I Electric Brake Control System for Trailers FIELD OF THE INVENTION [0001] The present invention relates to a controller for electric brakes, in particular a controller for electric brakes fitted to a trailer. BACKGROUND TO THE INVENTION [0002] Trailers that are fitted with electric brakes require an electric brake controller to be installed in the tow vehicle to apply power to the trailers electric brakes for them to operate. There are basically two types of electric brake controllers, fixed output and progressive types. Both types can be adjusted to suit various trailer loads and brake efficiency by what is commonly referred to as a gain control. Electric brake controllers also have a manual override for applying the trailer's brakes without applying the tow vehicle's brakes. An electric brake controller sends power to a trailer's electric brakes when the stop lights of the tow vehicle are activated. Both of these types of controllers are limited in their operation and can result in dangerous conditions such as inadequate braking force or wheel lockup. [0003] A fixed output electric brake controller is very simple in operation, setting the amount of power going to the electric brakes in accordance with the gain control. The power applied does not take into account the speed of the towed vehicle, nor adjust in any manner during the braking cycle as the speed of the towed vehicle decreases. [0004] Progressive electric brake controllers offer advantages over fixed controllers and self-adjust in accordance with the deceleration of the towed vehicle. They typically incorporate a pendulum device or accelerometer to measure the deceleration of the towed vehicle. The amount of deceleration of the tow vehicle is directly proportional to the resultant power that the electric brake controller sends to the trailer electric brakes for stopping the trailer. This is an improvement over the fixed output type controllers as the trailers electric brakes apply progressively with the tow vehicle brakes. [0005] The problem when using a pendulum or an accelerometer to measure deceleration is their instability when travelling on undulating roads. They are also affected by the vertical angle a trailer is being towed; for example when the trailer is travelling up or down a hill. These problems can result in inconsistent and uncomfortable application of the trailers electric brakes.
[0006] Existing electric brake controllers do not take into account how fast a trailer is travelling. For safe and comfortable braking existing electric brake controllers require the driver to increase the gain on the controller at higher speeds and then decrease the gain when entering a town at slower speeds. This may require the driver to take their eyes off the road to find the gain control on the electric brake controller to make the adjustment, with its obvious safety implications. Also, if the driver forgets or is unable to adjust the gain the resulting braking performance will most likely be sub optimal. [0007] Recent times have seen the introduction of electronic sway control systems (ESC) being fitted to trailers which apply the trailer brakes when excessive sway is detected. These systems use various sensors to measure the lateral acceleration or yaw of a trailer and apply the trailer's electric brakes when a predetermined threshold is breached. Such systems are disclosed in Australian patent application nos. 2012225142, 2014100943 and 2014204434. [0008] None of the existing sway control systems measure the speed of the trailer as a reference. The output of the existing sway controls is set for an average speed. This results in the current sway control systems being too sensitive at low speeds and not sensitive enough at higher speeds. [0009] The output of the existing sway control systems is factory set for the specific weight of a trailer and brake efficiency, or alternatively generic settings are used. When a sway control system is pre-programed for a specific trailer weight it is sub optimal when the trailer weight changes. The generic systems are programed for an average weight and brake efficiency and are thus too sensitive for lighter trailers and not sensitive enough for heavier trailers. Both regimes suffer from excessive brake application when not needed at slower speeds and then not enough braking at higher speeds when swaying starts. [0010] The object of the invention is to provide an electric brake control system for a trailer that takes into account the speed of the trailer to produce optimal braking performance.
SUMMARY OF THE INVENTION [0011] In a first aspect the invention comprises a brake control system for a trailer comprising an electronic controller module and trailer wheel speed sensor, wherein the trailer wheel speed sensor produces a trailer wheel speed signal, and the controller module produces a brake control signal in response to a brake activation signal and the trailer wheel speed signal. [0012] Preferably the controller module calculates the deceleration of the trailer from the wheel speed signal and asserts the brake control signal in proportion to the deceleration of the trailer. [0013] In preference the controller module monitors the wheel speed signal and de asserts the control signal for a pre-determined period if the wheel speed signal indicates a zero speed. [0014] The brake control system may further comprise a gyroscope producing a yaw rate signal, wherein the control module monitors the yaw rate signal and wheel speed signal and asserts the brake activation signal if the yaw rate exceeds a predetermined safe yaw rate corresponding to the wheel speed. [0015] It should be noted that any one of the aspects mentioned above may include any of the features of any of the other aspects mentioned above and may include any of the features of any of the embodiments described below as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS [0016] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows. [0017] Figure 1 is a schematic diagram of an electric brake control system of the current invention fitted to a towing vehicle and a towed vehicle. [0018] Figure 2 shows a speed sensor used in conjunction with the invention. [0019] Figure 3 is a graph of brake control voltage produced by the invention as a _r function of deceleration at various speeds and gain settings. [0020] Figure 4 is a graph of safe trailer yaw rate versus speed used by the invention to determine brake application. [0021] Figure 5 is a graph of brake control voltage produced by the invention as a function of excess yaw rate. COMPONENT LISTING [0022] The drawings include the following integers: 10 Electric brake control system 20 Towing vehicle 30 Trailer 32 Trailer wheels 34 Trailer electric brakes 36 Slotted brake drum 37 Brake drum teeth 38 Trailer stop lights 40 Control module 42 Gyroscope 44 Wheel speed sensor / Hall effect sensor 52 Gain control 53 Gain signal 54 Manual override 55 Override signal 60 Power 62 Brake activation signals 70 Plug and socket connection 80, 81 Wheel speed signals 82 Yaw signal 84 Brake control signals DETAILED DESCRIPTION OF THE INVENTION [0023] The following detailed description of the invention refers to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration. [0024] The present invention provides an electric brake control system for a trailer that accurately measures the speed, deceleration and yaw rate of the trailer and correspondingly controls the operation of the brakes to produce optimal braking performance and minimise sway of the trailer at varying speeds and with varying loads. [0025] Figure 1 shows a schematic of the system 10 fitted to a towing vehicle 20 and trailer 30. The system comprises a control module 40 with gyroscope 42 and wheel speed sensors 44 fitted to the trailer; and gain control 52 and manual override 54 fitted to the towing vehicle. The control module receives power 60 and control signals comprising brake activation signal 62, gain signal 53 and override signal 55 via a conventional trailer plug and socket connection 70. Alternatively wireless transmission of some or all of the various signals could be employed. Sensor signals comprising wheel speed signals 80, 81 and yaw rate signal 82 are monitored by the control module which in turn produces brake control signals 84. [0026] The gain control 52 is set by the vehicle operator and produces gain signal 53 indicative of the weight of the tailer and efficiency of the brakes. The gain is initially set to a nominal 50% and then adjusted during road tests to ensure that the towing vehicle and trailer stop together smoothly. The manual override 54 produces an override signal 55 for applying the brakes independent of the brake activation signal 62 (which is just the brake light signal for the trailer). The manual override is provided to allow for testing and emergency situations; it is also a statutory requirement in some jurisdictions. [0027] The wheel speed signals 80, 81 are produced by the wheel speed sensor 44 which is preferably implemented as a Hall effect sensor mounted adjacent to the brake drum 36 as shown in Figure 2. The periphery of the brake drum 36 is slotted to produce a series of teeth 37 which are detected by the sensor 44 as the drum rotates in unison with the trailer wheel. Preferably the teeth are arranged to produce a 300Hz signal when the trailer is travelling at 60 km/h. Measuring rotation of the wheels to determine speed has been chosen over other methods so that lockup of the wheels can also be detected. A separate sensor is fitted to each wheel as either wheel may lockup independently. The system shown is for a two wheel trailer and can obviously be extended to trailers with 4 or more wheels. [0028] A gyroscope 42 is used for stability control functionality to avoid erroneous signals associated with accelerometers which are encountered when the trailer is used on an undulating surface, is ascending or descending hills, or hits a pot hole or rut. The yaw rate signal 82 of the gyroscope reflects the hitch angle of the trailer with respect to the towing vehicle. [0029] The electric brakes 34 produce a braking force in proportion to the amplitude of the brake control signal 84, which typically varies between 0.2 V and 12 V. [0030] The control module 40 is preferably microcontroller based, although other implementations may be used as is known in the art such as discrete logic, programmable logic or even purely analogue circuitry. [0031] The first function of the control module is to apply the brakes in response to the control signals whilst considering the sensor signals. Optimal braking performance is achieved by setting the brake control signal 84, and hence braking force, according to the speed and deceleration of the trailer. The speed is determined from the wheel speed signals 81 and 82 as is the deceleration. The brake control signal is modified by the gain signal 53 to compensate for vehicle weight and braking efficiency. When the brake activation signal 62 or override signal 55 is asserted the controller will in turn assert the brake control signal 84. The magnitude of the brake control signal is set to an initial value dependent on the speed of the trailer. As the trailer decelerates the brake control signal is increased until the trailer comes to a halt. When the brake activation signal is de-asserted the brake control signal is de-asserted in turn (assuming that no override signal is present). [0032] Figure 3 show the brake control signal produced by the controller as a function of trailer deceleration at various speeds for gain settings of 33% and 66% respectively. In essence the brake control voltage and hence brake force is increased with gain, speed and deceleration. [0033] The second function of the control module is to avoid wheel skid. When the controller is applying the brakes by use of the brake control signal 84 the wheel speed signals 80 and 81 are monitored to determine the speed of each wheel. If the speed of a wheel is zero then it is assumed that wheel skid is occurring and the brake control signal is de-asserted for 200 milliseconds, before being applied again. The speed is then monitored again to ensure that the wheels have stopped skidding. This is repeated until the trailer has come to a halt or the brake activation signal has been de-asserted. This system ceases to operate when the brake activation signal is below 0.5 Volts so that the brakes are operational when the trailer is stationary. [0034] The third function of the control module is to alleviate trailer sway. This is primarily achieved by monitoring the yaw rate signal 83 from the gyroscope in conjunction with a speed signal 80 or 81. The yaw rate of the trailer will vary as it either goes around a bend or if it is swaying. The yaw rate is constantly compared with a predetermined safe yaw rate for the speed at which the trailer is travelling; a graph of which is shown in Figure 4. If the safe yaw rate is exceeded the brake activation signal is asserted in proportion to the speed, acceleration and gain setting (as previously discussed and shown in Figure 3) and the extent to which the yaw rate exceeds the safe level. Figure 5 shows the brake activation voltage versus excess yaw rate for different gain settings at 90 km/h. The brake activation signal is asserted until the lateral acceleration returns below the safe level for the present speed of the trailer. [0035] The reader will now appreciate the present invention which provides a brake control system for a trailer with optimal braking performance according to speed, controls wheel skid; and controls trailer yaw. [0036] Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus. Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in this field. [0037] In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers.