Abstract:
A robotic mower boundary sensing system includes a boundary driving circuit on a charging station transmitting an encoded signal on a boundary wire, a boundary sensor on a robotic mower and including an inductor receiving the encoded signal, and a vehicle control unit on the robotic mower receiving the encoded signal from the boundary sensor and decoding the signal and cross correlating the received signal to determine the distance of the boundary sensor from the boundary wire.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to robotic lawn mowers, and more specifically to a boundary sensing system for a robotic mower. 
       BACKGROUND OF THE INVENTION 
       [0002]    Robotic mowers may include boundary sensing systems to detect the boundary wire. Boundary sensing systems may detect if the robotic mower is inside or outside the boundary wire. When the robotic mower reaches the boundary wire, a vehicle control unit may prompt the traction drive to stop and/or turn around. A boundary sensing system is needed that also can accurately and reliably determine the distance of a robotic mower to the boundary wire. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention provides a robotic mower boundary sensing system with a boundary driving circuit on a charging station transmitting an encoded signal on a boundary wire, a boundary sensor on a robotic mower and including an inductor receiving the encoded signal, and vehicle control unit on the robotic mower receiving the encoded signal from the boundary sensor and decoding the signal and cross correlating the received signal to determine the distance of the boundary sensor from the boundary wire. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is schematic drawing of a robotic mower within a main boundary wire according to a preferred embodiment of the invention. 
           [0005]      FIG. 2  is a block diagram of a boundary sensing system for a robotic mower according to a preferred embodiment of the invention. 
           [0006]      FIG. 3  is a block diagram of an orientation and heading system for a robotic mower that may be used with the boundary sensing system of  FIG. 2 . 
           [0007]      FIG. 4  is a block diagram of an improved area coverage system for a robotic mower according to a first embodiment of the invention. 
           [0008]      FIG. 5  is block diagram of an embodiment of a wide area coverage that may be used with the improved area coverage system of  FIG. 4 . 
           [0009]      FIG. 6  is a block diagram of an embodiment of a local area coverage that may be used with the improved area coverage system of  FIG. 4 . 
           [0010]      FIG. 7  is a block diagram of an embodiment of a boundary following system that may be used according to one embodiment of the invention. 
           [0011]      FIG. 8  is a block diagram of a boundary following system that may be used according to an alternative embodiment of the invention. 
           [0012]      FIG. 9  is a block diagram of a boundary following system for a robotic mower with a single according to another alternative embodiment of the invention. 
           [0013]      FIG. 10  is a block diagram of a stuck detection system for a robotic mower according to a preferred embodiment of the invention. 
           [0014]      FIG. 11  is a schematic diagram of a boundary sensor according to a preferred embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    In one embodiment shown in  FIG. 1 , robotic mower  100  may be powered by battery pack  109  that may be charged periodically at charging station  105 . Vehicle control unit  101  may control all of the electronic functions of the robotic mower. For example, vehicle control unit  101  may command a pair of traction motors  110 ,  111  to turn traction drive wheels, blade motor  112  to rotate a cutting blade or blades, battery pack  109 , a user interface and various sensors. 
         [0016]    Vehicle control unit  101  may be a printed circuit board assembly that serves as the main control board for the robotic mower. The vehicle control unit may interpret and process information from various sensors, and use that information to control and operate the pair of traction motors to drive the robotic mower over a yard in order to maintain the lawn, and to drive the blade motor. For example, the vehicle control unit may be connected to a number of sensors including one or more boundary sensors  119 , as well as one or more obstacle sensors or accelerometers. The vehicle control unit also may communicate with the battery pack in order to monitor the status of the battery pack to maintain a charge for one or more lithium ion batteries in the battery pack. The vehicle control unit also may be connected to a user interface module including an LCD display along with several indicator lights and key buttons for operator input. 
         [0017]    In one embodiment, the vehicle control unit may include a microcontroller such as an LQFPSTM32F103ZET6 processor from ST Microelectronics. The microcontroller may have 512 kB of internal flash memory and 64 kbytes of internal RAM. The microcontroller may contain an ARM Cortex M3 core, may run at a maximum core clock frequency, and may use an onboard AtoD converter. The vehicle control unit may contain external static random access memory (SRAM) connected to the microcontroller with a 16 bit FSMC bus and have a minimum capacity of 1 Megabit. 
         [0018]    In one embodiment, the vehicle control unit may include three external EEPROM integrated circuits. For example, the EEPROMs may each be 125 kilobyte ICs for a total capacity of 384 kilobytes. The EEPROMs may use and SPI interface to the microcontroller and may be used to store configuration data. The vehicle control unit may use the microcontroller&#39;s internal real time clock module. 
         [0019]    In one embodiment, the vehicle control unit may interface and control a blade motor controller to power and control blade motor  112  that drives the cutting blade on the robotic mower. For example, blade motor  112  may be a permanent magnet brushless DC motor, such as the EBM Papst 63.20 BLDC motor having a typical output shaft speed range of about 4000 rpm. The vehicle control unit may have three inputs which receive signals from hall effect rotor position sensors contained in the blade motor, such as Melexis US 2884 Hall effect sensors. The vehicle control unit may sense the speed of the blade motor using feedback from the Hall effect sensors, and may sense the current through the blade motor phases combined. 
         [0020]    In one embodiment, the vehicle control unit may be connected to traction motor controllers for each of the left and right traction motors  110 ,  111 . Each traction motor may be a permanent magnet brushless DC motor, such as a EBM Papst 42.20 BLDC motor having a typical output shaft speed range of about 2080 rpm. The vehicle control unit may have three inputs which receive signals from Hall effect rotor position sensors, such as the Melexis US2884 Hall effect sensor contained in each traction motor. The vehicle control unit may sense the speed of each traction motor using a feedback from a hall sensor, and may sense the current through the traction motor phases combined. 
         [0021]    Still referring to  FIG. 1 , in one embodiment, robotic mower  100  may operate in a specified area  102  that is surrounded by main or outer boundary wire  103  which may form a loop positioned at or below the ground or turf surface. Additionally, inner wire  104  may be a shorter loop provided within the area of the main boundary wire where charging station  105  is positioned. The main boundary wire and inner wire may be connected to charging station  105 . 
         [0022]    In one embodiment, boundary drive circuit  106  may be contained in charging station  105 , and may drive signals on the main boundary wire and the inner wire. The fundamental frequency of the waveform on the main boundary wire may be about 2 kHz. The robotic mower may have a boundary wire sensor  119  to detect the waveform and provide a signal to the vehicle control unit to indicate the distance of the sensor to the main boundary wire. 
         [0023]    In one embodiment shown in the block diagram of  FIG. 2 , the charging station may drive the main boundary wire and inner wire from a single h-bridge device. The h-bridge may drive both boundary wires, but only one of the boundary wires at a time, to minimize power requirements and component costs. 
         [0024]    In one embodiment, the boundary driving circuit may transmit a unique ID on the main or outer boundary wire loop ten times per second in block  201 . In block  203 , the boundary driving circuit may encode the ID with a 4 bit Barker code to improve the signal to noise ratio and reduce susceptibility to noise interference. The resulting 1&#39;s and 0&#39;s are called chips. A process gain of 6 dB may be achieved with four chips, where process gain is the ratio of chip rate to data rate. In block  205 , the microprocessor may encode the Barker coded ID using Manchester encoding to ensure there always is a line voltage transition for every bit or chip. 
         [0025]    In one embodiment, in block  207 , one or more boundary sensors on the robotic mower may receive the encoded boundary wire magnetic signal, and send the signal to the vehicle control unit. In block  209 , the vehicle control unit may amplify the received signal. In block  211 , the vehicle control unit&#39;s analog to digital converter may sample the amplified signal, preferably at a rate of 200 kHz. In block  213 , the vehicle control unit may buffer the sample data for further processing. The boundary wire magnetic signal may be very small and similar in amplitude to the background noise if the robotic mower is a significant distance from the main boundary wire loop. This limits the amount of amplification that can be applied to the signal, and it may be difficult to detect the signal using traditional hardware/software methods. 
         [0026]    In one embodiment, in block  215 , the vehicle control unit may cross correlate the received signal (at the boundary sensor&#39;s present position) with a known waveform (at a known distance to the boundary wire) to identify the start bit in the data buffer and determine if the data is inverted, indicating the sensor is outside the main boundary loop, or normal, indicating the sensor is inside the main boundary loop. The peak to peak amplitude of the known waveform may be the theoretical maximum that the boundary sensor and vehicle control unit can receive without distorting the signal. Cross correlation is done by converting the known waveform data and the sampled waveform data from the time domain into the frequency domain. This may be accomplished by running a FFT on the data, multiplying the FFT results together, and then running an inverse FFT on the result of that product. 
         [0027]    In block  217 , the vehicle control unit may decode the Manchester and Barker coding, and verify the ID against the identification stored in non-volatile memory. In block  219 , the vehicle control unit may determine the relative distance of the sensor from the outer boundary wire. The cross correlation function may provide the time lag difference between the known waveform (at a known distance to the boundary wire) and the unknown received sampled waveform (the boundary sensor&#39;s distance to the boundary wire). The location in time of the maximum peak value of the lag provides the starting location in time of the transmitted data packet located in the sampled waveform data. The amplitude of the lag is proportional to the difference between the known waveform&#39;s maximum amplitude and the received sample data&#39;s maximum amplitude. For example, if the known waveform has a maximum peak value of 1.65 volts or 2048 A/D (0.000805 volts per count) counts, and the resulting cross correlation lag value is 360, the peak amplitude of the sampled data is 360*0.00805=0.2898 volts. 
         [0028]    In one embodiment, the robotic mower may have one boundary sensor to indicate proximity of the sensor to the wire.  FIG. 11  is a schematic diagram of an embodiment of the electronic circuit of a boundary sensor on the robotic mower. The boundary sensor may include a sense coil L 1  and a circuit to amplify and filter the signal from the sense coil before it is applied to the ND input of the vehicle control unit. The battery pack on the robotic mower may have a minimum power input voltage of 20V and a maximum power input voltage of 30 v. The vehicle control unit may have a +5V power supply to the boundary sensors, and the vehicle control unit may provide a 2.5V reference to each boundary sensor. The signal range for each sensor may be 0V to 5.25V. 
         [0029]    In one embodiment, sense coil L 1  may be an inductor that detects the magnetic field generated by the current flowing in the main or outer boundary wire and/or inner boundary wire. For example, sensor coil L 1  may be a 100 mH 10% inductor Bournes RL622-104K-RC. The maximum peak voltage of the sense coil L 1  may be approximately 75 mV when the sensor is located six inches from the boundary wire. 
         [0030]    In one embodiment, the boundary sensor circuit may include a quad op amp with transimpedance amplifier U 1 -A, band pass filter U 1 -B, variable gain amplifier U 1 -C, and comparator U 1 -D. For example, the quad op amp may be a National Semiconductor LMV6484IMX Op Amp (Quad). A valid signal from the final stage output of the quad op amp may be greater than about 100 mV. 
         [0031]    In one embodiment, transimpedance amplifier U 1 -A may convert the current induced in sense coil L 1  into a voltage and amplify that voltage. Resistor R 1  may convert the output current from sense coil L 1  into a voltage. The output voltage of the transimpedance amplifier may be equal to the input current multiplied by the feedback resistor R 1 . For example, resistor R 1  may be 100 kΩ. Capacitor C 1  may provide stability to prevent the transimpedance amplifier from oscillating. Oscillation may be the result of capacitance of the input sensor and the op amp itself. For example, C 1  may be a 47 pF 50V 10% C 0 G ceramic capacitor. 
         [0032]    In one embodiment, band pass filter U 1 -B may provide a second order Sallen-Key high pass filter to cancel noise such as low frequency noise from the traction wheel motors of the robotic mower. Capacitors C 2  and C 3  and resistors R 2  and R 3  may set the corner or roll off frequency of the filter. For example, R 2  and R 3  may be 1 Meg Ohm 1/16 W 1% resistors, and C 2  and C 3  may be 100 pF 50 V 5% C 0 G ceramic capacitors. The output of the high pass filter may be followed by resistor R 4  and capacitor C 4 , which may perform low pass filtering. For example, R 4  may be a 10.0 k 1/16 W 1% resistor, and C 4  may be a 47 pF 50V 10% C 0 G ceramic capacitor. Capacitor C 5  may be a decoupling capacitor with a voltage rating high enough for the maximum voltage on the +5V power supply. For example, C 5  may be a 0.1 pF 16V 10% X 7R  MLC capacitor. 
         [0033]    In one embodiment, the quad op amp also may include variable gain amplifier U 1 -C. Resistors R 5  and R 6  may set the gain of the variable gain amplifier, and resistor R 6  may provide the negative feedback. For example, R 5  may be a 10.0 k, 1/16 W, 1% resistor, and R 6  may be a 100 kΩ, 1/16 W 1% resistor. Dual diode D 1  may automatically reduce the gain when the received signal strength is higher, such as when the robotic mower is very near the boundary wire. If the output voltage of variable gain amplifier U 1 -C is too high, one of the pair of diodes D 1  may conduct and clamp the voltage across resistor R 6 , reducing the gain. As the input voltage to the amplifier increases, a point will be reached where the diodes conduct. Beyond this point the feedback from the output to the inverting input will be equal to the voltage across the diode. For example, D 1  may be an NXP BAV99LT1G high-speed switching dual diode. 
         [0034]    In one embodiment, the boundary sensor circuit also may include unity gain buffer U 2 -A to buffer the output of variable gain amplifier U 1 -C before connection to the vehicle control unit via a wiring harness. For example, unity gain buffer U 2 -A may be a National Semiconductor LM771 op amp. Capacitor C 7  may be a bypass capacitor for unity gain buffer U 2 -A. For example, capacitor C 7  may be a 0.1 μF 16V 10% X7R MLC capacitor. 
         [0035]    In one embodiment, the boundary sensor circuit may include comparator U 1 -D which may form a Schmitt trigger comparator circuit to provide an output that indicates whether or not the received signal strength is great enough to be considered a valid signal. If the received signal is greater than the threshold, the output of the comparator will be high. Resistors R 7  and R 8  may form a voltage divider used to set the threshold for a valid signal, indicating a valid signal instead of noise. For example, R 8  may be a 5.62 k, 1/16 W 1% resistor, and R 7  may be a 200Ω1/16 W1% resistor. Resistors R 9  and R 10  may configure the hysteresis of the comparator, with R 10  providing the positive feedback. R 9  and R 10  together set the upper and lower thresholds of the Schmitt trigger comparator. For example, R 9  may be a 5.62 k 1/16 W 1% resistor and R 10  may be a 1 Meg Ohm 1/16 W 1% resistor. 
         [0036]    In an alternative embodiment, the robotic mower may have a plurality of boundary sensors  119 , and most preferably three boundary sensors mounted at or near the front of the robotic mower and a fourth boundary sensor mounted at or near the back of the robotic mower. The vehicle control unit may receive input from each of the boundary sensors regarding strength of the signal from the main boundary wire to indicate proximity of the sensor to the wire. 
         [0037]    In the alternative embodiment described in  FIG. 3 , the vehicle control unit may use signals from four boundary sensors to determine orientation and heading of the robotic mower with respect to the boundary wire. In block  302 , the vehicle control unit may sign the boundary distance signal from each boundary sensor to indicate if the sensor is inside or outside the main boundary wire. In block  304 , the vehicle control unit calculates Δ 1  as the difference between the distance from the center front sensor to the main boundary wire, compared to the distance from the back sensor to the main boundary wire. In block  306 , the vehicle control unit calculates Δ 4  as the difference between the left front sensor to the main boundary wire, compared to the distance from the right front sensor to the main boundary wire. In block  308 , the vehicle control unit confirms the dimensions between the sensors on the mower based on fixed values stored in memory. For example, these dimensions may include L 1  between the front center and back sensors, and L 2  between the left and right front sensors. In block  310 , the vehicle control unit confirms that the values calculated for Δ 1  and Δ 4  are within the ranges that are possible given the specified dimensions, L 1  and L 2 . In block  312 , the vehicle control unit calculates a pair of angles using trigonometric equations with Δ 1 , 66   4 , L 1  and L 2 . The angles may be θ 1 =arcsin (Δ 1 /L 1 ) and θ 2 =arccos (Δ 4 /L 2 ). 
         [0038]    In one embodiment, in block  314 , the vehicle control unit determines which of the four possible heading quadrants the robotic mower is in relative to the main boundary wire. For example, if Δ 1  is greater than or equal to 0 and Δ 4  is less than or equal to zero, the heading is in quadrant 1. In block  316 , the vehicle control unit calculates the heading angle of the robotic mower given the heading quadrant from the preceding step. For example, in quadrant 1, the angle θ=360 degrees−arcsin (Δ 1 /L 1 )×180 degrees/π. The angle θ of the mower will be within the range from 0 degrees to 360 degrees. In block  318 , the vehicle control unit may flip the angle for readings outside the main boundary wire. 
         [0039]    In one embodiment, the vehicle control unit may select the type of area coverage used by the robotic mower for mowing within the main boundary wire. Using the steps described below in the block diagram of  FIG. 4 , the vehicle control unit may command the robotic mower to switch from one type of area coverage to another without operator intervention and without discontinuing mowing. The vehicle control unit may select the type of area coverage based on input from one or more boundary sensors  119  regarding distance of the robotic mower to the main boundary wire, current draw of electric blade motor  112  that rotates one or more cutting blades, and the type of area coverage used during a specified preceding time period which may be stored in the vehicle control unit memory. 
         [0040]    In one embodiment shown in the block diagram of  FIG. 4 , in block  400  the robotic mower may be activated to start area coverage, such as by an operator or by an internal or external timer. The vehicle control unit then may run the routine described in the block diagram about every 40 milliseconds. In block  401  the vehicle control unit may determine if the robotic mower is in the charging station, preferably by checking if the voltage on the charger contacts is within a specified range. If the robotic mower is in the charging station, in block  402  the vehicle control unit may command the traction wheel motors to leave the charging station by rotating in reverse for a specified distance or duration to back up the robotic mower out and away from the charging station, then turn the robotic mower around. The vehicle control unit may determine how much each wheel motor has rotated based on pulse feedback from the Hall effect sensor in each motor. If the vehicle control unit determines the robotic mower is not in the charging station, in block  403  the vehicle control unit may determine if the leave dock instruction is still active. If the leave dock instruction is still active, in block  404  the vehicle control unit may command the wheel motors of the robotic mower to continue executing the leave dock instruction. 
         [0041]    In one embodiment, in block  405  the vehicle control unit may determine if a bump is detected, indicating the robotic mower has contacted an obstacle. Bump detection may be provided to the vehicle control unit by one or more accelerometers attached to the chassis and/or top cover of the robotic mower. The accelerometer may be a three axis accelerometer such as the ST LIS302DL which also may be used to sense lifting and orientation, and may communicate to the microcontroller with a SPI bus at the voltage logic level of the microcontroller. If the accelerometer indicates an obstacle is bumped, in block  406  the vehicle control unit may command both traction motors to reverse direction for a specified distance or duration and then turn the robotic mower around. 
         [0042]    In one embodiment, if no bump is detected, in block  407  the vehicle control unit may determine if a specified coverage such as boundary coverage was executed within a specified preceding time period such as seven days. The vehicle control unit memory may store data on the type of coverage executed for a specified preceding time period. If boundary coverage was not executed during the specified preceding time period, in block  408  the vehicle control unit may command the traction motors to execute boundary coverage. Preferred boundary coverages are described below. 
         [0043]    In one embodiment, if the specified boundary coverage was executed within the preceding time period specified in block  407 , in block  409  the vehicle control unit may determine if the robotic mower is closer to the boundary or perimeter wire than a specified distance, using input from one or more boundary sensors  119 . If the robotic mower is closer than the specified distance, in block  410  the vehicle control unit may command the traction motors to reverse direction for a specified duration and then turn the robotic mower around. 
         [0044]    In one embodiment, if the robotic mower is not closer than the specified distance to the boundary wire, in block  411  the vehicle control unit may determine if the wheel motors are currently executing the reverse and turn around function. If the motors are still in reverse for the prespecified distance or duration, or have not finished turning the robotic mower around, in block  412  the vehicle control unit may command both traction wheel motors to continue the reverse and turn around functions. 
         [0045]    In one embodiment, if the vehicle control unit determines the reverse and turn around function is currently active, in block  413  the vehicle control unit may determine if the blade load is greater than a first predetermined specified value X which indicates uncut grass. Higher current means higher blade load and torque, indicating longer, uncut grass. Lower current, lower blade load and torque, indicates shorter, cut grass. If the blade load is greater than the first value, in block  414  the vehicle control unit may command the traction wheel motors to execute local area coverage. A preferred local area coverage is described below. 
         [0046]    In one embodiment, if the blade load is not greater than the predetermined specified value X, in block  415  the vehicle control unit commands the traction wheel motors traction motors to execute wide area coverage. A preferred wide area coverage is described below. 
         [0047]    In one embodiment, the vehicle control unit may execute wide area coverage by commanding the left and right wheel motors to drive the robotic mower in a straight line until an obstacle or boundary wire is encountered. When the robotic mower encounters the boundary wire or obstacle, the vehicle control unit may command the wheel motors to reverse and back up the mower for a prespecified distance and then turn the robotic mower around, preferably less than 180 degrees, to follow a path that diverges from the preceding forward path. Alternatively, the vehicle control unit may specify and execute other methods of wide area coverage, including but not limited to traveling in arcs instead of straight lines. 
         [0048]    In a preferred embodiment shown in the block diagram of  FIG. 5 , wide area coverage may begin executing in block  500 . In block  502 , the vehicle control unit may set the forward ground speed of the traction wheel motors at a nominal speed, and to maintain the same yaw or steering angle (i.e., 0 degrees for a straight path) so that the robotic mower travels in a straight line. 
         [0049]    In one embodiment, in block  504  the vehicle control unit determines if the robotic mower has bumped an obstacle, as indicated by one or more accelerometers, for example. If the robotic mower has detected an obstacle, in block  508  the vehicle control unit may command both traction wheel motors to rotate in reverse to back up at a reduced ground speed, and to maintain the same yaw angle. If the robotic mower has not bumped an obstacle in block  504 , the vehicle control unit may determine if one or more boundary sensors indicate the mower is closer to the main boundary wire than a prespecified threshold distance. If one or more boundary sensors indicate the robotic mower is not close to the main boundary wire, the vehicle control unit commands the left and right wheel motors to continue rotating forward as indicated in block  502 . If the boundary sensor(s) indicate the robotic mower is close to the main boundary wire, in block  508  the vehicle control unit may command the wheel motors to rotate in reverse at a reduced ground speed, and to maintain the same yaw angle. In block  510 , the vehicle control unit may determine if the traction wheel motors have rotated in reverse a prespecified or threshold time or distance. If the traction wheel motors have not rotated the prespecified time or distance in reverse, the vehicle control unit may command the motors to continue in reverse as indicated in block  510 . 
         [0050]    In one embodiment, once the traction wheel motors have rotated for the threshold distance or time in reverse, in block  512  the vehicle control unit may set a target yaw angle at a prespecified angle, preferably less than 180 degrees, and command the left and right wheel motors to turn the robotic mower around at a ground speed of zero. In block  514 , the vehicle control unit determines the turn error from the target yaw angle. In block  516 , once the turn angle reaches the target yaw angle, the vehicle control unit may command the traction wheel motors to rotate in forward again at a nominal speed and maintain the same yaw angle (i.e., 0 degrees), as specified in block  502 . If the turn angle has not reached the target yaw angle yet, the vehicle control unit will command the traction wheel motors to continuer turning the robotic mower around, and then calculate the turn error again in block  514 . 
         [0051]    In one embodiment, local area coverage may be a path that spirals outwardly, either clockwise or counterclockwise, from the robotic mower&#39;s initial position. Alternatives for local area coverage may include other patterns starting from an initial position of the robotic mower. As shown in the block diagram of  FIG. 6 , in block  600  the vehicle control unit begins executing local area coverage. In block  604 , the vehicle control unit may determine the radius from the spiral center, where local area coverage began, to the current location of the robotic mower. When local area coverage begins the radius value is zero, and may be incremented based on the difference in radius between sequential passes of the robotic mower around the spiral. Thus, the radius value is a function of how many degrees the robotic mower has traveled around the spiral, and the spacing of the spiral based on the robotic mower&#39;s effective cutting width. In block  606 , the vehicle control unit may determine if the radius is less than a prespecified minimum value. If it is less than the minimum value, in block  608  the vehicle control unit may command the traction wheel motors to rotate at a minimum forward ground speed. In block  610 , the vehicle control unit may determine if the radius is less than an intermediate value. If the radius is less than the intermediate value, in block  612  the vehicle control unit may command the traction wheel motors to rotate at a reduced forward ground speed, which may be greater than the minimum speed. In block  614 , the vehicle control unit may command the traction wheel motors to rotate at a nominal forward ground speed, which may be higher than the reduced speed, if the radius is at least the intermediate value. In block  616 , the vehicle control unit determines the desired change in yaw angle for the sample, which may be a function of the time period between function calls, the ground speed, and the radius. In block  618 , the vehicle control unit may add the desired change in yaw angle to the spiral total. In block  620 , the vehicle control unit may determine the desired yaw angle for the robotic mower, which may be based on the current yaw angle plus the desired change in yaw angle. 
         [0052]    In one embodiment, the robotic mower may execute boundary coverage, or return to the charging station, on a path along or parallel to the boundary wire using the system described in the block diagram of  FIG. 7 . The vehicle control unit may use this system based on input from one boundary sensor on the robotic mower regarding strength of the signal from the main boundary wire to indicate proximity of the sensor to the wire. The vehicle control unit may use input from the boundary sensor to direct the traction wheel motors to follow a path along or at a specified distance parallel to the boundary wire. 
         [0053]    As shown in  FIG. 7 , in block  700 , the vehicle control unit may command the left and right traction motors to start rotating in forward on a path at a specified distance parallel to the boundary wire. In block  701 , the vehicle control unit compares the input from the boundary sensor to the specified distance, to decide if the robotic mower is too close or too far from the boundary wire. If the boundary sensor indicates it is within the specified distance to the boundary wire, in block  702  the vehicle control unit commands the left and right wheel traction drive motors to continue rotating straight ahead. If the boundary sensor indicates it is too close or too far from the boundary wire, in block  703  the vehicle control unit determines if the error or deviation from the specified distance has decreased, by comparing the boundary sensor input to one or more previous boundary sensor inputs, preferably spanning a time period of at least about one second. If the error has not decreased, in block  704  the vehicle control unit commands the left and right wheel motors to turn the vehicle at a larger acute angle (such as 30 degrees) away from or back toward the boundary wire, depending on whether the robotic mower is too close or too far from the boundary wire. If the error has decreased, in block  705 , the vehicle control unit commands the left and right wheel motors to turn the vehicle at a reduced acute angle (such as  4  degrees) away from or back toward the boundary wire, depending on whether the boundary sensor is too close or too far from the boundary wire. 
         [0054]    In an alternative embodiment, the vehicle control unit may command the robotic mower to execute boundary coverage using one or more patterns along the boundary or perimeter wire. This boundary coverage may use a pattern that minimizes turf damage or rutting along the boundary due to repetitive wear from the robotic mower&#39;s traction drive wheels and caster wheels. For example, boundary coverage may use variable traffic patterns such as a zig-zag pattern to shift the wheel tracks each time the robotic mower executes boundary coverage along the boundary or perimeter wire. Other alternatives also may be specified by the robotic mower controller for boundary coverage, including but not limited to sine or square wave patterns along the boundary or perimeter wire. 
         [0055]    In one embodiment, the vehicle control unit may use information received from one or more boundary sensors regarding the distance of the robotic mower to the boundary wire, to alternate the robotic mower&#39;s path between driving toward and away from the boundary wire at a specified angle. For example, as shown in  FIG. 8 , the vehicle control unit may execute boundary coverage beginning in block  800 . In block  802 , the vehicle control unit may set a flag as a function of the distance between the boundary sensor and the boundary wire. For example, the flag may be set at 0 if the boundary sensor indicates it is within a threshold distance to the boundary wire, or 1 if it is further than the threshold distance. In block  804 , the vehicle control unit may specify the yaw angle of the robotic mower in relation to the main boundary wire at either plus 45 degrees or minus 45 degrees, depending on the flag setting. In block  806 , the vehicle control unit may command the left and right wheel motors to move the robotic mower forward at a reduced forward ground speed. In block  808 , the vehicle control unit may determine if the robotic mower is within a minimum distance to the boundary wire. If the robotic mower is within the minimum distance, the vehicle control unit may reset the flag in block  802 . If not, the vehicle control unit may determine if the robotic mower is farther than a maximum distance from the boundary wire. If the robotic mower is further than the maximum distance, the vehicle control unit may reset the flag in block  802 . Otherwise, the vehicle control unit may command the wheel motors to continue rotating forward at the reduced speed, as shown in block  806 . Thus, the vehicle control unit may command the traction motors to toggle back and forth between plus 45 and minus 45 degrees as a function of the robotic mower&#39;s distance to the perimeter wire. 
         [0056]    In one embodiment, the robotic mower&#39;s path along the boundary wire may change or shift each time it executes boundary coverage. The shift ensures that the same turf is not repeatedly contacted and compacted by the robotic mower&#39;s wheels. The shift may occur because the robotic mower will often have a different starting position each time it starts executing boundary coverage. Additionally, a shift may result from changing the boundary coverage pattern by including variables in the vehicle control unit logic such as the minimum and maximum distances used to toggle the desired orientation, or using a different angle other than 45 degrees. 
         [0057]    In one embodiment, the vehicle control unit may vary the distance of the robotic mower&#39;s path when the robotic mower executes home finding to return to the charging station. The vehicle control unit may specify a return path that is offset from the main boundary wire, and varies over a range of available paths between a minimum offset and a maximum offset. By varying the offset from the main boundary wire, the traction drive wheels of the robotic mower will not wear or damage the turf along the wire. The minimum and maximum allowable offset from the main boundary wire may be preselected or constant. Alternatively, the offset may be incremented or reduced each time the robotic mower returns to the charging station. 
         [0058]    In one embodiment, as shown in  FIG. 9 , the vehicle control unit may execute home finding in block  900 . In block  902 , the vehicle control unit may find the main boundary wire using one or more boundary sensors. In block  904 , the vehicle control unit may select a random variable. Alternatively, in block  906  the vehicle control unit may increment a variable from the last execution of the home finding task. In block  908 , the vehicle control unit may determine the desired offset from the boundary wire based on the random or incremented variable. In block  910 , the vehicle control unit may command the wheel motors to rotate at the nominal forward speed, and at a yaw angle needed to maintain the desired offset. In block  912 , the vehicle control unit determines if the inner loop wire is detected by the boundary sensors. If the inner loop wire is not detected, the vehicle control unit may continue commanding the wheel motors to rotate forward as shown in block  910 . If the inner loop wire is detected, the vehicle control unit commands the wheel motors to reduce speed, and sets the yaw angle to orient the robotic mower to enter the charging station. 
         [0059]    In one embodiment, the vehicle control unit memory may record and store the time when an obstacle or boundary wire has been last detected, and may determine the robotic mower is stuck if a prespecified amount of time elapses before the robotic mower encounters an obstacle or boundary wire again. Preferably, an accelerometer or similar device may be used to detect obstacles, and one or more boundary sensors may be used to detected the boundary wire. The timer duration may be prespecified by the operator or as a function of the size of the area to be mowed, obstacle density, vehicle speed and navigation rules. Additionally, the timer duration may be a function of the type of area coverage being executed by the robotic mower. 
         [0060]    In one embodiment, the timer duration may be the product of the expected maximum distance between obstacles or boundaries, and the robotic mower&#39;s expected travel speed. The timer duration may be relatively short during boundary coverage because the vehicle control unit expects to encounter the boundary again after traveling only a short distance. The timer duration for wide area coverage may be determined from the maximum span between opposite boundaries if the robotic mower travels in a straight line. 
         [0061]    In one embodiment, as shown in  FIG. 10 , the vehicle control unit may execute stuck detection in block  1000 . In block  1002 , the vehicle control unit may set a timer based on maximum distance and mower speed. In block  1004 , the vehicle control unit may determine if an obstacle or boundary wire is detected by an accelerometer or boundary sensor. If an obstacle or boundary wire is detected, in block  1006  the vehicle control unit may command the robotic mower to reverse and turn around, and then reset the timer again in block  1002 . If an obstacle or boundary wire is not detected, in block  1008  the vehicle control unit may determine if the timer exceeds a specified maximum time. If the timer does not exceed the specified maximum, the vehicle control unit may resume checking if an obstacle or boundary wire is detected in block  1004 . If the specified maximum time is exceeded, in block  1010  the vehicle control unit may execute a stuck vehicle task to safely move or stop the robotic mower. 
         [0062]    Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.