Method and apparatus for transmitting data between a position measuring system and an evaluation unit

A method for transmitting data between a position measuring system and an evaluating unit that includes the steps of generating a system-specific parameter, transmitting the system specific parameter between a position measuring system and an evaluating unit over at least one supply line.

FIELD OF THE INVENTION
 The present invention relates to a method and an apparatus for transmitting
 data between a position measuring system and a downstream evaluating unit.
 BACKGROUND OF THE INVENTION
 Conventional position measuring systems for determining the position of two
 parts movable relative to one another, for instance on a machine tool, are
 as a rule connected via a certain number of signal transmission lines with
 a downstream evaluating unit, for instance a numerical controller. In
 principle, in transmitting various signals between the position measuring
 system and the downstream evaluating unit, the attempt is made to keep the
 number of signal transmission lines required as low as possible. Besides
 avoiding unnecessary cable-laying effort and expense, another object in
 the transmission of possibly desired additional signals is to keep the
 compatibility with the previously conventional number of signal
 transmission lines assured as much as possible.
 From European Patent Application EP 0 660 209 A1 of the present applicant,
 a position measuring system is known which includes among other elements a
 read/write memory unit. The memory unit is used in particular for storing
 system-specific parameters, which can be transferred to the position
 measuring system for instance from the downstream evaluating unit. When
 the memory unit is read out, in turn, certain system-specific parameters
 are transferred from the position measuring system to the evaluating unit.
 The system-specific parameters that can be stored in the memory unit and
 read out again are parameters of the entire system, for instance
 comprising an evaluating unit, a machine tool, and a position measuring
 system, if the corresponding position measuring system is used in a
 machine tool. Specifically, this may involve the specification of the
 measurement system type, the width of the signal period, specifications on
 the measurement steps in the position measuring system, the form of output
 signals furnished, reference signal information, machine tool parameters,
 correction values for the generated signals, and many more.
 With the aid of such a memory unit in the position measuring system, it is
 now possible for instance prior to the actual measurement mode to program
 the position measuring system in a suitable way, by modifying certain
 parameters in the memory unit. On the other hand, the evaluating unit used
 can also be adapted to the existing overall system, by transferring
 certain parameters from the memory unit to the evaluating unit.
 For bidirectional transmission of the system-specific parameters between
 the position measuring system and the downstream evaluating unit, a
 separate signal transmission line may for instance be provided. However,
 this increases the aforementioned cable-laying effort and expense, and
 moreover in that case the desired compatibility with previous interface
 standards no longer exists.
 The object of the present invention is therefore, in position measuring
 systems having a read/write memory unit and with the provision for
 transmission of system-specific parameters from and to an evaluating unit,
 to keep the expense for the requisite signal connections as low as
 possible. In particular, given the possible transmission of such
 system-specific parameters, compatibility with previous interfaces is also
 desired.
 The transmission of incremental position information over a supply line
 between a position measuring system and an evaluating unit is known from
 German Patent Disclosure DE 195 04 822. However, this reference neither
 discusses how system-specific parameters should be transmitted as reliably
 as possible nor provides details on designing a suitable apparatus or a
 corresponding method.
 SUMMARY OF THE INVENTION
 One aspect of the present invention regards a method for transmitting data
 between a position measuring system and an evaluating unit that includes
 the steps of generating a system-specific parameter, transmitting the
 system specific parameter between a position measuring system and an
 evaluating unit over at least one supply line.
 A second aspect of the present invention regards an apparatus for
 transmitting data between a position measuring system and an evaluating
 unit, the apparatus includes a position measuring system, an evaluating
 unit and at least one supply line connected to the position measuring
 system and the evaluating unit. A modulator unit is connected to the at
 least one supply line, the modulator unit prepares a system specific
 parameter that is transmitted over the at least one supply line.
 Thus, instead of transmitting the system-specific parameters over a
 separate signal transmission line between the position measuring system
 and the evaluating unit, at least one already existing supply line between
 the position measuring system and the evaluating unit is used for this
 purpose.
 Each of the aspects of the present invention provide the advantage of
 render the otherwise required signal transmission line and the
 corresponding additional effort and expense accordingly become
 unnecessary. On the contrary, the provisions of the invention assure that
 with the connections already typically present between the position
 measuring system and the evaluating unit, reliable bidirectional
 transmission of system-specific parameters is possible.
 Each of the aspects of the present invention provide a number of
 possibilities, especially with regard to the way in which the
 system-specific data are transmitted over the supply line. Thus there are
 various possible ways to adapt the apparatus and method of the invention
 to applicable given conditions.
 Further advantages, details and possible embodiments of the method and
 apparatus according to the invention will become apparent from the ensuing
 description of an exemplary embodiment in conjunction with the
 accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The position measuring system 1 shown includes a scale graduation 2 and a
 scanning unit 3 movable relative to the scale graduation 2. The position
 data generated by the position measuring system 1 are transmitted to a
 downstream evaluating unit 6 for further processing. The scale graduation
 2 and the scanning unit 3 are for instance connected to parts of a machine
 tool that are movable relative to one another and whose relative position
 with regard to one another is to be determined. In this case, a known
 numerical machine tool controller is used as the evaluating unit 6.
 Along with the schematically shown exemplary embodiment for detecting
 linear motions, it is understood that position measuring systems that
 detect rotary motions can also be embodied according to the invention.
 In the relative motion of the scale graduation 2 and the scanning unit 3,
 position-dependent scanning signals are generated, via a schematically
 indicated signal generating unit 4. The generated data may involve the
 most various kinds of position data, such as incremental position data,
 absolute position data or suitable combinations of them, and so forth.
 The generation of the various types of position data may also be
 accomplished in the most various ways. That is, known optical, magnetic,
 inductive or capacitive scanning principles may be employed for this
 purpose.
 In the case where incremental position data are generated on the basis of
 optical scanning, a reflectively embodied scale graduation 2 may for
 instance be provided. The signal generating unit 4 of the scanning unit 3
 in turn includes a plurality of light sources, scanning structures, and
 optoelectronic detector elements.
 Absolute position data, on the other hand, may be generated via optical
 incident-light scanning of a code disk, a process which is also known.
 Alternatively to this, however, a signal generating unit 4 which furnishes
 a single signal period over the measurement range and in this way enables
 absolute position data to be determined and so forth, can also be used for
 this purpose.
 Because of the manifold possible embodiments in terms of the generation of
 position data, the drawing shows no further details of the signal
 generating unit 4 but merely schematically indicates it. It is understood
 that as noted, more than one of the various possibilities for generating
 position data may be used in combination with one another; that is, in
 this respect there are multiple possible ways of embodying the signal
 generating unit 4 accordingly or optionally of providing a plurality of
 different signal generating units.
 The incremental position data generated by the signal generating unit 4 in
 the exemplary embodiment shown are transmitted in parallel form over a
 total of four signal transmission lines 10a, 10b, 11a, 11b to the
 downstream evaluating unit 6. Accordingly, in the case of incremental
 position data in the relative motion of the scale graduation 2 and the
 scanning unit 3, the signal generating unit 4 furnishes at least two
 scanning signals phase-offset by 90.degree. from one another. For each of
 the two scanning signals, there is also an inverted signal, that is, a
 signal phase-offset by 180.degree., which is likewise transmitted to the
 evaluating unit 6 for further processing over the signal transmission
 lines 10a, 10b, 11a, 11b.
 In the schematic illustration of FIG. 1, a second signal generating unit 7
 is also indicated in the scanning unit 3 that is, in the position
 measuring system 1. By way of this second signal generating unit, a
 reference pulse signal is generated in a known way for one or more
 defined, known relative positions of the scale graduation 2 and scanning
 unit 3. The reference pulse signals generated by the second signal
 generating unit 7 are transferred, via a further pair of signal
 transmission lines 12a, 12b, to the evaluating unit 6, where they are
 further processed. As in the case of the other signals, once again a
 transmission of signals inverted relative to one another over the two
 signal transmission lines 12a, 12b is contemplated. That is, along with
 the actual reference pulse signal, the signals phase-offset from it by
 180.degree. are also transmitted. With regard to the characteristics
 essential to the invention in terms of transmitting the system-specific
 parameters over the supply lines 9a, 9b, however, the transmission of the
 reference pulse signal or the way in which it is transmitted plays no
 further role, but is merely mentioned in conjunction with possible
 embodiments of the apparatus or method according to the invention.
 Along with the signal generating units 4, 7 mentioned so far, the position
 measuring system 1 shown also has a read-write memory unit 5. In this
 memory unit 5, the most various system-specific parameters can be both
 stored and read out again. In addition, parameters stored in it can
 naturally also be modified, but the possibility also exists of allowing
 certain parameters to be changed only by way of suitable authorization.
 For instance, it is possible to divide up the memory unit 5 into a
 plurality of regions with different access rights; while a first memory
 region is accessible only to the particular manufacturer of that position
 measuring system, a second memory region may also be written into and read
 out of by the user, and so-forth. In one possible embodiment, for
 instance, the memory unit 5 is embodied as an EPROM in a known way.
 The system-specific parameters that can be stored in the memory unit 5 and
 modified and read out again may for instance be characteristic parameters
 of the position measuring system, which are specified by the applicable
 manufacturer, such as information on how the memory unit 5 is organized,
 the type of position measuring system, information on the signal period
 and the measurement steps, the location of the reference marks, etc. In
 the memory unit 5, parameters that are significant for the planned
 measurement mode, such as information on the location of the zero point,
 can also be modified by the user or rewritten. Furthermore, correction
 values can be stored in the memory unit 5, and these values can be imposed
 on the generated signals in order to compensate for possible signal
 errors. With regard to other system-specific parameters, reference may
 also be made to the aforementioned EP 0 660 209 filed by the present
 applicant.
 This kind of modification, writing or readout of the system-specific
 parameters of the position measuring system 1 is preferably done prior to
 the actual measurement; that is, a suitable transmission of the associated
 signals is as a rule needed only intermittently. Otherwise, such a
 procedure might be necessary in the event of an error, for diagnostic
 purposes, for instance.
 The modification, writing or readout of the system-specific parameters is
 done via the downstream evaluating unit 6, and hence an appropriate signal
 connection with the memory unit 5 is necessary. According to the
 invention, at least one of the existing supply lines 9a, 9b is now used
 for this purpose, lines over which otherwise the current and voltage
 supply to the position measuring system 1, or the corresponding components
 such as light sources, detector elements, etc., is accomplished. In the
 evaluating unit 6, a current and voltage supply unit 14 is provided for
 this purpose, though it need not necessary integrated with the evaluating
 unit 6. It is well known in the art that such a current and voltage supply
 unit 14 can be referred to as a power supply since it generates electric
 power that is transmitted along at least one of the power supply lines 9a,
 9b to the position measuring system.
 The communication between the evaluating unit 6 and the position measuring
 system 1 or memory unit 5 over the supply lines 9a, 9b can preferably be
 accomplished in both directions, or in other words can be designed
 bidirectionally. Thus, both a readout and a modification or writing in of
 system-specific parameters into the memory unit 5 in serial form is
 assured. To transmit the most various system-specific data over at least
 one of the available supply lines 9a, 9b, the system specific data has to
 be prepared for transmission over the supply lines 9a, 9b to the position
 measuring system 1 prior to a measurement performed by the position
 measuring system 1. On the transmission side preparation in the form of a
 modulation of the data or parameters to be transmitted is contemplated,
 while on the reception side a corresponding separation or demodulation of
 the transmitted signals from the respective supply line 9a, 9b is
 necessary. To that end, in the case of bidirectional serial transmission
 of system-specific parameters, modulator/demodulator units 8, 13 which
 perform these tasks, are disposed in both the position measuring system 1
 and the evaluating unit 6, respectively. Thus, while a corresponding
 modulator stage of such a unit prepares the system-specific parameters
 read out of the memory unit 5 for transmission via supply lines 9a, 9b to
 the evaluating unit 6, the respective demodulator stage separates or
 decouples these data or parameters from the supply line 9a, 9b. Although
 in FIG. 1 the modulator/demodulator units 8, 13 are shown merely
 schematically as combined units, it is naturally also possible to provide
 separate units for modulation and demodulation, respectively, of the
 transmitted data. For transmitting the various system-specific parameters,
 the most various known modulation methods may be employed.
 In the embodiment shown, for transmitting the system-specific parameters,
 the two existing supply lines 9a, 9b of the direct-voltage distributor are
 used. Thus, the system-specific parameters and the electric power travel
 within the same power lines and so it is inherent that the system-specific
 parameters are transmitted along, at least partially, an electrically
 conductive path that is identical to an electrically conductive path which
 the electric power of the supply lines travels. While the system-specific
 parameters are transmitted in modulated form over one of the two supply
 lines 9a, 9b, a transmission of the signals inverted from it, that is,
 phase-offset by 180.degree., is done over the respectively other supply
 line 9a, 9b.
 Along with this possibility of transmitting the inverted signals as well,
 it is understood that other possibilities for transmitting system-specific
 parameters also exist. For instance, where there are two existing supply
 lines 9a, 9b in the case of direct-voltage supply to the position
 measuring system 1, one of the two supply lines 9a can be used for
 transmission in one direction from the position measuring system 1 to the
 evaluating unit 6 and the other supply line 9b can be used for
 transmission of data from the evaluating unit 6 to the position measuring
 system 1.
 Along with the provisions described thus far, it also proves to be
 advantageous, via suitably disposed filter elements in the supply lines
 9a, 9b, to prevent the modulated signals from interfering with the current
 or voltage supply to the position measuring system 1 and/or the evaluating
 unit 6. To that end, upstream of the corresponding elements that are
 connected on both sides to the supply lines 9a, 9b, suitable filter
 elements may be provided.
 On the basis of the provisions of the invention, it is now possible to
 transmit system-specific parameters using at least one of the existing
 supply lines; that is, no additional cable-laying effort or expense is
 involved. In addition, the compatibility with the number of previous
 connections between the position measuring system 1 and the evaluating
 unit 6 is assured.
 The invention may be embodied in other forms than those specifically
 disclosed herein without departing from its spirit or essential
 characteristics. The described embodiments are to be considered in all
 respects only as illustrative and not restrictive, and the scope of the
 invention is commensurate with the appended claims rather than the
 foregoing description.