Abstract:
A suspension system for towed vehicles automatically adjusts for different payloads and road conditions without the need for manual shims used in prior art suspensions. automatically adjusts a mechanical bias on the vehicle to keep the suspension within a preferred ride height range. A hydraulic cylinder between the frames of the towing and the towed vehicles provides a mechanical biasing force to the towed vehicle. A sensor measures the ride height and causes the hydraulic cylinder to retract or extend according to the bias needed to maintain the suspension in the preferred ride height range.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a vehicle suspension system, and in particular to a suspension system for towed vehicles such as trailers, jeeps and dollies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vehicle suspension systems are important for both maintaining vehicle control and for isolating a vehicle&#39;s contents from bumps and vibrations caused by imperfections in the road. Typical suspension systems may utilize one or more of springs, shock absorbers (or dampers), hydraulic cylinders and air bags. 
         [0003]    Isolating the vehicle&#39;s contents from bumps and vibrations caused by road imperfections allows the vehicle to ride relatively undisturbed. Suspension systems are designed to absorb the energy generated when the vehicle rides over unevenness on the road and to dissipate the energy without causing undue vertical movement of the vehicle body. 
         [0004]    A proper suspension system also assists in maintaining vehicle control by minimizing weight transfer of the vehicle from side to side and from front to back that occurs during movement, braking and acceleration. This allows all of the wheels to better maintain contact with the ground, and maintains the vehicle&#39;s ability to steer, brake, and accelerate. 
         [0005]    The present invention relates to towed vehicles used to carry a payload such as trailers, as well as trailer accessory vehicles known as jeeps, dollies, tag dollies and boosters. For simplicity, the term “dollies” will be used to refer collectively to all of such trailers and trailer accessory vehicles. 
         [0006]    An effective dolly suspension system may help to prevent “bridging”, a condition that occurs when the wheels from one or more axles unload, or lose contact with the ground, typically caused by uneven road conditions. As a result, the entire load is borne by the remaining axles which may damage the dolly, and cause reduced brake performance or even loss of control if the axles are self-steering. 
         [0007]    Air bag suspension systems have an “air bag performance zone” defined by the manufacturer. The air bag performance zone determines the preferred range of ride heights, defined as the distance between axle and the frame. It is important that the suspension system maintain a ride height within this preferred ride height range in order to provide the most effective suspension and to ensure optimal performance and operating life for the air bags. One prior art suspension system for a dolly includes both a hydraulic bias mechanism (comprising one or more hydraulic cylinders) and an air suspension mechanism (comprising one or more air bags). In normal operation, the hydraulic cylinders are fully extended thereby acting effectively as struts between the towing vehicle and the dolly to bias the dolly frame toward the ground. The air bags provide the suspension performance. In the event that the air bags threaten to collapse, the cylinders are immediately retracted to reduce the downward bias of the dolly and to relieve the air bags. The operator of the dolly manually adjusts the suspension system upon loading or unloading cargo so that the ride height falls within the preferred range by removing or inserting metal shims in the hydraulic bias system so as to increase or decrease the downward bias. The manual removal and insertion of shims according to the payload is time-consuming. 
         [0008]    The distribution of the load can also be affected by a significant change in road conditions, such as an abrupt change in road grade or cornering an elevated curve. This change in distribution can affect the ride height and create handling problems. 
         [0009]    When the dolly is traveling, variations on the surface of the road may affect the ride height as well. For example, if the dolly encounters a bump on the road, the wheel (and axle) will experience upward vertical acceleration and rise, resulting in the compression of the air bags and instantaneous variations in the ride height that are normally compensated for by the air suspension system. If the bumps on the road are extreme, the air suspension system may be driven outside the air bag performance zone, bottom out and cease to provide suspension. This condition has a negative impact on the handling of the dolly and may lead to damage to the suspension and to the dolly. In prior art suspension systems that use a hydraulic bias system to achieve a nominal ride height, when the air suspension mechanism has collapsed, the hydraulic bias mechanism is activated on an emergency basis to decrease the load on the air suspension. This usually results in a violent jolt to the dolly, which is uncomfortable and potentially damaging to the cargo. 
         [0010]    It is therefore an object of the present invention to provide a suspension system that overcomes the aforementioned limitations. 
         [0011]    The particular objects of the invention will be better understood by reference to the detailed description of the preferred embodiment that follows. 
       SUMMARY OF THE INVENTION 
       [0012]    In one aspect, the invention comprises a suspension system comprising a suspension extending between a frame and at least one axle and at least one biasing mechanism (comprising at least one hydraulic cylinder with a piston) to bias the frame toward the axle. A sensor determines the ride height of the frame in relation to the axle and a controller automatically operates the hydraulic cylinder to effect the extension or retraction of the piston to adjust the bias and thereby control the ride height. 
         [0013]    In another aspect, the invention is a method for controlling the nominal ride height of a dolly having a frame supported by at least one suspension on one or more axles. According to the method, the ride height is determined by measuring the distance of the axle from the frame. A piston within a hydraulic cylinder that provides biasing of the frame toward the axles is selectively retracted or extended to adjust the ride height by reference to a predetermined ride height range. 
         [0014]    In relation to dollies specifically, the invention comprises a suspension system for a dolly comprising at least one suspension supporting at least one axle of the dolly and at least one hydraulic cylinder with a piston in operative relationship between a trailer and the dolly for providing downward bias to the dolly frame. An automatic sensor determines the ride height of the dolly and a controller operates the hydraulic cylinder to effect the extension or retraction of the piston to adjust the ride height if the instantaneous or an average ride height falls outside a predetermined range. 
         [0015]    In another aspect, the invention is a method for controlling the ride height of a dolly having a frame supported by at least one suspension on one or more axles. According to the method, the ride height is determined by measuring the distance between the axle and the frame. A piston within a hydraulic cylinder that provides load biasing to the dolly in relation to the trailer is selectively retracted or extended to adjust the ride height if the instantaneous or an average ride height is outside a predetermined range. 
         [0016]    In a further aspect of the invention, a history of recent ride height readings is maintained and the piston is selectively retracted or extended to adjust the nominal ride height as a function of the history of recent ride height readings. 
         [0017]    The foregoing was intended as a broad summary only and of only some of the aspects of the invention. It was not intended to define the limits or requirements of the invention. Other aspects of the invention will be appreciated by reference to the detailed description of the preferred embodiment and to the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will be described by reference to the detailed description of the preferred embodiment and to the drawings thereof in which: 
           [0019]      FIG. 1  is a side elevation of a prior art suspension system for a dolly with the dolly travelling over level ground; 
           [0020]      FIG. 2  is a side elevation of a prior art suspension system for a dolly with the dolly travelling over uneven ground; 
           [0021]      FIG. 3  is a side elevation of a prior art suspension system for a dolly with the dolly travelling over extremely uneven ground; 
           [0022]      FIG. 4  is a side elevation of a suspension system for a dolly according to the preferred embodiment of the invention, with the dolly travelling over level ground; 
           [0023]      FIG. 5  is a side elevation of a suspension system for a dolly according to the preferred embodiment of the invention, with the dolly travelling over uneven ground; and 
           [0024]      FIG. 6  is a side elevation of a suspension system for a dolly according to the preferred embodiment of the invention, with the dolly travelling over extremely uneven ground. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]      FIGS. 1 to 3  illustrate a suspension system for a dolly  10  according to the prior art. The right and left sides of the suspension system are similar so only the left side is shown. 
         [0026]    The suspension system comprises an air suspension mechanism generally indicated by the numeral  65 , comprising an air bag  70 . The air bag  70  supports the frame  30  of the dolly  10  over axles  20 . One or more wheels  40  are attached to each of the axles  20 . A hydraulic cylinder  50  is located forward of the air suspension mechanism  65  and includes a piston  55 . The dolly  10  is attached to the rear of a tractor unit or the rear of a trailer using a grip  90 . The hydraulic cylinder  50  is pivotably connected to the grip  90  at cylinder pivot  96  and is pivotally connected to one end of arm  97  at piston pivot  94 . The rear of the tractor unit or the rear of the other trailer (indicated generally by the numeral  15 ) is attached to the dolly  10  by pivotably connecting the grip  90  to a horizontal tube  91  on the tractor unit or the other trailer. The hydraulic cylinder and the associated linkages act to provide a load biasing force on the dolly  10  in relation to the trailer. 
         [0027]    In normal travel, the piston  55  is maintained at a constant extension and acts like a strut. Upon loading or unloading the dolly and the associated trailer, a ride height  80  must be adjusted to the preferred ride height range. This is done by inserting one or more shims  60  into the arm  97  before the piston is extended. Shims  60  (shown in an exploded view in  FIGS. 1 to 6 ) adjust the position of the hydraulic cylinder  50  with respect to the frame  30 . As more shims  60  are inserted, the piston pivot  94  is urged clockwise to change the load biasing on the dolly. Conversely, when shims  60  are removed, the piston pivot  94  is urged counterclockwise. 
         [0028]    The number of shims  60  that are required is dependent on the load supported by the dolly  10 . A heavier load exerts a greater downward force on the frame  30  near the axle  20 . This would normally tend to compress the air bag  70 , lower the rear of the frame  30 , and decrease the ride height  80 , or require greater compression of the air bags to maintain the ride height. To adjust the ride height  80  to keep it within the preferred range and to maintain the air bags within their performance zone, fewer shims  60  are inserted. When lighter payloads are involved, more shims  60  are inserted to urge the piston pivot  94  lower and towards the rear of the dolly  10  to keep the frame from rising above the preferred ride height range. 
         [0029]    When operating over extremely uneven road (as shown in  FIG. 3 ), it is possible that the air suspension mechanism  65  will be unable to fully compensate for large bumps in the road and will bottom out. In such circumstances, a fail-safe causes the piston  55  to quickly retract (in a violent decompression), causing the arm  97  to withdraw from abutment with block  99 . This relieves any downward bias force exerted by the hydraulic cylinder  50  on the frame  30  and allows the air bag  70  to recover. 
         [0030]      FIGS. 4 to 6  illustrate the preferred embodiment of the present invention. The suspension system for a dolly  110  according to the preferred embodiment comprises a hydraulic load biasing mechanism, an air suspension mechanism, a sensor  200  and a controller  202  located on the dolly  110 . The hydraulic load biasing mechanism includes at least one hydraulic cylinder  150  having a piston  155 , and the air suspension mechanism consists of one or more air bags  170 . The controller  202  includes a microprocessor that is in electrical communication with the sensor  200  and with the hydraulic load biasing mechanism through a control line  201 . A frame  130  of the dolly  110  is supported on an axle  120  by the air bag  170 . Sensor  200  is attached between the frame  130  and the axle  120  and measures the amount of axle travel or deflection with respect to the frame  130 . The magnitude of axle deflection is used to calculate the ride height  180  of the dolly  110 . The controller  202  receives data input from the sensor  200  regarding the magnitude of axle deflection and transmits control signals through a control signal line to a hydraulic control system  210  connected to the hydraulic cylinder  150  to effect the extension or retraction of the piston  155  to control the amount of downward bias force exerted on the frame by the hydraulic biasing system. 
         [0031]    The dolly  110  is attached to the rear of a trailer using a grip  190 . As in the prior art, the rear of the trailer is attached to the dolly  110  by pivotably connecting the grip  190  to a horizontal tube  91  on the trailer. The hydraulic cylinder  150  is pivotably connected to the grip  190  at cylinder pivot  196  and pivotably connected to one end of an arm  197  and to the frame  130  at arm pivot  192 . The other end of arm  197  is pivotably connected to the piston  155  at piston pivot  194 . 
         [0032]    The configuration and linkages between the grip  190 , arm pivot  192 , piston pivot  194 , cylinder pivot  196  and arm  197  results in a downward load bias force on the dolly frame  130 , allowing the ride height of the unloaded dolly to remain within the preferred ride height range that has been provided by the air suspension system supplier, or that is determined by the user. 
         [0033]    When the dolly  110  is loaded, the air bag  170  tends to compress, decreasing the ride height  180 . Depending of the weight of the payload, the resulting ride height  180  may be below the preferred ride height range, were it not for the system of the invention. In accordance with the invention, sensor  200  detects the change in the amount of axle deflection with respect to the frame  130  and corresponding data is sent to the controller  202 . The controller  202  recognizes that the ride height is below the preferred ride height range and sends a control signal to the solenoid operated control valve  210  to cause piston  155  to retract within the hydraulic cylinder  150 . Retraction of the piston  155  decreases the downward bias on the dolly frame, resulting in an increase in the ride height  180 . When the controller  202  (through the sensor  200 ) detects that the axle deflection (and consequently the ride height  180 ) has reached a nominal or target ride height within the preferred ride height range, the controller  202  sends a signal to the solenoid operated control valve  210  to stop the retraction of piston  155 . Typically the nominal ride height will be centered within the preferred ride height range. Conversely, when the dolly is unloaded, the system of the invention can sense a overly high ride height and extend the piston to provide a compensating downward bias on the dolly frame. 
         [0034]    By automatically monitoring the amount of axle deflection (and consequently ride height  180 ) and by automatically controlling the amount of the extension of the piston  155  within the hydraulic cylinder  150  such that the ride height  180  falls within the preferred ride height range, the need to manually insert and remove shims  60  into the arm  197  after loading and unloading is eliminated. 
         [0035]    In addition to operating after loading and unloading of the dolly  110 , the suspension system also operates while the dolly  110  is subjected to dynamic loading during travel. The sensor  200  continuously monitors the axle deflection, and consequently the ride height  180 . When traveling on an even road (as shown in  FIG. 4 ), there is little axle travel, and the ride height  180  does not deviate from the preferred ride height range. Under such conditions, the air bag  170  may compensate for any minor unevenness on the road and maintain the ride height  180  within the preferred ride height range without the need for intervention by the controller  202  and the hydraulic cylinder  150 . 
         [0036]    When the dolly  110  is traveling on an uneven road (as shown in  FIG. 5 ), there may be instances of greater instantaneous dynamic loads caused by bumps. For example, when the wheel travels over a bump  106 , the wheel  140  and axle  120  experience an upward vertical acceleration, which should be counteracted by the air suspension mechanism  165  in order to maintain a smooth ride for the frame  130 . However, this upward vertical acceleration by the wheel  140  and axle  120  caused by the bump  106  in the road  105  may result in the compression of the air bag  170 , a decrease in the amount of axle deflection and a decrease in the ride height  180 . This is monitored by the sensor  200  and the corresponding data is sent to the controller  202 . If the ride height  180  moves out of a predetermined ride height range, the controller  202  sends a control signal to the hydraulic control system  210  to effect retraction of piston  155 . This reduces the downward bias on the dolly frame. The converse applies when riding down a bump. With the controller  202  continually monitoring, and if necessary adjusting, the extent and magnitude of the retraction of the piston  155  based on data received from the sensor  200  regarding axle deflection, the ride height  180  is maintained in the preferred ride height range. 
         [0037]    In the preferred embodiment, the controller  202  maintains in memory a history of the most recent readings regarding axle deflection detected by the sensor  200  and uses this history to adjust the current extent of the retraction or extension of the piston  155  within the hydraulic cylinder  150 . This is particularly useful when loading or unloading the dolly, or when the dolly is stationary for a period of time, for example on a partially inclined surface. For example, this history may comprise the readings regarding axle deflection detected by the sensor  200  within the last 15 seconds. While the response of the system will accommodate instantaneous out of range variations in ride height as described above, it will be useful to rely on a running average over a predetermined period of time to change the set point of the cylinders to keep the ride height within the optimal range to maintain the air bags in their performance zone, without the system continually sensing and reacting to an extended an out of range condition. 
         [0038]    When engaging a steep change of grade, the load on the dolly will change as the trailer has engaged the new grade but the dolly is still substantially on the old grade. Although not a bump in the road, the effect on the ride height is similar. The change in load on the dolly will translate to a change in ride height. The system of the invention accommodates this scenario by detecting the change in ride height and adjusting the load bias accordingly. A similar situation occurs when the trailer engages a turn on a sloped corner. As the trailer exits the turn, it re-enters even grade while the dolly may still be on the sloped corner, or on the edge of a ditch. The change in ride height on one side of the dolly will cause the system of the invention to actuate to keep the ride height within acceptable limits. Because these latter two scenarios are likely to present themselves in less than a 15 second period, operation of the system contemplates either analyzing a shorter period of ride height history (for example 5 seconds or less) or the actuation of a more instantaneous response when a separate indicator signals that a turn or a significant change in grade have been engaged. 
         [0039]    It should be understood that although only one hydraulic cylinder  150  and one air bag  170  are depicted in  FIGS. 4 to 6 , there may be more than one hydraulic cylinder  150  and more than one air bag  170  in the suspension system of the present invention. Particularly, there may be two hydraulic cylinders, one located on either side of the dolly  110 . The hydraulic cylinders act in concert with each other (i.e. the pistons of the hydraulic cylinders retract and extend at the same time and are commonly controlled by the controller  202 ). 
         [0040]    The invention is also not restricted to an air bag type of suspension system but may be applicable to any passive suspension system that undergoes a range of travel, such as a passive hydraulic system, springs or other such suspension means. Similarly, it should be understood that there may be more than one axle  120  present on the dolly  110 . If there are multiple axles (and thereby multiple pairs of wheels connected to the axles), there may be more than one air bag  170  on the dolly  110  (e.g. there may be one air bag per axle). 
         [0041]    It will be appreciated by those skilled in the art that the preferred and alternative embodiments have been described in some detail but that certain modifications may be practiced without departing from the principles of the invention.