Method and apparatus for controlling engine speed of a self-propelled power trowel during high load conditions

A self-propelled concrete finishing trowel has an electronically controlled engine droop control to prevent stalling of the trowel's engine during overload conditions. The engine droop control includes an engine speed sensor that measures operating speed of the engine and a controller that adjusts operation of a hydrostatic drive system of the trowel based on feedback received from the engine speed sensor to reduce the power draw on the engine during overload conditions. The hydrostatic drive system is powered by the engine to rotate one or more finishing blade arrangements, and under normal operating conditions, is driven by a controller to rotate the blade arrangements at an operator desired speed, such as input by a foot pedal. During overloading conditions, the controller overrides the operator input to drive the hydrostatic drive system to match an operating speed supported by the overloaded engine to reduce the power draw on the engine and thereby prevent engine stalling.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to concrete finishing machines and, more particularly, to riding concrete trowels having engine droop control.

2. Discussion of the Related Art

A variety of machines are available for smoothing wet and partially cured concrete. These machines range from simple hand trowels, to walk-behind trowels, to self-propelled riding trowels. Regardless of the mode of operation of such trowels, the powered trowels generally include one or more rotors that rotate relative to the concrete surface. Riding finishing trowels can generally finish large sections of concrete more rapidly and efficiently than manually pushed or guided hand-held or walk behind finishing trowels.

Riding concrete finishing trowels typically include a frame having a cage that generally encloses two, and sometimes three or more, rotor assemblies. Each rotor assembly includes a driven vertical shaft and a plurality of trowel blades mounted on and extending radially outwardly from the bottom end of the driven shaft. The driven shafts of the rotor assemblies are driven to rotate at a commanded speed. The machine is steered by tilting one or more of the rotor assembles side-to-side to move the machine forward or reverse or fore-to-aft to propel the machine to the left or to the right. The pitch or flatness of the blades can also be adjusted to adjust the machine's finishing characteristics.

Trowels traditionally were powered by a gearbox mechanically coupled to an internal combustion engine and were steered manually using a lever assembly coupled to the gearbox assemblies by linkage assemblies. More recently, larger trowels have been introduced that are potentially fatiguing to steer manually. These trowels are steered via electrically or hydraulically powered actuators responsive to operator manipulation of joysticks. Some of the hydraulically steered trowels are also powered hydraulically via a hydrostatic drive system powered by the machine's internal combustion engine. The engine is driven at full throttle whenever the rotors are being driven, and rotor speed is adjusted by proportional control of the hydrostatic drive system. Specifically, a foot pedal or similar input device allows the operator to input a commanded rotational speed for the rotor assemblies. A controller provides command signals to a proportional control valve of the hydrostatic drive system based on the foot pedal position to adjust the output control of a variable output hydraulic pump to rotate the rotor assemblies at the operator-desired rotational speed. Operators typically operate the machine at full rotor speed through the vast majority of the machine's operational cycle.

The frictional load between the finishing blades and the concrete surface will vary continuously with concrete curing time, concrete mix, temperature and other ambient conditions, such as humidity. Therefore, as the concrete conditions change, the load placed on the engine will also change. For instance, the load placed on the engine can be much higher for wetter concrete, especially if the pitch of the finishing blades is not appropriate, e.g., is too steep. As the load on the engine increases, it is not uncommon for the operator to continue to demand maximum or full rotor speed notwithstanding the fact that the power being required of the engine is greater than the engine can provide. As a result, the increased load placed on the engine causes the engine to slow down, resulting in a noticeable reduction in power and rotor speed. An operator's natural response to such a decrease is to decrease the foot pedal further, if possible, to increase the rotor speed. Such an increase in demand will impose still more load on the already-overloaded engine. Whether or not additional power output is demanded, the overloaded engine may continue to slow and, in some cases, stall if the operator does not reduce the demand placed on the engine by letting up on the pedal. Additionally, exposing the engine to overloaded conditions over extended periods of time can reduce the engine life.

Accordingly, there is a need in the art to reduce engine overloading in hydraulically powered rotary trowels.

One proposed solution uses a drive motor pressure monitoring valve that monitors the pressure in a selected drive motor, e.g., the most downstream motor. In this proposed solution, the pressure in the selected drive motor is taken as indication of motor torque and, thus, as an indication of the demand being placed on the engine by the hydrostatic drive system. If the motor torque, as measured by the pressure monitoring valve, exceeds a desired torque, a relief valve is actuated to cut or decrease the input control pressure on a pilot pressure circuit in order to reduce rotor speed and reduce the load on the engine. It has been found that this proposed solution is unduly sensitive to system parameters such as motor efficiency and relief valve setting. The system may “hunt” or continuously and rapidly cycle between full-rotor-speed and reduced speed. Moreover, the proposed solution was found to display undesirable rotor performance during high load conditions, such as rotor stalling or an unacceptable decrease in engine speed.

Another drawback of this proposed solution is that since the relief valve is actuated based on a “threshold pressure”, an increase in applied torque is not possible once the relief valve is actuated. In other words, the pressure in the load circuit is a direct indication of the frictional torque demand on the concrete. Therefore, when the pressure threshold is reached, the available torque applied is at a maximum and additional torque is not available.

SUMMARY OF THE INVENTION

The present invention provides an electronically controlled engine droop control that overcomes the aforementioned drawbacks. The engine droop control is effective in preventing engine stalling by reducing the demand placed on the engine by the hydrostatic drive system of a rotary trowel during high load conditions irrespective of the operator demanded rotor speed. More particularly, the invention includes a controller that monitors engine speed and that reduces the power draw of the hydrostatic drive system when the engine speed drops below a designated threshold. The threshold may, for example, be a pre-selected speed that is relatively close to the maximum rated engine speed. This control decreases the load placed on the engine, thereby enabling continued stall-free operation of the engine. After the engine load lessens, the controller returns operation of the hydrostatic drive system to rotate the finishing blades at the operator desired speed. Hence, the engine droop control of the present invention adjusts the pressure/flow ratio in the hydrostatic drive system to decrease engine power draw during high load conditions and then readjusts the pressure/flow ratio to a ratio that corresponds to an operator-desired blade rotating speed once engine load is lessened to enable increased power draw. The system thus performs an operation that is analogous to that performed by a vehicular automatic transmission that automatically downshifts when engine load exceeds a designated threshold.

In accordance with one aspect of the invention, the present invention provides a method and apparatus for preventing engine stalling in a power trowel during high load conditions.

In accordance with a further aspect of the invention, an engine droop control system includes an engine sensor that monitors the speed of an engine providing power to a hydrostatic drive system of self-propelled power trowel. The control system further includes a controller that controls the hydrostatic drive system to reduce the speed of rotor rotation when the load placed on the engine, as reflected by monitored engine speed, exceeds a predefined threshold.

The present invention may also be embodied in a control method. Accordingly, in another aspect of the present invention, a control method includes driving a hydrostatic drive system to rotate a rotor assembly of a concrete finishing trowel at a commanded speed. The method further includes driving the hydrostatic drive system to rotate the rotor assembly at a slower-than-operator-commanded rotational speed if the speed of the engine powering the hydrostatic drive system falls below a threshold speed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2show a self-propelled riding concrete finishing trowel20equipped with a propulsion and steering system22and two or more rotor assemblies24,26. The propulsion and steering system22drives the rotor assemblies to rotate and also steers machine20by tilting the rotor assemblies24,26of machine20, as described in greater detail below. The rotor assemblies24and26rotate towards the operator, or counterclockwise and clockwise, respectively, to perform a finishing operation. Propulsion and steering system22is controlled by a foot pedal46for inputting a rotor speed command.

Each rotor assembly24,26includes a driven shaft54extending downwardly from a hydraulic motor56and a plurality of circumferentially-spaced blades58supported on the driven shaft54via radial support arms60. Blades58extend radially outwardly from the bottom end of the driven shaft54so as to rest on the concrete surface. During operation, blades58support the entire combined weight of the operator and trowel20on the surface to be finished. Each drive motor56is mounted within frame46so as to be tiltable relative to frame46, such as described in U.S. Publication No. 2010/0254763, the disclosure of which is incorporate herein.

As is typical of riding concrete finishing trowels of this type, trowel20is steered by tilting a portion or all of each of the rotor assemblies24and26so that the rotation of the blades58generates horizontal forces that propel machine20. The steering direction is generally perpendicular to the direction of rotor assembly tilt. Hence, side-to-side and fore-and-aft rotor assembly tilting cause machine20to move forward/reverse and left/right, respectively. As described in U.S. Pat. No. 7,775,740, the disclosure of which is incorporated herein, the most expeditious way to effect the tilting required for steering control is by tilting the entire rotor assemblies24and26, including the respective drive motors56.

Rotor tilting is initiated via the steering command signal generators that comprise joysticks28and30in the illustrated embodiment but that could conceivably take the form of levers or other devices. The joysticks28,30are positioned proximate an area to be occupied by an operator of finishing trowel20. Steering system22may also include a selector (not shown) that can be operated to alter the responsiveness of trowel20to steering input signals associated with movement of joysticks28,30.

Still referring toFIGS. 1-2, as is commonly understood with respect to riding finishing trowels, operator area32includes a seat34that flanked by a pair of towers36so that an operator is generally centrally positioned between or flanked by the joysticks28,30. The towers36each have an upper flat surface38located adjacent opposite lateral sides of the seat34to provide arm rests for the operator while seated on the chair. Seat34is supported by a generally rigid metallic frame or pedestal40. A deck42for supporting the operator's feet is located in front of pedestal40. A shroud or cage44is attached to frame assembly46and extends in an outward direction relative to operator area32. Preferably, cage44extends at least slightly beyond a rotational footprint associated with operation of rotor assemblies24,26. Cage44prevents or reduces the incidence of unintended impacts or contacts of rotor assemblies24,26with other devices and structures associated with operation of trowel20. Cage42is positioned at the outer perimeter of machine20and extends downwardly from frame46to the vicinity of the surface to be finished. A fuel tank48is disposed adjacent the right side of operator area32, and a water retardant tank50is disposed on the left side of the operator area32. As best shown inFIG. 1, the fuel tank48and the water retardant tank50are mounted on opposite sides of the towers36. Hand grips (not shown) may be attached to the front surfaces of the towers36to assist the operator in climbing into and out of the seat34.

Retractable wheels66may be pivotally supported on the frame to facilitate machine transport to and from the work area. Two sets of wheels66are provided on the front and rear of the machine, respectively. Each wheel set includes two wheels pivotally mounted to the frame46and deployable by a double acting hydraulic cylinder68.

Both rotor assemblies24and26, as well as other powered components of the finishing trowel20, are driven by a power source, such as internal combustion engine62, mounted under operator's seat34, as seen inFIG. 2. The size of engine62will vary with the size of the machine20and the number of rotor assemblies powered by the engine. The illustrated two-rotor60″ machine typically will employ an engine of about 66 hp. The speed of the engine preferably is controlled so that the engine is at full throttle whenever the rotor assembles are being drive to rotate.

As noted above, each rotor assembly24,26is powered by the engine42indirectly through a respective hydraulic drive motor56. In a preferred embodiment, the drive motors56form the outputs of a hydrostatic drive system70. As best seen inFIG. 3, in addition to the aforesaid drive motors56, the hydrostatic drive system70includes a hydrostatic pump72that is powered by engine62to circulate hydraulic fluid to the hydraulic drive motors56through supply lines74and return lines76. Operation of the hydrostatic pump72is governed by a solenoid controlled electro-hydraulic proportional control valve78that controls the output of the pump72based on a proportional current signal received across a communication line80from a controller82. The controller82provides the proportional current signal to the valve78based on a proportional voltage signal received from a foot pedal64via a communication line84. As noted above, the foot pedal64enables the operator to input a commanded rotating speed for the rotor assemblies24,26, but it is understood that other input devices could be used to input a desired speed. An engine speed sensor86monitors the operating speed of the engine62and provides an output signal to the controller82across communication line88. Under certain operating conditions described in detail below, the controller82adjusts operation of the pump72via command signals through control valve78based on the operating speed of the engine.

During normal operation, the seated operator depresses foot pedal64an amount that corresponds to a desired rotor assembly rotational speed. Depressing the foot pedal64causes a voltage signal to be sent to the controller82across communication line84that is proportional to the degree of foot pedal64depression. Typically, the operator will fully depress the foot pedal64to drive the rotor assemblies at a maximum velocity. The controller82then converts the voltage signal to a proportional current signal that is communicated to a solenoid of the proportional control valve78across communication line80. As known in the art, the magnitude of the current signal dictates the volume of fluid the pump72delivers to the hydraulic drive motors56, which in turn rotate the rotors24,26accordingly. The engine62powers the pump72to supply pressurized hydraulic fluid to the drive motors56.

The blades58rotate against the surface of the concrete at the operator-commanded speed. However, as conditions of the concrete vary, the amount of friction between the blades and the concrete can change. If the amount of friction increases, the torque load on the engine will also increase, decreasing the operating speed of the engine. If the torque load is sufficiently large, the engine could stall. Excessive engine speed reduction is prevented by overriding input to the solenoid of the control valve78if the engine speed falls below a threshold value.

The preferred control technique is illustrated diagrammatically via the flowchart ofFIG. 4. The process90represented by that flowchart begins at block92with the controller82receiving the proportional voltage signal from the foot pedal64. The controller also receives the engine speed signal from sensor86at block94and compares the actual engine speed to a threshold speed in block96. That threshold speed may be a pre-set speed that is a designated amount of, for example, 100 RPM below the maximum rated engine speed. In the illustrated example in which the maximum rated engine operating speed is 2,800 RPM, the threshold may be 2,700 RPM. Alternatively, the threshold speed could be selected from a look-up table based on at least the commanded rotor speed as determined by pedal position and possibly taking one or more other factors into account as well, such as a blade pitch. A look-up table could also take commanded engine speed into account in a machine having a variable engine speed capability. If the monitored engine speed is above the threshold speed, the controller82provides a signal to the valve78to control the hydraulic motors56to drive the rotors22,24to rotate at the commanded speed in block98.

If, on the other hand, the monitored engine speed is below the threshold speed, the controller82provides a current signal to the valve78at block100that is independent of the proportional voltage signal input to the controller82by the operator via the foot pedal64. This “over-ride” signal causes the pump72to deliver a reduced volume of hydraulic fluid to the motors56and thereby drives the motors56to rotate the rotors24,26at a slower speed. Doing so reduces the power draw on the engine62so that the engine does not stall. The process then returns to block92and cycles through blocks92,94,96, and100until the engine speed increases above the threshold. That is, once the frictional load from the concrete surface decreases, the blades58will begin to rotate faster. The reduction in frictional load can occur because of a number of factors, such as a change in concrete conditions or a change in blade pitch. In any event, when the engine speed increases above the threshold, the controller82will return operation of the control valve78based on the operator input to the foot pedal64. The over-ride input to the control valve78thus reduces the power draw on the engine but does not reduce the power supplied to the engine. This enables the engine to accelerate automatically when the frictional load on the engine is decreased.

The effects of the above-described droop control are illustrated graphically by the curves120,122,124,126, inFIG. 5. Curves120and122plot engine speed and rotor speed, respectively, versus time in a trowel constructed as discussed in conjunction withFIGS. 1 and 2but lacking the droop control capabilities discussed above in connection withFIGS. 3 and 4. Both curves show the engine and rotors operating at full speed under conditions in which the load imposed on the engine by the rotors start to overload the engine, resulting in reduction in both engine speed and rotor speed at points128and130, respectively. Engine and rotor speed thereafter both fall dramatically, resulting in complete engine stall at point132.

Curves124and126show the response of the same machine under the same operating conditions in which the droop control technique discussed above in connection withFIGS. 3 and 4is implemented. At point134on curve124, the engine speed drops below the threshold speed which, in the illustrated embodiment in which the engine's maximum rated speed is 2,800 RPM, is 2,700 RPM. The rotors rotate at about 150 RPM at this time. The controller82then overrides the operator command signal and to decrease the output of proportional control valve. Engine speed immediately rebounds to the threshold speed. From points136to point138on curve126, the controller82controls the proportional control valve to continue to reduce rotor speed, indicating that further torque reduction is needed to keep the engine speed from falling beneath the threshold. From points138to140on curve126, further rotor speed reduction is unnecessary to maintain engine speed operation at the threshold reduce speed. That rotor speed is approximately 110 RPM in the illustrated example, but might vary significantly depending upon the actual operating conditions of the trial. The controls signal to the proportional control valve78thereafter remains at this reduced level until point140, when rotor speed begins to increase due to improved operating conditions. At some point (not shown in these curves), operating conditions may improve to the point to which the above-described droop control is no longer necessary, at which time rotor speed and engine speed will both be in the regions illustrated to the left of the points134and136in curves124and126.

The self-propelled concrete finishing trowel described above and shown inFIGS. 1-2represents one exemplary apparatus that can benefit from the present invention. In this regard, it is understood that the present invention may be used with other types of ride-on trowels and even walk behind self-propelled trowels. Moreover, it is contemplated that conventional self-propelled trowels can be retrofitted to include the engine load management system of the present invention. Further, it will be appreciated that, while the engine droop control system of the present invention reduces the flow of hydraulic fluid to the hydraulic motors during engine overload conditions, the control system does not prevent pressure from increasing to address an increasing torque.

It is appreciated that many changes and modifications could be made to the invention without departing from the spirit thereof. Some of these changes, such as its applicability to riding concrete finishing trowels having other than two rotors and even to other self-propelled powered finishing trowels, are discussed above. Other changes will become apparent from the appended claims. It is intended that all such changes and/or modifications be incorporated in the appending claims.