Background of the Invention Federal Motor Vehicle Safety Standard 121 (FMVSS 121) requires newly manufactured semi-trailers and trailers that are hitched to a tractor-trailer combination to be equipped with an antilock brake system (ABS). The term"semi-trailer"refers to the type of trailer commonly found on combination long haul trucks. The semi-trailer mounts on a fifth wheel of a tractor. In the case of two or three trailers hitched to one tractor, the fifth wheel is part of a dolly which is attached to the back end of a leading trailer. For this reason, the second and subsequent trailers in such an assembled combination are not literally semi-trailers because the combination of a dolly and a semi-trailer results in substantially the full weight of the trailer being independently supported by the dolly and trailer wheels.

FMVSS 121 applies to trucks, buses, and trailers that are equipped with air brake systems. Trailers that are hitched to long haul type trucks having a tractor and trailer or semi-trailer combinations are required to have ABS with direct control of the wheels on at

least one axle of semi-trailers and on at least one front and one rear axle of full trailers.

Wheels on the other axles of the trailer may be indirectly controlled by the ABS.

Trailer ABS and the controls therefore, are well known in the art. ABS limits the braking pressure if a wheel starts to lock up, keeping the wheels turning in an emergency stop. During braking, friction between the tread of the tire and the road surface is at its maximum just before the wheel locks up. The distance the vehicle will skid with locked up wheels is much greater than if the wheels were kept rotating at a point just before they lock up. Thus, to develop optimum wheel slip it is beneficial to keep the wheels rotating at a rate of about 5%-10% of their free rolling velocity for the vehicle's velocity. This allows maximum braking force to be applied without brake lockup, or skidding. If standard non- ABS brakes are applied too hard, the wheels lock or skid, and the driver loses the ability to steer. If steering is lost, the vehicle skids in a straight line. To prevent skidding with standard non-ABS brakes, the driver had to either pump the brakes or sense the lockup and release the brakes entirely. During ABS controlled trailer braking, the wheels keep their grip on the road. Thus, ABS helps the driver to maintain steering and avoid skidding during emergency braking.

Conventional ABS utilize the following basic methodology. The ECU continuously monitors all available wheel velocity sensors. Data from the sensors is used by the ECU to calculate wheel velocity and acceleration and to make a best estimate of the actual vehicle velocity. During routine braking operations, if there is no indication of excessive wheel slip, and the ECU interprets this condition as normal and the ABS remains inactive. If the difference between the velocity and acceleration of one or more wheels and the estimated vehicle velocity exceeds predetermined limits, the ECU detects impending

wheel lock and activates the ABS. When the ABS is activated, the ECU controls the modulator to reduce the braking force that is delivered to the wheels of the trailer. As a result, the vehicle skidding is prevented while stopping distance is minimized.

Conventional trailer ABS utilize four basic configurations for ABS control of a braking system for two or more axles of a trailer, as shown in the plan view diagrams of Figs. 1-4. The components of a trailer ABS include an electronic control unit (ECU) 12, a modulator 14, and two or more wheel mounted wheel velocity sensors 16. During active ABS control of braking, the ECU 12 utilizes the modulator to 14 modulate the braking force that is applied to the wheels 18 by the air brake chambers 20.

The term"direct control"refers to a wheel that is equipped with wheel velocity sensors 16 and is controlled by the ECU 12 with a modulator 14 in response to the wheel velocity sensors 16. If this modulator 14 also controls the air pressure to the brakes of another wheel that does not have wheel velocity sensors, the other wheels are indirectly controlled wheels. Fig. 1 illustrates the most common configuration for a trailer ABS that includes an axle having directly controlled wheels 22 and an axle having indirectly controlled wheels 24.

Fig 1 illustrates the most basic configuration, a single axle having directly controlled wheels 22 plus at least one axle having indirectly controlled wheels 24. The ABS monitors a single axle of the trailer, with a pair of opposing wheel velocity sensors 16, and a single brake pressure modulator 14 sends the same braking pressure to all of the wheels 18. Fig. 2 illustrates an improvement upon the basic configuration, an axle having directly controlled wheels 22 plus as least one axle having indirectly controlled wheels 24 with separate modulation of the braking pressure on the left and right sides of the trailer.

As shown in Fig. 2, the system includes a brake pressure modulator 14 for the left hand side of the trailer, a brake pressure modulator 14 for the right hand side of the trailer, and the ABS monitors a single axle of the trailer with a pair of opposing wheel velocity sensors 16. Fig. 3 illustrates an additional improvement upon the basic configuration, wherein all of the wheels are directly controlled and brake pressure is modulated separately on each side of the trailer. As shown in Fig. 3, the system includes a brake pressure modulator 14 for the left hand side of the trailer, a brake pressure modulator 14 for the right hand side of the trailer, and a pair of opposing two wheel velocity sensors 16 for each axle. Fig 4 illustrates another configuration, direct control of all of the wheels, wherein the brake pressure is separately modulated at each axle 22. Each axle 22 has its own brake pressure modulator 14 and two wheel velocity sensors 16.

As explained above, the trailer ABS must determine the velocity of the entire vehicle during ABS control. Unfortunately, conventional trailer ABS do not always accurately estimate the velocity of the trailer. The most common trailer ABS configuration, a single axle having directly controlled wheels 22 plus at least one axle having indirectly controlled wheels 24, is particularly vulnerable to this problem.

Inaccuracies may occur due to the following circumstances. The trailer ABS monitors a pair of opposing wheel velocity sensors at only one axle so the wheel velocity information is minimal, the wheels of semi-trailers are located at one end of the vehicle. The trailer ABS does not independently control braking pressure at each wheel. Finally, the directly monitored wheels are located on either end of the same axle, so they are susceptible to locking up at the same time and at the same rate.

Figs 5 and 6 illustrate a problem that may occur during braking of a trailer having only a single axle with directly controlled wheels in icy or other poor traction conditions.

In this scenario, both directly monitored wheels are approaching lock-up at the same time and at the same rate, and ECU erroneously senses that the velocity of the trailer is approaching zero rather than that the trailer is still moving and that it is about to skid. Fig 5 is a plot of two wheel velocities (mph) vs. time (seconds) that is calculated with the signal from each wheel speed sensor. Both of the wheels are locked at approximately eight seconds. The wheel velocities shown in Fig. 5 are combined to generate an estimated trailer velocity, and Fig. 6 is a plot of the estimated trailer velocity and actual trailer velocity (mph) vs. time (seconds). Fig. 6 illustrates the wheels of the trailer locking up on ice while the trailer continues to move. At time a time of eight seconds, the wheels are locked and thus the estimated velocity of the trailer is zero, but the actual velocity of the trailer is approximately 15 mph. In this case, both wheels have locked, but the ABS cannot detect that the trailer is still moving because it obtains all of its trailer velocity data from the two locked wheels. Based upon such an erroneous velocity estimate the ECU would fail to activate ABS control. However under such circumstances, the ABS must to be activated to prevent skidding. It is therefore an object of the invention to provide an ABS which determines the vehicle of the trailer with a greater degree of accuracy.

Summary of the Invention The present invention provides a trailer ABS which uses the vehicle velocity calculated by the tractor to augment the vehicle velocity that is calculated by the trailer alone. As a result, the trailer ABS calculates vehicle velocity more accurately. The improved trailer velocity data permits optimum trailer ABS performance.

The trailer antilock brake system preferably includes at least one wheel velocity sensor that generates a trailer wheel velocity signal indicative of a reference trailer velocity, an electronic control unit that generates a corrected trailer velocity based on a tow vehicle velocity and the trailer wheel velocity signal, a trailer brake system, and an antilock modulation device coupled to the electronic control unit the trailer brake system.

The control unit controls the activate of the antilock modulation device based on the corrected trailer velocity.

In a preferred embodiment, the electronic control unit calculates the reference trailer vehicle velocity in response to the trailer wheel velocity signal, calculates the tow vehicle velocity in response to the reference trailer vehicle reference velocity and a received reference tow vehicle velocity, and calculates the corrected trailer velocity utilizing the reference trailer velocity and the tow vehicle velocity. The control unit then detects impending wheel lock by comparing the corrected trailer velocity with an individual wheel velocity and acceleration determined from the trailer wheel velocity signal, and activates the antilock modulation device to modulate braking force upon detection of impending wheel lock.

The system may incorporate a plurality of wheel velocity sensors to produce a plurality of trailer wheel velocity signals. In such cases, the control unit calculates the reference trailer

velocity based on the plurality of trailer wheel velocity signals. The trailer wheel velocity signals may be rate limited. Further, the corrected trailer velocity may be an average of the trailer wheel velocity signals that is biased toward the reference trailer velocity or a weighted average of the trailer wheel velocity signals.

The trailer antilock brake system can be incorporated with any type of convention system including antilock brake control strategy including individual control, select low control, select high control, and select smart control. Other advantages and features of the invention will become apparent from the following detailed description of the preferred embodiments and the accompanying drawings.

Brief Description of the Drawings The invention will now be described with reference to certain preferred embodiments thereof and the accompanying drawings, wherein: Fig. 1 is a plan view of the underside of a trailer that is equipped two wheel speed sensors and one modulator; Fig. 2 is a plan view of the underside of a trailer that is equipped two wheel speed sensors and two modulators configured side to side; Fig. 3 is a plan view of the underside of a trailer that is equipped four wheel speed sensors and two modulators configured side to side; Fig. 4 is a plan view of the underside of a trailer that is equipped two wheel speed sensors and two modulators configured axle to axle ; Fig. 5 is a plot of two wheel velocities (mph) vs. time (seconds);

Fig. 6 is a plot of estimated trailer velocity and actual trailer velocity (mph) vs. time (seconds); Fig. 7 is a plot of the estimated trailer velocity, corrected trailer velocity, and actual trailer velocity (mph) vs. time (seconds); Fig. 8 is a schematic plan view of a trailer that is equipped with an ABS in accordance with the present invention ; Fig. 9 is a side elevation view of an ECU and a modulator; Fig. 10 is a side elevation view of a wheel speed sensor; and Fig. 11 is a flow chart of an ABS calculating a corrected trailer velocity signal in accordance with the present invention.

Detailed Description of the Preferred Embodiments Fig. 8 is a plan view of a trailer ABS 26 in accordance with the present invention.

The ABS 26 includes the following components: an electronic control unit 12, a modulator 14, wheel velocity sensors 16, control lines 28, and a tractor velocity input device 30. The ABS 26 preferably interfaces with the following components that control the braking force of a braking system: air supply reservoirs 32, air brake chambers 20, wheels 18, and air reservoir supply lines 34, and air brake chamber supply lines 36. The ECU 12 is a compact unit that monitors wheel velocity sensors 16, controls the modulator 14, diagnoses ABS malfunctions, and stores failure specific fault codes. As shown in Fig. 9, the ECU 12 is preferably attached to the modulator 14 with a mounting bracket 38. Alternatively, the ECU 12 may be directly mounted to the frame of the trailer. One ECU 12 typically

monitors either two or four wheel velocity sensors 16 and control either one or two modulators 14. If necessary more than one ECU 12 may be used on a single trailer.

The modulator 14 is an antilock modulation valve that modulates the braking force of the brake system by regulating the air pressure that is supplied from the air supply reservoirs 32 to the brake chambers 20 by the air brake chamber supply lines 36. Air pressure is preferably controlled and modulated with brake pressure hold and release solenoids that are housed within the modulator 14. Each modulator 14 can control either two or four brake chambers 20 on an ABS equipped trailer. A modulator 14 can have the ECU 12 mounted to it, or it may be a stand alone modulator that is controlled remotely by the ECU 12.

As shown in Fig. 10, the wheel velocity sensor 16 preferably includes a wheel end velocity sensor 40 and a tone wheel 42. The wheel end velocity sensor 40 is a preferably a single point variable reluctance magnetic sensor that generates an alternating current wheel velocity signal in response to the movement of teeth on the tone wheel 42. The wheel velocity signal is interpreted by the ECU 12 to monitor wheel velocity. The tractor velocity input device 30 is preferably a control wire, however other devices may also be utilized including infra red transmission, radio frequency transmission, or a pulse signal that is transmitted through existing electrical lines or the chassis ground.

The operation of the ABS will now be described. Fig. 11 is a flow chart of the system that illustrates how a tractor velocity signal 44 is utilized to calculate a corrected trailer velocity signal 46, in accordance with the present invention. Trailer wheel velocity signals 48 are input from the wheel velocity sensors 16 into the ECU 12, and the ECU 12 calculates an estimated trailer velocity signal 50. The trailer velocity signal 50 is

preferably based upon a weighted average of the signals from each of the wheel velocity sensors 16. Next, the tractor velocity signal 44 is transmitted from the tractor to the ECU 12. The tractor velocity signal 44 and the trailer velocity signal 50 are calculated entirely independently of one another ; thus, differences may occur between the tractor velocity signal 44 and the trailer velocity signal 50. These differences are caused by factors such as differing tire sizes between the tractor and the trailer. During a period when the vehicle is operated at constant velocity, the ECU 12 utilizes an algorithm such as an integrator function to calculate scaling factors to equate the tractor vehicle velocity estimate with the trailer vehicle velocity. The scaling factors are utilized to calculate a compensated tractor velocity signal 52. The compensated tractor velocity signal 52 and the trailer velocity signal 50 are utilized to generate the corrected trailer velocity signal 46. In accordance with another preferred embodiment of the invention, the corrected trailer velocity signal is an average of the signals that is biased toward the trailer velocity signal 50 avoiding problems due to incorrect tractor velocity information.

During vehicle operation, two or more trailer wheel velocity sensors 16 communicate with the ECU 12 to determine if one or more wheels are trying to lock up during braking. Utilizing the trailer wheel velocity signals 48, the ECU 12 calculates individual wheel velocities and accelerations. As explained above, the ECU 12 utilizes the wheel velocity signals 48 and the tractor velocity signal 44 to calculate a corrected trailer velocity signal 46. Utilizing the relationship of corrected trailer velocity 46, wheel velocity, and wheel acceleration ; the ECU 12 makes a new assessment of conditions and updates a control signal to the modulator 14, preferably at a rate of 100 times per second.

Under normal non-ABS conditions, the modulator 14 operates exactly like a conventional

mechanical valve. However, if the ECU 12 detects that one or more wheels are rapidly decelerating relative to the corrected trailer velocity signal 46 during braking, then the ECU 12 senses impending wheel lockup and activates ABS.

During active ABS, the ECU 12 signals the modulator 14 to limit or reduce the braking pressure on the wheels, by over-riding the supply of air to the brake chambers 20.

The modulator 14 then causes the braking action, on the wheels, to be momentarily reduced. As a wheel approaches lock up the ECU 12 directs the modulator 14 to adjust air pressure to the brake chambers 20, preferably at a rate of up to five times a second. The ABS 26 utilizes a release mode and a hold mode. During ABS release mode, supply air is held off while the brake chambers 20 are vented to the atmosphere. In ABS hold mode, supply air is blocked and brake chamber air is held constant. Air is applied to the brake chamber 20 at a controlled rate by modulating the hold side of the modulator 14. The modulator 14 pulses the braking action to the wheels so the ECU 12 can determine how much traction is available and, in turn, how much braking action the wheels should be given. This pressure adjustment allows the wheels to rotate and develop optimal slip without locking. As a result, ABS prevents skidding of the trailer and allows the driver to maintain steering control.

In accordance with another preferred embodiment of the invention, the trailer velocity signal 50 may be rate limited. Rate limiting is utilized because an individual wheel on a trailer is capable of changing its velocity more rapidly that a large vehicle such as a trailer. For example, the wheel velocity can change faster than the vehicle itself when the vehicle is driving on ice or bouncing over bumps in the road. A rate limited velocity signal cannot exceed a predetermined maximum rate of change of velocity that is bounded

by the physical capability of the vehicle. Thus, rate limiting prevents an excessively high wheel velocity reading from adversely influencing the overall trailer velocity estimate.

ABS utilize preferably utilize one of the following four main control strategies : individual control, select low control, select high control, select high control, and select smart control. When operating on high traction surfaces with a loaded trailer, there is little difference between types of ABS control. However, performance differences related to vehicle stability and stopping distance appear when vehicles are lightly loaded and operating on variable and poor traction surfaces such as wet pavement, ice, and snow. In the case of individual control, a single wheel velocity sensor 16 controls a modulator 14 that operates the brakes at one wheel site. Thus, the braking pressure to each wheel is controlled by a separate modulator 14. Individual control offers optimal vehicle stability and stopping distance; however, it is also the most costly arrangement. Select low systems utilize wheel velocity sensors 16 to monitor several wheels and control them with a single modulator 14. Control is based on the wheel which is at the lowest velocity. Modified select low systems incorporate a delay before releasing braking pressure to bias activating ABS control slightly away from the low velocity wheel. Select low systems offer good vehicle stability, but their stopping distance is greater when compared to individual control. Select high systems also monitor several wheels and control them with a single modulator 14. Control is based on the wheel which is at the highest velocity. Modified select high systems activate a release before the low velocity wheel becomes severely locked. Select high systems generally have good stopping distances at the expense of vehicle stability, and select high systems may also have an increased risk of tire flat spotting. Select smart systems, operate as select low systems when there is little difference

in wheel velocity signals 40, and they operate as select high systems when there is a significant difference between sites. Thus, select smart systems offer many of the advantages of individual control systems while using a simpler design and fewer components. A corrected the trailer vehicle velocity signal 46 that has been generated in accordance with the present invention may be utilized to enhance the performance of any of the above referenced control methods.

Significantly improved ABS performance may be realized by correcting the trailer velocity in accordance with the present invention. Fig. 6 is a plot of estimated trailer velocity and actual trailer velocity (mph) vs. time (seconds) for a conventional ABS, and Fig. 7 is a plot of estimated trailer velocity, corrected trailer velocity, and actual trailer velocity (mph) vs. time (seconds) for an ABS in accordance with the present invention.

As shown in Fig. 6, at a time of eight seconds both wheels are locked, the estimated trailer velocity is zero mph, and the actual velocity of the trailer is 15 mph. In this case, the conventional ABS cannot detect that the trailer is still moving at 15 mph because it obtains all of its trailer velocity data from the two locked wheels. Based this erroneous velocity estimate the conventional ABS fails to activate ABS control. In Fig. 7, at a time of eight seconds both wheels are locked and the original estimated velocity of the trailer is zero ; however, in accordance with the invention, the corrected trailer velocity is 13 mph.

Therefore, the magnitude of the corrected velocity of the trailer is comparable to the actual velocity of the trailer, and the ABS is able to detect that the trailer is still moving regardless of the signal it obtains from the two locked wheels. As a result, the ABS is activated when it is needed.

The invention has been described with reference to certain preferred embodiments thereof. It will be understood, however, that modification and variations are possible within the scope of the appended claims. Correcting the trailer vehicle reference velocity in accordance with the present invention will improve the performance of any ABS equipped trailer, such as are found in articulated busses and other types of articulated combination trucks. The present invention is not limited to ABS that control braking force on air brake equipped trailers, it may also be utilized for other types of braking systems such as hydraulic or electromagnetic brakes.