Switching device for feeder in machine tools
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a directional switching device for a feeder in a machine tool, and more particularly, to an improved directional switching device for a feeder in a machine tool driven by a direct current motor. 
     2. Prior Art 
     Generally, oil pressure cylinder type devices are employed for driving feeders to move machine axles, work tables and the like to prescribed positions. The oil pressure type cylinder device has several advantages such as relative ease of control and great driving power which makes it suitable as a thrust generator device in a feeder. However, its drawbacks include unstable feeder control caused by temperature variations and the use of compressive fluids, which affect productivity and durability of the machinery. In addition it presents such environmental problems as oil leaks and noise. 
     As opposed to conventional oil pressure cylinder devices, a motor driven feeder method has been proposed. However this method does not completely satisfy the advantages derived by the oil pressue cylinder method in that, particularly with respect to alternating current (A.C.) motors, simple changes in rotational speeds are difficult to achieve which necessitates expensive and complex gear boxes and the like to be installed. Another drawback is the difficulty in securing and maintaining a prescribed stopping position of the feeder which causes inaccurate forward and backward movements and mechanical friction which results in a shorter machine life. 
     As opposed to these drawbacks associated with the A.C. motor, the direct current (D.C.) motor is more favorable, although it too has disadvantages such as a complex circuit structure for switching the rotational direction, i.e., the direction in which the feeder moves. In the conventional D.C. motor the D.C. current is supplied to the motor&#39;s armature by a D.C. bridge made up of four transistors. By a switch controlling the transistor bridge circuit, flow of the armature current is reversed thereby switching the rotational direction. Thus, the structure of the control circuit in the conventional D.C. motor driven device is not only more complex but also requires a protective circuit in order to control any abnormal excessive voltage that is generated when the circuit is stopped for reversing action. Because of this and other drawbacks the D.C. motor has been employed only in special machines such as NC machinery which is equipped with expensive control devices. 
     SUMMARY OF THE INVENTION 
     Accordingly it is a general objective to provide a directional switching device for a feeder in a machine tool utilizing a direct current motor capable of accurately switching the direction of feed by means of an exceedingly simple system. 
     In keeping with the principles of the present invention, the objects are accomplished by a unique directional switching device for a feeder in a machine tool driven by a direct current motor. The armiture circuit in the direct current motor is coupled to a relay contact circuit for switching the forward and backward movements of the feeder. In addition, the relay contact circuit is controlled by signals from a sequenting circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned features and objects of the present invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings. Wherein like reference numerals denote like elements, and in which: 
     FIG. 1 is an outline of the circuitry for a directional switching device for a feeder in a machine tool in accordance with the teachings of the present invention; 
     FIG. 2 is a circuit diagram of a sequencing circuit utilized with the embodiment of FIG. 1; and 
     FIG. 3 illustrates a time chart of the sequencing circuit of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the directional switching device for a feeder in a machine tool includes a DC motor coupled to a well known feeder (not shown). The motor 10 moves the feeder at a prescribed speed and provides a desired force to the feeder. Direct current is supplied to the armiture of the motor 10 via a relay contact circuit 12 and a serial control circuit 14. Servo control circuit 14 contains an output transistor 16 whose collector-emitter is coupled in series with the armature circuit of the motor 10. 
     Base driving current from a driving circuit 18 is supplied to the base of transistor 16 thereby providing desired control signals to transistor 16 in response to load conditions, speed and power instructions. Transistor 16 controls the intermittent flow of armature current thereby controls the amount of current supplied to motor 10 according to its duty cycle. An inductance 20 is inserted into the armature circuit of the motor 10 and evens out the intermittent flow of direct current from the servo control circuit 14 which is passed on to the relay contact circuit 12 and supplied to the motor 10. 
     Driving circuit 18 is controlled by the output from a comparator 22 which receives control signals from the outside. A tachometer generator 24 is directly coupled to the axle of motor 10 and is provided adjacent thereto for the purpose of counting the rotational speed of the motor 10 at all times. The count is supplied to servo control circuit 14 via relay contact circuit 26 which then instructs comparator 22 and others of the rotational speed of motor 10. Servo control circuit 14 also has an interlocking circuit 27 for shutting off the base current of transistor 16 so as to prevent unnecessary rotation of motor 10 when it must stop. Relay contact circuit 12 contains two sets of interlocking contacts MS 12  and MS 3  and MS 22  and MS 23 . Relay contact circuit 26, coupled to tachometer generator 24, also contains two sets of interlocking relay contacts CR 33  and CR 34  and CR 64  and CR 62 . Interlocking circuit 27 includes &#34;b&#34; contacts CR 12  and CR 41  and &#34;a&#34; contacts CR 73 , the later being coupled in parallel with the &#34;b&#34; contacts. A damping resistance 38 is coupled to motor 10 via contact MS 32  to form a closed circuit for dynamo breaking when the motor 10 is to be stopped. The relay contacts MS 12  and MS 13  and CR 33  and CR 34  within the relay contact circuits 12 and 26, respectively, are actuated when a command is received to rotate motor 10 forward. Contacts MS 22  and MS 23  and CR 62  and CR 23  are activated to cause the motor to rotate in a reverse direction when a reverse command is received. 
     FIG. 2 illustrates sequencing circuits whose function is to insure the accurate forward and reverse switching of relay contact circuits 12 and 26 shown in FIG. 1. In the sequencing circuit, CR 1  through CR 9  denote current relays, MS 1  through MS 3  denote main relays and TR 1  denotes a timing relay. 
     FIG. 3 is a timing chart showing the functions of the sequencing circuit of FIG. 2. The functions of the relay contact circuits of the present invention are described hereinbelow in conjunction with FIGS. 2 and 3. 
     For the purposes of prior discussion and the following description, the contacts of a relay are identified as follows: XY nm  where XY is the relay tape, n is an integer indicating the relay number and m is an integer indicating the contact number of the relay; i.e., CR 71  is the number 1 contact of the 7 current relay. Furthermore, relays MS 1 , MS 2 , MS 3 , CR 1 , CR 3 , CR 4 , CR 6  and CR 7  in the following description are those relays which control the relay contacts in the circuits of FIG. 1. 
     In operation, prior to the start of feeder movement, motor 10 is in a stationery state and relay CR 7  is on. Thereupon interlocking circuit 27 shuts off the basic current of transistor 16 so as to prevent any armature current from flowing into motor 10. The ON status of contact CR 74  also activates relay MS 3  which in turn causes the dynamo breaking circuit in motor 10 to activate. At time t 1  (FIG. 3) a forward command signal from contact 30 (FIG. 2) is relayed via relay CR 13  to contact CR 1  which activates relay CR 2  at t 2 . The activation of relay contact CR 21  causes relay CR 3  to be activated at time t 3 . Contact CR 21  also causes relay MS 1  to be activated at time t 4 , which is slightly later than relay CR 3 . This time delay is accomplished by a delay circuit comprised of a condenser 32 and a resistor 34. 
     Deactivation of contact CR 31  causes relay CR 7  to deactivate (at time t 5 ) whereupon interlocking circuit 27 is deactivated so as to enable the base current of transistor 16 to flow. By activation of CR 32 , relay CR 8  is activated and contact CR 83  is deactivated (at time t 6 ) whereupon relay TR 1  cannot be activated even though contact CR 35  is activated. Relay MS 3  is deactivated by the deactivation of contact CR 81  (at time t 7 ). Thus, with the forward command signal the base interlock as well as the dynamo brake are released and relay MS 1  is activated which in turn activates contacts MS 12  and MS 13  and forward turn from servo control circuit 14 is supplied to motor 10. 
     While motor 10 is rotating in the forward direction a speed switching signal from a limit switch LS 1  causes relay CR 8  to deactivate (at time t 8 ) whereupon relay CR 7  is activated (at time t 9 ), and deactivation of contact CR 71  causes relay MS 1  to deactivate (at time t 10 ). Activation of contact CR 74  causes relay MS 3  to activate dynamo braking action after a time delay introduced by a delay circuit consisting of condenser 36 and resistor 38 (at time t 11 ). Contact CR 82  is activated at time t 8 , but relay TR 1  is activated only after a prescribed time delay (at time t 12 ) and relay CR 9  is activated at time t 13 ). Relay CR 1  is deactivated by deactivation of contact CR 91  while relay CR 9  continues in the activator state by contact CR 94 . Deactivation of contact CR 92  causes relays MS 3  and CR 7  to deactivate at time t 14  thereby releasing both the dynamo brake and transistor interlock. Activation of contacts CR 71  causes relay MS 1  to again activate (at time t 15 ) and thus the switching of the forward speed is accomplished. The switching operation described in the embodiment indicates decreasing times in the forward speed. 
     As the forward command is cancelled and contact 30 is deactivated, relay CR 1  is deactivated (at time t 16 ) followed by deactivation of relay CR 2  and relay CR 9  (at time t 17 ). Activation of contact CR 92  causes relay C 7  to activate, whereupon the base interlock is activated. A short while later relay MS 3  is activated (at time t 18 ) and still later relay TR 1  is activated (at time t 19 ). Thus, the dynamo braking and interlocking functions are performed in the same manner as in the above described speed switching operation. 
     Meanwhile, a reverse command activates contact 40, whereupon relay CR 4  is activated (at time t 20 ) while contact CR 42  activates relay CR 5  (at time t 21 ). Activation of contact CR 51  causes relay CR 6  to activate (at time t 22 ). Whereupon the activated contact CR 64  activates relay CR 8  (at time t 23 ) and deactivation of contacts CR 83  and CR 81  deactivates relays TR 1 , CR 7  and MS 3 . This action releases the brake interlock and motor 10 is in an unrestricted state. The above reverse preparations being completed, MS 3  is activated after a time delay introduced by a delay circuit made up of condensor 42 and resistor 44 (at time t 24 ). Thereafter motor 10 continues its reverse rotation. 
     Since the feeder has completed its desired reverse movement and has returned to its standard position, reverse command contact 40 is deactivated and relay CR 4  is deactivated (at time t 25 ). This deactivation causes contacts CR 12  and CR 41  of interlocking circuit 27 to activate thereby interlocking transistor 16. Thereafter contacts CR 51  and MS 22  and MS 23  and CR 64  are deactivated in succession at times t 26  and t 27  respectively whereupon the supply of armature current to motor 10 is stopped. Deactivation of CR 64  causes relay CR 8  to also deactivate (at time t 28 ). Activation of contact CR 81  activates relay CR 7  (at time t 29 ). Activation of contact CR 74  activates relay MS 3  at time t 30  thereby causing the feeder to stop and maintain its prescribed position. 
     As described above the forward and back controls, as well as the speed switching control of motor 10, are accomplished by the sequencer circuit of FIG. 2. Furthermore, it should be apparent that the driving circuit 18 and the comparator 22 could be common circuits which exist in the prior art. 
     In all cases it is understood that the above described embodiment is merely illustrative of but one of the many possible specific embodiments which represent the applications of the principles of the present invention. Furthermore numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention by those skilled in the art without departing from the spirit and scope of the invention.