Patent Description:
Vehicle parking systems for commercial trucks are known, for example from <CIT>. One type of vehicle parking system for trucks is an electronic parking system in which the parking brake is automatically applied using a primary parking mechanism when certain criteria associated with the truck or the truck driver are met. In some electronic parking systems, a second parking mechanism as a backup is provided for applying the parking brake in the event that the primary parking mechanism is unable to cause the parking brake to be applied. These known second parking mechanisms require the truck driver to take some manual action to activate the secondary parking mechanism after the truck driver is alerted that the primary parking mechanism has been unable to cause the parking brake to be applied.

The known secondary parking mechanisms can be used in any type of truck including autonomously drivable trucks. However, in the case of an autonomously driven truck, some manual action from an occupant of the autonomously driven truck would still be needed to activate the secondary parking mechanism if the primary parking mechanism were unable to cause the parking brake to be applied. Accordingly, those skilled in the art continue with research and development efforts in the field of parking systems of a vehicle, such as a commercial truck, that includes a primary parking mechanism, and may or may not include a secondary parking mechanism as a backup to the primary parking mechanism.

In accordance with one embodiment, a parking brake apparatus is provided for an autonomously drivable vehicle having components of a parking brake system for applying a parking brake. The parking brake apparatus comprises a first controller arranged to provide one or more control signals to be applied to components of the parking brake system to apply the parking brake in response to a signal requesting the parking brake to be applied. The parking brake apparatus also comprises a second controller arranged to provide one or more control signals to be applied to other components of the parking brake system to apply the parking brake in response to unavailability of the first controller to cause the parking brake to be applied.

The parking brake apparatus further comprises a parking brake valve controllable by the first controller, a first <NUM>/<NUM> normally-open solenoid valve disposed between a primary compressed air supply and the parking brake valve, and a second <NUM>/<NUM> normally-open solenoid valve disposed between a secondary compressed air supply and the parking brake valve, wherein the second controller is arranged to provide a first control signal to the first normally-open <NUM>/<NUM> solenoid valve, and a second control signal to the second normally-open <NUM>/<NUM> solenoid valve, a first relay valve disposed between the first <NUM>/<NUM> normally-open solenoid valve and the parking brake valve, and a second relay valve disposed between the second <NUM>/<NUM> normally-open solenoid valve and the parking brake valve.

In accordance with another embodiment, a computer-implemented method is provided for an autonomously drivable vehicle having a parking brake, a parking brake apparatus as described before with a first controller as primary parking brake controller, and a second controller as secondary parking brake controller which is different from the primary parking brake controller. The computer-implemented method comprises detecting unavailability of the primary parking brake controller to cause the parking brake to be applied. The method also comprises electronically by the secondary parking brake controller, causing the parking brake to be applied in response to the unavailability of the primary parking brake controller.

The present invention is directed to a parking brake apparatus for an autonomously drivable vehicle such as a commercial truck. The specific construction of the parking brake apparatus may vary. It is to be understood that the description below provides a number of embodiments or examples for implementing different features of various embodiments. Specific examples of components and arrangements are described to simplify the present description. These are merely examples and are not intended to be limiting.

Referring to <FIG>, a schematic block diagram showing an example parking brake apparatus <NUM> for an autonomously drivable vehicle, and constructed in accordance with an embodiment is illustrated. In <FIG>, electrical line connections are shown as solid lines, pneumatic lines connections are shown as dashed lines, and mechanical couplings are shown as double solid lines.

Parking brake apparatus <NUM> includes a controller area network (CAN) bus <NUM> to which a number of vehicle devices are connected to communicate with each other. The CAN bus <NUM> may be in a standardized serial communication format, such as SAE J1939, or in a proprietary format. It is conceivable that some or all of the vehicle devices be hardwired for communication instead of using the CAN bus <NUM> for communication.

Vehicle devices that may be connected to the CAN bus <NUM> include, but are not limited to, a first controller as a primary parking brake controller <NUM>, a second controller as a redundant parking brake controller <NUM>, and a third controller as an automated driver controller <NUM>. The primary parking brake controller <NUM> may provide to the CAN bus <NUM> a variety of signals including configuration messages, diagnostic status, and brake-specific signals such as parking brake status, and parking brake pressure. Similarly, the redundant parking brake controller <NUM> may provide to the CAN bus <NUM> a variety of signals including configuration messages, diagnostic status, and brake-specific signals such as parking brake status, and parking brake pressure. The automated driver controller <NUM> may provide to the CAN bus <NUM> a variety of signals including configuration messages, diagnostic status, the driving mode (i.e., autonomous, semi-autonomous, or driver-controlled), and desired intent of the status of the vehicle (e.g., stop, go, park). The CAN bus <NUM> enables the primary parking brake controller <NUM>, the redundant parking brake controller <NUM>, and the automated driver controller <NUM> to communicate with each other.

A primary compressed air supply <NUM> provides a source of compressed air in line <NUM> through a first <NUM>/<NUM> normally-open solenoid valve <NUM> and then in line <NUM> to a first supply port <NUM> of a parking brake valve <NUM>. As an example, the parking brake valve <NUM> may comprise a valve such available as part of the Bendix Intellipark® system, commercially available from Bendix Commercial Vehicle Systems LLC located in Elyria, Ohio. The first <NUM>/<NUM> normally-open solenoid valve <NUM> is disposed between the primary compressed air supply <NUM> and the parking brake valve <NUM>. Similarly, a secondary compressed air supply <NUM> provides a source of compressed air in line <NUM> through a second <NUM>/<NUM> normally-open solenoid valve <NUM> and then in a line <NUM> to a second supply port <NUM> of the parking brake valve <NUM>. The second <NUM>/<NUM> normally-open solenoid valve <NUM> is disposed between the secondary compressed air supply <NUM> and the parking brake valve <NUM>. Each of the first and second <NUM>/<NUM> normally-open solenoid valves <NUM>, <NUM> may comprise a Bendix AT-<NUM>™ solenoid valve, commercially available from Bendix Commercial Vehicle Systems LLC.

Although the above description describes the use of a <NUM>/<NUM> normally-open solenoid valve, it is conceivable that another type of valve may be used. For example, an antilock brake system (ABS) valve may be used, such as a Bendix M-<NUM>™ modulator valve, commercially available from Bendix Commercial Vehicle Systems LLC. For purpose of explanation, the use of <NUM>/<NUM> normally-open solenoid valves will be described herein.

The primary parking brake controller <NUM> is in the form of an electronic controller unit that is arranged to monitor signals on the CAN bus <NUM> to provide one or more control signals to apply the parking brake based upon control logic <NUM> that is stored in a data storage unit of the primary parking brake controller <NUM>. The primary parking brake controller <NUM> provides one or more signals on lines <NUM>, <NUM> to first and second control ports <NUM>, <NUM> of the parking brake valve <NUM> to control delivery of compressed air (originating from first and second compressed air supplies <NUM>, <NUM>) to first and second delivery ports <NUM>, <NUM> of the parking brake valve <NUM>.

The parking brake valve <NUM> is controlled by control logic <NUM> of parking brake controller <NUM> to vary pneumatic pressure in line <NUM> to one or more chambers of spring brake chambers <NUM> and also to vary pneumatic pressure in line <NUM> to trailer supply gladhands <NUM>. More specifically, when the parking brake of the vehicle is applied, the primary parking brake controller <NUM> provides one or more signals on lines <NUM>, <NUM> to parking brake valve <NUM> so as to exhaust air in one or more chambers of spring brake chambers <NUM>. The spring brake chambers <NUM> are operatively coupled via line <NUM> in known manner to parking brake springs <NUM>. When air in spring brake chambers <NUM> is exhausted and system air pressure drops to less than about <NUM> psi to <NUM> psi, the parking brake springs <NUM> are activated to apply the vehicle parking brake, as is known. Structure and operation of primary parking brake controller <NUM> and parking brake valve <NUM> for controlling operation of spring brake chambers <NUM> and parking brake springs <NUM> to apply the parking brake are conventional and, therefore, will not be further described.

At the same time the pneumatic pressure in line <NUM> to the one or more spring brake chambers <NUM> is varied to apply the parking brake, the pneumatic pressure in line <NUM> to the trailer supply gladhands <NUM> (which are connectable to a trailer parking brake of the vehicle) is varied to enable the trailer parking brake to be applied. Structure and operation of primary parking brake controller <NUM> and parking brake valve <NUM> for controlling operation of a trailer parking brake via the trailer supply gladhands <NUM> are conventional and, therefore, will not be further described.

One or more pressure-to-voltage transducers are coupled to corresponding one or more parking brake components. Each pressure-to-voltage transducer provides a voltage indicative of pressure associated with the corresponding parking brake component. More specifically, a first pressure-to-voltage transducer <NUM> senses pressure in pneumatic line <NUM> and provides a corresponding voltage on electrical line <NUM> to the primary parking brake controller <NUM>. A second pressure-to-voltage transducer <NUM> senses pressure in pneumatic line <NUM> and provides a corresponding voltage on electrical line <NUM> to the redundant parking brake controller <NUM>. A third pressure-to-voltage transducer <NUM> senses pressure in pneumatic line <NUM> and provides a corresponding voltage on electrical line <NUM> to the primary parking brake controller <NUM>. A fourth pressure-to-voltage transducer <NUM> senses pressure in pneumatic line <NUM> and provides a corresponding voltage on electrical line <NUM> to the redundant parking brake controller <NUM>.

The redundant parking brake controller <NUM> is in the form of an electronic controller unit that is arranged to monitor signals on the CAN bus <NUM> to provide one or more control signals to apply the parking brake based upon control logic <NUM> that is stored in a data storage unit of the redundant parking brake controller <NUM>. The redundant parking brake controller <NUM> provides a first control signal on line <NUM> to the first <NUM>/<NUM> normally-open solenoid valve <NUM> and a second control signal on <NUM> to the second <NUM>/<NUM> normally-open solenoid valve <NUM>.

The automated driver controller <NUM> is in the form of an electronic controller unit that is arranged to monitor signals on the CAN bus <NUM> indicating that the primary parking brake controller <NUM> is unavailable to apply the parking brake (or the trailer parking brake). The automated driver controller <NUM> then provides one or more signals on the CAN bus <NUM> to activate the redundant parking brake controller <NUM> to apply the parking brake.

In accordance with an aspect of the present disclosure, the redundant parking brake controller <NUM> and the automated driver controller <NUM> cooperate to provide a backup parking brake solution in the event of unavailability of the primary parking brake controller <NUM> to cause the parking brake to be applied The automated driver controller <NUM> monitors the primary parking brake controller <NUM>, detects unavailability of the primary parking brake controller <NUM> to cause the parking brake to be applied, and activates the redundant parking brake controller <NUM> to apply the parking brake when the unavailability is detected. More specifically, the redundant parking brake controller <NUM> has control logic <NUM> and the automated driver controller <NUM> has control logic <NUM> that cooperates with the control logic <NUM> of the redundant parking brake controller <NUM> to provide the backup parking brake solution. Although shown separately, it is conceivable that the redundant parking brake controller <NUM> and the automated driver controller <NUM> may be combined as a single controller, and that the control logic <NUM> and the control logic <NUM> may be combined as a single control logic block.

The first <NUM>/<NUM> normally-open solenoid valve <NUM> and the second <NUM>/<NUM> normally-open solenoid valve <NUM> are shown in <FIG> in their de-energized positions. In their de-energized positions shown in <FIG>, compressed air is supplied through the parking brake valve <NUM> to the spring brake chambers <NUM> and to the trailer supply gladhands <NUM>. Both parking brakes (i.e., the parking brake of the truck tractor and the parking brake of the truck trailer) are released (i.e., not applied). When the primary parking brake controller <NUM> signals the parking brake valve <NUM> to apply the parking brakes, compressed air in line <NUM> and compressed air in line <NUM> are exhausted to atmosphere, which allows the parking brakes to be applied in known manner.

However, if the parking brakes do not apply in response to the primary parking brake controller <NUM> to do so, the redundant parking brake processor <NUM> and the automated driver controller <NUM> cooperate to energize the first <NUM>/<NUM> normally-open solenoid valve <NUM> and the second <NUM>/<NUM> normally-open solenoid valve <NUM> so as to move them to their energized positions shown in <FIG>. In their energized positions shown in <FIG>, compressed air from the primary compressed air supply <NUM> and compressed air from the secondary compressed air supply <NUM> are blocked by the first and second <NUM>/<NUM> normally-open solenoid valves <NUM>, <NUM> from reaching the parking brake valve <NUM> to enable the parking brakes to be applied when the primary parking brake controller <NUM> signals the parking brake valve <NUM> to do so. When compressed air is blocked from reaching the spring brake chambers <NUM> and the trailer supply gladhands <NUM>, the parking brakes are applied.

More specifically, program instructions of a secondary parking brake control algorithm associated with the control logic <NUM> of the redundant parking brake controller <NUM> and the control logic <NUM> of the automated driver controller <NUM> are executed to provide a backup for the control logic <NUM> of the primary parking brake controller <NUM> in the event that the parking brakes are not applied in response to execution of program instructions of a primary parking brake control algorithm associated with the control logic <NUM> of the primary parking brake controller <NUM>.

The unavailability of the parking brakes to be applied can be due to a number of reasons. One reason may be that the primary parking brake controller <NUM> does not execute program instructions of the primary parking brake control algorithm to apply the parking brakes in response to a signal requesting the parking brakes to be applied. Another reason may be that one or more control signals from the primary parking brake controller <NUM> do not reach parking brake components so that the parking brakes can be applied. Yet another reason may be due to unresponsiveness of a portion of the parking brake valve <NUM> (e.g., an internal relay valve of the parking brake valve <NUM>). Still another reason may be due to loss of communication between certain vehicle components including components of the parking brake system. Other reasons for unavailability of the parking brakes to be applied are possible.

Referring to <FIG>, a flow diagram <NUM> depicts an example computer-implemented method of operating a parking brake apparatus in accordance with an embodiment. The computer-implemented method is for an autonomously drivable vehicle having a parking brake, a primary parking brake controller, and a secondary parking brake controller which is different from the primary parking brake controller.

In block <NUM>, the process begins by detecting unavailability of the primary parking brake controller to cause the parking brake to be applied. The detecting may be performed by looking at the memory of the primary parking brake controller or at a CAN bus for a signal that is indicative of unavailability of the parking brake to be applied in response the primary parking brake controller to do so. Then, in block <NUM>, the secondary parking brake controller responds by causing the parking brake to be applied in response to the unavailability of the primary parking brake controller. As an example, the secondary parking brake controller is responsive to the primary parking brake controller sending a signal stating that it is unavailable. As another example, the secondary parking brake controller is responsive to the primary parking brake controller simply not communicating at all when the secondary parking brake controller sees that the vehicle needs to park (e.g., when the secondary parking brake controller sees a message from an automated driver controller indicating that the vehicle needs to park). The process then ends.

In some embodiments, the secondary parking brake controller causes the parking brake to be applied when the unavailability of the primary parking brake controller to cause the parking brake to be applied is due to inability of the primary parking brake controller to provide one or more control signals for applying to one or more parking brake valves to enable one or more parking brake springs to apply the parking brake.

In some embodiments, the secondary parking brake controller causes the parking brake to be applied when the unavailability of the primary parking brake controller to cause the parking brake to be applied is due to inability of one or more control signals from the primary parking brake controller to reach one or more parking brake valves to enable one or more parking brake springs to apply the parking brake.

In some embodiments, the secondary parking brake controller causes the parking brake to be applied when the unavailability of the primary parking brake controller to cause the parking brake to be applied is due to absence of response of a parking brake valve of the parking brake system.

In some embodiments, unavailability of the primary parking brake controller to cause the parking brake to be applied is detected by the secondary parking brake controller receiving a signal from the primary parking brake controller stating that the primary parking brake controller is unavailable.

In some embodiments, unavailability of the primary parking brake controller to cause the parking brake to be applied is detected by the secondary parking brake controller receiving a signal from an autonomous driver controller stating that the primary parking brake controller is unavailable.

In some embodiments, the method is performed by a processor having a memory executing one or more programs of instructions which are tangibly embodied in a program storage medium readable by the processor.

Program instructions for enabling the secondary parking brake controller (e.g., the redundant parking brake controller <NUM> together with the automated driver controller <NUM> shown in <FIG> and <FIG>) to perform operation steps in accordance with the flow diagram <NUM> shown in <FIG> may be embedded in memory internal to the controllers. Alternatively, or in addition to, program instructions may be stored in memory external to the controllers. As an example, program instructions may be stored in memory internal to a different electronic controller unit of the vehicle. It is conceivable that any number of electronic controller units may be used. Moreover, it is conceivable that any type of electronic controller unit may be used. Suitable electronic controller units for use in vehicles are known and, therefore, have not been described. Accordingly, the program instructions of the present invention can be stored on program storage media associated with one or more vehicle electronic controller units. Program instructions may be stored on any type of program storage media including, but not limited to, external hard drives, flash drives, and compact discs. Program instructions may be reprogrammed depending upon features of the particular electronic controller unit.

A second embodiment of a parking brake apparatus is illustrated in <FIG> and <FIG>. Since the embodiment illustrated in <FIG> and <FIG> is generally similar to the embodiment illustrated in <FIG> and <FIG>, similar numerals are utilized to designate similar components, the suffix letter "a" being associated with the embodiment of <FIG> and <FIG> to avoid confusion.

Parking brake apparatus 100a comprises primary parking brake controller 120a, redundant parking brake controller 160a, and automated driver controller 180a. Primary parking brake controller 120a controls operation of parking brake valve 138a and spring brake chambers 143a in similar manner that primary parking brake controller <NUM> controls parking brake valve <NUM> and spring brake chambers <NUM> as described hereinabove in the embodiment of <FIG> and <FIG>.

Similarly, redundant parking brake controller 160a controls operation of first and second <NUM>/<NUM> normally-open solenoid valves 134a, 144a in similar manner that redundant parking brake controller <NUM> controls operation of first and second <NUM>/<NUM> normally-open solenoid valves <NUM>, <NUM> as described hereinabove in the embodiment of <FIG> and <FIG>. Automated driver controller 180a communicates with primary parking brake controller 120a and redundant parking brake controller 160a in same manner that automated driver controller <NUM> communicates with primary parking brake controller <NUM> and redundant parking brake controller <NUM> as described hereinabove in the embodiment of <FIG> and <FIG>.

In the embodiment of <FIG> and <FIG>, a first relay valve <NUM> is disposed between first <NUM>/<NUM> normally-open solenoid valve 134a and parking brake valve 138a. Similarly, a second relay valve <NUM> is disposed between second <NUM>/<NUM> normally-open solenoid valve 144a and parking brake valve 138a.

Compressed air is supplied from primary compressed air supply 130a in line 131a to first <NUM>/<NUM> normally-open solenoid valve 134a and then in line <NUM> to control port <NUM> of first relay valve <NUM>. Pneumatic line <NUM> interconnects delivery port <NUM> of first relay valve <NUM> and supply port 136a of parking brake valve 138a. Compressed air is also supplied from primary compressed air supply 130a in line <NUM> to supply port <NUM> of first relay valve <NUM>.

Compressed air is supplied from secondary compressed air supply 140a in line 141a to second <NUM>/<NUM> normally-open solenoid valve 144a and then in line <NUM> to control port <NUM> of second relay valve <NUM>. Pneumatic line <NUM> interconnects delivery port <NUM> of second relay valve <NUM> and supply port 146a of parking brake valve 138a. Compressed air is also supplied from secondary compressed air supply 140a in line <NUM> to supply port <NUM> of second relay valve <NUM>.

In the event that primary parking brake controller 120a is unavailable to cause the parking brake to be applied, redundant parking brake controller 160a and automated driver controller 180a cooperate to energize first and second <NUM>/<NUM> normally-open solenoid valves 134a, 144a to move them from their de-energized positons shown in <FIG> to their energized positions shown in <FIG> to apply the parking brake in the same manner as described hereinabove in the embodiment of <FIG> and <FIG>. However, in the embodiment shown in <FIG> and <FIG>, the use of first and second relay valves <NUM>, <NUM> in conjunction with first and second <NUM>/<NUM> normally-open solenoid valves 134a, 144a increases compressed air flow capacity to parking brake valve 138a to apply the parking brake while reducing the electrical power needed to energize first and second <NUM>/<NUM> normally-open solenoid valves 134a, 144a.

It should be apparent that the above description describes a backup parking brake system for a main parking brake system of an autonomously driven vehicle that may or may not have a human "driver" occupying the autonomously driven vehicle. If a human driver is occupying the autonomously driven vehicle, the human driver is not an integral part of the backup parking brake system (i.e., no manual action is required from the human driver to activate the backup parking brake system in the event of unavailability of the main parking brake system to apply the parking brake). Accordingly, the backup parking brake system causes the parking brake to be applied when the main parking brake system is unable to cause the parking brake to be applied, such as when a control signal is unable to reach a parking brake valve or when a parking brake valve is unresponsive.

It should also be apparent that the parking brake control algorithms associated with the parking brake apparatus <NUM> of <FIG> and <FIG> and the parking brake apparatus 100a of <FIG> and <FIG> are integrated into a practical application of implementing a low-cost backup parking brake mechanism for autonomously drivable vehicles. The backup parking brake mechanism is low cost since implementation requires the addition of essentially only a pair of <NUM>/<NUM> normally-open solenoid valves and a pair of controllers (or just a single controller if the redundant parking brake controller and the automated driver controller are combined).

A number of advantages result by providing an autonomously drivable vehicle with the above-described parking brake apparatus <NUM> of <FIG> and <FIG> (and the parking brake apparatus 100a of <FIG> and <FIG>) to provide the backup parking brake mechanism.

One advantage is that, even if the main parking brake system were to be unavailable, service brake pressure can be retained (i.e., does not need to be exhausted to atmosphere) so that the service brake can continue to hold the vehicle if needed. This eliminates the need to unload compressed air or the need to shut down the vehicle engine.

Another advantage is that since the first and second <NUM>/<NUM> normally-open solenoid valves <NUM>, <NUM> are controlled by one controller (i.e., the redundant parking brake controller <NUM>), there is no need to coordinate solenoid valve diagnostics between two controllers. This simplifies parking brake system design, and facilitates troubleshooting when servicing of the parking brake mechanisms is needed.

Still another advantage is that since two pressure-to-voltage transducers <NUM>, <NUM> are coupled to pneumatic line <NUM> to the spring brake chambers <NUM>, an independent indication of air pressure in spring brake chambers <NUM> is provided. Similarly, since two pressure-to-voltage transducers <NUM>, <NUM> are coupled to pneumatic line <NUM> to the trailer supply gladhands <NUM>, an independent indication of air pressure in trailer supply gladhands <NUM> is provided. This is advantageous because additional information may be used to satisfy additional functional safety requirements of the system.

Moreover, although the above description describes the use of pressure-to-voltage transducers <NUM>, <NUM>, <NUM>, <NUM>, it is conceivable that other types of transducers may be used, such as wheel speed-to-voltage transducers (i.e., wheel speed sensors). As an example, with the use of wheel speed sensors (either alone or in conjunction with pressure-to-voltage transducers), it is possible to monitor for the following sequence of events: (<NUM>) the vehicle is stationary with the parking brake released, (<NUM>) the vehicle is stationary with the main parking brake system indicating the parking brake is activated, and (<NUM>) the vehicle is moving with the main parking brake system indicating the parking brake is activated. Observation of this sequence of events indicates a roll away-from-park rather than a rollaway due to unavailability or inability of the parking brake to be applied when needed. If this occurs, the automated driver controller <NUM> could use the service brake to stop the vehicle or use the backup parking brake system to attempt to park the vehicle while continuing to monitor the wheel speed sensors to determine whether the vehicle is remaining stationary.

Aspects of disclosed embodiments may be implemented in software, hardware, firmware, or a combination thereof. The various elements of the system, either individually or in combination, may be implemented as a computer program product tangibly embodied in a machine-readable storage device for execution by a processor. Various steps of embodiments may be performed by a computer processor executing a program tangibly embodied on a computer-readable medium to perform functions by operating on input and generating output. The computer-readable medium may be, for example, a memory, a transportable medium such as a compact disk or a flash drive, such that a computer program embodying aspects of the disclosed embodiments can be loaded onto a computer.

Claim 1:
A parking brake apparatus (<NUM>) for an autonomously drivable vehicle having components of a parking brake system for applying a parking brake, the parking brake apparatus comprising:
a first controller (<NUM>) arranged to provide one or more control signals to be applied to components of the parking brake system to apply the parking brake in response to a signal requesting the parking brake to be applied; and
a second controller (<NUM>) arranged to provide one or more control signals to be applied to other components of the parking brake system to apply the parking brake in response to unavailability of the first controller to cause the parking brake to be applied, characterized in that the parking brake apparatus further comprising:
a parking brake valve (<NUM>) controllable by the first controller (<NUM>);
a first <NUM>/<NUM> normally-open solenoid valve (<NUM>) disposed between a primary compressed air supply (<NUM>) and the parking brake valve (<NUM>); and
a second <NUM>/<NUM> normally-open solenoid valve (<NUM>) disposed between a secondary compressed air supply (<NUM>) and the parking brake valve (<NUM>);
wherein the second controller (<NUM>) is arranged to provide a first control signal to the first normally-open <NUM>/<NUM> solenoid valve (<NUM>), and a second control signal to the second normally-open <NUM>/<NUM> solenoid valve (<NUM>),
a first relay valve (<NUM>) disposed between the first <NUM>/<NUM> normally-open solenoid valve (<NUM>) and the parking brake valve (<NUM>); and
a second relay valve (<NUM>) disposed between the second <NUM>/<NUM> normally-open solenoid valve (<NUM>) and the parking brake valve (<NUM>).