This invention relates to an improved elevator speed control apparatus for regulating the running speed of an elevator cage to accommodate load changes when passengers exit before the cage comes to a complete halt at an accessed floor.
A conventional elevator speed control system is shown in FIG. 1, wherein an electric power converter 2 which comprises a plurality of thyristors connected in a 3-phase bridge configuration, is coupled to a 3-phase AC power source 1 and generates DC power that is supplied to an armature 3 of a DC elevator drive motor through a line 2a. The field winding for the motor is not shown in the drawing.
A tachometer generator 4 driven by the armature 3 produces a speed signal on line 4a proportional to the rotation speed of the armature. A traction sheave 5 also driven by the armature 3 drives an elevator cage 7 and a counterweight 8 through a main cable 6 as is well known. A speed arithmetic circuit 10 receives the speed signal on line 4a from the tachometer generator 4 and a speed instruction signal on line 9a from a speed instruction signal generator 9 as inputs, and generates a current instruction signal on output line 10a. The speed arithmetic circuit 10 along with the speed instruction signal generator 9 and the tachometer 4 constitute a speed control system.
A current arithmetic circuit 12 receives as inputs the current instruction signal on line 10a from the speed arithmetic circuit 10 and a current signal on line 11a from a current detector 11 proportional to the current supplied to the converter 2. A phase shifter 13 receives the output signal on line 12a from the current arithmetic circuit 12 as an input, and outputs a firing control signal on line 13a for the converter 2. The current arithmetic circuit 12 along with the current detector 11 constitute a current control system.
By controlling the firing angle or phase of the thyristor converter 2 by means of both the speed control and current control systems, the voltage applied to the armature 3 is correspondingly controlled and thus the running speed of the elevator cage 7 is controlled through the traction sheave 5. In other words, the elevator cage 7 is speed controlled in accordance with the difference between the speed instruction signal on line 9a and the actual speed signal on line 4a with a high degree of accuracy.
In the aforementioned speed control system, in order to compensate for the non-linearity of the converter 2, the response time of the minor loop constituted by the current control system is set at an extremely short value, generally in the range of 0.01 to 0.03 second. On the other hand, the response time of the main loop constituted by the overall speed control system must be set at a higher value in order to avoid resonances in the suspension and traction cables. Consequently, the speed control system is generally designed so as to have a response time in the range of 0.2 to 0.33 second.
With such a conventional elevator system, in order to improve the transport efficiency and speed up the overall operation both the internal cage door and the external door on the accessed floor are sometimes controlled to be simultaneously opened just before the cage reaches the floor. A brake system (not shown) is also provided to engage the traction sheave 5, but such engagement does not occur until the cage comes to a complete stop. Passengers may thus step out of the cage before the brake system acts upon the traction sheave, and as a result an abrupt variation in torque is exerted on the sheave due to the change in the cage load or weight, as shown in FIG. 2(a).
Upon the occurrence of a torque variation, the current flowing through the armature 3 correspondingly varies in response to the output of the speed arithmetic circuit 10 due to the functioning of the current control system, as described above. In this case, however, based on the relatively slow response time characteristics of the speed control system the current flowing through the armature 3 varies or adjusts relatively slowly as shown in FIG. 2 (b). The running speed of the cage 7 therefore varies as shown in FIG. 2 (c), as a result of which the cage may overshoot or undershoot the exact position of the accessed floor, which constitutes a potentially dangerous situation. Even in the best case where the cage ultimately stops at the exact position of the floor sill, the passengers will experience a discomforting "acceleration-deceleration bump".
It will be understood that the curves of FIG. 2 have been simplified by removing or subtracting therefrom the normal transient values, to leave just the "abnormal"variants caused by a premature passenger exit (or entry).