Abstract:
A vacuum pump includes at least two stages ( 2, 3 ), a motor ( 8 ) for driving pump-active elements of the two stages ( 2, 3 ), a transmission unit ( 7 ) for transmitting driving power from the motor ( 8 ) to the pump-active elements, a motor control ( 9 ) for controlling a rotational speed of the motor ( 8 ), and an evaluation unit ( 12 ) electrically connectable with pressure-sensitive signal sensor ( 10 ) located between the two stages for receiving a signal generated by the signal sensor and connected with the motor control ( 9 ) for transmitting an evaluated signal thereto, so that the motor control ( 9 ) can change a rotational speed of the motor ( 8 ) in accordance with the received evaluated signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a vacuum pump having at least two stages, a motor for driving pump-active elements of the two stages, a transmission unit for transmitting driving power from the motor to the pump-active elements and a motor control for controlling a rotational speed of the motor. The present invention also relates to a method of operating such a vacuum pump with the motor having at least two different rotational speeds. 
         [0003]    2. Description of the Prior Art 
         [0004]    In the vacuum technology, vacuum pumps with discharge against the atmospheric pressure are mostly designated as fore-vacuum pumps. This designation follows from the fact that these vacuum pumps are often used in combination with vacuum pumps capable of producing high vacuum, but without compression to the atmospheric pressure. A turbomolecular pump can serve as an example of high-vacuum pump. Vane-type rotary pumps, piston pumps, and diaphragm pumps are examples of fore-vacuum pumps. Very often, fore-vacuum pumps are formed as multi-stage pumps because the high-vacuum pumps, which are combined therewith, produce a pressure at their outlet of only few millibars and, therefore, a further pressure region should be overlapped. 
         [0005]    Both at their use as fore-vacuum pumps of multi-stage pumps and when used independently for producing an end vacuum in a recipient, the fore-vacuum pumps should compress a large amount of gas. For common designs such as diaphragm pumps or piston pumps, this means that they should have a correspondingly large-dimensioned compression chamber. With these pumps, the amount of gas, which is compressed per time unit, depends on the maximal volume of the compression chamber and frequency with which the compression chamber changes from its maximal volume to the minimal volume. In case the amount of the to-be-processed gas is small, the fore-vacuum pumps become overdimensioned with regard to the compression chamber volume and the pump rotational speed. The compression chamber volume and the rotational speed also determine the power consumption of a fore-vacuum pump, and it, of course, is desirable to minimize the power consumption. 
         [0006]    In order to solve this problem, the state of the art suggests to reduce the rotational speed. E.g., German Publication DE-OS 103 54 20 5 suggests to reduce a rotational speed of a piston pump dependent on the inlet pressure. 
         [0007]    The drawback of the technique of reduction of the rotational speed in accordance with the inlet pressure consists in that in addition to a fore-vacuum pump, further components such as measurement tubes, control electronics, etc. need to be provided for determining the inlet pressure. The problem of a user consists in that he/she can combine only components which are received from a manufacturer and which are adapted to each other. 
         [0008]    An object of the invention is, therefore, to provide a compact vacuum pump with a technically simple solution for reducing the power consumption. 
       SUMMARY OF THE INVENTION 
       [0009]    This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a vacuum pump of the type discussed above and including a gas-sensitive signal sensor located between the two stages for sensing pressure of gas transmittable from one stage to another stage and for generating a corresponding signal, and an evaluation unit electrically connectable with the signal sensor for receiving the generated signal and evaluating same and connected with the motor control for transmitting an evaluated signal thereto, with the motor control changing a rotational speed of the motor in accordance with the received evaluated signal; and a method of operating a vacuum pump that includes generating, with a gas-sensitive signal sensor, a signal dependent on gas pressure of a gas flow between the two stages, evaluating the generated signal, and setting a rotational speed of the motor by the motor control, dependent on result of a evaluation of the generated signal. 
         [0010]    The arrangement of a gas-sensitive signal sensor between the pump stages of a vacuum pump permits to obtain an integrated, compact construction. The region between the two pump stages is self-contained. The user needs not to install any additional components and can combine, on the pump stand, vacuum pumps of different manufacturers. Even a more compact structure is obtained when the evaluation unit and the motor control form a common unit. 
         [0011]    The reduction of the rotational speed is particularly important in dry-running vacuum pumps, i.e., in such vacuum pumps in which no working medium is used in the compression chamber for lubrication and/or sealing, because in these pumps wear greatly depends on the rotational speed. 
         [0012]    The advantages, which are achieved by the present invention, are further increased, in each type of a vacuum pump and, in particular, in a fore-vacuum pump in which the signal sensor is arranged at a gas inlet of a pump stage of the two pump stages that communicate with atmosphere. The pressure here is in a middle region between the end and atmospheric pressures. In this region, the pressure head and the pressure switch can be arranged in a technically simple manner and, therefore, cost-effectively. 
         [0013]    Advantageously, the present invention is used in such vacuum pumps in which art least one pump stage is sealed with a movable seal. In these vacuum pumps, the back flow through the gaps between the rotor and stator, piston and cylinder, diaphragm and the housing, etc. is particularly small, so that the theoretical suction capacity is rapidly and exactly reached at all rotational speeds. 
         [0014]    The present invention is also advantageously used in vacuum pumps in which one stage is formed as a piston vacuum pump as this pump is provided with a wear-prone seal on the piston. Further stages, therefore, can also be formed as piston or diaphragm pump stages. 
         [0015]    Advantageously, the signal sensor is located in an annular space that serves as a gas inlet and at least partially surrounds a cylinder at least partially at a height of a gas inlet opening that communicates the annular space (with a compression chamber defined by the cylinder or conduit means through which gas flows to the annular space. This space is constructively easily accessible so that manufacturing is simple and the pump can be subsequently retrofitted. 
         [0016]    The inventive method insures that the pump operates automatically, with effective power use, and in energy-saving manner. One of the advantages of the present invention consists in that the reduction of the rotational speed also leads to reduction of noise and vibrations. No special measures should be undertaken during or after installation of the pump to reduce the rotational speed which takes place in the pump itself. The speed is established automatically, i.e., the rotational speed is continuously changed, dependent on the gas pressure. In addition, by providing a suitable feedback loop, an effective control is achieved. For establishing of the feedback loop, a rotational speed sensor can be provided, e.g., in the motor, the drive power transmission unit, or in one of the pump stages. The rotational speed sensor would communicate back to the motor control the actual rotational speed with which the pump operates. In a simpler case, a gas-sensitive signal sensor is used to communicate the pressure value back. There should be provided a possibility to use the pressure value at a suitable point. This can achieved, e.g., by a suitable design of the motor control. An appropriate control insures an optimal use of the rotational speed setting. The rotational speed setting can be achieved by change-over of speeds. The advantage of this approach consists in the possibility to use a technically simple and, therefore, convenient pressure switch as a signal sensor. When only a change-over between two rotational speeds takes place, a cost-effective and simple control electronics for the motor can be used. 
         [0017]    The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The drawings show: 
           [0019]      FIG. 1  a schematic view of a vacuum pump according to the present invention; and 
           [0020]      FIG. 2  a cross-sectional view of a fore-vacuum pump stage of the vacuum pump according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    A vacuum pump according to the present invention, a schematic view of which is shown in  FIG. 1 , has a first pump stage  2  which is connected by a gas inlet  4  with, e.g., a recipient or a high-vacuum pump. A gas conduit  5  connects the outlet of the first pump stage with the inlet of the second pump stage  3 . The second pump stage  3  is discharged, through a gas outlet  6 , against atmospheric pressure. 
         [0022]    A driving power transmission unit  7  is provided between a motor  8  and pump-active elements of the vacuum pump  1 . The driving power transmission unit  7  takes care that pump-active elements of both pumping stages  2  and  3  are driven simultaneously. The driving power transmission unit  7  can be formed, e.g., as a crank drive with two connecting rods which drives either pistons or diaphragms. Dependent on the pumping system a gear drive with or without a transmission can be used. The motor  8  is controlled by a motor control  9  that can control the motor  8  in such a way that it would operate at least at two different speeds. 
         [0023]    An electrical conductor connects the motor control  9  with an evaluation unit  12 . The evaluation unit  12  evaluates the signal which is generated by a pressure-sensitive signal sensor  10  and which is communicated from the signal sensor  10  to the evaluation unit  12  via conductor means  1   1 . The signal sensor  10  can be formed as a pressure switch or a pressure head. The signal sensor  10  converts the gas pressure in the conduit  5  into an electrical signal. The signal sensor  10  can be so formed that its operation is independent from the type of the used gas, so that the advantages, which are achieved according to the present invention, are obtainable with any type of a gas mixture. The evaluation signal is communicated from the evaluation unit  12  to the motor control  9  that can change the rotational speed of the motor  8 , dependent on the evaluation signal. When, e.g., the gas pressure is low, the evaluation results in reduction of the rotational speed of the motor to a lower value. The signal, which is generated by signal sensor  10 , can be a voltage level signal. The signal can also be converted into a digital signal, with the evaluation unit being modified to be able to handle digital signals. 
         [0024]      FIG. 2  shows a stage  3  of a piston vacuum pump that can be used as a fore-vacuum stage of the inventive vacuum pump  1 .  FIG. 2  shows a housing  20  in which a cylinder  21  is located. In the cylinder  21 , a piston  22  reciprocates, periodically increasing or reducing the volume of a compression chamber  29 , whereby a pumping action takes place. In course of its movement, the piston  22  opens, in or in vicinity of a first of two reverse points, the gas outlet valve  24 , so that the gas can be expelled through an outlet flange  25  provided in the housing cover. On its way to the second reverse point, the piston opens gas inlet opening or openings  27  so that the gas can flow from the annular space  28  into the compression chamber  29 . In the second reverse point, the direction of movement of the piston  22  is reversed, and the piston, on its way to the first reverse point, closes the gas inlet opening(s)  27 , thus separating the gas inlet opening(s)  27  and the compression chamber  29  from each other. The annular space  28  surrounds the cylinder  21 , at least partially, at the height of the gas inlet openings  27 . The gas conduit  5 ,  FIG. 1 , connects the annular space  28  with the preceding pump stage. The signal sensor  10  is located in the annular space  28 . The signal sensor  10  can also be provided on a flange and be connected with the annular space  28  through a bore formed in the housing  20 . Thereby, retrofitting of pumps and replacement of defective pressure-sensitive signal sensors is simplified. Another possibility of mounting of the signal sensor consists in placing it in the same chamber in which the piston-driving crank drive is located. This chamber should be connected with the annular space  28  to insure gas flow therebetween. Alternatively, it is possible to arrange the signal sensor in the wall of the cylinder  21  with a direct connection of the signal sensor with the compression chamber. 
         [0025]    A movable seal  23 , which has a L-shaped cross-section, is provided between the cylinder  21  and the piston  22 . The movable seal  23  wears off due to its contact with the wall of the cylinder  21 . Further, different supports and bearings, e.g., in the drive power transmission unit, are also subjected to wear. By reducing the rotational speed, the wear can be noticeably reduced. 
         [0026]    The vacuum pump shown in  FIG. 1 , and in particular, vacuum pumps with a fore-vacuum pump stage shown in  FIG. 1 , can be operated in the following manner. In a first step, the pressure-sensitive signal sensor  10  generates a signal dependent on the pressure in the conduit means that connect the first stage  2  with the second stage  3 . To this end, the signal sensor  10  can be formed, e.g., as a diaphragm pressure switch in which dependent on the gas pressure, the diaphragm is deflected to a different extent and opens or closes an electrical contact at a threshold gas pressure, generating, in this case, a voltage level signal that changes upon opening or closing of the electrical contact. 
         [0027]    In a second step, the generated signal is evaluated by the evaluation unit which is adapted to process the voltage level signals. It is contemplated to use a relay the switch position of which is changed by the voltage level signal. Use of similarly operating electronic switches is also contemplated. Instead of change-over, a comparison with a predetermined value, e.g., voltage value can be used, with the comparison results being used for determining the rotational speed. 
         [0028]    In a further step, the motor control sets the rotational speed, dependent on the evaluation results. The change of the rotational speed can be a discrete or continuous process at which the motor control changes the rotational speed dependent on the voltage value. 
         [0029]    It is advantageous for vacuum pumps, to modify the method of operating the pump so that the reduction of the rotational speed takes place when the signal for the pressure-sensitive signal sensor corresponds to pressure below the atmospheric pressure. 
         [0030]    This pressure can be developed into a pressure close to the end pressure of the vacuum pump. At the pressure close to the end vacuum pressure, the to-be-delivered gas amounts are particularly small, so that the rotational speed can be greatly reduced. This effect is particularly high in vacuum pump with movable seals, in particular, in piston vacuum pumps. 
         [0031]    Though the present invention was shown and described with references to the preferred embodiment, such is merely illustrative of the present invention and is not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiment or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.