Control System For Material Handling Equipment

Abstract

A control system for a hydraulic motor actuated material handling mechanism including a source of fluid power, a directional flow control valve interposed between the source of fluid power and the hydraulic motor, a bridge circuit including a source of electrical energy and having a first variable impedance branch varied by a command control lever and a second variable impedance branch associated with the flow control valve, comparator means connected to the output of the bridge circuit for detecting and amplifying an unbalance in the bridge circuit and means interconnected between the comparator and the directional flow control valve for controlling said valve in a manner corresponding to command movement of the variable impedance branch.

Description

BACKGROUND OF THE INVENTION

This invention relates to a unique electro-hydraulic or electrical control system for controlling one or more hydraulic motors. More specifically the invention concerns a control system for a material handling mechanism mounted upon a vehicle in which the several linkages and hydraulic motors of the material handling device may be easily controlled in a manner to improve cycle times with a measurable amount of reduced operator fatigue.

DESCRIPTION OF THE PRIOR ART

Material handling apparatuses of the prior art as illustrated by a front end loader or a backhoe having one or more mechanical linkages interconnecting the material handling bucket or shovel with a vehicle, each linkage being controlled by a separate hydraulic motor, a separate directional flow control valve which in turn is controlled by a separate lever. For example, backhoes in production today utilize a separate lever controlling each flow control valve which separately actuates the boom, rotary actuator, dipper stick and bucket. Thus four separate levers are utilized to perform digging operations for the current backhoes, and excellent dexterity is required on the part of the operator to economically utilize such devices. Similarly, the prior art utilizes purely mechanical linkages for controlling directional flow control valves which actuate the rotary actuator, boom, dipper stick and bucket.

SUMMARY OF THE INVENTION

Accordingly, the instant invention relates to a novel and unique electrical control circuit for controlling actuation of these various linkages of the article-handling equipment. Preferred embodiments, hereinafter disclosed, incorporate a circuit between a command lever and the flow control valve in such a manner to easily control actuation of the flow control valve and is of such simple construction as to permit utilization of a single lever to operate several such valves. It is an object of the instant invention to provide a single lever control system for material-handling mechanisms in which a plurality of directional flow control valves and hydraulic motors must be controlled. Similarly, it is an object of the instant invention to further correlate the direction of movement of the single control lever with directional movement of the linkage to be controlled. Not only is such directional movement correlated to provide a follow-up system, but a unique and novel position control system is proffered herein. It is a further object of the instant invention to provide a relatively simple control system which eliminates the dexterious requirements of operators of material-handling equipment.

With respect to the control system per se, it is a general object to provide an economical and easily assembled electrical control circuit for any hydraulic motor which results in unique operating characteristics of precise and accurate control of a linkage or hydraulic motor as well as the ability to accurately meter fluid through a directional flow control valve. Such a control system may be easily adapted in fields unrelated to article handling and would include machine tools and other diverse areas. Finally, it is an object of the instant invention to provide an electrical control system for a hydraulic motor in which either a followup system or a position control system can be readily realized.

DESCRIPTION OF THE DRAWINGS

The manner in which the objects of the instant invention is attained will be made clear by consideration of the following specification and claims when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of the rear portion of a vehicle having mounted thereon a conventional backhoe;

FIG. 2 is a rear elevational view of the directional flow control valves which are utilized to direct flow to and from the hydraulic motors actuating the various linkages of the backhoe;

FIG. 3 is an exploded perspective view of the single handle control utilized to actuate the directional flow control valves which in turn direct fluid to and from the hydraulic motors;

FIG. 4 is a side elevational view of the rotary actuator of the backhoe having the boom attached thereto with a schematic disclosure of the electro-hydraulic control system utilized in controlling the boom of the backhoe;

FIG. 5 is a schematic view of an electro-hydraulic circuit for controlling the hydraulic motor which in turn operates the boom of the backhoe;

FIG. 6 is a side elevational view of the rotary actuator and the boom of the backhoe attached thereto with a schematic view of an electro-hydraulic control system which actuates the rotary actuator in turn controlling rotational movement of the backhoe boom;

FIG. 7 is a schematic view of the electro-hydraulic control system of FIG. 6;

FIG. 8 is a top view disclosing rotary actuation of the backhoe unit and its correlated movement of the control handle;

FIG. 9 is a perspective view of a mounting element for the control lever of FIG. 3; and

FIG. 10 is a schematic view of an additional control circuit.

DETAIL DESCRIPTION

As exemplified in the attached drawings, the preferred embodiments of our unique invention are incorporated into a tractor 10 to which a material-handling apparatus is attached. The tractor 10 is provided with an operator's station 13 having thereon a seat 11 adjacent to which is a single handle control lever 35 operative to control all of the movements of the associated material-handling apparatus. In the instant disclosure, this material-handling apparatus comprises a backhoe having a support stand 12 suitably attached to the tractor by conventional means and being further supported upon the ground surface by stabilizer arms 14 (only one of which is here shown), the stabilizer itself being controlled by hydraulic motor 15. Appropriate brackets 16, 16 extending rearwardly from support stand 12 carry a rotary hydraulic motor 30 constrained from rotation by suitable means and having a shaft 31 extending therethrough for rotatably driving the swing mount 17 as fluid energy is directed to the actuator 30. The swing mount 17 in turn rotatably mounts a boom 18, dipper stick 19 and bucket 20, these linkages and the bucket being controlled in their movement by hydraulic rams 21, 22, and 23 respectively.

Disposed underneath the operator's station 13 for conveniently controlling the movement of the rams 15, the rotary actuator 30, and the hydraulic rams 21, 22 and 23, is a valve bank 62 comprising six directional flow control valves 61. Upon one end of this valve bank is mounted a conventional end cap 63 to which is connected an intake conduit 67 for delivering fluid from a pump to the valves. Adjacent the opposite end of the valve bank 62 is another identical end cap 63 to which is connected a conduit 79 for returning hydraulic fluid to the reservoir. These valves 61 are substantially similar to that disclosed in U.S. Pat. No. 2,873,762 issued to F. H. Tennis and dated Feb. 17, 1959. Further reference may be had to a cross sectional view of these same valves in FIGS. 4 and 6 for a clearer understanding thereof.

ELECTRO-HYDRAULIC CONTROL CIRCUIT

Reference may now be had to FIGS. 4 and 5 in which one of the control valves 61 is integrated into the control circuit of the instant invention for the hydraulic motor 21 which effectuates movement of the boom 18 of the backhoe. (As later explained, such a circuit is associated with each element of the backhoe). A pump P delivers fluid from reservoir S to directional control valve 61 through a conduit 67. As disclosed in FIG. 2 such fluid would normally be delivered to an end cap 63 of valve bank 62, such being omitted from FIG. 4 for purposes of simplicity. This fluid is then directed into an open center passage 68 and normally flows directly out of this valve and into the opposite end cap and back to sump. As more fully disclosed in U.S. Pat. No. 2,873,762, a spool 70 is reciprocable within the valve 61 whereby the reduced diameters of said spool in conjunction with various lands and porting will control the direction of fluid flow to and from the hydraulic motor 21. Briefly setting out such structural and functional characteristics of this well known valve, it will be observed that if spool 70 is shifted either to the right or to the left and fluid can no longer flow through the open or low pressure passage 68 since the normal diameter of the spool precludes such, and fluid is then directed into a high pressure passage 69 for delivery to the hydraulic motor 21 via one of the motor ports 72 or 73. Thus, assuming the spool is shifted to the right, the low pressure, open center feature of the valve is closed by a normal diameter of the spool, and hydraulic fluid is then urged into the high pressure passage 69, across the reduced diameter of the spool at 81 and out port 72 to contract the hydraulic ram 21. Fluid is then returned from the opposite end of the ram 21 via conduit 77, motor port 73, across reduced diameter 83 of the spool and out of exhaust port 78 to the sump S through a conduit 79. Reciprocation of the spool in the opposite direction merely reverses the direction of flow above explained. These passages and portings of the open center valve being quite conventional, no further explanation is deemed necessary other than reference to the above identified patent.

Movement of the spool 70 of the valve 61 is controlled by expansionable chamber device or servo motor 85 and includes a housing 86 connected to the valve 61 and having a piston element 87 attached to the spool 70 whereby any unbalance of fluid energy delivered through a port 88 or a port 89 is effective to cause reciprocation of the piston 87 and the spool 70 to control the direction of fluid flow to hydraulic motor 21. Fluid is supplied to the servo motor 85 from a pump P1 delivering fluid through a conduit 113 to solenoid operated valves 110 and 111. These solenoid valves may appropriately be a conventional "normally open" three-way valve in which the fluid is normally directed through the valve to the ports 88 and 89 of servo motor 85 but upon selective actuation they will dump both supply fluid and fluid on the appropriate side of piston 87 to sump S whereby the normal pressure on the opposing side of the piston will cause the spool to reciprocate. Another preferable solenoid valve would include the "four-way normally open -- normally open solenoid valve, Type V955" made and sold by Skinner Precision Industries, Inc. of New Britain, Connecticut. This valve is basically a combination of two three-way valves disclosed above, but incorporated into one housing.

For controlling actuation of the solenoid valve 110 or 111 and consequent actuation of spool 70, a bridge circuit is interposed between a control member or lever 35 and spool 70 of the control valve 61. As more clearly depicted in FIG. 5, a bridge circuit is established in which two potentiometers 44 and 92 are connected in parallel and a voltage impressed across them. The wiper of potentiometer 44 is rotatably attached to the control lever 35 as hereinafter explained, and the wiper of potentiometer 92 is rotatably attached by assembly 91 (see FIG. 2) to the spool 70 of control valve 61.

The wiper leads are then connected to a null detector means or comparator means 109 which will detect any unbalance or voltage differential in the bridge circuit and amplify a signal created by said unbalance to actuate the appropriate solenoid valve 110 and 111. Thus, upon varying the resistance in potentiometer 44, the comparator means 109 will actuate solenoid valve 110 or 111 so as to dump fluid from the appropriate chamber on one side of piston 87 and normal pressure on the opposite side will effectuate reciprocation of spool 70, the spool movement rotating the wiper of potentiometer 92 such that the bridge circuit will again be balanced and fluid will continue to be pumped until the potentiometer 44 is returned to neutral. Thus, it should be appreciated that movement of a control lever 35 about a horizontal axis will produce a corresponding proportional movement of spool 70. With reference to the components of the electro-hydraulic control system, it is to be noted that the null detector or comparator means 109 is a conventional item and readily obtainable in the market in various forms. An example of other forms of the electrical circuits which may provide excellent alternatives would include a series connection of two variable resistances in parallel with a series connection of two fixed resistances, the comparator 109 being interconnected between the resistances of each parallel branch. The pump delivering fluid to the solenoid valves is preferably a low volume and low pressure pump, but may be the same pump utilized to deliver fluid to the backhoe itself if acceptable flow rates are provided. The solenoid valves selected for the system should preferably have a small flow rate for the intended pump and pressure since more accurate movement of the spool 70 may be obtained. It should be appreciated that solenoid members acting directly upon the spool could be utilized as well as other systems including conventional electro-hydraulic servo valves should the potentiometer reading be taken from the valve in such a manner as to be proportional to the flow rate through such valve.

Referring back to FIG. 4, the potentiometer 92 is attached to the spool 70 in a simple mechanical manner. For example, a bracket 93 is fixedly attached to the console 13 of the backhoe apparatus with the potentiometer secured therein and constrained against rotation. A link 94 is then constrained for rotation with the wiper of the potentiometer 92, reciprocation of the spool causing rotation of this link through a pivotal element 95 so as to vary the impedance of this branch of the bridge circuit.

Each of the directional flow control valves 61 associated with the rotary actuator, and the hydraulic motors 21, 22 and 23 may be conveniently provided with such an electrical control circuit.

SINGLE LEVER CONTROL

For manipulating the command potentiometer 44 as well as a command potentiometer for each control circuit associated with motors 30, 21, 22 and 23, applicants have provided a unique control means whereby movement of said control means produces a similar directional movement of an appropriate element of a material handling mechanism. As disclosed in FIG. 3 this control means 35 comprises a vertical support 36 which may be appropriately journaled in a housing 57 in a manner permitting rotational movement thereof. Attached to the lower end of this vertical support 36 is a gear 38 which upon rotation will drive a pinion 39 constrained for rotation with the stem of a potentiometer 40, the latter being fixedly supported in housing 57. Thus, as the lever 35 is rotated about a vertical axis, a varying resistance in a potentiometer 40 is incurred and since this variable command impedance forms one branch of a bridge circuit which is part of an electro-hydraulic circuit associated with a valve 61 controlling flow to actuator 30 and otherwise identical to that disclosed in FIGS. 4 and 5, the rotational movement of lever 35 about a vertical axis will controll rotational movement of the shaft 31 of rotary actuator 30.

In order to control command impedance 44 and boom 18 and motor 21, as previously explained in FIGS. 4 and 5, a yoke 37 is provided on the upper end of vertical support 36, and common apertures 45 connect same to another yoke member 41 as well as to an extensible arm 46, the latter connection being made rigid by pin connections extending the adjacent diagonal apertures (unnumbered). The yoke 41 then extends downwardly from the pivotal connection 53 with the extending arm carrying the potentiometer 44 constrained for movement therewith. The stem of the potentiometer 44 carries a pinion 43 which is driven by a rack 42 mounted on yoke 37, and thus as the extensible arm 46 is rotated about a horizontal axis passing through a pivotal connection 53, the stem of the potentiometer 44 is rotated so as to obtain a variable impedance or resistance. This potentiometer 44 appropriately controls movement of the boom 18 as previously discussed in relation to FIGS. 4 and 5 by connecting same with another bridge circuit element attached to the spool of the directional flow control valve 61 associated with hydraulic motor 21.

The extensible arm 46 may additionally consist of an outer extensible member 47 and an inner member 48 over which the outer member is telescoped. A potentiometer 49 is then fixed by a bracket 50 upon said outer member, and upon extension and retraction of outer member 47, a rack 52 and a gear 51 constrained for rotation with the stem of potentiometer 49 creates a variable impedance in said potentiometer. Appropriately, this potentiometer may be integrated in a bridge circuit with a potentiometer associated with the flow control valve 61 which is connected to hydraulic motor 22 effectuating movement of the dipper stick 19.

Finally a handle 54 is rotatably secured upon the end of extensible member 47, and a bolt member 55 constrained for rotation by any conventional means with handle 54 extends through the handle and is joined to the stem of potentiometer 56 secured to member 47. Again, rotation of the control handle 54 will vary the impedance of potentiometer 56, and if this potentiometer is integrated into the electro-hydraulic circuit of FIGS. 4 and 5 which is further associated with motor 23, rolling movement of control handle 54 will cause rolling of the bucket 20.

Thus it should be appreciated that, as constructed, the control means 35 permits motions analogous to that of the backhoe itself. For example if the operator desires to rotate the backhoe, he merely rotates control lever 35 about its vertical axis causing an unbalance in the bridge circuit of the associated potentiometers and fluid may be directed to the hydraulic actuator 30 to rotate the backhoe. Similarly rotational motion of the extensible arm 46 about a horizontal axis passing through the pivotal connection 53 causes actuation of the hydraulic valve controlling the boom and such movement is somewhat analogous to movement of the control means. Similarly extension and retraction of hydraulic ram of extensible member 47 will operate the directional flow control valve associated with hydraulic motor 22 to extend and retract the dipper stick 19 in an analogous fashion, and finally rotational movement of member 54 will cause similar rotational movement of bucket 20. This analogous and corresponding movement directional of the control member and the backhoe is a most significant advantage since such is effective to reduce cycle time of the handling operation as well as enabling an operator with little experience to rapidly master the techniques of such a machine. Thus it should be appreciated that by utilization of a control system as disclosed in FIGS. 4 and 5 with a single control lever 35 and applying same to each of the control valves 61 located within the valve bank 62, significant advantages may be obtained.

POSITION CONTROL SYSTEM

FIGS. 6 and 7 disclose an additional embodiment of the instant invention in which the directional control valve movement not only corresponds to movement of the control means 35, but the hydraulic motor and associated linkage itself will accurately correspond to movement of control means 35 whereby a position control system may be obtained, i.e. a 2.degree. movement of the manual control means will result in a 2.degree. movement of the linkage itself. In this embodiment, a bracket 132 is disposed upon flange 16 and extends above the rotational shaft 31 of rotary actuator 30, and has mounted thereon a potentiometer 131 whose stem is constrained for rotation with the shaft 31 of the motor 30. As schematically disclosed in FIG. 7, this potentiometer 131 is placed in parallel with the potentiometer 40 but stem rotation is such that a positive increasing voltage of potentiometer 40 will result in a decreasing voltage in potentiometer 131 upon rotation of actuator 30. Thus, the comparator 109 compares the voltage difference between potentiometer 92 and potentiometers 40 and 130. Accordingly, if the control lever 35 is rotated to the right, the wiper of potentiometer 40 will be moved downwardly as shown in FIG. 7, and the comparator will detect an unbalance and actuate solenoid valve 111 to permit spool 70 to be moved downwardly. This downward movement will deflect the stem of potentiometer 92 as shown by dotted lines and simultaneously fluid will be directed to the rotary actuator so as to rotate same in a clockwise direction. This clockwise movement of the rotary actuator in turn controls the wiper of potentiometer 130 to reduce its impedance as the actuator approaches the corresponding position of control lever 35. Thus, the total resistance seen by the comparator on one side of the bridge circuit diminishes upon rotation of the actuator and the null detector will then gradually actuate solenoid valve 110 to return the spool to neutral such that potentiometer 92 is again in balance with potentiometers 40 and 130 in parallel. That is, as the difference between 131 and 35 gets smaller, the amount of high pressure fluid delivered to the hydraulic motor decreases proportionally because potentiometer 92 is closing off spool 70. As disclosed in FIG. 8, an annular movement alpha of control arm 35 will thus result in a corresponding annular movement alpha of the boom 18 and a position control system is effectuated.

As more fully pointed out in FIG. 9, a tubular member 57 may be mounted upon the console 13 to rotatably support control lever 35, apertures 58 being provided in the upper end surface of member 57. Appropriately, dowel pins 59 may be inserted into these apertures to limit the rotational movement of control means 35 about a vertical axis. Consequently, if the position control system of FIGS. 6 and 7 is utilized to control swing movement of the backhoe, a position controlled system may additionally yield a return to dig system. For example, if dowel pins are placed in the end surface of tubular member 57 corresponding to a position of the proposed trench, and corresponding to a position for bucket dump, the outer limits of rotational movement of the backhoe will be fixed and by merely rotating the lever 35 between these limits, the rotary motor 30 will always return the boom to these limits. Under such circumstances the dowel pin may reflect a return to dig position and the operator need not concern himself with accurate positioning of the bucket in a trench.

Although not a part of the instant invention, FIG. 10 discloses a circuit for controlling hydraulic motors 15 which position the stabilizer arms 14, and support the boom structure during the digging operation. Since continuous movement is not desired, the two end valves of bank 62 are not provided with potentiometers, and the solenoid valves are operated by on-off switches, 120 and 121. As opposed to the bridge arrangement previously discussed the remainder of the system, e.g., valve 61, solenoids, and chamber device 85 remain the same.

SUMMARY

In summary, applicants have disclosed a novel, unique, and most advantageous electrical hydraulic system for controlling movement of associated hydraulic motors by electrically controlling actuation of a flow control valve. A single lever capable of several distinct movements may effectively be designed so as to control several directional flow control valves and their associated hydraulic motors. Further, such movements may be correlated with the anticipated movement of the elements to be controlled and a follow-up system is effected in which a command signal is followed by a correlated movement of the associated motor controlled element. In addition to this follow-up system, provision has been made for an accurate position control. Finally, in addition to the aforesaid advantages and results, applicants, by incorporating into their system a solenoid actuated valve which is effective to control movement of the flow control valve, accurate metering of fluid through the flow control valve may be effected and infinite and incremental movements of the associated hydraulic motors and linkages may result therefrom. It should be readily appreciated that the system herein disclosed may be employed with numerous vehicles so as to control many material-handling operations by a single lever. Even in the utilization of continuously rotating hydraulic motors, the herein disclosed control system may be effective to accurately control said motor so as to afford braking and incremental movement, such not being heretofore attainable. Naturally, various utilizations as well as variations of the system herein disclosed will find application in many fields.

Claims

We claim:

1. In a vehicle having a hydraulic motor actuated material-handling mechanism, a control system for effecting movement of said motor comprising:

1. a source of fluid energy on said vehicle,

2. a directional flow control valve including a reciprocating spool and interconnected between said source and the hydraulic motor for controlling flow to and from the hydraulic motor upon displacement of said spool from a normally closed position,

3. command means on said vehicle capable of a directional movement analogous to a movement of said mechanism,

4. electric circuit means having two variable impedances, one of said impedances associated with and responsive to actuation of said spool, the other impedance being associated with and responsive to movement of said command means,

5. detector means interconnectible across said impedances for detecting a voltage difference in the respective electrical signals developed across said impedances,

6. electro-hydraulic means interconnected between said detector means and said flow control valve for actuating said flow control means to displace said spool in a direction and for a distance corresponding to the direction and distance movement of said command means so as to nullify any voltage difference in said impedances and to effect a corresponding directional movement of said material handling mechanism at a rate determined by the distance of movement of said command means.

2. An apparatus as recited in claim 1 in which:

1. each of said impedances is a potentiometer, one of said potentiometers having its resistance varied by the spool of said valve and said other potentiometer having its resistance controlled by said command means.

3. An apparatus as recited in claim 2 in which said electro-hydraulic means comprises:

1. expansible chamber means on said spool of said flow control valve for effecting reciprocation thereof, and

2. electrically operated hydraulic means supported by said vehicle for directing fluid to said expansible chamber means to actuate said spool.

4. An apparatus as recited in claim 1 in which said electro-hydraulic means comprises:

1. an expansible chamber means operatively associated with said directional flow control valve for controlling actuation thereof,

2. an electrically operated hydraulic means for directing fluid flow to and from said expansible chamber device for actuating said directional flow control valve, said valve directional flow control and said expansible chamber device being of such a capacity as to permit incremental movement of said directional flow control valve so as to meter hydraulic fluid to and from the hydraulic motor.

5. In a vehicle having a hydraulic motor actuated tool and hydraulic circuit therefor, a control system for controlling movement of said motor comprising:

1. flow control valve in said hydraulic circuit having a movable member therein for controlling the direction and flow rate of hydraulic fluid,

2. lever means on said vehicle capable of a movement related to a movement of said tool,

3. circuit means including an electrical source and having a first variable impedance associated with the movable member of the flow control valve, and a second variable impedance associated with said lever means,

4. comparator means interconnected with said circuit means for detecting and amplifying an unbalance in the respective electrical signals developed across said variable impedances,

5. electrical means interconnected between said comparator and said movable member for controlling said movable member in a manner so as to eliminate said unbalance in said respective electrical signals.

6. An apparatus as defined in claim 5 in which:

1. said flow control valve includes a reciprocating spool therein for controlling fluid flow,

2. each of said impedances is a potentiometer, the potentiometer associated with the flow control valve having its resistance varied by movement of the spool of said valve and the potentiometer of the manually variable impedance having its resistance varied by movement of the lever means.

7. An apparatus as recited in claim 5 in which said electrical means includes:

1. an expansible chamber device operatively associated with said flow control valve for actuating its movable member, and

2. an electrically operated hydraulic means for directing fluid flow to and from said expansible chamber device for actuating said directional flow control valve, said valve means and said expansible chamber device being of such a capacity as to permit incremental movement of said directional flow control valve so as to meter hydraulic fluid to and from the hydraulic motor.

8. An apparatus as recited in claim 7 in which said electrically operated hydraulic means comprises:

1. hydraulic valve means interconnected between said hydraulic circuit and said expansible chamber device, said hydraulic valve means being responsive to any unbalance in said circuit and directing fluid to said expansible chamber device so as to eliminate any unbalance.

9. An apparatus as recited in claim 5 in which said lever means includes means for permitting movement thereof analogous to potential movement of the motor actuated tool.

10. In a material handling vehicle having an operator control station and having a moveable linkage coupled at one end portion to said vehicle and supporting a work implement at an opposite end portion, and having a source of hydraulic fluid energy and a control system comprising:

1. actuating means comprising a manually operable actuating member located at said operator control station and capable of positional movement functionally analogous to that of said linkage;

2. directional flow control valve means including a valve member moveable in opposite directions from a central closed position which precludes flow of hydraulic fluid energy to said motor for permitting flow of said hydraulic fluid energy to said motor to move said linkage in corresponding opposite directions;

3. first sensing means coupled to said manually operable actuating member for developing a first sensing signal representative of the position of said actuating member;

4. second sensing means coupled to said moveable linkage for developing a second sensing signal representative of the position of said linkage;

5. third sensing means coupled to said valve member for developing a third sensing signal representative of the position of said valve member;

6. control means responsive to a differential in amplitude between said first and third sensing signals attendant movement of said actuating member from a neutral position for displacing said valve member in a corresponding direction to permit flow of hydraulic fluid energy to said motor to initiate movement of said linkage in a corresponding direction and responsive to said second sensing signal for gradually restoring said valve member to said closed position as said linkage moves to a position corresponding to that of said actuating member.

11. In a material handling vehicle according to claim 10, and further comprising:

stop means associated with said linkage for establishing adjustable limit positions of movement for said linkage and at least one direction of movement thereof.