Patent Publication Number: US-9849598-B2

Title: Hair cutting device

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB2012/057422, filed on Dec. 18, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/578,910, filed on Dec. 22, 2011. These applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a hair cutting device comprising cutter unit and a skin stretcher. The cutter unit comprises a skin-contact surface for dragging over a skin surface in a shaving direction, a front surface arranged in front of the skin-contact surface in the shaving direction, and a laser beam exit window arranged in the front surface for allowing a hair cutting laser beam to cut a hair near the skin surface in front of the front surface. The skin stretcher is positioned in front of the cutter unit, according to the shaving direction, and comprises a stretcher surface for dragging over the skin surface together with the skin-contact surface. 
     BACKGROUND OF THE INVENTION 
     Such a hair cutting device is, e.g., known from the international patent application published as WO 2011/010246. Said patent application describes a hair cutting device for cutting hair near skin of a human or animal body part. An optical blade, embodying the functionality of the cutter unit, is dragged over the skin surface, while a reflector in the optical blade directs the hair cutting laser beam to the hair. The optical blade has a tapered end with a curved surface for exerting a local pressure on the skin surface and thereby manipulating the skin in an attempt to improve closeness and minimize skin irritation. Closeness is thereby defined as the length of the remaining stubbles after shaving. Irritation is caused by the hair cutting laser irradiating the skin instead of the hair. WO 2011/010246 further discloses the use of a skin stretcher, installed in front of the optical blade, for stretching the skin and making skin doming more predictable. Skin doming is the piling up of an amount of skin before the front surface of the optical blade when dragging the blade surface over the skin surface in the shaving direction. The aim of this skin manipulation is to ensure that the hair cutting light beam remains parallel to and above the skin surface. 
     One of the disadvantages of this known hair cutting device is that longer hairs may be trapped between the skin stretcher and the skin while the optical blade passes and will therefore be missed by the hair cutting laser beam. In addition, skin properties like flexibility and smoothness may vary from person to person, within persons on different body sites or positions and perhaps also from day to day, resulting in either reduced closeness or increased irritation. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the invention to provide a hair cutting device which provides improved closeness and reduced irritation. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, this object is achieved by providing a hair cutting device comprising a cutter unit, a skin stretcher and slit adapting means. The cutter unit comprises a skin-contact surface for dragging over a skin surface in a shaving direction, a front surface arranged in front of the skin-contact surface in the shaving direction, and a laser beam exit window for allowing a hair cutting laser beam to cut a hair near the skin surface in front of the front surface. The skin stretcher is positioned in front of the cutter unit, according to the shaving direction, and comprises a stretcher surface for dragging over the skin surface together with the skin-contact surface, such that a skin dome is formed by the skin surface in a slit between the skin stretcher and the cutter unit. The slit adapting means are provided for adapting at least one dimension of the slit for controlling a shape of the skin dome. 
     The inventors have realized that the above mentioned problems of the prior art can be avoided by adapting the slit dimensions to obtain an optimum balance between improving closeness and minimizing irritation. For example, when the hairs to be cut are relatively long or the skin surface is relatively smooth and inelastic, larger slits are preferred. Larger slits decrease the chance that a hair is still trapped underneath the stretcher surface when the cutter unit passes. Larger slits also allow more skin to pile up between the skin stretcher and the cutter unit, thereby increasing the skin dome height. So while smooth and inelastic skin surfaces reduce the tendency of skin to pile up, a larger slit may compensate for that effect and increase skin dome height to improve closeness. When the skin properties are such that the skin dome height is larger than usual, a decrease of the slit size reduces the skin dome height and the irritation caused by the hair cutting laser beam hitting the skin. 
     The inventors have found out that various slit dimension parameters like slit size and exposure are important parameters for manipulating the skin dome shape and height. Slit size is defined as the distance between a rear surface of the skin stretcher and the front surface of the cutter unit. Exposure is defined as the distance between the skin-contact surface and the stretcher surface measured in a direction perpendicular to the shaving direction. By careful tuning the slit size, the exposure or both, the dome shape can be controlled in order to obtain an optimum balance between closeness and irritation. 
     The slit size and the exposure may both be adapted by either moving the skin stretcher or the cutter unit relative to each other in the hair cutting device. In principle, also moving both parts is an option, but this would require a more complex construction of the device from a mechanical point of view. 
     Instead of or in addition to adapting the slit size and exposure, also the angle (hereinafter referred to as the cutter angle) between the skin-contact surface and the shaving direction may be adapted. Changing the cutter angle has two effects on the shaving process. A first effect is that the inclined skin-contact surface exerts a different pressure on the skin surface than a flat skin-contact surface would do which may lead to a different dome shape and dome height. A second effect is that, because the light source is rotated together with the cutter unit, the optical base line of the hair cutting laser will also rotate which means that the focus of the hair cutting laser moves away from or towards the skin surface. Both the change of the dome shape and the modification of the optical baseline affect the closeness and the irritation. It is to be noted that the optical baseline may also be modified independent of the cutter unit, e.g. by mechanically rotating or displacing optical elements in the cutter unit. 
     In an embodiment of the hair cutting device according to the invention, the device may comprise control means for manually setting the appropriate slit dimensions. The user may, e.g., be enabled to select a slit size and/or an exposure from a discrete or continuous range of available settings. Alternatively, the device may offer different settings for short hair and for longer hair. Each setting then corresponds to a different predetermined slit size and/or exposure. Also, different settings for shaving a beard, a head, arms and/or legs may be provided. 
     Instead of manually setting the slit dimensions, a processor may be provided which is coupled to the slit adapting means for controlling slit dimensions and skin dome height. The processor is preferably also coupled to one or more sensors for measuring parameters that are indicative of the skin dome height. Some examples of such sensors are described below. 
     The hair cutting device may further comprise a light based hair detector for detecting the hair near the skin surface, a hair cutting laser source for generating the hair cutting laser beam, and a processor coupled to the light based hair detector and to the hair cutting laser source. The processor is arranged to activate the hair cutting laser source in a focal position of the hair cutting laser beam in which the light based hair detector has detected the presence of the hair. Because the device knows the positions of the hairs, the cutting laser can be selectively targeted at the hairs instead of systematically scanning larger areas where hairs may be present. As a result, skin irritation is considerably reduced. 
     In an embodiment of the hair cutting device according to the invention, the light based hair detector is further adapted to detect a height of the skin dome and the processor is further arranged to control the slit adapting means in dependence of the detected height of the skin dome. The light based hair detector is already equipped to distinguish hairs from skin and may thus also be used for detecting the skin dome height. When the skin dome height is known, it can also be controlled by adapting the slit dimensions. In general, lower skin dome height improves closeness but also increases irritation. 
     Optionally, the skin stretcher comprises a pressure sensor for determining a pressure exerted on the stretcher surface and wherein the processor is further arranged to control the slit adapting means in dependence of the determined pressure. When a user pushes the hair cutting device into the skin with too much force, the chances of the focal point of the hair cutting laser falling within the skin is considerably increased. To avoid irritation of the skin, the slit dimension may then be adapted in order to reduce skin dome height and thus irritation. 
     Additionally or alternatively, the skin stretcher comprises a friction sensor for determining a friction between the stretcher surface and the skin surface and wherein the processor is further arranged to control the slit adapting means in dependence of the determined friction. Friction of the skin surface may depend on, e.g., skin type, use of shaving lubricants, hair length and pressure applied by the hair cutting device. More friction leads to a higher skin dome, which can be compensated by appropriate changes to the slit dimensions. 
     Optionally, also a means for determining a speed of the hair treatment device relative to the skin surface is provided and the processor is further arranged to control the slit adapting means in dependence of the determined speed. 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  schematically shows a cross section of a hair cutting device according to the invention, 
         FIG. 2  schematically shows a top view cross section of the hair cutting device of  FIG. 1 , and 
         FIG. 3  shows the cross section of  FIG. 1 , with the cutter unit in a tilted position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically shows a cross section of a hair cutting device  10  according to the invention. The hair cutting device  10  comprises a cutter unit, here in the form of an optical blade  11 , optionally with a tapered end as also described in the international patent application published as WO 2011/010246. The optical blade  11  comprises optical elements like mirrors, reflectors and lenses for directing a hair cutting laser beam  13  through an exit window of the optical blade  11  and focusing the hair cutting laser beam  13  in a focal point near the skin surface  21 . The optical blade  11  preferably also comprises the light source for the hair cutting laser beam  13 . The same or additional optical elements may also be used by a light based hair detector which is used for distinguishing hairs  22  from skin  21 . The hair detector and the hair cutting laser may both use the same light source, possibly at a different intensity. Alternatively, separate light sources may be provided for detecting and cutting hairs  22 . The light based hair detector and the hair cutting laser are controlled by a processor  15 , which is coupled to the light source(s) and the light detector(s) of the light based hair detector. This processor  15  is preferably provided in the body of the hair cutting device  10  and also serves for controlling other parts if the device  10 . It is to be noted that the hair cutting device  10  may also function very well without using a hair detector. For example, the hair cutting laser beam  13  may just be focused at a standard height above the skin-contact surface or blade surface and may systematically scan larger areas where hairs  22  may be present. 
     During shaving, the user moves the hair cutting device  10  over the skin surface  21 . The preferred shaving direction  55  is against the grain because this leads to beneficial hair manipulation, e.g. due to additional hair lift, and an improved shaving result. In front of the optical blade  11 , a skin stretcher  12  is provided. One of the functions of the skin stretcher  12  is to bring the hairs  22  in a detection area and to achieve hair lift. The detection area is the open space between the optical blade  11  and the skin stretcher  12  where a focused light beam  13  of the hair detector is able to detect the hair  22 . The hairs  22  are brought in the detection area by assuring stretching of the skin  21  by the skin stretcher  12  and subsequent contact between the hair  22  and the optical blade  11 . Hair lift is achieved due to a shear force, exerted by the skin stretcher  12 , and hair rotation around its anchorage point in the skin  21  (due to skin stretching as well as due to hair-optical blade contact). 
     When dragging the skin stretcher  12  and the optical blade  11  over the skin surface  21 , a skin dome  23  is formed in the slit between the stretcher  12  and the blade  11 . The skin dome  23  is a small amount of piled up skin  21  in front of the optical blade  11 . The height  53  and shape of the skin dome  23  depends on multiple factors, such as the shaving speed, shaving direction, skin smoothness, skin flexibility, use of shaving lubricants, pressure exerted on the skin by the hair cutting device, etc. An important function of the skin stretcher  12  is to reduce the variation in skin dome  23  dimensions. The more constant the skin dome  23  shape and dimensions are, the closer to the skin surface  21  the hair cutting laser beam  13  can be focused without causing too much skin irritation. 
     According to the invention, the slit dimensions can be adapted for controlling the skin dome  23  shape and height  53 . Two important slit dimensions that may be adapted are the slit size  51  and the exposure  52 . The slit size  51  is the width of the slit as defined by the distance between a back surface of the skin stretcher  12  and a front surface of the optical blade  11 . Exposure  52  is the distance between the stretcher surface of the skin stretcher  12  and the blade surface of the optical blade  11 , measured in a direction perpendicular to those surfaces (and the skin surface  21 ). Skin dome height  53  may, e.g., be increased by increasing the slit size  51  and/or decreasing the exposure  52  or decreased by decreasing the slit size  51  and/or increasing the exposure  52 . Changing the slit size  51  and/or exposure  52  may be realized by moving the skin stretcher  12  and/or the optical blade  11  relative to each other in the hair cutting device  10 . The slit adapting means may comprise actuators for providing the required movements which are preferably also controlled by the processor  15 . In practice, it may be easier to only move the skin stretcher  12  and not the optical blade  11 . The optical blade  11  already comprises a lot of mechanical and electronic parts for enabling hair detection and cutting of the hairs  22 . Adding mechanical actuators for moving the optical blade  11  may be more complex from a constructional point of view than providing actuators for moving the skin stretcher  12 . Suitable parameter ranges for the slit size may, e.g., be about 0.5 and 2.0 mm. The exposure may, e.g., be chosen in the range 0-250 μm (micrometer). 
     It is to be noted that the skin stretcher  12  and/or the cutter unit  11  may have some additional freedom of movement for following irregularities of the skin or body parts to be shaved. For example, the skin stretcher  12  and/or the cutter unit  11  may be mounted to the hair cutting device  10  with some springs allowing the skin stretcher  12  and/or cutter unit  11  to follow the contours of the body part during shaving. The skin stretcher  12  and the cutter unit  11  may move in union relative to the hair cutting device  10  in order to follow the skin contours. It is, however, important that this additional freedom of movement is not so large that it interferes with the skin dome control process. 
     In an embodiment of the hair cutting device according to the invention, the hair cutting device  10  may comprise control means for manually setting the appropriate slit dimensions. The user may, e.g., be enabled to select a slit size  51  and/or an exposure  52  from a discrete or continuous range of available settings. For this purpose, control means  42  may be provided at the outer surface of the hair cutting device  10 . The control means  42  are coupled to the processor  15  and may also enable controlling other functions of the hair cutting device  10 . The control means  42  may, e.g., comprise as an on/off switch for turning the device  10  on or off. Alternatively, the device may offer different settings for short hair and for longer hair. Each setting then corresponds to a different predetermined slit size and/or exposure. Also, different settings for shaving a beard, a head, arms and/or legs may be provided. The user may be allowed to put the skin stretcher  12  or the optical blade  11  in one of the predetermined settings manually or the selected settings may be received by the processor  15  which then controls one or more actuators for moving the appropriate parts of the device  10 . Alternatively, the appliance could offer a “test shave” in which the detection sensors are enabled and the cutting laser beam  13  is temporarily disabled. The information is then used to either set or propose an optimal setting. 
     In preferred embodiments, the processor  15  is arranged to control the skin dome height  53  by adapting slit dimensions in response to measurements of parameters that are indicative of or have an influence on the skin dome height  53 . For example, the light based hair detector may be adapted to detect the skin dome height  53 . The processor  15  may then be arranged to control the actuators in dependence of the detected skin dome height  53 . The light based hair detector is already equipped to distinguish hairs  22  from skin  21 . The processor can thus easily be programmed to be able to detect the skin dome height  53 . For example, the light based hair detector may be used to determine, at one or more selected heights above the skin-contact surface, whether there is skin tissue  21  present or not. When the skin dome height  53  is known, for example from the detection statistics, it can also be controlled by adapting the slit dimensions accordingly. 
     Optionally, the skin stretcher  12  comprises a pressure sensor for determining a pressure exerted on the stretcher surface and the processor  15  is further arranged to control the slit adapting means in dependence of the determined pressure. When a user pushes the hair cutting device  10  into the skin  21  with too much force, the chances of the focal point of the hair cutting laser  13  falling within the skin  21  is considerably increased. To avoid irritation of the skin  21 , the slit dimension may then be adapted in order to reduce skin dome height  23  and thus irritation. 
     Additionally or alternatively, the skin stretcher  12  comprises a friction sensor for determining a friction between the stretcher surface and the skin surface  21  and the processor  15  is further arranged to control the slit adapting means in dependence of the determined friction. Friction of the skin surface  21  may depend on, e.g., skin type, use of shaving lubricants, hair length and pressure applied by the hair cutting device  10 . More friction leads to a higher skin dome  23 , which can be compensated by appropriate changes to the slit dimensions. 
     Optionally, also a means for determining a speed of the hair treatment device  10  relative to the skin surface  21  is provided and the processor  15  is further arranged to control the slit adapting means in dependence of the determined speed. For example, one or more accelerometers  41  may be used for determining the speed or changes in speed of the hair cutting device  10 . The accelerometers  41  are coupled to the processor  15  and may be positioned in any part of the hair cutting device  10 . 
       FIG. 2  schematically shows a top view cross section of the hair cutting device  10  of  FIG. 1 .  FIG. 3  shows the cross section of  FIG. 1 , with the cutter unit  11  in a tilted position. In addition to adapting the slit size  51  and the exposure  52 , also the cutter angle  54  between a neutral position  56  and a current position  57  of the front surface of the cutter unit  11  may be adapted. This cutter angle  54  is the same as the angle  54  between the skin-contact surface and the shaving direction  55 . Changing the cutter angle  54  has two effects on the shaving process. A first effect is that the inclined skin-contact surface exerts a different pressure on the skin surface  21  than a flat skin-contact surface would do which may lead to a different dome shape and dome height  53 . A second effect is that, because the light source is rotated together with the cutter unit  11 , the focus of the hair cutting laser  13  will move away from or towards the skin surface  21 . Both the change of the dome shape and the displacement of the focus of the hair cutting laser beam  13  affect the closeness and the irritation. For optimum skin dome control, the rotation angle of the cutter unit is preferably selected somewhere in the range of −15° to +15°. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.