This invention relates to a circuit for electronically controlling the rate of activation of an electromagnetic solenoid and, more particularly, the control of the release of an elevator brake.
While this invention is primarily intended for, and has been developed for use with an elevator brake magnet, its principles are not limited to elevators. As will be understood from the discussion below, this invention has general applicability wherever it is necessary to obtain smooth motion from a direct current electro-magnet.
An elevator brake is applied by springs which force the brake shoes against the brake drum or disc to prevent rotation of the hoist motor. The brake shoes, which are attached to a plunger influenced by an electro-magnet, are moved away from the drum or disc by the energization of the brake electro-magnet, when it is required that the hoist motor rotate. Although some slow speed elevator systems depend entirely on the brake for stopping the elevator at each normal stop, most systems use the brake only as a holding brake and occasionally as an emergency stop. The present state of the art is such that the hoist motor can be brought to a complete stand still, regardless of the load on the car, prior to the de-energization of the magnet that causes the application of the brake. Thus it is not necessary to be concerned with the smoothness of the application of the brake, because it has no effect on the smoothness of the final stop as experiencd by a passenger in the elevator car. The same cannot be said of the release of the brake when the car is about to start. Although smooth starting performance is usually obtained with ease when the weight of the car plus its load equals the weight of the counterweight, considerable difficulty can be experienced at other loads. The situation is somewhat equivalent to starting an automobile on a hill, i.e. release of the brake may allow the car to move even before the motor is started.
The most obvious method to overcome the difficulty of achieving smooth starting performance is to measure the load in the car while the doors are closing in preparation for a trip. Then, the motor torque can be caused to assume an appropriate value such that the hoist motor will not move when the brake lifts. Once the brake has been lifted, the start can proceed normally with performance identical to a start at balanced load. This method has disadvantages, however. Accurate measurement of the load is difficult. If the motor torque is brought up to the appropriate level while the doors are closing, there is a potential hazard because faulty operation of this system might apply sufficient torque to rotate the motor in spite of the brake. There must of course be some feedback signal related to the measurement of torque, which might consist of a measurement of the armature current of a D.C. motor, and failure of this feed-back signal could permit maximum torque to be applied. Further, if the motor torque is brought up to the appropriate level only after the doors are fully closed, the start may have to be delayed by a noticeable amount, thereby reducing the performance of the elevator.
Another method for improving the smoothness of starting is to use a very special design for the brake electro-magnet. Experience has shown that extreme smoothness in the motion of the brake shoes under the influence of the magnet is required to get a smooth start. Any sudden change in the braking force, when there is motor torque or a weight unbalance trying to rotate the motor, results in a rough start that is noticeable to a passenger in the car. Brake magnets have the characteristic that as the plunger on which the shoe is located moves toward its fully energized position, it inherently reduces the air gap. As the air gap decreases, less and less current is required to produce a given force. Thus a basic instability exists, i.e. regardless of how slowly the current rises, a point is reached where the reduced air gap causes increased force which further reduces the air gap, and the brake moves rapidly to the fully released position.
By very careful design of the brake magnet, generally by having "steps" of increasing diameter on the plunger, it is possible to overcome this instability. Such a brake has a smooth curve, not necessarily straight, relating brake current to brake plunger travel. The inherent inductance of the brake coil forces the current to rise relatively slowly, and thus the brake lifts smoothly.