Patent Publication Number: US-6981558-B2

Title: Controller for an unbalanced mass adjusting unit of a soil compacting device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a controller for an unbalanced mass-adjusting unit in a vibration exciter. 
   2. Description of the Related Art 
   Vibration exciters of this type are known in particular for use in ground-compaction machines, such as vibration plates or rollers. Conventionally, two—but also several—unbalanced shafts are driven in a positive manner rotating in opposite directions, wherein the superposition of the centrifugal forces of each individual unbalanced shaft serves to produce a resulting force which depending upon the design of the vibration exciter can be adjusted with respect to its value and/or direction. Vibration exciters of this type have been known for a long time particularly in the case of vibration plates, so that a further description thereof would be superfluous. 
   In order to adjust the desired value or the direction of the resulting force vector, it is necessary to be able to change the relative positions of the unbalanced shafts with respect to each other or of the unbalanced masses in relation to the shafts which support them. To this end, either the counter-rotating unbalanced shafts can be adjusted with regard to their mutual phase position or the position of the unbalanced mass on the shaft supporting it can be changed. For this purpose, unbalanced mass-adjusting units are known, wherein a transverse spigot which can be adjusted in an axial manner by an adjusting piston in an unbalanced shaft which is formed as a hollow shaft passes through a longitudinal slit in the wall of the hollow shaft and engages into a spiral groove on the inner side of a hub which supports an unbalanced mass. In the event that the transverse spigot is displaced axially, the transverse spigot is guided in the longitudinal slit, whereby the effect of the spiral groove causes the hub to change its relative position on the shaft which supports it. Therefore, it is possible to achieve the desired, above-described adjustment of the phase position. This structure has also been known for a long period of time and is illustrated e.g. in EP-A-0 960 659. 
   In the case of vibration exciters, the adjusting units thus serve to adjust the phase positions of unbalanced masses with respect to each other in order to reverse the direction of travel or in order to change the so-called “m·r-value”. They are based upon an hydraulic adjustment by means of a piston which is influenced on one side and which in the event of falling oil pressure is reset either by way of a spring device or by way of the restoration moment exerted by means of the unbalanced masses. 
   EP-A-0 960 659 also describes a controller for an unbalanced mass-adjusting unit, wherein the adjusting piston is hydraulically coupled to a reference piston. The position of the reference piston is controlled by way of a pilot valve. In turn, the position of the pilot valve is governed by forces which act upon the valve body from two sides, namely on one side by means of a force which can be predetermined by the operator and can be introduced by way of a spring into the valve body, and on the other side by means of a force which is exerted on to the valve body by means of the reference piston by way of a spring. The known controller is difficult to adjust and does not render it possible to perform different control algorithms. In order to carry out an adjustment or effect a change in the adjustment, it is necessary on each occasion to change the characteristic curves of the sets of springs which influence the valve body, which constitutes either substantial structural or substantial assembly outlay. 
   In the case of more complex vibration exciters, in which in addition to adjusting the phase position it is also possible to adjust the mr-value and thus under certain circumstances several unbalanced mass-adjusting units are provided, it is necessary to obtain a reliable signal relating to the respective position of the adjusting piston and therefore of the unbalanced masses in the exciter which are adjusted by said adjusting piston. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   It is the object of the invention to provide a controller for an unbalanced mass-adjusting unit in a vibration exciter, wherein it is possible to detect in a precise manner the position of the adjusting piston and to perform a convenient and reliable actuation in the event of changes in the position of the adjusting piston. Furthermore, it should be possible to detect the respective position of the adjusting piston and thus of the unbalanced masses or unbalanced shafts which are actuated by said adjusting piston. The object is achieved in accordance with the invention by means of a controller as claimed in claim  1 . Advantageous further developments of the invention are evident in the dependent claims. 
   In the case of the controller in accordance with the invention, the position of the reference piston which is hydraulically coupled to the adjusting piston is detected by a signal-generating device and is relayed in the form of a position signal. The reference piston can be provided with a piston rod, whose position corresponds to the position of the reference piston and thus renders it possible to draw precise conclusions with respect to the position of the adjusting piston and thus the unbalanced masses or unbalanced shafts which are influenced by the adjusting piston. 
   The position signal is transmitted to the control device which can be e.g. an electronic regulating device and which is also influenced by means of an operating signal provided by an operator. The control device compares the position signal, which corresponds to an actual value of the position of the unbalanced masses in the vibration exciter, to the operating signal which in this respect can be regarded as a desired value. The operating signal can be information which is specified by the operator relating to the direction of travel but also the vibration intensity which is to be generated by the vibration exciter. 
   The control device then generates a control signal which is used for the purpose of actuating a pilot valve, in order to control the supply of oil to and the discharge of oil from the second side of the reference piston. It is thus possible by way of the control device to change the position of the reference piston and consequently also to change the position of the adjusting piston. 
   By reason of the presence of the position signal and thus the reliable information relating to the position of the unbalanced masses in the vibration exciter, it is possible with the aid of the control device to perform any control algorithms. If the control device is an electronic controller, then for this purpose it is only necessary to perform changes to the electronically stored program. 
   In the case of a particularly advantageous embodiment of the invention, the adjusting piston is disposed on the vibration exciter, whereas the reference piston is disposed in a region which in terms of vibration is uncoupled from the vibration exciter. This embodiment can be accomplished in a particularly advantageous manner in the case of a ground-compaction device, e.g. a vibration plate which is defined in claim  12  and comprises an upper mass, which comprises a drive, and a lower mass which supports the vibration exciter and is coupled to the upper mass by way of a spring device, wherein the adjusting piston of the controller is disposed on the lower mass and the reference piston is disposed on the upper mass. Since, for the purpose of detecting the position of the reference piston or its piston rod, it is necessary to provide vibration-sensitive components, e.g. proximity sensors, pressure sensors or optical sensors, it is particularly advantageous if the reference piston is disposed on the upper mass, which vibrates to a lesser extent, of the ground-compaction device and not in the proximity of the vibration exciter. This brings about lower stress-loading of the sensitive components and a resulting greater degree of reliability and accuracy in the measurements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and further advantages and features of the invention will be explained in detail hereinunder by means of an example with reference to the single FIGURE, in which 
       FIG. 1  shows a block diagram of a controller in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a section of a ground-compaction device, e.g. a vibration plate, in which the controller in accordance with the invention is used. 
   A vibration exciter  1  which is known per se and is described above is disposed on a lower mass of the ground-compaction device. A ground contact plate, not illustrated, for the purpose of compacting the ground is also provided on the lower mass. 
   Located above the lower mass—separated in a known manner by a spring-damper system, not illustrated,—is an upper mass  2  which comprises inter alia the drive. Disposed directly on the vibration exciter  1  is an adjusting piston  3 , of which one side is influenced by means of a spring  4 . Its other side is connected by way of an hydraulic line  5  to a first side  6  of a reference piston  7  which is disposed on the upper mass  2 . Disposed on a second side  8  of the reference piston  7 , which is axially movable in a cylinder  9 , is a piston rod  10  which is supported by way of a spring  11  against a pressure sensor  12 . The pressure sensor  12  which serves as a signal-generating device detects the pressure exerted by the spring  11  and thus by the reference piston  7  and converts this pressure into a position signal  13  which indicates the position of the piston rod  10 . The position signal  13  is transmitted to a control device  14 . The control device  14  also obtains an operating signal  15  which is generated from operating procedures performed by the operator, e.g. an actuation of a lever or push-buttons or even by remote-control. 
   The position signal  13  can also be output via an optical or acoustic display, in order to provide the operator with information relating to the behaviour of the ground-compaction device. 
   In dependence upon the position signal  13  and upon the operating signal  15  which reflects the wish of the operator, the control device  14  which is configured as an electronic regulating device determines which measures are required, in order to satisfy the operator&#39;s wish. Accordingly, it generates a control signal  16  which is transmitted to magnet coils  17  of a pilot valve  18 . The pilot valve  18  illustrated in  FIG. 1  and comprising the magnet coils  17  is provided for illustrative purposes only. Of course, the control signal  16  can be transmitted various ways to a valve and its position can be changed. 
   The pilot valve  18  is a 3 port, 3 position directional control valve which can be switched between three positions  18   a ,  18   b  and  18   c . It serves to control an oil supply from an oil pump  19 , which is used as a pressure oil source, to the second side  8  of the reference piston  7 , or it serves to control a discharge of oil from the second side  8  to an oil return line  20 . 
     FIG. 1  illustrates the pilot valve  18  in the position  18   b , in which a supply of oil to and a discharge of oil from the second side  8  of the reference piston  7  is interrupted. The position  18   a  enables the supply of oil, whereas the position  18   c  enables the discharge of oil into the return line  20 . 
   Furthermore, a safety valve  21  is provided between the pressure-side of the oil pump  19  and the oil return line  20 . 
   The first side  6  of the reference piston  7  is likewise connected to the oil return line  20  by way of a non-return valve  22 , wherein the non-return valve  22  is installed in such a manner that it prevents the flow of oil back from the first side  6  of the reference piston  7  to the oil return line  20 . 
   Finally, the oil return  20  is provided with a restrictor  23 . 
   The presence of oil in the hydraulic line  5  ensures that in terms of their movements the adjusting piston  3  and the reference piston  7  are coupled in a precise manner. Any change to the adjusting piston  3  also effects a change to the reference piston  7 , which change is detected via the piston rod  10  and the pressure sensor  12  and is transmitted to the controller  14 . 
   Then, in dependence upon the operator&#39;s wish and the operating signal  15  resulting therefrom, the controller  14  introduces suitable measures by actuating the pilot valve  18 . For example, a supply of oil to the second side of the reference piston  7  is switched via the pilot valve to position  18   a  which serves to displace the reference piston  7  to the left, as shown in  FIG. 1 . As a consequence, the adjusting piston  3  is also displaced to the left. 
   If, conversely, the extreme right-hand position of the pistons is to be achieved, the pilot valve  18  is switched to position  18   c , so that the oil is able to flow from the second side  8  of the reference piston  7  into the oil return line  20 . As a consequence, the pressure of the oil in the hydraulic line  5  and on the first side  6  of the reference piston  7  becomes zero, so that by virtue of the effect of the spring  4  or by the inertia effect of the unbalanced masses supported by the adjusting piston  3 , the said adjusting piston is likewise displaced to the right until it comes to rest against the mechanical stop of the cylinder which accommodates it. 
   The reference piston  7  follows this movement for as long as until the adjusting piston comes to this standstill. If the quantity of oil in the hydraulic line  5  and on the first side  6  is too small, e.g. by reason of leakage losses, the reference piston  7  is not able to achieve its extreme right-hand position. However, by reason of the restrictor  23  which forms a flow-obstacle, the oil in the oil return line  20  is at a certain residual pressure which is sufficient to open the non-return valve  22 . It is necessary to take into consideration that there is practically no more oil pressure in the hydraulic line  5  and on the first side  6 . It is thus possible by way of the non-return valve  22  for oil to flow subsequently to the first side  6  of the reference piston  7  and for any leakage losses to be compensated for. The leakage losses are compensated for by means of the different pressurisation surfaces on the first side  6  and the second side  8  of the reference piston  7  until the reference piston achieves its extreme right-hand position.