Abstract:
A D.C. motor control circuit including a wheatstone bridge type speed regulating circuit with a transistor arranged for detection of the counter-EMF developed across the motor armature to control operation of a drive control circuit. In one embodiment of the invention, an actuating circuit comprises a constant voltage source formed from a constant current circuit connected parallel to an electrical power supply source for the motor and a variable resistor connected in series to the constant current voltage source. A diode applies the output of the variable resistor to the base of the transistor to thereby effect conduction of this transistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to D.C. motor speed control apparatus, and more particularly to a D.C. motor speed control apparatus of the type in which the armature winding of the motor is connected in one branch of a Wheatstone bridge circuit with an output proportional to the shaft speed being utilized in controlling the electrical power supply to the motor. Still more particularly, it relates to improvements of an actuating circuit associated with such a motor speed control circuit. 
     2. Description of the Prior Art 
     A D.C. motor speed control circuit of the Wheatstone bridge type is, for example, disclosed in Japanese Patent Publication No.Sho 50-40442 (Date of Publication: Dec. 24, 1975). FIG. 1 shows an example of this type of circuit. Here a D.C. motor DCM with an armature winding M having a resistance Ra forms a Wheatstone bridge together with three resistors R 1 , R 2  and R 3 . Connected across a pair of output terminals or points a and b, is an npn transistor Tr 2  and a spring of two diodes D 1  and D 2 . The base of transistor TR 2  is connected to the point a, between registors R 2  and R 3 . Its emitter is connected through a resistor R 4  to a negative input terminal or point d between resistor R 3  and the motor armature M. The emitter is also reverse biased by the string of two diodes D 1  from the point b between resistor R 1  and the motor armature. Hence the collector current depending upon the counter-EMF developed across the armature can be utilized to control the electrical power supply to the motor by using a pnp transistor Tr 1 . The latter is connected between a positive input terminal c of the Wheatstone bridge circuit and a positive electrode of an electrical power source or battery E through a control switch S 1 . 
     The voltage Et across the motor armature M is composed of two components; one is an equivalent developed counter electromotive force (EMF) Ea and the second is a voltage across the armature resistance Ra, and therefore may be expressed as 
     
         Et=EA+RaIa                                                 (1) 
    
     Here Ia is the current flowing through the armature resistance Ra. When the resistance Ra is selected so that the relationship Ra/R 1  =R 3  /R 2  is satisfied, the voltage appearing across the output terminals a and b of the Wheatstone bridge circuit is equal to the counter-EMF developed across the motor armature M. This counter-EMF Ea may be expressed as 
     
         Ea=(Z/a)·p·φ·n              (2) 
    
     wherein Z is the number of conductors; p is the number of poles; φ is the magnetic flux density per pole; a is the number of turns of wire; and n is the number of revolutions of the rotor. All of the above parameters except the n are constants of the design, thus the equation (2) may be written, 
     
         Ea=Kn                                                      (3) 
    
     With this Wheatstone bridge circuit, therefore, it is possible to control the number of revolutions of the rotor, that is, the speed of rotation of the motor, as a function of only one variable Ea in a manner such that the output of the Wheatstone bridge circuit is applied to the base of the transistor Tr 1 . This can then control the current flowing through the emitter-collector of transistor Tr 1 . 
     In operation, after a stationary state has been attained to drive the motor DCM at a constant speed, the output voltage V D  of the Wheatstone bridge circuit is maintained constant depending upon the difference between the voltage V F  across diodes D 1  and the base-emitter voltage V EB  of transistor Tr 2  regardless of how much current flows through the diodes D 1  and the transistor Tr 2 . Assuming that load on the motor DCM is increased a decrease occurs in the number of revolutions of the rotor. The voltage Et across the motor winding is then decreased. This decreases the potential at the point b, along with the emitter potential of transistor Tr 2 . This in turn causes an increase in the base potential of transistor Tr 1  relative to the emitter potential. As a result, the current supply to the Wheatstone bridge circuit is increased until the speed of rotation of the motor is restored to the initial predetermined level. 
     The conventional actuating circuit for use with the D.C. motor speed control circuit of FIG. 1 is shown in the same figure. It includes a resistor R 5  and a capacitor C 1 . These are connected in series between the base electrode of transistor Tr 1  and the negative terminal of the direct current supply source E. This actuating circuit is, however, not suited for assurance of initiation of operation of the speed control circuit at the time of closure of the main switch S 1 . For example, when the voltage of the energy source E increases at a slow rate to the critical level for the normal operation of the motor DCM, a long time lag results. Alternatively when switch S 1  is opened at the time the motor DCM is restrained, the charge stored on the capacitor C 1  is retained except for spontaneous leakage because of the lack of the discharge circuit therefor. This is so because the transistor Tr 2  is cut off at that time. Consequently, the subsequent closure of switch S 1  before capacitor C 1  is not completely discharged fails to result in actuation of the motor DCM, for transistor Tr 1  is not rendered conductive between the emitter and base thereof. 
     SUMMARY OF THE INVENTION 
     The present invention concerns the above mentioned Wheatstone bridge type of D.C. motor speed control apparatus. It is an object of the invention to improve the actuating circuit and to assure initiation of operation of the D.C. motor any time it is activated. 
     Another object is to provide an improved actuating circuit associated with the speed control circuit without adversely affecting the speed control operation. 
     Still another object is to reduce the consumption of electrical energy which would otherwise result from use of the actuating circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional D.C. motor control apparatus. 
     FIG. 2 is a circuit diagram of one embodiment of a D.C. motor control apparatus according to the present invention. 
     FIG. 3 is a similar diagram showing a second embodiment of the invention. 
     FIG. 4 is a similar diagram showing a third embodiment of the invention. 
     FIG. 5 is a similar diagram showing a fourth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows a D.C. motor control circuit embodying the invention. Here the right-hand portion of the circuit is a Wheatstone type speed control circuit of conventional structure similar to that of FIG. 1. The same reference characters have been employed to denote parts similar to those of FIG. 1. The left-hand portion is an actuating circuit using characteristics of the present invention. In the actuating circuit of FIG. 2 a pnp transistor Tr 3  has an emitter electrode connected through a resistor R 6  to the positive bus at a point between the main switch S 1  and the emitter of the power supply control transistor Tr 1 . A base electrode of the transistor Tr 3  is connected to a junction between a diode D 2  and a resistor R 7  which are connected in series across the positive and negative buses. The parts Tr 3 , R 6 , R 7  and D 2  form a constant current circuit. A variable resistor VR 1  in the actuating circuit is connected between the collector of transistor Tr 3  and the negative bus. The output of the variable resistor VR 1  appears at a point e between the collector of transistor Tr 3  and the variable resistor VR 1 . This point is connected to the output a of the Wheatstone bridge circuit, and the base electrode of the transistor Tr 2  through a unidirectional conductor or diode D 3  poled toward the latter. 
     The operation of the apparatus of FIG. 2 is as follows. When the power switch S 1  is closed, a current of predetermined level flows through the variable resistor VR 1  from the constant current circuit. This renders transistor Tr 2  conductive if the variable resistor VR 1  is adjusted to produce an output voltage higher than the threshold level of the transistor Tr 2 . When the transistor Tr 2  conducts, the transistor Tr 1  operates to supply power to the Wheatstone bridge circuit and simultaneously acutuate the D.C. motor DCM. 
     An advantage of the actuating circuit of the invention derives from the use of the constant current circuit, Tr 3 , R 6 , R 7 , D 2 . Because of this circuit even when the electrical power source E operates along a slow voltage ramp, the output voltage of the variable resistor VR 1  is coincident with the predetermined level therefor. This assures initiation of the motor DCM at the time the main switch S 1  is closed. 
     Another advantage is that the the invention permits opening of switch S 1  at the time the motor DCM is restrained, though this renders the transistors Tr 1 , Tr 2  and Tr 3  non-conducting as in the prior art apparatus of FIG. 1. This is possible because when switch S 1  is thrown again, transistor T 2  is forcibly turned on by the voltage generated across the variable resistor VR 1  so that the motor DCM is actuated without fail. 
     A further advantage is that after the actuation of the motor DCM has been initiated, the potential at the point a becomes higher than that at the point. Hence no current flows from the actuating circuit to the Wheatstone bridge circuit. This cuts the actuating circuit off from its connection with the latter. During the operation of the motor DCM, therefore, the actuating circuit produces no adverse effect on the speed control of the motor DCM. This is true even when the voltage of the supply source E is permitted to increase to a considerable level after the actuation of the motor DCM, as the voltage across the variable resistor VR 1  is maintained constant by the constant current circuit Tr 3 , R 6 , R 7 , D 2 . Also, the one-way conductive means or diode D 3  functions to prevent occurrence of a forward current flow from the actuating circuit to the speed control circuit after the production of the voltage at point a higher than that at point e. 
     The use of the variable resistor VR 1  in the actuating circuit provides an advantage. Assume that the potential at the point e is made slightly higher than the threshold level of the transistor Tr 2  and that the desired speed of the motor DCM is made very high then the difference between the potential at point a corresponding to the selected speed of the motor DCM and the potential at point e becomes quite large. Even in this case, it is of course possible to effect actuation of motor DCM, though a considerable time is required for the actual speed of motor DCM to reach the desired speed. This is so because the transistor Tr 2  is driven only by the current flowing through the resistor R 2 . The actuating current flowing through diode D 3  is stopped as soon as the transistor Tr 2  is rendered conductive. In order to reduce this time lag, however, according to the invention, the operator need only to adjust the variable resistor VR 1  so that the potential at point e becomes almost equal to that at point a which corresponds to the desired speed of motor DCM. If so, the actuating circuit continues to supply the voltage to the transistor Tr 2  until a time just before the motor DCM reaches the desired speed. 
     In order to remove electrical noise and to prevent oscillation, there are provided two capacitors C 2  and C 3  respectively connected between the base and collector of transistor Tr 2  and between the base of transistor Tr 2  and the negative bus. 
     In FIG. 3, there is shown another embodiment of the actuating circuit according to the invention. Here the constant current circuit is constructed with a field effect transistor Tr 4  arranged with its drain electrode connected through a resistor R 8  to the positive bus. Its source electrode is connected through a dynamic characteristics-adjusting variable resistor VR 2  to a variable resistor VR 3  corresponding to that VR 1  of FIG. 2. Its substrate electrode is connected both to the junction between the variable resistors VR 2  and VR 3  and to the anode of diode D 3 . Upon proper adjustment of the variable resistor VR 2 , the transistor TR 4  is set to an optimum operating position as it serves as a constant current source. 
     In the above two embodiments of FIGS. 2 and 3, there is a disadvantage that the actuating current exiting from the diode D 3  is applied not only to the transistor Tr 2  but also to the resistor R 3  constituting one branch of the Wheatstone bridge circuit. That portion of the actuating current which flows through the resistor R 3  is entirely wasteful as no use is made in actuating the motor DCM. In order to eliminate this disadvantage, the present invention contemplates to use an additional diode D 4  for connection between the diode D 3  and the resistor R 3 . 
     FIGS. 4 and 5 show additional two embodiments which differ respectively from those of FIGS. 2 and 3 in using this diode D 4  as arranged with its anode connected to the output point, a, of the Wheatstone bridge circuit and with its cathode connected to the base electrode of transistor Tr 2 . The cathode of diode D 3  is connected to the junction between the diode D 4  and the base electrode of transistor Tr 2 . 
     It will be seen from the foregoing that the present invention provides a D.C. motor control apparatus which enables the D.C. motor to be actuated for rotation without delay from the time when an actuating switch is operated. Even when the voltage of the power supply source falls considerably below a satisfactory operating level for the motor, the actuation of the motor can be secured. Further the apparatus of the invention permits the motor to be accelerated along a high ramp regardless of how fast a speed is selected, as the variable resistor VR 1  or VR 3  is adjusted to the corresponding resistance value, and then to be operated with minimum energy consumption.