Patent Publication Number: US-11028862-B2

Title: Metering hydraulic control system for mining machine

Description:
RELATED APPLICATION DATA 
     This application is a § 371 National Stage Application of PCT International Application No. PCT/EP2017/054341 filed Feb. 24, 2017. 
     FIELD OF INVENTION 
     A hydraulic system to provide actuation of a mechanical member, and in particular although not exclusively, to a hydraulic system having an actuator with independent metering control. 
     BACKGROUND ART 
     Independent metering valve (IMV) assemblies have been used for hydraulic control of actuators of heavy machinery that act on excavation buckets, loader front ends and the like. Typically, an IMV assembly receives pressurised hydraulic fluid from a pump and is coupled in fluid communication with a hydraulic load actuator such as a hydraulic cylinder that is mechanically linked to a mechanical actuator, i.e. loader bucket, mechanical arm etc. Conventionally, an IMV assembly includes four independently controllable valves in which a first pair is coupled with a first chamber (head end) of the cylinder and a second pair is coupled with a second chamber (rod end) such that selective hydraulic fluid flow between the respective pairs of valves provides either extension or retraction of the cylinder rod. Typically, the valves of the IMV assembly are electrically controlled independently and are configured to receive input signals from one or more sensors provided at the hydraulic circuit. 
     US 2013/0081383 discloses a hydraulic system in which a variable displacement pump is connected in a closed-loop manner to first and second linear actuators that operate to manipulate a pivoting boom of an excavator machine. 
     US 2003/0106423 discloses a fluid control system operable in either an independent function mode or a regenerative function mode. 
     US 2002/0148223 discloses a hydraulic system having a IMV assembly coupled to a first and second hydraulic load such as a cylinder or fluid motor driving a fan or the like. 
     However, conventional arrangements offer limited control of fluid flow speed, pressure and fluid flow direction through the circuit that in turn limits the range of functionality of the hydraulic and according the mechanical actuator. Accordingly, what is required is a hydraulic system that solves these problems. 
     SUMMARY OF THE INVENTION 
     It is one objective of the present invention to provide a hydraulic system capable of being fluidly connected to an actuator so as to provide a wide range of dynamic (adjustable) properties including in particular load force, speed of movement, accurate static and moving positional control in response to an external static and/or dynamic load forces. It is a further specific objective to provide a hydraulic system suitable for controlling a mechanically actuating member such as a boom, arm, linkage or other member associated with heavy machinery such as an excavator, mining machine or other bulk processing apparatus. 
     It is a specific objective to provide a hydraulic system for a cutting mining machine and in particular an undercutting mining machine capable of controlling actuation of one or more pivoting support arms of the machine that mount one or a plurality of cutting heads. 
     According to one aspect, a hydraulic system is provided having an independent metering valve (IMV) assembly in which electrically controlled meter-in and meter-out valves are configured to control a mechanical actuator (such as a cutter arm) to provide accurate movement and static positioning of the mechanical actuator both when the actuator is not externally loaded (other than by gravity) and in response to an external static and/or dynamic load forces. The present hydraulic system is specifically adapted for variation of speed of actuation and the force with which actuation is provided. For example, and in one mode of operation, hydraulic actuating cylinders, controlled by the present hydraulic system, may be maintained in a relatively ‘stiff’ configuration so as to be capable of withstanding significant loading forces, e.g., when the cutter arm is in a cutting mode, whilst maintaining position or whilst being actuated to provide pivoting of the cutter arm against the rock strata e.g., movement laterally inward and outward relative to a main body of the bulk processing machine. In a further mode of operation, the hydraulic actuating cylinders, may be controlled to provide fast movement of the cutter arm when not loaded e.g., when the cutting arm is in an idling (not cutting) mode. 
     The present hydraulic system is advantageous to provide independent metering of movable mechanical actuators such as pivoting support arms of cutting or other mining machines and other bulk processing apparatus. In particular, the subject invention provides control of one or a plurality of hydraulic load actuators (e.g. hydraulic cylinders) according to different operational modes such as a ‘cutting’ mode and an ‘idling’ mode. In the cutting mode the system is configured to control the hydraulic actuator to achieve a slow speed or movement with high loading force resistance (i.e., enhanced relative ‘stiffness’ of the actuator) and in the cutting mode to achieve a fast speed or movement with low loading force resistance (i.e., reduced relative ‘stiffness’ of the actuator) via the same independent IMV assembly. Accordingly, the subject invention is optimised for maximising efficiency of a mining machine so as to be capable of obtaining a desired fluid flow speed and pressure that may be proportionally adjusted so as to provide operation according to the cutting and idling modes in which at least one mechanical actuator is displaceable with a wide range of movement speeds and against a wide range of external forces (movement-resistive forces). 
     According to a first aspect of the present invention there is provided a hydraulic system to control at least two hydraulic actuators, the system comprising: a pump; a first and a second hydraulic actuator; a metering control valve assembly having a plurality of electrohydraulic valves, an inlet fluidly connected to the pump and an outlet fluidly connected to a drain or reservoir, the metering control valve assembly fluidly connected to the first hydraulic actuator; a transmission valve assembly having an electronically controllable valve fluidly connected to provide pilot control of a first and a second logic valve, the first and second logic valves fluidly connected to the metering control valve assembly; and the second hydraulic actuator fluidly connected to the metering control valve assembly via the first and second logic valves. 
     Preferably, the pump is a variable displacement pump. Optionally, the pump may comprise a constant pump or other pump configurations a will be appreciated by those skilled in the art. 
     Reference within this specification to a ‘hydraulic actuator’ encompass a hydraulic component having a main body or housing and at least one movable member capable of moving relative to the body or housing. The term hydraulic actuator includes a hydraulic cylinder such as a linear actuator having a cylinder body defining internal chambers separated by a piston with the piston coupled to an extendable and retractable shaft or arm. Optionally, the hydraulic actuator comprises a hydraulic motor. 
     Preferably, the system comprises at least four electrohydraulic valves in which a first pair of said valves is fluidly connected to the inlet and a second pair of said valves is fluidly connected to the outlet. Such an arrangement is beneficial to provide selective control of fluid flow speed and pressure to and from the actuator so as to provide a corresponding control of the movement or position of at least parts of the actuator. 
     Preferably, the system comprises a first and a second conduit fluidly connecting the first hydraulic actuator and the transmission valve assembly to the electrohydraulic valves; and a third and a fourth conduit fluidly connecting the second hydraulic actuator to the first and second logic valves respectively. Preferably, the first hydraulic actuator is coupled directly to the transmission valve assembly and in particular the electrohydraulic valves via the first and second conduits. Optionally, the second hydraulic actuator is coupled to the logic valves directly via the third and fourth conduits. Such an arrangement maintains to a minimum the number of components within the system for ease of maintenance and efficiency. 
     Preferably, the system comprises at least one or a plurality of sensors including in particular fluid flow sensors providing monitoring, output and response control of fluid pressure and flow speed (flow rate) within the system. Optionally, the sensors may be positioned at the valves, the conduits and/or the hydraulic actuators. 
     Optionally, the first hydraulic actuator is fluidly connected to the metering control valve assembly without fluid connection via the first and second logic valves and the second hydraulic actuator is fluidly connected to the metering control valve assembly via the first and second logic valves. Preferably, the first hydraulic actuator is coupled directly to the metering control valve assembly and the second hydraulic actuator is coupled directly to the transmission valve assembly. As such, a fluid flow speed and fluid pressure may be regulated exclusively via control of the metering control valve assembly without fluid routing via the transmission valve assembly. 
     Preferably, the system further comprises a first float logic valve fluidly connected to the second hydraulic actuator to allow fluid transfer between regions of the second hydraulic actuator when operated in a first mode. 
     Preferably, the system further comprises at least one further (i.e., third) hydraulic actuator fluidly connected to the first and second logic valves via the third and fourth conduits respectively. 
     Preferably, the electronically controllable valve is a solenoid control valve. 
     Preferably, the second and/or third hydraulic actuators are fluidly connected to the metering control valve assembly via the first and second logic valves. Preferably, the system further comprises a second float logic valve. The float logic valves are coupled directly to the respective second and third hydraulic actuators so as to minimise the fluid flow path length between the regions (i.e. internal chambers) of the actuators when operated in the first mode. Preferably, the float logic valves are pilot controlled via the transmission valve assembly and in particular the electronically controllable valve (i.e., solenoid control valve). Preferably, the float logic valves are connected to the electronically controllable valve via the same pilot fluid control circuit as the logic valves of the transmission valve assembly. Accordingly, the electronically controllable valve is configured to provide control of all logic valves within the system. 
     Optionally, the system further comprises a pressure reducing/relieving valve fluidly connected to an auxiliary logic valve, the pressure reducing/relieving valve fluidly connected to the pump and the auxiliary logic valve fluidly connected to at least the second hydraulic actuator. Preferably, the auxiliary logic valve is fluidly connected to the second hydraulic actuator via the third conduit. Preferably, the pressure reducing/relieving valve and the auxiliary logic valve are coupled to the drain or reservoir so as to provide further regulation of the fluid pressure at the hydraulic actuators and in particular to avoid cavitation. 
     Preferably, the hydraulic actuators comprises hydraulic cylinders having a first chamber and a second chamber, each respective chamber fluidly connected to the metering control valve assembly. Where the assembly comprises float logic valves, these logic valves are coupled to each of the first and second chambers such that when activated by the electronically controllable valve, at least some of the hydraulic actuators are capable of operating in a ‘float’ mode. Such a configuration is advantageous to control the number of ‘positively activated’ hydraulic actuators within the system with at least one hydraulic actuator being positively actuated by the metering control valve assembly and at least some of the hydraulic actuators being floating or idle when not supplied with fluid from the metering control valve assembly. 
     Preferably, the electrohydraulic valves are electrically controlled independently of one another. More preferably, the electronically controllable valve (i.e., the solenoid control valve) is electrically controlled independently of the electrohydraulic valves. Each of the electrohydraulic valves and the solenoid control valve may comprise suitable fluid flow rate and speed sensors that are in turn coupled or provided wired or wireless communication with an external control such as a computer, a network, a remote computer, processor based utility or server. 
     Preferably, the system comprises three hydraulic cylinders each having a first chamber and a second chamber wherein each of the first and second chambers of a first cylinder are fluidly connected directly to the electrohydraulic valves of the metering control valve assembly and each of the first and second chambers of a second and third cylinder are fluidly connected to the electrohydraulic valves of the metering control valve assembly via the logic valves of the transmission valve assembly. Such a system is particularly suitable to control a pivoting support arm of a bulk processing machine such as an undercutting mining machine. 
     Optionally, the electrohydraulic valves may comprise any one or a combination of proportional solenoid valves, directional control values or servo valves. Within this specification, reference to electrohydraulic valves encompasses valve types configured to control flow direction, flow volume and fluid pressure via electrical signal control of moving components (i.e., spools) within the valves. Such valves may be operated using AC or DC power. 
     According to a second aspect of the present invention there is provided mechanically actuating apparatus having a moveable member controlled by at least one hydraulic actuator of the hydraulic system as claimed herein. 
     Preferably, the mechanical actuating apparatus is an undercutting mining machine and the movable member is a support arm mounting at least one cutting head. Preferably, the hydraulic system is configured to control a pivoting of the arm at least in a direction laterally inward and outward relative to a main body or chassis of the mining machine. Optionally and additionally or alternatively, the hydraulic system may be configured to control a pivoting of the arm in a vertical plane (upward and downward direction). 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a mobile undercutting mining machine suitable for creating tunnels and subterranean roadways having a pair of forward mounted pivoting cutting arms each mounting a set of roller cutter units wherein each of the cutter arms are controlled by hydraulic actuators that are in turn controlled by a hydraulic system according to one aspect of the present invention; 
         FIG. 2  is a schematic illustration of a hydraulic control system implemented to control movement of an actuator for fast movement and with low external force resistance in a first direction according to one aspect of the present invention; 
         FIG. 3  is a schematic illustration of a hydraulic control system implemented to control movement of an actuator for fast movement and with low external force resistance in a second direction according to one aspect of the present invention; 
         FIG. 4  is a schematic illustration of a hydraulic control system implemented to control movement of an actuator for slow movement and with high external force resistance in a first direction according to one aspect of the present invention; 
         FIG. 5  is a schematic illustration of a hydraulic control system implemented to control movement of an actuator for slow movement and with high external force resistance in a second direction according to one aspect of the present invention; 
         FIG. 6  is a schematic illustration of a hydraulic system according to a further embodiment of the present invention comprising three hydraulic actuators implemented to control movement of an actuator for slow movement and with high external force resistance in a first direction. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION 
     A hydraulic system according to the present invention is capable of providing independent metering for controlled movement and static positioning of hydraulic actuators that in turn are coupled to mechanical members such as pivoting arms and the like. A specific implementation of the present hydraulic system is described with reference to movement and positional control of pivoting cutting arms of a mining undercutting machine. In particular, the present hydraulic system provides a configuration to change modes of operation of the movement of the cutting arms between for example a cutting mode in which an arm is controlled to move at slow speed and with high external load force resistance and an idling mode in which an arm is controlled to move at high speed and with low external load force resistance via the same hydraulic system. 
     Referring to  FIG. 1 , cutting apparatus  10  is configured to cut into rock within a mining environment to create drifts, subterranean roadways and the like so as to form an underground mine network. Apparatus  10  is configured for operation in an undercutting mode in which a plurality of rotatable roller cutter units  13  may be forced into the rock to create a groove or channel and then to be pivoted vertically upward so as to overcome the reduced tensile force immediately above the channel and break the rock. Accordingly, the cutting apparatus  10  is optimised for forward advancement into the rock using less force and energy typically required for conventional compression type cutters that utilise cutting bits or picks mounted at rotatable heads. 
     Apparatus  10  comprises a main frame  11   a  (or chassis) that mounts a sled  11   b  capable of sliding forward and aft along a forward region of the sled  11   a . A pair of support arms  12  are mounted at a forward region of sled  11   b  and are configured with parts to pivot independently via a generally horizontal pivot axis and a generally vertical pivot axis. A respective cutting head  15  is mounted at the distal end of each arm  12  and by rotation about the respective horizontal and vertical pivot axes is capable of being raised in a vertical plane (up and down) and to be slewed laterally in a horizontal plane (side-to-side). Each cutting head  15  mounts a plurality of cutter units  13 , with each unit  13  rotatably mounting a respective cutter ring  14  (otherwise referred to as a roller cutter). As will be appreciated, apparatus  10  further comprises additional components associated with conventional undercutting apparatus including in particular an electric motor, jacking legs, tracks etc. 
     The lateral slewing movement of each arm  12  is provided by selective actuation of a first pair of externally mounted hydraulic cylinders  16 ,  17  and an internally mounted hydraulic cylinder  18 , with each of the three cylinders being configured to control one of the two arms via linear extension and retraction of the piston shafts as will be appreciated. 
       FIGS. 2 to 5  illustrate a first embodiment of the subject invention configured to provide actuation control of two hydraulic cylinders  16 ,  18 .  FIG. 6  illustrates a further embodiment configured for control of three hydraulic cylinders  16 ,  17 ,  18  illustrated by way of example to control each arm  12  of the mining machine of  FIG. 1 . Referring to the embodiment of  FIGS. 2 to 5  the hydraulic system comprises a variable displacement pump  21  fluidly connected to a drain or reservoir  20 . A metering control valve assembly  22  is fluidly connected via an inlet conduit  43  to pump  21 . Metering control valve assembly  22  is also fluidly connected to a drain or reservoir  20  via outlet conduit  44 . Valve assembly  22  comprises four electrohydraulic valves implemented as proportional solenoid valves  29 ,  30 ,  31 ,  32  fluidly connected to each other, the inlet and outlet conduits  43 ,  44  and a first  40  and a second  41  conduit extending from assembly  22  to provide fluid connection with hydraulic actuator  16 . As illustrated, actuator  16  comprises a cylindrical housing mounting a piston  35  coupled to a piston rod or shaft  34  capable of linear extension and retraction from the cylinder housing. First and second internal chambers  36   a ,  36   b  are partitioned by piston  35  with each chamber  36   a ,  36   b  coupled respectively to the conduits  41 ,  40  and in turn the metering control valve assembly  22 . The first and second conduits  40 ,  41  are positioned in fluid connection with the proportional solenoid valves  29 ,  30 ,  31 ,  32  at a position (in fluid connection) between respective pairs of valves  29 ,  32  and  30 ,  31 . That is, each of the proportional solenoid valves  29  to  32  are fluidly connected together in a loop with the first and second conduits  40 ,  41  fluidly coupled at regions on opposite sides of the loop. 
     The hydraulic system further comprises a transmission valve assembly  23  comprising a pair of logic valves  26 ,  27 , a solenoid control valve  24 , a pressure reducing/relieving valve  28  and an auxiliary logic valve  25 . Solenoid control valve  24  is fluidly connected via pilot conduit  46  to logic valves  26 ,  27  and is also fluidly connected to pump  21  via supply conduit  45 . Logic valve  26  is fluidly connected to the metering control valve assembly  22  via the first conduit  40  and the second logic valve  27  is fluidly connected to the metering control valve assembly  22  via the second conduit  41 . The pressure reducing/relieving valve  28  is fluidly connected to the first, second and auxiliary logic valves  25 ,  26 ,  27  via a reducing/relieving conduit  38 , with valve  28  also being fluidly coupled to pump  21  via supply conduit  45 . 
     The second hydraulic actuator  18  comprises the same configuration as the first actuator  16  and is coupled in fluid connection to the transmission valve assembly  23  via a third conduit  42  and a fourth conduit  48 . Third conduit  42  provides fluid connection between the second actuator first chamber  36   a  and the first logic valve  26  and the fourth conduit  48  provides fluid connection between the second actuator second chamber  36   b  and the second logic valve  27 . A first float logic valve  33  is provided at the second actuator  18  and is coupled to provide a fluid flow circuit between the first and second chambers  36   a ,  36   b . Float logic valve  33  is coupled to the pilot conduit  46  so as to be pilot controlled by the solenoid control valve  24 . Float logic valve  33  is also coupled to the third and fourth conduits  42 ,  48 . Each of the valves  24 ,  25 ,  26 ,  27 ,  28  and  33  are all fluidly coupled to a further reducing/relieving conduit  47  which is in turn fluidly connected to the drain or reservoir  20 . Additionally, auxiliary logic valve  25  is further fluidly connected to the third conduit  42  so as to be positioned in a fluid flow direction intermediate the pressure reducing/relieving valve  28  and the second actuator  18 . 
     Accordingly, first actuator  16  is fluidly connected directly to the metering control valve assembly  22  via first and second conduits  40 ,  41 . Second actuator  18  is coupled indirectly to the metering control valve assembly  22  via the transmission valve assembly  23  and in particular fluid connection via the third and fourth conduit  42 ,  48  and the respective first and second logic valves  26 ,  27  (that are in turn coupled to the first and second conduits  40 ,  41 ). 
     According to the specific implementation, the first and second logic valves  26 ,  27  are ‘normally open’; the auxiliary logic valve  25  is ‘normally closed’ such that when solenoid control valve  24  is actuated, logic valves  26 ,  27  are closed whilst auxiliary logic valve  25  is open. Accordingly, when solenoid control valve  24  is deactivated valves  26 ,  27  are open and valve  25  is closed. Additionally, float logic valve  33  is ‘normally closed’ and like auxiliary logic valve  25 , is configured to be open by actuation of solenoid control valve  24 . 
       FIG. 2  illustrates control of cylinders  16  and  18  when apparatus  10  (mining machine) and in particular arm  12  is in an ‘idling’ mode so as to provide fast movement of arm  12  under low external load force in a first movement direction;  FIG. 3  illustrates operation of the hydraulic system with the cylinders  16  and  17  operating in the same idling mode but in the reverse direction;  FIG. 4  illustrates control of the cylinders  16  and  18  in a ‘cutting’ mode in which arm  12  is moved slowly and is maintained in a more ‘stiff’ configuration so as to withstand high external load force moving in a first direction and;  FIG. 5  illustrates control of cylinders  16  and  18  in the same ‘cutting’ mode of  FIG. 4  with the arm  12  moving in the opposite direction with approximately the same slow speed and similar high load balancing.  FIG. 6  illustrates the embodiment of the present system controlling three linear hydraulic actuators  16 ,  17  and  18  as will be described in further detail below. 
     As will be appreciated, the proportional solenoid valves  29  to  32  are configured to selectively regulate fluid flow speed and pressure via independent electronic controls. The fluid flow and valve control configuration is described with reference to  FIG. 2  and it will be appreciated that corresponding fluid flow directions and valve control configurations are applied equally to the different modes and operation of the linear actuators  16 ,  18  according to the desired direction of movement of the shafts  34  according to  FIGS. 3 to 5 . As illustrated in  FIG. 2 , fluid is supplied to the metering control valve assembly  22  via pump  21  and inlet conduit  43 . Fluid is then supplied at the desired rate and pressure to cylinder second chamber  36   b  via first conduit  40 . Fluid from cylinder first chamber  36   a  is directed to the drain or reservoir  20  via the second conduit  41 , the metering control valve assembly  22  and the outlet conduit  44 . In this configuration, cylinder  16  is ‘active’ whilst cylinder  18  is maintained in a ‘floating’ mode without fluid supply from the metering control valve assembly  22 . In this mode of operation, fluid flows from first chamber  36   a  of second cylinder  18  to the second chamber  36   b  via float logic valve  33 . Excess fluid (due to the difference in chamber size) from the first chamber  36   a  is transferred to the reducing/relieving conduit  47  via third conduit  42 , auxiliary logic valve  25  and pressure reducing/relieving valve  28 . This mode of operation according to  FIG. 2  is provided by activating solenoid  24  to close valves  26 ,  27  and open valve  25 . In the reverse flow direction illustrated in  FIG. 3 , the pressure reducing/relieving valve  28  is controlled to provide the reverse fluid flow direction through auxiliary logic valve  25  so as to provide the opposite fluid flow direction between the first and second chambers  36   a ,  36   b  of second cylinder  18 . Naturally, the reverse fluid flow direction through first and second conduits  40 ,  41  is provided by the selective electronic control of the four proportional solenoid valves  29  to  32 . 
     Referring to  FIGS. 4 and 5 , where it is required to actuate cylinders  16  and  18  according to the ‘cutting’ mode (i.e. with slow speed and high stiffness/high external load force resistance) solenoid control valve  24  is deactivated so as to allow fluid flow from metering control valve assembly  22  to the second cylinder  18  via logic valve  26 . In this configuration, the metering control valve assembly  22  is provided in fluid connection to both cylinders  16 ,  18  and accordingly the fluid flow speed and pressure is divided approximately equally to achieve a higher force resistance and a proportionally lower actuation speed relative to the ‘fast’ mode of operation of  FIGS. 2 and 3 . In such a configuration the pressure reducing/relieving valve  28  and the auxiliary logic valve  25  are isolated from the fluid flow at the first, second, third and fourth conduits  40 ,  41 ,  42  and  48 . 
     The components, construction and functionality of the embodiment of  FIG. 6  is the same as the embodiment of  FIGS. 2 to 5  save for the additional actuator  17  and a corresponding additional float logic valve  37  (associated with the actuator  17 ). As illustrated, the second and third actuators  18 ,  17  are fluidly connected via the third and fourth conduits  42 ,  48  and in particular the first chamber  36   a  of the second actuator  18  is coupled to the second chamber  36   b  of the third actuator  17  via the third conduit  42  and the second chamber  36   b  of the second actuator  18  is coupled to the first chamber  36   a  of the third actuator via the fourth conduit  48 . Accordingly, excess fluid from the first respective chamber  36   a  may be transferred to the respective to the second chamber  36   b  of the alternate actuator  18 ,  17  when the second and third actuators  18 ,  17  are operated in ‘float’ or ‘idling’ mode and not driven directly by the metering control valve assembly  22 . As with the embodiment of  FIGS. 2 to 5 , regulation of the fluid supply pressure and flow rate to the second and third actuators  18 ,  17  is controlled via the metering control valve assembly  22  when solenoid control valve  24  is deactivated and the logic valves  26  and  27  are opened as default. As will be appreciated, the third actuator  17  is configured for the same corresponding directional movement of the first actuator  16  such that first and third actuators  16 ,  17  are motion paired and the second actuator  18  is operable in the corresponding reverse stroke direction. As with the embodiment of  FIGS. 2 to 5 , both the second and third actuators  18 ,  17  and in particular the respective first and second chambers  36   a ,  36   b  are fluidly coupled to the drain or reservoir  20  via the reducing/relieving conduit  47 , the third conduit  42 , the auxiliary logic valve  25  and the pressure reducing/relieving valve  28  so as to provide regulation of the fluid flow given the respective differences in the volumes of the first and second chambers  36   a ,  36   b . Additionally, and as with the embodiment of  FIGS. 2 to 5 , the auxiliary logic valve  25  and the associated pressure reducing/relieving valve  28  are capable of supplying a constant pressure to the second and third actuators  18 ,  17  and of avoiding cavitation via fluid connection to the drain or reservoir  20  via reducing/relieving conduit  47 . 
     The present hydraulic system provides a mechanism and a method of controlling the speed of pivoting movement of the arms  12  in the lateral inward and outward direction in addition to providing control of the ‘stiffness’ of the movement and hence a relative resistance to the external load applied to the arms primarily resultant from contact with the rock strata when in cutting mode. For example, and in a fast mode of operation, only the first actuator  16  is positively displaced by metering control valve assembly  22  corresponding to a non-cutting operation as the arm  12  is returned to an in-board position. In a cutting mode, all three actuators  16 ,  17 ,  18  are positively displaced so as to distribute the applied fluid pressure from the metering control valve assembly  22  and to provide greater resistance to the external load as the cutting units  13  are forced against the rock strata during cutting. A maximum load would be encountered for example when arm  12  is extended fully (orientated approximately 90° to the orientation of  FIG. 1 ) via pivoting of the arm  12  about the generally horizontal axis). In such a position, proportional solenoid valves  29  to  32  would be controlled so as to deliver maximum pressure to the actuators  16 ,  17 ,  18  with low fluid flow speed. 
     By utilising proportional solenoid valves, at least one solenoid control valve and logic valves, the present hydraulic system provides a versatile mechanism and method for quantitative adjustment of the speed of movement and force of displacement of the actuators  16 ,  17 ,  18  according to predefined operating conditions.