Patent Publication Number: US-2023149942-A1

Title: Material reduction machine with dynamic startup control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of co-pending U.S. Provisional Patent Application No. 63/279,452, filed Nov. 15, 2021, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     The present invention relates to material reduction machines, for example, stump cutters, brush chippers, and grinders, and more particularly to startup control for rotating a material reduction device, such as a drum or wheel of such material reduction machines. 
     Material reduction machines such as brush chippers, grinders, and stump cutter machines are well known. These machines commonly include a rotating cutter wheel or drum driven by a prime mover (e.g., a gas or diesel engine). Stump cutters (also known as stump chippers or stump grinders) typically include a cutter wheel, which, while rotating, is advanced toward the stump and moved laterally across the face of the stump. Often, the cutter wheel is automatically advanced across the face of the stump in a sweeping motion. The cutter wheel is mounted to one end of a boom which is, in turn, pivotally mounted on a support frame. Hydraulic boom swing cylinders are used to pivot the boom about the pivot point to move the cutter wheel back and forth across the face of the stump to cut it away. Example prior art stump cutters are shown in U.S. Pat. Nos. 5,845,689 and 10,039,239; and U.S. Publication No. 2020/0178482 each owned by Vermeer Manufacturing Company; these documents are each incorporated herein by reference in their entirety and form part of the current disclosure. 
     Brush chippers typically contain sharp knives that cut material such as whole trees and branches into smaller woodchips. Grinders, on the other hand, typically contain hammers which crush aggregate material into smaller pieces through repeated blows. Example prior art brush chippers are shown in U.S. Pat. Nos. 10,350,608; 8,684,291; 7,637,444; 7,546,964; 7,011,258; 6,138,932; 5,692,549; 5,692,548; 5,088,532; and 4,442,877; and U.S. Publication Nos. 2014/0031185 and 2021/0229108 each owned by Vermeer Manufacturing Company; these documents are each incorporated herein by reference in their entirety and form part of the current disclosure. Example grinders are disclosed in U.S. Pat. Nos. 10,350,608; 7,441,719; 7,213,779; 7,077,345; and 6,840,471, each owned by Vermeer Manufacturing Company; these patents are each incorporated herein by reference in their entirety and form part of the current disclosure as well. 
     Material reduction machines may have additional features, for example, stump cutters may have handles. The handles may include features such as sensors configured for determining the presence of the operator. The operator station of a stump cutter machine often includes hydraulic control levers that are operated by the operator and other switches/controls, such as a control for starting and stopping the rotation of the material reduction device. 
     Material reduction machines often include a drive system. The drive system includes a coupling, also referred to as a clutch throughout this disclosure, and power transfer element(s) (such as gear boxes and drive shafts) which are coupled to and driven by the engine. When the coupling is in an engaged state, the drive system is configured to transfer power from the engine through the drive system elements for driving the material reduction device. The coupling also has a disengaged state configured to inhibit transfer of power from the engine to the material reduction device. 
     Many existing material reduction machines initiate clutch engagement when the prime mover is at a low idle speed, thus providing for smooth engagement and start-up of the material reduction device. The present disclosure relates to initiating clutch engagement when the prime mover is at a predetermined idle speed that is above the low idle speed. Engaging the clutch at a predetermined idle speed may be necessary for various reasons, such as avoidance of a critical drive system frequency defined by range of engine speed (a limited range between low and high idle speeds) that causes high vibration and rough engagement. A challenge associated with engaging a clutch at a predetermined prime mover speed, particularly if the speed is relatively high, is the amount of power required to overcome the stationary inertial load of the drive system and material reduction device. The high inertial load may cause the prime mover speed to drop significantly, or in the case of an internal combustion engine, kill the engine, if the clutch were to be continuously engaged as the engine attempts to increase the speed of the material reduction device from stationary to operating speed. Additionally, if the prime mover speed were to cause the drive system to drop below the critical drive system frequency due to a continuously engaged clutch, the drive system would pass through the critical drive system frequency as the prime mover increases speed, causing high vibration, possible machine damage, and poor perception of machine quality. Therefore, the material reduction machine of the present disclosure is configured to initiate start-up of the material reduction device at high prime mover speeds by cyclically engaging the clutch via a controller that is dynamic and automatic in responding and adapting to the load placed on the prime mover to provide a smooth start-up of the material reduction device. 
     SUMMARY 
     In one aspect, the invention provides a material reduction machine including a cutting mechanism and a prime mover coupled with the cutting mechanism by a drive system to drive the cutting mechanism. The drive system has a clutch with an engaged state to transfer power from the prime mover to the cutting mechanism and a disengaged state where power is not transferred from the prime mover to the cutting mechanism. A sensor is operable to sense a machine load parameter. A controller is coupled to the sensor and configured to receive a signal representing the sensed machine load parameter. The controller is operatively coupled to the clutch to control engagement and disengagement of each of a plurality of sequential engagement cycles of the clutch. The controller is configured to utilize a stored first disengagement threshold value of the machine load parameter for stopping a first disengagement cycle of the plurality of sequential disengagement cycles when the sensor signals to the controller that the first disengagement threshold value is realized, and the controller is configured to continue monitoring the sensor signal as machine load increases momentarily after reaching the first disengagement threshold. The controller is configured to determine and adopt a second disengagement threshold value, the second disengagement threshold value being based on an observation of the machine load parameter indicative of maximum load during the continued monitoring after the first disengagement threshold is realized, and further being based on a stored correction factor. The controller is configured to utilize the second disengagement threshold value for disengaging the clutch during a second engagement cycle of the plurality of sequential engagement cycles following the first engagement cycle when the sensor signals to the control that the second disengagement threshold value is realized. 
     In another aspect, the invention provides a material reduction machine including a cutting mechanism, an internal combustion engine, and a drive system to selectively transfer power from the internal combustion engine to the cutting mechanism. The drive system has a clutch. The clutch has an engaged state wherein the clutch transfers power from the prime mover to the cutting mechanism, and a disengaged state where power is not transferred from the prime mover to the cutting mechanism. A sensor is operable to sense a load on the material reduction machine via detection of droop in the operation speed of the internal combustion engine. A controller is coupled to the sensor and configured to receive a signal indicative of the sensed droop in the operation speed of the internal combustion engine, the controller being operatively coupled to the clutch to control power transfer during of each of a plurality of sequential power transfer cycles. The controller is configured to utilize a stored first operation speed trip point for stopping a power transfer cycle of the plurality of sequential power transfer cycles when the sensor signals to the controller that the first operation speed trip point is realized, and the controller is configured to continue monitoring further droop in the operation speed via the sensor signal as machine load increases momentarily after reaching the first operation speed trip point. The controller is configured to determine and adopt a second operation speed trip point, the second operation speed trip point being based on an observation of a minimum operation speed during the continued monitoring after the first operation speed trip point is realized, and further being based on a stored correction factor. The controller is configured to utilize the second operation speed trip point for stopping a second power transfer cycle of the plurality of sequential power transfer cycles following the first power transfer cycle when the sensor signals to the control that the second operation speed trip point is realized. 
     In yet another aspect, the invention provides a method of controlling a material reduction machine including a cutting mechanism and a prime mover coupled with the cutting mechanism to drive the cutting mechanism. The prime mover is operated to drive the cutting mechanism at a no load operation speed. A clutch is engaged to transfer power from the prime mover to the cutting mechanism to start a first engagement cycle. A machine load parameter is sensed with a sensor that reports signals to a controller in control of the engagement and disengagement of the clutch to control stopping and starting of each of a plurality of sequential engagement cycles the prime mover. Disengagement of a first engagement cycle of the plurality of sequential engagement cycles is triggered via the controller in response to the sensor signaling to the controller that a stored first disengagement threshold value is realized. With the controller, monitoring of the machine load parameter sensor signal is continued as machine load increases to a maximum load momentarily after reaching the first disengagement threshold. The controller determines and adopts a second disengagement threshold value based on the value of the machine load parameter at the time of maximum load after the first disengagement threshold is realized, and further based on a stored correction factor. Via the controller, disengagement of a second engagement cycle of the plurality of sequential engagement cycles is triggered following the first engagement cycle in response to the sensor signaling to the controller that the second disengagement threshold value is realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a stump cutter according to one embodiment of the present disclosure, with components hidden for illustration. 
         FIG.  2    is a side view of the stump cutter of  FIG.  1   , with components hidden for illustration. 
         FIG.  3    illustrates a flowchart for an exemplary process carried out by the stump cutter of  FIG.  1   . 
         FIG.  4    is a graph of operating speed vs. time, illustrating sequential clutch engagement cycles carried out by the stump cutter. 
         FIG.  5    is a perspective view of a brush chipper according to one embodiment of the present disclosure. 
         FIG.  6    is a side elevation view of the brush chipper of  FIG.  5   , with components hidden for illustration. 
         FIG.  7    is a perspective view of a grinder according to one embodiment of the present disclosure. 
         FIG.  8    is a side elevation view of the grinder of  FIG.  7   , with components hidden for illustration. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
       FIGS.  1  and  2    illustrate a stump cutter  100 , according to one embodiment. The stump cutter  100  includes a cutting mechanism in the form of a cutter wheel  120  for reducing stumps into smaller pieces. A boom frame  110  supports the cutter wheel  120 . The boom frame  110  may further be supported by a main frame  117  which include tracks or wheels  112  for ground movement of the stump cutter  100 . Mobility may not be desirable in all cases, such as grinder embodiments, and stationary embodiments are also contemplated. The cutter wheel  120  ( FIG.  2   ) includes cutting teeth  124 . Cutting teeth are well known, and any appropriate cutting tooth (whether now known or later developed) may be used to process material into smaller pieces. The cutter wheel  120  is driven by a prime mover  128 , such as an internal combustion engine (e.g., gasoline or diesel) or an alternative power source(s), such as one or more electric motors. Portions of this disclosure refer to the prime mover  128  directly as the engine  128  with the understanding that specific references to the engine are not limiting to the overall scope of the disclosure, which has aspects extending to prime movers other than internal combustion engines. The cutter wheel  120  can be engaged or disengaged with the prime mover  128  by a clutch  116 . 
     The cutter wheel  120  is selectively powered by the engine  128 . A drive system can transfer power from the prime mover  128  to the cutter wheel  120 . The drive system may include the clutch  116 , a first or proximal gearbox  114 , a driveline  115  (driveline  115  may have couplers and joints, such as universal joints), and a second or distal gearbox  113 . Alternative drive components, such as belts, are considered within the scope of the present disclosure. The drive system may include a brake for slowing and stopping rotation of the cutter wheel  120  in certain situations. The clutch  116  includes engaged and disengaged states. When the clutch is in an engaged state, the drive system is configured to transfer power from the engine  128  through the drive system to rotate the cutter wheel  120 . When the clutch  116  is in a disengaged state, the drive system is configured to inhibit transfer of power from the engine  128  to the cutter wheel  120 . 
     Although other types of clutches are also contemplated, the clutch  116  can be an electromagnetic clutch in some constructions. The engaged and disengaged states of the clutch  116  is at least partially controlled by a control system. A controller  170  of the control system may be in direct or indirect control of the clutch  116 , among other components of the stump cutter  100 . The clutch  116  is engaged when an electrical signal is sent to the clutch  116  by the controller  170 . The clutch  116  is disengaged when no electrical signal is sent to the clutch  116  by the controller  170 . Although the clutch  116  may be capable of operating with various amounts of current or at various voltages, this is not the subject of the present disclosure, so it may be assumed that supplied voltage is “on” during clutch engagement and the supplied voltage is “off” during clutch disengagement, and the supplied voltage is not varied. In the case of using pulse width modulation (PMW), the voltage is rapidly turned on and off (example: 200 Hz), therefore the average voltage over time is considered the constant supply voltage. Additionally, the terms “engaged” and “disengaged” as used in connection with a clutch refer to the capacity, or lack of capacity, respectively, of the clutch to transfer a significant amount of torque. Mere random contact of the friction surfaces, in the absence of an engagement signal, is not considered engagement. 
     One of ordinary skill in the art will appreciate that many of the various electrical and mechanical parts discussed herein can be combined together or further separated apart. The controller  170  may include one or more electronic processors and one or more memory devices. The controller  170  may be communicably connected to one or more sensors or other inputs, such as described herein. The electronic processor may be implemented as a programmable microprocessor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGA), a group of processing components, or with other suitable electronic processing components. The memory device (for example, a non-transitory, computer-readable medium) includes one or more devices (for example, RAM, ROM, flash memory, hard disk storage, etc.) for storing data and/or computer code for completing the or facilitating the various processes, methods, layers, and/or modules described herein. The memory device may include database components, object code components, script components, or other types of code and information for supporting the various activities and information structure described in the present application. According to one example, the memory device is communicably connected to the electronic processor and may include computer code for executing one or more processes described herein. The controller  170  may further include an input-output (“I/O”) module. The I/O module may be configured to interface directly interface with one or more devices, such as a power supply, sensors, displays, etc. In one embodiment, the I/O module may utilize general purpose I/O (GPIO) ports, analog inputs/outputs, digital inputs/outputs, and the like. 
     Referring primarily to  FIG.  2   , one or more operator controls  174  may additionally be in data communication with the controller  170 . The operator controls  174  can include, for example, levers, switches, dials, buttons, or any other appropriate controls, whether now existing or later developed. In some embodiments, at least one of the operator controls  174  is not in direct physical communication with the controller  170 , and instead communicates with the controller  170  wirelessly, such as through one or more of near-field (e.g. Bluetooth, Bluetooth Low Energy, LoRA, Near Field Communication (“NFC”), Wi-Fi, Wi-Max, etc.), radio (e.g. RF), or cellular communication technology (e.g. 3G, 4G, 5G, LTE, etc.). In some constructions, the operator control(s)  174  and the controller  170  may be part of a controller area network (CAN) of the stump cutter  100  (e.g., SAE J1939 for heavy duty vehicles). 
     The stump cutter  100  can be set to a variety of idle engine speeds between and including a low idle speed and a high idle speed. The low and high idle speeds can correspond, respectively, to the minimum and maximum throttle settings of the engine  128 . Although the engine speed can be maintained at the desired idle speed when unloaded, the engine speed will droop according to the load applied. Many prior stump cutters have initiated clutch engagement when the engine  128  is at or near low idle speed, thus providing for smoother engagement and start-up of the cutter wheel  120  due to a small speed differential. However, stump cutter  100  of the present disclosure initiates clutch engagement at a predetermined idle engine speed, the “engagement idle,” which may be a relatively high engine speed (e.g., above the low idle speed, and in some constructions, nearer the high idle speed than the low idle speed). The clutch  116  is engaged at engine speeds sufficiently high to avoid the engine speed drooping into a mid-range engine speed (an engine speed or speed range between low and high idle speeds, referred to below as the “critical” speed) that may cause high vibration and rough engagement due to a critical drive system frequency. As described below, the controller  170  of the present disclosure can be programmed in relation to a predetermined minimum allowable engine speed during clutch engagement, the minimum allowable engine speed being set at or above the critical speed. A challenge associated with engaging the clutch  116  at high engine speeds is the amount of power required to overcome the stationary inertial load of the drive system and the cutter wheel  120  since the speed differential is high between the cutter wheel  120  and the engine  128 . The high inertial load may cause the engine speed to drop significantly, or even kill the engine  128 , if the clutch  116  were to be continuously engaged as cutter wheel speed increases from stationary to operating speed. If the engine  128  were to drop below the critical speed due to continuous engagement of the clutch  116 , the drive system will then be forced to pass through the critical drive system frequency as the engine speed and cutter wheel speed increase together—similar to an engagement that starts from a low engine speed below the critical speed. When the drive system is connected such that the cutter wheel  120  imposes a load on the engine  128 , engine operation passing through the critical speed will cause brief but undesirable high vibration, which can lead to possible machine damage, and poor perception of machine quality. As mentioned, the stump cutter  100  of the present disclosure is configured to initiate start-up of the cutter wheel  120  at a predetermined higher engine speed, above and offset from the critical engine speed, by cyclically engaging the clutch  116  via the controller  170  that is dynamic and automatic in responding and adapting to the load placed on the engine  128  to provide a smooth start-up of the cutter wheel  120 . As such, this start-up process avoids driving the cutter wheel  120  with the engine  128  at the critical speed. 
     Although it is possible to size the engine  128  so as to enable start-up of the cutter wheel  120  without a significant amount of droop (i.e., load-induced drop in operating speed), this is generally impractical and/or unreasonable due to the cost and size of such an engine. Additionally, this would lead to a gross oversizing of the engine for most work operations. Thus, while other control operations may be carried out after the cutter wheel  120  has reached operating speed, such as those disclosed in U.S. Pat. No. 5,845,689 and U.S. Publication No. 2020/0178482 related to control of a cutting wheel and boom during cutting, the subject of this disclosure is the start-up control system of the cutter wheel  120  which is configured to perform cyclic engagement and disengagement of the clutch  116 . That is, the clutch  116  engages to transfer power from engine  128  to cutter wheel  120 , followed by the clutch  116  disengaging (no transfer power from engine  128  to cutter wheel  120 ), followed by the clutch  116  engaging again, and so on until the cutter wheel  120  reaches operating speed and is ready to begin a cutting operation. The length of each engagement cycle in the series of engagement cycles may vary as engagement of the clutch  116  is in accordance with a disengagement threshold, or “trigger,” or “trip-point” of a monitored parameter. In some examples, the length of an individual engagement cycle is a fraction of a second, and the complete cutter wheel start-up process (having multiple engagement cycles) may average approximately 2-3 seconds in some constructions. This type of cyclic clutch control allows a smaller-sized engine  128  to be used to initiate rotation of the cutter wheel  120  by powering cutter wheel start-up in bursts so as to keep the engine speed within a predetermined acceptable speed range. Satisfactory start-up of the cutter wheel  120  may otherwise be impossible due to overloading the engine  128 , which would lead to stalling, or a dragging down of the engine  128  out of the acceptable speed range, for example, along with other possible consequences such as inefficient operation and even component damage under certain circumstances. 
     The monitored parameter related to the disengagement threshold can correlate to a load exerted on the engine  128 . In other words, the disengagement threshold controls how much load is allowed on the prime mover  128  due to engagement of clutch  116 . As discussed in further detail below, the disengagement threshold is variable in accordance with certain aspects of the invention to provide a dynamic start-up control that learns according to an iterative learning program executed by the controller  170  of the stump cutter  100 , providing cycle-to-cycle adjustment during the start-up of cutter wheel  120 . The start-up control system operates to meet the objective of maintaining the prime mover  128  within a predetermined range of operation. An internal combustion engine responds naturally to increased load with a reduction, or droop, in operating speed of the engine, given in crankshaft revolutions per minute (RPM) for example, from a predefined (high idle) engine speed setting at which the engine is set to run with no applied cutting load. As described in at least one specific example below, the predetermined range of operation may be defined by a minimum acceptable engine operating speed. The minimum acceptable engine operating speed can be preset and stored within a memory of the controller  170 . The minimum acceptable engine operating speed can be set to maintain operation (avoid stalling) and more particularly to maintain operation within a desired power band of the engine  128 . 
     Aspects of the present disclosure include at least one load sensor for directly or indirectly monitoring the load applied to the prime mover  128 . In the case of the prime mover  128  of the stump cutter  100  being an internal combustion engine, the load sensor can be configured to measure operating speed of the engine  128  rather than directly measuring load in the form of a force or torque. As mentioned above, engine operating speed acts as a surrogate engine load parameter since the operating speed changes in response to load in a predictable manner correlated to and/or indicative of the load as the engine throttle setting remains fixed. Alternatively, or additionally, one or more load sensors can measure operating speed downstream of the prime mover  128  as described further below. 
     In some constructions, the machine load parameter monitored during the start-up control is not a parameter of the prime mover  128  at all, but rather, a parameter of a component connected to, for example driven from, the prime mover  128 . For example, the load sensor can be configured to measure a speed of on one or both of the cutter wheel  120  and the driveline  115  between the prime mover  128  and the cutter wheel  120 . In some constructions, the load sensor is configured to measure load on the driveline  115 . Such a driveline load sensor can be provided as a torque or slip sensor on the clutch  116 . Such a driveline load sensor can alternately be provided as a load sensing (e.g., strain) gauge on a drive shaft, gearbox, or crankshaft. It should be appreciated that various disclosed implementations can be used individually or combined together in various combinations to achieve the objectives of the present disclosure. 
     In the case of an electric motor as the prime mover  128 , the predetermined range of operation for the prime mover may be defined by an acceptable amount of electrical current draw. Thus, the load sensor can take the form of a current sensing circuit or “current sensor.” The dynamic start-up control, as described in further detail below, allows the stump cutter  100  to optimize the clutch engagement cycles to bring cutter wheel  120  up to operating speed, even without any initial input information to the controller  170  regarding the characteristics of the drive system, such as variations or changes to the cutter wheel, cutters, gearbox, driveline, etc. 
     An exemplary sequence for the dynamic start-up control is schematically illustrated in  FIG.  3   , with the understanding that variations thereof are also within the scope of the present disclosure. The steps of the sequence shown in  FIG.  3    are carried out within and by the controller  170  to accomplish the dynamic start-up control. The sequence starts at step  51 , which may occur upon startup of the stump cutter  100  or may be triggered by a particular initialization, e.g., from the operator. At step S 2 , the controller  170  waits for a command to engage the clutch  116 . At step S 3  the controller  170  determines whether or not the clutch  116  can be engaged (e.g., checks safety features such as operator presence, and/or ensures engine speed has reached engagement threshold, etc.). In some constructions, step S 3  is eliminated, or occurs prior to the steps outline in the start-up schematic of  FIG.  3   , and the process flows directly from step S 2  to step S 5 . If incorporated, the determination of step S 3  can be made on the basis of information from the prime mover sensors and operator presence sensors. When conditions to engage the clutch  116  are not met at step S 3 , the clutch  116  will not be engaged, the process may revert to step S 1 . Optionally, at step S 4  an alert may be sent to the operator related to the reason for not engaging the clutch  116 . As described in further detail following the description of the dynamic start-up control, the controller  170  can also detect and keep track of a recent unsuccessful cutter wheel start-up such that step S 3  prevents repeated start-up attempts under conditions where the cutter wheel  120  is prevented from achieving a successful start-up. 
     Engagement of the clutch  116  at step S 5  begins the initial engagement cycle of the cutting mechanism. As mentioned above, some amount of load is inherent during the initial engagement cycle, but it is desirable to keep load on the prime mover  128  within prescribed boundaries. As such, the controller  170  monitors load via a load parameter (e.g., engine speed sensor) at step S 6 . At step S 7 , values of the load parameter are monitored by the controller  170 , periodically or continuously, to determine whether a disengagement threshold value for the load parameter has been reached. In response to the disengagement threshold value being reached, the clutch  116  is disengaged by the controller  170  at step S 8 . The disengagement threshold value for the first engagement cycle can be a stored value accessed by the controller  170 . For example, the disengagement threshold value may be stored in a memory (not shown) of the controller  170 . The initial disengagement threshold value is not representative of the actual minimum engine speed (droop). It is expected that the actual minimum speed of prime mover  128  will continue to drop briefly. This continued speed drop may be due to the lag in response of components after recognition of the initial disengagement threshold, for example, the time it takes for the clutch  116  to disengage. There may also be contributing lag in the reporting from the engine speed sensor and/or within the controller  170 . In any case, the construction of the stump cutter  100  makes it impractical to control the minimum allowable engine speed by using the minimum allowable engine speed as the disengagement point. In practice, step S 7  may be a timed step where the controller  170  is programmed to only monitor for the disengagement threshold for a predefined amount of time (e.g., not more than 3 seconds, not more than 2 seconds, or not more than 1.5 seconds). If the disengagement threshold is not reached within this amount of time, the cutter wheel  120  is deemed to be successfully engaged, and the dynamic start-up control routine of  FIG.  3    is terminated. This also corresponds to the monitoring for full reset condition at step S 12  described further below. 
     After the clutch  116  is disengaged at step S 8 , two subsequent actions take place. First, the controller  170  detects and stores the engine speed as the load parameter value at the time of minimum engine speed at step S 9 . Second, the controller  170  monitors the engine speed to determine whether a recovery condition indicative of reduced load is achieved at step S 10  (e.g., an increase in engine speed). The minimum engine speed occurs after the disengagement threshold is reached (S 8 ) and prior to the recovery. In response to determining that the recovery condition is met, the stump cutter  100  is ready to begin the next engagement cycle on the cutter wheel  120 , however, before beginning the next engagement cycle, the controller  170  is configured to first determine, based on the preceding engagement cycle, how to run a modified next engagement cycle. In particular, the controller  170  calculates a second disengagement threshold (e.g., new disengagement threshold) value at step S 11 . In one embodiment, the minimum engine speed following clutch disengagement at step S 8  is the controlling parameter used in step S 11  to calculate the second disengagement threshold. The second disengagement threshold replaces the initial disengagement threshold. The controller  170 , in carrying out step S 11 , may compare and ascertain a difference between the load parameter value from step S 9  and a stored target value for the load parameter that corresponds to the minimum allowable load parameter value (e.g., according to a manufacturer&#39;s recommendation based on empirical data). In the example of engine speed as the load parameter, this equates to a comparison between a lowest recorded engine speed below the disengagement threshold engine speed and a target value for lowest allowable engine speed. A correction factor can be applied to the calculated difference by the controller  170  in order to determine the second disengagement threshold to be used for the next engagement cycle. The equation may be expressed as n i+1 =n i +k*(nX−nY) where n i  is the current, or in the case of the completion of the first engagement cycle, the original disengagement threshold operating speed, nX is the target value or lowest allowable operating speed, nY is the lowest recorded engine speed during an engagement cycle, k is the correction factor, and n i+1  is the calculated subsequent disengagement threshold operating speed. 
     The correction factor, which may be pre-programmed to the controller  170 , may be 1 or less, for example 0.25 or 0.3 or 0.5. The sign of the difference (of nX−nY) may be carried through the calculation so that, if resulting in a negative result, the subsequent disengagement threshold n i+1  will be lower than the current or initial disengagement threshold n i , although the initial disengagement threshold may be set as a value highly likely to prevent the actual minimum engine speed from exceeding the minimum allowable engine speed. Thus, the first engagement cycle may utilize a disengagement threshold that leaves a positive safety margin with respect to the actual minimum allowable engine speed. The purpose of successive engagement cycles is to gradually bring the cutter wheel  120  up to operating speed. The newly calculated disengagement threshold for the second engagement cycle then brings the actual minimum engine speed during the second engagement cycle closer to the minimum allowable engine speed. As shown in  FIG.  3   , the controller  170  returns to step S 2  in response to calculating the new disengagement threshold so that the stump cutter  100  performs sequential engagement cycles on the cutter wheel  120  as long as the S 10  recovery condition is met. Thus, the engine speed data gathered at the end of the second engagement cycle is used by the controller  170  to calculate a third disengagement threshold in a manner similar to how the second disengagement threshold was calculated on the data from the first engagement cycle, and so on and so forth for as many engagement cycles as are required to get the cutter wheel  120  to an operating speed (the speed required to begin a chipping/cutting operation). As such, the actual minimum engine speed determined will, on a cycle-by-cycle basis, gradually home in on or creep toward the minimum allowable engine speed prescribed for the stump cutter  100  as the controller  170  learns how the stump cutter  100  responds to start-up of cutter wheel  120 . In other words, the safety margin is dynamically reduced by the controller  170  so that the stump cutter  100  increases the speed of cutter wheel  120  up to an operating speed at or near its full capability despite not having manual or operator-controlled variable settings. 
     After a number of engagement cycles, the cutter wheel  120  is operating above the disengagement threshold and nearing or at an operating speed. When this occurs, the controller  170  may end the start-up process of  FIG.  3    and move to a stump cutting process such as those described in U.S. Pat. No. 5,845,689 and/or U.S. Publication No. 2020/0178482, optionally, the controller may return to step S 2 , awaiting a command to begin the start-up process again. When the next start-up process begins, it is preferable to reset the disengagement threshold to the default stored value. Reset of the disengagement threshold is illustrated in step S 12  of  FIG.  3   . The circumstances for the disengagement threshold to be reset to the initial stored disengagement threshold (the disengagement threshold prior to any iterative learning), is illustrated schematically by steps S 12  and S 13 , which if included in the controller program, may obstruct the controller  170  from carrying out step S 11  in the case of a YES response to step S 12 . The full reset condition of step S 12  can be one or any combination of the following non-limiting examples: (1) detection of the clutch  116  engaged for a prescribed length of time as described above (i.e. the disengagement threshold is not reached), or (2) if there is no command to engage the clutch  116  (i.e. step S 2  is NO, the operator has not given a command or has turned off a command to engage the clutch), or (3) if any of the conditions mentioned related to step S 3  cause the clutch  116  to not engage or disengage, or (4) detection of the prime mover  128  being at a no load state for a prescribed time. 
       FIG.  4    illustrates an exemplary plot of operating speed (n) of an internal combustion engine as the prime mover  128  versus time (t). Beginning at time t0, the engine  128  operates at a steady predetermined idle speed n1. In the context of  FIG.  4    and its description, times of interest are labeled sequentially as t1, t2, etc. The operating speed n1 is the operating speed at time t1, the operating speed n2 is the operating speed at time t2, and so forth. This is done for simplicity in the description and comprehension of  FIG.  4   , and it bears noting that this convention results in the first disengagement threshold identified as n2 and the second disengagement threshold identified as n5, although they are sequential disengagement thresholds as per the n i  and n i+1  notation from above. Prior to t1, the engine  128  is not yet loaded by cutter wheel  120 . Once the clutch  116  is engaged at time t1 (e.g., step S 5  above), the drive system and the cutter wheel  120  begin to load the engine  128 . Thus, the first engagement cycle begins at time t1 . The load exhibits as a reduction in engine speed, which can be seen between times t1 and t3. Although shown as linear for simplicity, the engine speed may slow down nonlinearly in other constructions, and the shape of the curve may depend at least in part on the characteristics of the drive system and the rate at which the clutch  116  engages. At time t2, the initial disengagement threshold n2 is reached (step S 7 ) and the clutch  116  is disengaged (step S 8 ). However, due to the mechanics of the stump cutter  100 , the engine speed continues to decrease somewhat up until time t3. At time t3 the minimum engine speed is recorded and saved for the subsequent engagement cycle. The engine speed begins to increase during “recovery” of the engine  128 . The recovery threshold may be represented by n4 in  FIG.  4   . The first engagement cycle completes at time t4. The recovery threshold may be above or below the disengagement threshold. Also, it should be noted that while the chart depicts the engine speed peaking at t4, n4, the recovery threshold used for initiating the next engagement cycle may be lower than the peak engine recovery speed, i.e., the engine speed may actually continue to go up due to delays in the system before the engine speed actually peaks. Initiation of the next engagement cycle may rely on the recovery threshold “leading” the curve slightly in order to minimize the time the engine speed is fully recovered or at high idle and more importantly, minimize the amount of time the clutch  116  is disengaged. It is important to minimize the amount of time the clutch  116  is disengaged to prevent the cutter wheel  120  from slowing too much while the engine  128  is recovering. As will be appreciated, the event (clutch disengagement) at time t2 is directly controlled by the controller  170 . On the other hand, the minimum engine speed at time t3 is not directly controlled by the controller  170 , although it is resultant from the performance of the stump cutter  100  directly following the event at time t2. From time t3 to time t4, the engine speed naturally recovers and increases to a reset speed n4 at or near idle speed n1. 
     Also, after capturing the minimum engine speed n3, the controller  170  determines the difference between the minimum engine speed n3 and the minimum allowable engine speed nX. The correction factor is then applied to the difference to determine the disengagement threshold n5 for the second engagement cycle. The second engagement cycle commences at time t4, and the load again causes droop in engine speed until the second disengagement threshold n5 is reached at time t5. As with the first engagement cycle the engine speed continues to decrease from time t5 to time t6 where the minimum operating speed n6 is observed, similar to that experienced from time t2 to time t3 at the end of the first engagement cycle. Thus, the iterative learning program allows the minimum engine speed n6 following the second disengagement threshold to encroach upon the minimum allowable engine speed nX. From time t6, the engine  128  again recovers, and the controller  170  determines a new disengagement threshold for the next (third) engagement cycle (along the dotted line) based on the difference between nX and n6, and based on the correction factor. These steps repeat continuously, as the controller  170  learns how to set the disengagement threshold to come as close as possible to the predetermined minimum allowable engine speed nX, which is the speed preset to maintain operation within the desired performance range. Eventually, as represented by the dashed line and tX, which may occur several engagement cycles later, the clutch  116  engages, and since the cutter wheel  120  increases speed with each engagement cycle, the engine speed (represented by tY and nY) will not drop to the disengagement threshold nZ and the clutch  116  will be continuously engaged. At this point the stump cutter controller  170  may end the start-up logic (upon sensing parameters mentioned above) and begin the cutting/chipping control logic in preparation for a cutting operation. 
     As one non-limiting example, the following are an exemplary set of numerical parameters for the stump cutter  100  and the operating method thereof in accordance with the above description. In this example, the low and high idle speeds of the engine  128  can be 1200 RPM and 2900 RPM, respectively. The engagement idle speed n1 for the initial clutch engagement can be set as 2900 RPM, in other words equal to the high idle speed. The engagement idle speed n1 for the initial clutch engagement can be more broadly expressed as at least 70 percent, at least 80 percent, or at least 90 percent of the high idle speed. The critical engine speed can be approximately 2100 RPM, and the minimum allowable engine speed nX can be set higher, for example 2550 RPM to give a suitable margin. In order for the controller  170  to keep the engine speed at or above the minimum allowable engine speed nX, the initial disengagement threshold n2 can be 2815 RPM. The initial disengagement threshold n2, which may also be referred to as the initial droop setpoint, can be experimentally determined as the lowest possible value that does not allow the engine  128  to subsequently droop below the minimum allowable engine speed nX. As described above, the controller  170  determines each subsequent disengagement threshold based on the minimum recorded engine speed of the preceding engagement cycle. As the engine speed recovers following disengagement of the clutch  116  (e.g., from time t3 toward time t4), the controller  170  may be programmed with a reset speed or recovery threshold slightly lower than the engagement idle speed n1 as described above to lead the curve and have the subsequent clutch engagements occur at about the time that the engine speed recovers back to the engagement idle speed n1. For an engagement idle speed of 2900 RPM, the recovery threshold to start the next engagement cycle may be set to about 2725 RPM. 
     As mentioned above, step S 3  may include logic in the controller  170  to prevent clutch engagement when the controller  170  determines that an unsuccessful cutter wheel engagement or start-up process has recently occurred (e.g., within a predetermined timeframe prior to the clutch engagement command S 2 ). Unsuccessful cutter wheel engagement refers to a failure to put the cutting mechanism into a predefined running state that provides cutting, for example operating the cutter wheel  120  at a predefined speed and/or operating the cutter wheel  120  in direct relation to the prime mover  128 . Among other reasons, an unsuccessful cutter wheel engagement may be the result of the cutter wheel  120  being lodged in a stump, or a broken driveline, for example. As such, engagement of the clutch  116  to apply torque from the prime mover  128  does not result in a normal increase in rotational speed of the cutter wheel  120 . In order to make the determination that an unsuccessful cutter wheel engagement has recently occurred, the controller  170  may count the number of clutch engagements (e.g., the counter may cycle for each clutch engagement (S 5 ) and/or each clutch disengagement (S 8 )). The controller  170  can be programmed to compare the number of consecutive clutch engagements without a successful start-up (e.g., without reaching step S 12 ) to a stored limit value such that the consecutive clutch engagements are limited to the stored limit value as the maximum. Also, or alternatively, the controller  170  can monitor the speed of the driveline  115  and/or the cutter wheel  120  to determine unsuccessful cutter wheel engagement by speed failing to increase (remaining at zero) or by speed increasing less than a limit value over the course of one or a plurality of clutch engagement cycles. In practice, detecting unsuccessful cutter wheel start-up by speed monitoring may require two or more clutch engagement cycles. Thus, there may be an allowable threshold of time or number of clutch engagement cycles when detecting unsuccessful cutter wheel start-up via speed monitoring. When the controller  170  determines the cutter wheel start-up to be unsuccessful, the rest of the start-up process can be aborted and/or an alert can be triggered by the controller  170  and provided to the operator, for example via an alarm and/or a display notification. 
       FIGS.  5  and  6    illustrate another material reduction machine, in particular a brush chipper  1000 , to which aspects of the present disclosure may be applied. Despite the physical differences between stump cutter  100  and brush chipper  1000 , some of which are detailed below, the brush chipper  1000  may provide dynamic start-up control that follows the preceding description. Thus, the description of the brush cutter  1000  is kept to minimum so as to avoid unnecessary repetition. The brush chipper  1000  primarily differs from stump cutter  100  it that the brush chipper  1000  has a material infeed chute  1010  and material infeed system  1005 . The material infeed system  1005  moves material to the material reduction portion of the brush chipper  1000 . The material reduction portion of the brush chipper  1000  includes a chipping drum  1120 , which may have a plurality of cutters or knives. Similar to the cutter wheel  120  of stump cutter  100 , the chipping drum  1120  rotates at a high-speed during operation. The chipping drum  1120 , and other components of the brush chipper  1000 , are powered by the prime mover  1128 . The input portion of a clutch  1116  is attached to an output shaft from the prime mover  1128 . The clutch  1116  can be optionally engaged and disengaged similar to the clutch  116  of the stump cutter  100  to transfer power. The output portion of the clutch  1116  is connected to the chipping drum  1120  by a drive system, such as belt  1111 . Despite some fundamental constructional differences between the brush chipper  1000  and the stump cutter  100 , the brush chipper  1000  can also be provided with sensors and a controller  1170  according to the description of the stump cutter  100  so that the brush chipper  1000  is configured to provide dynamic start-up control that changes the disengagement points for stopping sequential engagement cycles of the cutting mechanism (e.g., the chipping drum  1120 ) according to maximum load data on a cycle-by-cycle basis. 
       FIGS.  7  and  8    illustrates yet another material reduction machine, in particular a grinder  2000 , to which aspects of the present disclosure may also be applied. Despite the physical differences between the stump cutter  100  and the grinder  2000 , some of which are detailed below, the grinder  2000  may provide dynamic start-up control that follows the preceding description. Thus, the description of the grinder  2000  is kept to a minimum so as to avoid unnecessary repetition. The grinder  2000  primarily differs from the stump cutter  100  it that the grinder  2000  has a material infeed conveyor  2010  and a material infeed system  2005 . The material infeed system  2005  moves material to the material reduction portion of the grinder  2000 . The material reduction portion of the grinder  2000  includes a grinding drum  2120 , which may have a plurality of cutters or hammers. Similar to the cutter wheel  120  of the stump cutter  100 , the grinding drum  2120  rotates at a high-speed during operation. The grinding drum  2120 , and other components of the grinder  2000 , are powered by the prime mover  2128 . The input portion of a clutch  2116  is attached to an output shaft from the prime mover  2128 . The clutch  2116  can be optionally engaged and disengaged similar to the clutch  116  of the stump cutter  100  to transfer power. The output portion of the clutch  2116  is connected to the grinding drum by a drive system, such as a belt  2111 . Despite some fundamental constructional differences between the grinder  2000  and the stump cutter  100 , the grinder  2000  can also be provided with sensors and a controller according to the description of the stump cutter  100  so that the grinder  2000  is configured to provide dynamic start-up control that changes the disengagement points for stopping sequential engagement cycles of the cutting mechanism (e.g., the grinding drum  2120 ) according to maximum load data on a cycle-by-cycle basis. 
     Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects of the invention as described.