Abstract:
The invention relates to a circuit for enabling or disabling hydraulic actuating devices, in particular for enabling the braking system of a hydraulic propel drive, having a device for checking the pressure level in working lines or control lines of hydraulic circuits, a maximum or minimum pressure level being checked in two or more working lines or control lines. An ‘OR’ logic circuit is provided for checking the pressures. In said ‘OR’ logic circuit, two or more pistons are arranged in series in a common bore. In each case one of the pressures to be checked is introduced into one of the spaces between the pistons and the space between one of the pistons and the base of the bore.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a circuit for enabling or disabling hydraulic actuating devices according to the features of claim  1 .  
         [0002]     In certain operating states, actuating devices in hydraulic machines must be disabled and/or it must be checked before enablement that no dangerous and uncontrollable travelling states could be encountered. For example, in hydraulically driven vehicles, it must be checked several times before releasing the brakes after stopping whether the hydraulic propel drive is actually in a neutral setting. If such checks are not carried out, sudden unpredictable acceleration, and in the case of a dual path drive, unexpected steering movements, can occur.  
         [0003]     There have been a very wide variety of proposals for control in the above described context. For example, the checking can involve determining the positions of relevant parts such as swashplate, servo pistons, loop flushing valves and the like at which transducers are provided, whose signals are processed in order to enable the actuation of the relief valve for the brakes or to prevent said enablement if the transducers indicate impermissible deviations. Small positional deviations, however, are often sufficient to cause impermissibly high pressures in the propel drive. Here, position monitoring has the disadvantage that it is often not sufficiently precise for this purpose.  
         [0004]     The present invention is intended to provide a simple circuit, based on pressure monitoring, for enabling or disabling hydraulic actuating devices.  
         [0005]     Said aim is achieved according to the invention by means of a circuit for enabling or disabling hydraulic actuating devices having a device for monitoring the pressure level in working or control lines of hydraulic circuits, a maximum or minimum pressure level being checked in two or more working lines or control lines. An ‘OR’ logic circuit is provided for checking the pressures, said ‘OR’ logic circuit being formed as a series arrangement of two or more pistons in a common bore. In each case one of the pressures to be checked is introduced into one of the spaces between the pistons and the space between one of the pistons and the base of the bore. This results in a particularly simple, compact measuring arrangement, by means of which it is possible to determine whether, for example, there is an impermissibly high pressure prevailing in one of the ports. This result remains independent of whether an increased pressure in the base of the bore displaces all the pistons together towards the wall situated opposite the base of the bore, or the pressure in one or more of the intermediate spaces splits the piston arrangement apart, because in any case, at least the outer piston is displaced in the direction of the wall situated opposite the base of the bore. The advantage is that a plurality of shuttle valves and/or sensors can be dispensed with in this way. Only one single component is required instead of, for example, three shuttle valves for four pressures.  
         [0006]     In the enabling circuit according to the invention, a return spring is preferably provided which pushes the pistons into contact with one another in the base of the bore, the return spring preferably being arranged at one side between the outer piston and a wall which terminates the bore. The pistons of the ‘OR’ logic element are therefore automatically returned to their initial position when a sufficiently unpressurized state is obtained in all the lines which are to be checked. Here, the spring force of the return spring is in a defined ratio to the permitted level of the pressures which are monitored.  
         [0007]     One advantageous embodiment of the circuit according to the invention ensures that the space upstream of the outer piston, which is situated at the side remote from the base of the bore, or the space between the outer piston and a wall which terminates the bore and is situated opposite the base of the bore, is connected to a low, approximately constant pressure level which serves as a reference pressure relative to the pressures to be monitored. The tank pressure is particularly suitable for this purpose.  
         [0008]     In an advantageous development of the invention, the pistons have annular grooves at each end, said annular grooves making it possible to fully utilize the short piston length as a guide length in the bore, and at the same time ensuring, in all switching positions, the connection between the supply bores and the piston end sides, with the pistons floating in the bore in a balanced fashion as a result of the uniform pressure distribution effected by means of the annular grooves. The condition here is that the annular webs which remain between the grooves and piston end sides are narrower than the diameters of the supply bores, so that the supply bores cannot be completely covered by the annular webs in any position of the pistons.  
         [0009]     Transverse grooves are preferably formed in the piston end sides, as a result of which the pressures can be distributed better over the piston end sides.  
         [0010]     It is advantageous if all the pistons are of identical design. This results in the same piston arrangement always being obtained during assembly, regardless of the sequence of the pistons and regardless of which side of a piston is inserted first when the latter is assembled into the bore.  
         [0011]     The position of the outer piston, which is situated at that wall which is situated opposite the base of the bore, is decisive for the enablement or non-enablement of the actuating device. If said outer piston is in its initial position, this signals that no impermissibly high pressure is prevailing in any of the lines being monitored. If said outer piston is situated outwith its initial position, this means that the pressure in at least one of the lines being monitored is impermissibly high.  
         [0012]     The position of the outer piston can advantageously be measured electronically by means of suitable transducers, for example by means of proximity switches or displacement transducers. Alternatively, the determination can also be carried out by measuring the support force of the return spring which, for example, is determined by means of a load cell.  
         [0013]     The signal which reflects the pressure state in the lines being monitored can also be passed on to the actuating device which is to be enabled or disabled, by virtue of the outer piston of the ‘OR’ logic circuit displacing a control piston which, as a function of the position of the outer piston, generates or dissipates a hydraulic signal which can be used directly to disable or enable the braking system. The hydraulic signal from the control piston can, however, be processed instead or simultaneously by means of pressure sensors. Here, in an advantageous embodiment, the outer piston itself forms the control piston.  
         [0014]     The output signals generated by the transducer are preferably displayed visually and/or audibly.  
         [0015]     In one preferred embodiment, a load spring is arranged, in addition to the return spring, between the outer piston and the control piston, so that the mechanical connection between said two pistons is produced only by means of said load spring. So that the outer piston does not come into direct contact with the control piston when it is displaced towards the control piston, a mechanical stop is provided on two sides between the two pistons, said mechanical stop delimiting the stroke of the outer piston so that the force acting on the control piston is determined by the stroke and the spring rate of the load spring and is independent of the level of the pressures being monitored and their force action on the outer piston and the return spring. In addition, the control piston is actuated from the side situated opposite the outer piston by means of an electromagnet. The spring force of the load spring is matched to the magnet force characteristic curve of the electromagnet such that the magnet can move the control piston counter to the load spring at relatively low spring force values, when the outer piston is in its initial position, so as to provide enablement or disablement, and can still hold the control piston at relatively high spring force values when the operating pressures act after enablement or disablement. The magnet force is, however, not sufficient to displace the control piston counter to the load spring if the latter has already been preloaded by means of a displacement of the outer piston against the load spring as far as the mechanical stop. Said adaptation can be improved in a particularly advantageous way by virtue of the electromagnet being operated with higher current after the braking system is enabled.  
         [0016]     One advantageous development of said arrangement is to adapt the diameter of the piston, in relation to the permissible pressures, to the force level of the load spring such that the load spring simultaneously replaces the return spring.  
         [0017]     Further features and advantages of the invention can be gathered from the following description of the exemplary embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     In the figures:  
         [0019]      FIG. 1  shows a first exemplary embodiment of the hydraulic mechanical ‘OR’ logic circuit according to the invention,  
         [0020]      FIG. 2  shows a second exemplary embodiment of the hydraulic mechanical ‘OR’ logic circuit according to the invention using a return spring and a load spring,  
         [0021]      FIG. 3  shows an exemplary embodiment of the pistons K 1 , K 2 , K 3 , K 4 ,  
         [0022]      FIG. 4  shows a third exemplary embodiment of the hydraulic mechanical ‘OR’ logic circuit according to the invention using only one spring. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]      FIG. 1  shows a hydraulic mechanical ‘OR’ logic circuit  1 according to the invention. In the present case, said circuit serves to monitor four ports of dual pumps in a dual path hydraulic drive. Four identical pistons KI, K 2 , K 3 , K 4 , corresponding to the number of pressures to be monitored, are arranged in series in a cylindrical bore  2 , said pistons KI, K 2 , K 3 , K 4  being pushed into contact with one another in the base  3  of the bore by means of a pressure spring, the return spring  4 . Here, the base  3  of the bore can also be formed as a detachable cover which closes off the cylindrical bore  2 . The return spring  4  is supported on a wall  5  which is situated opposite the base  3  of the bore, terminates the bore  2  and, for assembly purposes, can likewise optionally be detachable. The bore  2  has a stop  10  for limiting the piston stroke.  
         [0024]     The pistons KI, K 2 , K 3 , K 4  have extensions  6 , by means of which they are supported against one another and against the base  3  of the bore in the axial direction of the bore  2 , said pistons KI, K 2 , K 3 , K 4  producing between them in each case, and between the base  3  of the bore and the adjacent piston Ki, a space into which the pressure ports A 1 , A 2 , Bi, B 2  open out. Abnormally high pressure in one of the lines results in at least the outer piston K 4  being displaced counter to spring force, this result being independent of whether an increased pressure in the base of the bore displaces all the pistons together (as illustrated in  FIG. 1 ) or the pressure in one of the other intermediate spaces splits the piston arrangement apart counter to the spring  4  at one side and towards the base  3  of the bore at the other side.  
         [0025]     In the exemplary embodiment, the wall  5  comprises a transducer  19 , whose measurement signals allow conclusions to be drawn regarding the pressure state in the pressure ports A 1 , A 2 , BI, B 2 . In the exemplary embodiment, the transducer  19  is a load cell which measures the force of the spring  4 .  
         [0026]      FIG. 2  shows a further exemplary embodiment in section. As in the previous example, the circuit serves to monitor four ports of dual pumps in a dual path hydraulic drive. Four pistons KI, K 2 , K 3 , K 4  are therefore likewise arranged, in turn, in the cylindrical bore  2 , said pistons KI, K 2 , K 3 , K 4  forming between them in each case, and between the first piston Ki and the base  3  of the bore, a space for introducing one of the pressures to be monitored, said pressures being applied via the pressure ports A, B and the supply ducts  14 .  
         [0027]     The pistons KI, K 2 , K 3 , K 4  are in turn pushed into contact with one another in the base  3  of the bore by means of a pressure spring, the return spring  4 , it being possible here for the base  3  of the bore to be embodied as a detachable cover which closes off the bore.  
         [0028]     The pistons KI, K 2 , K 3 , K 4  each have a transverse groove  15  at the end side  13  in order to ensure optimum pressure action on the piston. In each case one annular groove  11  which generates narrow annular webs  12  between the piston end sides  13  and the annular grooves  11  is formed at the periphery of the piston, said annular webs  12  having smaller dimensions than the diameter of the supply ducts  14 . The pistons are guided in a floating manner in the bore  2  in this way while an unbroken connection is simultaneously ensured between the supply ducts  14  and the piston end sides  13 .  
         [0029]     The stroke of the outer piston K 4  is limited by a stop I  0   a  in the bore  2 . The load spring  9  is supported at one side on the outer piston K 4  and at the other side on a control piston  8 . The stroke of the control piston  8  is likewise limited by a stop I  0   b , so that the outer piston K 4  and the control piston never come into direct contact.  
         [0030]     When the electromagnet  7  is switched on, the control piston  8  moves towards the pistons KI, K 2 , K 3 , K 4  as long as the force generated by the electromagnet  7  on the control piston  8  prevails over the spring force of the load spring  9 . Here, the spring forces of the load spring  9  are adapted such that only a relatively small force is exerted on the control piston when all the pistons K 1 , K 2 , K 3 , K 4  are in their initial position. In this case, when the electromagnet  7  is switched on, the control piston  8  compresses the load spring  9  until it reaches its stop  10   b  in the bore  2 . Here, the characteristic curve of the electromagnet  7  is dimensioned such that, in said state, the electromagnet  7  applies a sufficient holding force to hold the control piston  8  against the load spring  9  even when a corresponding high pressure is required during travelling and the pistons K 1 , K 2 , K 3 , K 4  are retrospectively displaced counter to the load spring  9  ( FIG. 2  shows this state). For this purpose, in said exemplary embodiment, the characteristic curve of the electromagnet  7  is additionally expanded by means of simultaneous variation of the switching current level.  
         [0031]     If, on the other hand, when the magnet  7  is switched on, the load spring  9  has already been compressed by the displacement of the piston KI, K 2 , K 3 , K 4  as a result of a malfunction in the propel drive and, as a result, an increased spring force is exerted on the control piston  8 , then the electromagnet  7  is not capable, as a result of the respective adapted characteristic curves, of displacing the control piston  8  out of its initial position counter to the more highly preloaded load spring  9 .  
         [0032]     In the exemplary embodiment, the control piston  8  has the function of a ⅔ directional valve. Its two switching positions determine the enablement or disablement of the braking system. When the electromagnet  7  is inactive, or the load spring  9  exerts too great a force counter to the active electromagnet  7 , said control piston  8  is situated in its initial position. When the electromagnet  7  is active and the load spring exerts a sufficiently low force, said control piston  8 , in its working position, is displaced towards the step  10   b  in the bore  2 .  
         [0033]     The control piston  8  has three pressure ports: the tank pressure port  16 , the pressure supply port  18 , which is preferably supplied with feed pressure, and the pressure port  17  which leads to the braking system. In its initial position, the control piston  8  connects the pressure port  17 , which leads to the braking system, to the tank pressure connection  16 , resulting in the brakes being relieved of pressure and preventing starting. In the working position of the control piston  8  against the stop  10   b , the pressure port  17 , which leads to the braking system, and the pressure supply port  18  are connected to one another, resulting in the brakes being released.  
         [0034]      FIG. 3  shows, in detail, one of the pistons KI, K 2 , K 3 , K 4  having the transverse grooves  15  at the piston end sides  13 . Here, for radial distribution of the pressure for the purpose of floating guidance of the pistons KI, K 2 , K 3 , K 4 , the annular grooves  11  generate annular webs  12  which are penetrated by the transverse grooves  15 , resulting in optimum pressure action on the piston end sides  13 . The annular webs  12  are kept very small so that they do not close off the supply bores  14  (not illustrated in  FIG. 3 ) in any piston position.  
         [0035]      FIG. 4  shows an embodiment similar to  FIG. 1 , but with the difference that the load spring  9  simultaneously serves to return the pistons KI, K 2 , K 3 , K 4  into contact in the base  3  of the bore  2 , it being possible for the base  3  of the bore to also in turn be formed as a detachable cover which closes off the cylindrical bore  2 .  
         [0036]     The enabling circuit operates as follows: if a sufficiently high pressure to displace the piston out of its initial position counter to the springs is not present in any of the lines before the brakes are enabled, then the electromagnet can, when it is switched on, move the control piston counter to the small force of the springs. The brakes are released by the displacement of the control piston and the measurement signal which is reduced as a result.  
         [0037]     After the brakes are released, high pressure is required in any case when moving the vehicle. In the ‘OR’ logic circuit according to the invention, said high pressure displaces the pistons counter to the force of the springs and of the magnet. The brakes cannot, however, re-engage, because the active magnet, as a result of its characteristic curve, can provide a sufficient holding force counter to a retrospective compression of the springs.  
         [0038]     If, on the other hand, when at rest, the pistons are already displaced before the magnet is switched on because the high pressure in at least one of the lines has not dissipated, then the initial force of the magnet is not sufficient to displace the control piston counter to the force of the springs, and is not sufficient to enable the brakes such that they can be released.