Abstract:
A device and method for measuring friction on a surface, such as a road, allow for accurate and reliable measurements by preventing the slipping of a pilot wheel. The device includes a pilot wheel and a measurement wheel mounted on shafts in a wheel box and arranged one behind the other, the pilot wheel rotating without slip and the measurement wheel being braked in order to provide slip. The configuration of the device, including the positioning of an attachment axle, results in the pilot wheel being subjected to greater ground pressure than the measurement wheel, thus preventing slipping of the pilot wheel.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to measuring friction on a physical surface, preferably a roadway, where a measuring device on wheels is pulled by a traction vehicle. One of the wheels of the measuring device is braked during driving, so that this wheel provides slip against the surface. 
     For the office in charge of roads (in Norway, the Public Roads Administration or the municipalities), there is a requirement that roads must exhibit a friction minimum value between the roadway and a car wheel. This is particularly important in winter. A standardized method of measuring has been established, namely with a fixed slip value and with measurement in a wheel track. This is to be documented, with respect to contracts, by the office in charge of roads as well as the contractor. 
     Friction measurement is simple in theory, but difficult to execute in practice. Measuring devices available today are complicated and expensive, and in most cases they are adapted to airports, with a measuring wheel mounted between the wheel tracks of the traction vehicle. This is a set-up poorly adapted for use in a road, since the traction vehicle/the car must drive outside the usual tracks in order to place the measuring wheel in a wheel track. In other words, there is a need of a measuring device better adapted to the situation in a normal road. 
     (2) Description of the Related Art 
     A measurement principle relied on by several previously known measuring devices has two wheels connected to each other by means of a chain or similar, where one wheel is a “pilot wheel” rotating with a periphery speed equal to the driving speed, that is without slip, and where the other wheel is braked to a slip value (which means that this other wheel skids on the roadway and has a lower periphery speed than the driving speed). Since the two wheels are connected to each other, a “break” will arise between the two wheels, which “break” can be measured in the chain (or another force/moment transfer means) by means of a load cell. 
     Related art is known from British patent number 1,269,334, DE “Offenlegungsschrift” 2742110 and U.S. Pat. No. 4,909,073, all disclosing devices for measuring friction on a road surface, using two wheels. The wheels are arranged in a frame, a box or a housing, and they are typically arranged on one and the same shaft. The wheels rotate with different rotation speeds against the underlying road surface. 
     Because the two wheels are arranged beside each other on one and the same shaft, they are actually not capable of measuring friction precisely in one wheel track. And in the closest one of these publications, namely U.S. Pat. No. 4,909,073, which uses a measurement principle that lies rather close to the one used in the present invention, with measurement of load in a force/moment transfer means between two connected wheels with a “transmission ratio”, no device has been disclosed to ensure that one of the two wheels, namely the “pilot wheel” intended to roll without slip, will actually be unable to have slip. With the solution in U.S. Pat. No. 4,909,073, it seems clear that in certain situations, one may have a “return” reaction moment to the pilot wheel, resulting in a slip situation for this wheel also. A rather unfavourable instability will then be induced in the measurements. 
     Further, Japanese publication JP 4,102,034 discloses a friction measurement device in which two wheels are mounted behind each other in a frame, for measurement in one wheel track. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has the aim to provide a solution that will have a reasonable price and a simple construction, makes a genuine measurement in one wheel track, and is safeguarded, by way of its construction, against measurement instabilities by preventing a slip condition for the pilot wheel. 
     Hence, in accordance with the present invention there is provided a method for measuring friction such as defined precisely in the appended claims, and a friction measuring device such as defined precisely in the appended claims. Favourable embodiments of the friction measuring device in accordance with the invention are also set forth in the claims. 
     As a result of the feature that the two wheels in the measuring device are arranged right behind each other, a possibility is secured for a genuine measurement in one roadway wheel track, and by attaching the chain cover to the main unit using an attachment axle positioned markedly closer to one wheel than the other, this closest wheel will achieve an increase in ground pressure relative to the other wheel, the other wheel being the braked wheel. Hence, stable measurements are ensured, and variations induced by a sudden slip for the pilot wheel are avoided. 
     Further assurance of stability in the measurements can be achieved by mounting the force sensor near the measurement wheel. Vibrations in the force/moment transfer means close to the force sensor are then avoided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall now be described in more detail by going through embodiments thereof, and in this connection it is also referred to the appended drawings, of which 
         FIG. 1  shows an exemplary embodiment of a friction measuring device in accordance with the invention, mounted behind a traction vehicle, 
         FIG. 2  shows substantially the same embodiment of the friction measuring device as in  FIG. 1 , however in closer detail, 
         FIG. 3  shows the same device as  FIG. 2 , however in a top view, 
         FIG. 4  is a principle sketch showing an embodiment of a force/moment transfer means that is part of the friction measuring device, and 
         FIG. 5  shows a concrete embodiment example of a friction measuring device in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a traction vehicle  1 , in this case an ordinary car, pulling a friction measuring device  2  on a surface that has not been drawn. The surface is normally a road surface. Important elements in the friction measuring device  2  as shown, are a forward wheel  3 , a rear wheel  4 , a common wheel box or chain cover  5 , as well as a main unit  6  to which the chain cover  5  is attached, and which in its turn is attached to the rear end of vehicle  1 . 
     In  FIG. 2  appears a somewhat different embodiment of the friction measuring device  2 , than the embodiment shown in  FIG. 1 , however the most important elements are the same. Main unit  6  is shaped more like a box, having a housing  7  that extends somewhat downwards, and inside housing  7  there is a data processing box  8 . An important element is an attachment axle  9  that attaches chain cover  5  to the main unit  6 . The location of this attachment axle  9  appears clearly as being substantially closer to the axis of one wheel (in this case forward wheel  3 ) than to the axis of the other wheel, see distance indications a and b. 
     The gist of this “eccentric” placing of the main unit attachment point on the chain  5  cover, is to ensure that one wheel, in this embodiment forward wheel  3 , is guaranteed to have a heavier ground pressure than the other wheel. 
     Concurrently with this placing of the attachment point  9 , the wheels are arranged in such a manner that the forward wheel  3  shall be a pilot wheel intended to rotate without slip, while the rear wheel  4  shall be kept in a slip condition during operation. 
     The manner of achieving a slip condition for the rear wheel  4 , appears partly from  FIG. 3  that shows the same friction measuring device  2  as  FIG. 2 , but in a top view. It appears inside the chain cover  5  that the two wheels are provided with differently sized sprocket wheels on their respective shafts, for instance the forward sprocket wheel  10  may have 20 teeth, while sprocket wheel  11  at the rear wheel  4  may have 24 teeth. It is of course possible to use other sizes of the sprocket wheels in order to provide other transmission ratios. 
     At the same time it should be noted that the force/moment transfer means between the two wheels is not necessarily a chain device, for instance a cardan means may be used instead. 
     In  FIG. 3 , the chain encircles sprocket wheels  10  and  11  is not shown, but it is referred instead to  FIG. 4 , which drawing shows a simplified sketch of the chain drive principle in the friction measuring device disclosed in  FIGS. 2 and 3 . It is referred to sprocket wheels  10  and  11 , of which ref  10  is attached to forward wheel  3  that in this embodiment is a pilot wheel intended to roll without slip. Sprocket wheel  10  is smaller than sprocket wheel  11  which will then rotate somewhat slower, and thereby the rear wheel  4  is braked to have a slip situation. In  FIG. 4  appears also attachment point  9 , to ensure that the ground pressure is largest for forward wheel  3 : The chain is indicated by reference numeral  12 , and two additional sprocket wheels  13  and  14  are inserted to provide a possibility for measuring stress in chain  12 . At the center of sprocket wheel  13  there is a load cell  15  able to measure vertical load c, and this vertical load is directly related to the tension in chain  12 . Sprocket wheel  13  is arranged rather close to the large sprocket wheel  11 , and furthermore, sprocket wheel  14  is arranged rather close to sprocket wheel  13 . This means that the free lengths or runs of chain  12  between sprocket wheel  14  and the large sprocket wheel are short. These lengths are indicated by reference numerals  16  and  17 . The points of having lengths  16  and  17  short, is that the chain will then have a very small tendency to vibrate or swing, and consequently further stability is achieved for the load cell measurement. 
     In the shown embodiment, pilot wheel  3  is the forward wheel, while measurement wheel  4  is the rear wheel. This might equally well be reversed, but in such a case, the attachment axle  9  must be located closer to the rear wheel instead. 
     In  FIG. 5  appears a practical embodiment of the friction measuring device in accordance with the invention. In addition to the already mentioned details like wheels  3  and  4 , wheel box/chain cover  5 , main unit  6 , housing  7  and attachment axle  9 , there appears an antenna  18  for short distance transmission of data to a PC/display equipment that may preferably be located inside the traction vehicle. Furthermore, the friction measuring device  2  is equipped with a suspension system, in which a combined spring/shock absorber  21  constitutes the main cushioning for chain cover  5  and wheels  3 ,  4 , while an additional shock absorber  20  is mounted to a front stay  19  on the chain cover in order to counteract vibrations and shocks that would otherwise cause pivoting about the attachment axle  9 . 
     A measurement signal from load cell  15  ( FIG. 4 ) is transmitted, on a not shown wire, to data processing box  8  ( FIGS. 2 ,  3 ) in which calculations and recordings are executed. The data box may also, as appears from the above, transmit signals via antenna  18 , or via wiring, to further equipment carried in the traction car. 
     It is possible to make compensations on a continuous basis in the friction result, for changes in the ground pressure during the measuring operation. The ground pressure is measured continuously by a (not shown) load cell at the top of shock absorber/spring  21 . This shock absorber is pushed down by a load from the car, and weighs down the wheels of the measuring device against the roadway with an adjusted ground pressure for the measurement wheel. When the ground pressure is changed due to irregularities in the road surface and car springing action caused thereby, these changes are compensated for by computer, in order to obtain stable friction measurements. 
     In an alternative embodiment (not shown in any drawing), the main unit may be mounted to a frame arranged underneath the traction vehicle, which may possibly be a large vehicle (truck). The frame may then be arranged between the wheel sets of the vehicle, i.e. in front of a rear wheel set, or between front and rear wheels. In any case, the sideways location must be such that the two wheels of the friction measuring device roll in line with a wheel track of the vehicle. 
     The most important features of the invention can be summarized as follows: 
     By having the two wheels  3 ,  4  of the friction measuring device mounted behind each other in the wheel track, measurements are made in the correct place, and by giving one of the wheels (the forward wheel)  3  the largest ground pressure, one ensures that this wheel will always have a periphery speed equal to the driving speed, which means that this wheel rolls without slip. The other wheel (the rear wheel) will always be the wheel that slips (skids), and this fact will guarantee a stable measurement result. 
     In order to achieve this in a simple manner, the two wheels are mounted, in accordance with the invention, to a common wheel box/chain cover  5  that has a common attachment axle  9  to the main unit  6 . The attachment axle  9  is placed with an offset, that is closer to one wheel than the other wheel, in order to provide a larger ground pressure for said one wheel than for the other, which other wheel is a measurement wheel with slip. 
     In addition, it is favourable that the force sensor (load cell  15 ) is mounted as close to the measurement wheel  4  as possible, especially in a chain solution. Swinging in a chain will ordinarily influence the force sensor and add pulses, that is variations in the friction measurements, but with a location close to the measurement wheel, the chain influence on the load cell will be reduced significantly. 
     The friction measuring device in accordance with the invention is preferably mounted directly at the rear of the traction vehicle/car with two attachment points. One attachment may be an ordinary towing bracket, and the other one will be a coupling  5  point mounted at the corner of the car, or in a bumper bracket.