Abstract:
An electric mower having constant speed control. The mower is powered by a battery and a controller connected to the electric motor. The controller monitors mower operation and ensures that a constant speed is maintained while mowing and transporting even on includes and declines.

Description:
This is a division of U.S. patent application Ser. No. 09/008,188, filed Jan. 16, 1998, now U.S. Pat. No. 6,109,009. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to electrically powered mowers and relates more particularly to such a mower with the ability to maintain a constant speed on inclines and declines. 
     2. Background of the Invention 
     Mowing golf courses requires a precise mower designed is and engineered to ensure a closely cropped and consistently cut green. In addition, because of the time required to grow a mature green and the cost associated with starting and maintaining a golf course, it is extremely important that the greens mower operate properly and not damage the green. 
     Golf course greens maintenance equipment traditionally has utilized internal combustion engines. A number of greens mowers are known in the art. Some are discussed in U.S. Pat. Nos. 3,429,110, and 4,021,996. Riding greens mowers with multiple or gang cutting units are the subject of U.S. Pat. Nos. 3,511,033, 3,668,844, 4,866,917, and 5,042,236. The mowers discussed in these patents all rely on an internal combustion engine as a source of power, and a complex drive mechanism or hydrostatic system for supplying power to the ground engaging wheels and the reel mowing units. However, there are a number of problems associated with the use of a golf course vehicle incorporating an internal combustion engine. First, fuel or hydraulic fluid can leak from the vehicle onto the green and damage the grass. Furthermore, it is inconvenient to service internal combustion engines, and it is inconvenient and hazardous to obtain and store the necessary fuel. Finally, internal combustion engines are a source of both noise and air pollution, and many U.S. cities have recently enacted noise and air pollution prevention statutes that severely limit the time of day and the duration that internal combustion golf course vehicles may be operated. 
     Taking into consideration the above noted risks and concerns associated with internal combustion engines, battery-powered mowers have become a viable alternative to conventional internal combustion powered greens mowers. Historically, most electric vehicles have utilized series motor designs because of their ability to produce very high levels of torque at low speeds. The current electric greens mowers, however, have a distinct disadvantage relative to internal combustion engine mowers: since they operate by electric motor, their speed varies is on inclines and declines that are greater than about 5 degrees. Such a variation in speed is undesirable because the cutting reels are typically operating at a constant speed. A reel mower is designed to have a certain frequency of clip, wherein a reel blade passes across the bedknife at a certain rate as the machine moves forward. For example, if the height of cut is 0.250 inches, a reel blade will pass the bedknife every 0.250 inches of forward travel. Therefore, since riding Greens Mowers typically have fixed reel speed, any increase in ground speed will cause the machine to move forward farther than what is optimal and cause an uneven, wavy cut that is called marcelling. 
     Two of the main goals of any type of riding greens mower, whether electrically powered or otherwise, are maintaining the desired cutting height and sustaining an even quality of cut provided by the reels. The ability of the cutting reels to maintain the desired quality of cut depends upon the consistency of the ground speed of the mower. Those skilled in the art will appreciate that variations in terrain and turf conditions affect the ground speed and the quality of cut of an electric greens mower such as disclosed in U.S. Pat. No. 5,406,777. 
     Internal combustion or hydraulically powered greens mowers incorporate dynamic braking to ensure that the mower maintains a constant ground speed. Dynamic braking is a technique in which retarding force is supplied by the same device that originally was the driving motor. Such braking occurs in an hydraulic system when the operator releases the acceleration pedal. The hydraulic circuit builds pressure internally, creating the braking action, and the motor turns relative to the speed of the hydraulic pump and not faster. The engine, therefore, acts as a brake. 
     In an electrically powered greens mower, however, dynamic braking, as previously described, is not possible. Rather, when the operator releases the pedal, the voltage applied to the motor drops and the motor enters into a free wheel mode, where the revolution of the electric motor is not inhibited as in the hydraulic motor. As long as the electric mower is coasting on a decline, the electric motor will continue to increase speed. When the mower reaches level ground or an incline, the electric motor will begin to slow gradually until it stops, thus stopping the mower. When operating a mower using this sort of electric motor, the operator must constantly monitor and adjust the ond speed. Those skilled in the art will recognize that the varying speed of the mower is a detraction from using the electrically powered greens maintenance devices of the prior art since the relationship between ground speed and reel speed is not always constant, and marcelling will often result. 
     Accordingly, there is a need for an electric greens mower that includes a constant speed control mechanism that will prevent the electric motor from entering a free wheel mode, minimize marcelling, and improve the quality of the cut provided by the electric mower. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an electrically driven degrees mower having a novel traction speed control mechanism. 
     The present invention provides a traction speed control mechanism for an electrically powered mower to allow the mower to travel at a fixed speed over varying turf conditions. The mechanism includes a separately excited shunt, motor, a feedback loop system from the motor, and a controller to electrically adjust the field and armature current to regulate the speed of the mower. This feature allows the mower to run at a near constant speed and to provide a high quality cut, and it does not require the operator to manually regulate the speed. 
     These and other advantages and features which characterize the present invention are pointed out with particularity in the following detailed description of the preferred embodiment, the drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
     FIG. 1 is a general drawing of an electric greens mower incorporating the traction speed control system of the present invention; 
     FIG. 2 is a schematic of the electric circuitry of the greens mower in FIG. 1 incorporating the traction speed control system of the present invention; 
     FIG. 3 is a flow chart showing the different modes of operation for the electric greens mower incorporating the traction speed control system of the present invention; and 
     FIG. 4 is a flow chart showing the interfaces between the controller and the electric greens mower incorporating the traction speed control system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the figures, FIG. 1 is a general illustration of a mower  10  incorporating the traction speed control system of the present invention. Although the invention is described with respect to the preferred embodiment, those skilled in the art will recognize that other versions of the mower  10  are possible and that the invention is not limited to any specific embodiment 
     In the embodiment shown in FIG. 1, the mower  10  includes three wheels  20 ,  21 , and  22 . Two front drive wheels  20  and  21  are powered by a drive motor  30 . The rear wheel  22  is positioned behind and between the two front drive wheels  20  and  21  and is pivotable to steer the mower  10 . The cutting units  60 ,  61 , and  62  are ideally reel cutting units. The present invention, however, could also be used with rotary cutting units or other sorts of cutting units while still remaining within the spirit and scope of the invention. In FIG. 1, the cutting units  60 ,  61 , and  62  are positioned ahead of each of the wheels  20 ,  21 , and  22  respectively. Of course other positions are possible. The cutting units  60 ,  61 , and  62  are optionally mounted on lift arms  63 ,  64 , and  65 . The operator selectively raises and lowers the lift arms  63 ,  64 , and  65  depending on which cutting unit the operator wishes to use or to service. 
     The electric drive motor  30  drives the mower  10 , is powered by a battery set  40 , and is controlled by a controller  100  (not shown). 
     Instead of a battery set  40 , a generator could also be used, or any other electrical power supply. The motor is preferably a separately excited DC shunt motor, and the controller  100  (not shown) is preferably a separately excited (SX) Transistorized Motor Controller made by General Electric (GE), and disclosed in the March 1997 Electric Vehicle Motors and Controls Installation and Operation Manual that is hereby incorporated by reference. Of course, other types of motors, including but not limited to switched reluctance motors, brushless DC permanent magnet motors, stepper motors, and AC induction motors, and compatible controllers could be used while still remaining within the spirit and scope of the invention. 
     The operator&#39;s seat  95  is positioned above the front axle  32 . In one preferred embodiment, the traction speed control system  90  including controller  100  (not shown) is centrally located in an enclosure  110  underneath the seat  95  and above the front axle  32 . The traction speed control system  90  could be located almost anywhere on the mower  10 , but, in the preferred embodiment, it is placed underneath the seat  95  for easy access during servicing. The enclosure  110  is for mounting convenience and protection from the environment. 
     As described above, the controller  100  (not shown) is powered by battery set  40 . In the preferred embodiment, the traction speed control system  90  operates on a 48 volt DC battery system. (See FIG. 2) Usually eight batteries are in the battery set  40 , although a different number can be used depending on the desired operating voltage, the range of the battery set  40 , and the size of the mower  10 . Although lead-acid batteries are preferred, other types of batteries can be used as well, including, nickel cadmium, nickel metal hydride, lithium-ion, zinc air, iron-sulfur, etc. 
     Operation of the electric mower  10  using the traction speed control system  90  will now be discussed in detail with respect to FIGS. 2-4. 
     FIG. 2 shows a traction speed control system  90  (as shown in FIG. 1) including a controller  100  connected to the battery set  40 , which provides 48 volts to the controller  100 . When the operator turns the key in the ignition (not shown), an interlock system checks various signals to determine if certain preconditions to operate have been met. For example, the interlock system checks whether the operator is in the seal  95 , whether the mowers are deactivated (a master mow switch is not active), and whether the brake is activated. As shown in FIG. 3, these preconditions to operation include the condition  200  that the brake must be on, the condition  202  that accelerator pedal must be in neutral, the condition  204  that an operator must be in the seat, and the condition  206  that the master mow switch must be off. It is only after all of these preconditions have been met that the interlock system is active and power is sent to controller  100  for power-up in state  208 . Of course, other preconditions to operation that have not been specified could be programmed while still remaining within the spirit and scope of the invention. 
     Returning to FIG. 2, when the operator turns the key in the ignition, a signal on line  115  is sent to the controller  100 . This signal is also sent to switch  125 , which is a safety check to ensure that the operator&#39;s foot is not on the acceleration pedal. If the operator&#39;s foot is on the acceleration pedal, a signal on line  127  is sent to the controller  100  which issues a warning notification to the operator. The notification can be in the form of an LED or an audible alarm or any other suitable notification means. The signal on line  115  is also sent to relay  130 . Once relay  130  receives power, the controller  100  can energize the relay coil  130 A which closes the relay contacts  130 B. Preferably, the power stage capacitors (not shown) internal to the controller  100  are pre-charged through a current limiting means in order to reduce arcing at the relay contacts  130 B and eliminate other undesirable effects of not pre-charging. The relay contacts  130 B provide the power to the drive stage of the controller  100 . 
     When the controller  100  receives the signal on lines  108  and  120  and power has been applied, the controller  100  ensures that the voltage from the battery set  40  is between 88 percent and 112 percent of nominal battery volts. The controller  100  sends a mow enable signal on line  150  to the mower control panel (not shown). The interlock system (discussed above) is activated and sends a ready signal on line  135  to control shifter  140 . The control shifter  140  sends signals to the controller  100  indicating in which direction and selection of mow or transport mode of the mower  10 . The mower  10  of the current invention preferably has at least two speeds of operation: a mow speed that is variable up to 4 mph +/−0.5 mph when the mower  10  is mowing and the operator has depressed the accelerator pedal fully, and a transport speed that is variable up to 8-10 mph when the mower  10  is not mowing and the operator has depressed the accelerator pedal fully. The mower  10  also operates in reverse at a mow speed of up to 4 mph +/−0.5 mph when the operator has selected the reverse mode R from the control shifter  140  and has depressed the accelerator pedal fully. Although these speeds and modes have been described with respect to the presently disclosed embodiments, those skilled in the art will recognize that a controller  100  such as the SX Transistorized Motor Controller made by General Electric can be programmed for different operating modes and speeds and is not limited solely to those disclosed herein. For example, the maximum speed in both the mow and the transport mode could be higher or lower, or different restrictions could be placed on the mow mode and the reverse mode. The speeds and restrictions disclosed herein are illustrative only and should not be taken as limiting in any way. 
     If the operator elects to operate the mower at transport speed, i.e., up to 8-10 mph without operation of the reels  60 ,  61 , or  62 , the operator selects the transport T mode from the control shifter and signals on lines  142  and  149  are sent from the control shifter  140  to the controller  100 . The signal on line  149  indicates operation in the forward direction, and the signal on line  142  indicates that the mower should operate in the “Transport” mode. If the operator elects to operate the mower at the “Mow” mode, i.e., up to about 4 mph with or without operating the reels, the controller  100  receives the signal on line  144  but does not receive the signal on line  142 , thus indicating operation at the lower speed. If operating in reverse, the signal on line  144  is sent to the mower  10  to operate in the “Mow” mode, i.e., 4 mph at full accelerator stroke, and the signal on line  146  is sent to controller  100  to operate in “Reverse.” In “Reverse,” the controller ignores the signal sent from the “Transport” mode on line  142 , because in the presently preferred embodiment, the mower  10  will not operate at transport speed (8-10 mph) in reverse. Due to the programmability of the controller  100 , however, this restriction could be eliminated as needed by one skilled in the art. 
     During operation of the mower, the operator has access to a dashboard display  170 . From that display  170 , the operator can determine information relevant to the operation of the mower  10 . Connections  160 ,  162 ,  164 ,  166  and  168  represent the transfer of information from the controller  100  to the dashboard display  170 . Those skilled in the art will recognize that the selection of information available to the operator is a design choice. Such information can include operating speed, operating mode (i.e., forward, reverse, mow, transport), power indication, and power consumption, etc. The accelerator pedal and related accelerator control  180  are connected to the controller  100  via lines  182  and  184 . In the presently disclosed embodiment, the accelerator input voltage signal is present on line  182 , and the accelerator negative voltage signal is present on line  184 . The tachometer  190  provides information relating to the rotational speed of the armature of the motor  30  to the controller  100 . Voltage is supplied to the tachometer  190  through lines  192  and  194 , and the tachometer provides information to the controller along input line  196 . 
     Referring again to FIG. 3, once the operator has powered up the mower  10  in state  208 , the available modes of operation are again described. If the operator chooses the transport mode, state  212 , the following options exist. The mower can move forward (state  216 ) at a maximum of 10 mph with the cutting units disabled (state  218 ), or the mower can move in reverse (state  220 ) at 4 mph +/−0.5 mph, with the cutting units disabled (state  218 ). If the operator elects to move in mow mode (state  214 ), a different set of options are available. The mower can move forward at 4+/−0.5 mph with the cutting units enabled (state  222 ), it can move forward at 4+/−0.5 mph without the cutting units enabled (state  224 ), or it can move in reverse at 4+\−0.5 mph with the cutting units disabled (state  226 ). Under each operational circumstance, the controller  100  will sense the conditions and automatically adjust operation of the motor  30  so that it is in electrical balance and maintains a constant speed +\=0.5 mph. The +/−0.5 mph limitation is related to design choices for various parts of the preferred embodiment only, however, and one skilled in the art could set other limits. 
     Referring now to FIG. 4, a flow chart showing the interfaces between the controller  100  and the mower  10  is shown. As discussed above, in order for the controller to be operational, certain preconditions must be met. In FIG. 4, these preconditions are represented as a key that is turned in the key switch  300 , and a power source  302  that provides power to the interlock system  304 . As previously discussed and shown in FIG. 3, the interlock system checks whether the parking brake is on  200 , the accelerator pedal is in neutral  202 , the operator is in the seat  204 , and the master mow switch is off  206 . In FIG. 4, once the interlock system  304  is activated, a signal is sent on line  115  (shown in FIG. 2) that activates the controller  100  as shown at  308 . Once the controller  100  is operational, it receives control inputs from a number of sources. For example, if the mower  10  includes a tachometer  190  connected to the controller  100 , the controller  100  can receive information relating to the speed of the rotation of the motor  30 . In the mower  10 , the accelerator pedal is operably connected to the accelerator control  180 . As discussed with respect to FIG. 2, the accelerator control  180  provides information to the controller about the speed of the mower  10  through lines  182  and  184 . This interrelation is shown in FIG. 4 at  312 . The controller  100  also receives information from the operator&#39;s selection of the mow or transport mode  314  and the forward or reverse mode  316 . With this information, the controller  100  can manipulate the field current on lines  406 ,  408  and the armature current on lines  402 ,  404  of the motor  30 , and as discussed below. 
     Referring back to FIG. 2, operation of the traction speed control system  90  (shown in FIG. 1) including the controller  100 , which in this embodiment is preferably the SX transistorized motor controller, will now be discussed. 
     The controller  100  is programmed to control both the armature and field currents independently to normally adjust for maximum efficiencies at certain operating parameters discussed below. In the preferred embodiment, the motor  30  is a separately excited DC shunt motor, since by independently controlling the field and armature currents in a separately excited motor, the best attributes of both a series and a shunt motor can be combined. Therefore, in the preferred embodiment, the field winding is connected separately from the armature and is, therefore, independent of variations in load and armature current. The controller  100  controls the armature current of the motor  30  on lines  402  and  404 . Similarly, the controller  100  controls the field strength of the motor  30  on lines  406  and  408 . After activation of the mow enable signal  150  indicating that all preconditions to operation of the mower  10  have been met, as discussed above, the operator directs operation of the mower  10  through the control shifter  140 . 
     On level ground, the motor  30  is operated as a fixed field shunt motor. In a shunt motor the variation in armature speed from no load to full load does not exceed 10%. In the preferred embodiment, however, when additional torque is required, for example to climb non-level terrain, such as a hill on a golf course, the controller  100  increases the field current to provide a higher level of torque. With the additional torque obtained by increasing the field current, the mower  10  can maintain a steady speed even on an incline. Under these circumstances, the armature to field current ratio of the preferred separately excited motor  30  can be very similar to that of a comparable size series motor. 
     When starting from a stop either in “Transport” or “Mow” mode, the motor  30  of the present invention operates as a fixed field shunt motor. In other words, the controller  100  keeps the field current on lines  406  and  408  constant, and therefore, the torque developed from starting from a stop varies directly with the armature current on lines  402  and  404 . As the load on the motor  30  increases, the motor speed slows down, thus reducing the back EMF which allows the armature current to increase and providing the greater torque needed to drive the mower  10  from a stop. When the mower  10  reaches its designated speed, preferably either “Mow” at about 4 mph or “Transport” at around 8-10 mph, the load on the motor  30  remains constant. Thus the speed of the motor  30  and the armature current  402  and  404  remain constant, and the motor  30  is in electrical balance. 
     If the mower  10  is operating at “Mow” mode and the operator decides to begin mowing operations, lowering the cutting units adds an additional load on the motor  30 . The motor  30  reacts to the additional load by keeping the motor speed, and hence the mower speed, constant. The controller  100  increases the field current on lines  406  and  408  to provide increased torque to accommodate beginning mow operations. 
     When the operator is finished mowing and raises the cutting units, the motor  30  must again adjust for the change in load. The field current remains the same, but the motor speed and the back EMF increase, while the armature current and the torque decrease. Thus, whenever the load changes on the mower  10  of the present invention, either by changing velocity or by adding or removing the cutting units, the speed of the motor  30  changes also, but the controller  100  automatically adjusts the field strength and armature current until the motor is again in electrical balance. 
     If the operator directs the mower  10  to operate in reverse, when accelerating from a stop, the motor  30  behaves as a fixed field motor in the same manner as in the forward direction, discussed above. The field current on lines  406  and  408  is kept constant, and therefore, the torque developed on starting from a stop varies directly with the armature current on lines  402  and  404 . As the load on the motor  30  increases the motor  10  speed slows, reducing the back EMF. The reduced back EMF of the motor allows the armature current to increase, providing the greater torque needed to power the mower  10  from a stop. 
     If the operator is operating at “Transport” speed or at uMow speed in either forward or reverse and wishes to slow down, when the accelerator is released, regenerative braking occurs in the motor  30 . Regenerative braking is a system of dynamic braking in which the motor  30  is used as a generator and returns the energy of the armature and load to the electric system. Regenerative braking initiates a plugging signal by reversing the motor field. Plugging slows the vehicle to a stop when reversing the motor  30  by providing a small amount of retarding torque for deceleration. Once the current reaches a particular current level, the plugging mode transitions back to regenerative braking mode. The controller  100  keeps the motor  30  in regenerative braking mode as long as the motor  30  can maintain a regenerative current limit set in the controller  100 . When the regenerative current drops below the level set in the controller  100 , the regenerative braking mode transitions back to plugging mode. One of the major advantages of regenerative braking is longer motor life due to reduced motor heating. 
     Thus, the motor controller  100  provides for constant speed either when traveling on an incline or a decline. When operating the mower  10  on an incline, the field current on lines  406  and  408  is decreased to give the effect of “overdrive.” This is called field weakening. Independent control of the field current on lines  406  and  408  by the controller  100  provides for infinite adjustments of “overdrive” levels, between the motor base speed and maximum weak field. When operating the mower  10  on a decline, the speed will also be constant. By its nature the shunt motor  30  will try to maintain a constant speed downhill. This characteristic, however, is enhanced because the controller  100  increases the field strength on lines  406  and  408  to the motor  30 . As the armature rotation slows with the increase of current in the field, the motor speed decreases. Thus, no matter what the terrain, the mower  10  of the present invention will maintain a constant speed, thus creating a more even cut and preventing undesired effects such as marcelling. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive manner.