Abstract:
Tread structures, wave motion navigational controls, and ramp up recovery controls are provided to an autonomous floor cleaner to reduce wheel slippage. The floor cleaner delivers liquid to the floor as part of the cleaning process. Wheels on the device are provided with relatively deep peripheral grooves to minimize the contact surfaces of a sprocket wheel and to accommodate the layer of liquid on the floor. In the event of wheel slippage, or to prevent wheel slippage, the device is designed to move forward with a slight side-to-side wave action caused by periodically altering the relative speeds of two drive wheels. There is also provided a slippage recovery mode where the drive wheels shut down or greatly slow when severe slippage is sensed, followed by a slow ramp up of speed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   It is desirable to minimize the amount of human labor expended in maintaining and cleaning buildings. The art has therefore developed autonomous robotic devices that can clean or otherwise maintain or treat hard floors, carpeting and similar surfaces without the necessity for a human to be present during the operation of the device. 
   In some such devices a liquid is applied to the flooring area being treated. For example, U.S. Pat. No. 5,279,672 discloses a robotic cleaning apparatus where a cleaning solution is dispensed to the floor by a scrub deck. In U.S. Pat. No. 6,741,054 there is disclosed an autonomous floor mopping apparatus where cleaning fluid is applied to the floor by way of a pre-moistened towel. 
   Such robotic devices typically have a programmable controller for directing the device in a preferred movement pattern. This helps insure coverage of the full area to be treated, as well as helping to insure that obstacles (e.g. furniture legs) and undesired contact points (e.g. stairways) are avoided. The controllers are typically linked to motors that drive the wheels of the device on the floor. 
   While devices of this type can work quite well on dry surfaces, the wheels of such devices may slip when traveling over areas of the floor that are wet from the fluid being applied. This is particularly likely when the liquid itself is of the type which, when wet, is significantly more slippery than water (e.g. contains oil for polishing purposes). Such slipping can cause the device to remain in place for an extended period, or more likely cause the device to divert in an unexpected direction from the optimal desired path. This can extend the time needed to treat the surface, and/or can lead to portions of the surface not being adequately treated. 
   In some robotic devices, a third wheel that is not driven by a motor is used to monitor the movement of the robot. This third wheel has an optical or mechanical sensor (encoder) that will send a digital signal to the controller as long as the robot is moving. Hence, if the robot is in a moving mode, but this third wheel does not sense movement, then the controller knows it is slipping. This method detects slip. This third wheel may be called a stator wheel. 
   In connection with automobile and truck tires there has been substantial work on trying to improve the traction of the tires through the use of varied tread patterns. However, many of these approaches are designed to take advantage of the very heavy weight of such vehicles, and are not easily transferred to environments where a cleaning robot is involved that weighs much less. Others of these approaches rely on expensive materials, or structures that are relatively expensive to create. 
   Similarly, in connection with automobiles and trucks, there have been attempts to provide improved anti-slip control by monitoring wheel movement and automatically altering power to the wheels when sensing such slip. Because such controller systems were designed for extremely heavy vehicles, they were not easily transferred to environments where a cleaning robot was involved that weighed much less. Further, some systems that could be transferred to a small cleaning robot were of too great a cost to be used in that environment as a practical matter. 
   Hence, a need still exists for improved structures and systems for addressing wheel slip concerns in the context of an autonomous floor cleaner. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention addresses the foregoing needs by modifying the wheel structure to provide radial transverse recesses of substantial depth between gear-like teeth, by providing a side-to-side wave pattern for forward motion of the device, and/or by providing a reset and ramp up mode once severe wheel slippage is sensed. 
   During a wet treating operation, a thin film of fluid (e.g. cleaning fluid) is deposited on the floor surface. The maximum layer thickness is controlled so as not to be greater than the depth of radial transverse channels on the wheel. This can be achieved by first applying the liquid to an application cloth, and then controlling the amount of liquid on the cloth and the speed of take-up of the cloth relative to device movement. It can also be achieved by directly applying liquid to the floor, but in a manner where the amount of liquid dispensed is limited based on the area that the device passes over. 
   In connection with such a tread design, it is desirable to make the contact area of the tire as small as possible, so that more of the weight of the device is borne in a small area of contact. This helps drive the contact surface down through any pooling liquid. A preferred way to achieve this is to form the tire in a sprocket shape with the radial edges of the sprocket being very small rectangular areas. Such a structure also has advantages for gripping a carpet. 
   A particularly preferred form of wheel can be molded from a thermoplastic elastomer. Such materials are particularly suitable for inexpensive injection molding. 
   Applying this approach to the invention, there is provided a robotic device for treating (e.g. preferably cleaning) a surface wherein the robotic device includes means for reducing the incidence of wheel slip. The robotic device has a wheel having a tread which has sprocket teeth separated by a radially extending peripheral groove. The peripheral grooves are of a depth exceeding 0.15 cm. The sprocket teeth are suitable to contact the surface being treated. 
   The robotic device has a housing supported by the wheel, and means for delivering a layer of fluid onto the surface. The robotic device also has a controller in communication with the means for delivering a fluid. The controller provides fluid delivery signals to the means for delivering a fluid such that the layer of fluid can be provided on the surface being treated having a thickness not exceeding the peripheral groove depth. 
   In another aspect the invention provides a robotic floor treater that delivers a liquid to the floor. The treater has a navigation pattern comprising a side-to-side wave pattern. This is preferably achieved by periodically changing the wheel speed of at least one wheel, and preferably of at least two wheels. The device can be designed, when it is moving forward, to move a short distance (e.g. 1 cm) to the right, followed by movement a short distance to the left. The cycle is repeated continuously. Alternatively, the wave pattern can be initiated only after a certain degree of slippage is sensed. 
   Adopting such a wave pattern has been surprisingly found to reduce the incidence of wheel slippage, and/or help the device recover from slippage once it occurs. Because the side-to-side movements are so small, this can be achieved with disrupting the ability of the device to essentially move linearly (e.g. along a wall). 
   In yet another aspect an autonomous robotic device has a first wheel driven by a first motor, a second wheel driven by a second motor, and a housing supported by the wheels. The robotic device further includes means for measuring wheel rotation for the first wheel and the second wheel. 
   In one preferred form the means for measuring wheel rotation is an encoder. The controller receives first wheel rotation signals associated with the first wheel and second wheel rotation signals associated with the second wheel from the means for measuring wheel rotation. The controller outputs first speed signals to the first motor for driving the first wheel and second speed signals to the second motor for driving the second wheel. The controller executes a stored program to calculate a first amount of slip for the first wheel from the first wheel rotation signals received from the means for measuring wheel rotation and calculate a second amount of slip for the second wheel from the second wheel rotation signals received from the means for measuring wheel rotation. 
   When the first amount of slip exceeds a predetermined first value of slip or the second amount of slip exceeds a predetermined second value of slip, the controller provides first speed signals to the first motor and/or provides second speed signals to the second motor such that the device navigates in a side-to-side wavy pattern. This can be achieved by varying relative wheel speed between the left wheel and the right wheel in a periodic fashion. 
   In still another embodiment such a robotic device for treating a floor has a software routine that works in conjunction with speed signals from the wheels of the autonomous floor cleaner to reduce the speed of the wheels (e.g. stop the wheels) in the situation where the wheels are slipping too much on a wet surface. Once the anti-slip routine is implemented and the wheels have either stopped or greatly slowed, the routine will cause the wheel speed to ramp up in a slow manner. This allows the machine to try to recover at a slow speed, rather than just spinning slipping wheels at a slow speed. 
   The nature of the ramp up may be affected by the cleanliness of the floor or the amount of liquid used. Hence, the machine may be programmed to ramp up more slowly when a high level of liquid is known to be present. Further, in such a situation the device can go to a dead stop before ramping up, rather than merely a slowed speed. 
   This latter software system may be particularly effective when the liquid being applied is formulated for quick evaporation. By stopping the device one gives an opportunity for some of the liquid causing the slippage to evaporate. 
   It should be noted that the above tread designs, wave patterns, and ramp up recovery software not only provide anti-slip assistance for wet floors, they are of assistance when the floors are slippery due to the presence of sand or other similar solid material. 
   In still another aspect the invention provides a robotic device for treating a surface wherein the robotic device includes a first wheel driven by a first motor, a second wheel driven by a second motor, and a housing supported by the first wheel and the second wheel. The robotic device further includes an encoder for measuring wheel rotation for the first wheel and the second wheel. The robotic device has a controller in communication with the first motor, the second motor, and the means for measuring wheel rotation. The controller receives first wheel rotation signals associated with the first wheel and second wheel rotation signals associated with the second wheel from the means for measuring wheel rotation. The controller also provides first speed signals to the first motor for driving the first wheel and second speed signals to the second motor for driving the second wheel. 
   During operation of the robotic device, the controller executes a stored program to calculate a first amount of slip for the first wheel from the first wheel rotation signals received from the means for measuring wheel rotation and calculate a second amount of slip for the second wheel from the second wheel rotation signals received from the means for measuring wheel rotation. The controller then provides first speed signals to the first motor that slow or stop the first wheel if the first amount of slip exceeds a predetermined first value of slip. The controller may also provide further first speed signals to the first motor that increases speed of the first wheel if the first wheel has been slowed or stopped because the first amount of slip has exceeded the predetermined first value of slip. 
   In still another aspect, the invention provides a robotic device for treating a surface wherein the robotic device includes means for reducing and/or preventing wheel slip. The robotic device has a first wheel driven by a first motor, a second wheel driven by a second motor, and a housing supported by the first wheel and the second wheel. The robotic device further includes means for measuring wheel rotation for the first wheel and the second wheel such as an encoder. The robotic device also includes a controller in communication with the first motor, the second motor, and the means for measuring wheel rotation. The controller receives first wheel rotation signals associated with the first wheel and second wheel rotation signals associated with the second wheel from the means for measuring wheel rotation. The controller provides first speed signals to the first motor for driving the first wheel and second speed signals to the second motor for driving the second wheel. 
   During operation of the robotic device, the controller executes a stored program to calculate a first amount of slip for the first wheel from the first wheel rotation signals received from the means for measuring wheel rotation, and calculate a second amount of slip for the second wheel from the second wheel rotation signals received from the means for measuring wheel rotation. The controller provides first speed signals to the first motor that cyclically decreases and increases speed of the first wheel if the first amount of slip has exceeded the predetermined first value of slip, and provides second speed signals to the second motor that cyclically decreases and increases speed of the second wheel if the second amount of slip has exceeded the predetermined second value of slip. 
   In yet another aspect the invention provides a robotic device for treating a surface wherein the robotic device includes means for reducing and/or preventing wheel slip. The robotic device includes a first wheel driven by a first motor, a second wheel driven by a second motor, and a housing supported by the first wheel and the second wheel. The robotic device has means for delivering a fluid onto the surface, and means for measuring wheel rotation for the first wheel and the second wheel. The robotic device also has a controller in communication with the first motor, the second motor, the means for delivering a fluid, and the means for measuring wheel rotation. 
   The controller receives first wheel rotation signals associated with the first wheel and second wheel rotation signals associated with the second wheel from the means for measuring wheel rotation. The controller provides first speed signals to the first motor for driving the first wheel and second speed signals to the second motor for driving the second wheel. The controller also provides fluid delivery signals to the means for delivering a fluid. 
   During operation of the robotic device the controller executes a stored program to calculate a first amount of slip for the first wheel from the first wheel rotation signals received from the means for measuring wheel rotation, and calculate a second amount of slip for the second wheel from the second wheel rotation signals received from the means for measuring wheel rotation. When the first amount of slip exceeds a predetermined first value of slip or the second amount of slip exceeds a predetermined second value of slip, the controller provides fluid delivery signals to the means for delivering a fluid such that fluid is not delivered onto the surface. In this manner, the device can temporarily stop applying liquid floor cleaner until wheel slip subsides. 
   In still another aspect, the invention provides a robotic device for treating a surface wherein the robotic device includes means for reducing and/or preventing wheel slip. The robotic device includes a first wheel driven by a first motor, a second wheel driven by a second motor, and a housing supported by the first wheel and the second wheel. The robotic device has a sheet cleaning material disposed on the device, and means for delivering a fluid onto the sheet cleaning material. The robotic device also has means for measuring wheel rotation for the first wheel and the second wheel. 
   A controller of the robotic device is in communication with the first motor, the second motor, the means for delivering a fluid, and the means for measuring wheel rotation. The controller receives first wheel rotation signals associated with the first wheel and second wheel rotation signals associated with the second wheel from the means for measuring wheel rotation. The controller provides first speed signals to the first motor for driving the first wheel and second speed signals to the second motor for driving the second wheel. The controller also provides fluid delivery signals to the means for delivering a fluid. 
   During operation of the robotic device, the controller executes a stored program to calculate a first amount of slip for the first wheel from the first wheel rotation signals received from the means for measuring wheel rotation, and calculate a second amount of slip for the second wheel from the second wheel rotation signals received from the means for measuring wheel rotation. When the first amount of slip exceeds a predetermined first value of slip or the second amount of slip exceeds a predetermined second value of slip, the controller provides fluid delivery signals to the means for delivering a fluid such that fluid is not delivered onto the sheet cleaning material. In this manner, the device can temporarily stop applying liquid floor cleaner until wheel slip subsides. 
   Hence, a robotic device with improved anti-slip control is provided. The foregoing and other advantages of the invention will become apparent from the following description. In the following description reference is made to the accompanying drawing which forms a part thereof, and in which there is shown by way of illustration preferred embodiments of the invention. These embodiments do not represent the full scope of the invention. Reference should therefore be made to the claims herein for interpreting the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded rear perspective view of an autonomous robotic surface treating device of the present invention; 
       FIG. 2  is an exploded frontal perspective view of the device of  FIG. 1 ; 
       FIG. 3  is a view similar to  FIG. 1 , but with upper housings removed; 
       FIG. 4  is a view similar to  FIG. 2 , but with upper housings removed; 
       FIG. 5  is a schematic view illustrating how a take-up reel of the assembly is ratcheted for one-way motion; 
       FIG. 6  is an enlarged perspective view of an end of a supply reel of the present assembly; 
       FIG. 7  is a view similar to  FIG. 1 , but showing the device in fully assembled form; 
       FIG. 8  is a sectional view taken along line  8 - 8  of  FIG. 7 ; 
       FIG. 9  is an enlarged view of the reel-to-reel portion of the present device, highlighting a portion of the  FIG. 8  drawing; 
       FIG. 10  is a front, left, upper perspective view of an alternative cartridge useful with the  FIG. 1  device, when cleaning carpeting; 
       FIG. 11  is a view similar to  FIG. 10 , but with an upper cover removed; 
       FIG. 12  is a right, front, upper perspective view of a sleeve for a drive wheel for such devices (with wheel hub removed); 
       FIG. 13  is a front elevational view of the wheel sleeve of  FIG. 12 ; 
       FIG. 14  is a side elevational view of the wheel sleeve of  FIG. 12 ; and 
       FIG. 15  is a schematic comparison of two prior art modes of travel with a preferred wave mode of travel of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   We first describe example autonomous cleaning devices with reference to  FIGS. 1-11 . This provides examples of environments where the invention of the present invention can be applied. Thereafter, we describe with reference to  FIGS. 12-15  specific features of the present invention. 
   It should be understood that the present invention is also suitable for use with many other types of autonomous treating devices. Thus, the invention is not intended to be restricted to just cleaning devices, much less devices having the specific attributes shown in  FIGS. 1-11 . 
   Referring particularly to  FIGS. 1 and 3 , there is a cleaning cartridge  10  suitable to be inserted into a cleaning device  12 . The cleaning cartridge  10  has a roll of sheet cleaning material  44  which is provided in a reel-to-reel configuration. A portion of the roll is maintained in contact with the surface below the cleaning device  12  at any given time during operation. A motor  52  is provided in the cleaning device  12  to consistently index the cleaning sheet material, so as to maintain a relatively fresh sheet against the floor. 
   Referring now also to FIGS.  FIGS. 2 and 4 , the cleaning device  12  is in the form of an autonomous robot which includes a housing  13  having an aperture  14  sized and dimensioned to receive the cleaning cartridge  10 . In the housing  13  and located above the aperture  14  are two windows  22  and  24  which allow the user to view the cleaning cartridge  10 , and the roll of cleaning sheet material  44  maintained therein. 
   An aperture  25  is also provided which, in conjunction with a latching device  27  on the cartridge  10 , provides a latch for selectively connecting the cartridge  10  to the cleaning device  12 . The cleaning device  12  also includes a bumper  15  at a front end and side brushes  16 . As shown in  FIG. 8 , the cleaning device  12  also includes a sweeper brush  60  for cleaning large particulate matter. The cleaning sheet material  44  follows the brush  60  and typically cleans smaller particulate matter such as hair and dust which have not been picked up through the use of the brush  60 . 
   The cartridge  10  includes windows  26  and  28  which, when positioned in the cleaning device  12 , are aligned with the windows  22  and  24  in the housing  13  of the cleaning device  12 , thereby allowing a user visual access to the cleaning sheet material  44  within the cartridge  10 . A dust bin  30  is provided in the cleaning cartridge  10  at the end of the cartridge which is received inside of the housing  13  of the cleaning device  12 . 
   The dust bin  30  is designed to be positioned adjacent the brush  60  ( FIG. 8 ) in the cleaning device  12 . It is selectively covered by a hinged lid  38 , which is forced open as the cleaning cartridge  10  is moved into the cleaning device  12  but which swings shut and is therefore normally closed when the cartridge is removed from the cleaning device  12 , thereby retaining dust collected by the cleaning device  12  within the dust bin  30  for cleaning, replacement, or disposal of the cartridge  10 . 
   A flexible blade  32  is provided in front of the dust bin  30 , directed from an upper edge of the dust bin  30  to the surface below the cartridge  10 . The flexible blade  32  directs dirt collected by the brush  60  of the cleaning device  12  into the dust bin  30 . 
   The reel-to-reel device provided in the cartridge  10  includes both a take-up reel  34 , to which used cleaning sheet material  44  is directed, and a supply reel  36 , to which an unused roll of cleaning sheet material is connected and from which the cleaning process is supplied. The take-up reel  34  ( FIG. 5 ) is ratcheted in order to prevent used cleaning sheet material  44  from being directed back over the surface to be cleaned, while the supply reel  36  ( FIG. 6 ) provides a resistive force limiting rolling of the sheet unless driven by the stepper motor  52 . Teeth  35  in the take-up reel  34  are engaged with spring-loaded teeth  33  to ratchet the reel and limit motion. 
   The cleaning sheet material  44  can comprise, for example, an electrostatic or electret material. Examples of such materials are those described in WO 02/00819. The cleaning sheet material  44  can also provide a liquid treating or dispensation function. For example, the cleaning cloth can be treated with cleaning fluid or polishes to treat the floor with surfactants, insecticides, insect repellants, and/or fragrances. 
   The cartridge  10  can further comprise a fluid reservoir  42  for providing a fluid to the cleaning sheet material  44  during operation. The fluid supply provided in the reservoir  42  is connected to a pump  50  provided in the cleaning device  12  through fluid inlets  40  provided on the cartridge  10  and fluid outlets  48  provided on the cleaning device  12 . In operation, therefore, the control of fluid flow to the cleaning sheet material  44  is controlled by the cleaning device  12 , and is provided to the sheet material to maintain a selected level of moisture over the life of the cartridge. 
   A bank of batteries  54  provides power to the cleaning device, which is selectively activated by a switch  18  ( FIG. 1 ) provided on the cleaning device  12 . The batteries are preferably rechargeable, and are accessed through a port  55  provided in the side of the housing of the cleaning device  12  ( FIG. 2 ) 
   The cloth supply reel  36  is driven by the stepper motor  52  provided in the cleaning device and the amount of the roll of the sheet material  44  which is unwound during operation is monitored by an optical sensor  46 , which is also provided in the cleaning device  12 . The stepper motor  52 , optical sensor  46 , and pump  50  are each driven by a programmable controller (not shown, but positioned above the battery pack) based on timing which drives the stepper motor to replace the sheet material as necessary to maintain proper cleaning processes during a cleaning operation while monitoring actual movement of the sheet. 
   Similarly, the controller drives the pump  50  to supply fluid to the roll of sheet material  44  as necessary during cleaning. The timing for replenishment of the fluid source is based on the type of material and fluid being employed, and in the expected life of the roll of cleaning sheet material  44 . The controller preferably maintains the cleaning sheet material  44  in a constant tension, and, while in use, indexes at a predetermined rate, as for example, 0.75 inches per 5 minutes or thereabout, over the life of the cartridge. The stepper motor  52  is coupled to the take-up reel  34  through a series of gears, while the supply reel is coupled to the optical sensor which detects the amount of rotation of the supply wheel. 
   Referring now to  FIGS. 8 and 9 , the cleaning device  12  includes a beater or sweeping brush  60 . A wheel  62  at the front of the cleaning device  12  is adjustable by activation of a switch  20  between at least two positions, one selected for use with a carpet, and another for use with a hard floor surface. As the cartridge  10  is inserted into the robot  12  the flexible blade  32  is positioned adjacent the main brush  60  and receives the relatively large particulate matter collected by the brush as the cleaning device  12  is run across a floor surface. The particulates are directed up the flexible blade  32  by the main brush  60  and into the dust containment bin  30 . 
   In operation the hinged lid  38  is retained in an open position such that the dust and particulate matter can be readily directed into the containment bin  30 . Following behind the main brush  60  is the cartridge  10  including the cleaning sheet material  44 . The cleaning sheet material  44  is retained against the surface to be cleaned by a platen  66  which includes a leaf-spring  64  that insures contact between the surface to be cleaned and the cleaning cloth  44 . Also as described above, the reservoir  42  is provided adjacent the cleaning material  44  such that fluids can be applied to replenish the cloth when a wet or moist mop cloth is employed in the cleaning device  12 . 
   Although a cleaning sheet material  44  has been shown and described particularly designed for use on a hard, smooth floor, a cartridge  10  for use with a carpet is shown in  FIGS. 10 and 11 . Here, the cartridge comprises a larger dust containment bin  30 , and is weighted appropriately to maintain the cleaning device  12  against the surface to be cleaned, and in an upright position during the cleaning operation. 
   The cartridge  10  preferably is a replaceable element that can be thrown away as a unit when the sheet material is used up, the fluid in the fluid reservoir  42  is spent, or the dust bin is full. Furthermore, even before the cleaning material is spent, the cartridge  10  can be removed and the dust bin  30  emptied by the user with minimal dust dispersion. 
   In an alternative embodiment (not shown), the fluid reservoir  42  can deliver fluid directly to the floor during operation. The fluid supply provided in the reservoir  42  is connected to the pump  50  provided in the cleaning device  12  through fluid inlets  40  provided on the cartridge  10  and fluid outlets  48  provided on the cleaning device  12 . The controller drives the pump  50  to supply fluid to the floor as necessary during cleaning. 
   Turning now to key features of the present invention, the cleaning device  12  includes motors  70  and  71  for driving the left wheel  101  and the right wheel  102  of the cleaning device  12 , respectively. The motors  70 ,  71  are each controlled by the programmable controller which includes a microprocessor under the control of a software program stored in a memory. Among other things, the controller provides voltage signals to the motors  70  and  71  that cause the left wheel  101  and the right wheel  102  to start, stop, rotate in a direction causing forward motion of the cleaning device  12 , rotate in a direction causing reverse motion of the cleaning device  12 , and rotate at increased or decreased speeds. 
   An encoder is associated with each wheel  101 , 102  and is connected to the controller. Encoders are commercially available and in one version, the encoder outputs a signal having a pulse every time each wheel  101 , 102  rotates a predetermined angle. For example, an optical encoder outputs pulses each time an optical beam is broken by an element that rotates with the wheel. The controller respectively calculates the wheel speed of each wheel  101 , 102  based upon an interval between pulses outputted from each encoder. Changes in the interval between pulses can also be used by the controller to calculate wheel acceleration. 
   Among other things, the controller can use calculated wheel speeds to control motion of the left wheel  101  and the right wheel  102 . In one example algorithm, the controller provides a positive voltage in the range of 0 to +10 volts to each motor  70  and  71  to drive the left wheel  101  and the right wheel  102  in forward motion. The controller uses calculated wheel speeds to determine the amount of voltage to be applied the motors  70  and  71  to control motion of the left wheel  101  and the right wheel  102 . Voltage controls the motor speed as voltage will typically be proportional to motor speed. The controller provides a negative voltage in the range of 0 to −10 volts to each motor  70  and  71  to drive the left wheel  101  and the right wheel  102  in reverse motion. 
   Turning next to focus on  FIGS. 12-14 , the tread portions of wheels are shown. These could be integral with hubs and thus suitable to link directly to axles connected with the driving motors. Alternatively, these could be separate treads positioned on separate hubs. 
   In any event, these drawings show tread for a particular left wheel  101 , which preferably will be identical to tread for the right wheel  102 . The tread of the left wheel  101  has a tread  111  having lateral grooves  113  of depth  117  on each side of the tread  111 . A circumferential longitudinal channel  115  extends around the tread  111  between the grooved sections. The entire wheel  101  may be formed from a thermoplastic styrenic material. This material provides good grip as well as chemical resistance. In one form, the radius of the wheel  101  is 31.70 millimeters, the transverse width of the wheel  101  is 23.50 millimeters, the transverse width of the longitudinal channel  115  is 7.7 millimeters, the depth  117  of the lateral grooves is 2.01 millimeters, and the floor contacting surfaces  116  are 1.07 millimeters in the circumferential direction and 7.90 millimeters in the transverse direction. 
   During a wet cleaning operation of the autonomous floor cleaner  12 , a thin film of fluid is deposited on the floor surface by way of direct delivery from the fluid reservoir  42  to the floor surface or by way of the moistened cleaning sheet material  44 . Traction is improved by reducing the thickness of the film layer to the point where tire tread and floor surface are able to make contact. In one embodiment, this is achieved as the controller of the cleaning device  12  provides fluid delivery signals to the pump  50  such that the layer of fluid on the floor surface has a thickness less than the depth  117  of the tread grooves  113 . In this manner, traction is improved by reducing the thickness of the fluid layer to the point where tire tread and floor surface are able to make contact. Also, the longitudinal channel  115  of the tread  111  channels fluid away to further improve traction of the wheels  101 ,  102 . 
   There can be an algorithm in the controller for measuring slippage of the left wheel  101  and/or the right wheel  102  of the cleaning device  12 , whereby the controller calculates and compares wheel speeds for the left wheel  101  and the right wheel  102 . Wheel slipping results when a motor drives its wheel at too high of a speed relative to the other wheels. 
   Thus, if motor  70  drives the left wheel  101  at too high of a speed relative to the right wheel  102 , the controller outputs a variable indicating left wheel slippage. Likewise, if motor  71  drives the right wheel  102  at too high of a speed relative to the left wheel  101 , the controller outputs a variable indicating right wheel slippage. The relative difference between left wheel speed and right wheel speed that indicates a wheel slippage condition can be programmed in the controller. 
   In another example algorithm for measuring slippage of the left wheel  101  and/or the right wheel  102  of the cleaning device  12 , an encoder is provided for the wheel  62  at the front of the cleaning device  12 . The controller calculates the wheel speed of wheel  62  based upon an interval between pulses outputted from the encoder associated with the wheel  62 . Wheel  62  is not driven by a motor and therefore, it provides a good value for the forward or reverse speed of the cleaning device  12 . Wheel slipping can be deemed to have resulted if motor  70  drives the left wheel  101  at too high of a speed relative to the wheel  62  or if motor  71  drives the right wheel  102  at too high of a speed relative to the wheel  62 . Therefore, the controller outputs variables indicating left wheel slippage or right wheel slippage based on a programmed relative difference between the left wheel speed and the speed of the wheel  62  and the right wheel speed and the speed of the wheel  62 . 
   In still another example algorithm for measuring slippage of the left wheel  101  and/or the right wheel  102  of the cleaning device  12 , the relationship between wheel torque and slippage is used to calculate wheel slippage. Typically, under-torque conditions indicate wheel slippage. The torque on the left wheel  101  and the torque on the right wheel  102  may be calculated as a function of current in the wheel motors  70  and  71 . Thus, the controller outputs variables indicating left wheel slippage or right wheel slippage based on wheel motor current readings. 
   Yet another example algorithm for measuring slippage of the left wheel  101  and/or the right wheel  102  of the cleaning device  12  is described in U.S. Pat. No. 6,046,565 which is incorporated herein by reference along with all other patents cited herein. 
   Having calculated slippage of the left wheel  101  and/or the right wheel  102 , the controller executes one of the specified software routines that reduce and/or prevent wheel slip in the autonomous floor cleaner  12 . For example, if the software determines that the amount of wheel slip of the left wheel  101  exceeds a predetermined first value of slip (that may be programmed in the controller) or that the amount of wheel slip of the right wheel  102  exceeds a predetermined second value of slip (that may be programmed in the controller), the controller provides speed signals to the motor  70  and provides speed signals to the motor  71  such that the device navigates in a side-to-side wavy pattern. This can be achieved by varying wheel speed between the left wheel  101  and the right wheel  102 . 
   For instance, when the speed of the right wheel  102  exceeds the left wheel  101 , the cleaning device  12  will veer left, and when the speed of the left wheel  101  exceeds the right wheel  102 , the cleaning device  12  will veer right. In  FIG. 15 , this wave mode for anti-slip motion is shown to the right of a conventional straight line mode and a conventional spiral mode for autonomous robot navigation. It is most preferred that the side-to-side motion be limited to only a few centimeters at most so that the algorithm will work a vertical wall. 
   Alternatively, if the software determines that the amount of wheel slip of the left wheel  101  exceeds a predetermined first value of slip, the controller provides speed signals to the motor  70  that slow or stop the left wheel  101 . The controller may also provide further speed signals to the motor  70  that increase speed of the left wheel  101  after the left wheel  101  has been slowed or stopped. There may be a pause period while the system allows some cleaner to evaporate. In any event, at some point the software slowly ramps up the speed to gain traction. 
   In yet another software routine, if the software determines that the amount of wheel slip of the left wheel  101  exceeds a predetermined first value of slip, the controller provides speed signals to the motor  70  that cyclically decrease and increase speed of the left wheel  101 . If the software determines that the amount of wheel slip of the right wheel  102  exceeds a predetermined second value of slip, the controller provides speed signals to the motor  71  that cyclically decrease and increase speed of the right wheel  102 . In this manner, the device can modulate wheel speed until wheel slip subsides. 
   In yet another software routine, if the software determines that the amount of wheel slip of the left wheel  101  exceeds a predetermined first value of slip or the amount of wheel slip of the right wheel  102  exceeds a predetermined second value of slip, the controller provides fluid delivery signals to the pump  50  such that fluid is not delivered onto the surface. In this manner, the device can temporarily stop applying liquid floor cleaner until wheel slip subsides. 
   In yet another software routine, if the software determines that the amount of wheel slip of the left wheel  101  exceeds a predetermined first value of slip or the amount of wheel slip of the right wheel  102  exceeds a predetermined second value of slip, the controller provides fluid delivery signals to the pump  50  such that fluid is not delivered onto the sheet cleaning material  44 . In this manner, the device can temporarily stop applying liquid floor cleaner until wheel slip subsides. 
   Although specific embodiments of the present invention have been described in detail, it should be understood that this description is merely for purposes of illustration. Many modifications and variations to the specific embodiments will be apparent to those skilled in the art, which will be within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. Rather, the claims should be looked to in order to judge the full scope of the invention. 
   INDUSTRIAL APPLICABILITY  
   Disclosed are wheel tread configurations, wave mode navigational controlling systems, ramp up recovery systems, and other software for reducing and/or preventing wheel slip in an autonomous floor treater that applies fluid to a surface being treated.