APPARATUS FOR MEASURING THE FRICTION PROPERTIES OF A SURFACE

Field of the Invention

The present invention relates to an apparatus for measuring the friction properties of the top layer of a road or airport runway.

Technical Background

Friction measurements are routinely performed by road and airport authorities, either to incur protective measures if the road/runway becomes too slippery, or for investigating the properties of various surface layers.

Such measurements are commonly undertaken by trailing a measuring wheel behind a vehicle. The wheel is braked achieving a slip of about 18% between the wheel and road. This braking action may be achieved with mechanical or hydraulic means. Then, the work done by the wheel is measured. Such systems have several drawbacks. Both mechanical and hydraulic systems are very temperature sensitive, as the friction of braking bands or the viscosity of hydraulic oil varies with temperature. Hydraulic measuring units in particular are complicated and costly to manufacture and maintain. In addition, prior art measuring wheels are apt to jump or jolt on uneven road surfaces; which introduces noise in the measurements.

Summary of the Invention

It is an object of the present invention to provide an improved apparatus for measuring friction, which alleviates at least some of the drawbacks associated with prior art measuring wheels mentioned above.

This is achieved in an apparatus for measuring the friction properties of a surface, as claimed in the appended claim 1. In particular the apparatus includes a measuring wheel which is rolled along the surface while being pressed against the surface with a predefined force, an electrical generator which is connected to the measuring wheel and driven by said measuring wheel, a load resistor which is connected to said generator and adapted to dissipate electric power generated by the generator, means for measuring the power

generated by the generator, and means for determining the surface friction properties from the measured power.

Brief Description of the Drawings

Fig. 1 is a schematic drawing illustrating the measuring principle used in the invention,

Fig. 2 illustrates the mechanical design of a measuring unit according to the present invention.

Detailed Description

The invention will now be described in detail in reference to the above-mentioned drawings.

Fig. 1 illustrates schematically how the measurements are performed in an apparatus according to the present invention. The measuring setup includes a measuring wheel 1 that is trailed behind a vehicle (not shown) while it is pressed against the road surface with a predefined force Fi _oad - The heavy arrow indicates the direction of motion. The wheel 1 is driving an electrical generator 2 through a belt 3. The generator 2 is connected to a load resistor 4, and thus acts to brake the wheel 1. The braking force F _dra g is controlled by a controller 5. The controller 5 controls the electrical power deposited in the load 4 by controlling the magnetization of the generator's rotor. The power delivered by the generator is proportional to the revolution rate and the magnetization current. The setup also includes means for measuring the voltage across the load resistor 4, and means for computing the friction properties of the surface from these measurements. These means are not shown in the illustration.

The load resistor 4 is converting the power generated by the generator 2 into heat. The load resistor is made of a particular alloy, which is stable with temperature. This means that the load will stay constant even if the power level is varying. This also means that a simplification of the power measurement, as we may measure the voltage u across the load and calculate the power p:

Ii ³

(D

where r is the resistance of the load.

Fdrag may be computed from the following relationship:

Fdrag = ^■ η * r * v (2)

where η is the degree of efficiency (a constant) and v is the velocity.

Then, the friction coefficient μ is given as:

Fig. 2 illustrates the mechanical design of the invention. The friction measuring - apparatus has a mechanical construction which aims at reducing errors from an uneven support. Since the friction coefficient is a relationship between F _drag and Fi _oad , the system is dependent on Fi _oad being as stable as possible, without being influenced by accelerations caused by the support.

In Fig. 2, the measuring wheel 1 is suspended in a control arm 6. The control arm 6 is fastened to a measuring carriage 7 in a hinge point 8. A spring damper unit 9 is connected between the arm 6 and the carriage 7 damping the action of the arm. The carriage is connected to the vehicle 10 in a free swinging pivot point 11. In this way, the measuring wheel is spring suspended and dampened against the carriage, while the carriage may move freely vertically. If there is a proper ratio between spring suspended and not spring suspended mass, unwanted noise caused by variations in the vertical distance between the wheel and carriage will become attenuated. If the mass of the carriage is at least five times the mass of the measuring wheel, the noise contribution will be about 1/6 compared with a traditional stiff measuring setup.