Abstract:
The invention relates to a rotary transmitter ( 2 ) for machine tools, having an inductive energy transmission section ( 31 ), which is arranged between a stator part ( 4 ) fixed to the machine and a rotor part ( 6 ) fixed to the tool, and a contactless bidirectional data transmission section ( 35 ). A special feature of the invention consists in that, in order to make maximum use of the capacity of the energy transmission section ( 31 ), precautions are taken with which the optimal operating frequency (f opt ) of the energy transmission operating according to the transformer principle is determined at every system start in a test run with a connected test resister ( 51 ) and a variable frequency (f p ). Furthermore, for the purpose of interference-free data transmission, buffer storage of the data to be transmitted via the data transmission section ( 35 ) is proposed, which data are synchronised in predefined time windows with interference-free periods of the energy transmission.

Description:
[0001]    This is a divisional of prior U.S. application Ser. No. 14/238,537, which was the national stage of International Application No. PCT/EP2012/061970, filed Jun. 21, 2012. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a rotary transmitter for machine tools, having an inductive transmission section for electrical power, having a stator part, which is fixed to the machine, and a rotor part, which is fixed to the tool and can rotate about a rotation axis, wherein the stator part has a primary winding, which is arranged in a primary circuit, and the rotor part has a secondary winding, which is arranged in a secondary circuit and is separated from the primary winding by means of an air gap. 
       DESCRIPTION OF RELATED ART 
       [0003]    Rotary transmitters of this kind are used, for example, in machine tools having adjustment tools (EP 1 856 705 B1). In the known rotary transmitters, a stator-end and a rotor-end power winding are in each case provided for inductive power transmission in accordance with the transformer principle. The power windings are separated from one another by in each case a stator-end and a rotor-end core part, wherein the stator-end and rotor-end core parts face one another across an air gap at mutually facing ends. In addition, stator- and rotor-end transmission and reception elements are provided in the known rotary transmitter, said stator- and rotor-end transmission and reception elements having stator- and rotor-end coupling turns, which are associated with one another in pairs, for inductive data transmission and being connected to a transmission and reception electronics system. An essential requirement of these rotary transmitters is that high electrical powers and large quantities of data can be transmitted given the lowest possible installation space. In addition, the systems should be robust and easy to handle since they have to operate in a reliable manner under harsh environmental and use conditions. Both in respect of production using components which are subject to tolerances and the partial mechanical manufacture of the coupling elements and also in respect of practical use, in which rotor, stator and actuation electronics systems have to be able to be replaced in the event of repair and maintenance, there are differences in the components, it being possible for these differences to influence both power and data transmission. In addition, owing to the physical proximity of power and data transmission elements, the compact design on offer exhibits the problem that power transmission can interfere with data transmission. Despite various structural measures for reducing this influence by means of shielding, symmetrization and interference-reduced circuit technology, this means that the transmission power, the reduction in installation space, the data rate, the flexibility and not least the system costs are subject to limits which restrict the field of application. 
       OBJECT OF THE INVENTION 
       [0004]    Proceeding from this, the invention is based on the object of increasing the power which can be transmitted on the power transmission section, without interfering with data transmission in the process. 
         [0005]    The combinations of features indicated in patent claims  1 ,  2 ,  7 ,  9  and  10  are proposed for achieving this object. Advantageous refinements and developments of the invention can be found in the dependent claims. 
       SUMMARY OF THE INVENTION 
       [0006]    The solution according to the invention is based on the knowledge that in order to transmit the electrical power from the stationary stator to the rotating rotor in accordance with the transformer principle said power has to be in the form of an alternating current or AC voltage. Since mains power is not suitable for direct transmission owing to the excessively low frequency (50 Hz) and the excessively high voltage (230 volts), a suitable alternating current has to be produced in the rotary transmitter itself. To this end, a power supply unit is used to provide a DC voltage which is converted by means of an inverter into an AC voltage with a suitable frequency. On the secondary side, the AC voltage is converted back into a DC voltage by means of a rectifier and a buffer capacitor. In addition, the contact-free transmission of power across an air gap in accordance with the transformer principle has the disadvantage that the degree of coupling between the primary winding and the secondary winding is considerably less than 1 and changes as the size of the air gap changes. In addition, the inductances of the primary and the secondary winding change as the air gap changes. In order to achieve optimum power output, the coupler has to be operated at a specific frequency close to the natural frequency of the secondary resonant circuit. Since said natural frequency changes as the size of the air gap changes and on account of the various component tolerances, the process cannot be based on a constant natural frequency. The invention therefore makes provision for the frequency to be newly determined and prespecified each time the system is started, with the result that the system can always be operated at an optimum operating frequency on the power transmission section even under changed conditions, such as a change in the tool head, a deviation in the air gap or an exchange of assemblies. 
         [0007]    In order to achieve this, the invention proposes a procedure which comprises the following method steps each time the system is started:
       in a first step a test resistor is connected into the secondary circuit,   in a second step a measure for the electrical load power in the primary circuit or in the secondary circuit is ascertained as a function of the alternating current frequency in the primary circuit,   in a third step an optimum alternating current frequency in the primary circuit, at which frequency the electrical load power is at a maximum, is ascertained,   in a fourth step an operating frequency of the alternating current for transmitting electrical power from the stator part to the rotor part is set in the primary circuit, the value of said operating frequency corresponding to the optimum alternating current frequency.       
 
         [0012]    In respect of circuitry, the above object can be achieved 
         [0013]    a) in that an inverter is arranged in the primary circuit, the DC input of said inverter being connected to a DC source, the AC output of said inverter being connected to a primary resonant circuit which contains the primary winding, and it being possible to vary and adjust the operating frequency of said inverter by means of a control assembly; 
         [0014]    b) in that a rectifier is arranged in the secondary circuit, a secondary resonant circuit which contains the secondary winding being connected to the AC input of said rectifier, and the DC output of said rectifier being in the form of a load connection, wherein the secondary circuit comprises a test resistor which can be selectively connected and bridges the load connection. 
         [0015]    c) In a first variant embodiment of the invention, an ammeter also is arranged in the primary circuit at the DC input of the inverter, while the control assembly has a programmable actuating output, which is connected to a frequency input of the inverter, and a measurement input which is connected to or communicates with the ammeter. 
         [0016]    c′) In a second variant embodiment of the invention, a voltmeter, which records the voltage drop across the test resistor, is arranged in the secondary circuit, while the control assembly has a programmable actuating output, which is connected to a frequency input of the inverter, and also a measurement input which communicates wirelessly with the voltmeter. 
         [0017]    In order to arrive at the desired objective, according to a preferred refinement of the invention, the control assembly has an evaluation circuit or routine for storing and/or evaluating the measurement values, which are received by means of the measurement input in the form of measurement signals, as a function of the frequency values, which are output by means of the actuating output in the form of frequency signals, when the test resistor is connected, and also has a data memory for storing an optimum operating frequency which is calculated using the evaluation circuit or routine. In other words, this means that the control assembly sets the frequency of the alternating current, which is output by the inverter, to an optimum operating frequency which is close to the resonant frequency of the secondary resonant circuit. 
         [0018]    A further preferred refinement of the invention makes provision for a respective transmission and reception element for contact-free bidirectional data transmission to be arranged in the stator part and in the rotor part. When the AC voltage for power transmission is inherently produced in an inverter in the manner of a square-wave voltage, relatively high-frequency interference signals, which can lead to interference in data transmission owing to the compact arrangement of power and data transmission sections in the rotary transmitter, occur primarily at the zero crossings. In order to avoid this, it is also proposed according to the invention 
         [0019]    a) that the data is transmitted by means of the bidirectional data transmission section in the form of data packets in time windows, 
         [0020]    b) that the information relating to the temporal position of the zero crossing of an alternating current in the power transmission section is ascertained, 
         [0021]    c) and that, on the basis of the information relating to the temporal position of the zero crossing of the alternating current in the primary circuit or in the secondary circuit of the power transmission section, the time windows for the transmission of the data packets are established such
       that the time windows lie between two successive zero crossings of the alternating current   and each have a start and an end point which are at a time interval from the zero crossings of the alternating current.       
 
         [0024]    In respect of circuitry, this can be realized according to the invention in that the stator-end and the rotor-end transmission element are connected to a respective control assembly which contains a buffer memory in which data packets for transfer to the associated rotor-end or stator-end reception element in a defined time window are stored, wherein the length of the time window is smaller than half the period of the alternating current which flows through the primary winding in the primary circuit and/or the secondary winding in the secondary circuit, and wherein the start and the end of the time window are at a time interval from the successive zero crossings of the alternating current which flows through the primary winding and/or the secondary winding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be explained in greater detail below with reference to the exemplary embodiments which are schematically illustrated in the drawing, in which 
           [0026]      FIG. 1  shows a partially sectioned illustration of a side view of a tool head, which is clamped into a machine spindle, with a rotary transmitter for power and data transmission; 
           [0027]      FIG. 2  shows a circuit diagram of the rotary transmitter with a stator-end primary circuit and a rotor-end secondary circuit; and 
           [0028]      FIG. 3  shows an optimization diagram for determining the optimum operating frequency of the power transmission section. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    The rotary transmitter illustrated in  FIG. 2  is intended to be used in the region of a replaceable tool head of a machine tool, as illustrated by way of example in  FIG. 1 . The tool head  60 , which is shown in  FIG. 1  and is in the form of a precision turning head, substantially comprises a main body  68 , a slide  70  which can be adjusted transversely in the direction of the arrow  74  in relation to the rotation axis  64  of the tool head  60  and has a cutting tool  72 , at least one electrical load  50  which is arranged within the tool head  60 , for example in the form of a measuring device  78  for direct adjustment movement measurement, and an electric adjusting motor  76  for the slide  70 . Power is supplied for the electrical load  50  and the data interchange by means of the rotary transmitter  2  which comprises a stator part  4  and a rotor part  6 . The tool head  60  can be coupled to the machine spindle  62  of a machine tool  63  by way of a tool shank  80  which projects axially beyond the main body. In order to set an air gap  37  between the stator part  4  and the rotor part  6 , the stator housing  82  is arranged on a holder  86 , which is fixed to the stator, by means of an adjustment mechanism  88  such that both its distance from the rotor and its rotary position about an axis which is parallel in relation to the rotation axis  64  can be adjusted. In the exemplary embodiment shown in  FIG. 1 , the stator part  4  extends in the manner of a segment only over a portion of the circumference of approximately 60° to 100° of the tool shank  80  and leaves the majority of the shank circumference free, so as to form a free space  90  for access by a tool gripper  92  for automatic tool changing. When the tool is changed, the tool head  60  is grasped at the gripper groove  96  by the tool gripper  92  from that side which is opposite the stator part  4 , and is moved axially with respect to the machine spindle  62  when the tool coupling is released. The tool head  60  is coupled to the machine spindle  62  by means of a clamping mechanism which can be operated on the machine side by means of a tie rod  98 , engages from the machine side into the hollow space  100  in the tool shank  80 , and couples the tool head  60  to the machine spindle  62  so as to produce planar surface clamping and radial clamping. 
         [0030]    As shown in  FIG. 2 , the rotary transmitter  2  has an inductive power transmission section  31  which comprises the stator part  4 , which is fixed to the machine and has already been described in connection with  FIG. 1 , and the rotor part  6 , which is fixed to the tool and can rotate about a rotation axis  64 . The stator part  4  has a primary winding  10 , which is arranged in a primary circuit  8 , and the rotor part  6  has a secondary winding  38 , which is arranged in a secondary circuit  36  and is separated from the primary winding  10  by means of the air gap  37 . 
         [0031]    An inverter  12  is arranged in the primary circuit  8 , the DC input  14 ,  16  of said inverter being connected to a DC source  7 ,  9 , and the AC output  18 ,  20  of said inverter being connected to a primary resonant circuit which contains the primary winding  10 . The operating frequency of the inverter  12  can be set by means of a stator-end control assembly  24 . A rectifier  40  is located in the rotor-end secondary circuit  36 , a secondary resonant circuit which contains the secondary winding  38  being connected to the AC input  42 ,  44  of said rectifier, and the DC output  46 ,  48  of said rectifier being in the form of a connection for the rotor-end electrical load  50 . 
         [0032]    A data transmission section  35  is also located between the stator part  4  and the rotor part  6 , said stator part and rotor part having a respective transmission and reception element  106 ,  108  and, respectively,  106 ′,  108 ′ for contact-free bidirectional data transmission, said transmission and reception elements each having a transmission and reception electronics system  110 ,  112  and, respectively  110 ′,  112 ′. The transmission and reception elements are actuated by means of the stator-end control assembly  24  or the rotor-end control assembly  56 . The transmission and reception elements are expediently constituent parts of an inductive, capacitive or optical data transmission section. 
         [0033]    Contact-free power transmission across the air gap  37  in accordance with the transformer principle has the advantage that the degree of coupling between the stator-end primary winding  10  and the rotor-end secondary winding  38  is considerably less than  1  and changes as the size of the air gap  37  changes. In addition, the inductances of the primary and the secondary winding  10 ,  38  change as the size of the air gap  37  changes. In order to achieve optimum power transmission, the primary circuit  8  and the secondary circuit  36  have to be operated at an optimum frequency which corresponds approximately to the natural frequency of the secondary circuit. Since the natural frequency changes as the size of the air gap changes and the tolerances of various components in the stator and rotor part  4 ,  6  change, the operating frequency has to be adjusted each time the system is started. 
         [0034]    To this end, the secondary circuit  36  has a test resistor  51  which can be selectively connected by means of the switch  53  and bridges the DC output  46 ,  48  in the region of the load connection. In addition, according to a first variant embodiment, an ammeter  28  is arranged in the primary circuit at the DC input  14 ,  16  of the inverter  12 , the output  30  of said ammeter communicating with a measurement input  32  of the stator-end control assembly  24 . As an alternative to this, according to a second variant embodiment, a voltmeter  102 , which records the voltage drop across the test resistor  51 , is arranged in the secondary circuit, the output of said voltmeter communicating with a measurement input  39  of the stator-end control assembly  24 , for example, by means of a data transmission section  41 . 
         [0035]    In both variant embodiments, the stator-end control assembly  24  has a programmable actuating output  26  which is connected to the frequency input  22  of the inverter  12 . For its part, the control assembly  24  has an evaluation circuit or routine for storing and/or evaluating the measurement values from the ammeter  28  or from the voltmeter  102 , which measurement values are received by means of the measurement input  32  and, respectively,  39  in the form of measurement signals, as a function of the frequency values f p , which are output by means of the actuating output  26  in the form of frequency signals S, when the test resistor  51  is connected, and also has a data memory for storing an optimum operating frequency f opt  which is calculated by way of the evaluation circuit or routine. This circuit arrangement operates as follows: 
         [0036]    once the rotary transmitter  2  is activated, the stator-end control assembly  24  prespecifies a fixed alternating frequency, which is roughly in the range of the later operating frequency, to the inverter  12  for a short time. In the process, power is transmitted from the stator end to the rotor end, it being possible for the rotor-end control assembly  56  to begin to operate using said power. Said control assembly first connects the test resistor  51  by means of the output  59  and the switch  53 , said test resistor receiving the transmitted power by means of the DC output  46 ,  48  of the rectifier  40  and therefore providing the secondary resonant circuit with a low impedance for the natural frequency, that is to say providing it with a certain quality. 
         [0037]    Subsequently, the stator-end control assembly  24  begins to run through a prespecified frequency range in steps. In the process, at each frequency step, either the current consumption I of the inverter  12  is measured by means of the ammeter  28  or the voltage drop U across the test resistor  51  is measured by means of the voltmeter  102  and stored with the associated frequency values f p  so as to form a curve  140  ( FIG. 3 ). If the natural frequency of the rotor-end secondary resonant circuit is now within the frequency range which has been run through, it is possible to establish either a current maximum  142  or a voltage maximum  144  in that range (cf.  FIG. 3 ). 
         [0038]    The evaluation circuit or routine which is present in the stator-end control assembly  24  now determines the frequency belonging to the current maximum  142  or voltage maximum  144 , possibly further adjusts said frequency with a correction value and stores it as the optimum operating frequency f opt  in a data memory for the subsequent actuation of the inverter  12 . The test resistor  51  is then disconnected by means of the switch  53 , with the result that the total power which can be transmitted is now available to the electrical load  50 . 
         [0039]    As already explained above, both an inductive power transmission section  31  and a contact-free bidirectional data transmission section  35  are provided between the stator part  4  and the rotor part  6  of the rotary transmitter  2 . The AC voltage for the power transmission is inherently produced in the inverter  12 , which is connected to a DC source, in the manner of a square-wave voltage. When the DC voltage is chopped into the AC voltage, relatively high-frequency interference occurs primarily at the zero crossings, but this interference quickly dissipates. At a time interval from each zero crossing, there is a relatively long time period until the next zero crossing in which there is no interference. Secondly, the data does not have to be continuously transmitted by means of the data transmission section  35  when there is a sufficiently high bit rate. According to the invention, this can be used for the purpose of transmitting the data in time windows with interposed transmission breaks by means of the bidirectional data transmission section  35  in the form of data packets  180 ,  182 ,  176 ,  178 . In order to ensure interference-free transmission, the information relating to the temporal position of the zero crossing of the alternating current in the power transmission section is first ascertained. On the basis of the information relating to the temporal position of the zero crossing of the alternating current of the power transmission section, the time windows for the transmission of the data packets  180 ,  182 ,  176 ,  178  are established such that they are between two successive zero crossings of the alternating current, and that they each have a start and an end point which are at a time interval from the zero crossings of the alternating current. 
         [0040]    In respect of circuitry, this is realized according to  FIG. 2  in that the stator-end and the rotor-end transmission element are connected to a respective control assembly  24 ,  56  which contains a respective buffer memory  34 ,  58  in which data packets  180 ,  182  and, respectively,  176 ,  178  for transmission by means of the associated rotor-end or stator-end transmission elements  106 ,  106 ′ in a defined time window are stored. In this case, the data packets  180 ,  182 ,  176 ,  178  are dimensioned such that the length of the associated time window is smaller than half the period of the alternating current which flows through the primary winding  10  in the primary circuit  8  and/or the secondary winding  38  in the secondary circuit  36 , and wherein the start and the end of the time window are at a time interval from the successive zero crossings of the alternating current which flows through the primary winding  10  and/or the secondary winding  38 . Data transmission which is not adversely affected by changeover interference in the power transmission section is achieved by way of this measure, despite the compact design of the rotary transmitter  2 . 
         [0041]    In summary, the following can be stated: the invention relates to a rotary transmitter  2  for machine tools, having an inductive power transmission section  31 , which is arranged between a stator part  4 , which is fixed to the machine, and a rotor part  6 , which is fixed to the tool, and also having a contact-free bidirectional data transmission section  35 . A special feature of the invention is that precautions are taken for the purpose of utilizing the capacity of the power transmission section  31  to the maximum extent, whereby the optimum operating frequency f opt  of the power transmission process, which operates in accordance with the transformer principle, is ascertained in a test run with the test resistor  51  connected and at the variable frequency f p  each time the system is started. Furthermore, for the purpose of interference-free data transmission, it is proposed that the data which is to be transmitted by means of the data transmission section  35  is temporarily stored, this temporary storage being synchronized in prespecified time windows with interference-free time periods of power transmission. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           2  Rotary transmitter 
           4  Stator part 
           6  Rotor part 
           7 ,  9  DC source 
           8  Primary circuit 
           10  Primary winding 
           12  Inverter 
           14  Slide 
           14 ,  16  DC input 
           18 ,  20  AC output 
           22  Frequency input 
           24  Stator-end control assembly 
           26  Actuating output 
           28  Ammeter 
           30  Output 
           31  Power transmission section 
           32  Measurement input (I) 
           34  Buffer memory, data memory 
           35  Data transmission section 
           36  Secondary circuit 
           37  Air gap 
           38  Secondary winding 
           39  Measurement input (U) 
           40  Rectifier 
           41  Data transmission section 
           42 ,  44  AC input 
           46 ,  48  DC output 
           50  Electrical load 
           51  Test resistor 
           53  Switch 
           56  Rotor-end control assembly 
           58  Buffer memory 
           59  Output 
           60  Tool head 
           62  Machine spindle 
           63  Machine tool 
           64  Rotation axis, rotary axis 
           68  Main body 
           70  Slide 
           72  Cutting tool 
           74  Arrow 
           76  Electrical adjusting motor 
           78  Measuring device 
           80  Tool shank 
           82  Stator housing 
       
     
         [0087]    Holder which is fixed to the stator
     88  Adjustment mechanism     90  Clearance     92  Tool gripper     96  Gripper groove     98  Tie rod     100  Hollow space     102  Voltmeter     106 ,  108  Transmission and reception element (stator-end)     106 ′,  108 ′ Transmission and reception element (rotor-end)     110 ,  112  Transmission and reception electronics (stator-end)     110 ′,  112 ′ Transmission and reception electronics (rotor-end)     140  Curve     142  Current maximum     144  Voltage maximum     176 ,  178 ,  180 ,  182  Data packet   I Current   U Voltage   f p  Frequency   f opt  Optimum operating frequency