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BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
The purpose of this invention is to accomplish electronically what was
previously obtained by the special mechanical design of the brake plunger.
This purpose is achieved by controlling the current activating the brake
magnet, or any other electro-magnetic solenoid, so that the solenoid is
applied at a constant rate.
While the relationship between brake current and plunger travel has an
inherent discontinuity, the relationship between magnetic flux and plunger
travel does not have any such discontinuity. While this relationship is
not linear, it does not matter because smooth application of the brake can
still be accomplished by controlling the flux of the solenoid in a
feedback circuit. Although it is not convenient to directly measure the
magnetic flux in a magnet for this purpose, it is very easy to accurately
measure the rate of change of magnetic flux and to integrate this signal
to arrive at a flux signal. This is done by adding a second winding to the
brake coil. This extra winding can consist of a relatively few turns of
very fine wire that occupy very much less space than the main winding.
This extra winding will be referred to as the "sensing" winding.
The voltage induced into this sensing winding, and measureable at its
terminals, is precisely proportional to the rate of change of magnetic
flux in the main winding forming the electro-magnet. Thus, if the brake
coil is energized in such a way that the voltage in the sensing winding is
constant, smooth operation must occur. To accomplish this it is necessary
that a variable voltage be applied to the magnet coil.
It is very efficient to employ a static switching device, such as a power
transistor, to control the brake energization. Then, smooth operation can
be obtained by controlling the switching on and off of the transistor,
which is connected in series with the brake coil, in accordance with the
voltage induced in the sensing winding. The result is the same as that
achieved for a variable voltage supply.
The switching transistor commonly used to control the brake current of
elevator brake coils normally is turned fully on until the brake current
reaches the desired level, and then switches on and off with the dwell
ratio automatically adjusted to hold the current at the desired level in
spite of changes in temperature, coil resistance and line voltage. This
same transistor could be used to achieve the smooth brake release of this
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features of the present invention will be more
readily apparent from the following detailed description and drawings of
illustrative embodiments of the invention in which:
FIG. 1 shows a circuit which illustrates the basic principles for the
control of the magnetic flux in an electro-magnet; and
FIG. 2 shows a circuit in which the release of an elevator brake is
controlled by this principle.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
In FIG. 1 there is shown an electro-magnet with a primary winding BC1 and a
sensing winding BC2. The primary winding BC1 is energized by current
flowing through winding BC1 and power transistor 18 from a suitable D.C.
power supply level +V.sub.1 to a level OV. A diode D6 is connected across
winding BC1 to provide a path for the inductive current to decay into when
the transistor 18 turns off.
Transistor 18 can be switched on or off in accordance with the output of
operational amplifier 12 via an amplifier comprised of operational
amplifier 14, resistors R13, R14, and R15, diodes D3, D4 and D5, and
transistor 16. Power for this amplifier, and for the remainder of the
circuit of FIG. 1 is obtained from a DC power supply which provides a
positive voltage +V.sub.2 and a negative voltage -V.sub.3 at a suitable
level, such as plus and minus 5 volts, respectively, with respect to level
OV, which is preferably at ground potential.
Operational amplifier 12 is operated as a toggle device so that its output
has only two states; one state is positive with respect to level OV and
the other state is negative. When the output of operational amplifier 12
is positive, transistor 18 is turned off because the output of operational
amplifier 14 is also positive and current through resistor R13 flows into
the base of transistor 16 to turn it on. When transistor 16 is turned on,
transistor 18 is turned off because its base is held slightly negative due
to the forward drop in voltage across diodes D4 and D5 from current
flowing through resistor R15.
When the output of operational amplifier 12 is negative, transistor 18 is
turned on because the output of operational amplifier 14 is also negative
and this turns transistor 16 off. When transistor 16 is off, current flows
from +V.sub.2 through resistor R14 and through diodes D4 and D5 into the
base of transistor 18, which causes it to turn on.
An understanding of the remainder of FIG. 1 can best be obtained by first
assuming that potentiometer R1 has its slider exactly in the middle so
that it is at the same potential as line OV. Also, it is assumed that the
current through winding BC1 is at some intermediate level, neither zero
nor maximum. Under these conditions, the circuit oscillates at a frequency
determined by capacitor C1, resistor R3, voltage +V.sub.1 and the turns
ratio between the solenoid winding BC1 and the sensing coil BC2. For
satisfactory operation, this frequency must be high enough to achieve
substantially steady current in winding BC1, with the alternating
component relatively small. The inductance of the electro-magnet coil, of
course, has the effect of smoothing out the current in spite of the very
high alternating component in the voltage across the coil. Generally, for
typical magnets, a very wide range of frequencies is acceptable.
This oscillation can be explained by starting with the condition where
transistor 18 is turned on. Then current is increasing in winding BC1 and
a negative voltge is induced in sensing winding BC2. This voltage is
applied to an integrator comprised of operational amplifier 10, resistors
R3 and R8, and capacitor C1. Diodes D1 and D2 are for protection only and
do not affect the normal operation of this integrator. The output of this
integrator swings in a positive direction at a rate proportional to its
input voltage, and when it reaches a predetermined positive voltage, as
determined by resistors R9 and R11, the output of operational amplifier 12
toggles abruptly to its positive state. This causes transistor 18 to turn
off, as described earlier.
When transistor 18 turns off, the current in winding BC1 decays through
diode D6, and the voltage induced in winding BC2 changes to positive. This
causes the output of the integrator to swing in a negative direction at a
rate proportional to its input voltage, which is the voltage induced in
sensing winding BC2. When the output of the integrator reaches a
predetermined negative voltage, again determined by resistors R9 and R11,
the output of operational amplifier 12 toggles abruptly to its negative
state. This turns transistor 18 on again, and thus one complete cycle of
the oscillation has occurred. Further similar cycles occur in repetition.
Since the voltage induced in sensing winding BC2 is proportional to the
rate of change of flux in the electro-magnet, the integration of this
voltage results in a measurement of the actual change in flux. Each time
transistor 18 turns on, the flux rises in the electro-magnet by a small,
but specific amount. Each time transistor 18 turns off, the flux drops by
an identical amount. Thus, on the average, the flux remains constant.
The preceeding description was based on the assumption that potentiometer
R1 had its slider at the same potential as OV. If it is assumed that the
slider is moved toward voltage +V.sub.2 so as to put a positive voltage on
resistor R4, the output of the integrator will be biased so that it tends
to swing faster in the negative direction and slower in the positive
direction. This causes the increase in flux, when transistor 18 is turned
on, to be greater than the decrease in flux when transistor 18 is turned
off. This is perhaps easier to understand by stating that the off time of
transistor 18 will be shortened and the on time lengthened by the bias
applied to the integrator. The result of this biasing of the integrator is
that the flux is increased at each cycle of the oscillation by an amount
proportional to the positive voltage on the slider of potentiometer R1. By
similar reasoning, a negative voltage on the slider of potentiometer R1
causes the flux to be decreased at each cycle by an amount proportional to
this negative voltage.
There is, of course, a limit as to how quickly the flux can be reduced. If
too high a rate is demanded, transistor 18 will turn off completely, and
the rate of decay will be determined by the L/R ratio of the electromagnet
and winding BC1. There is, however, no limit as to how slowly the flux can
be reduced. Similarly, there is a limit as to how quickly the flux can be
increased because if too high a rate is demanded, transistor 18 turns on
continuously; but, no corresponding limit on how slowly the flux can be
increased, exists.
FIG. 2 contains all of the components of FIG. 1, but additions have been
made to make it suitable for controlling the energization of an elevator
brake coil in order to achieve a smooth release of the brake.
Brake contactors B1 and B2 are required to deenergize the brake coil for
emergency stops. Contacts B1-1, B1-2, B2-1 and B2-2 are shown in series
with the brake winding BC1. These contacts would be all closed to run, and
open for an emergency stop and perhaps also at every normal stop, if
desired.
Optical coupler 22, analogue switch 26 and resistor R12 have been added to
the circuit of FIG. 2 to provide a means of turning transistor 18 off to
de-energize the brake coil. Optical coupler 20, analogue switch 24, and
resistors R5, R6 and R7 have been added to bypass the flux control feature
for the initial part of the energization in order to get a faster brake
release. Further, potentiometer R1 has been connected to level OV through
resistor R2 to give a suitable range of adjustment for positive voltages
only, since this circuit only controls the flux when it is increasing.
When the elevator brake is not being energized, the two optical couplers
have no current flowing through their light-emitting diodes, and both
photo-transistors are turned off. Resistor R12 thus holds the control
input of analog switch 26 at ground potential, and the internal solid
state connection is as depicted by the mechanically equivalent blade of a
toggle switch. This puts positive voltage from line +V.sub.2 into the
input of operational amplifier 14 and this turns transistor 18 off.
Similarly, resistor R7 holds the control input of analogue switch 24 at
ground potential, and this connects resistors R5 and R6 into the
integrator circuit to force the integrator output to a negative value
sufficient to make the output of operational amplifier 12 toggle to the
negative condition.
When the brake is required to lift, the contactors B1 and B2 are energized,
and current is applied through terminals T3 and T4 of optical coupler 22
by circuits not shown in FIG. 2. The illumination of the light emitting
diode in circuit 22 causes the photo-transistor to turn on. This applies a
positive voltage to the control input of switch 26 which causes its
internal solid state connection to switch to the alternate input, which is
connected to the output of operational amplifier 12. Since the output of
amplifier 12 is presently negative, transistor 18 turns on, and brake
current starts to flow. The control of flux, however, is not yet in
effect.
After the brake current has risen to a predetermined value, as determined
by circuits not shown in FIG. 2, current is supplied to terminals T1 and
T2 of optical coupler 20. This causes analogue switch 24 to connect the
input of the integrator into the feedback circuit for sensing winding BC2
so that the further increase of flux proceeds at a rate determined by the
setting of potentiometer R1.
For general industrial use, the delay of the start of flux control may not
be required. For elevator brake control, however, this delay is desirable
so that the brake current rises initially as fast as possible to minimize
delays in starting. The flux control delay, however, cannot be too great
because the current sensing means must cause the switch to flux control to
occur before the previously described magnet instability occurs. Without
this flux control delay feature, there may be too much time delay between
the initial energization of the brake and the start of brake plunger
movement.
Although the example of FIG. 2 is particularly useful for elevator brake
release, the invention is in no way limited to elevator brakes, and is
equally applicable to the control of the rate of activation or release of
any direct current electro-magnet.
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