Patent Description:
Steer-by-wire steering systems are increasingly used in the automotive field, especially with the rise of autonomous vehicles. Such steer-by-wire typically use sensors detecting a rotation of a steering wheel and send a signal representing this detected rotation to an actuator acting on a steering rack connecting steered wheels.

In other types of autonomous vehicles, another version of the steer-by-wire system is used where a separate steering actuator is assigned to each steerable wheel of the autonomous vehicle. This variation allows an independent control of a steering angle of each steerable wheel. Advantageously, such a steer-by-wire system may orient the steerable wheels either according to the Ackermann principle or to allow crabbing movement of the vehicle when all wheels are steered parallelly.

As is often the case with product design, one of the objectives is often to keep the cost of the vehicle as low as possible such that the number of sensors and actuators in both of these designs is kept to an acceptable minimum. However, in some other types of autonomous vehicles, such as those used in the mass transit market, higher safety levels are required where the automotive safety levels are not sufficient and where the above described steer-by-wire systems may not be capable of meeting safety requirements which often require higher availability and integrity. There is therefore a need for autonomous mass-transit vehicles meeting more stringent safety requirements.

Vehicles in accordance with the preamble of claim <NUM> are known from <CIT> and from <CIT>.

It is an object of the present invention to provide a steering system for an autonomous vehicle that overcomes or mitigates one or more disadvantages of known steering systems for autonomous vehicles, or at least provides a useful alternative.

The present invention provides the advantage of meeting safety requirements for at least some categories of autonomous mass-transit vehicles.

In accordance with an embodiment of the present invention, there is provided an autonomous vehicle comprising a body having a passenger compartment, four steering systems, at least two controllers, a first power source and a second power source. Each steering system is equipped with a wheel and a first actuator connecting the wheel to the body for steering the wheel. Each one of the four wheels supports a different end and a different side of the body. The at least two controllers are operatively connected to the first actuators for controlling their actuation. The first power source powers a first subset of the first actuators while the second power source powers a second subset of the first actuators.

Each one of the four steering systems further comprises a second actuator and one controller. The second actuator also connects the wheel to the body for steering the wheel. Each controller is operatively connected to the first actuator of its respective steering system and to one of the second actuators of another steering system. The first power source powers a first subset of the second actuators while the second power source powers a second subset of the second actuators. Each second actuator acts on its respective wheel in opposition to the first actuator of the respective steering system.

Optionally, the four steering systems of the autonomous vehicle may be a front right steering system, a front left steering system, a rear right steering system and a rear left steering system.

Alternatively, the four steering systems of the autonomous vehicle may be a front right steering system, a front left steering system, a rear right steering system and a rear left steering system.

The first and the second actuators may be hydraulic actuators, in which case the first and the second power sources would be hydraulic pumps. Alternatively, the first and second actuators may be electro-mechanical actuators, in which case the first and the second power sources would be electrical power sources.

Optionally, the controllers may be organized in two pairs. Each pair of controllers control two of the four steering systems located at a different end of the body.

Optionally, the controller of each steering system may be connected in active mode to the first actuator of the respective steering system, while being connected in passive mode to one of the second actuators of a different one of the four steering systems.

Optionally, the first actuators and the second actuators of each steering system may operate in active-active mode. In this case, each controller of each steering system further comprises a force-fight compensation system adapted to adjust one of a stroke and a force of at least one of the first actuator and the second actuator of their respective steering systems.

Optionally, the controllers may be high-integrity controllers.

These and other features of the present invention will become more apparent from the following description in which reference is made to the appended drawings wherein:.

<FIG> schematically depicts an autonomous vehicle <NUM>, which typically comprises a body <NUM> having a passenger compartment for transporting passengers. The vehicle <NUM> comprises at least four wheels <NUM>, each supporting a different end and a different side of the body as well as a steering system <NUM> for steering steerable wheels <NUM>.

A command to steer the autonomous vehicle <NUM> may either come from a main vehicle controller which either detects that steering is required, or which uses a memorized path and sends a command to steer accordingly. Alternatively, the autonomous vehicle <NUM> could be remote controlled and receive the steering command remotely.

The steering system <NUM> of the instant embodiment steers two steerable wheels <NUM> through two actuators <NUM>. Each actuator <NUM> is connected at one end to the body <NUM> and at its moveable end to a different one of the steerable wheels <NUM>. In the present embodiment, the steerable wheels <NUM> are solely located at a front <NUM> of the vehicle <NUM>. The wheels <NUM> located at a back <NUM> of the vehicle <NUM> are non-steerable. It should be noted that the front <NUM> and the back <NUM> of the vehicle <NUM> could be inverted.

Steerable wheels <NUM> are typically organized in pairs on a common "axle". Although there is clearly no axle used in the present embodiment, that term will nevertheless be used to denote two corresponding wheels located at a common longitudinal distance in the vehicle <NUM>, but on two different sides of the vehicle <NUM>.

Each actuator <NUM> is independently controlled by a controller <NUM> and independently powered by its own power source <NUM>. The actuators <NUM> may either be hydraulic, in which case the power source <NUM> is a hydraulic pump, or electro-mechanic, in which case the power source <NUM> is electric. The actuators <NUM> may be linear or rotational. In case of failure, each actuator <NUM> may be back driven.

Upon receiving the steering command, for example, by the main vehicle controller, each controller <NUM> independently connected to one actuator <NUM> is operative to send a signal to its respective actuator <NUM> that is indicative of a stroke to reach. In turn, this stroke corresponds to a desired steering angle of that steerable wheel <NUM>. The controller <NUM> therefore sends a signal to the actuator <NUM> to which it is connected that is proportional to the desired steering angle. Each actuator <NUM> may be equipped with its own stroke sensor <NUM> operative to send back a signal (a stroke reading) to its controller <NUM>, indicative of the stroke it has reached. This creates a feedback loop allowing the controller <NUM> to readjust its command in case the actuator <NUM> has not reached the desired stroke. Moreover, angle sensors <NUM> may be installed in proximity to a steering pivot <NUM> of each steerable wheel <NUM> to monitor the actual steering angle of its respective steerable wheel <NUM>. Typically, each angle sensor <NUM> also feeds its actual steering angle reading to its respective controller for monitoring. If the controller <NUM> detects a significant discrepancy between the actual angle reading and a theoretical angle yielded by the stroke read by the stroke sensor <NUM>, then the controller <NUM> may decide on a mitigating action, such as switching the actuator <NUM> in passive mode, or stopping the vehicle by communicating a request to the main vehicle controller. In the present document, the expression "controlling in Passive mode" shall be interpreted as switching one actuator in Passive mode, that is letting this actuator being back-driven.

In the present description, and in all embodiments of the invention, the one or many controllers <NUM> may be of the high-integrity type. High integrity types of controllers <NUM> further add to the integrity of the whole system.

In the present embodiment, redundancy is provided by a steering link <NUM> connecting both steerable wheels <NUM>. It is used as a redundant steering means in case one of the actuators <NUM> fails. Because each steerable wheel <NUM> is independently controlled, their steering angle may be slightly different. However, in normal operation, this steering angle will follow the Ackermann principle so an Ackermann geometry may be built in the steering link <NUM>. According to the Ackermann geometry, in a turn, the inner wheel steers more than the outer wheel. In this case, the steering link <NUM> comprises a steering rod and two steering arms, one connected to each steerable wheel <NUM>. In other cases, however, and as will be further discussed below, it may be desirable for the steered wheels <NUM> to acquire a parallel toe angle. To accommodate this, the steering link <NUM> is designed with free play so that the steered wheels <NUM> may steer either parallel to each other or with an Ackermann angle in between them. This free play insures a certain independence of steering of the corresponding steered wheels <NUM> on a common axle. If one of the actuators <NUM> fails, the steered wheel <NUM> on the failed side may want to steer in a totally unpredictable manner. To prevent this, a predetermined threshold of angle difference between both steerable wheels <NUM> of a common axle is designed in the steering link <NUM>. This predetermined threshold may correspond to the end of the free play in the steering linkage. Passed this threshold, the steering link <NUM> acts as a solid linkage and couples both steerable wheels <NUM>. To achieve this, the steering link <NUM> may be equipped with one or both of a resilient spring element and a damping element. The steering link <NUM> may either be a mechanical link or a hydraulic connection between the steerable wheels <NUM>. For light vehicles, a pneumatic connection could even be considered.

To provide even further redundancy, the steering link <NUM> could be dualized, that is two steering links <NUM> may be used in combination to both steer the steerable wheels <NUM> to which they are attached. For example, the steering links <NUM> may be attached one atop the other or in some appropriate way. They may be concentric rods or placed side-by-side. Under some circumstances, it may even be appropriate to use such dualized steering links <NUM> and to remove one of the actuators <NUM> driving one of the steerable wheels <NUM> to which the steering links <NUM> are attached. In case of failure of one steering link <NUM>, the other steering link <NUM> is still sufficient to transfer steering movement between both steerable wheels <NUM> to which it is attached. This would however be to the detriment of the integrity of the system since there would be no way to steer the steerable wheels <NUM> in case of failure of the only remaining actuator <NUM>.

In a first embodiment outside the scope of the claims, a second steering system <NUM>, similar to the one used at the front <NUM> of the vehicle <NUM>, is used at the back <NUM> of the vehicle. This embodiment is depicted in <FIG>, now concurrently referred to. In this embodiment, all four wheels <NUM> of the vehicle <NUM> are steerable. Similarly to the steerable wheels <NUM> of the front axle, the steerable wheels <NUM> of the rear axle are steered independently, each being steered through their own actuator <NUM> controlled by their own controller <NUM>. As can be seen in <FIG>, the exact same steering system architecture is used at the rear of the vehicle <NUM> as at the front of the vehicle and works exactly the same way. The rear steering system <NUM> will therefore not be further described here.

This four-wheel steering system however provides an advantage over that of the two-wheel steering system: the crabbing capacity (also known as Dog Tracking). Crabbing happens when the vehicle <NUM> moves sideways at an angle. This maneuver may be induced by steering all wheels <NUM> at the same angle. This capacity is useful when, for example, the vehicle <NUM> is required to dock along a platform, for example to embark or disembark passengers.

A variant of this design still provides some level of redundancy while using less components (and is therefore arguably cheaper to manufacture). This embodiment, which is not part of the claimed invention, is depicted in <FIG>, now concurrently referred to. This embodiment is similar to the previous embodiment depicted in <FIG>, except that only two controllers <NUM> and two power sources <NUM> are used instead of four of each. Each controller <NUM> is linked to two steered wheels <NUM> located on the same side of the body <NUM>. Hence, one controller 24a controls the steering of the right steerable wheels 18a by being connected to the right actuators 20a while the other controller 24b controls the steering of the left steerable wheels 18b by being connected to the left actuators 20b. Both right stroke sensors 27a are connected to the controller 24a for providing feedback on the stroke of the front right and rear right actuators 20a. Both right angle sensors 28a are also connected to the right controller 24a for providing feedback on the steering angle of the front right and rear right steerable wheels 18a. Similarly, both left stroke sensors 27b are connected to the left controller 24b for providing feedback on the stroke of the front left and rear left actuators 20b. Both left angle sensors 28b are also connected to the left controller 24b for providing feedback on the steering angle of the front left and rear left steerable wheels 18b. The right and left controllers 24a, 24b are referred to as such not based on their actual location, but rather because they belong respectively to either the right or the left steering system 16a, 16b.

The right power source 26a powers both of the right actuators 20a while the left power source 26b powers both of the left actuators 20b. The right and left power sources 26a, 26b are referred to as such not based on their actual location, but rather because they belong respectively to either the right or the left steering system 16a, 16b. Again, the right and left power sources 26a, 26b may either be hydraulic, in which case the actuators <NUM> are hydraulic actuators or the right and left power sources 26a, 26b may be electric in which case the actuators <NUM> are electro-mechanical actuators.

Interestingly, in the present embodiment, it cannot be said that the steering links <NUM> belong to either the right or left steering systems 16a, 16b since they still connect the right and left steerable wheels <NUM> on a common axle. In other words, the front steering link 32a connects the front right and front left steerable wheels <NUM> while the rear steering link 32b connects the rear right and rear left steerable wheels <NUM>.

<FIG> is now concurrently referred to. Alternatively, each steering link <NUM> could belong to a different one of the steering systems 16a, 16b by having a right steering link 32a connecting the right front steerable wheel 18a to the right rear steerable wheel 18a and a left steering link 32b connecting the left front steerable wheel 18b to the left rear steerable wheel 18b. Note that for clarity reasons, the controllers <NUM> and the power sources <NUM> as well as their connections have been omitted to clearly see the steering links 32a, 32b, but that it should be understood that implementations of such controllers <NUM>, power sources <NUM> and their respective connections may be achieved according to either <FIG> or <FIG>.

According to the present invention, the steering links <NUM> are completely avoided. This embodiment is represented in <FIG>, now concurrently referred to. The autonomous vehicle <NUM> of this embodiment comprises four steerable wheels <NUM> supporting a different end and a different side of the body <NUM>, the steering system <NUM>, four controllers <NUM> and at least two power sources <NUM>. This steering system <NUM> is made of four sets of actuators <NUM>, each having a first actuator 20c and a second actuator 20d connected in opposition. Each set of actuators <NUM> is connected to a different one of the four steerable wheels <NUM>.

The first power source 26c powers all first actuators 20c, while the second power source 26d powers all second actuators 20d. Again, these power sources 26c, 26d may be a hydraulic pump, when the actuators <NUM> used are hydraulic actuators or the power sources 26c, 26d may be an electric power source when the actuators <NUM> used are electro-mechanical actuators. In the present example, the actuators <NUM> are controlled to work in Active-Passive mode. Active means that the controller is controlled and provides the force to steer the wheel. Passive means that the actuator is not controlled or is not responsive to the command and is simply back driven by the moveable element to which it is attached. Hence, in Active-Passive mode, one actuator provides a force to steer the wheel while the other is being back driven and provides no force. Active-Active mode means that both controllers provide a force to steer the wheel.

Upon receiving the command from the main vehicle controller, each one of the four controllers <NUM> controls a steering of a different one of the four steerable wheels <NUM>. Each controller <NUM> is connected in Active mode to the first actuator 20c of the respectively steerable wheel <NUM> it controls. Each controller <NUM> may additionally be connected in Passive mode to the second actuator 20d of at least one of the other steerable wheels <NUM>. None of the controllers <NUM> is connected to both the first actuator 20c and the second actuator 20d of one given steerable wheel <NUM>. This intertwined connection ensures that each controller <NUM> is in control of a different steerable wheel <NUM> through its Active connection to the first actuator 20c of that steerable wheel <NUM> while acting as a back-up controller for at least one other steerable wheel <NUM> by controlling in Passive mode its second actuator 20d, should the controller <NUM> or the first actuator 20c of that other steerable wheel <NUM> fail. Stated differently, each controller <NUM> is assigned to the control of a different steerable wheel <NUM> and controls two actuators <NUM>: one first actuator 20c steering a first steerable wheel <NUM> is controlled in Active mode and one second actuator 20d connected to a second steerable wheel <NUM> is controlled in Passive mode. If, for example, a first controller <NUM> detects a failure of its Actively controlled first actuator 20c (e.g. as discussed earlier, if there exist a significant discrepancy between the steering angle reading received from the steering angle sensor <NUM> and the theoretical steering angle yielded by the stroke reading received by the stroke sensor <NUM>), then it can switch its connected first actuator 20c in Passive mode immediately. Simultaneously and in a similar manner, a second controller <NUM> will be informed of the mode transition of the first controller in Passive mode and will switch this second actuator 20d in Active mode. Transition from one controller to the other must be within an acceptable time threshold to prevent losing control of the vehicle <NUM>.

In the present embodiment, and as an example, controller 24x controls in Active mode the steering of the front right steerable wheel 18x through the first actuator 20cx. Controller 24x is also connected in Passive mode to the second actuator 20dy of the front left steerable wheel 18y. Controller 24x receives the feedback signals from the stroke sensor 27cx and angle sensor 28x of the steerable wheel 18x as well as from the stroke sensor 27dy and angle sensor 28y of the steerable wheel 18y.

Controller 24y controls in Active mode the steering of the front left steerable wheel 18y through the first actuator 20cy. Controller 24y is also connected in Passive mode to the second actuator 20dx of the front right steerable wheel 18x. Controller 24y receives the feedback signals from the stroke sensor 27cy and angle sensor 28y of the steerable wheel 18y as well as from the stroke sensor 27dx and angle sensor 28x of the steerable wheel 18x. The control of both front steerable wheels 18x, 18y is therefore interconnected so that one controller 24x, 24y acts as a redundant controller for the other one by controlling the second actuator 20dx, 20dy of the other controller 24y, 24x.

Similarly, the rear steerable wheels 18z, 18w are also interconnected. Controller 24z controls in Active mode the steering of the rear right steerable wheel 18z through the first actuator 20cz. Controller 24z is also connected in Passive mode to the second actuator 20dw of the rear left steerable wheel 18w. Controller 24z receives the feedback signals from the stroke sensor 27cz and angle sensor 28z of the steerable wheel 18z as well as from the stroke sensor 27dw and angle sensor 28w of the steerable wheel 18w.

Controller 24w controls in Active mode the steering of the rear left steerable wheel 18w through the first actuator 20cw. Controller 24w is also connected in Passive mode to the second actuator 20dz of the rear right steerable wheel 18z. Controller 24w receives the feedback signals from the stroke sensor 27cw and angle sensor 28w of the steerable wheel 18w as well as from the stroke sensor 27dz and angle sensor 28z of the steerable wheel 18z. The control of both rear steerable wheels 18z, 18w is therefore interconnected so that one controller 24z, 24w acts as a redundant controller for the other one by controlling the second actuator 20dz, 20dw of the other controller 24w, 24z.

These interconnections are only provided as an example and the same level of redundancy may be provided by interconnecting a controller <NUM> in Passive mode with any one of the wheels to which it is not connected to the first actuator <NUM> in Active mode. For example, the controller 24x could be connected to any one of the second actuators 20dy, 20dz or 20dw. The other controllers 24y, 24w, 24z must also therefore be connected in Passive mode to any one of the other second actuators <NUM> so that each steerable wheel <NUM> is controlled in Active mode by one controller <NUM> and in Passive mode by another controller <NUM>.

In another variant of the present embodiment, the actuators may be controlled in Active-Active mode so that both the first actuator 20c and the second actuator 20d attached to the same steerable wheel <NUM> may use a force to induce a steering movement to the steerable wheel <NUM>. In this case, the controllers <NUM> and actuators <NUM> may be interconnected as previously described, but a force-fight control is added to the controllers <NUM> so as to prevent having the first and second actuators 20c, 20d of a set of actuators force against each other or forcing differently and creating mechanical fatigue. Using the feedback provided by force-fight feedback reduction controller (FFRC) sensors <NUM> connected to each one of the first and second actuators 20c, 20d, the controllers <NUM>, using a force-fight reduction algorithm, are capable of reducing the induced additional force created by a possible misalignment of a set of actuators <NUM>, by either adjusting the stroke of or the force applied by one or both actuators <NUM> so that the steerable wheel <NUM> to which the set of actuators <NUM> is connected is steered at the desired angle without additional force being incurred in the actuators <NUM>. This is especially important in the case where the vehicle <NUM> is a self-propelled electric vehicle which needs to manage properly its battery, which also feeds the power sources <NUM>, in order to maximize its range. Consequently, the following force-fight solution is applied.

When adding the force-fight function to the controller <NUM>, the FFRC sensors <NUM> are required in the actuators <NUM>. The FFRC sensors <NUM> may detect one or more of a load, force, pressure or current, depending on the type of actuator <NUM> used. For each steerable wheel <NUM>, the FFRC sensors <NUM> of both connected first and second actuators 20c, 20d are operative to send the detected signal to both their respective controller <NUM> as well as to the controller <NUM> of the other connected actuator. In other words, whereas each steerable wheel <NUM> is steered by both connected first actuator 20c and second actuator 20d, the FFRC sensors <NUM> of both the first and second actuator 20c, 20d send their respective signals to both the controller <NUM> controlling the first actuator 20c and to the controller <NUM> controlling the second actuator 20d.

<FIG> is now concurrently referred to. As already mentioned, during operation, the loads supplied by both actuators <NUM> of a set of actuators acting on a steerable wheel <NUM> may not be equal or acting in the same direction. A force-fight reduction controller <NUM>, which may be implemented in each individual controller <NUM>, implements a so-called force-fight limiting compensation function whose goal is to equalize the load share between all active actuators <NUM> acting on a given steerable wheel <NUM> to less than a percentage of the actuator maximum force. Typical numbers range between <NUM> to <NUM>%.

This is achieved by continually biasing a control current command output <NUM> (such as for commanding an electro-hydraulic servo valve (EHSV) or motor torque for example) of every actuator position controller <NUM> of actuators <NUM> which are part of one set of actuators so as to balance the measured loads applied by each active actuator <NUM> on a given steerable wheel <NUM>. The compensation can be considered as being equivalent to a position command bias for aligning the motion and position of any given actuator <NUM> with that of the other actuator <NUM> in a set of actuators. As such, the force-fight reduction controller <NUM> provides an adaptive improvement to the sets of actuators <NUM>. In other words, the force-fight reduction controller <NUM> acts as a continuous mutual recalibration with one another of both actuators <NUM> of one set of actuators.

One force-fight reduction controller <NUM> is associated with every actuator position controller <NUM> controlling the steering angle of one steerable wheel <NUM> steered by one set of at least two actuators nominally acting in an All Active configuration. Both the actuator position controller <NUM> and the force-fight reduction controller <NUM>, for a given actuator <NUM> or a given set of actuators <NUM>, may be implemented as a pair in their corresponding controller <NUM>. The actuator position controller <NUM> and the force-fight reduction controller <NUM> may operate at a sampling frequency which typically ranges between <NUM> and <NUM> (i.e. <NUM> and <NUM> sampling period, respectively).

At each computation cycle, the force-fight reduction controller <NUM> computes and adds a limited-authority compensation to the control current command <NUM> computed by the actuator position controller <NUM>. The force-fight reduction controller <NUM> authority is typically limited to approximately +/- <NUM>% of control current command <NUM>, while the combined authority of the actuator position controller <NUM> and force-fight reduction controller <NUM> is limited to the nominal value of the control current command.

The force-fight reduction controller <NUM> comprises a force-fight enable logic function <NUM> and a force-fight compensation function <NUM>. The force-fight enable logic function <NUM> determines if the force-fight reduction controller <NUM> supplies a compensation based on availability of the required actuator sensor data and the status of the force-fight reduction controller <NUM>, while the force-fight compensation function computes the value of this compensation. If failure occurs in either one of the first actuator 20c or second actuator 20d connected to one common steerable wheel <NUM>, then the force-fight reduction control function is disactivated since the failed actuator <NUM> will have been placed in Passive mode by its controller <NUM>.

Claim 1:
A vehicle (<NUM>) comprising:
a body (<NUM>) having a passenger compartment;
four steering systems (<NUM>), each steering system having:
a wheel (<NUM>,<NUM>), said wheels of said four steering system (<NUM>) supporting a different end and a different side of said body (<NUM>); and
a first actuator (<NUM>) connecting said wheel (<NUM>,<NUM>) to said body (<NUM>) for steering said wheel;
at least two controllers (<NUM>) operatively connected to said first actuators (<NUM>) for controlling an actuation of said first actuators (<NUM>);
a first power source (<NUM>) powering a first subset of said first actuators (<NUM>); and
a second power source (<NUM>) powering a second subset of said first actuators (<NUM>).
characterized in that the vehicle (<NUM>) is an autonomous vehicle, in that said at least two controllers (<NUM>) are four controllers (<NUM>) and in that each one of said four steering systems (<NUM>) further comprises:
a second actuator (<NUM>) connecting said wheel (<NUM>, <NUM>) to said body (<NUM>) for steering said wheel (<NUM>, <NUM>); and
one of said four controllers (<NUM>), each controller (<NUM>) being operatively connected to said first actuator (<NUM>) of the respective steering system (<NUM>) and to one of said second actuators (<NUM>) of another one of said steering systems (<NUM>);
wherein said first power source (<NUM>) powers a first subset of said second actuators (<NUM>) and wherein said second power source (<NUM>) powers a second subset of said second actuators (<NUM>).
wherein each one of said second actuators (<NUM>) acts on said respective wheel (<NUM>, <NUM>) in opposition to said first actuator (<NUM>) of said respective steering system.