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CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 13/382,207, filed on Jan. 4, 2012, which is a national phase application of International Application No. PCT/DE2010/000685, filed on Jun. 18, 2010, which claims priority to German Patent Application No. 10 2009 033 572.2, filed on Jul. 16, 2009, and the present application also claims priority to German Patent Application No. 10 2012 003 087.8, filed on Feb. 18, 2012, each of which is hereby incorporated herein in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a hydraulic circuit for longwall support by means of a support device (support shield) for use in underground mining. 
     2. Description of Related Art 
     Such circuits are well known from PCT/DE2010/000685 (the publication of which is W02011006461A2). The proposed pressure monitoring system prevents unforeseen operating conditions where pressure conditions may occur that are sufficient to manipulate the hydraulic pilot control, i.e. the opening of essential valves even in case of failures to the pumping system, or if in case of an emergency the overall electric and hydraulic control systems are switched off, or in case of extremely high pressures from the rock which the load maintaining valves are not capable of handling. 
     The arrangement of W02011006461A2 according to one embodiment also monitors the annular piston line for each cylinder/piston assembly by means of a pressure sensor. When a pre-determined maximum pressure is achieved, the entire longwall is depressurized so that particularly the unlocking process for the check valves that retain the rock pressure is disabled. This can, however, cause operating conditions that may require the system to be controlled either manually or automatically. 
     The purpose of the invention is to design the circuit in such a manner that a comprehensive monitoring of the system as well as any necessary manipulation of the control system is possible. 
     SUMMARY OF VARIOUS EMBODIMENTS 
     This is achieved by means of the various embodiments described herein. 
     An alternate embodiment includes a supplementary improvement that allows for a so-called negative emergency operation, and therefore enables the control if, due to a pressure signal activated by the pressure monitor, an emergency signal would cause a system failure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention is hereafter described by means of a preferred embodiment. Explicit reference is made to the drawing descriptions relating to FIGS. 1A, 1B and 2 of W02011006461A2 and particularly to FIG. 1B and its description. 
       The terms used and their reference signs are also taken up in this application. Any divergences will be expressly noted in the following detailed description of the invention. 
         FIG. 1  is an electrical/hydraulic circuit of a support shield in a longwall mine in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The electric/hydraulic circuit of a support unit in a longwall according to W02011006461A2 comprises the following elements, which are also illustrated or indicated in the drawing:
         1. the longwall supply line  1  (pumps—manifold, flow pipe), which extends through a portion of the longwall or the entire length of the longwall and which is connected to the pump station—without reference mark.   2. the return flow manifold  2  (return flow—manifold, return flow), which extends through a portion of the longwall or the entire length of the longwall and which is connected to the tank—without reference mark—of the pump station.   3. the hydraulic control device of the shield control device for a support shield. Shown is one of the power transmitters  4 . The hydraulic control device  3  is connected through the feed line stub  12  with the feed line and through the return line stub  13  with the return line.   4. A power transmitter, which is here illustrated as cylinder-piston unit.   5. The electrical control device  5  of the shield control unit for controlling the hydraulic control device  3 . The hydraulic control device  3  and the electrical control device  5  together comprise the excavation control device, which is designed for inputting switching and control commands, however, may also receive its switching and control commands from the central longwall control device  15 .       

     Other existing secondary valves, particularly check valves, have not been illustrated or further described. 
     The hydraulic control device comprises multiple valves. The connection for each power transmitter  4  with the pump manifold of the longwall between the power transmitter outlet, which is acted upon by the rock pressure, and the hydraulic control device  5  is generally blocked by a pressure holding valve  14  which is designed as an unlockable check valve so that in case the pump pressure fails or is turned off the load pressure of the power transmitter acts upon the tightly locking check valve  14 . This check valve  14  can be unblocked by means of hydraulic pilot operation through the system pressure when the pressure variation between load pressure and pilot pressure fall below a value that is predetermined by the valve construction. The check valve  14  is hydraulically designed in such a manner that when it is hydraulically unblocked the working space of the power transmitter is through outlet  6  and the return line stub connected with the return line manifold. Such unlockable check valve is, for example, well known through DE 38 04 848 A1. 
     The pressure monitoring device  19  prevent the pressure between the unlockable check valve  14  and the cylinder annular space and/or the hydraulic control device  3  from reaching a level that could cause the check valve  14 , which acts as a load maintaining valve, from being unintentionally unlocked (turned on). See also W02011006461A2. 
     The detailed drawing of  FIG. 1  illustrates the individual valves of the hydraulic control device  3 . 
     The pilot control valve  16 . 1  for setting the power transmitter and the pilot control valve  16 . 2  for removing the power transmitter are both activated through bus line  20  by means of the electric control device  5  of the support shield and/or by means of the central longwall control unit  15  through the signal line  21  and hydraulically activate the main valve  17 . 1  for setting the power transmitter and main valve  17 . 2  for removing the power transmitter between two settings. 
     The encoding of the switching signals causes the magnet of the pilot control valves to be interlocked in the following manner:
         when hydraulically actuated in the course of setting (lifting):
           Main valve  17 . 1  opens the connection (feed line stub  12 , setting line  22 ) between longwall supply line (pump line, pressure line)  1  and power transmitter input  6 ;   Main valve  17 . 2  releases the connection (annular piston line  10 , return line stub  13 ) of the annular space  24  to the return line manifold  2 .   The piston of power transmitter  4  and the load acting upon it are elevated.   
           at standstill:
           Main valve  17 . 1  blocks the connection between connection  6  of the power transmitter and longwall supply line (pump line, pressure line)  1  and opens the connection to the return line manifold  2 .   Main valve  17 . 2  releases the connection of the annular space  23  to the return line manifold  2 .   The load acting upon the piston of the power transmitter is held by the blocked check valve  14 /load maintaining valve.   
           when hydraulically actuated in the course of removing the timbering (lowering the piston):
           Main valve  17 . 1  blocks the connection between longwall supply line (pump line, pressure line)  1  and opens the connection to the return line manifold  2 .   Main valve  17 . 2  releases the connection of the annular space  23  to the longwall supply line (pump line, pressure line)  1 .   The load acting upon the piston of the power transmitter is held by the blocked check valve  14 /load maintaining valve until the check valve  14  is unblocked by the rising pressure in the annular piston line  10  through line stub  24 .   The load acting upon the piston of the power transmitter is thus lowered.   
               

     Standstill is a critical condition since persons staying inside the longwall are subject to injury or death from the unintentional movement of the expansion equipment. This hazard is prevented by the pressure sensor  19  which is installed into annular piston line  10  of each cylinder/piston assembly  4 . Each of these pressure sensors  19  is switched through another bus line  20  to the longwall shutoff valve  11  through the central longwall control device  15  in such a manner that upon reaching a predetermined maximum pressure in the annular piston line  10  the entire longwall is pressureless switchable. This also ensures that internal pressure, which could cause the unblocking of the check valves that are holding the rock pressure, is no longer present. Therefore the maximum allowable pressure at which all support units of the longwall are depressurized is set significantly lower, in fact at least 20% lower, for example, at 50 bar, than the inherent pressure of, for example, 80 bar that is sufficient to unlock the check valve. 
     It is, however, possible for operating conditions to occur for which it may become desirable or even necessary to manually adjust the control system. For this reason it is intended that the longwall shutoff valve  11  can only be activated when a triggering signal for the main valves  16 . 1 ,  16 . 2  is no longer present. For this reason the central longwall control device  15  is activated through a UND—member  25 , which only sends a positive output signal if the pressure signal of the pressure sensor  19  is positive and at the same time the signal for triggering the pilot control valves  16 . 1 ,  16 . 2  is negative. This is here illustrated by means of a negative (NANO) member  26 , which is connected with the signal line  21  and only sends a positive output signal to the UND member  25  of the longwall control device  15  when a negative input signal is present. This prevents the possibility that the pressure sensor  19  interferes with any intentional manually or automatically controlled operating condition or process of the power transmitter. Such a situation could cause serious hazards. 
     It is furthermore intended that the function of the pressure sensor  19  can be completely disabled. For this purpose a push button switch  28  is installed in the line between longwall control device  15  and longwall shutoff valve  11 . If necessary, this push button switch can be opened when it is disadvantageous that the entire longwall is shut off accidentally. It is also possible to bypass the pressure sensor by means of a circuit which is not illustrated here.

Summary:
A hydraulic circuit for longwall support for use in underground mining for supporting a longwall by means of a plurality of support shields includes in the annular piston line of each cylinder/piston unit a pressure sensor. The pressure sensor, upon reaching a predetermined maximum pressure activates a pressure deviation signal, causes the entire longwall to be depressurized by means of a longwall shut-off valve. The pressure deviation signal is blocked for one of the hydraulic valves against each triggering signal.