for distributing a braking request between the friction braking system and at least one electrical machine of a powertrain, said electrical machine being capable of delivering a resistive torque,a modulating means for modulating the torque setpoints intended for the braking systems and for the powertrains as a function of the signals from the sensors,a control means for controlling the powertrains,the brake force distributing means, the torque setpoint modulating means and the powertrain control means being capable of dynamically interacting in order to issue torque commands to the powertrains and to the friction braking systems in order to promote the stability of the vehicle.

The control system may be applied to a vehicle equipped with driver assist means. The means of determining the stability may comprise a braking co-ordinating device capable of taking into consideration in a concerted and prioritized manner the signals from the driver assist means.

The powertrain control means may further comprise an engine torque co-ordinating device capable of taking into consideration in a concerted and prioritized manner the signals from the driver assist means, from the sensors and from the means of determining the stability of the vehicle.

The first powertrain may be connected to the front wheelset and the second powertrain may be connected to the rear wheelset, the torque setpoint modulating means then being capable of limiting the recuperative braking of the rear wheelset in order to promote the grip of said rear wheelset.

The means of determining the stability of the vehicle may comprise a control means able to exert an influence on the friction braking system which does not generate force torque but which does reduce the response time for a later demand.

Another aspect of the invention defines a method for controlling a motor vehicle of the four-wheel drive hybrid propulsion type equipped with at least one powertrain on each wheelset, a first powertrain comprising at least one combustion engine, a second powertrain comprising at least one electrical machine. The control method comprises steps during which:the driver braking request is distributed between the friction braking and the recuperative braking of the electrical machines of the powertrains according to the estimated speed of the vehicle, to the depression of the brake pedal and to the angle through which the steered wheels are turned,ranges of recuperative braking torque supplied by the electrical machines of a powertrain are determined for the front wheelset, for the rear wheelset, and under static and dynamic conditions;braking torques for each friction braking device are determined as a function of the stability of the vehicle,recuperative braking torques for the front wheelset under static conditions, for the rear wheelset under static conditions, for the front wheelset under dynamic conditions and for the front wheelset under dynamic conditions are determined within the ranges of recuperative braking torque previously determined, the braking torques being determined as a function of the friction braking torques of each friction braking device.

Furthermore, the control method may be applied to a vehicle equipped with driver assist means. The taking into consideration of the braking torque setpoints from the driver assist means may then be prioritized in order to determine braking setpoints that will promote the stability of the vehicle.

The recuperative braking on the rear wheelset may be limited in order to promote the stability of the vehicle.

A minimum friction braking torque setpoint may also be determined in order to increase the speed of response of the braking devices in case of a braking request involving significant use of the friction braking.

A range of torques supplied by the powertrains may be determined as a function of the torque requests on the part of the driver and of the driver assist means.

DETAILED DESCRIPTION

FIG. 1shows a motor vehicle termed VEH comprising the main parts of a control system. The vehicle VEH comprises a front powertrain1connected to a front wheelset3a;3bvia an axle21and a rear powertrain2connected to a rear wheelset4a;4bby an axle22. The wheel3ais equipped with a braking device5a, the wheel3bwith a device5b, the wheel4awith a device6aand the wheel4bwith a device6b.

An electronic control unit identified by the reference UCE controls the braking devices5a,5b,6aand6bvia the connections12,13,14and15. The electronic control unit UCE also controls the front1and rear2powertrains via the links19and20respectively.

The electronic control unit UCE is connected to sensors by connections7a,7b,7c,7d,7eand7f. The electronic control unit UCE comprises a means8of controlling the powertrains, a means9of distributing a braking request, a means10of modulating the torque setpoints and a system11for controlling the braking devices. The powertrain control means8is connected at output by the connection19to the front powertrain1, by the connection20to the rear powertrain2. The braking device control system11is connected by the connection12and by the connections13,14and15to the braking devices5a,5b,6aand6b.

The means9for distributing a braking request and the means10for modulating the torque setpoints intended for the braking systems and for the powertrains are interconnected by the connections18a,18band18c. The brake request distribution means9is connected to the powertrain control means8by the connections17a,17b,17c,17d,17e,17f,17g,17hand17i. The means10for modulating the torque setpoints intended for the braking systems and for the powertrains is connected to the powertrain control means8by the connections16a,16band16c. The means10for modulating the torque setpoints intended for the braking systems and for the powertrains is connected to the braking device control system11by the connection23.

FIG. 2shows the various means involved in the control system, notably the powertrain control means8, a brake request distribution means9and a torque setpoint modulating means10.

The means9for distributing a braking request comprises the following components:a brake pedal interpretation means30is connected via the connection18ato a computing means39that computes the reference speed contained in the modulating means10. The interpretation means30is also connected at input to the sensor7by the branch7bof the connection7a, and to the powertrain control means8by the connection17b.

The interpretation means30is connected at output to a compensation means31aby the connection57and to a computing means32afor computing the acceleration of the vehicle by the connection59.

The means32afor computing the acceleration of the vehicle is connected by the branch56of the connection18ato the computing means39that computes the reference speed contained in the modulating means10. The computing means32afor computing the acceleration is also connected at input, by the connection17d, to the means24of interpreting the acceleration pedal of the powertrain control means8. The computing means32athat computes the acceleration is connected at output to the compensation means31bby the connection60.

The compensation means31bis connected to the means35of determining the distribution of the recuperative braking between the front and rear wheelsets by the connection61, and to the friction braking compensating means37by the branch62of the connection61.

The compensation means31ais connected by one of its inputs to the sensors7via the branch7c. The compensation means31ais connected at output to the means34of determining the maximum recuperative braking by the branch58aof the connection58, and to the means38for determining the prebraking setpoint by the connection58.

The means34for determining the maximum recuperative braking is connected by one of its outputs to the powertrain setpoint optimizing means27of the powertrain control means8by the connection17e. The means34of determining the maximum recuperative braking is also connected at output by the branch63of the connection17eto the means35of determining the distribution of the recuperative braking between the front and the rear wheelsets.

The means36for interpreting the situation is connected at input by the connection7dto the sensors7. The interpretation means36is connected at output by the connection64to the means35of determining the distribution of the recuperative braking between the front and rear wheelsets. The interpretation means36is also connected at output by the connection18bto the means39of computing the reference speed contained in the modulating means10.

The means38for determining the prebraking setpoint is connected at output to the friction braking compensating means37by the connection66.

The means35for determining the distribution of the recuperative braking between the front and rear wheelsets is connected by its outputs to the friction braking compensating means37by the connection65, to the means27of optimizing the powertrain setpoints of the control means8by the connection17fand to the engine torque coordinating device29by the connection17h.

The means37for compensating for the friction braking is connected by one of its inputs to the means28of dynamic compensation of the powertrain setpoints contained in the powertrain control means8by the connection17i. The compensation means37is connected at output by the connection18cto the switch48of the modulating means10.

The means10for modulating the torque setpoints intended for the braking systems and for the powertrains comprises the following main components:the computing means39for computing the reference speed is connected at input to the sensors7by the connection7eand to the situation interpreting means36by the connection18b. The computing means39is connected at output to the interpretation means30by the connection18a, to the situation determining means by the connection81, to an electronic stability control device41(usually known by its electronic stability program abbreviation ESP) by the connection67a, to an ABS device42by the connection67b, to a traction control device44by the connection67c, to a device preventing recuperative braking on the rear wheelset45by the connection67dand to a device supporting the reference speed46by the connection67e.

The situation determining means40is connected at input to the sensors7by the connection7f. The situation determining means40is connected at output to the switch48by the connection82, to the electronic stability control device41by the connection68a, to the ABS device42by the connection68b, to an HBD (Hybrid Brake-force Distribution) device by the connection68c, to the traction control device44by the connection68d, to the device preventing recuperative braking on the rear wheelset45by the connection68eand to the reference speed maintaining device46by the connection68f.

The device47for coordinating the braking is connected by its inputs to the ESP device41by the connections74and104, to the ABS device42by the connections75and103, to the HBD device43by the connection76, to the traction control device44by the connections77a,77band105, to the device preventing recuperative braking on the rear wheelset45by the connection78and to the reference speed maintaining device46by the connection79.

The device for coordinating the braking47is connected by its outputs to the switch48by the connection80and to the device29for coordinating engine torque by the connections16aand16c.

The switch48is connected at output to the braking device control system11via the connection23.

The means8for controlling the powertrains comprises the following main components:the means24for interpreting the accelerator pedal is connected by one of its inputs to the sensors7by the connection7a. The interpretation means24is connected by one of its outputs to the means32bfor computing the acceleration of the vehicle by the connection50.

The vehicle acceleration computing means32bis connected at input to the brake pedal interpreting means30by the branch17cof the connection59. The vehicle acceleration computing means32bis connected at output to the compensation means31cby the connection51.

The compensation means31cis connected at output to the means27of optimizing the powertrain setpoints by the connection52, and to the means28for dynamically compensating the powertrain setpoints by the branch53of the connection52.

The powertrain setpoint optimizing means27is connected by at least one of its inputs to the means34of determining the maximum recuperative braking by the connection17e. The optimizing means27is connected at output by the connection54to the powertrain setpoint dynamic compensating means28.

The powertrain setpoint dynamic compensating means28is connected by at least one of its inputs by the branch53of the connection52to the compensating means31c. The powertrain setpoint dynamic compensating means28is connected by at least one of its outputs to the engine torque coordinating device29by the connection55and to the friction braking compensating means37by the connection17i.

The engine torque coordinating device29is connected by at least one of its inputs to the means35of determining the distribution of recuperative braking between the front and rear wheelsets by the connection17hand is connected by the connection16cto the braking coordination device47. The engine torque coordinating device29is connected at output to the front1and rear2powertrains by the connections19and20.

The sensors7supply information regarding the position of the brake pedal XBP_sens or the position of the master cylinder P_MC_sens to the interpreting means30. The interpreting means30also receives an estimate of the longitudinal speed of the vehicle VVH_x_est by the connection18aand the minimum deceleration generated by the mechanical resistance of the powertrains for zero acceleration GPT_min, also known as the foot-off deceleration.

The interpreting means30then determines the deceleration due to the depressing of the brake pedal GBP_sp and the derivative with respect to time of the deceleration due to the depressing of the brake pedal dGBP_sp. The variables GBP_sp and dGBP_sp are emitted by the connection57and the variable GBP_sp is emitted by the connection59.

The means32afor computing the acceleration of the vehicle receives the estimate of the longitudinal speed of the vehicle VVH_x_est by the branch56and receives the acceleration generated by the powertrains GPT_sp. The vehicle acceleration computing means32athen determines the vehicle acceleration setpoint GWH_sp according to the driver request.

The compensating means31bthen receives the vehicle acceleration setpoint GWH_sp and determines the total vehicle torque setpoint TWH_sp by applying the following relationship:
TWH_sp=M·R·GWH_spwhere M is the estimated mass of the vehicle andR is the estimated radius of the wheel.

At the same time, the compensating means31areceives as input the variables GBP_sp and dGBP_sp. The compensating means31athen determines the torque associated with the depressing of the brake pedal TBP_sp and the derivative of the torque associated with the depressing of the brake pedal dTBP_sp.
TBP_sp=M·R·GBP_sp
dTBP_sp=M·R·dGBP_sp

The means34for determining the maximum recuperative braking receives as input the torque associated with the depressing of the brake pedal TBP_sp and the derivative of the torque associated with the depressing of the brake pedal dTBP_sp. The means34of determining the maximum recuperative braking then determines the minimum braking torque excluding friction braking TNBP_min.

The means36of interpreting the situation receives, from the sensors7, the angle through which the wheels are turned ASW_sens. Further, the situation interpreting means36receives logic signals reflecting the fact that recuperative braking on the rear wheelset has been prevented Flag_int_recup and the fact that optimized four-wheel drive mode has been activated Flag—4wd_opt, each of these two signals originating from the torque setpoint modulating means10.

The means36for interpreting the situation then determines the traction grip potential threshold Mu_trac, the recuperative braking grip potential threshold Mu_recup, the traction grip potential dynamic threshold Mu_trac_dyn, and the recuperative braking grip potential dynamic threshold Mu_recup_dyn.

The determining means35receives the vehicle total torque setpoint TWH_sp, the minimum braking torque excluding friction braking TNBP_min, the traction grip potential threshold Mu_trac, the recuperative braking grip potential threshold Mu_recup, the traction grip potential dynamic threshold Mu_trac_dyn, and the recuperative braking grip potential dynamic threshold Mu_recup_dyn.

The determining means35then determines the minimum torque on the rear axle in near-static conditions TPT_r_min, the maximum torque on the rear axle under near-static conditions TPT_r_max, the minimum torque on the rear axle under transient conditions TPT_r_min_trans and the maximum torque on the rear axle under transient conditions TPT_r_max_trans.

At the same time, the determining means38receives the torque associated with the depressing of the brake pedal TBP_sp and the derivative of the torque associated with the depressing of the brake pedal dTBP_sp and determines the braking torque directly applied to the brakes ΔFBP_sp.

The friction braking compensating means37receives the vehicle total torque setpoint TWH_sp, the minimum torque on the rear axle under near-static conditions TPT_r_min, the maximum torque on the rear axle under near-static conditions TPT_r_max, the torque setpoint of the rear powertrain TPT_r_osp, the torque setpoint of the front powertrain TPT_f_osp and the braking torque directly applied to the brakes ΔFBP_sp.

The friction braking compensating means37then determines the braking torque of the rear left wheel TFB_rl_osp compensated as a function of the resistive torque of the rear powertrain TPT_r_osp, the braking torque of the rear right wheel TFB_rr_osp compensated as a function of the resistive torque of the rear powertrain TPT_r_osp, the braking torque of the front left wheel TFB_fl_osp as a function of the resistive torque of the front powertrain TPT_f_osp and the braking torque of the front right wheel TFB_fr_osp compensated as a function of the front powertrain TPT_f_osp.

In the powertrain control means8, the accelerator pedal interpreting means24receives information relating to the depressing of the accelerator pedal and to the gear ratio from the sensors7. The interpreting means24further receives the estimate of the longitudinal speed of the vehicle VVH_x_est. The interpreting means24at output determines the acceleration generated by the powertrains GPT_sp.

The vehicle acceleration computing means32breceives the acceleration generated by the powertrains GPT_sp and determines the vehicle acceleration setpoint GWH_sp.

It should be noted that the operation of the means32aand32bmay be merged into a single means distributed across the means8and9.

The compensating means31creceives the vehicle acceleration setpoint GWH_sp and determines the vehicle total torque setpoint TWH_sp by applying the following relationship:
TWH_sp=M·R·GWH_spwhere M is the estimated-mass of the vehicle andR is the estimated radius of the wheel.

Here again, it should be noted that the means31a,31b, and31ccan be merged, their functions then being distributed across the means8and9.

The powertrain setpoint optimizing means27receives at input, in addition to the value TWH_sp, the minimum braking torque excluding friction braking TNBP_min and the minimum torque on the rear axle under near-static conditions TPT_r_min, the maximum torque on the rear axle under near-static conditions TPT_r_max, the minimum torque on the rear axle under transient conditions TPT_r_min_trans and the maximum torque on the rear axle under transient conditions TPT_r_max_trans.

The powertrain setpoint dynamic compensating means28emits at output the values of torque of the front powertrain TPT_f_osp, of torque of the rear powertrain TPT_r_osp and the gear ratio RCL_f_osp.

The engine torque coordinating device29receives from the braking coordinating device47the values of minimum torque on the rear axle under static conditions TPT_r_min_stat, of maximum torque on the rear axle under static conditions TPT_r_max_stat, of minimum torque on the rear axle under dynamic conditions TPT_r_min_dyn, of maximum torque on the rear axle under dynamic conditions TPT_r_max_dyn, of minimum torque on the front axle under static conditions TPT_f_min_stat, of maximum torque on the front axle under static conditions TPT_f_max_stat, of minimum torque on the front axle under dynamic conditions TPT_f_min_dyn, of maximum torque on the front axle under dynamic conditions TPT_f_max_dyn, and of gear ratio RCL_f_tgt. The engine torque coordinating device29also receives the values of torque of the front powertrain TPT_f_osp, of torque of the rear powertrain TPT_r_osp from the dynamic compensating means28. The coordinating device29comprises the components described inFIG. 3.

The coordinating device29comprises a computing means and a computing means85. The computing means84receives on its inputs the values of minimum torque on the rear axle under static conditions TPT_r_min_stat, of minimum torque on the rear axle under dynamic conditions TPT_r_min_dyn, of minimum torque on the front axle under static conditions TPT_f_min_stat and of minimum torque on the front axle under dynamic conditions TPT_f_min_dyn. The computing means also receives the values of torque of the front powertrain TPT_f_osp, of torque of the rear powertrain TPT_r_osp from the dynamic compensating means28. The computing means84determines the maximum value of torque that can be applied to the front and rear powertrains. These two values are transmitted to the computing means85by the connection110.

The computing means85receives on its inputs the values of maximum torque on the rear axle under static conditions TPT_r_max_stat, of maximum torque on the rear axle under dynamic conditions TPT_r_max_dyn, of maximum torque on the front axle under static conditions TPT_f_max_stat and of maximum torque on the front axle under dynamic conditions TPT_f_max_dyn.

The computing means85then determines the minimum values from among the values received, these values being emitted at output by way of target torque values TPT_f_tgt and TPT_r_tgt for the front and rear powertrains respectively.

The means10for modulating the torque setpoints intended for the braking systems and for the powertrains receives, via the reference speed computing means39, the traction grip potential threshold Mu_trac, the recuperative braking grip potential threshold Mu_recup, the traction grip potential dynamic threshold Mu_trac_dyn, and the recuperative braking grip potential dynamic threshold Mu_recup_dyn. It also receives, from the sensors7, wheel speed values. At output, it determines an estimate of the longitudinal speed of the vehicle VVH_x_est, and emits two logic signals preventing recuperative braking from being used on the rear wheelset Flag_int_recup and for activating the optimized four-wheel drive mode Flag—4wd_opt. The computing means39is also connected by the connections67a,67b,67c,67dand67eto the ESP device41, the ABS device42, the traction control device44, the device for preventing recuperative braking on the rear wheelset45and the device for maintaining the reference speed46.

A means40for determining the situation determines the situation of the vehicle from the data received from the reference speed computing means39and from the wheel speed received from the sensors7. It is connected by the connections68a,68b,68c,68d,68eand68fto the ESP device41, the ABS device42, the HBD device43, the traction control device44, the device for preventing recuperative braking on the rear wheelset45and the reference speed maintaining device46. The determining means40is also connected to the switch48by the connection82.

The driver assist and vehicle safety devices such as the ESP device41, the ABS device42, the HBD device43, the traction control device44, the device preventing recuperative braking on the rear wheelset45and the device for maintaining the reference speed46are known per se and will not be described here.

The device47for coordinating the braking comprises two parallel structures. A first structure is used to determine the engine torques intended for the engine torque coordinating device29and a second structure is used to determine the resistive torques intended for the switch48and for the braking systems5a,5b,6aand6b.

The first structure is described inFIG. 4. The ESP41, ABS42, HBD43and traction control44devices are connected to a computing means86by the connections76,77b,78and79. The computing means86is also connected to a memory88by the connection111.

The devices45for preventing recuperative braking on the rear wheelset and for maintaining the reference speed46are connected to a computing means87by the respective links74and77b. The computing means87is also connected to a memory89by the connection112.

The computing means86receives the torque couples setpoints from the ESP41, the ABS42, the HBD43and the traction control44devices. The computing means86also receives, from the memory88, a threshold value corresponding to the minimum value expected at output of the computing means86. The values of minimum torque on the rear axle under static conditions TPT_r_min_stat, of minimum torque on the rear axle under dynamic conditions TPT_r_min_dyn, of minimum torque on the front axle under static conditions TPT_f_min_stat and of minimum torque on the front axle under dynamic conditions TPT_f_min_dyn are emitted at output of the computing means86via the connection16c.

At the same time, the computing means87receives the torque couples setpoints from the device45for preventing recuperative braking on the rear wheelset and the device46for maintaining the reference speed. The computing means87also receives a threshold value corresponding to the minimum value expected at output of the computing means87. The values of maximum torque on the rear axle under static conditions TPT_r_max_stat, of maximum torque on the rear axle under dynamic conditions TPT_r_max_dyn, of maximum torque on the front axle under static conditions TPT_f_max_stat and of maximum torque on the front axle under dynamic conditions TPT_f_max_dyn are emitted at output of the computing means86via the connection16a.

The second structure of the device47for coordinating the braking is described inFIG. 5. The braking coordinating device47comprises computing means90,92,93and94and a memory91.

The computing means90is connected to the ABS device42by the connection103and to an electronic brake-force distributor95by the connection102.

The computing means92is connected to the ESP device by the connection104, to the traction control device44by the connection105and to a memory91by the connection106.

The computing means93is connected to the computing means92by the connection107and to the sensors7via the connection98.

The computing means94is connected by the connection109to the computing means90and by the connection108to the computing means93.

The computing means90,92,93and94receive, on each of their inputs, a value containing four braking torque setpoints each one intended for one of the friction braking devices.

The computing means90determines the maximum value from among the signals received on these inputs. To do that, each of the four setpoints received at input is compared against the setpoint of comparable rank on the other input or inputs. For example, the rank i setpoint of the value j is compared against the rank i setpoint of the value k. The minimum setpoint for a rank i is considered from among all the setpoints. This method of comparison is valid for the computing means92,93and94.

The means92and93each determine the minimum value from among the values received on their inputs.

Finally, the computing means94determines the maximum value from among the signals received on its inputs. This value contains the rear right wheel safe braking torque TFB_rr_tgt, the rear left wheel safe braking torque TFB_rl_tgt, the front left wheel safe braking torque TFB_fl_tgt and the front right wheel safe braking torque TFB_fr_tgt. This value is then emitted by the connection80.

The switch48thus receives on its inputs, via the connection80, the rear right wheel safe braking torque TFB_rr_tgt, the rear left wheel safe braking torque TFB_rl_tgt, the front left wheel safe braking torque TFB_fl_tgt and the front right wheel safe braking torque TFB_fr_tgt and, via the connection18c, the rear right wheel braking torque TFB_rr_osp, the rear left wheel braking torque TFB_rl_osp, the front left wheel braking torque TFB_fl_osp and the front right wheel braking torque TFB_fr_osp. Further, the switch48via the connection82receives control signals originating from the means40.

Thus, according to the situation detected by the means40, the switch emits at output either the set of safe braking torques determined by the computing means47or the set of braking torques determined by the friction braking compensating means37.

These braking setpoints are emitted via the connection23to the braking device control system11which in turn forwards the appropriate braking setpoints to each of the friction braking devices5a,5b,6aand6bvia the connections12,13,14and15.

The control system and method described here allow the full extent of the drive and of the braking of a hybrid vehicle to be taken into consideration. A bipolar approach split between a device that determines torque and braking setpoints according to driver requests and a device that interprets the various signals from the sensors and driver assist and safety devices of the vehicle allows said driver requests to be modulated in such a way as to keep the vehicle under driving conditions that are compatible with vehicle safety.