Abstract:
The device includes an operating head movable by a control unit with respect to a board for machining; the operating head includes a board holder device and an electric motor for a tool spindle; the rotor of the motor rotates with respect to the stator on air-cushion supports, and the stator slides axially on a further air-cushion support and is insulated with respect to the operating head; a signal generator senses the capacitance between the rotor and the stator, and generates an electric signal upon the tool on the spindle contacting the upper surface of the board; and the signal is used to define a reference dimension relative to the upper surface, and to so condition the control unit as to feed the tool to a predetermined depth as of the upper surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional application of prior application Ser. No. 08/856,793, filed May 15, 1997, now U.S. Pat. No. 6,015,249. The entire disclosure of the prior application is considered to be part of the disclosure of this application and is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device and method for controlling the machining depth of a machine for machining multilayer printed circuit boards. 
     In the fabrication of multilayer printed circuit boards, the board must be drilled and/or milled to a predetermined, precisely controlled depth, the purpose of the machining operation, in fact, normally being to reach a given conducting layer and no further. Owing to the uneven upper surface of the board, however, controlling the machining depth with reference to the machine itself, e.g. the worktable, would inevitably result in depth control errors. 
     In one known machine for drilling multilayer boards, machining depth is controlled according to the position of the tip of the tool on the spindle with respect to the board holder bush, which therefore acts as a feeler for the upper surface of the board, from which the machining depth is controlled. Such a machine, however, presents the drawback of the position of the tip of the tool with respect to the board holder bush varying as a result of both wear and the temperature of the spindle. 
     In another known drilling machine, the references position of the tip of the tool is determined by bringing the spindle up to the steady-state temperature and probing with the tool a reference board fitted to the edge of the worktable. More specifically, the tool locates an air-cushion piston fitted to the reference board; and an electric reference signal is generated by a proximity sensor as the tool contacts the piston. 
     This device presents the drawback of involving a small amount of movement of the piston to generate the reference signal. Moreover, the assembly comprising the air-cushion piston and proximity sensor is fairly complicated and expensive to produce. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a highly straightforward, reliable device for controlling the machining depth of multilayer boards, designed to overcome the aforementioned drawbacks typically associated with known devices. 
     According to the present invention, there is provided a device for controlling machining depth, and comprising an operating head movable by an electronic control unit with respect to a board to be machined; said head comprising a board holder device, and an electric motor for a tool spindle; said electric motor comprising a rotor integral with said spindle, and a stator fitted to said head and movable axially towards said board; and air-cushion supporting means being provided between said rotor and said stator; characterized by comprising a generator for generating electric signals and for sensing contact of a tool con said spindle with a conducting layer of said board, to define a machining depth reference dimension; and enabling means for so conditioning said control unit as to feed said stator to a predetermined depth as of said reference dimension so defined. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows a machine for machining printed circuit boards and featuring a machining depth control device in accordance with the present invention; 
     FIG. 2 shows a schematic, larger-scale section of a detail of the FIG. 1 machine; 
     FIG. 3 shows a block diagram of the machine control unit and the device control circuit; 
     FIG. 4 shows a detailed diagram of the logic blocks of the device control circuit; 
     FIG. 5 shows a graph of the electric signals generated by the blocks in FIG. 4; 
     FIG. 6 shows a partially sectioned front view of an operating head fitted with the control device; 
     FIG. 7 shows a partial section of the FIG. 6 head; 
     FIG. 8 shows a printed circuit board ready for machining; 
     FIG. 9 shows a section along line IX—IX in FIG. 8; 
     FIG. 10 shows a block diagram of the steps in the machining depth control method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Number  5  in FIG. 1 indicates a machine for mechanically machining multilayer printed circuit boards  6 , which, as is known, are normally four-sided, preferably rectangular, and may comprise two or more rigid layers  7  (FIG. 2) made, for example, of fiberglass-reinforced plastic, and a flexible layer  8  made, for example, of MYLAR (registered trademark) and located between the two rigid layers  7 . 
     Layers  7 ,  8  are deposited separately, on one or both faces, with an electrically conducting layer  9  of metal material to form the respective conducting tracks and pads; layers  7 ,  8  are bonded to one another; the edges of board  6  are trimmed; and board  6  is then mechanically machined, e.g. drilled and/or milled, to a strictly predetermined depth K from the upper surface  10  of board  6  to a particular conducting layer  9 . 
     Machine  5 , e.g. a drilling machine, comprises at least one operating head  11  (FIG. 1) movable with respect to a worktable  12  along two coordinate horizontal axes X and Y (only the X axis shown in FIG.  1 ). Head  11  comprises a spindle  13  supporting a drilling tool  14 , and which is movable in known manner along a vertical axis Z to effect the forward travel of tool  14 . All the movements along the X, Y and Z axes are controlled by an electronic numeric-control feedback unit indicated as a whole by  16  (FIG.  3 ). 
     Spindle  13  (FIG. 1) is rotated by an asynchronous a.c. electric motor indicated as a whole by  17 , and which is integrated with spindle  13  to form an electric spindle  13 ,  17 . More specifically, motor  17  comprises a rotor  18  integral with spindle  13  and rotating inside a stator  19  fitted to head  11 . 
     Stator  19  is moved along the Z axis together with rotor  18 , which rotates on a series of air-cushion supports defining a gap  15 , as described in detail later on. As rotor  18  and spindle  13  rotate, therefore, any mechanical contact between rotor  18  and stator  19  is excluded, and rotor  18  is electrically insulated with respect to stator  19 . 
     Operating head  11  also comprises a board holder device  21 , which, when activated, engages the upper surface  10  of board  6  by means of a board holder bush  22  to press board  6  on to worktable  12  before the board is engaged by tool  14 . As is known, for safety reasons, both head  11  and worktable  12  must be electrically grounded. 
     Drilling machine  5  is equipped with a machining depth control device indicated as a whole by  23  (FIG. 3) and controlled by control unit  16 . For which purpose, unit  16  is so programmed as to enable device  23  to control drilling depth K (FIG.  2 ), and comprises hardware or software registers  20  for storing various depth K values. 
     According to the present invention, device  23  comprises a generator  24  for generating an electric signal in response to tool  14  contacting surface  10  of a metal layer  9 ; a memory  25  for storing the signal so generated; and enabling means in turn comprising a circuit  26  enabled by memory  25  and for so conditioning unit  16  as to control drilling depth according to a memorized K value. 
     More specifically, generator  24  (FIG. 4) comprises a known low-pass, wide-band, so-called pi filter  27  substantially comprising an inductor L 1  located in series between two capacitors C 1  and C 2 ; filter  27  is supplied by a high-frequency oscillator  29  via an output power amplifier  31 ; and a regulated, so-called switching, power supply  28  supplies oscillator  29  and the other electronic components of device  23 . 
     Oscillator  29  is frequency adjustable within certain limits, and may be selected to generate a sinusoidal signal of 400 to 1200 kHz frequency. Amplifier  31  is a so-called class A type, provides for linear amplification of the amplitude of the sinusoidal signal, and may advantageously be selected to supply a peak-to-peak signal amplitude of 15 to 20 volts, and preferably of about 18 volts. 
     More specifically, the frequency generated by oscillator  29  must be selected as a function of the physical characteristics of electric spindle  13 ,  17 , i.e. as a function of the in-service parasitic capacitance CP (FIGS. 1 and 3) between stator  19  and rotor  18 , due to the gap  15  defined by the air-cushion supports. As capacitance CP normally ranges between 10 and 12 picofarads, oscillator  29  (FIG. 4) may advantageously be selected to generate a signal of about 1000 kHz frequency. 
     Filter  27  must be so selected as to be tuned to the output frequency of oscillator  29 , and is therefore referred to hereinafter as the tuned filter. Filter  27  therefore transmits the selected output frequency at maximum energy, attenuates or suppresses the other frequencies, in particular the harmonics of the selected frequency, and may advantageously be tuned to a frequency range differing by plus or minus 50 kHz from the selected frequency. 
     The output of tuned filter  27  comprises two wires connected to a connector  32 , to which may be connected a complementary connector  33  connected to a further two wires  34  and  36 . Wire  34  is connected electrically to bush  22  (FIG.  1 ), which is normally made of conducting material and fitted removably to device  21 . According to the invention, bush  22  is insulated with respect to device  21 , e.g. by means of a ring  37  of insulating material, or device  21  may be made entirely of insulating material, e.g. ceramic. 
     Wire  36 , on the other hand, is connected electrically to stator  19 , which in turn is connected to head  11  by a joint  38  insulated electrically from head  11 , as described later on. For safety reasons, however, stator  19  is grounded by an inductor L, e.g. of about 500 microhenries, which insulates the stator at the operating frequency, but acts as a resistor at mains frequency. 
     The capacitance at the output of tuned filter  27  (FIG. 4) therefore normally assumes a first low value when tool  14  is detached from surface  10  of board  6 , so that the peak-to-peak amplitude A of the output signal of filter  27  (FIG. 5) is about 18 volts. Conversely, when the tip of tool  14  contacts surface  10  of metal layer  9  of board  6 , the parasitic capacitance CP between stator  19  and rotor  18  is inserted parallel with capacitance C 2  of tuned filter  27 . Since capacitance CP is much higher than that in the absence of contact by tool  14 , the reactance of filter  27  is reduced, and the peak-to-peak amplitude a of the output signal falls to about 2 volts, well below the above mentioned amplitude A. 
     Generator  24  (FIG. 4) also comprises a circuit  39  for rectifying and leveling the incoming signal, and which substantially comprises a diode and a capacitor, and generates a rectified, substantially leveled output signal (see also FIG. 5, in which the signals are indicated on the left by the reference numbers of the respective circuits). The output signal of circuit  39  is; supplied to a comparator  41  for comparison with at threshold signal S. 
     Threshold S must be so selected as to permit comparator  41  to pass from a high signal when tool  14  is detached from surface  10 , to a low signal when tool  14  contacts surface  10 . The low signal is stored in memory  25 , which substantially comprises a flip-flop set to memorize only the first contact of tool  14  with surface  10 , so that, until it is reset, any bounce or subsequent contact has no effect on the state of the flip-flop. 
     Enabling circuit  26  normally blocks the digital signals received from a normal Z axis position transducer  42 ; and, when set, memory  25  conditions circuit  26  to supply the digital signals to unit  16  to feedback control the travel of electric spindle  13 ,  17 . A second photocoupler  46  is located between control unit  16  and memory  25 ; and, at the end of each drilling cycle, unit  16  supplies an end-of-cycle signal to reset memory  25 . 
     Control device  23  operates as follows. 
     Once board  6  (FIG. 1) is fixed to worktable  12 , unit  16  moves head  11  with respect to worktable  12  along the X and Y axes to position tool  14  vertically over the point to be drilled; unit  16  lowers device  21  so that bush  22  (FIG. 2) presses on surface  10  of board  6 ; and oscillator  29  (FIG. 4) causes amplifier  31  to generate a train of sinusoidal waves (FIG. 5) of the selected frequency and amplitude, and which are filtered by tuned filter  27 . Since the tip of tool  14  is detached from board  6 , the output wave of filter  27  has the highest amplitude A. 
     At this point, unit  16  lowers electric spindle  13 ,  17  until the tip of tool  14  contacts upper surface  10  of conducting layer  9  of board  6 . Since, at this point in time, tuned filter  27  is connected to capacitance C between stator  19  and rotor  18 , circuit  41  generates a trailing edge of the respective output signal to set flip-flop  25 . Via photocoupler  43 , flip-flop  25  conditions circuit  26  to supply the digital signals from transducer  42  to unit  16 , which, by feedback control, lowers electric spindle  13 ,  17  by the number of elementary steps corresponding to the depth K memorized in register  20 , arrests the travel of electric spindle  13 ,  17 , and emits an end-of-cycle signal to reset flip-flop  25  via photocoupler  46 . 
     With reference to FIG. 6, operating head  11  comprises an aluminum alloy body  48  with a bottom crosspiece  49  comprising a seat in which is fitted an air-cushion bush  51  for creating, in use, a further air-cushion gap  52  (FIG.  7 ). Stator  19  is fitted in axially-sliding manner inside bush  51  (FIG. 6) as described in Italian Patent Application TO93A 000831 filed on Nov. 5, 1993, by the present Applicant. 
     Stator  19  comprises two air-cushion supports  53  for supporting rotor  18  by means of radial air cushions; an air-cushion support  54  for supporting rotor  18  by means of an axial air cushion, supports  53  and  54  creating, in use, gap  15  (FIG.  1 ); a fitting  56  (FIG. 6) for insertion of the electric supply wires and inductor L; and a fitting  57  for connecting wire  36 . 
     Head  11  also comprises a fixed top crosspiece  58  in turn comprising a central opening  59  housing for rotation the top end of a screw  61 . Screw  61  engages a nut screw  62  fitted to a movable crosspiece  63  also fitted with two columns  64  sliding inside two bushes on fixed crosspiece  58 . Head  11  also comprises a top wall  66  fitted with a d.c. electric motor  67  for controlling the travel of electric spindle  13 ,  17 , for which purpose, the shaft of motor  67  is connected to the top end of screw  61  by a further joint  68 . 
     Movable crosspiece  63  is connected by two pneumatic actuators  69  to two columns  70  fitted with board holder device  21 , which substantially comprises a ring  71  fitted removably with bush  22  by means of an electrically conducting washer  65  to which wire  34  is connected. Ring  71  comprises two integral arms  72  fitted to the bottom ends of columns  70 ; and ring  71  and arms  72  may advantageously be made of known 35-70 anodized anticorodal aluminum, which is sufficiently insulating electrically. 
     Crosspiece  63  is fitted at the bottom with a bracket  73  engaged by a pin  74  for preventing stator  19  from rotating inside bush  51 , and which is fitted to a rod  75  integral with stator  19 . Joint  38  is connected to crosspiece  63  by a column  76  comprising a bottom threaded seat  77  which is screwed on to a screw  78  of joint  38 , and a top threaded seat  79  which is screwed on to a further threaded pin  80  fitted to crosspiece  63 . To insulate stator  19  electrically with respect to head  11 , both pin  74  and column  76  are made of insulating material, e.g. DELRIN (registered trademark). 
     According to a further characteristic of the invention, for more accurately controlling machining depth K, board  6  (FIGS. 8 and 9) may be provided with at least two series  81  of holes, in which each hole extends up to a respective conducting layer  9 . In FIG. 9, board  6  comprises four conducting layers  9   a ,  9   b ,  9   c ,  9   d  alternating with three fiberglass-reinforced plastic layers  7   a ,  7   b ,  7   c ; and each series  81  of holes comprises a hole  82  through layers  9   a ,  7   a  up to layer  9   b , a hole  83  through the same layers as hole  82  and through layers  9   b ,  7   b  up to layer  9   c , and a hole  84  through the same layers as hole  83  and through layers  9   c ,  7   c  up to layer  9   d.    
     At each hole  82 - 84 , the respective conducting layer  9   b ,  9   c ,  9   d  comprises a pad  85  connected by a respective conducting track  86 ,  87 ,  88  to the two metal pins  89  normally provided on board  6  for positioning it with respect to worktable  12 , and which, in use, provide for electrically connecting pads  85  to bush  22  (FIG.  2 ). As board  6  is normally four-sided, provision may advantageously be made for four series  81  of holes  82 - 84 , each located adjacent to a respective corner of board  6 . 
     Prior to the actual drilling cycle, control unit  16  (FIG. 4) is programmed to perform a self-teaching cycle to determine the actual distances between layers  9   b - 9   d  and surface  10  of board  6 . That is, unit  16  first inserts tool  14  through holes  82 - 84  to locate and acquire the depth of layers  9   b - 9   d  in the four series  81  of holes  82 - 84 ; determines the mean thickness of pairs of layers  9   a - 7   a ,  9   b - 7   b ,  9   c - 7   c  and the mean depth of the four corresponding holes  82 - 84 ; and memorizes the various mean values in respective registers  20 , for use as the depth value K by which to drill the specific board  6 . 
     Device  23  therefore provides for a machining depth control method comprising a step  90  (FIG. 10) for determining electrical contact between tool  14  and board  6 ; a step  91  for generating a signal indicating a predetermined capacitance between stator  19  and rotor  18  following contact; a step  92  for memorizing said signal; and a step  93  for controlling the machining depth K at which said signal was generated. If the board comprises at least one series  81  of holes  82 - 84 , the method also comprises a self-teaching step  95  to determine depth K by probing holes  82 - 84 . 
     As compared with known devices, the advantages of machining depth control device  23  according to the invention will be clear from the foregoing description. In particular, it provides for eliminating errors due to determining the position of the tool with respect to the board holder; for eliminating the complications involved in the manufacture of the proximity sensor and air-cushion piston of known devices; and, finally, by automatically determining the thickness of layers  7 ,  9  in holes  82 - 84 , for eliminating errors due to varying thicknesses of boards  6 , even in the same production lot. 
     Clearly, changes may be made to the device as described and illustrated herein without, however, departing from the scope of the present invention. For example, changes may be made to logic control circuits  24  and/or to the structure of electric spindle  13 ,  17 ; to the insulation of stator  19  with respect to head  11 ; and, finally, to the number and location of series  81  of holes  82 - 84  for the self-teaching step. If necessary, even only one series  81  of holes  82 - 84  may be provided.