WO2021038226A1

WO2021038226A1 - Friction measurement device

Info

Publication number: WO2021038226A1
Application number: PCT/GB2020/052051
Authority: WO
Inventors: Marc MASEN
Original assignee: Imperial College Innovations Limited
Priority date: 2019-08-29
Filing date: 2020-08-27
Publication date: 2021-03-04
Also published as: GB201912426D0

Abstract

Embodiments described herein relate to a device for measuring friction between surfaces, the device comprising: a contact surface arranged for sliding contact with a test surface; an actuator arranged to move the contact surface relative to the test surface; a sensor arranged to measure the normal load applied to the test surface by the contact surface; and a processor arranged to measure the friction force between the contact surface and the test surface from the electrical current demand of the actuator.

Description

FRICTION MEASUREMENT DEVICE

FIELD

The present disclosure relates to the measurement of friction between surfaces, and in particular to the measurement of friction between a material and human skin. BACKGROUND

It is important to evaluate the skin’s friction response because the skin is always in contact with different materials and products. Measuring the skin’s friction response provides insight into how the skin responds to such materials and products. Applications can range from clothing and bedding to prosthetic devices and cosmetics.

Existing methods of measuring friction between a material and human skin are largely laboratory based. The use of laboratory equipment is not well suited to measuring the friction response of human skin, because this equipment is typically used for measuring friction between engineering contacts such as ball bearings. Using such laboratory equipment is therefore not suitable for measuring the friction response of human skin, for the following reasons: a) the load applied to the skin is too high; b) measurement is typically confined to an area of human skin that can easily be positioned in the equipment, such as the forearm; c) the laboratory and equipment requires extensive cleaning to prevent contaminants from affecting the friction measurement; and d) the laboratory equipment can intimidate the test subject, leading to a sweat response which affects the friction measurement. Accurate measurement of the skin’s friction response is therefore not possible using existing laboratory equipment. Portable devices for investigating the skin’s friction response exist. However, these devices typically measure the skin’s response at conditions of high velocity and load.

Accordingly, there exists a need for improving how the skin’s friction response is measured. SUMMARY

This summary introduces concepts that are described in more detail in the detailed description. It should not be used to identify essential features of the claimed subject matter, nor to limit the scope of the claimed subject matter. The features of the present disclosure are discussed below with reference to the measurement of the human skin’s friction response. It will be appreciated, however, that the effects of the disclosed features discussed below are also applicable to the measurement of the friction response of other surfaces.

In one aspect of the present disclosure, there is provided a device for measuring friction between surfaces, the device comprising: a contact surface arranged for sliding contact with a test surface; an actuator arranged to move the contact surface relative to the test surface; a sensor arranged to measure the normal load applied to the test surface by the contact surface; and a processor arranged to measure the friction force between the contact surface and the test surface from the electrical current demand of the actuator.

Measuring the normal load applied to the test surface allows the actual normal load applied to the test surface to be identified. By measuring the normal load (as opposed to deriving or estimating the normal load), a more accurate value of the normal load is provided. This means that when calculating the coefficient of friction between the contact surface and the test surface (i.e. by dividing the frictional force by the normal load), a more accurate value of the coefficient of friction is provided, because a more accurate value of the normal load is used in the calculation.

Measuring the friction force from the electrical current demand of the actuator avoids the use of flexures and force transducers for measuring the friction force. This means that the actuator can be mounted within the device without using a complex suspension system that incorporates a flexure for measuring the friction force. In other words, the actuator can be mounted in the device in a more stable manner than if the coupling between the contact surface and the actuator were used to measure the friction force. Mounting the actuator in a more stable manner reduces the degrees of freedom of the coupling between the actuator and the contact surface, meaning that a more accurate measurement of the friction force (and consequently also the coefficient of friction) is provided. Mounting the actuator within the device also allows the footprint of the device to be reduced. Reducing the footprint of the device improves the ease of using the device one-handed, meaning that the device is more ergonomic. Mounting the actuator within the device also simplifies the construction of the device.

Reducing the footprint of the device also means that the device can be used to investigate the skin’s response to forces at more areas of a test subject’s skin (for example, areas such as the nose where the skin area is relatively small). This allows the skin’s response to forces to be determined at locations which previously have not been accessible.

The test surface may be the skin of a test subject. The normal load is the force that acts perpendicular to the test surface. The normal load comprises the force due to gravity acting on components of the device and any additional force applied to the contact surface by the device itself. The friction force results from the sliding contact of the contact surface over the test surface.

The normal load may be applied to the test surface by applying a force to the actuator. Applying a normal load to the actuator (rather than directly applying the normal load to the contact surface) simplifies the coupling between the actuator and the contact surface.

When the device is positioned, in use, above the test surface in a substantially vertical orientation, the actuator may be vertically spaced apart from the test surface.

The sensor may be positioned in series with the contact between the contact surface and the test surface. This ensures that the forces are not dissipated by any other part of the device construction, meaning that the normal load is accurately measured.

The device may be handheld. A handheld device is more user friendly because it does not require the use of complex laboratory equipment in order to measure the skin’s response to forces. This means that anyone can use the device to carry out a measurement of the response to forces of their own skin or another person’s skin. Avoiding the use of laboratory equipment also means that the coefficient of friction between the skin and the contact surface can be measured with increased accuracy. Test subjects whose skin response is measured using laboratory equipment can experience discomfort, which can alter the skin’s properties (for example, if the subject’s discomfort triggers sweat release). A handheld device is less intimidating than laboratory equipment, meaning that test subject discomfort is reduced. Therefore, adverse effects of sweat release on friction measurement can be reduced. Even if only some test subjects experience sweat release when laboratory equipment is used, the handheld device improves the repeatability of friction measurements by reducing the discomfort experienced by those subjects.

The device may further comprise a housing. The housing may be arranged to contact the test surface while the contact surface is in contact with the test surface. Contact between the housing and the test surface prevents over application of the normal load to the test surface. This is because any additional load applied by the user of the device is carried through the housing. This means that only the intended normal load is applied to the test surface. This improves repeatability of measurements because a specified normal load can be consistently applied to the test surface.

Contact between the housing and the test surface also avoids the need for a separate reference surface for conducting the friction response measurement. This means that the device is not sensitive to positional errors.

The housing may be arranged to surround the contact surface when the contact surface is in contact with the test surface. By surrounding the contact surface, the contact surface can be centred where the measurement is taking place. This allows a user to more accurately position the device in the location where the measurement is being carried out. Surrounding the contact surface also means that a user is prevented from unintentionally applying a tilting load to the contact surface, which would affect the measurement of the normal load.

The actuator may be slidably mounted within the housing. The actuator may be slidably mounted using sliding bearings. Using sliding bearings reduces the friction resulting from movement of the actuator relative to the housing. This increases the proportion of the intended normal load that is applied to the test surface. In addition, this improves the accuracy of measurement of the normal load because friction between the actuator and the housing does not contribute to the normal load measurement.

The device may comprise a chassis arranged within the housing and configured to move relative to the housing, wherein the actuator is coupled to the chassis. The chassis provides a stable mounting for the actuator within the housing.

The device may further comprise a resiliently deformable member coupled between the actuator and the housing. The resiliently deformable member controls the movement of the actuator relative to the housing. The resiliently deformable member may exert a force on the actuator. The resiliently deformable member may be a spring.

The normal load may be applied to the test surface by compression of the resiliently deformable member. The force may be exerted on the actuator by compression of the resiliently deformable member.

The device may further comprise a load adjustment mechanism arranged to adjust the normal load applied to the test surface. The load adjustment mechanism allows the normal load applied to the test surface to be adjusted, meaning that an increased number of skin responses can be investigated.

The load adjustment mechanism may be arranged to adjust the applied normal load by compressing the resiliently deformable member. Compression of the resiliently deformable member provides a simple mechanism for adjusting the normal load applied to the test surface.

The device may further comprise an orientation sensor arranged to measure the orientation of the device. By measuring the orientation of the device, the device does not need to be positioned substantially vertically in use, because the device orientation can be factored into the calculation of the applied normal load. This allows a greater number of skin areas to be tested more easily.

The device may further comprise a battery arranged to supply power to the actuator.

By supplying power to the actuator using a battery, the device is portable and is not restricted to being used in a particular location. The battery therefore allows remote testing to be carried out. For example, measurements are not confined to a laboratory setting.

The device may further comprise a user interface arranged to display the normal load applied to the test surface by the contact surface. Displaying the applied normal load provides feedback to the user, meaning that the user can see the normal load that is currently being applied. Providing feedback on the applied normal load also allows the user to more accurately adjust the normal load using the load adjustment mechanism.

The device may further comprise a wheel, wherein the curved surface of the wheel comprises the contact surface. By using a wheel as the contact surface, a friction force can be applied to the skin without requiring a large area of skin. This also allows the measurements to be specific to a particular skin location because the contact surface does not need to move across an area of skin. In addition, the use of a wheel results in a constant velocity over the contact surface. The wheel may be mounted on an axle.

The device may further comprise a gearing mechanism between the actuator and the axle. Implementing a gearing mechanism means that the actuator does not need to be mounted in line with the wheel. This allows for different relative locations of the actuator and the wheel. The actuator may be mounted substantially perpendicular to the axle. Mounting the actuator substantially perpendicular to the axle means that the device is more compact, thereby reducing the size of the device.

The wheel may alternatively be mounted in line with the actuator.

The actuator may be a motor arranged to rotate the wheel to provide the sliding contact between the contact surface and the test surface.

In another aspect of the present disclosure, there is provided a method of measuring friction between surfaces using a device comprising an actuator and a contact surface, the method comprising: measuring the normal load applied to a test surface by the contact surface; and measuring the friction force between the contact surface and the test surface from the electrical current demand of the actuator.

The method may further comprise calculating a coefficient of friction using the measurement of the normal load and the measurement of the friction force. Calculating the coefficient of friction may comprise dividing the measurement of the friction force by the measurement of the normal load.

In a further aspect of the present disclosure, there is provided a computer-readable medium comprising instructions which, when executed by a processor of a device, cause the device to carry out the method described in the previous paragraph.

In another implementation of the present disclosure, the friction force is not measured from the electrical current demand of the actuator. For example, the friction force may alternatively be measured using a force transducer. Therefore, a device for measuring friction between surfaces may alternatively comprise: a contact surface arranged for sliding contact with a test surface; an actuator arranged to move the contact surface relative to the test surface; and a sensor arranged to measure the normal load applied to the test surface by the contact surface; wherein the normal load is applied to the test surface by applying a normal load to the actuator. In this alternative arrangement, the device may comprise any of the features described in the above paragraphs.

BRIEF DESCRIPTION OF FIGURES

Specific embodiments are described below by way of example only and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a front view of a device for measuring friction between surfaces.

FIG. 2(a) is a schematic diagram of a front view of the device of FIG. 1 before the device is brought into contact with a surface. FIG. 2(b) is a schematic diagram showing the device of FIG. 2(a) after the applied load has been adjusted.

FIG. 2(c) is a schematic diagram showing the device of FIG. 2(b) after the device has been brought into contact with a surface. FIG. 3 is a schematic diagram of a front view of an alternative device for measuring friction between surfaces.

FIG. 4 is a schematic diagram of a front view of a further alternative device for measuring friction between surfaces.

DETAILED DESCRIPTION

Implementations of the present disclosure are explained below with particular reference to measuring the coefficient of friction between a contact surface and human skin. It will be appreciated, however, that the implementations described below are also applicable to measuring the coefficient of friction between the contact surface and any other surface.

FIG. 1 shows a device 20 for measuring friction between surfaces. The device 20 can be hand-held and comprises a housing 8. The device 20 has a cavity 18 in which a chassis 5 is mounted. The chassis 5 is slidably mounted within the housing 8 using sliding bearings 6. The sliding bearings 6 are arranged to slide along bearing runners 7 located within the housing 8. The bottom end of each bearing runner 7 acts as a stop to limit movement of the chassis 5 out of the cavity 18 in the housing 8. The top end of each bearing runner 7 acts as a stop to limit movement of the chassis 5 into the cavity 18 in the housing 8.

A spring 11 is mounted between the chassis 5 and the interior of the housing 8. The spring 11 acts to force the chassis 5 out of the housing 8. The spring is coupled to a load adjustment mechanism 15 in the form of a nut mounted on a threaded bolt. When the nut is screwed onto the threaded bolt, it pushes downwards against the top of the spring 11 , thereby compressing the spring 11. Similarly, when the nut is unscrewed, the spring 11 occupies an increased amount of space, thereby releasing the compression on the spring 11.

A sensor (in FIG. 1 in the form of a force transducer 10) is mounted in series with the spring 11. The force transducer 10 measures the normal load applied to a test surface via the chassis 5, as described further below. The signal from the force transducer 10 is received by a processor 13 located within the housing 8. The force measured by the force transducer 10 is displayed on a user interface 14.

An actuator (in FIG. 1 in the form of a motor 4) is fixedly mounted within the chassis 5, so that the motor 4 slides up and down within the housing 8 with the sliding movement of the chassis 5. The motor 4 is powered by a battery 9 mounted within the housing 8.

A motor controller 12 is also mounted within the housing 8. The motor controller 12 controls the speed of the motor 4. The motor controller 12 also measures the electrical current demand of the motor 4. The electrical current measurement from the motor controller 12 is received by the processor 13.

An axle 21 is coupled to the chassis 5 using roller bearings 3. The axis of rotation of the axle 21 is perpendicular to the axis of rotation of the motor 4. The motor 4 is coupled to the axle 21 via a gear system 2. The gear system 2 allows the motor 4 to drive rotation of the axle 21. The axle 21 is coupled to a contact surface (in FIG. 1 in the form of a wheel 1). Rotation of the motor 4 therefore drives rotation of the axle 21 and therefore rotation of the wheel 1. The speed of rotation of the wheel 1 is controlled by controlling the speed of the motor 4 using the motor controller 12.

The processor 13 includes circuitry which amplifies the signals from the force transducer 10 and the motor controller 12. The electrical current demand of the motor 4 is proportional to the friction force (i.e. shear force) between the wheel 1 and a test subject’s skin. The processor circuitry converts the electrical current measurement to a friction force between the wheel 1 and the subject’s skin. The processor 13 calculates a coefficient of friction between the wheel 1 and the skin by dividing the friction force by the normal load measured by the force transducer 10.

FIGS. 2(a) to 2(b) show the arrangement of the device 20 of FIG. 1 in use. For simplicity, FIGS. 2(a) to 2(c) show only the following components of FIG. 1 : the wheel 1, the chassis 5, the housing 8, the spring 11, and the load adjustment mechanism 15. Also shown in FIGS. 2(a) to 2(c) is a test surface in the form of the skin 16 of a human test subject.

FIG. 2(a) shows the device 20 before the user has made any adjustment to the normal load that will be applied to the skin 16 of the test subject (which could be the user’s own skin). As shown in FIG. 2(a), the wheel 1 protrudes from the end of the housing 8 owing to the action of gravity on the chassis 5.

If desired, the user adjusts the normal load that will be applied to the test subject’s skin 16 by adjusting the load adjustment mechanism 15. The user can increase the normal load by screwing the nut onto the threaded bolt, which forces the spring 11 downwards. FIG. 2(b) shows the device 20 after the user has increased the normal load that will be applied to the skin 16. As shown in FIG. 2(b), by forcing the spring 11 downwards, the chassis 5 is also forced downwards. This means that the wheel 1 protrudes further from the end of the housing 8 than in FIG. 2(a).

The user then switches on the device 20 using a switch (not shown). The switch may be provided on the user interface 14 of FIG. 1. Once the user has switched on the device 20, the user has a short time period to bring the device 20 into contact with the skin 16 before the motor 4 (shown in FIG. 1) starts.

FIG. 2(c) shows the device 20 once the user has moved the device into contact with the skin 16 of the test subject. To position the device 20 correctly, the user firstly places the wheel 1 on the skin 16. The user then allows the housing 8 to come into contact with the skin 16. The housing 8 slides downwards relative to the wheel 1 and the chassis 5 under the action of gravity. As shown in FIG. 2(c), the downward movement of the housing 8 compresses the spring 11. The compression of the spring 11 causes the selected normal load to be applied to the test subject’s skin 16 via the wheel 1.

Once the device 20 is in the position shown in FIG. 2(c), the motor 4 (shown in FIG. 1) starts. The rotation of the motor 4 causes rotation of the wheel 1. Rotation of the wheel 1 results in sliding contact between the wheel 1 and the skin 16. The friction between the wheel 1 and the skin 16 as a result of the sliding contact is measured by the motor controller 12 of FIG. 1, which measures the electrical current demand of the motor 4. The normal load applied to the skin 16 by the wheel 1 is measured by the force transducer 10. The normal load measurement from the force transducer 10 and the electrical current measurement from the motor controller 12 are both received at the processor 13. The processor 13 converts the electrical current demand to a friction force. The processor 13 then calculates a coefficient of friction by dividing the normal load measurement by the friction force, and records the calculated coefficient of friction. The processor 13 may start recording the measurements before the motor 4 starts, meaning that static friction between the skin 16 and the wheel 1 can also be measured.

FIGS. 2(a) to 2(c) show the operation of the device 20 where the applied load is adjusted using the load adjustment mechanism 15. It will be appreciated, however, that the user does not have to adjust the applied load prior to measuring the friction response of the skin 16. That is, the user may use the device 20 in the configuration shown in FIG. 2(a), where the applied load has not been adjusted using the load adjustment mechanism 15. In this case, it can be seen from FIG. 2(a) that when the device 20 is brought into contact with the skin 16, the downward movement of the housing 8 will compress the spring 11 , meaning that a normal load will be applied to the skin 16. When the device 20 is used in the configuration shown in FIG. 2(a), the normal load applied to the skin will be smaller than the normal load applied using the configuration shown in FIG. 2(c).

In addition, the skin 16 may be pre-treated before the device 20 is brought into contact with the skin 16 (i.e. prior to the arrangement shown in FIG. 2(c)). For example, the skin 16 may be cleaned with soap, cream may be applied to the skin 16, or the measurement may be carried out after a more extensive (e.g. long duration) skincare regime.

FIG. 3 shows an alternative device 100 for measuring friction between surfaces. The device 100 includes all components of the device 20 shown in FIG. 1, except for the axle 21, roller bearings 3, and gear system 2. The device 100 of FIG. 3 includes a motor 104 which is arranged in a different manner to the motor 4 of the device 20 of FIG. 1 and a force transducer 110 which is arranged in a different manner to the force transducer 10 of FIG. 1.

In particular, it can be seen from FIG. 3 that the motor 104 of the device 100 is mounted in line with the wheel 1. This means that the wheel 1 rotates about the same axis as the axis of rotation of the motor 104. That is, in contrast to the device 20 of FIG. 1, the motor 104 is not mounted perpendicular to an axle on which the wheel 1 is mounted. In addition, in the device 100 shown in FIG. 3, the force transducer 110 is mounted between the motor 104 and the chassis 5. As with the device 20 shown in FIG. 1, the force transducer 110 is mounted in series with the spring 11. Mounting the force transducer 110 (or load cell) closer to the wheel (in comparison with the device 20 shown in FIG. 1) prevents some of the normal force being dissipated in the sliding bearings 6. In the device 100 shown in FIG. 3, the force on the load cell is equal to the load acting on the wheel 1 minus the mass of the motor-wheel combination, which can be accounted for.

The operation of the device shown in FIG. 3 is the same as for the device shown in FIG. 1. That is, the device of FIG. 3 operates in accordance with the steps illustrated in FIG. 2(a) to FIG. 2(c).

A further alternative device 200 is shown in FIG. 4. As with the device 100 of FIG. 3, the device 200 shown in FIG. 4 does not include the axle 21 , roller bearings 3 or gear system 2 of the device of FIG. 1. Similar to the device 100 shown in FIG. 3, the device 200 of FIG. 4 includes a motor 204 which is mounted in line with the wheel.

The motor 204 is coupled to the chassis 5 using a ball joint 222. The use of the ball joint 222 ensures that all forces from the skin-object interaction are transmitted through the ball joint 222. The normal load would be measured using a load cell 210 mounted between the chassis 5 and the motor 204. A second load cell (not shown in FIG. 4) is mounted either behind the motor 204 shown in FIG. 4 or in front of the motor 204, depending on the rotational direction of the wheel 1. This second load cell is used to measure the shear force. In the device shown in FIG. 4, the ball joint 222 takes up a force but from the moment balance the acting force can be derived. Alternatively, the construction can be designed such that the force through the load cell is equal to the force acting on the wheel 1 meaning that no derivation of the acting force is necessary.

Additional variations or modifications to the systems and methods described herein are set out in the following paragraphs.

It will be appreciated that the device described in relation to FIGS. 1 and 2(a) to 2(c) allows the skin’s friction response to be measured. The skin’s friction response will be specific to the material of the contact surface (i.e. the wheel 1 in FIGS. 1 and 2(a) to 2(c), which may be, for example, plastic or steel. To allow the skin’s response to different materials to be measured, the wheel may be replaceable, so that a wheel formed of a different material may be attached to the axle 21 instead of the wheel 1. In particular, wheels formed of soft surfaces such as textiles may be used to measure the skin’s friction response.

The contact surface does not have to be in the form of a wheel (i.e. a cylinder). For example, the contact surface may alternatively be a crowned wheel, which provides point contacts with the skin as opposed to sliding contacts, to allow more complex skin- material responses to be investigated.

The device may alternatively further include an orientation sensor such as an accelerometer. By measuring the orientation of the device, the device orientation can be factored into the calculation of the normal load. This means that the device does not have to be positioned vertically when measuring the skin’s response. To explain, when the device is used in a vertical orientation, the normal load applied to the skin is the combination of the weight of the chassis, motor, bearings, axle, gearing system and wheel, along with the force exerted by compression of the spring. The normal load can therefore be expressed as: F_R = Mg + kx

Where:

M is the combined mass of the chassis and all components coupled to the chassis (i.e. all components coupled to the housing by the spring); g is acceleration due to gravity; k is the spring constant of the spring; and x is the compression of the spring.

If the orientation of the device is expressed as an angle from the vertical, Q, then the above expression can be re-written as:

F_R = Mg cos Q + kx

The gear system has been described above as coupling the motor to the gear system so that the motor (which is mounted perpendicular to the axle in the device of FIG. 1) can drive rotation of the axle. The gear system may alternatively further include gearing to reduce the speed of the motor. Gearing to reduce the speed of the motor may also be included in the alternative devices shown in FIGS. 3 and 4.

In the above description, the friction force is measured from the electrical current demand of the motor. The friction force may alternatively be measured by assessing the deflection of a flexure system. In such an alternative implementation, the motor (in the arrangement of FIG. 3) or the chassis (in the arrangement of FIG. 1) would be supported with a flexible element. All forces from the skin-object interaction are transmitted through the flexible element. Although shear forces would be dissipated in the sliding bearings of the device, the motor would experience the same shear force because it is in series with the sliding bearings. This means that the shear force on the motor or chassis can be measured. As an alternative to using a flexure system, a ball joint may be used to couple the motor to the chassis (e.g. as shown in FIG. 4).

The device may be constructed so that it does not look industrial or intimidating. For example, the housing may be formed of a material such as plastic and may be a colour (e.g. white) which is similar to devices that test subjects may have encountered in daily life. This reduces the likelihood of the test subject expressing a negative response to being tested (which could affect the measurement of the friction response).

The touch screen of the device is described above as displaying a reading of the applied normal load. The user interface may alternatively be more simplistic, including only an on/off switch. Alternatively, the user interface may include a touchscreen for receiving user commands and displaying measurement data.

The processor may be configured to transfer the measurements to another device wirelessly and in real-time. The device may therefore include a wireless transceiver coupled to the processor. The wireless transceiver may allow data to be sent over a wireless connection (for example, a short-range wireless connection such as Bluetooth). A USB interface may alternatively be used for transferring data.

The described methods may be implemented using computer executable instructions.

A computer program product or computer readable medium may comprise or store the computer executable instructions. The computer program product or computer readable medium may comprise a hard disk drive, a flash memory, a read-only memory (ROM), a CD, a DVD, a cache, a random-access memory (RAM) and/or any other storage media in which information is stored for any duration (e.g., for extended time periods, permanently, brief instances, for temporarily buffering, and/or for caching of the information). A computer program may comprise the computer executable instructions. The computer readable medium may be a tangible or non-transitory computer readable medium. The term “computer readable” encompasses “machine readable”.

The singular terms “a” and “an” should not be taken to mean “one and only one”. Rather, they should be taken to mean “at least one” or “one or more” unless stated otherwise. The word “comprising” and its derivatives including “comprises” and

“comprise” include each of the stated features, but does not exclude the inclusion of one or more further features.

The above implementations have been described byway of example only, and the described implementations are to be considered in all respects only as illustrative and not restrictive. It will be appreciated that variations of the described implementations may be made without departing from the scope of the invention. It will also be apparent that there are many variations that have not been described, but that fall within the scope of the appended claims.

Claims

CLAIMS:

1. A device for measuring friction between surfaces, the device comprising: a contact surface arranged for sliding contact with a test surface; an actuator arranged to move the contact surface relative to the test surface; a sensor arranged to measure the normal load applied to the test surface by the contact surface; and a processor arranged to measure the friction force between the contact surface and the test surface from the electrical current demand of the actuator.

2. A device according to claim 1 , wherein the normal load is applied to the test surface by applying a force to the actuator.

3. A device according to claim 1 or claim 2, wherein the device is handheld.

4. A device according to any of claims 1 to 3, wherein the device further comprises a housing arranged to contact the test surface while the contact surface is in contact with the test surface.

5. A device according to claim 4, wherein the actuator is slidably mounted within the housing.

6. A device according to claim 4 or claim 5, wherein the device further comprises a chassis arranged within the housing and configured to move relative to the housing, wherein the actuator is coupled to the chassis.

7. A device according to any of claims 1 to 6, wherein the device further comprises a housing and a resiliently deformable member, wherein the resiliently deformable member is coupled between the actuator and the housing.

8. A device according to claim 7, wherein the normal load is applied to the test surface by compression of the resiliently deformable member.

9. A device according to any of claims 1 to 8, wherein the device further comprises a load adjustment mechanism arranged to adjust the normal load applied to the test surface.

10. A device according to claim 9, wherein the device further comprises a resiliently deformable member, wherein the load adjustment mechanism is arranged to adjust the applied normal load by compressing the resiliently deformable member.

11. A device according to any of claims 1 to 10, wherein the device further comprises an orientation sensor arranged to measure the orientation of the device.

12. A device according to any of claims 1 to 11, wherein the device further comprises a user interface arranged to display the normal load applied to the test surface by the contact surface.

13. A device according to any of claims 1 to 12, wherein the device further comprises a wheel, wherein the curved surface of the wheel comprises the contact surface.

14. A device according to claim 13, wherein the wheel is mounted on an axle.

15. A device according to claim 14, further comprising a gearing mechanism between the actuator and the axle.

16. A device according to claim 15, wherein the actuator is mounted substantially perpendicular to the axle.

17. A device according to claim 13 or claim 14, wherein the wheel is mounted in line with the actuator.

18. A device according to any of claims 13 to 17, wherein the actuator is a motor arranged to rotate the wheel to provide the sliding contact between the contact surface and the test surface.

19. A method of measuring friction between surfaces using a device comprising an actuator and a contact surface, the method comprising: measuring the normal load applied to a test surface by the contact surface; and measuring the friction force between the contact surface and the test surface from the electrical current demand of the actuator.

20. A computer-readable medium comprising instructions which, when executed by a processor of a device, cause the device to carry out the method of claim 19.