Patent Application
20120163914
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.
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.
These and other aspects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof. It is hereby disclosed that the invention include all such modifications.
Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:
Each rotor assembly 24, 26 includes a driven shaft 54 extending downwardly from a hydraulic motor 56 and a plurality of circumferentially-spaced blades 58 supported on the driven shaft 54 via radial support arms 60. Blades 58 extend radially outwardly from the bottom end of the driven shaft 54 so as to rest on the concrete surface. During operation, blades 58 support the entire combined weight of the operator and trowel 20 on the surface to be finished. Each drive motor 56 is mounted within frame 46 so as to be tiltable relative to frame 46, 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, trowel 20 is steered by tilting a portion or all of each of the rotor assemblies 24 and 26 so that the rotation of the blades 58 generates horizontal forces that propel machine 20. 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 machine 20 to 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 assemblies 24 and 26, including the respective drive motors 56.
Rotor tilting is initiated via the steering command signal generators that comprise joysticks 28 and 30 in the illustrated embodiment but that could conceivably take the form of levers or other devices. The joysticks 28, 30 are positioned proximate an area to be occupied by an operator of finishing trowel 20. Steering system 22 may also include a selector (not shown) that can be operated to alter the responsiveness of trowel 20 to steering input signals associated with movement of joysticks 28, 30.
Still referring to
Retractable wheels 66 may be pivotally supported on the frame to facilitate machine transport to and from the work area. Two sets of wheels 66 are provided on the front and rear of the machine, respectively. Each wheel set includes two wheels pivotally mounted to the frame 46 and deployable by a double acting hydraulic cylinder 68.
Both rotor assemblies 24 and 26, as well as other powered components of the finishing trowel 20, are driven by a power source, such as internal combustion engine 62, mounted under operator's seat 34, as seen in
As noted above, each rotor assembly 24, 26 is powered by the engine 42 indirectly through a respective hydraulic drive motor 56. In a preferred embodiment, the drive motors 56 form the outputs of a hydrostatic drive system 70. As best seen in
During normal operation, the seated operator depresses foot pedal 64 an amount that corresponds to a desired rotor assembly rotational speed. Depressing the foot pedal 64 causes a voltage signal to be sent to the controller 82 across communication line 84 that is proportional to the degree of foot pedal 64 depression. Typically, the operator will fully depress the foot pedal 64 to drive the rotor assemblies at a maximum velocity. The controller 82 then converts the voltage signal to a proportional current signal that is communicated to a solenoid of the proportional control valve 78 across communication line 80. As known in the art, the magnitude of the current signal dictates the volume of fluid the pump 72 delivers to the hydraulic drive motors 56, which in turn rotate the rotors 24, 26 accordingly. The engine 62 powers the pump 72 to supply pressurized hydraulic fluid to the drive motors 56.
The blades 58 rotate 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 valve 78 if the engine speed falls below a threshold value.
The preferred control technique is illustrated diagrammatically via the flowchart of
If, on the other hand, the monitored engine speed is below the threshold speed, the controller 82 provides a current signal to the valve 78 at block 100 that is independent of the proportional voltage signal input to the controller 82 by the operator via the foot pedal 64. This “over-ride” signal causes the pump 72 to deliver a reduced volume of hydraulic fluid to the motors 56 and thereby drives the motors 56 to rotate the rotors 24, 26 at a slower speed. Doing so reduces the power draw on the engine 62 so that the engine does not stall. The process then returns to block 92 and cycles through blocks 92, 94, 96, and 100 until the engine speed increases above the threshold. That is, once the frictional load from the concrete surface decreases, the blades 58 will 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 controller 82 will return operation of the control valve 78 based on the operator input to the foot pedal 64. The over-ride input to the control valve 78 thus 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 curves 120, 122, 124,126, in
Curves 124 and 126 show the response of the same machine under the same operating conditions in which the droop control technique discussed above in connection with
The self-propelled concrete finishing trowel described above and shown in
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.
