Patent Publication Number: US-2019191618-A1

Title: Agricultural System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 14/593,492, filed Jan. 9, 2015 (Attorney Docket No. 250600-000102USPT), U.S. Provisional Application No. 62/085,334, filed Nov. 28, 2014 (Attorney Docket No. 250600-000101PL01); and U.S. Provisional Application No. 62/076,767, filed Nov. 7, 2014 (Attorney Docket No. 250600-000100PL01), each of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to agricultural planters and, more particularly, to gauge wheel load sensors and down pressure control systems for agricultural planters. 
     BRIEF SUMMARY 
     In accordance with one embodiment, a hydraulic control system for controlling the down force on an agricultural implement comprising a hydraulic cylinder containing a movable ram, a source of pressurized fluid coupled to the hydraulic cylinder on a first side of the ram by a first controllable valve, a fluid sump coupled to the hydraulic cylinder on the first side of the ram by a second controllable valve, and an electrical controller coupled to the valves for opening and closing the valves. The valves are preferably self-latching valves, such as magnetic latching valves, that remain in an open or closed position until moved to the other position in response to a signal from the controller. Alternatively, the valves may be non-latching valves that are spring-biased toward their closed positions. A pair of energy storage devices, such as accumulators, may be coupled to the cylinder on opposite sides of the ram. A pressure transducer is preferably coupled to the cylinder on one side of the ram. A pair of check valves may couple the cylinder to the energy storage device and to the controllable valves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical longitudinal section through a portion of an agricultural planter that includes a gauge wheel and an opener device. 
         FIG. 2  is an enlargement of the left side of  FIG. 1 . 
         FIG. 3  is a bottom perspective of the control portion of the equipment shown in  FIG. 1 . 
         FIG. 4  is an enlarged side elevation of the equipment shown in  FIG. 3 . 
         FIG. 5  is an enlarged top plan view of the equipment shown in  FIG. 3 . 
         FIG. 6  is an enlarged vertical longitudinal section through the equipment shown in  FIG. 3 . 
         FIG. 7  is a plan view of a gauge wheel transducer system for an agricultural planter that includes a gauge wheel and an opener device. 
         FIG. 8  is a side elevation of the transducer system shown in  FIG. 7 . 
         FIG. 9  is a sectional view taken along line A-A in  FIG. 7 . 
         FIG. 10  is a side elevation, partially in section, of the transducer system of  FIGS. 7-9  mounted on a gauge wheel and its supporting structure. 
         FIG. 11  is a perspective view of portions of the devices shown in  FIG. 10 . 
         FIG. 12  is a plan view similar to  FIG. 7  but with portions removed to show the equalizer arm. 
         FIG. 13  is a plan view of a modified transducer system. 
         FIG. 14  is a longitudinal section taken along line  14 - 14  in  FIG. 13 . 
         FIG. 15A  is a side elevation of a modified sensing system for detecting the pressure exerted on a pair of gauge wheels. 
         FIG. 15B  is an end elevation of the system shown in  FIG. 15A . 
         FIG. 16  is a schematic diagram of a hydraulic and electrical control system for controlling a down pressure actuator. 
         FIG. 17  is a schematic diagram of a first modified hydraulic and electrical control system for controlling a down pressure actuator. 
         FIG. 18  is a schematic diagram of a second modified hydraulic and electrical control system for controlling a down pressure actuator. 
         FIG. 19  is a schematic diagram of a third modified hydraulic and electrical control system for controlling a down pressure actuator. 
         FIG. 20  is a perspective view of a planting row unit adapted to be attached to a towing frame. 
         FIG. 21  is an enlarged perspective view of the down-pressure control assembly in the row unit of  FIG. 20 ; 
         FIG. 22  is the same perspective view shown in  FIG. 16 , rotated 90 degrees in a clockwise direction; 
         FIG. 23  is an enlarged side elevation of the control assembly shown in  FIGS. 21 and 22 , from the left side of the assembly as shown in  FIG. 21 . 
         FIG. 24  is a section taken along line  19 - 19  in  FIG. 23 . 
         FIG. 25  is a side elevation of the right side of the control assembly shown in  FIG. 23 . 
         FIG. 26  is a side elevation of the right side of the control assembly shown in  FIG. 25 . 
         FIG. 27  is a section taken along line  22 - 22  in  FIG. 25 . 
         FIG. 28  is a section taken along line  23 - 23  in  FIG. 25 . 
         FIG. 29  is an enlarged exploded perspective of the central portion of the left side of the control assembly shown in  FIG. 21 . 
         FIG. 30  is a horizontal section taken through the two ports shown in  FIG. 29 , with all the parts assembled. 
         FIG. 31  is a vertical section taken through the middle of the control assembly shown in  FIG. 7 , with the rod of the hydraulic cylinder in its fully extended position. 
         FIG. 32  is the same vertical section shown in  FIG. 21 , with the rod of the hydraulic cylinder in an intermediate position. 
         FIG. 33  is the same vertical section shown in  FIG. 21 , with the rod of the hydraulic cylinder in its fully retracted position. 
         FIG. 34A  is a schematic diagram of a hydraulic and electrical control system for use in the device of  FIGS. 20-33  to provide rebound damping. 
         FIG. 34B  is a schematic diagram of a modified hydraulic and electrical control system for use in the device of  FIGS. 20-33  to provide both rebound and compression damping. 
         FIG. 35A  is a schematic diagram of a modified hydraulic and electrical control system for use in the device of  FIGS. 14-33  to provide rebound damping. 
         FIG. 35B  is a schematic diagram of another modified hydraulic and electrical control system for use in the device of  FIGS. 14-33  to provide both rebound and compression damping. 
         FIG. 36  is a waveform diagram illustrating different modes of operation provided by a PWM control system for the hydraulic valves in the system of  FIG. 34B . 
     
    
    
     DETAILED DESCRIPTION 
     An agricultural planter typically includes a number of individual row units, each of which includes its own row cleaner device, row opener device and row closing device. The down pressure is typically controlled separately for each row unit or each of several groups of row units, and is preferably controlled separately for one or more of the individual devices in each row unit, as described in more detail in pending U.S. application Ser. No. 14/146,822 filed Jan. 3, 2014, which is incorporated by reference herein in its entirety. 
       FIGS. 1-6  illustrate an improved gauge wheel load sensor that takes the upward force from a pivoting planter gauge wheel support, such as the pivoting support arms  10  in the row unit equipment shown in  FIGS. 1 and 2 , and translates that force into a fluid pressure in a fluid chamber  11 . The gauge wheel support arms push against an equalizer support  12 , which is connected via a pivot  13  with a rocker/cam  14 . The force on the gauge wheel due to the weight of the row unit and applied down force causes the rocker/cam  14  to pivot around a pivot bolt  15  and push against a hydraulic ram  16 . This force on the ram  16  causes the fluid in the chamber  11  to pressurize. The pressure is proportional to the amount of gauge wheel load. A pressure transducer  18  reads the amount of pressure and sends a signal to a row unit down pressure controller via signal line  19 . This signal allows the planter row unit down pressure to be controlled to a desired level. 
     Depth adjustment is accomplished in the conventional sense by pivoting the assembly around a pivot  20 , and locking a handle  21  into the desired position with a mechanism  22 . With this design it is imperative that that there is no air trapped in the fluid chamber  11 . For this reason the mechanism includes a bleed valve  23 . The process for removal of air is to extend the ram to the maximum extent with calibration/travel limiter plates  24  ( FIG. 4 ) removed. The system is then filled completely with fluid with the bleed valve  23  closed. Then the bleed valve  23  is opened, and the rocker arm  14  is pushed against the ram  16  to move the ram to the exact place where the calibration/travel limit plates  24  allow a calibration plate retaining screw  25  to fit into a hole. This ensures that each assembly is set the same so all the row units of the planter are at the same depth. At this point the bleed valve  23  is closed. With all air removed, the mechanical/fluid system will act as a rigid member against forces in compression. The travel limiter plate  24  keeps a cam pivot weldment  27  from falling down when the planter is lifted off the ground. 
     Standard industry practice is to use a strain gauge to directly measure the planter gauge wheel load. The design shown in  FIGS. 1-6  is an improvement over the state of the art because it allows the sensor to measure only the down force on the gauge wheels. In typical designs using strain gauge type sensors, the mechanical linkage that allows the gauge wheels to oscillate causes the measured wheel force to have substantial noise due to changes in the force being applied. For this reason it can be difficult to determine which parts of the signal correspond to actual changes in down force on the gauge wheels, versus signal changes that are due to movement of components of the gauge wheel support mechanism. The reason for this is that strain gauge sensors will only measure the force that is being applied in a single plane. Because of the linkage and pivot assembly that is used on typical planters, the force being applied to the strain gauge type designs can change based on the depth setting or whether the planter gauge wheels are oscillating over terrain. In this way they will tend to falsely register changes in gauge wheel down force and make it difficult to have a closed loop down pressure response remain consistent. 
     Additionally, the fluid seal of the pressure sensor creates friction in the system which has the effect of damping out high frequency noise. Agricultural fields have very small scale variations in the surface which causes noise to be produced in the typical down force sensor apparatus. By using fluid pressure this invention decouples the sensor from the mechanical linkage and allows the true gauge wheel force to be more accurately measured. Lowering the amount of systematic noise in the gauge wheel load output sensor makes it easier to produce an automatic control system that accurately responds to true changes in the hardness of the soil as opposed to perceived changes in soil hardness due to noise induced on the sensor. 
       FIGS. 7-12  illustrate a modified gauge wheel load sensor that includes an integrated accumulator  125 . The purpose of the accumulator  125  is to damp pressure spikes in the sensor when the planter is operating at low gauge wheel loads. When the forces that the gauge wheel support arms  110  are exerting on the hydraulic ram  117  are near zero, it is more common for the surface of the soil or plant residue to create pressure spikes that are large in relation to the desired system sensor pressure. As the target gauge wheel down force increases, and consequently the pressure in the fluid chamber  111  and the transducer output voltage from sensor  118 , the small spikes of pressure due to variation in the soil surface or plant residue decreases proportionally. 
     In the present system, rather than have a perfectly rigid fluid coupling between the ram  117  and the pressure transducer  118 , as load increases on the ram  117 , the fluid first pushes against an accumulator  122  that is threaded into a side cavity  123  in the same housing that forms the main cavity for the ram  117 , compressing an accumulator spring  126  until the piston  125  rests fully against a shoulder on the interior wall of the accumulator housing  127 , thus limiting the retracting movement of the accumulator piston  125 . At this point, the system becomes perfectly rigid. The amount of motion permitted for the accumulator piston  125  must be very small so that it does not allow the depth of the gauge wheel setting to fluctuate substantially. The piston accumulator (or other energy storage device) allows the amount of high frequency noise in the system to be reduced at low gauge-wheel loads. Ideally an automatic down pressure control system for an agricultural planter should maintain a down pressure that is as low as possible to avoid over compaction of soil around the area of the seed, which can inhibit plant growth. However, the performance of most systems degrades as the gauge wheel load becomes close to zero, because the amount of latent noise produced from variation in the field surface is large in relation to the desired gauge wheel load. 
     Planter row units typically have a gauge wheel equalizer arm  130  that is a single unitary piece. It has been observed that the friction between the equalizer arm  130  and the gauge wheel support arms  110 , as the gauge wheel  115  oscillates up and down, can generate a substantial amount of noise in the sensor. At different adjustment positions, the edges of the equalizer arm  130  contact the support arms  10  at different orientations and can bite into the surface and prevent forces from being smoothly transferred as they increase and decrease. When the equalizer arm  130  is a single unitary piece, there is necessarily a high amount of friction that manifests itself as signal noise in the sensor. This signal noise makes it difficult to control the down pressure system, especially at low levels of gauge wheel load. 
     To alleviate this situation, the equalizer arm  130  illustrated in  FIG. 13  has a pair of contact rollers  131  and  132  are mounted on opposite ends of the equalizer arm. These rollers  131  and  132  become the interface between the equalizer arm and the support arms  110 , allowing forces to be smoothly transferred between the support arms  110  and the equalizer arm  130 . The roller system allows the gauge wheel support arms  110  to oscillate relative to each other without producing any sliding friction between the support arms  110  and the equalizer arm  130 . This significantly reduces the friction that manifests itself as signal noise in the sensor output, which makes it difficult to control the down pressure control system, especially at low levels of gauge wheel load. 
       FIG. 14  is a longitudinal section through the device of  FIG. 13 , with the addition of a rocker arm  150  that engages a ram  151  that controls the fluid pressure within a cylinder  152 . A fluid chamber  153  adjacent the inner end of the ram  151  opens into a lateral cavity that contains a pressure transducer  154  that produces an electrical output signal representing the magnitude of the fluid pressure in the fluid chamber  153 . The opposite end of the cylinder  152  includes an accumulator  155  similar to the accumulator  125  included in the device of  FIG. 9  described above. Between the fluid chamber  153  and the accumulator  155 , a pair of valves  156  and  157  are provided in parallel passages  158  and  159  extending between the chamber  153  and the accumulator  155 . The valve  156  is a relief valve that allows the pressurized fluid to flow from the chamber  153  to the accumulator  155  when the ram  151  advances farther into the chamber  153 . The valve  157  is a check valve that allows pressurized fluid to flow from the accumulator  155  to the chamber  153  when the ram  151  moves outwardly to enlarge the chamber  153 . The valves  156  and  157  provide overload protection (e.g., when one of the gauge wheels hits a rock) and to ensure that the gauge wheels retain their elevation setting. 
       FIGS. 15A and 15B  illustrate a modified sensor arrangement for a pair of gauge wheels  160  and  161  rolling on opposite sides of a furrow  162 . The two gauge wheels are independently mounted on support arms  163  and  164  connected to respective rams  165  and  166  that control the fluid pressure in a pair of cylinders  167  and  168 . A hydraulic hose  169  connects the fluid chambers of the respective cylinders  167  and  168  to each other and to a common pressure transducer  170 , which produces an electrical output signal corresponding to the fluid pressure in the hose  169 . The output signal is supplied to an electrical controller that uses that signal to control the down forces applied to the two gauge wheels  160  and  161 . It will be noted that the two gauge wheels can move up and down independently of each other, so the fluid pressure sensed by the transducer  170  will be changed by vertical movement of either or both of the gauge wheels  160  and  161 . 
       FIGS. 16-19  illustrate electrical/hydraulic control systems that can be used to control a down-pressure actuator  180  in response to the electrical signal provided to a controller  181  by a pressure transducer  182 . In each system the transducer  182  produces an output signal that changes in proportion to changes in the fluid pressure in a cylinder  183  as the position of a ram  184  changes inside the cylinder  183 . In  FIG. 16 , the pressurized fluid chamber in the cylinder  183  is coupled to an accumulator  185  by a relief valve  186  to allow pressurized fluid to flow to the accumulator, and by a check valve  187  to allow return flow of pressurized fluid from the accumulator to the cylinder  183 . In  FIG. 17 , the accumulator  185  is replaced with a pressurized fluid source  188  connected to the check valve  187 , and a sump  189  connected to the relief valve  186 . In  FIG. 18 , the accumulator  185  is connected directly to the pressurized fluid chamber in the cylinder  183 , without any intervening valves. In the system of  FIG. 19 , the pressure sensor  182  is connected directly to the pressurized fluid chamber in the cylinder  183 . 
       FIG. 20  illustrates a planting row unit  210  that includes a furrow-opening device  211  for the purpose of planting seed or injecting fertilizer into the soil. A conventional elongated hollow towing frame (typically hitched to a tractor by a draw bar) is rigidly attached to the front frame  212  of a conventional four-bar linkage assembly  213  that is part of the row unit  210 . The four-bar (sometimes referred to as “parallel-bar”) linkage assembly  213  is a conventional and well known linkage used in agricultural implements to permit the raising and lowering of tools attached thereto. 
     As the planting row unit  210  is advanced by the tractor, the opening device  211  penetrates the soil to form a furrow or seed slot. Other portions of the row unit  210  then deposit seed in the seed slot and fertilizer adjacent to the seed slot, and close the seed slot by distributing loosened soil into the seed slot with a pair of closing wheels. A gauge wheel  214  determines the planting depth for the seed and the height of introduction of fertilizer, etc. Bins  215  on the row unit carry the chemicals and seed which are directed into the soil. The planting row unit  210  is urged downwardly against the soil by its own weight, and, in addition, a hydraulic cylinder  216  is coupled between the front frame  212  and the linkage assembly  213  to urge the row unit  210  downwardly with a controllable force that can be adjusted for different soil conditions. The hydraulic cylinder  216  may also be used to lift the row unit off the ground for transport by a heavier, stronger, fixed-height frame that is also used to transport large quantities of fertilizer for application via multiple row units. 
     The hydraulic cylinder  216  is shown in more detail in  FIGS. 21-33 . Pressurized hydraulic fluid from the tractor is supplied by a hose  301  to a port  304  that leads into a matching port of a unitary housing  223  that forms a cavity  224  of a hydraulic cylinder containing a hollow rod  225 . The housing  223  also forms a side port  226  that leads into a second cavity  227  that contains hydraulic fluid that can be used to control the down pressure on the row unit, as described in more detail below. 
     The hydraulic control system includes a pair of controllable 2-way hydraulic lines  301  and  302  leading to the hydraulic cylinder in the unitary housing  223 , which includes an integrated electronic controller  303 . The hydraulic lines  301  and  302  are coupled to a pressure/inlet valve and a return outlet valve which are controlled by signals from the controller  303 . The controller  303  receives input signals from a pressure transducer  304  that senses the pressure in the cavity  224 , and a gauge wheel sensor that monitors the elevation of a tool relative to the elevation of the gauge wheel. 
     Slidably mounted within the hollow interior of the rod  225  is a down-pressure accumulator piston  230 , which forms one end of a sealed chamber  231  containing pressurized gas that is part of the down-pressure accumulator. The lower end of the chamber  231  is sealed by a rod end cap  232  that contains a valve  233  for use in filling the chamber  231  with pressurized gas. Thus, the down-pressure accumulator is formed entirely within the hollow rod  225   
     The hydraulic pressure exerted by the hydraulic fluid on the end surface of the rod  225  and the accumulator piston  230  urges the rod  225  downwardly, with a force determined by the pressure of the hydraulic fluid and the area of the exposed end surfaces of the rod  225  and the piston  230 . The hydraulic fluid thus urges the rod  225 , and thus the row unit, in a downward direction, toward the soil. 
     When an upward force is exerted on the rod  225 , such as when a rock or increased soil hardness is encountered, the rod  225  is moved upwardly within the cavity  224 , as depicted in  FIGS. 32 and 33 . Because the cavity  224  is filled with pressurized hydraulic fluid in the cavity  224 , the accumulator piston  230  does not move upwardly with the rod  225 , as depicted in FIGS.  32  and  33 . Thus, the pressurized gas between the accumulator piston  230  and the cap  232  at the lower end of the rod  225  is further compressed. This process continues as the rod  25  moves upwardly within the cavity  224 , until the upper end of the rod engages the housing  216 , as depicted in  FIG. 33 . In this fully retracted position of the rod  225 , the accumulator piston  230  engages the end cap  232  on the lower end of the rod  225 . 
     During upward movement of the rod  225  and downward movement of the accumulator piston  230 , hydraulic fluid flows from the second cavity  227  through the conduit  226  into the space between the outer surface of the rod  225  and the wall of the cavity  224 . The hydraulic fluid if urged in this direction by a second accumulator formed by a piston  240  and a charge of pressurized gas between the piston  240  and an end cap  241  that seals the top of the cavity  227 . As can be seen in  FIGS. 32 and 33 , the compressed gas urges the piston  240  downwardly as the rod  225  moves upwardly, thus forcing hydraulic fluid from the cavity  227  through a check valve  228  into the increasing space between the outer surface of the rod  225  and the wall of the cavity  224 . In  FIG. 33 , the rod  225  has been withdrawn to its most retracted position, and the accumulator piston  240  has moved to its lowermost position where it engages the bottom end wall of the cavity  227 . At this point, the row unit is in its uppermost position. 
     The process is reversed when the rod  225  returns to its extended position, with the accumulators providing dynamic “rebound” damping during this return movement. As the rod  225  moves downwardly, hydraulic fluid is returned to the cavity  227  through a restriction  229  to damp the downward movement of the rod. The restriction  229  can be adjusted by turning the screw formed by the outer end portion of the tapered pin  229   a  that forms the restriction  229 . The return flow rate of the hydraulic fluid is also affected by the pressure of the gas in the space above the accumulator piston  240 , which must be overcome by the returning hydraulic fluid to move the piston  40  upwardly. 
     It will be appreciated that the system described above does not require any hydraulic fluid to flow into or out of the housing  223  during advancing and retracting movement of the rod  225  that controls the vertical position of the row unit relative to the soil. Thus, there is no need to open or close any valves to control the flow of hydraulic fluid in and out of the tractor reservoir of hydraulic fluid. This is not only more efficient than moving hydraulic fluid to and from the main reservoir, but also makes the operation of the row unit much smoother, which in turn improves the delivery of seed and/or fertilizer to the desired locations in the soil. The actuator assembly is normally closed with no fluid entering or leaving the actuator/accumulator assembly unless one or more valves are opened. There is also an advantage in using two valves because a 2-position, 1-way valve can be made fast-acting more readily that a 3-position, 2-way valve. Moreover, the computer controller can be directly integrated into the actuator assembly. The single double-acting actuator with two accumulators, one acting in the downward direction and one acting in the upward direction, can be mounted in the same location as previous actuators used on row units. 
     The present system has an accumulator on both sides of the actuator, with valves that control flow, not pressure, so that the actuator can become a totally closed system with no oil entering or leaving. The compensator design is linear because the piston accumulator is packaged within the inner diameter of the ram of a larger cylinder, which reduces the number of parts as well as the size of the actuator unit. The linear compensator design allows perfectly open and unrestricted flow of oil in the compression direction, which is advantageous because of the need to rapidly absorb energy when the row unit hits a rock or obstacle. 
     When the valves have a “latching” feature, the spools of the valves can be rapidly magnetized and demagnetized. This allows the valve to latch magnetically in either the open or closed condition so that the valve does not consume power continuously, as a typical proportional coil valve does. Moreover, the latching valve design takes advantage of the ability of the accumulators to allow the planter linkage to float up and down without requiring any gain or loss of fluid. Rather, the down pressure on the planter may be changed by holding either the pressure or return valve open for varying pulse width modulated durations to achieve a rise or drop in down pressure. These valves may have a very fast rate of change between open and closed conditions. If the valve changes state very quickly, typically less than 10 milliseconds, and requires no power to remain either open or closed, it is possible to achieve negligible power consumption system because the probability that any two valves will be in the process of opening or closing at the exact same time is very low. 
     Planter row units have varying unsprung weights (the portion of the planter row unit weight that is carried by the gauge wheels and not the frame). In some tillage and soil conditions which are very soft or prone to compaction, it can be advantageous to suspend some or all of this weight by pushing upward against it. 
     By pressurizing the uplift accumulator by filling gas through the gas valve, the gas pressure increases, pushing the piston accumulator against the fluid which is connected to the main cylinder by a fluid passageway. This pressure exerts an upward force on the smaller cross sectional area of the rod side of the main piston seals, and the gas pressure can be adjusted to change the amount of uplift force. It is also possible to have a gas pressure system that allows remote adjustment of the gas pressure. The fluid in the uplift circuit forms a closed system, and a manual or automatic flow control valve can be added between the main cylinder and the uplift accumulator to restrict flow, causing damping of the rebound cycle of the suspension cylinder. 
     Fluid is introduced into the cylinder by opening the pressure valve for some duration of time, allowing high-pressure fluid from the tractor to flow into the fluid chamber. This high-pressure fluid pushes against the linear compensator accumulator piston, which in turn compresses the gas to equalize the pressures on opposite sides of the piston. The accumulator piston will move back and forth inside the hollow rod when the down pressure is changing, even if the rod is not moving up and down. The length of time the pressure valve remains open corresponds to the size of the adjustment needed. Control is being accomplished in a closed loop fashion based on the planter gauge wheel load. Once the required pressure is achieved, the valve closes so that the actuator is a closed system again. The actuator can then allow the row unit to float up and down, compressing and decompressing the gas in the down-pressure and up-pressure accumulators. This will generate heat in the process—the heat is energy that is being damped from the system. To facilitate the removal of this heat from the system, the portion of the housing  223  that forms the cavity  227  forms multiple cooling fins  242  around its exterior surface. 
       FIG. 34A  is a schematic diagram of a hydraulic control system that uses a single hydraulic cylinder  1601 , two two-position control valves  1602 ,  1603  and a pair of accumulators  1604 ,  1605 . The valves are both latching type valves with a single actuator  1602   a  or  1603   a  for each valve, for moving the valve to either the open or closed position when the valve is unlatched. When valve  1602  is in the open position, it connects a source  1606  of pressurized hydraulic fluid to the hydraulic cylinder  1601  via pump  1607 . When valve  1603  is open, it connects cylinder  1601  to a sump  1607 . Electrical signals for energizing the actuators  1602   a  and  1603   a  are supplied to the respective actuators via lines  1607  and  1608  from a controller  1609 , which in turn may be controlled by a central processor, if desired. The controller  1609  receives input signals from a pressure transducer  1610  coupled to the hydraulic cylinder  1601  via line  1611 . The accumulator  1604  is coupled to the hydraulic cylinder  1601  through a valve  1612 , as described in more detail below. 
       FIG. 34B  is a schematic diagram of a modified version of the system of  FIG. 34A  to provide both rebound damping and compression damping. The only difference is that the system of  FIG. 34B  includes a valve  1613  between the accumulator  1603  and the compression side of the hydraulic cylinder  1601 , so that the accumulator  1603  provides compression damping when the rod of the cylinder  1601  is moved from right to left in  FIG. 34A . 
       FIGS. 35A and 35B  illustrate systems that are identical to those of  FIGS. 34A and 15B , except that the latching valves are replaced with non-latching valves  1702  and  1703 . These non-latching valves are biased toward their closed positions by respective springs  102   a  and  1703   a,  and can be moved to their open positions by energizing their respective actuators  1702   b  and  1703   b.    
     In the control system of  FIG. 34B , a PWM control system may be used to supply short-duration pulses P to the actuators  1602   a  or  1603   a  of the control valves  1602  or  1603  to move the selected valve to its open position for short intervals corresponding to the widths of the PWM pulses. This significantly reduces the energy required to increase or decrease the pressure in the hydraulic cylinder  1601  for adjusting the down pressure on the soil-engaging implement. As depicted in  FIG. 36 , pulses P 1 -P 3 , having a voltage level V 1 , are supplied to the actuator  1602   a  when it is desired to increase the hydraulic pressure supplied to the hydraulic cylinder  1601 . The first pulse P 1  has a width T 1  which is shorter than the width of pulses P 2  and P 3 , so that the pressure increase is smaller than the increase that would be produced if P 1  had the same width as pulses P 2  and P 3 . Pulses P 4 -P 6 , which have a voltage level V 2 , are supplied to the actuator  1602   a  when it is desired to decrease the hydraulic pressure supplied to the hydraulic cylinder  1601 . The first pulse P 4  has a width that is shorter than the width T 2  of pulses P 2  and P 3 , so that the pressure decrease is smaller than the decrease that would be produced if P 4  had the same width as pulses P 5  and P 6 . When no pulses are supplied to either of the two actuators  1602   a  and  1603   a,  as in the “no change” interval in  FIG. 36 , the hydraulic pressure remains substantially constant in the hydraulic cylinder  1601 . 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.