Abstract:
An elevator speed control device in which the cage of the elevator can be stopped at a desired floor with a high accuracy without increasing the capacity of power supply equipment to compensate for periods of low AC source voltage. A detecting circuit produces an output signal in response to the generation of a deceleration instruction signal. This signal is converted into a short duration pulse signal which is used to set the output of an integrator included in the control circuit loop to a predetermined value.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improved speed control device for an elevator control system employing an integrator. 
     A speed control device in an elevator of the general type to which the invention pertains is shown in FIGS. 1 and 2. In FIG. 1, reference numeral 1 designates a three-phase AC source, 2 a transformer, 3 a speed instruction generating device for generating a speed instruction signal 3a, 4 an adder which produces a difference signal 4a, 5 an integrator for improving the characteristics of the control system with the integrator 5 providing an output signal 5a, 6 a thyristor control device, 7 a thyristor converter, 8 a current detector providing an output signal 8a which is a current signal, 9 the armature of a DC motor, 10 the field of the DC motor, 11 the sheave of a winding machine, 12 a main cable wound on the sheave 11 with the main cable being connected to a cage 13 and a balance weight 14, and 15 a tachometer generator which is driven by the armature 9 in response to which it generates a speed signal 15a. 
     The difference signal 4a representing the difference between the speed instruction signal 3a and the speed signal 15a is applied to the integrator 5 as a result of which the integrator 5 produces the output signal 5a. The thyristor control device 6 receives the output signal 5a and the current signal 8a and from them controls the thyristor converter 7. The thyristor converter 7 converts the voltage of the three-phase AC source into a DC voltage which is applied to the armature 9. In this manner, a well-known static Reonard system is formed with which the speed of the armature, and correspondingly the lifting speed of the cage 13, is automatically controlled by the speed instruction signal 3a with a high accuracy. 
     If the voltage of the AC source 1 in the system described above is decreased for some reasons, the thyristor converter 7 cannot provide the necessary output voltage and accordingly the armature 9 will not be driven at the correct speed. This will become more apparent from the waveform diagrams of FIG. 2. As indicated in FIG. 2a, at the time instant t 1 , because of a decrease in the AC source voltage, the thyristor converter 7 is saturated and the correct speed, represented by the speed signal 3a, cannot be provided. As a result, the output 5a of the integrator corresponding to the difference signal 4a increases after the time instant t 1  as shown in FIG. 2b. Even if the speed instruction signal 3a is decreased becoming a speed reduction instruction signal at the time instant t 2 , the output 5a of the integrator 5 will not decrease because of its inherent integrating characteristics until the speed instruction signal 3a becomes smaller than the speed signal 15a at the time instant t 3 . Furthermore, even when the integrator output 5a begins dropping, it still takes a certain period of time until it reaches the correct value so that adequate speed reduction cannot be effected satisfactorily within a sufficient time. It may then result that the cage 13 cannot be stopped at a desired floor. FIG. 2c shows the armature current or the current signal 8a. 
     The above-described difficulties may be overcome by a technique in which the output voltage of the transformer 2 is maintained at a high value by increasing the transformer turns ratio so that the saturation value of the thyristor converter is increased. However, this technique is disadvantageous in that the transformer 2 then becomes uneconomically large in both electrical capacity and physical size making it inconvenient to install. Moreover, increasing the voltage of the transformer 2 also undesirably reduces the power factor of the elevator control device. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to provide an elevator speed control device in which the above-described difficulties have been eliminated and with which the cage can be stopped at a desired floor with a high accuracy without increasing the capacity of the power supply equipment such as for instance a power transformer. 
     In accordance with this and other object of the invention there is provided a speed control device for an elevator in which the speed of a driving electric motor is controlled according to a deceleration instruction signal applied through an integrator including a detecting circuit for detecting the generation of the deceleration instruction signal and an output setting circuit for setting an output of the integrator to a predetermined value in response to an output of the detecting circuit. In a preferred embodiment, a hysteresis or dead zone circuit is included in the input of the detecting circuit so that the detecting circuit operates only in response to signals having a value above a predetermined level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional elevator speed control device; 
     FIGS. 2a-c are explanatory diagrams for a description of the operation of the device shown in FIG. 1; 
     FIG. 3 is a circuit diagram of an elevator speed control device according to the invention; and 
     FIGS. 4a-g are explanatory diagrams for a description of the operation of the circuit shown in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the invention will be described with reference to FIGS. 1, 3 and 4. 
     In FIG. 3, reference numeral 500 designates an integration circuit which is employed in place of the integrator 5 in FIG. 1, 501 an operational amplifier, 502 a resistor, 503 a capacitor with the operational amplifier 501, the resistor 502 and the capacitor 503 forming an integrator 5 similar to that in FIG. 1, 504 throgh 507 resistors, 508 and 509 diodes. 510 and 511 normally closed switches which are opened upon application of an input control signal thereto, 512 and 513 DC sources, 514 and 515 reference potential terminals, 516 an acceleration and deceleration instruction detecting circuit of known design for detecting whether an acceleration and deceleration instruction has been issued, 516a and 516b the output signals from the circuit 516, and 517 a timer or pulse generator which produces a single pulse 517a, 517b having a predetermined short time width in reponse to each rise in the input signal 516a, 516b thereto. The components other than the integrator 5 in the integration circuit 500 form an output setting circuit. The remaining components are similar to those in FIG. 1. 
     The case where the cage 13 is being wound upwardly and the speed of the cage 13 has not reached the correct value because of a reduction of the voltage of the AC source 1 will now be described. An acceleration instruction is issued during the period from t 0  to t 1 . As a result, the output 516a of the acceleration and deceleration instruction detecting circuit 516 has a positive value as shown in FIG. 4d. The timer 517 generates a positive pulse 517a for the predetermined short operation time at the time instant t 0  as shown in FIG. 4e. As a result, the switch 510 is opened for the same predetermined short period of time as shown in FIG. 4f. In this case, the diode 508 is not rendered conductive because the integrated circuit output 5a is positive. As the switch 511 is closed, the diode 509 also cannot be rendered conductive because it is reversely biased. Accordingly, the operation of the integration circuit is not affected at all. 
     When the circuit 516 produces the positive output 516a, only the monostable multivibrator 517c in the timer circuit 517 is triggered to thereby produce the positive pulse signal 517a. In response to the positive pulse signal 517a, the relay coil 517e is deenergized to thereby open the closed switch 510. When switch 510 is opened, the cathode voltage level of the diode 508 is equal to a voltage obtained by subjecting the positive voltage of the power source 512 and the integrator output 5a to voltage division by the resistors 504 and 505. In this case, when the cage moves upwardly, the output 5a is positive during acceleration, and the diode 508 is thus reversely biased. Accordingly, the integrating capacitor is not discharged through the diode 508. Thus, no operation is effected by the timer circuit 517. 
     For the period of time from t&#39; 1  to t 2 , a constant running speed instruction is issued. In this case, the integration circuit output 5a gradually increases as shown in FIG. 4b. The output 516a of the acceleration and deceleration instruction detecting circuit 516 becomes zero at the time instant t&#39; 1  as shown in FIG. 4d. Therefore, upon the issuance of a deceleration instruction at the time instant t 2 , the acceleration and deceleration instruction generating circuit output 516b assumes a negative value as shown in FIG. 4d in response to which the timer 517 produces a negative pulse 517b for the predetermined short time. As a result, the switch 511 is opened for a predetermined period of time as shown in FIG. 4g. In this case, the integrator output 5a will be positive. Therefore, the diode 509 becomes forwardly biased and is rendered conductive whereupon the capacitor 503 is instantaneously discharged and as a result the integrator output 5a is set to a value V 1  defined by the resistors 506 and 507 as shown in FIG. 4b. 
     More specifically, upon the issuance of a decleration instruction at the time instant t2, the circuit 516 produces the negative output 516b in response to which only the multivibrator 517d in the timer circuit 517 is triggered to thereby produce the negative pulse output 517b. In this case, the relay coil 517f is deenergized during the negative output pulse 517 so as to open the switch 511. When the switch 511 is opened, the anode voltage of the diode 509 is a voltage obtained by subjecting the negative source voltage 513 and the integrator output 5a (being positive when the cage moves upwardly) to voltage devision by the resistors 506 and 507. When the output 5a exceeds a predetermined value, the diode 509 is forwardly biased. As a result, the integrating capacitor 503 is discharged through the resistor 506 and the diode 509 until the diode is reversely biased. In response to the discharge, the output 5a is also reduced to the predetermined value V1. After a certain period of time, in the case where the negative pulse 517 is eliminated so as to close the switch 511 again, the resistor is coupled through the switch 511 to the negative source again. Consequently, the diode is reversely biased, and thus the integrator 5 continues to operate in a normal condition. 
     At the time instant t 4 , deceleration is started with the time corresponding to the delay time from t 2  to t 4  as shown in FIG. 4a considerably reduced with respect to that in the prior art system as indicated by the time from t 2  to t 4  in FIG. 2a. Thus, even if the voltage of the AC source 1 is decreased, deceleration will nonetheless be effected without delay and, accordingly, the cage 13 can be stopped accurately at a desired floor. Furthermore, since the voltage of the transformer 2 is maintained unchanged, it is unnecessary to increase the capacitance and size of the transformer 2 and the difficulty of power factor reduction is eliminated with the use of the invention. 
     As the circuit operates similarly to the abovedescribed case when the cage moves downwardly, a description of that operation is unnecessary and will be omitted. 
     In a variation of the embodiment described above, the acceleration and deceleration instruction detecting circuit 516 is provided with slight hysteresis or small dead zone so that it will not be operated by inputs smaller than a predetermined value. In other words, since the speed instruction signal 3a is not completely constant for the time period from t&#39; 1  to t 2 , the acceleration and deceleration instruction detecting circuit 516 may operate erroneously. However, by providing a hysteresis or dead zone by disposing a circuit of well-known construction for performing this function in the input of the circuit such erroneous operation can be prevented. 
     As is apparent from the above description, according to the invention, the output of the integrator in the control circuit is set to a predetermined value when the deceleration instruction signal is issued. Therefore, even if the supply voltage is decreased, the cage can be stopped at a desired floor with high accuracy and the difficulties accompanying a conventional system in which the capacity of the power equipment must be increased beyond that required for normal operation is eliminated. 
     Furthermore, according to the invention, the deceleration instruction detecting circuit is so designed that it will not be operated by an input of less than predetermined value. Therefore, the output setting circuit will never be operated erroneously even if the deceleration instruction signal is slightly variable.