1. Field of the Invention
The present invention generally relates to an apparatus and method for controlling elevator doors and, in particular, relates to one such apparatus including an incremental encoder for determining the position of the elevator door.
2. Description of the Prior Art
Conventional elevator door control systems are primarily mechanical in nature. Typically, elevator doors require different motor speeds during the opening and closing thereof. That is, an elevator door, because of its mass and in order to ensure passenger safety, requires a relatively slow motor rotation but high torque motor force at the beginning of its run. Once moving, however, the motor force requires less torque but higher speed to accelerate the door until the elevator door has travelled about three quarters of its full distance. Thereafter, the elevator door needs to be slowed prior to reaching the end of its run. This speed/torque trade-off for the motor according to the movement and position of the door is generally referred to as the door profile.
The door profile, in many modern elevators, is controlled or regulated by the use of mechanical switches, for example, located on cams or drive shafts. The switches, or relays, are adjusted to control the motor depending upon the path position of the door. Further, the transmission between the motor shaft and the door was typically accomplished by use of a sinusoidal drive linkage because of the difficulty in controlling the motor to start at a low speed, accelerate and decelerate according to the door position. As a result, not only are such systems subject to the wear and contamination of the relays but subject to wear and slippage of the linkages as well.
A further drawback of such mechanical systems is that fact that reversals are difficult to achieve. As well known in the elevator art, a reversal of an elevator door generally refers to a situation where the door encounters an obstruction in the path of the door while in motion, such as a passenger entering or leaving the elevator while the door is in motion. One reason for such difficulties in reversals in the conventional mechanical systems is that the drive transmissions are typically implemented via sinusoidal linkages and thus there is a continuously changing mechanical reduction factor.
A further drawback of conventional systems is that, at the time of installation of the elevator, all of the various mechanical components must be adjusted to ensure the proper opening and closing of the elevator door. This usually requires trained personnel and specialized tools.
Consequently, it is highly desirable to provide an apparatus and method for controlling an elevator door that not only overcomes the mechanical difficulties of conventional elevator door but is also more efficient and less expensive.