A high performance, multiple rotor, hydraulically driven riding trowel for finishing concrete includes a rigid trowel frame with two or more downwardly-projecting, bladed rotor assemblies that frictionally engage the concrete surface. The rotor assemblies are tilted with double acting hydraulic cylinders to effectuate steering and control. Double acting hydraulic cylinders also control blade pitch. Hydraulic pressure to the rotors is monitored and controlled by an unloader valve system that monitors drive pump pressure with a shuttle valve to derive an unloader pilot signal. A sequence valve responds to the unloader pilot signal to control a pressure valve that bypasses the normal foot control valve in an overpressure situation. The pressure control head signal normally applied to the hydraulic drive motor control heads is modified with a feedback signal to automatically control the associated pump swash plates.

CROSS-REFERENCE TO RELATED APPLICATION 
   This utility patent application is based upon, and claims the filing date of, prior pending provisional application entitled “Hydraulic Riding Trowel with Motor Control Hydraulic Feedback,” Ser. No. 61/009,182, which was filed Dec. 27, 2007. 


   BACKGROUND OF THE INVENTION 
   I. Field of the Invention 
   The present invention relates generally to hydraulically-powered, multiple rotor, riding trowels with various rotor diameters, and hydraulic control circuits used in such trowels. More particularly, the present invention relates to a riding trowel using hydraulic circuitry including an unloader circuit responsive to hydraulic feedback for critically regulating the pump output flow to operate within the engine horsepower envelope. Riding trowels of this general type are classified in United States Patent Class 404, Subclass 112. 
   II. Description of the Prior Art 
   High power, multiple rotor, hydraulic riding trowels for finishing concrete are well recognized by those skilled in the art. Proper finishing insures that desired surface characteristics including appropriate smoothness and flatness are achieved. It is also important that delamination be minimized. High power, hydraulically driven riding trowels are capable of finishing large areas of plastic concrete quickly and efficiently, while insuring high quality surface characteristics. 
   Modern hydraulic power riding trowels comprise two or more bladed rotors that project downwardly and frictionally contact the concrete surface. In advanced machines the rotors are driven hydraulically from hydraulic drive motors pressured by hydraulic pumps that are in turn powered by a separate, internal combustion engine. The riding trowel operator sits on top of the frame and controls trowel movement with a joystick steering system that tilts the rotors for control. The weight of the trowel and the operator is transmitted frictionally to the concrete by the revolving blades or pans. Frictional forces caused by rotor tilting enable the trowel to be steered. 
   Holz, in U.S. Pat. No. 4,046,484 shows a pioneer, twin rotor, self propelled riding trowel. U.S. Pat. No. 3,936,212, also issued to Holz, shows a three rotor riding trowel powered by a single motor. Although the designs depicted in the latter two Holz patents were pioneers in the riding trowel arts, the devices were difficult to steer and control. 
   Prior U.S. Pat. No. 5,108,220 owned by Allen Engineering Corporation, the same assignee as in this case, relates to a manual steering system for riding trowels. Mechanical gearboxes were used for rotor propulsion. 
   Allen Engineering Corporation Pat. No. 5,613,801 issued Mar. 25, 1997 discloses a power riding trowel equipped with twin motors. The latter design employs a separate motor to power each rotor. Steering is accomplished with structure similar to that depicted in U.S. Pat. No. 5,108,220 previously discussed. 
   Older manually operated trowels used hand levers to develop rotor tilting movements for steering. Rotors were driven by internal combustion motors transmitting force through rotor gear boxes. Manually operated systems with gearbox-driven rotors have been largely replaced with hydraulic trowels. For example, U.S. Pat. No. 5,890,833 entitled “Hydraulically controlled riding trowel” issued to Allen Engineering Corporation Apr. 6, 1999 discloses a high performance, hydraulic riding trowel using a joystick system that controls steering, propulsion, and blade pitch. A rigid trowel frame mounts two or more downwardly-projecting, bladed rotor assemblies that frictionally engage the concrete surface. The rotor assemblies are tilted with double acting hydraulic cylinders to effectuate steering and control. Double acting hydraulic cylinders also control blade pitch. The joystick system activates solenoid control valves that energize various hydraulic cylinders that tilt the rotors and alter blade pitch. 
   U.S. Pat. No. 6,089,786 entitled “Dual rotor riding trowel with proportional electro-Hydraulic Steering” issued Jul. 18, 2000 and U.S. Pat. No. 6,053,660 issued Apr. 25, 2000 and entitled “Hydraulically controlled twin rotor riding trowel” disclose joystick-operated, twin rotor riding trowels for finishing concrete. The trowel frame mounts two spaced-apart, downwardly projecting, and bladed rotors that frictionally contact the concrete surface. The rotors are tilted with double acting, hydraulic cylinders for steering and control. Double acting hydraulic cylinders also control blade pitch. A joystick system enables the operator to hand control the trowel with minimal physical exertion. The joystick system directly controls electrical circuitry that outputs proportional control signals to electrically control the steering or tilting cylinders. The hydraulic circuitry comprises a motor driven pump delivering pressure to a flow divider circuit. 
   U.S. Pat. No. 6,048,130 issued Apr. 11, 2000 and entitled “Hydraulically driven, multiple rotor riding trowel” and U.S. Pat. No. 5,816,739 entitled “High performance triple rotor riding trowel” disclose related, triple rotor hydraulic trowels. 
   U.S. Pat. No. 6,106,193 entitled “Hydraulically driven, Multiple Rotor riding trowel issued Aug. 22, 2000 discloses high performance, hydraulic riding trowels for finishing concrete. Separate hydraulic motors revolve each rotor assembly. Associated hydraulic circuitry engenders convenient joystick control. 
   U.S. Pat. No. 6,857,815 entitled “Acoustic impedance matched concrete finishing” issued Feb. 22, 2005 discloses a method for matching the acoustic impedance of concrete treating equipment to the acoustic impedance of the concrete slab being treated. A twin rotor riding trowel is provided with a pair of circular finishing pans that are attached to the conventional rotor blades. The pans are characterized by acoustic impedance approximating the acoustic impedance of plastic concrete, thereby optimizing the energy transferred to the concrete. The matching material comprises ultra high molecular weight polyethylene (UHMWPE) plastic. During troweling, the pans are frictionally revolved over the plastic concrete for finishing the surface without prematurely sealing the uppermost slab surface, to produce a highly stable concrete surface with minimal delamination. 
   U.S. Pat. No. 7,108,449 entitled “Method and apparatus for acoustically matched slip form Concrete Application” issued Sep. 19, 2006 involves the concept of acoustic matching discussed in Allen U.S. Pat. No. 6,857,815 and employs it with slip form pavers. 
   U.S. Pat. No. 7,114,876 entitled “Acoustically matched concrete finishing pans” issued Oct. 3, 2006 to Allen Engineering Corporation discloses improved acoustically matched pans for riding trowels. The pans are provided with means for matching the acoustic impedance of the concrete slab being treated as discussed in Allen U.S. Pat. No. 6,857,815. 
   German Pat. No. G9,418,169.1 entitled “Concrete smoothing machine” issued Jan. 26, 1995 to Betontechnik Shumacher GmbH discloses a riding trowel. 
   U.S. Pat. No. 5,816,740 entitled “Hydraulically controlled steering for power trowel” issued Oct. 6, 1998 to Timothy S. Jaszkowiak discloses dual-acting hydraulic cylinders interconnected to the rotors and the frame for steering. 
   U.S. Pat. No. 6,048,130 entitled “Hydraulically driven, multiple rotor riding trowel” issued Apr. 11, 2000 to Allen Engineering Corporation discloses a hydraulically-propelled multiple rotor riding trowel utilizing hydraulic motors and circuitry. 
   U.S. Pat. No. 2,869,442 entitled “Floating and troweling machine” issued Nov. 29, 1956 to John M. Mincher discloses a floating and troweling machine for finishing plastic floors which is constructed so that it can controlled by an operator seated on the machine. 
   U.S. Pat. No. 4,320,986 entitled “Motor powered rotary trowel” issued Mar. 23, 1982 to Donald R. Morrison discloses a trowel with radially arranged trowel blades which can be adjustably tilted on their support arms in either direction and are mounted on a drive shaft which can be driven in either direction. 
   U.S. Pat. No. 4,676,691 entitled “Dual rotary trowel” issued Jun. 30, 1987 to Donald R. Morrison discloses a concrete troweling machine having two sets of troweling blades with a mechanism for setting the tilt of individual blades in a rotor assembly. 
   U.S. Pat. No. 4,977,928 entitled “Load sensing hydraulic system” issued Dec. 18, 1990 to Caterpillar Inc. discloses a hydraulic load sensing system and more particularly a hydraulic system in which one of the pressure compensated flow control valves is rendered inoperative during certain operating conditions of the system. 
   Barikell located in Australia has two versions of a hydraulic controlled riding trowel. The “MK8-120 HCS” and the “OL-120 HCS Overlapper” are the trowels noted. (http://www.barikell.com.au/) 
   Tremix located in Sweden has a hydraulic controlled riding trowel called the “Pro Rider” in which the machine is controlled by two joysticks that act directly upon the guiding valves. There are two foot pedals, one adjusting the revolutions of the engine, the other opening/closing the valves to the hydraulic engines. (http://www.tremix.com/eng/concrete/prorider.html) 
   An article found on an internet web page entitled “Insider secrets to Hydraulics” reveals how to understand hydraulic load sensing control in control circuits. (http://www.insidersecretstohydraulics.com/hydraulic-load-sensing.html) 
   MBW Inc. whose headquarters are in Slinger, Wis. U.S.A. has a riding trowel called the “MK8 121” in which the machine is controlled by two hydraulic joysticks. 
   Multiquip Inc. whose headquarters are in Carson, Calif. U.S. A. has two riding trowels that are hydraulically controlled and driven. The STX-55Y-6 and the HTX-44K-5 models are detailed in a MQ-WRPT-1797 Rev. H (01-08) brochure entitled “Ride-on Power Trowels”. 
   An article in a January 2005 issue of Concrete Construction Magazine written by Ted Worthington states a riding trowel called the Tarantula. It is manufactured by a company called Full-Track BVBA located in Belgium. (http://www.concreteconstruction.net/industry-news.asp?sectionID=707&articleID=566833) 
   Bosch Rexroth Corporation has a product they manufacture entitled the “Power Valve” and is used to control a variable displacement pump's operating pressure. This item is detailed in a September 1999 brochure RE 95 514/09.99 distributed by Rexroth. 
   Bondioli and Pavesi Inc. has a product they manufacture entitled the “Power limiter control valve” and is used to maintain maximum power from a power source by sensing operating pressure of the hydraulic circuit. This item is detailed in a quick reference hydraulic catalog provided by Bondioli and Pavesi entitled “QH008”. 
   Sauer Danfoss has a product they manufacture entitled the “MCV106A Hydraulic Displacement Control (HDC)” and is uses mechanical feedback to establish closed-loop control of the swashplate angle of various pumps provided by Sauer Danfoss. This control is explained in article BLN-95-8972-3 issued March 1991 by Sauer Danfoss. 
   Notwithstanding numerous attempts at maximizing the speed of troweling, along with the pursuit of high quality concrete finishes, new problems have developed in the art. 
   Speed increases in surface finishing have made it possible for larger quantities of concrete to be placed in a given job environment in a given time. Modem placement speeds exceed the speed at which concrete was placed several years ago. Contractors routinely expect to finish thousands of square feet of surface area after placement. Panning and troweling stages commence when the concrete is still plastic. 
   Concrete undergoes numerous well recognized changes in its physical chemistry between the initial mixing stages and the final hardening stages. For example, as diagrammed in  FIG. 1  below, in the initial mixing stage, high heat is generated followed by rapid cooling. This initial stage lasts about fifteen minutes, and it is critical that the mixture be adequately mixed. During the ensuing dormancy period, which lasts about two to four hours, the concrete mixture is plastic and workable, and high heat is no longer generated. At the beginning of the dormancy period, the plastic concrete is typically confined within a delivery vehicle during transportation to the job site. After transportation, delivery, and placement, various diverse finishing techniques follow. As concrete is laid, it can be struck off for initial shaping. Typically, screeding follows. At this time significant moisture may rise to the surface. 
   The subsequent hardening or hydration stage, which generates significant heat, lasts about two to four hours. The mixture sets, begins to harden, and the slab gains strength. Panning ideally starts at the “initial set” point indicated in  FIG. 1 , which is approximately between the dormancy and cooling stages. Large, circular metal pans are temporarily secured to the trowel rotors for panning. Alternatively, plastic pans, or acoustically matched pans, can be used. As the concrete hardens, pans are removed and blade troweling finishes the job. Often, multiple trowels, equipped with different pans or blades, are employed in stages. 
   After panning, when the concrete has gotten harder, blade troweling follows. Vigorous blade troweling continues through the hardening period. In the following cooling stage, stresses are developed within the slab, and stress relief, typically relieved by sawing, is required. 
   However, in typical construction, as large areas of concrete are poured and finished, wet, freshly poured concrete regions will often border harder regions. Large riding trowels rapidly traverse large areas of fresh concrete surface, and it is not uncommon for their spaced-apart rotors to simultaneously contact surface regions of varying hardness and frictional characteristics. Severe, potentially damaging stresses on the trowel drive train can result. 
   Further, when a trowel enters a plastic region of wet concrete characterized by a high friction, as can happen when panning stages encounter wet concrete too early, the severe power drain significantly slows the internal combustion engine powering the trowel. The same thing can happen when a trowel encounters concrete that is too plastic during blade troweling of a large, curing slab. When the rotors are overloaded, even if momentarily, engine droop can occur, stalling follows, and normal engine output drops. Internal combustion engines are particularly vulnerable to stalling and power drops in such circumstances. With hydraulic trowels, this sudden power drop reduces the hydraulic operating pressure below optimum levels, affecting trowel steering and control. Sometimes the sudden fluctuation in operating pressure, particularly if the engine stalls completely, can result in surface damage to the concrete from irregular rotor movements. 
   As a practical matter, stalling can occur when the required horsepower from the engine in a given situation exceeds the maximum horsepower available. Normally with hydraulic riding trowels it is desirable to maintain drive engine RPM within a relatively limited range at a favorable operating point. Sudden demands placed on the engine by the hydraulic system can place too much demand on the drive engine. Such condition causes reduced engine life, degraded trowel performance, overheating, and a reduction of finish quality. The horsepower required is a function of rpm and rotor torque. To optimize trowel operation, as rotor torque increases, rotor rpm can then be reduced to promote operation within the desired engine horsepower limits. When rotor load conditions occur where maximum rotor torque and maximum rotor rpm are required simultaneously, the corresponding engine horsepower availability may be inadequate. 
   In a typical hydraulic riding trowel an internal combustion engine drives one or more hydraulic pumps. The typical hydrostatic piston pump in a twin-rotor trowel drives two hydraulic rotor motors. A mechanical stroking device, including a mechanical arm that pivots a swash plate, can increase or decrease rotor rpm. Two mechanical arms connected by a common linkage are linked to a foot pedal controlled by an operator. When the foot pedal is depressed, the linkage creates a turning torque to the swash plates on both pumps. Resulting increased pump displacement creates increased flow to turn the rotor motors at an increased rpm. The stroking mechanism forces are dictated by piston pump pressure. As pressure increases, the holding torque needed to maintain position increases. This is a natural condition for a direct, mechanically operated stroking operation for a piston pump swash plate. To maintain swash plate position, and therefore rotor speed, the trowel operator takes corrective action by pushing harder on the foot-pedal. A rider's instinctive action is to further push the trowel foot pedal, which can stall the engine, with the consequences, discussed above. 
   Thus a solution is required to prevent riding trowel internal combustion engines from overloading and over-stressing in response to diverse RPM and torque requirements encountered upon varying concrete surfaces. 
   In using hydraulically driven riding trowels in the field, a problem with internal combustion engine overload was discovered. Severe overloading stresses the hydraulic components. One way to overcome the overloading problem is to increase pressure in the hydraulic system. The latter approach results in two problems however: not enough torque to the rotors, and the machine could not perform at higher engine RPM and torque without stalling the engine. The torque required to turn the rotors is directly proportional to the weight of the machine. By using the operating parameters of the hydraulic riding trowel, the torque needs to finish the concrete can be measured. As the torque needs are less, the frictional forces are less, and can be measured in coefficient of friction values. During the window of finishability, two times of peak load occur, each during the pan and blade operations. At one point during the panning operation the surface is such that the coefficient of friction is larger than usual. At this time the invention is very useful to moderate this condition of heavy loading. Similarly during the blade operation this peak occurs at a point of finishing the concrete. During the process of finishing the concrete there is a typical amount of loading. Only at these moments of peak loading is there a spike in the demand of horsepower. This condition is somewhat unpredictable due to the different mixture content of the concrete and environmental conditions. Only in very large pours with rapid concrete placement can this be observed with any regularity. Most times this is elusive to observe in a small pour. It does occur in spots, however, and this will be very detrimental to efficient work using a smaller powered ride on trowel. Space and weight limitations prevented using higher horsepower engines. There was a need to increase torque and reduce weight so a redesign of the machine was required. A partial solution is to increase the displacement on the rotor motors, which increases torque and reduces RPM. It was concluded that the torque envelope required for proper operation sacrifices rotor RPM and internal combustion drive engine RPM. A solution could not be achieved with the existing system. The goal was to provide a system that could not be burdened by the operator and which would optimize performance levels of torque and rotor RPM without engine overload. 
   A system was designed that would allow the control of flow from the pump to the rotor motors based upon operating pressure. This would control the total horsepower required by the machine. When the set torque limit was obtained, rotor RPM was reduced to stay within the available engine horsepower. In a light load situation, there would be low torque and high RPM. In a heavy load situation, there would be high torque and lower RPM. 
   Thus it is proposed to monitor hydraulic system conditions, and to derive a corrective hydraulic feedback signal. A responsive unloading valve system is proposed to decrease rotor RPM at a maximum preselected torque limit and to increase rotor speed at a minimum predetermined torque limit. Simultaneously, it is important that the internal combustion engine operate within the optimum engine horsepower curve. Engine stalling is reduced, if not avoided altogether, notwithstanding the continually fluctuating surface frictional characteristics as depicted by the hydration curve ( FIG. 1 ) of the concrete regions being traversed by the trowel. 
   SUMMARY OF THE INVENTION 
   This invention provides an improved, high power, hydraulically-driven riding trowel equipped with a hydraulic unloader valve system for controlling the hydraulic pump or pumps driving the rotor drive motors. A hydraulic feedback circuit responsive to sensed pressures facilitates automatic control. 
   In the best mode each rotor has a separate hydraulic drive motor and a corresponding hydraulic pump for supplying operating fluid flow and pressure. An auxiliary pump supplies fluid pressure for accessory operation, including the foot-pedal that controls the rotor hydraulic pumps. The feedback system includes an unloader valve arrangement that senses potential over-pressure conditions in the rotor drive motors. A shuttle valve determines when either of the hydraulic rotor motors is pressured excessively. A sequence valve driven by the shuttle valve controls a diverter valve that dynamically triggers a pressure adjustment. 
   In operation the unloader valve circuit bypasses the normal foot-pedal control to instantly dethrottle the hydraulic drive motors by adjusting the swash plates within the hydraulic drive pumps. Reduced flow is then experienced by the rotor drive motors, and consequently reduced rotor rpm occurs, minimizing surface damage and maintaining optimum drive-engine horsepower. 
   Thus a basic object of our invention is to provide a riding trowel that dynamically controls the hydraulic drive pumps in response to the load conditions being experienced by the torque induced on the rotors. 
   A related object is to moderate the demands of the hydraulic system on the trowel's internal combustion engine. 
   A similar object is to provide a trowel hydraulic controlling system that optimizes operation of the internal combustion engine. 
   More particularly, it is an object of our invention to substantially stabilize the horsepower developed by the internal combustion engine in a hydraulic riding trowel notwithstanding sudden variances and fluctuations in rotor drive motor torque requirements. 
   A related object is to provide a hydraulic control system for riding trowels that helps to maintain the internal combustion drive engine within its intended horsepower and torque operating range. 
   A related object is to control rotor drive motor rpm in reaction to dynamically changing load conditions. 
   Another object is to prevent engine stalling. 
   Yet another object is to minimize fluctuations in trowel operation. 
   It is also an object to prevent or minimize the surface degradation that can result when the trowel encounters widely varying load and friction conditions. 
   These and other objects and advantages of the present invention, along with features of novelty appurtenant thereto, will appear or become apparent in the course of the following descriptive sections. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following drawings, which form a part of the specification and which are to be construed in conjunction therewith, and in which like reference numerals have been employed throughout wherever possible to indicate like parts in the various views: 
       FIG. 1  is a combined tabular and graphical view illustrating known characteristics of concrete from initial mixing to advanced curing, showing the “window of finishability” of concrete as it cures; 
       FIGS. 2 and 3  are front, isometric views of a hydraulically-driven and hydraulically steered, twin-rotor riding trowel incorporating the best mode of the invention; 
       FIG. 4  is a rear isometric view of the trowel of  FIGS. 2 and 3 ; 
       FIG. 5  is an enlarged, fragmentary, isometric view of a four-way trowel rotor and a typical hydraulic drive motor and associated hardware, with portions thereof broken away for clarity or omitted for brevity; 
       FIG. 6  is an enlarged, top plan view showing the rotor structure of  FIG. 5 ; 
       FIG. 7  is an enlarged, fragmentary, isometric view of a two-way trowel rotor showing its hydraulic drive motor and associated hardware, that is similar to  FIG. 5 ; 
       FIGS. 8 and 9  are detailed hydraulic schematic diagrams of the preferred hydraulic circuitry known to us at this time; 
       FIG. 10  is a diagrammatic view illustrating how  FIGS. 8 and 9  should be positioned for viewing; 
       FIG. 11  is a simplified block diagram illustrating basic operation of the hydraulic control circuitry, showing only fundamental components; 
       FIG. 12  is a detailed block diagram of the unloader valve assembly of  FIG. 11 ; 
       FIG. 13  is a diagrammatic view showing the control heads of  FIG. 11 ; 
       FIG. 14  is a theoretical trowel operating graph showing pressure, flow and horsepower relationships associated with the invention; 
       FIG. 15  is a simplified graph indicating actual system performance without the invention installed; 
       FIG. 16  is a simplified graph indicating actual system performance with the invention installed; 
       FIG. 17  is a simplified graph showing unloader operation; and, 
       FIG. 18  is an enlarged portion of  FIG. 1  showing approximate times to begin trowelling with pans and blades. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With primary attention directed now to  FIGS. 1-4  of the accompanying drawings, reference numeral  20  denotes a hydraulic riding trowel equipped with our new hydraulic circuit described hereinafter. The above discussed Allen Engineering Corporation patents are hereby incorporated by reference, as if fully set forth herein, for purposes of disclosure. The hydraulic unloading valve circuitry is discussed in conjunction with  FIGS. 8-13  detailed hereinafter. 
   Referring to  FIG. 1 , the concrete curing graph  21  plots heat against time through the five stages of hydration of freshly placed concrete. Time segment  22  indicates a time period in which troweling is preferably conducted, known as the “window of finishability.” As discussed earlier, troweling ideally begins with panning as known in the art when the concrete is plastic, towards the left portion of segment  22 . Troweling graduates to blading as concrete hardens during the hardening stage, towards the right of segment  22 . However, as concrete hardens, there is no clear demarcation point mandating the transition from pan troweling to blade troweling. Similarly, on a job site, the exact condition of curing concrete contacted by a given trowel during its travel is far from uniform. Therefore a panning trowel will sometimes encounter concrete that should be trowelled with a blade, and blading trowels often contact more plastic regions of concrete that ideally require panning. The transition between regions of different surface frictional characteristics can result in inconsistent trowel movements and operation, sometimes damaging the surface being finished. Furthermore, sudden power increases needed to maintain RPM when the frictional load varies widely and suddenly can stall the internal combustion engine and overload the hydraulic power train. 
   Thus, as explained below, our new system prevents over loading of the internal combustion engine by monitoring the pressure applied to the rotor drive motors. When a maximum pressure set point occurs, a feedback signal is derived, and the pressure applied to pump control heads on the high pressure, hydraulic pump section is varied to prevent stalling of the internal combustion engine. 
   An operator (not shown) comfortably seated within seat assembly  23  ( FIG. 2 ) can operate trowel  20  ( FIG. 2 ) with a pair of easy-to-use joysticks  26 ,  27  respectively disposed at the operator's left and right side. Details for the joystick controls are illustrated profusely in one or more of the above-referenced Allen patents. A foot-operated, hydraulic pilot control valve  30  ( FIGS. 2 ,  3 ) functions as rotor throttle for machine control. Valve  30  is accessible from seat assembly  23  that is located atop the frame assembly  34  ( FIG. 3 ). Engine throttle is regulated by a hand operated lever  25  and controls only the engine RPM. Rotor throttle is only acquired when the operator depresses the foot-pedal  30 . The RPM of the rotors is determined by the amount of pressure the operator applies to the foot-pedal. A pair of spaced-apart rotor assemblies  36  and  38  dynamically coupled to the frame extend downwardly into contact with the concrete surface  40  ( FIGS. 2 ,  4 ) as is well known in the art. Each rotor assembly is independently, pivotally suspended from the trowel  20 . 
   A preferably internal combustion engine assembly has been generally designated by the reference numeral  42  ( FIG. 4 ). Hydraulic riding trowels typically use diesel or gasoline drive engines, but alternate combustible fuels such as natural gas, hydrogen or E-85 blends can be used as well. In the preferred design, engine  42  drives hydraulic pumps for driving the hydraulic circuitry and hydraulic parts discussed hereinafter. Preferably, each rotor assembly is driven by a separate hydraulic motor whose hydraulic pressure is derived from one or more hydraulic pumps driven by the internal combustion engine  42 . The self propelled riding trowel  20  is designed to quickly and reliably finish extremely large areas of concrete surface  40 , while being both driven and steered with hydraulic means. 
   Referring primarily now to  FIGS. 5-7 , suitable hydraulic drive motors  50 ,  51  respectively power rotor assemblies  38  or  36  respectively. The four-way rotor assembly  38  and hydraulic motor  50  are pivotal fore-and-aft and left-to-right as established by twin pivot rods  52 ,  54  ( FIG. 6 ). The two-way rotor assembly  36  ( FIG. 7 ) and hydraulic motor  51  are pivoted by a single pivot rod  56 , which in assembly is oriented parallel with rod  52  ( FIG. 5 ). A plurality of radially spaced-apart blades  60  associated with each rotor are driven by the hydraulic motors  50  and  51 . As is well known, each blade  60  can be revolved about its longitudinal axis via a linkage  62  controlled by conventional blade pitch apparatus. Preferably a circular reinforcement ring  67  ( FIG. 7  ) braces the revolving blades. As best seen in  FIG. 7 , a vertically oriented hydraulic cylinder  70  controls blade pitch on rotor assembly  36 ;  FIG. 5  shows a similar pitch control cylinder  71  on rotor assembly  38 . Tilting for steering and control is effectuated by horizontally disposed hydraulic cylinders. Two rotor tilting cylinders  74  and  75  are used with rotor assembly  38  ( FIGS. 5 ,  6 ), but only one tilting cylinder  78  is required with rotor assembly  36  ( FIG. 7 ). Details of various hydraulic circuits, circuitry interconnections, and control apparatus are disclosed in the above mentioned patents. 
   Trowel  20  includes a unique hydraulic system for controlling dynamically varying friction and load fluctuations encountered in demanding use. The preferred load control circuitry is seen in  FIGS. 8 and 9  and it has been broadly designated by the reference numeral  80 . The circuitry  80  prevents overloads and engine stalling. 
   The internal combustion engine  42  ( FIG. 4 ) has been schematically indicated by the reference numeral  82  ( FIG. 8 ). Engine  82  ( FIG. 8 ) drives primary hydraulic pumps  83  and  84 , a charge pump  85 , and an auxiliary pump  131 . High pressure fluid from pump  83  is delivered via high pressure line  88  to the hydraulic drive motor  50 . Pump  84  drives motor  51  through high pressure line  89 . The motors  50 ,  51  may or may not return case drain fluid to a reservoir tank through lines  92  and  93  respectively. A low pressure output from each motor  50 ,  51  is connected via line  90  through oil cooler  95  and oil line  96  to inlets of pumps  83  and  84 . Both hydraulic rotor drive motors  50 ,  51  ( FIG. 8 ) are protected by pairs of cross over relief valves  100 ,  101  that prevent damage from extreme overpressure. 
   Viewing the left side of  FIG. 8  it is seen that the high pressure rotor-motor drive lines  88 ,  89  are both connected to an unloader pressure signal (i.e., “UPS”) circuit  105  which senses pressure and derives a feedback signal. The “UPS” control circuit  105  is part of an unloader valve assembly  107  ( FIGS. 11 ,  12 ). Assembly  107  includes a “Pressure Control Head” (i.e., “PCH”) circuit  189  explained later and detailed in  FIGS. 9 and 12 . UPS control circuit  105  comprises a manifold  106  preferably made of hardened steel that is subjected to high pressures. Circuit  105  monitors pressure applied to the rotor drive motors  50 ,  51  with a shuttle valve  110  in communication with both high pressure drive lines  88 ,  89  that alternates between them. Valve  110  communicates through a sequence valve  108  via a line  111 . 
   When an overpressure condition is detected on either line  88  or  89  (i.e., when either hydraulic drive motor  50  or  51  is over-pressured), pressure-sequence valve  108  ( FIGS. 8 ,  12 ) is activated. The system checks for an optimum pressure set point. Return line  112  runs back from sequence valve  108  to the reservoir tank  114 . Importantly, a corrective feedback signal is outputted from valve  108  on line  109 . The “unloader pilot signal”, hereinafter designated “UPS”, ultimately provides corrective feedback to moderate rotor RPM and prevent stalling of internal combustion engine  82 . The “PCH Control Section”  189  ( FIG. 9 ) of the unloader valve assembly  107  (i.e.,  FIGS. 11 ,  12 ) responds to the UPS signal appearing on line  109  ( FIGS. 8 ,  9 ,  12 ). PCH Section  189  generates a “Pilot Control Signal” (i.e., PCH signal) that is transmitted along line  130  ( FIGS. 8 ,  9 ,  12 ) to the control heads on high pressure pumps  83 ,  84  as detailed hereinafter. 
   The preferably the internal combustion engine  82  ( FIG. 8 ) also drives an auxiliary pressure pump  131  used for steering (i.e., rotor tilting), rotor blade pitch control, and the rotor foot pedal control that is schematically designated as  166  in  FIG. 9 . Pump  131  outputs on line  128  leading to  FIG. 9 . Charge pump  85  and auxiliary pump  131  ( FIG. 8 ) are supplied with suction oil via filter  124 . 
   Breather tank  116  ( FIG. 8 ) facilitates air release on line  129  from separate pilot control heads  120 ,  121  associated with the pumps  83 ,  84  ( FIG. 8 ). Line  129  is interconnected via lines  118  to pilot control heads  120 ,  121 . Line  117  from breather tank  116  returns to reservoir  114 . The pilot control heads are part of a standard pump. UPS control circuit  105  applies the unloader pilot signal (i.e., “UPS” signal) on line  109  originating on the left side of  FIG. 8  that leads to  FIG. 9 . Line  130  at the top right of  FIG. 8 , a pilot control head line (i.e., hereinafter “PCH” line), drives the pump control heads  120 ,  121  ( FIG. 8 ). Pressure applied to these heads via PCH line  130  normally controls rotor speed by the foot pedal control  166  ( FIG. 9 ). PCH line  130  drops in pressure in response to the PCH circuit diverter valve arrangement discussed below. The pilot control heads  120 ,  121  are normally controlled by the operators' foot-pedal  30  ( FIG. 3 ) that is schematically designated as  166  in  FIG. 9 . Varying pressure applied along PCH line  130  normally established by operator depression of the foot-pedal  30  ( FIG. 1 ) enables the operator to vary rotor RPM. 
   Referring to  FIGS. 8 and 9 , pressure appears on line  128  from auxiliary hydraulic pump  131  that powers steering, pitch and foot-pedal control. Joystick steering control  140  ( FIG. 9 ) controls rotor assembly  36  ( FIG. 7 ) with a left-mounted joystick  26  ( FIGS. 2 ,  9 ). Joystick  26  operates a pair of pressure reducing valves  142  that control the steering cylinder  78  ( FIG. 7 ). The joystick steering control  145  ( FIG. 9 ) uses right side joystick  27  ( FIGS. 2 ,  9 ) to control four pressure reducing control valves  147  to operate the twin steering cylinders  74 ,  75  associated with rotor assembly  38  ( FIGS. 5 ,  6 ). Pitch control cylinders  70 ,  71  are controlled by four-way solenoid valves  151 ,  152 . Lines  155 ,  156  respectively supply steering controls  140 ,  145  which are connected to an equalizer  158  and a flow divider  160  leading to pressure lines  128 . Line  128  connects to line  161  that applies pressure to the foot pedal controller  166 . A pilot valve  167  controlled manually by a foot pedal linkage  168  outputs pressure on line  170 . A foot pedal controller tank return is indicated at line  171 . 
   The UPS line  109  drawn at the top of  FIG. 9  runs to PCH Control  189  that is associated with the unloader valve assembly  107  discussed earlier. PCH Control  189  is activated by, and hydraulically associated with the UPS control circuit  105 . This relationship is indicated by the dashed lines in  FIG. 12  surrounding the unloader valve assembly  107 . In the best mode, trowels made in accordance with the invention have the PCH control  189  mechanically or physically separate from the UPS control circuit  105 . The manifold portion of the PCH control is subjected to relatively lower pressures than manifold  106 , and hence may be made of lower weight aluminum. In retrofit kits for practicing the invention, the manifolds associated with UPS control  105  and PCH control  189  may be combined in one unit. 
   As seen in  FIG. 9 , UPS line  109  inputs to PCH control  189 . The PCH output line  130  extends from PCH circuit  189  ( FIG. 9 ) back to the control heads  120 ,  121  ( FIG. 8 ). UPS line  109  connects to a diverter valve  176  that is coupled to a low pressure adjustment valve  178  that drains to line  201 . Auxiliary pump  131  supplies foot pedal control  166  ( FIG. 9 ) with pressure across relief valve  211  ( FIG. 9 and 11 ) through line  161  into foot pedal control valve  167 . Fluid flow through valve  167  is selected by the operator foot pedal activating linkage  168 . Line  170  outputs fluid from the foot-pedal control valve  167  to PCH circuit  189 . Normally, fluid traveling through foot pedal control valve  167  travels through PCH valve  176  into the PCH line  130 , being delivered to control heads  120 ,  121  for normal control of the pumps  83 ,  84  ( FIG. 8 ). However, when UPS line  109  triggers valve  176 , the normal path of fluid on line  170  directly through valve  176  is interrupted, and fluid from line  170  is diverted to pressure reduction valve  178 . 
   When the UPS signal appears on line  109 , fluid from line  170  is diverted to valve  178 . The fluid diverted from the foot-pedal control valve line  170  is passed by valve  178  to valve  176  and then to PCH line  130  at a reduced pressure. Any pressure above the set reduced pressure of valve  176  is relieved to line  201 . The PCH circuit  189  automatically triggers in response to the optimum pressure set point in circuit  105  previously discussed, reducing the pilot control heads  120 ,  121  pressures automatically without operator intervention to control rotor output RPM. 
   Operation 
   Trowel unloader valve operation is illustrated in the simplified block diagrams of  FIGS. 11-13 . 
   The rotor hydraulic drive motors  50  and  51  are respectively operated by primary pumps  83 ,  84 , with high pressure appearing on lines  88 ,  89 . As seen in  FIG. 11 , the high pressure value is sensed by unloader valve assembly  107 , specifically the UPS control  105 . The UPS control  105 _signals PCH control  189 , varying the PCH line  130  which dynamically controls the pump control heads  120 ,  121 . 
   The foot-pedal assembly  166  in  FIG. 11  receives pressure from line  161 , and outputs variable, user selected pressure on line  170 . The output pressure on line  170  is either applied directly to PCH line  130  by PCH control  189 , or it is reduced in pressure in response to the UPS signal from control  105 . 
   Referring additionally now to  FIGS. 12 , and  13 , the pressured lines  88 ,  89  entering the unloader valve assembly  107  reach the UPS control  105 . Shuttle valve  110  monitors input drive pressure on both hydraulic rotor motors. When either or both rotor motors  50 ,  51  (i.e.,  FIG. 8 ) reach optimum set point pressure, sequence valve  108  responds by outputting a UPS signal on line  109 . The UPS signal reaches normally open flow diverter valve  176  in the PCH circuit within assembly  107 . As long as sensed pressures within lines  88 ,  89  are normal, valve  176  (and thus unloader valve assembly  107 ) provides normal control via lines  118 ,  130  ( FIG. 11 ) to the control heads  120 ,  121  on the hydraulic pumps  83 ,  84 . The operator foot pedal controls rotor speed. However, when the optimum set point pressure condition occurs, the diverter valve  176  ( FIG. 12 ) blocks normal flow by closing its normally open path, and fluid from line  170  is redirected through the normally closed path via adjustment valve  178  and then through valve  176  to PCH line  130 . The pressure on line  130  is reduced immediately. 
   The lowered pressure achieved by valve  178  ( FIGS. 9 ,  12 ) supersedes foot pedal control for adjusting rotor speed. Lowered pressure on lines  118  ( FIG. 13 ) and PCH line  130  causes the control heads  120 ,  121  to forcibly adjust the swash plates within the drive pumps  83 ,  84  to reduce pump flow. Because of the load sensing system shown in  FIGS. 8 ,  9  the operator will not experience foot-pedal kickback. 
   Referring to  FIG. 14 , graph  300  depicts theoretical trowel operating parameters with the invention. Averaged horsepower of the internal combustion engine is plotted against time on line  302 . Lines  304  and  306  respectively designate rotor drive motor pressure and flow. It can be observed that when a pressure surge occurs, as at  307 , a corresponding flow rate drop is observed at  309 . Through the various spikes and variances in the flow rate and pressure parameters, observed horsepower achieved by the internal combustion engine is substantially constant, so engine RPM is substantially constant, and efficiency is promoted while stalling is prevented. 
     FIG. 15  is a typical graph of data collected that indicates the need for an unloader circuit of the type described herein. With no unloader installed, point  360  indicates a spike of approximately 2641 PSI loading the system. This load represents a drag on the rotor rotation and demands more pressure to accommodate the load. As a result of the loading, the engine cannot provide adequate horsepower to sustain the hydraulic demand, as indicated at  364 . This results in a drop in the engine rpm which is shown by the resulting drop in flow to 12 GPM at  365  from normal 15.5 GPM. The opposite rotor suffers the same problem due to the engine rpm drop. All of this was caused by the load from the concrete causing a sharp increase in pressure at  360  which exceeded the available torque of the engine. 
     FIG. 16  is a simplified graph of actual data collected in the field with the invention in use. The purpose of the invention is to provide an automatic hydraulic load sensing system where by the ride on trowel can continue to operate at optimum performance throughout the concrete hardening stage as depicted on the hydration curve ( FIG. 1 ). 
   A load demand is seen at  361  and is caused by excess pressure on the rotor. A low UPS signal at  367  of 660 PSI activates in response to excess pressure at  361 . The UPS signal at  368  is now shown to be 3059 PSI. Now at  369  the system pressure is reduced to 2116 PSI with a resulting rise in the rotor RPM to 109. It is noticed that only a slight drop in horsepower occurs at  370 . The flow however remains steady shown at  371 . The next occurrence of the UPS activity is at  372 . 
     FIG. 17  is a actual graph showing the operation of the invention. The unloader has acted due to loading as shown by the decrease of flow and high pressure at  380 . The unloader is inactive as noted at point  381  due to lowered pressures. Thru the 3 to 7 second cycle there is normal operation as shown through time interval  382 . Light load is being experienced as depicted by the low pressure and high flow at point  383 . A sudden load is obvious at point  384  due to the increase in pressure and lack of unloader reaction. The unloader reaction is seen at point  385 , decreasing flow and high pressure is visible. The pressure has dropped at point  386  and the unloader is reducing control. 
     FIG. 18  is an enlarged view of  FIG. 1  depicting the “hardening” stage of the hydration curve in which the approximate time for operating the riding trowel with pans  326  and as curing continues; the use of finish blades  327  is shown. It is well known that several factors contribute to the exact time panning and finishing are initiated, including local weather conditions (i.e. humidity, temperature, etc.) and mixture content of the concrete. 
   From the foregoing, it will be seen that this invention is one well adapted to obtain all the ends and objects herein set forth, together with other advantages which are inherent to the structure. 
   It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
   As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.