EP3555807A1

EP3555807A1 - A method, an arrangement and a measuring apparatus for measuring sliding of a ski

Info

Publication number: EP3555807A1
Application number: EP17828941.9A
Authority: EP
Inventors: Marko Höynälä
Original assignee: Exiops Oy
Current assignee: Exiops Oy
Priority date: 2016-12-16
Filing date: 2017-12-12
Publication date: 2019-10-23
Also published as: WO2018109269A1; FI127357B; FI20165975A

Abstract

The ski sliding measurement arrangement (20A, 20B, 20C) according to the invention is configured to define a sliding index between a ski and the snow. A sliding measurement apparatus is attached to a ski or an ankle of a skier for measuring accelerations during skiing. The measured accelerations are saved in a signal processing unit, cloud service, or the memory of the sliding measurement apparatus. From the saved acceleration data a velocity path graph is processed. Cycle time periods are defined from the velocity path graph. The velocity path graphs and acceleration path graphs of the defined cycle time periods are saved. An averaged velocity path graph of the defined cycle time periods is then calculated. At least one angle of gradient is defined from the averaged velocity path graph. The angel of the gradient defines a sliding index that depicts friction between the ski pair and the snow.

Description

A method, an arrangement and a measuring apparatus for measuring sliding of a ski

The invention relates to a measuring method, a measuring device and a measuring arrangement by which a skier can assess how his or her skis are sliding. The invention also relates to a server utilised in the measuring arrangement and also to computer programs in the measuring device and in the server.

Prior art

Before a skiing competition, a skier often tests several ski pairs so that in the competition, he or she would have skis that glide best. This requires that in the skiing event friction between the skis and snow is the lowest possible.

Soles of skies are commonly manufactured from ultra-high molecular polyethylene. The under-surface of the ski can be manipulated and optimized for maximal sliding properties. Other features affecting the sliding are different patterning of the under-surface of the skis and utilized waxes. The waxing chemicals on the under- surface of the ski may be micro-machined by several techniques for improving sliding of the skis.

Nowadays comparison of the best possible surface properties of the skis is based on a direct comparison between the different skis, waxes, or base structures in a ski trail close to the competition arena. This method is practical to use out in the field but is sensitive to variation of the initial velocity, air resistance, and changing weather conditions. If in the sliding test two test skiers are testing skis at the same time side by side, body masses and body structures of the skiers can affect the measurement results.

In the measurement event a velocity of the skier can be measured for example by timing gates utilizing a light emitter and receiver, RF emitter and receiver, a Dop- pler measurement system, or a GPS based measurement system.

The above-mentioned setups can provide information regarding time used to glide a certain distance. This distance is often about 100 m and the distance most often at least has a downward part. In a test situation, a test skier normally starts the test by standing still. When the test skier starts the test and glides downwards a test slope, the measurement system detects an acceleration phase and also a sliding phase with higher velocity. The test depicted above can only give a rough mean glide velocity value which could differ significantly from a sliding situation in a racing situation.

In patent application US 2016/0192866 there are described examples of the peri- odic movements of a cross-country skier using a classic or skate ski technique. The measurement system discloses a three-dimensional accelerometer.

In the depicted measurement arrangement an inertial sensor, such as an accelerometer or a gyroscope, is placed on the body of the skier to sense the periodical ski skating movements of the skier. Streaming data from the sensor is then collected and partitioned into periods of the skier's periodical movement. The data of each period is transformed into a data representation that allows a comparison between consecutive periods. In the depicted cross-country ski case it is depicted how the movement period that is due to pushing the poles in the snow contributes to the propulsion of the skier in the direction of movement of the skier. The time the poles are in the snow is put in relation to the time of the total period between two consecutive pole pushes. This measurement result gives a measure of how much of the movement period is ac- tually used for propulsing the skier forward.

In WO 2016/174612 is disclosed a measurement system utilized in downhill skiing. In the publication it is depicted how the downhill skier bypasses gates placed along a track. The depicted measuring method comprises gearing the athlete with a wearable magnetometer sensor unit and 3D acceleration sensor. 3D acceleration sensor is configured to obtain speed and a speed drift at a point passage and at the beginning and end of a race.

In EP 3000396 is disclosed a skiing performance measuring method where a 3D accelerometer is utilized. It discusses about deceleration signal when the foot of the skier is pushed against the snow. The depicted velocity signal depicts how quickly velocity v is falling to a value zero (or near zero). From the depicted velocity curve it can be assessed how well the ski suits the skier. Numerous different parameters directly or indirectly affect the ski-snow friction that has to be taken into account when optimizing the glide in cross-country skis. A measurement method to choose a ski pair with the lowest possible friction coefficient is not known, however. Summary of the invention

The object of the invention is to introduce a new measuring method, measuring device and measuring system, by means of which a sliding index depicting a friction between a ski and the snow can be defined both in cross-country skiing and in downhill skiing.

The objects of the invention are attained with a measuring method, measuring device, and measuring arrangement, where changes of the velocity of a ski are measured by a 3D acceleration sensor. The measurement data discloses consecutively acceleration and deceleration periods that compose one measurement cy- cle that depicts skier's periodical movements in a skiing event. Each measurement cycle discloses an acceleration period and a deceleration period. From momentary acceleration and deceleration measurement is calculated a momentary velocity of the ski. Measurement data of several measurement cycles are then averaged so that the defined graph depicts an averaged velocity graph of the ski under test. In cross-country skiing the sliding index of the tested ski pair can be deduced from negative slope of the averaged velocity graph. In downhill skiing the sliding index of the tested ski pair can be deduced from positive slope of the averaged velocity graph.

An advantage of the invention is that a cross-country skier or a downhill skier can accomplish sliding tests by himself or herself.

It is further an advantage of the invention that by measuring the sliding index value by an accelerometer it enables an easy and simple comparison for different skis and different skiing waxes. It is further an advantage of the invention that by measuring the sliding index value by an accelerometer a skier can improve his or her skiing technique so that the skis have as low a friction against the snow as possible.

The measuring method according to the invention defining a sliding index between a ski and the snow in which method:

- a sliding measurement apparatus attached to a ski or an ankle of a skier measures acceleration of the ski during a skiing event

- the sliding measurement apparatus saves the measured acceleration data to a signal processing unit or to a cloud service or to the memory of the sliding meas- urement apparatus - the signal processing unit or cloud service or the sliding measurement apparatus defines from the measured acceleration data a velocity graph and/or an acceleration graph

- the signal processing unit or cloud service or the sliding measurement apparatus defines from the velocity graph or the acceleration graph cycle times of repeated skiing movements, and

- the signal processing unit or cloud service or the sliding measurement apparatus saves velocity path graphs and/or acceleration path graphs of the defined cycle times,

is characterised in that the signal processing unit or cloud service or the sliding measurement apparatus further:

- defines an averaged velocity path graph from the saved acceleration path graphs of the defined cycle times

- defines at least one angle of gradient from the averaged velocity path graph - defines a sliding index from the selected angle of gradient that depicts friction between the ski pair and the snow, and that

- sliding indexes of the different ski pairs are utilized for selecting a best sliding ski pair in prevailing weather conditions. The ski performance measuring arrangement according to the invention comprises:

- a sliding measurement apparatus attached to a ski or an ankle or foot or shoe of a skier comprising:

- a processor for controlling measuring

- an accelerometer sensor measuring acceleration and deceleration of the ski

- a memory for storing acceleration, deceleration and velocity measurement data

- a transmitter for sending measured acceleration and deceleration data, and - a receiver for receiving control commands during measuring state

- a signal processing unit or cloud service configured to:

- receive measured acceleration and deceleration data from the measuring unit

- define from the received acceleration and deceleration data an averaged acceleration graph

- save averaged acceleration path graphs of the defined cycle time

- define from the received acceleration and deceleration data a velocity graph - define from the velocity graph a cycle time period of a skiing movement

- a display unit for displaying a sliding index of the ski,

is characterised in that the signal processing unit or the cloud service or the sliding measurement unit are configured to:

- define from the received acceleration and deceleration data a velocity path graph of a defined cycle time

- save the velocity path graph of the defined cycle time

- define an averaged velocity path graph from saved velocity path graphs

- define at least one angle of gradient from the averaged velocity path graph - define a sliding index from the selected angle of gradient that depicts friction between the ski pair and the snow, and that

- the defined sliding indexes of the different ski pairs are configured to be utilized in selecting a best sliding ski pair in prevailing weather conditions. The sliding measurement apparatus according to the invention attached to a ski or an ankle comprising:

- a processor configured to control measuring

- an accelerometer configured to measure accelerations and deceleration of the ski during a skiing event

- a memory configured to store acceleration, deceleration and processed velocity measurement data

- a transmitter configured to transmit the measured acceleration and deceleration data to a signal processing unit or to a cloud service, and

- a receiver configured to receive control commands during measuring state, is characterized in that the sliding measurement apparatus is configured to:

- receive from the signal processing unit or the cloud service a defined sliding index of the ski based on the transmitted acceleration data, and

- indicate the received sliding index of the ski, and that

- the indicated sliding indexes of the different ski pairs are configured to be utilized in selecting a best sliding ski pair in prevailing weather conditions.

The computer program according to the invention for defining a glide index of a ski comprising:

- code means for defining from measurement data an acceleration graph and a velocity graph

- code means for defining from the velocity graph a cycle time of repeated skiing movements, and

- code means for saving the acceleration path graph of the defined cycle time, is characterised in that computer program further comprises:

- code means for defining an averaged velocity path graph from the saved acceleration path graphs of the defined cycle times

- code means for defining at least one angle of gradient from the averaged veloci- ty path graph

- code means for defining a sliding index from the selected angle of gradient that depicts friction between the ski pair and the snow, and

- code means for indicating sliding indexes of the different ski pairs to be utilized in selecting a best sliding ski pair in prevailing weather conditions.

Some advantageous embodiments of the invention are presented in the dependent claims.

The basic idea of the invention is the following: A measuring device according to the invention is attached to an ankle or a ski of a skier. The measuring device has been equipped advantageously with an accelerometer. The measuring device may comprise also some other sensors such as GPS, temperature sensor, pressure sensor, and magnetic sensor. In cross-country skiing a skier increases or maintains his/her speed by step or kick efforts by the ski against the snow track and/or pushing by ski poles. The effort movements are naturally cyclic. So the movement efforts will cause the velocity of the skier to accelerate and decelerate. The deceleration is caused by many parameters such as air resistance, ski-snow friction, and altitude change (elevation).

A momentary velocity of the skis can be calculated from the acceleration and deceleration measurement results. When the air resistance and altitude variation can be kept constant in cross-country skiing, the ski-snow friction can be defined from a portion of the velocity graph depicting decreasing velocity of the skis.

In downhill skiing, the ski-snow friction can be defined from a portion of the velocity graph depicting increasing velocity of the skis.

The ski-snow friction, i.e. the sliding friction, is called in this application a sliding index because by it can be expressed by differences in sliding between different ski pairs. Description of advantageous embodiments of the invention

In the following, the invention will be described in detail. In the description, reference is made to the enclosed drawings, in which

Figure 1 shows an exemplary skiing event where the skier utilizes sliding index measurement apparatus according to the invention,

Figure 2a shows as an example a first embodiment of the measurement arrangement according to the invention,

Figure 2b shows as an example a second embodiment of the measurement arrangement according to the invention, Figure 2c shows as an example a third embodiment of the measurement arrangement according to the invention,

Figure 3 shows exemplary main elements of a server utilized in the cloud service,

Figure 4a shows an exemplary velocity graph of a ski, Figure 4b shows an exemplary acceleration and deceleration graph of several skiing moment cycles,

Figure 4c shows an exemplary averaged acceleration graph of several skiing moment cycles,

Figure 4d shows an exemplary averaged velocity graph wherefrom a glide index slope is defined from a negative velocity gradient

Figure 4e, shows an exemplary averaged velocity graph wherefrom a glide index slope is defined from a positive velocity gradient, and

Figure 5 shows as an exemplary flowchart main steps of the method utilized to define a glide index depicting friction between a ski and the snow. The embodiments in the following description are given as examples only and someone skilled in the art can carry out the basic idea of the invention also in some other way than what is described in the description. Although the description may refer to a certain embodiment or embodiments in several places, this does not mean that the reference would be directed towards only one described embod- iment or that the described feature would be usable only in one described embodiment. The individual features of two or more embodiments may be combined and new embodiments of the invention may thus be provided. A gliding index can be defined for certain point or short length of the skiing track. It can be compared to other gliding indexes defined on the same point or area. The gliding index defined can also be compared to gliding indexes defined at different times, let's say in the morning and in the afternoon, at different temperatures, in any different skiing conditions, such as under the direct sun or shadow and differ- ent locations over the skiing track, for example similar flat area or just by comparing uphill and downhill parts of the skiing track.

Figure 1 shows an exemplary skiing event where a cross-country skier 1 uses an apparatus 3 or 3A that advantageously may be utilized to define a sliding index of the skis 2 that the skier 1 uses. The sliding index to be defined is proportional to friction between skis 2 and the snow.

The sliding index measurement system utilizes velocity, acceleration and in one advantageous embodiment also elevation data from a sliding measurement appa- ratus 3 or 3A attached to a skier's 1 ankle, shoe, or ski 2. The sliding measurement apparatus 3 or 3A comprises advantageously at least an accelerometer.

Also some other sensors such as a GPS, temperature sensor, pressure sensor, and magnetic sensor may advantageously be integrated to the sliding measure- ment apparatus 3 or 3A according to the invention.

In a cross-country skiing event the skier 1 increases or maintains his/her skiing speed by making a step or kick effort with the ski 2 against the snow track and/or pushing by ski poles. The ski effort movements are naturally cyclic. So the effort movements cause skier's 1 velocity to accelerate and decelerate cyclically. The deceleration is caused by many different parameters such as air resistance, friction between the skis 2, and snow and altitude or elevation change.

In a cross-country skiing case the air resistance and altitude variation may in a test situation be kept essentially constant. Therefore the friction between the skis 2 and the snow may advantageously be defined from an averaged velocity graph that is calculated from momentary acceleration and deceleration signals that the sliding measurement apparatus 3 or 3A measures. The friction between skis 2 and the snow may be called a sliding index because it relates to a prevailing sliding condition of the skis.

In one advantageous embodiment the sliding measurement apparatus 3 or 3A transmits acceleration, deceleration and elevation measurement results to an external signal processing unit 4 and/or to a cloud service 6 (shown in Figures 2a- 2c). The signal processing unit 4 or the cloud service 6 processes from the received measurement data a sliding index according to the invention. In the example of Figure 1 the signal processing unit 4 is girded onto the waist of the skier 1 . In another advantageous embodiment the signal processing unit 4 is not carried by the skier 1 . In that case the signal processing unit may be in a container or in a car or on the side of the skiing track for example. In one advantageous embodiment the defined sliding index may be delivered as an audio signal so that the cross-country skier 1 gets the defined sliding index value during skiing for example via an earpiece. The audio signal may be transmitted advantageously from the signal processing unit 4 either directly to the earpiece or via the sliding measurement apparatus 3 or 3A.

In some advantageous embodiments the defined sliding index is transmitted to display device on the wrist of the skier, on the ski, or on the ski pole.

The sliding index measurement system 20A according to the invention gives the following benefits. The invention enables easy and simple comparison for different cross-country ski pairs, skiing waxes, and also skiers. The invention also makes it possible that a glide test can be accomplished by a skier 1 himself or herself and no reference tester or supporting person is needed. One other benefit is that the glide test can be accomplished without any special technical arrangements such as timing gates or Doppler measurement systems that are commonly utilized in the prior art test arrangements.

When the sliding index measurement system according to the invention is utilized in downhill skiing then in that case advantageously also the shape of the utilized track has to be taken into account when the glide index is defined. The slope of the hill varies and also a distance between slalom gates varies. Therefore, to get accurate result advantageously also all events during the whole run have to be taken account when defining the sliding index from an averaged velocity path graph.

Figure 2a shows one advantageous ski performance measurement arrangement embodiment 20A according to the invention. The ski performance measurement arrangement 20A advantageously comprises a sliding measurement apparatus 3 or 3A, a signal processing unit 4, and an auxiliary display unit 5.

The sliding measurement apparatus 3 or 3A carried by the skier 1 or assembled on a ski 2 advantageously comprises an energy source such as a battery or an accumulator. The electric components in the sliding measurement apparatus 3 or 3A get energy they need for their function from the battery.

There is at least one accelerometer in the sliding measurement apparatus 3. The accelerometer is advantageously a 3D accelerometer.

Measurement data from the 3D accelerometer is processed in a measuring event in the processor of the sliding measurement apparatus 3. The processor is connected also to a memory. The memory is used for storing the computer programs needed in processing the acceleration and deceleration measurement data. All the measurement results during testing are advantageously stored at least temporarily in the memory.

The processor is also connected to data transfer components such as a transmit- ter and a receiver. With the aid of these data transfer components a data transfer connection may be established to a signal processing unit 4 or to a cloud service 6 belonging to the ski performance measurement arrangement 20A. The data transfer components advantageously support at least one data transfer method. Some advantageous transfer methods usable in data transfer are infrared technology (IR), Bluetooth^® technology, ZigBee^® technology, UWB technology, WLAN technology, and various time or code division data transfer technologies used in common cellular networks. Acceleration and deceleration measurement data stored in the memory of the sliding measurement apparatus 3 are transferred via the transmitter and the wireless data transfer connection to the signal processing unit 4 and/or to the cloud service 6 that are capable of defining a sliding index according to the invention from the measurement data. The signal processing unit 4 may advantageously be a smart phone, tablet or laptop computer. The basic electronic elements such as a processor, memory, receiver, transmitter, user interface and battery and their mutual electrical connections in the smart phone, tablet or laptop computer are well known.

The signal processing unit 4 may transmit via the transmitter control commands to the sliding measurement apparatus 3 or 3A and receive measurement data from the sliding measurement apparatus 3 or 3A. The processor of the signal processing unit 4 may advantageously define the sliding index from the received measurement data. The signal processing unit 4 comprises a display on which the defined sliding index may be shown.

Alternatively the signal processing unit 4 may receive the defined sliding index from the cloud service 6.

The defined sliding index according to the invention from the signal processing unit 4 or from the cloud service 6 may advantageously be shown on an auxiliary display unit 5 when the signal processing unit 4 has transmitted it to the auxiliary display unit 5. The display unit 5 comprises advantageously a processor, memory, receiver, battery, and display. The display unit 5 advantageously receives the defined sliding index according to the invention from the signal processing unit 4 or from the cloud service 6.

The sliding measurement apparatus 3 may advantageously comprise also other measuring sensors than the above-mentioned 3D accelerometer. The other sensors can be either a part of the sliding measurement apparatus 3 or 3A or they may be separate sensors on a skier's 1 body or on the ski 2. The measuring data from these sensors is advantageously transferred to the sliding measurement apparatus 3 either via a wired or a wireless data transfer connection.

Examples of other types of sensors are a temperature sensor, an elevation sensor, a compass, a gyro sensor, and an air pressure sensor, with which a relative change in altitude can be measured. Also sensors measuring physiological properties of the skier 1 may also be included in the ski performance measurement arrangement 20A according to the invention (not shown in Figure 2a). Examples of such sensors are a sensor measuring body temperature, a sensor measuring breathing functions, a sensor measuring muscle movements, a sensor monitoring heart rate.

The signal processing unit 4 or cloud service 6 may advantageously process also the data from these auxiliary measuring sensors and store it for later use. In one advantageous embodiment at least some of the other measured parameters may be shown for example on the auxiliary display unit 5.

Figure 2b shows another advantageous ski performance measurement arrange- ment embodiment 20B according to the invention. The second embodiment differs from the first embodiment in that the display unit 5 in Figure 2a has been embedded into the signal processing unit 4A.

Figure 2c shows a third advantageous ski performance measurement arrange- ment embodiment 20C according to the invention. In the third embodiment functional elements from the signal processing unit 4 and display unit 5 of Figure 2a have been embedded into the sliding measurement apparatus 3B. In this embodiment 20C the sliding index according to the invention is processed from the measurement results that are stored in the memory of the sliding measurement appa- ratus 3B. Alternatively the apparatus does not have a display. But it may use another auxiliary display or mobile device or computer with a display which is has a connection to a cloud service.

The sliding measurement apparatus 3B comprises transmitter and receiver that are capable to receive command messages and to transmit measurement results via one known cellular communications system such as 2G, 3G, 4G, 5G network, Bluetooth, Wi-Fi, or any known RF radio link.

The sliding measurement apparatus 3B also comprises a user interface and dis- play for receiving instructions and displaying test results.

Figure 3 shows the functional main parts of an exemplary server 60 that is utilized in the cloud service 6. The server 60 comprises a processor or processor means

61 which comprise an arithmetic logic unit, a number of different registers and con- trol circuits. A data storing arrangement 62, such as a memory unit or memory means, wherein computer-readable information or programs or user information can be stored, has been connected to the processor means. The memory means

62 typically contain memory units, which allow both reading and writing functions (Random Access Memory, RAM), and memory units containing non-volatile memory, from which data can only be read (Read Only Memory, ROM). The server 60 also comprises an interface element 64 which comprises an input or input means 66 for receiving measurement data via a data communications network from the sliding measurement apparatus 3 or 3A. The measurement information received with the input means 66 is transferred to be processed by the processor means 61 of the server 60.

The interface element 64 of the server 60 also comprises an output or output means 65, with which sliding index information is transferred from the processing means of the server 60 via a telecommunication network either to the sliding measurement apparatus 3, 3A or to a signal processing unit 4, 4A.

The server 60 advantageously also comprises a user interface 63, which compris- es means for receiving information from the signal processing unit 4, 4A. The user interface 63 may comprise a keyboard, a touch screen, a microphone, and a speaker.

The processor means 61 , the memory means 62, the interface element 64, and the user interface 63 are electrically joined together to systematically execute received and/or stored data in accordance with predetermined and essentially preprogrammed operational sequences. Such operational sequences include the operations and operational sequences described in connection with Figure 5 and utilised in implementing the sliding index determination according to the invention. The detailed implementation of the logical units of the server in Figure 3 is prior art to one skilled in the art.

Figure 4a shows an exemplary velocity graph 40 of a cross-country skier 1 of Figure 1 . It can be seen that the velocity of the skier changes cyclically.

In a cross-country skiing event the skier 1 increases or maintains his/her skiing speed by kicking with the skis 2 against the snow track and/or by pushing by ski poles. These effort movements cause skier's 1 velocity to accelerate and decelerate cyclically. The deceleration is caused by many different parameters such as air resistance, friction between the skis 2 and the snow, and altitude or elevation change. One cycle time period measured from one maximum velocity value to the next maximum velocity value is marked by a reference number 41 . When a crosscountry skier skis on a flat terrain, cycle time periods 41 between consecutive velocity maximums may essentially have a constant value.

Figure 4b shows an exemplary acceleration and deceleration graph 42 of a crosscountry skier 1 that corresponds to the velocity behaviour that has been shown in Figure 4a. The shown measurement data discloses consecutively acceleration (having a positive slope in Figure 4b) and deceleration periods (having a negative slope in Figure 4b). The cycle time periods depict skier's periodical movements in a skiing event. Each time period cycle 41 discloses an acceleration period and a deceleration period.

The shown acceleration and deceleration values in Figure 4b are actually meas- ured by the sliding measurement apparatus 3, 3A or 3B according to the invention. Some exemplary portions of the acceleration path graph 42 are defined by letters a-b, c-d, and e-f and three exemplary deceleration phases of three consecutive cycles 41 are depicted. From the actual momentary acceleration and deceleration values is then calculated a momentary velocity 40 of the skier that is shown in Figure 4a.

Figure 4c shows as an example how the acceleration and deceleration data of several, for example 2-10, measurement cycles 41 can be combined and then av- eraged so that the defined graph depicts an averaged acceleration path graph 43 of the ski pair under test. The averaged acceleration path graph can be calculated for example by using periods a-c, c-e, or alternatively periods b-d and d-f of Figure 4b. In the first alternative cyclic time, periods starting from the maximum acceleration time point are utilized. In the second alternative cyclic time, periods starting from the zero crossing time point are utilized.

It is evident that it is possible to use any other timing point that is defined from the averaged acceleration path graph 43 to combine and then average cyclic velocity or acceleration/deceleration time periods.

Figure 4d shows as an example how velocity data of several time period cycles 41 can be combined and then averaged so that the defined graph 40A depicts an averaged velocity of the ski pair under test. Figure 4d discloses also defined averaged acceleration graph 43A. A zero crossing point 45 of the averaged acceleration graph 43A is shown.

In cross-country skiing the sliding index of the tested ski pair can be deduced from a negative slope of the averaged velocity graph 40A advantageously as follows. The zero crossing point 45 on the averaged acceleration path graph 43A can be used as a starting point to calculate the velocity path graph 40A and a gradient 44 of the averaged velocity graph 40A can be drawn related to that point. It is obvious that any other timing points in the averaged acceleration path graph 43A may be used for calculating the corresponding velocity path graph.

The angle of the defined gradient 44 on the averaged velocity path graph 40A depends on the deceleration values after point 45 on the averaged acceleration path graph 43A. The deceleration values around the above-mentioned zero crossing point 45 depend on the friction between the ski pair and the snow. So from the angle of the gradient 44 defined from the averaged velocity path graph 40A can be defined a sliding index that depends on the ski-snow friction of the tested ski pair.

In cross-country skiing a sliding index can be defined from the gradient 44 as fol- lows. If the gradient angle between a horizontal axis x-axis and gradient with - sign due a clockwise direction from the horizontal axis is 0 degrees, then the sliding index is advantageously 100. If the gradient angle is -84 degrees, then the sliding index is advantageously 1 . The values of gradient angles between 0 - -84 degrees can be linearly or non-linearly pointed to sliding index that is between 100 - 1 .

Figure 4e shows as an example how from measurement data of several measurement cycles 41 measured in downhill skiing the sliding index of the tested ski pair can be deduced from positive slope of the averaged velocity graph 43B. Figure 4e discloses also defined averaged acceleration graph 43B. A zero crossing point 46 is shown in an acceleration portion of the averaged acceleration graph 43B.

In downhill skiing the sliding index of the tested ski pair can be deduced from a positive slope of the averaged velocity graph 40B advantageously as follows. The zero crossing point 46 on the averaged acceleration path graph 43B can be used as a starting point to calculate the velocity path graph 40B and a gradient 47 of the averaged velocity graph 40B can be drawn related to that point. It is obvious that any other timing points in the averaged acceleration path graph 43B may be used for calculating the corresponding velocity path graph.

The angle of the defined gradient 47 on the averaged velocity path graph 40B de- pends on the acceleration values after point 46 on the averaged acceleration path graph 43B. The acceleration values after the above-mentioned zero crossing point 46 depend on friction between the ski pair and the snow and possible slope of the downhill and its acceleration effect on the skis. So the angle of the gradient 47 defined from the averaged velocity path graph 40A can be utilized as one parameter when a sliding index is defined.

However in downhill skiing also other parameters than the ski-snow friction of the tested ski pair have to be taken into account in the test situation. Most important parameter for defining the ski-snow friction can be deduced from the averaged ve- locity path graph 40B when also difference in altitude and track configuration made by gates are taken into account during calculation of the averaged path graph 40B.

In downhill skiing a sliding index can be defined from the gradient 47 as follows. If the gradient angle between a horizontal axis x-axis and gradient with +sign due a anticlockwise direction from the horizontal axis is 0 degrees, then the sliding index is advantageously 1 . If the gradient angle is 84 degrees, then the sliding index is advantageously 100. The values of gradient angles between 0 - +84 degrees can be linearly or non-linearly pointed to sliding index that is between 1 and 100. It is possible to use any kind of suitable scaling and calculation tables to define a practical sliding index.

Figure 5 shows as an exemplary flowchart the functional main steps of the method of defining a sliding index of a ski pair.

In step 50 the skier starts the skiing test. The skier can give a starting command to the sliding measurement apparatus 3, 3A or 3B in several ways. One alternative is to give the test starting command by utilizing the user interface of the signal processing unit 4. The signal processing unit 4 transmits the starting command ad- vantageously via a wireless connection to the sliding measurement apparatus 3 or 3A. Another alternative is to activate the sliding measurement apparatus 3B by utilizing the user interface embedded into the sliding measurement apparatus 3B.

In step 51 an accelerometer measures continuously momentary acceleration or deceleration values. The measured acceleration or deceleration values are advantageously transmitted either to the signal processing unit 4 or to the cloud service 6 for further processing. Measurement data transmission may be either real time or the measurement data may be transmitted after the ski test. In one advantageous embodiment the measured acceleration and deceleration values are processed directly in the sliding measurement apparatus 3B. In that embodiment the defined measurement result may advantageously be transmitted to the cloud service 6 after a ski pair test. In all the above-mentioned alternatives a momentary velocity value of the skier is integrated from the momentary acceleration and deceleration values that have been measured in the test period underway. By using the momentary velocity values a velocity path graph 40 shown in Figure 4a may be defined. In step 52 cycle time periods are defined from velocity path graph 40.

In step 53 one raw cycle time period at a time is selected. In this step advantageously such a cycle time period, which deviates more than a predetermined variation threshold, advantageously may be discarded.

In step 54 the velocity cycle path graph data of an accepted cycle time period is saved. The velocity path graph data may be saved in the memory of the signal processing unit 4, in a cloud service 6 or in the memory of the sliding measurement apparatus 3, 3A or 3B.

In one advantageous embodiment also the measured acceleration and deceleration path graph of the accepted cycle time period may be saved.

In step 55 it is checked if all measured cycle time periods are either stored in the memory or have been discarded in step 53. If the result of the check 55 is that there are unverified cycle time periods left, the process returns to step 53. If the result of the check 55 is that all cycle time periods have been verified, the process proceeds to step 56.

In step 56 an averaged velocity path graph 40A or 40B is calculated. In one ad- vantageous embodiment also an averaged acceleration path graph 43A or 43B is calculated. The calculated path graphs are then saved in the memory of the signal processing unit 4, in the cloud service 6 or in the memory of the sliding measurement apparatus 3B. In step 57 is first defined a gradient 44 or 47 either on the decreasing velocity slope portion of the velocity path graph 40A or on the increasing velocity slope portion of the velocity path graph 40B.

As a second sub-step, an angle of the defined gradient 44 or 47 is defined.

In step 58 the defined angle of gradient is converted to a sliding index according to the invention.

The sliding index definition process ends in step 59 where the defined sliding in- dex of the tested ski pair is saved in the memory of the signal processing unit 4, in the cloud service 6 or in the memory of the sliding measurement apparatus 3B.

When all ski pairs have been tested, the skier can compare the sliding indexes of the different ski pairs and select the best sliding ski pair for that particular weather and for the snow circumstances.

All the process steps presented above in Figures 5 can be implemented with computer program commands, which are executed in a suitable special-purpose or general-purpose processor. The computer program commands can be stored in a computer-readable media, such as a data disk or a memory, from where the processor can retrieve said computer program commands and implement them. The references to computer-readable media can for example also contain special components, such as programmable USB Flash memories, logic arrays (FPLA), application-specific integrated circuits (ASIC), and signal processors (DSP).

Some advantageous embodiments of the invention have been described above. The invention is not limited to the solutions described above, but the inventive idea can be applied in numerous ways within the scope of the claims.

Claims

1 . A measurement method defining a sliding index between a ski and the snow, in which method:

- a sliding measurement apparatus (3, 3A, 3B) attached to a ski (2) or an ankle of a skier (1 ) measures (51 ) acceleration of the ski (2) during a skiing event

- the sliding measurement apparatus (3, 3A, 3B) saves the measured acceleration data to a signal processing unit (4) or to a cloud service (6) or to the memory of the sliding measurement apparatus (3, 3A, 3B)

- the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) defines (52) from the measured acceleration data a velocity graph (40) and/or an acceleration graph (42)

- the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) defines (53) from the velocity graph (40) or the acceleration graph cycle times (41 ) of repeated skiing movements, and

- the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) saves (54) velocity path graphs (40) and/or acceleration path graphs (42) of the defined cycle times,

characterised in that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) further:

- defines (56) an averaged velocity path graph (40A, 40B) from the saved acceleration path graphs of the defined cycle times

- defines (57) at least one angle of gradient (44, 45) from the averaged velocity path graph (40A, 40B)

- defines (58) a sliding index from the selected angle of gradient that depicts fric- tion between the ski pair and the snow, and that

- sliding indexes of the different ski pairs are utilized for selecting a best sliding ski pair in prevailing weather conditions.

2. The measurement method according to claim 1 , characterised in that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) defines the sliding index of the ski (2) from an angle of gradient (44) having a negative angular coefficient depicting deceleration of the ski (2).

3. The measurement method according to claim 1 , characterised in that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) defines the sliding index of the ski (2) from an angle of gradient (45) having a positive angular coefficient depicting acceleration of the ski (2).

4. The measurement method according to claims 1-3, characterised in that the sliding measurement apparatus (3, 3A, 3B) measures acceleration and deceleration in three dimensions and that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) calculates acceleration and decelera- tion values both toward a direction of a longitudinal axel of the ski and toward a skiing direction.

5. The measurement method according to claim 4, characterised in that the sliding measurement apparatus (3, 3A, 3B) measures also an elevation angle of the ski (2) during sliding and that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) uses also the measured elevation angle when defining the sliding index of the ski (2).

6. A ski performance measuring arrangement (20A, 20B, 20C), which comprises:

- a sliding measurement apparatus (3, 3A, 3B) attached to a ski (2) or an ankle or foot or shoe of a skier (1 ) comprising:

- a processor for controlling measuring

- an accelerometer sensor measuring acceleration and deceleration of the ski (2)

- a memory for storing acceleration, deceleration and velocity measurement data

- a transmitter for sending measured acceleration and deceleration data, and

- a receiver for receiving control commands during measuring state

- a signal processing unit (4) or cloud service (6) configured to:

- receive (51 ) measured acceleration and deceleration data from the measuring unit (3, 3A)

- define from the received acceleration and deceleration data an averaged acceleration graph (43, 43A, 43B)

- save averaged acceleration path graphs (43, 43A, 43B) of the defined cycle time

- define from the received acceleration and deceleration data a velocity path graph (40)

- define (53) from the velocity graph (40) a cycle time period (41 ) of a skiing movement

- a display unit (5) for displaying a sliding index of the ski (2),

characterised in that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) is further configured to: - define (52) form the received acceleration and deceleration data a velocity path graph (40A, 40B) of the defined cycle time

- save (54) the velocity path graph of the defined cycle time period (41 )

- define (56) an averaged velocity path graph (40A, 40B) from the saved velocity path graphs of the defined cycle time periods

- define (57) at least one angle of gradient (44, 47) from the averaged velocity path graph (40A, 40B)

- define (58) the sliding index from the selected angle of gradient that depicts friction between the ski pair and the snow, and that

- the defined sliding indexes of the different ski pairs are configured to be utilized in selecting a best sliding ski pair in prevailing weather conditions.

7. The ski performance measuring arrangement according to claim 6, characterised in that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) is configured to define the sliding index of the ski (2) from an angel of gradient (44) having a negative angular coefficient depicting deceleration of the ski (2).

8. The ski performance measuring arrangement according to claim 6, charac- terised in that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) is configured to define the sliding index of the ski (2) from an angel of gradient (47) having a positive angular coefficient depicting acceleration of the ski (2).

9. The ski performance measuring arrangement according to claims 6-8, characterised in that the sliding measurement apparatus (3, 3A, 3B) is configured to measure acceleration and deceleration in three dimensions and that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) is configured to calculate acceleration and deceleration values both toward a di- rection of a longitudinal axel of the ski and toward a skiing direction.

10. The ski performance measuring arrangement according to claim 9, characterised in that the sliding measurement apparatus (3, 3A, 3B) is configured to measure also an elevation angle of the ski (2) during sliding and that the signal processing unit (4) or cloud service (6) or the sliding measurement apparatus (3B) is configured to use also the measured elevation angle when defining the sliding index of the ski (2).

1 1 . A sliding measurement apparatus (3, 3A, 3B) to be attached to a ski (2) or an ankle of a skier (1 ) comprising:

- a processor configured to control measuring

- an accelerometer configured to measure (51 ) accelerations and decelera- tion of the ski (2) during a skiing event

- a transmitter configured to transmit the measured acceleration and deceleration data to a signal processing unit (4) to a cloud service (6), and

- a receiver configured to receive control commands during measuring state, characterised in that the sliding measurement apparatus (3, 3A, 3B) is further configured to:

- receive from the signal processing unit (4) or from a cloud service (6) a defined sliding index of the ski (2) based on the transmitted acceleration data

- indicate the received sliding index of the ski (2), and that

12. The sliding measurement apparatus according to claim 1 1 , characterised in that it further comprises a display configured to show the sliding index of the ski

(2).

13. The sliding measurement apparatus according to claim 1 1 , characterised in that it further comprises means for measuring an elevation angle of the ski (2) dur- ing sliding and that the signal processing unit (4), cloud service (6) or the sliding measurement apparatus (3B) is configured to use the defined elevation angle when defining the sliding index of the ski (2).

14. A computer program product comprising computer program code means adapted to perform the following program code steps when said program is executed in a processor of a data processing device for defining a sliding index of a ski (2) that is configured to be utilized in selecting a best sliding ski pair in prevailing weather conditions, the computer program product_comprising:

- code means for defining (52) from measurement data an acceleration path graph (42) and a velocity path graph (40)

- code means for defining (53) from the velocity path graph (40) a cycle time period (41 ) of repeated skiing movements, and - code means for saving (54) the acceleration path graph (42) of the defined cycle time period,

characterised in that computer program further comprises:

- code means for defining (56) an averaged velocity path graph (40A, 40B) from the saved velocity path graphs of the defined cycle times

- code means for defining (57) at least one angle of gradient (44, 47) from the averaged velocity path graph (40A, 40B)

- code means for defining (58) a sliding index from the selected angle of gradient that depicts friction between the ski pair and the snow, and

15. The computer program product according to claim 14, characterised in that it further comprises code means for defining the sliding index of the ski (2) from an angle of gradient (44) having a negative angular coefficient depicting deceleration of the ski (2).

16. The measurement method according to claim 14, characterised in that it further comprises code means for defining the sliding index of the ski (2) from an an- gle of gradient (47) having a positive angular coefficient depicting acceleration of the ski (2).

17. The measurement method according to claims 14-16, characterised in that it further comprises code means for measuring acceleration and deceleration in three dimensions and code means for calculating acceleration and deceleration values toward to a longitudinal axel of the ski (2) and toward a skiing direction.

18. The measurement method according to claim 17, characterised in that it further comprises code means for measuring also an elevation angle of the ski (2) during sliding and code means for using also the measured elevation angle when defining the sliding index of the ski (2).