Patent Publication Number: US-9883778-B2

Title: Mobile robot

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 USC 371 of International Application No. PCT/GB2014/050214, filed Jan. 28, 2014, which claims the priority of United Kingdom Application No. 1301578.9, filed Jan. 29, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a mobile robot. In particular, although not exclusively, the invention has utility in the context of domestic mobile robot applications such as robotic floor sweepers, vacuum cleaners, and floor washers that are used in a home or office environment, for example. 
     BACKGROUND OF THE INVENTION 
     It is becoming increasingly common to see mobile robotic appliances around the home or office environment. Typically these robotic appliances are in the form of robotic floor sweepers or vacuum cleaners. Examples of known robotic vacuum cleaners are the Roomba™ range of machines manufactured by iRobot Corporation, the Navibot™ range of machines manufactured by Samsung, and the Electrolux Trilobite™, which is described in part in WO97/40734. It is notable that the vacuum cleaner in WO97/40734 includes its heavier components such as the electronics and vacuum motor in a rearwards portion of the housing whilst the dirt collecting chamber is located in a forward portion of the housing, with reference to its normal direction of travel. 
     A robotic vacuum cleaner is required to travel around an environment treating the floor as it goes. A home or office may not have entire floor space on one level so there may be various undulations and transitions that a robot must be able to negotiate in order to perform its task effectively. For example, there may be a small vertical step between rooms and/or between types of floor coverings within the floor space. Also, the robot may be required to climb onto a temporary floor covering such as a rug. 
     The ‘climbing ability’ of a domestic mobile robot depends on a large extent on its overall configuration. It will be appreciated for example that if a robots centre of mass is biased significantly towards a rearward portion of the robot there is a risk of the robot becoming ‘beached’ whilst negotiating a transition. This may affect vacuum cleaning robots which are configured such that their heavier components such as vacuum motor, battery and electronics are housed in a rear portion of the machine, whilst its relatively light components such as a dirt collecting bin are cited towards a forward portion of the machine. Such a configuration is apparent in WO97/40734. 
     SUMMARY OF THE INVENTION 
     It is against this background that the invention provides a mobile robot comprising a body having a drive arrangement for supporting and driving the body on a surface, and biasing means for biasing a rear portion of the body in a direction away from the floor surface. 
     The invention provides a particular advantage in mobile robots whose centre of mass is located in a relatively rearward position. A mobile robot with such a ‘rearward-biased’ centre of mass may have a relatively strong ability to climb over transitions since it is less massive at its front thereby requiring less driving energy to lift its forward section up and over a transition. However, the rearward bias of its mass can cause the robot to become stuck or ‘beached’ if its main drive arrangement is unable to pull the rear section of the robot up and over the transition. The present invention provides mechanically elegant solution to this problem by providing a means to upwardly bias a rear portion of the mobile robot away from the floor surface whilst retaining the benefits of a mobile robot with a rearwards biased centre of mass. 
     In one embodiment the biasing means may take the form of a floor engaging support member arranged support a rear portion of the body. This arrangement therefore serves to push the rear portion away from the floor surface with a predetermined force which has the affect of ‘tipping’ the body forward in circumstances when the robot becomes stuck on a transition, which may be a shallow step, for example. 
     So as to ensure a low physical profile of the floor engaging member, it may be stowable in a suitable bay or recess in the body and movable between stowed and deployed positions. A particularly convenient configuration to achieve the above functionality is provided by a swing arm that pivots relative to the body of the robot and arranged to stow in the recess, either partly or fully, when the robot is at rest on a surface. 
     The floor engaging support member may rotate relative to the body about a substantially vertical axis. The floor engaging support member may comprise a carrier rotatably mounted to the body and a swing arm pivotally mounted to the carrier, the carrier being able to rotate relative to the body about a substantially vertical axis and the swing arm being able to pivot relative to the carrier about a substantially horizontal axis. 
     To achieve the predetermined downward force a spring member may be arranged to act on the floor engaging support member. Although the spring member may be a helical compression spring, for example, it may also be a torsion spring braced between the swing arm and the body so as to bias the swing arm into the deployed position. 
     Although the swing arm may be fitted with a runner or skid to reduce frictional contact between it and the adjacent floor surface, a preferable option is to provide the swing arm with a roller or wheel. 
     In order to provide its biasing force in a balanced position, the swing arm may be located on a rear portion of the body aligned on a longitudinal axis of the body. A further option will be to position the swing arm between a pair of further floor engaging supports, for example fixed and passive wheels or rollers which reduces the load on the spring-loaded swing arm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more readily understood, embodiments will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  is a front perspective view of a mobile robot in accordance with an embodiment of the invention; 
         FIG. 2  is a view from beneath of the mobile robot in  FIG. 1 ; 
         FIG. 3  is an exploded perspective view of the mobile robot of  FIG. 1  showing its main assemblies; 
         FIG. 4  is a side view of a traction unit of the mobile robot in  FIGS. 1 to 3  and illustrates the range of movement of the traction unit; 
         FIG. 5  is a simplified perspective view, from underneath, of the mobile robot of  FIG. 1  showing a floor engaging support member in a deployed position; 
         FIG. 6  is an enlarged view of the floor engaging support member in  FIG. 5  and  FIG. 7  is a longitudinal section through the floor engaging support member; 
         FIG. 8  is a simplified perspective view like that in  FIG. 5  but which shows the floor engaging support member in a fully stowed position; 
         FIG. 9  is an enlarged view of the floor engaging support member in  FIG. 8  and  FIG. 10  is a longitudinal section through the floor engaging support member; 
         FIG. 11  is a perspective view of the floor engaging support member; 
         FIGS. 12 a  to 12 e    show sequential views of a simplified-form mobile robot of the preceding figures negotiating a transition in a floor surface; and 
         FIGS. 13 a  and 13 b    are schematic views of an alternative embodiment. 
         FIGS. 14, 15   a ,  15   b ,  16   a ,  16   b ,  17   a ,  17   b  and  17   c  show an alternative embodiment for the floor engaging support member. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1, 2, 3 and 4  of the drawings, an autonomous surface treating appliance, in the form of a mobile robotic vacuum cleaner  2  (hereinafter ‘robot’) comprises a main body  3  having four principal assemblies: a chassis (or sole plate)  4 , a body  6  which is carried on the chassis  4 , a generally circular outer cover  8  which is mountable on the chassis  4  and provides the robot  2  with a generally circular profile, and a separating apparatus  10  that is carried on a forward part of the body  6  and which protrudes through a complementary shaped cut-out  12  of the outer cover  8 . 
     For the purposes of this specification, the terms ‘front’ and ‘rear’ in the context of the robot will be used in the sense of its forward and reverse directions during operation, with the separating apparatus  10  being positioned at the front of the robot. Similarly, the terms ‘left’ and ‘right’ will be used with reference to the direction of forward movement of the robot. As will be appreciated from  FIG. 1 , the main body of the robot  2  has the general form of a relatively short circular cylinder, largely for maneuverability reasons, and so has a cylindrical axis ‘C’ that extends substantially vertically relative to the surface on which the robot travels. Accordingly, the cylindrical axis C extends substantially normal to a longitudinal axis of the robot ‘L’ that is oriented in the fore-aft direction of the robot  2  and so passes through the centre of the separating apparatus  10 . 
     The chassis  4  supports several components of the robot and is preferably manufactured from a high-strength injection moulded plastics material, such as ABS (acrylonitrile butadiene styene), although it could also be made from appropriate metals such as aluminium or steel, or composite materials such a carbon fibre composite to name a few examples. As will be explained, the primary function of the chassis  4  is as a drive platform and to carry cleaning apparatus for cleaning the surface over which the robot travels. 
     With particular reference to  FIG. 3 , a front portion  14  of the chassis  4  is relatively flat and tray-like in form and defines a curved prow  15  that forms the front of the robot  2 . Each flank of the front portion  14  has a respective traction unit  20  mounted to it. 
     The pair of traction units  20  are located on opposite sides of the chassis  4  and are operable independently to enable to robot to be driven in forward and reverse directions, to follow a curved path towards the left or right, or to turn on the spot in either direction, depending on the speed and direction of rotation of the traction units  20 . Such an arrangement is sometimes known as a differential drive, and detail of the traction units  20  will be described more fully later in the specification. 
     The relatively narrow front portion  14  of the chassis  4  widens into rear portion  22  which includes a surface treating assembly  24  or ‘cleaner head’ having a generally cylindrical form and which extends transversely across substantially the entire width of the chassis  4  relative to its longitudinal axis ‘L’. 
     With reference also to  FIG. 2 , which shows the underside of the robot  2 , the cleaner head  24  defines a rectangular suction opening  26  that faces the supporting surface and into which dirt and debris is drawn into when the robot  2  is operating. An elongate brush bar  28  is contained within the cleaner head  24  and is driven by an electric motor  30  via a reduction gear and drive belt arrangement  32  in a conventional manner, although other drive configurations such as a solely geared transmission or a direct drive are also envisaged. Moreover, although a wheel-based drive arrangement is shown, other drive systems are also acceptable such as a legged-based system. 
     The underside of the chassis  4  features an elongate sole plate section  25  extending forward of the suction opening  26  which includes a plurality of channels  33  (only two of which are labeled for brevity) which provide pathways for dirty air being drawn towards the suction opening  26 . The underside of the chassis  4  also carries a plurality (four in the illustrated embodiment) of passive wheel or rollers  31  which provide further bearing points for the chassis  4  when it is at rest on or moving over a floor surface. The rollers  31  therefore serve to space the underside of the chassis a predetermined minimum distance (approximately 5 mm in this embodiment, although this is not essential) from the floor surface which benefits the performance of the brush bar. 
     In this embodiment, the cleaner head  24  and the chassis  4  are a single plastics moulding, thus the cleaner head  24  is integral with the chassis  4 . However, this need not be the case and the two components could be separate, the cleaner head  24  being suitably affixed to the chassis  4  as by screws or an appropriate bonding technique as would be clear to the skilled person. 
     The cleaner head  24  has first and second end faces  27 ,  29  that extend to the edge of the chassis  4  and which are in line with the cover  8  of the robot. Considered in horizontal or plan profile as in  FIGS. 2 and 3 , it can be seen that the end faces  27 ,  29  of the cleaner head  24  are flat and extend at a tangent (labeled as ‘T’) to the cover  8  at diametrically opposed points along the lateral axis ‘X’ of the robot  2 . The benefit of this is that the cleaner head  24  is able to run extremely close to the walls of a room as the robot traverses in a ‘wall following’ mode therefore being able to clean right up to the wall. Moreover, since the end faces  27 ,  29  of the cleaner head  24  extend tangentially to both sides of the robot  2 , it is able to clean right up to a wall whether the wall is on the right side or the left side of the robot  2 . It should be noted, also, that the beneficial edge cleaning ability is enhanced by the traction units  20  being located inboard of the cover  8 , and substantially at the lateral axis X, meaning that the robot can maneuver in such a way that the cover  8  and therefore also the end faces  27 ,  29  of the cleaner head  24  are almost in contact with the wall during a wall following operation. 
     Dirt drawn into the suction opening  26  during a cleaning operation exits the cleaner head  24  via a conduit  34  which extends upwardly from the cleaner head  24  and curves towards the front of the chassis  4  through approximately 90° of arc until it faces in the forwards direction. The conduit  34  terminates in a rectangular mouth  36  having a flexible bellows arrangement  38  shaped to engage with a complementary shaped duct  42  provided on the body  6 . 
     The duct  42  is provided on a front portion  46  of the body  6 , and opens into a forward facing generally semi-cylindrical recess  50  having a generally circular base platform  48 . The recess  50  and the platform  48  provide a docking portion into which the separating apparatus  10  is mounted, in use, and from which it can be disengaged for emptying purposes. 
     It should be noted that in this embodiment the separating apparatus  10  consists of a cyclonic separator such as disclosed in WO2008/009886, the contents of which are incorporated by reference. The configuration of such separating apparatus is well known and will not be described any further here, save to say that the separating apparatus  10  may be removably attached to the body  6  by a suitable mechanism such as a quick-release fastening means to allow the apparatus  10  to be emptied when it becomes full. The nature of the separating apparatus  10  is not central to the invention and the cyclonic separating apparatus may instead separate dirt from the airflow by other means that are known in the art for example a filter-membrane, a porous box filter or some other form of separating apparatus. For embodiments of the apparatus which are not vacuum cleaners, the body  6  can house equipment which is appropriate to the task performed by the machine. For example, for a floor polishing machine the main body can house a tank for storing liquid polishing fluid. 
     When the separating apparatus  10  is engaged in the docking portion  50 , a dirty air inlet  52  of the separating apparatus  10  is received by the duct  42  and the other end of the duct  42  is connectable to the mouth  36  of the brush bar conduit  34 , such that the duct  42  transfers the dirty air from the cleaner head  24  to the separating apparatus  10 . The bellows  38  provide the mouth  36  of the duct  34  with a degree of resilience so that it can mate sealingly with the dirty air inlet  52  of the separating apparatus  10  despite some angular misalignment. Although described here as bellows, the duct  34  could also be provided with an alternative resilient seal, such as a flexible rubber cuff seal, to engage the dirty air inlet  52 . 
     Dirty air is drawn through the separating apparatus  10  by an airflow generator which, in this embodiment, is an electrically powered motor and fan unit (not shown), that is located in a motor housing  60  on the left hand side of the body  6 . The motor housing  60  includes a curved inlet mouth  62  that opens at the cylindrical shaped wall of docking portion  50  thereby to match the cylindrical curvature of the separating apparatus  10 . Although not seen in  FIG. 4 , the separating apparatus  10  includes a clean air outlet which registers with the inlet mouth  62  when the separating apparatus  10  is engaged in the docking portion  50 . In use, the suction motor is operable to create low pressure in the region of the motor inlet mouth  62 , thereby drawing dirty air along an airflow path from the suction opening  26  of the cleaner head  24 , through the conduit  34  and duct  42  and through the separating apparatus  10  from dirty air inlet  52  to the clean air outlet. Clean air then passes through the motor housing  60  and is exhausted from the rear of the robot  2  through a filtered clean air outlet  61 . 
     The cover  8  is shown separated from the body  6  in  FIG. 4  and, since the chassis  4  and body  6  carry the majority of the functional components of the robot  2 , the cover  8  provides an outer skin that serves largely as a protective shell and to carry a user control interface  70 . 
     The cover  8  comprises a generally cylindrical side wall  71  and a flat upper surface  72  which provides a substantially circular profile corresponding to the plan profile of the body  6 , save for the part-circular cut-out  12  shaped to complement the shape of the docking portion  50 , and the cylindrical separating apparatus  10 . Furthermore, it can be seen that the flat upper surface  72  of the cover  8  is co-planar with an upper surface  10   a  of the separating apparatus  10 , which therefore sits flush with the cover  8  when it is mounted on the main body. 
     As shown particularly clearly in  FIGS. 1 and 3 , the part-circular cut-out  12  of the cover  8  and the semi-cylindrical recess  50  in the body  6  provides the docking portion a horseshoe shaped bay defining two projecting lobes or arms  73  which flank either side of the separating apparatus  10  and leave between approximately 5% and 40%, and preferably 20%, of the apparatus  10  protruding from the front of the docking portion  50 . Therefore, a portion of the separating apparatus  10  remains exposed even when the cover  8  is in place on the main body of the robot  2 , which enables a user easy access to the separating apparatus  10  for emptying purposes. 
     Opposite portions of the side wall  71  include an arched recess  74  (only one shown in  FIG. 3 ) that fits over a respective end  27 ,  29  of the cleaner head  24  when the cover  8  is connected to the body  6 . As can be seen in  FIG. 1 , a clearance exists between the ends of the cleaner head  24  and the respective arches  74  order to allow for relative movement therebetween in the event of a collision with an object. 
     On the upper edge of the side wall  71 , the cover  8  includes a semi-circular carrying handle  76  which is pivotable about two diametrically opposite bosses  78  between a first, stowed or retracted position, in which the handle  76  fits into a complementary shaped recess  80  on upper peripheral edge of the cover  8 , and a deployed or extended position in which it extends upwardly, (shown ghosted in  FIG. 1 ). In the stowed position, the handle  76  maintains the ‘clean’ circular profile of the cover  8  and is unobtrusive to the user during normal operation of the robot  2 . Also, in this position the handle  76  serves to lock a rear filter door (not shown) of the robot  2  into a closed position which prevents accidental removal of the filter door when the robot  2  is operating. 
     In operation, the robot  2  is capable of propelling itself about its environment autonomously, powered by a rechargeable power source such as a battery pack (not shown). To achieve this, the robot  2  carries an appropriate control means which is interfaced to the battery pack, the traction units  20  and an appropriate sensor suite  82  comprising for example infrared and ultrasonic transmitters and receivers on the front left and right side of the body  6 . The sensor suite  82  provides the control means with information representative of the distance of the robot from various features in an environment and the size and shape of the features. Additionally the control means is interfaced to the suction fan motor and the brush bar motor in order to drive and control these components appropriately. The control means is therefore operable to control the traction units  20  in order to navigate the robot  2  around the room which is to be cleaned. It should be noted that the particular method of operating and navigating the robotic vacuum cleaner is not material to the invention and that several such control methods are known in the art. For example, one particular operating method is described in more detail in WO00/38025 in which navigation system a light detection apparatus is used. This permits the cleaner to locate itself in a room by identifying when the light levels detected by the light detector apparatus is the same or substantially the same as the light levels previously detected by the light detector apparatus. 
     Turning to the traction units, in overview, the traction unit  20  comprises a transmission case  90 , a linkage member  92  or ‘swing arm’, first and second pulley wheels  94 ,  96 , and a track or continuous belt  98  that is constrained around the pulley wheels  94 ,  96 . 
     The transmission case  90  houses a gear system which extends between an input motor drive module  100  mounted on an inboard side of one end of the transmission case  90 , and an output drive shaft  102  that protrudes from the drive side of the transmission case  90 , that is to say from the other side of the transmission case  90  to which the motor module  100  is mounted. The motor module  100  in this embodiment is a brushless DC motor since such a motor is reliable and efficient, although this does not preclude other types of motors from being used, for example brushed DC motors, stepper motors or hydraulic drives. As has been mentioned, the motor module  100  is interfaced with the control means to receive power and control signals and is provided with an integral electrical connector  104  for this purpose. The gear system in this embodiment is a gear wheel arrangement which gears down the speed of the motor module  100  whilst increasing available torque, since such a system is reliable, compact and lightweight. However, other gearing arrangements are envisaged within the context of the invention such as a belt or hydraulic transmission arrangement. 
     The traction unit  20  therefore brings together the drive, gearing and floor engaging functions into a self-contained and independently driven unit and is readily mounted to the chassis  4  by way of a plurality of fasteners  91  (four fasteners in this embodiment), that are received into suitable lugs on the chassis  4 . 
     The traction unit  20  is mountable to the chassis so that the first pulley wheel  94  is in a leading position when the robot  2  is travelling forwards. The ‘leading wheel’  94  may also be considered a sprocket since it is the driven wheel in the pair. 
     The swing arm  92  includes a leading end that is mounted to the transmission case  90  between it and the lead wheel  94  and is mounted so as to pivot about the drive shaft  102 . The continuous belt or track  98  provides the interface between the robot  2  and the floor surface and, in this embodiment, is a tough rubberized material that provides the robot with high grip as the robot travels over the surface and negotiates changes in the surface texture and contours. Although not shown in the figures, the belt  98  may be provided with a tread pattern in order to increase traction over rough terrain. 
     Similarly, although not shown in the figures, inner surface of the belt  98  is serrated or toothed so as to engage with a complementary tooth formation (not shown) provided on the circumferential surface of the leading wheel  94  which reduces the likelihood of the belt  98  slipping on the wheel  94 . In this embodiment, the trailing wheel  96  does not carry a complementary tooth formation, although this could be provided if desired. 
     As will be appreciated, the swing arm  92  fixes the leading and trailing wheels  94 ,  96  in a spaced relationship and permits the trailing wheel  96  to swing angularly about the leading wheel  94 . The traction unit  20  also comprises swing arm suspension in the form of a coil spring  118  that is mounted in tension between a mounting bracket  126  extending upwardly from the leading portion of the swing arm  92  and a pin  128  projecting from the trailing portion of the transmission case  90 . The spring  118  acts to bias the trailing wheel  96  into engagement with the floor surface, in use, and so improves traction when the robot  2  is negotiating an uneven surface such as a thick-pile carpet or climbing over obstacles such as electrical cables.  FIG. 4  shows three exemplary positions of the traction unit  20  throughout the range of movement of the swing arm  92 . 
     Referring once again to  FIG. 2 , in addition to the traction units  20  and the passive wheels  31 , the chassis  4  is supported on a floor surface by biasing means in the form of a floor engaging support member, indicated generally at  130 . In this embodiment the floor engaging support member  130  is a jockey wheel that is located on a rear portion  129  of the chassis  4  (and therefore also on the main body of the robot) and supports the rear portion  129  on a floor surface. More specifically, the jockey wheel  130  is located on the centerline L of the robot  2  equidistant from the two support wheels  31  also located on the rear portion of the chassis  4 . 
     Reference will now be made to  FIGS. 5 to 11  which show the jockey wheel  130  in more detail. It should be noted, here, that the robot  2  is shown in simplified form for clarity purposes. 
     The jockey wheel  130  is mounted in a recess or ‘bay’  132  defined in the underside of the chassis and is movable between a first position in which the jockey wheel  130  is stowed in the bay  132  (as shown in  FIGS. 8, 9 and 10 ) and a second position in which the jockey wheel is deployed from the bay  132  ( FIGS. 5, 6 and 7 ). The jockey wheel  130  is biased into the deployed position with a predetermined force by biasing means  134  which in this embodiment is a helical torsion spring although the skilled person would appreciate that the biasing means could have a different form such as a compression spring, a gas-filled spring and a resilient mass. Currently a helical torsion spring is preferred since it is compact and so lends itself to use in a tight volume. 
     In more detail, the jockey wheel  130  comprises an arm  136  that is pivoted at a first, inner, end  136   a  and includes a roller or wheel  138  that is mounted at a second, outer, end  136   b  of the arm  136 . The arm  136  may be pivotably mounted in various ways, although in this embodiment the inner end  136   a  of the arm includes bearing means in the form of a pair of c-shaped mounts  140  that are secured by way of a snap fit to a pivot pin  142  provided on the chassis  4 . Although not shown specifically in the Figures, it should be appreciated that the arrangement of the pivot pin  142  and the mounts  140  are such that the arm is pivoted about a horizontal axis that lies substantially parallel with the lateral axis X of the robot. 
     The torsion spring  134  is received over the pivot pin  142 , as shown in  FIG. 7 , for example, and is braced between an inner part  141  of the arm  136  and a component of the chassis  4  and so outwardly biases the arm  136  into the deployed position. The jockey wheel  130  therefore serves as a biasing means to bias the rear portion  129  of the robot  2  in a direction away from the floor surface with a predetermined force. Note that the predetermined force is selected so that the jockey wheel  130  is able to lift the rear portion  129  of the robot  2  off of the floor surface and so this depends on the overall mass of the robot and also where that mass is distributed within the body of the robot; in this embodiment, however, the predetermined force is approximately 5 Newtons (5 N). Expressed another way, the predetermined force selected is a function of the machine mass and the position of the centre of mass along the longitudinal axis of the robot. 
     The outer end  136   b  of the arm  136  includes a yoke  143  within which the roller  138  is rotatably mounted on an axle  143   a . Note that the roller  138  is mounted in the yoke  143  so that the roller  138  does not protrude significantly below the underside of the arm  136 . The maximum outward travel of the arm  136  is limited by a pair of catches  144  defined by opposed walls  145  on either side of the arm  136 . The catches  144  are engageable with a stop  146  that is provided on the chassis  4 . In this embodiment, the roller  138  provides minimal rolling resistance to the mobile robot as it travels over a surface. However, the roller could also be replaced by an alternative such as a skid or runner if it was considered suitable for a particular mobile robotic application. 
     By virtue of the torsion spring  134  and the catches  144 , the arm  136  applies a predetermined downward force throughout its range of angular movement until the arm  136  comes up against the stop  146 . However, in normal operation the arm  136  and stop  146  are configured so that the arm  136  remains within its range of travel, which is approximately 30 degrees in this embodiment, although the precise range of movement is selected so as to provide the rear of the robot with enough upwards assistance during a climbing maneuver and so is largely depending on the dimensions of the robot. Preferentially, the arm  136  is movable so that the roller  138  may extend up to 20 mm below the underside of the chassis. 
       FIG. 12 a    shows a schematic side view of the robot positioned on a floor surface F and it will be seen here that the jockey wheel  130  is in a relatively stowed position (although not fully stowed). In this position, the jockey wheel  130  exerts a downward force of approximately 5 N by virtue of the torsion spring  134 . 
     The jockey wheel  130  is particularly advantageous in circumstances when the robot  2  is required to drive over a transition in the floor surface, and particularly a moderate step change in height. In such circumstances, since the centre of mass of the robot is rearwards-biased due to the location of the motor and fan unit and the relatively light separating apparatus  10  positioned at the front, there is a risk of the robot  2  losing traction on a step of a certain height so that it becomes stuck. However, the jockey wheel  130  urges the rear portion  129  of the robot  2  away from the floor surface which effectively tips the robot forward about the pivot point defined by the traction units  20  thereby assisting the robot in overcoming the obstacle. 
     The above scenario will now be described with reference to  FIGS. 12 b  to 12 e    which are a sequence of side views illustrating the robot  2  approaching and climbing over a vertical transition T in the floor surface F. 
     Referring firstly to  FIG. 12 a   , the mobile robot  2  is shown travelling across a floor surface F. In this condition, the jockey wheel  130  is in a stowed position and the robot is supported by the traction units  20  and the jockey wheel  138 . 
     In  FIG. 12 b   , the robot  2  is approaching a vertical transition T until the traction units  20  engage the transition and thus begin a climbing maneuver. As in  FIG. 12 a   , the jockey wheel  130  remains retracted as the attitude of the robot remains flat. 
     In  FIG. 12 c   , the traction units  20  drive up the transition T so that the robot  2  is supported on the raised floor surface F 1  by the traction units  20  and supported on the first floor surface F by the roller  138 . At the point shown in  FIG. 12 c   , the jockey wheel  130  comes into play as the upwardly directed biasing force it provides acts to ‘tip’ the robot  2  forwards as the robot  2  continues to move in its driving direction which assists the robot  2  in negotiating the transition T. Importantly, the jockey wheel  130  provides its biasing force throughout its range of movement and it is shown in a deployed position in  FIG. 12 d    where it is seen that the robot  2  has tipped forward as compared the robot  2  in  FIG. 12 c   . In effect, therefore, the jockey wheel  130  has an effect comparable to that of a mass located on a forward portion of the robot  2  which would forward-shift the centre of mass of the robot, or even to an upwards force applied to the upper surface of the mobile robot. 
     Turning to  FIG. 12 e   , as the robot  2  continues in the forward direction on the raised transition surface F 1 , the jockey wheel  134  engages the transition T and is caused to move angularly in an anticlockwise direction so as to be stowed once again in the bay  132 . 
     Some modifications to the specific embodiments have been explained in the discussion above. In addition to these, the skilled person would understand that the specific embodiment may be altered without departing from the scope of the invention as defined in the claims. Some non-limiting examples of such alternatives will now be discussed. 
     The jockey wheel has been described as broadly comprising a roller that is mounted to a swing arm. However, this is only one way of achieving the technical advantage. A similar result could be achieved by a floor engaging wheel mounted in the chassis for substantially linear vertical movement. By way of example, in  FIG. 13 a    a jockey wheel arrangement  150  of an alternative embodiment, shown schematically, comprises a wheel support member  152  that defines a sliding fit in a recess  153  of the chassis  4  of the robot  2 . The support member  152  supports a wheel  154  on an axle  156  at one end and its other end is engaged with a biasing spring  158  so that the support member  152  is biased outwards with respect to the chassis  4 . As in the previous embodiment, the support member  152  adopts a stowed configuration when the robot  2  is travelling on a relatively flat region of the floor surface F, as is shown in  FIG. 13 a   . However, in circumstances where the robot  2  is required to traverse a transition in the floor surface, such as shown in  FIGS. 12 a  to 12 e   , the support member  152  may deploy or extend downwardly with respect to the chassis into the position shown in  FIG. 13   b.    
     In the previously described embodiments the floor engaging support member is a jockey wheel  138  which has a fixed orientation. The wheel  138  is free to rotate to allow movement in both the forward and reverse directions. However, it will be appreciated that if the robot  2  is traveling along a curved path, there will not only be a longitudinal component to the frictional force acting on the wheel from the floor surface but also a lateral component. As the wheel  138  is unable to rotate in a lateral direction, increased friction may build up between the floor surface and the wheel, which may cause the wheel to “rub” and be worn away over time. An extreme example of this would be when the robot  2  is turning about its vertical axis C. In this situation, there is no longitudinal component to the frictional force acting on the wheel, and so it does not rotate. However, the wheel continues to rub on the floor surface due to the lateral frictional force from the floor surface. 
     An alternative embodiment of a floor engaging support member is shown in  FIGS. 14 to 17   c .  FIG. 14  shows an exploded view of the alternative floor engaging support member, which comprises a carrier  200 , a bearing  202  and a swing arm  204 . The carrier  200  is connected to the rear portion  129  of the chassis  4  by way of the bearing  212 , such that the carrier  200  is freely rotatable about an axis J that is substantially parallel to the vertical axis C of the robot  2 . The carrier  200  comprises a pivot pin  206  to which the swing arm  204  is pivotably mounted by way of the corresponding eyelets  208  into which the pivot pin  206  can engage. A wheel  210  is mounted to the swing arm  204 . Therefore, as shown in  FIGS. 15 a  and 15 b   , during use the carrier  200  and swing arm  204  can freely rotate around the centre of bearing  202 , allowing the wheel  210  act as a caster wheel. 
     Similar to the earlier described embodiments, a torsion spring (not shown) acts to bias the swing arm  204  into a deployed position so as to bias the rear portion  129  of the robot  2  in a direction away from the floor surface.  FIG. 16 a    shows the swing arm  204  in a retracted position, and  FIG. 16 b    shows the swing arm in a deployed position. Whether retracted or deployed, the swing arm is still able to rotate freely about the axis J. 
       FIGS. 17 a, b  and  c    show the alternative floor engaging support member in place and fitted to the rear portion  129  of the chassis  4 . In  FIG. 17 a   , the swing arm  204  is in the retracted position. The direction of travel of the robot  2  is forward, as indicated by the arrow M, which means the wheel  210  is located at the point closest to the rear of the robot  2 . In  FIG. 17 b   , the swing arm  204  is in the deployed position, and as the direction of travel is the same as in  FIG. 17 a   , the wheel is still at the rearmost point.  FIG. 17 c    shows the swing arm in the deployed position again, but this time the direction of travel has reversed, N. In this instance the bearing  202  has enabled the carrier  200 , swing arm  204  and wheel  210  to rotate 180° with respect to the chassis  4 . Of course, it will be understood that when the robot  2  is turning about the central vertical axis C, the carrier  200 , swing arm  204  and wheel  210  will rotate 90° such that the wheel  210  is able to rotate in the direction of travel of the rear of the rear of the machine. 
     This alternative embodiment of a floor engaging support member helps to prevent the wheel from wearing away during use whilst still allowing the swing arm to bias the rear portion of the robot away from the floor surface. 
     The above examples include supporting devices that support a rear portion of the mobile robot and urge it away from the floor surface with a substantially constant predetermined force. Both examples make use of floor engaging member that serves to upwardly bias the rear portion of the mobile robot. A comparable effect could be achieved by other means without including a spring-loaded support member, for example a moveable mass could be housed within the body of the robot and a detection system could be configured to move the mass forward within the robot body when the detection system has identified that the robot has become stuck during a climbing maneuver. This solution would ensure that all of the necessary components would be located internal to the mobile robot, which would avoid the need to locate floor engaging support members external to the body of the mobile robot which may attract dust and debris. However, it would be appreciated that such an ‘internal’ solution would be less cost-effective and would require a considerable volume of the internal space of the mobile robot. 
     The mobile robot  2  of the embodiment described above has a substantially circular profile in plan view and, in common with examples of known robotic vacuum cleaners, this shape is generally preferred since it allows the robot to move effectively into tight spaces and to maneuver its way out again without getting stuck. However, although such a circular profile lends itself to domestic applications such as floor cleaning tasks, other profile shapes are acceptable, such as rectilinear shapes in general. Furthermore, the invention is not intended to be limited to domestic mobile robots such as vacuum cleaners and is envisaged to be useful to a wider category of mobile robots that are required to navigate terrain and negotiate transitions in a floor surface. Some non-limiting examples may be a floor washing robot, a mobile sentry robot, a mobile payload-carrying robot. 
     The mobile robot described above has been described as being capable of driving itself autonomously over a floor surface. Of course, this is not intended to be limiting and the invention applies also to mobile robotic applications that are guided remotely or ‘teleoperated’ and also to semi-autonomous applications. Also, the floor surface need not be a floor of a domestic environment, but could be any ground surface on which the robot may travel.