Abstract:
In the magnetic chuck control disclosed herein, the chuck winding is selectively energized in one direction or the other through respective triggerable semiconductor current switching devices. Successively reducing current levels are applied in successive phases of operation, the polarity of the current being reversed in successive phases. During a first portion of each phase the device is triggered to apply a corresponding preselectable voltage. During a second portion of each phase the switching devices are triggered quite late to provide an average voltage which opposes the current flow which was induced during the first portion thereby to quickly reduce the level of current flowing in the winding. The current level is sensed to generate a control signal representative thereof and a sequencing means, responsive to the control signal, operates when the current in the winding falls below a preselected value during the latter portion of each phase thereby to terminate the late energization portion and to initiate the application of current in the reverse direction thereby instituting the next phase in the demagnetizing sequence.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetic chuck controller and more particularly to such a controller in which current sensing is employed to determine the dissipation of a previously applied current in the chuck winding. 
     Magnetic chucks are widely utilized in the machine tool industry for holding a work piece which is to be machined or ground. The magnetic chuck is essentially an electro-magnet which is energized to retain the work piece. However, to release the work piece it is typically necessary to provide a demagnetizing sequence, i.e. to reduce the residual magnetism in the chuck and the work piece in order for the work piece to be removed. The demagnetizing sequence typically comprise the application of a succession of successively reducing current levels in successive phases, the polarity of the current being reversed in successive phases. Since the chuck, together with the work piece, constitutes a highly inductive load, the time periods required are relatively long as compared with the typical period of supply line alternating current. Further, in order to demagnetize effectively, time must be allowed for the magnetic flux to penetrate the work piece against the counteracting forces of eddy currents, etc. 
     In order to shorten the demagnetizing cycle as much as possible, it has previously been proposed to utilize current sensing during the build up of current during each phase in the demagnetizing cycle. Such proposals are for example contained in the Littwin U.S. Pat. No. 3,401,313 and the Wilterdink U.S. Pat. No. 4,402,032. It has been found, however, that this mode of speeding the demagnetizing cycle can reduce the effectiveness of the demagnetizing since it makes no allowance for the time required for the magnetic flex to penetrate the work piece to maximum depth. It has also been proposed to shorten the time required to dissipate a current previously induced in the chuck winding by shunting or &#34;crowbarring&#34; the winding following a period of energization thereof. 
     While the application of a reverse voltage through a second set of triggerable semiconductor current switching devices would, in theory, more quickly reduce the current flowing, as a practical matter such a technique may induce failures of the semiconductor devices since triggering the second set of devices may produce an effective short across the a.c. supply mains if the first set has not commutated. As is understood by those skilled in the art, it is the nature of an inductive load to freewheel through a triggerable current switching device and keep it forward biased and conducting even though it is not triggered. Thus, though such an arrangement has previously been proposed, i.e. in the Wilterdink patent identified above, there are concomitant problems. 
     Among the several objects of the present invention may be noted the provision of a novel magnetic chuck controller; the provision of such a controller which facilitates rapid ard complete demagnetization of a chuck and its work piece, notwithstanding variations in the size and magnetic characteristics of the work piece; the provision of such a controller which is highly reliable nothwithstanding the inductive character of the magnetic chuck; the provision of such a controller which is highly flexible in operation; and the provision of such a controller which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter. 
     SUMMARY OF THE INVENTION 
     Briefly, apparatus of the present invention operates to demagnetize a work piece of unknown characteristics, using a winding which is magnetically coupled to the work piece, through the application of a sequence of successively reducing current levels in successive phases of operation, the polarity of the current being reversed in successive phases. Triggerable semiconductor current switching devices are utilized for applying direct current of a selectable polarity to the winding from alternating current supply mains. The switching devices are controlled during a first portion of each phase of the demagnetizing sequence for applying a respective preselectable voltage to the winding to develop a corresponding current level. The control means operates during a second portion of each phase to effect late triggering of the semiconductor current switching devices thereby to provide an average voltage which opposes the current flow induced during the first portion of the phase thereby to quickly reduce the level of current flowing in the winding. The level of current in the winding is sensed and a control signal representative thereof is generated. Control means responsive to the control signal and operative when the current in the winding falls below a preselected absolute level during the latter portion of each phase in the demagnetizing sequence terminates the late triggering operation and initiates the application of current in the reverse direction, i.e. initiates the first portion of the next phase in the demagnetizing sequence. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a magnetic chuck controller constructed in accordance with the present invention; and 
     FIGS. 2A-2C are diagrams illustrating the timing of triggering of semiconductor current switching devices utilized in the device of FIG. 1. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, the magnetic chuck to be controlled by the apparatus of the present invention is designated generally by reference character 11. Chuck 11 includes a winding 13 which is selectively connectable to alternating current supply leads 15 and 17 through triggerable semiconductor current switching devices. In the embodiment illustrated, these triggerable switching devices are silicon controlled rectifiers (SCRs) Q1-Q8. As is understood, if the a.c. supply voltage readily available is not appropriate, a step-up or step-down transformer may be interposed in the a.c. supply circuit. For purposes described in greater detail hereinafter, the primary winding 21 of a current sensing transformer 23 is placed in series with the connection to supply lead 15. By means of this transformer, the current being applied to the chuck 11 may be sensed. 
     The SCRs Q1-Q8 are triggered through pulse transformers T1-T4. In the embodiment illustrated, each pulse transformer includes a single primary, e.g. T1P, and a pair of secondaries, e.g. T1S1 and T1S2, the SCRs thus being triggered in pairs. For the purpose of simplifying the drawings and the explanation, the various suppression and/or damping networks typically associated with the use of triggering pulse transformers have been omitted in this illustrative drawing. The SCRs are connected in a bridge circuit so that, by triggering an appropriate pair, a current can be applied to the winding 15 in a selected direction on either half cycle of the a.c. supply lines. 
     Sequencing of the triggering of the various SCR pairs is flexibly controlled by a microprocessor. This microprocessor, together with its associated memory components, is designated generally by reference character 25. Pulse output signals generated by the microprocessor are applied to the primaries of the pulse transformers through driver amplifiers 26 and 27. 
     A step-down transformer 31 provides a.c. current at a reduced voltage level to a power supply 33 which powers the various integrated circuit components employed in the controller. A zero crossing detector circuit 35 also responds to this a.c. voltage to provide a phase or timing reference to the microprocessor 21. 
     The secondary winding 24 of current sensing transformer 23 is connected to a diode bridge 41 and the rectified output signal from the bridge 41 is applied to a sense resistor R1 shunted by a filter capacitor C1. The d.c. voltage generated across the capacitor C1 is essentially representative of the current being applied to the chuck 15 through the bridge circuitry comprising SCRs Q1-Q8. In order to provide to the microprocessor 25 a digital value representing this current level, the d.c. voltage across capacitor Cl is applied to an analog-to-digital converter 43 whose output is connected to the microprocessor 25. The analog-to-digital converter 43 is also utilized to digitize preselectable voltage levels provided by a series of potentiometers R2-R6. The first of these is an operator selectable value which is used to produce a reduced or so-called VARIABLE level of energization of the chuck. As is understood in the art, a reduced level of energization is desirable for some machining operations where less than full holding power is required. The other values are typically preset for a given application or installation and provide operating parameters for the micro-computer 25. In the particular embodiment illustrated, the other values determine: an absolute minimum current level which is used as a reference to determine if the chuck winding has open circuited; a relative current level which is used in testing for some component failures; a value which represents the number of steps (pulses) to be taken in the demagnetizing sequence; and the length of time each pulse is to be applied. 
     To select between the various possible modes of operation provided by the controller of the present invention, four operator actuable push button switches PB1-PB4 are provided. One side of each push button switch is grounded and the other side is connected, through a respective biasing resistor R15-R18, to the 5-volt supply thereby to generate binary signals representative of the state of the respective push button switch. These signals are provided to the microprocessor 25. The states of the switches are read during a background process run by the microcomputer. 
     To indicate the existing mode of operation of the control, the microprocessor 25 controls four indicator LEDs (light emitting diodes) L1-L4 which are connected to the five volt supply through respective current limiting resistors R19-R22. The four states which can be selected and indicated are conventionally designated FULL (full energization), VARIABLE (variable energization), RESIDUAL (de-energize without demagnetization) and RELEASE (demagnetize). 
     To fully energize the chuck 15, the SCRs are selectively energized to apply a current in the direction indicated ty the arrow in FIG. 1. Thus, during the a.c. half cycle when the supply lead 15 is positive, the SCRs Q1 and Q2 are triggered early in the half cycle so that substantially the full positive going waveform drives the desired current flow. Similarly, during the a.c. half cycle when the supply lead 17 is positive, the SCRs Q3 and Q4 are triggered early in the half cycle so that, again, substantially the whole area of the waveform drives current in the forward direction through the chuck winding. This operation is illustrated in FIG. 2A where the point of triggering of the SCRs Q1 and Q2 is designated by reference character 51 and the point of triggering of SCRs Q3 and Q4 is designated by reference character 53. Since the chuck 11 is highly inductive, it will be understood that, once current flow is established, each SCR will remain conductive until the current flow is taken over by the triggering of another SCR, i.e. until commutation takes place. The forward current flow through each SCR will in fact continue even though the corresponding supply lead goes negative, since the inductive reactance to current change will cause the SCR itself to remain forward biased. Thus, the waveform will, in fact, include small negative going portions, i.e. portions which slightly oppose the forward current flow. 
     To effect partial or VARIABLE energization, the triggering of the SCRs is delayed as illustrated in FIG. 2B. In this case, the average DC component driving current through the winding is reduced and, in fact, the waveform includes not only positive but significant negative portions. 
     Using these same diode pairs, it is even possible to develop a voltage which significantly opposes a current previously induced through the chuck winding by these same SCR pairs. This mode of operation is important in the overall method of operation of the apparatus of the present invention and is illustrated in FIG. 2C. In this mode of operation, the triggering of the SCRs is delayed until very late in the respective a.c. half cycle, i.e. well after the peak in the a.c. waveform. Although each SCR pair is triggered while the respective supply lead is positive, i.e. so that it can take over from or commutate the other SCR pair, the net waveform is essentially negative and opposes the preexisting current flow. As will be understood by those skilled in the art, this mode of operation will successfully produce an opposing voltage so long as the inductively stored energy is sufficient to maintain the direction of the preexisting current flow. Given the highly inductive nature of magnetic chucks together with their associated work pieces, this mode of operation is, in fact, sustainable for a great many a.c. half cycles. 
     While it would be possible to merely trigger according to this mode for a sufficient time to guarantee that the current drops below the level at which the SCRs will continue to conduct unless commutated, the present invention substantially shortens this current quenching phase by measuring the actual current flow to the chuck. This measurement is provided by means of the current sensing transformer 23 and the analog-to-digital converter 43 described previously. Thus, the microprocessor can run the SCR bridge in the current quenching mode of FIG. 2C for a period of time which it determines empirically and which will vary in accordance with the magnetic characteristics of the chuck and work piece. In other words, the microprocessor can run the SCR bridge in the FIG. 2C mode until it determines, from the A/D converter 43, that the current being supplied to the bridge has dropped below a programmatically preselected level. From the foregoing, it can be seen that a current of a desired level can be induced in the winding 15 of chuck 11 and that, using the same SCRs which induce the current, the current can be actively quenched, i.e. by a voltage which opposes the current. As will be understood, quenching with an opposing voltage will cause the current level to drop much faster than merely shorting or &#34;crowbarring&#34; the chuck supply leads. 
     Since the SCR bridge is entirely symmetrical, it can also be seen that a current of programmatically preselectable level can be induced in the reverse direction and then selectively quenched using the same SCRs which induced the current. In generating a reverse current, i.e. a current which is opposite to the arrow in FIG. 1, however, SCRs Q5 and Q6 are triggered during the a.c. half cycle when the supply lead 15 is positive and the SCRs Q7 and Q8 are triggered during the a.c. half cycle when the supply lead 17 is positive. Since the actions are essentially the same on each a.c. half cycle, it is convenient to merely connect the respective primaries in parallel and have the microprocessor generate an appropriate pulse during each half cycle, these pulses being applied through the driver amplifier 27 when a forward current is desired or existing and through the driver amplifier 26 when a reverse current is desired or existing. 
     Since the quenching phase is active, i.e. a voltage is applied which opposes the existing current flow, the quenching phase can proceed quite quickly. Further, since the time of quenching is not predetermined but determined empirically by means of the current sensing transformer, no time is wasted during the demagnetizing (RELEASE) operation. 
     While a voltage opposing an existing forward current flow could, in theory, be generated by the SCRs which are utilized to generate a reverse current flow, a substantial danger exists that improper commutation will occur and that the reverse current driving SCRs will be turned on without the forward driving SCRs being commutated off. In this case an effective dead short will exist across the supply leads. This condition, even though momentary, can quickly destroy the SCRs. 
     In the embodiment illustrated, the microprocessor was a type 8748 from the Intel Corporation of Sunnyvale, California. This device incorporates EPROM memory so that the operating program can be permanently stored in the device itself. The actual program under which the microprocessor operates is set forth, in source code form, in an appendix to this application. 
     In view of the foregoing, it may be seen that several objects of the present invention are achieved and other advantageous results have been attained. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it should be understood that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. ##SPC1##