Automatic open loop force gain control of magnetic actuators for elevator active suspension

Automatic gain control is provided for a control means for controlling a magnetic actuator for an elevator horizontal active suspension. The gain is varied depending on the drive current in the coil of the electromagnet of the magnetic actuator, the airgap of the magnetic actuator, or both.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The invention relates to elevator active suspensions and, more 
particularly, to control of magnetic actuators. 
2. Discussion of Related Art 
It is known from U.S. Pat. No. 5,439,075, for example, to control 
horizontal motions of an elevator car guided vertically along hoistway 
rails by means of an active suspension system. The guiding means can be 
provided in the form of roller clusters at the corners of the car for 
engaging the hoistway rails on opposite walls of the hoistway. Horizontal 
acceleration of the elevator car and horizontal displacement between the 
car and the rail is sensed for controlling the horizontal motions by means 
of actuators of the active suspension system. Each roller cluster may 
include one or more actuators with associated springs wherein the roller 
cluster actuators are responsive to a controller for actuating the 
elevator car horizontally with respect to the associated hoistway rail. 
A controller shown in FIG. 20 of the above mentioned U.S. patent includes a 
summer responsive to a force command signal and to a force feedback signal 
for providing a force error signal to a proportional-plus-integral gain 
compensator. The compensator in turn provides a current command signal to 
a current driver which provides current to a coil of an electromagnet 
actuator of the active suspension. This current in the coil is sensed by a 
sensor and provided along with a sensed magnetic flux signal to a signal 
processor for providing a signal indicative of the size of an airgap 
between the electromagnet and an iron reaction plate. Another signal 
processor, i.e., a flux-to-force converter, is responsive to the sensed 
magnetic flux signal for providing the force feedback signal (which is 
simply related to the square of the flux) to the summer. 
As can be seen at column 17, lines 63-66 and the proportional gain of the 
compensator 486 of FIG. 20 of the above-mentioned U.S. patent, is a 
constant. Unfortunately, the output force characteristic of an 
electromagnet actuator is a doubly non-linear function of current and gap. 
Consequently, the open loop gain of such a force loop varies tremendously 
over the operational ranges of current and gap and can cause instabilities 
at the extremes. The performance of the force loop is thereby limited to 
worst-case gain considerations. 
SUMMARY OF INVENTION 
An object of the present invention is to allow the achievement of a higher 
system gain and thereby better performance of a control loop for an 
electromagnet actuator for an elevator active suspension. Another object 
is to extend operational magnet airgap ranges while avoiding instabilities 
in system operation. 
According to the present invention, a control for controlling a magnetic 
actuator for an elevator active suspension, wherein the magnetic actuator 
is responsive to a drive current from a magnet driver in response to a 
magnet command signal from the control, wherein the control is responsive 
to a force command signal, a sensed magnetic flux signal indicative of 
magnetic flux in an airgap of the magnetic actuator and to a sensed drive 
current signal for providing the magnet command signal, comprises: a 
summer, responsive to a force feedback signal having a magnitude 
indicative of force exerted by the magnetic actuator and responsive to the 
force command signal, for providing a force error signal; a compensator, 
responsive to the error signal and to an automatic gain control signal, 
for providing the magnet command signal; an automatic gain control, 
responsive to the force feedback signal and to the sensed drive current 
signal, for providing the automatic gain control signal; and a 
flux-to-force converter, responsive to the sensed magnetic flux signal, 
for providing the force feedback signal. 
In further accord with the present invention, the compensator includes an 
adaptive proportional gain which is reduced as the sensed drive current 
signal increases in magnitude. 
In still further accord with the present invention, the automatic gain 
control means is also responsive to the force feedback signal or to the 
sensed magnetic flux signal for providing a gap signal having a magnitude 
indicative of the magnitude of the airgap, wherein the adaptive 
proportional gain is increased as the gap signal increases in magnitude. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the detailed description of a best 
mode embodiment thereof, as illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 2 shows an elevator car frame 10 suspended to horizontally in the 
side-to-side axis by a pair of opposed active roller guides 12, 14. Not 
shown are the left front-to-back and right front-to-back control axis, 
which have identical (from the standpoint of control) hardware. Each 
active roller guide includes a roller for engaging an associated hoistway 
rail and attached to a spring in series, for example, with a digital 
linear magnetic actuator (DLMA) and in parallel with a vibration 
supressing electromagnet. The invention is not limited to the particular 
active roller guide configuration shown in FIG. 2, since other 
configurations are known and it should be understood that the invention is 
applicable to them as well. The function of the active roller guide 
suspension is to both keep the car frame horizontally "centered" in the 
hoistway, and to suppress horizontal vibrations of the car. 
FIG. 1 is an illustration of the non-linear characteristics of the 
electromagnets used in an active roller guide (ARG) for an elevator 
horizontal suspension of the prior art. As shown, the output force 
characteristic of the electromagnet is a doubly non-linear function of 
current and gap. Consequently, the open loop gain of any force control 
loop for controlling the active roller guide is dependent on the operating 
conditions of the electromagnet, where the "slope" of the force/current 
characteristic changes with gap and current. 
Any such control means for the magnet force must provide an effective 
control voltage for the electromagnet coil. The electromagnet coil current 
resulting from the control voltage is a function of the electromagnet 
inductance and resistance. The curves in FIG. 1 were computed based on an 
850 turn, 2 in.sup.2 core cross section magnet, based on the following 
equation: 
EQU Fmag=K.sub.f i.sup.2 /g.sup.2 ; 
where 
i is the magnet current in Amps 
g is the magnet gap in meters. 
The constant "K.sub.f " is a gap conversion factor and is a fixed function 
of the magnet design. 
As can be seen from the curves of FIG. 1, at extreme operating gaps, the 
maximum force which can be generated at large magnet gap is about 250 N 
before the 10 A current limit is reached. At the opposite extreme, 
assuming that the magnet is idling at 1 A (a typical constant ARG value) 
and the gap is 2 mm, then the idling force will be in excess of 250N. This 
presents an awkward operational situation since the magnets oppose each 
other (they are unipolar force generators): this would be a "lockup" 
configuration which the control could not break out of. 
This lockup condition cannot be resolved by simply reducing the magnet 
idling current for two reasons. First, reducing the idling current in the 
magnet results in more delay when the magnet is activated, since the 
current has to be slewed up to several amps at nominal gaps before 
significant force is developed. Secondly, the control uses flux feedback 
in conjunction with current feedback to calculate the lateral position of 
the car for use in "centering" control. Thus, if a fixed low idling 
current were used, then at large gaps the flux feedback would be too small 
for reliable position calculation. 
Hence, the concept of idling current is abandoned, and the concept of 
idling force is introduced into the control. As shown in FIG. 3, this 
concept requires the use of two force loops 16, 18 for control, one for 
each magnet. Depending on the polarity of a "Net.sub.-- Force" dictation 
signal on a line 20, a "Net.sub.-- Force.sub.-- 1" signal on a line 22 and 
"Net.sub.-- Force.sub.-- 2" signal on a line 24, for each loop is set to 
either "MinimumForce Cmd" or abs("Net.sub.-- Force")+"MinimumForceCmd". 
Thus, the net force resulting from the output of both magnets 26, 28 taken 
together is just "Net.sub.-- Force", assuming that the closed loop gain of 
the dual force loops is essentially 1. 
One effect of this approach is that the actual idling current in the magnet 
is not controlled, since force is controlled and gap is not controlled. If 
the idling force is set too high, excessive idling currents will be 
generated at large gaps; if the idling force is set too low, then idling 
currents can be very low at small gaps, which increases the time it takes 
to slew the magnets up to high force. According to the embodiment of the 
present invention described above, it has been determined by 
experimentation that an idling force between 20 and 50 N is the best 
compromise between excessive idling current and slew rate problems, as 
evidenced by crossover distortion. 
Referring back to FIG. 2, not shown are the flux sensors 30, 32 of FIG. 3 
but these are mounted inside the magnet airgaps for magnets 26 and 28. The 
flux sensors 30, 32 are Hall Effect devices which are used to sense the 
flux intensity within the airgaps of the vibration magnets. The force 
exerted by the magnet on its reaction bar is proportional to the square of 
the flux density which is sensed by the flux sensors. Thus, the flux 
sensing of the software force control loop is conditioned and used as flux 
force feedback for the dual force control loops. As shown in FIG. 2, the 
car frame is suspended laterally with respect to the rails by means of 
spring suspension. The controller uses the DLMAs to bias the spring 
suspension to effect the above-mentioned "centering" of the car with 
respect to the rails. This control is provided so that the working stroke 
of the magnets is maximized. Another way of rationalizing the centering 
control requirement is to imagine that the car is perfectly stabilized in 
an inertial sense: centering control then permits maximum rail deviations 
even in the presence of imbalance loads on the car frame. Position 
information is derived by sensing the current in the magnets, the flux in 
the magnets and solving for the gaps in the magnet according to the 
equation above, where the Flux Force is equal to FMag: 
EQU F.sub.mag .about.B.sup.2 
The proportionality constant is a function of the magnet design: 
EQU F.sub.mag =(B.sup.2 /2.mu..sub.o)A; 
where B is the flux density in the gap of the magnet, 
.mu..sub.o is the permeability of free space (4.pi..times.10.sup.-7 H/m), 
and 
A is the total area of the pole faces of the magnet. 
For a fixed magnet design, the constant (A/2.mu..sub.o) we refer to as the 
"Flux.sub.-- Force.sub.-- Factor". The flux is sampled, converted to force 
(F.sub.mag), and plugged into the first equation 
EQU F.sub.mag =K.sub.f i.sup.2 /g.sup.2 ; 
to solve for the gap, g. 
Referring back to FIG. 3, according to the present invention, it 
illustrates a control block diagram of a dual automatic gain control (AGC) 
force loop. The "Net.sub.-- Force" dictation command signal on the line 20 
is algebraically split by a "Net Force Algebra" block 34 into a 
"Net.sub.-- Force.sub.-- 1" signal on the line 22 and a "Net.sub.-- 
Force.sub.-- 2" signal on the line 24, as described above. A "Flux.sub.-- 
Force.sub.-- 1" feedback signal on a line 36 and a "Flux.sub.-- 
Force.sub.-- 2" feedback signal on a line 38 are derived by means of 
flux-to-force conversion blocks 40, 42 from sensed flux 25 signals 44, 46 
from the flux sensors 30, 32, respectively. The signals on the lines 36, 
38 are applied as negative feedback at two summers 48, 50. Respective 
error output signals on lines 52, 54 of the summers 48, 50, "Force.sub.-- 
Error.sub.-- 1" and "Force.sub.-- Error.sub.-- 2", are applied as inputs 
to respective compensation filters 56, 58 which may include an integrator. 
A respective output (filtered force error) signal on lines 60, 62 of each 
compensator is multiplied in a respective block 64, 66 by a proportional 
gain factor which, according to the present invention, is variable as a 
function of current and gap conditions for the magnet in question (further 
detail provided below). Respective magnet command signals on lines 68, 70 
are outputs of the force loop regulator and are applied as PWM signals to 
respective magnet driver power electronics 72, 74. Resulting currents on 
lines 76, 78 in the magnet coils are sensed and fed back as sensed coil 
current signals on lines 80, 82 and in a respective "Current & Gap AGC" 
block 84, 86 used to provide AGC (proportional) gain adjustment signals on 
lines 88, 90 to the blocks 64, 66 based on the sensed coil current level 
signals 80, 82 and the flux feedback signals 36, 38, as shown, or based on 
the sensed flux signals 44, 46 directly. By means of the AGC gain 
adjustment signals, the blocks 84, 86 cause the proportional gain to be 
reduced as the respective sensed drive current signal increases in 
magnitude. These blocks also determine the magnitude of the airgap (e.g. 
by solving for "g" in the last equation) in the respective magnets in 
response to the sensed current and force signals and increase the 
respective proportional gain as the respective argap increases in 
magnitude. As mentioned before, the magnet currents create flux in the 
magnet airgaps which are detected by the flux sensors 30, 32 and also fed 
back to the software control for the flux-to-force computation 40, 42. It 
should be realized that the determination of the respective airgap 
magnitudes in blocks 84, 86 could be made (in conjunction with the sensed 
current signals 80, 82) based directly on sensed flux density on lines 44, 
46, rather than force feedback signals 36, 38, as shown. 
The calculation of AGC.sub.-- Gain does not actually linearize the open 
loop gain of the force loop, but does help to stabilize the loop over a 
wide range of current gap conditions. First, the proportional gain term 
used in each force loop is derated as a linear function of the operating 
current. As the current increases from its minimum, the gain is reduced. 
Secondly, the proportional gain term used is derated or boosted as a 
linear function of the magnet gap, as the magnet gap drops below or above 
8 mm, respectively. The 8 mm is simply a scheduling factor that was 
empirically determined for this example. The AGC gain leveling 
calculations are performed for each force loop by means of the following 
equations: 
EQU AGC.sub.-- Gain1=Gain(1 A)/I.sub.mag ; 
and 
EQU AGC.sub.-- Gain2=AGC.sub.-- Gain1(gap(mm))/8 mm. 
FIG. 6 shows the gain adustment factor for varying gap. FIG. 7 shows the 
gain adjustment for varying current. It should be realized that other ways 
to accomplish similar results can also be carried out, this being but one 
example. 
FIG. 4 provides a block diagram of the controller hardware for the dual 
force loop. The .mu.P samples the inputs and stores the input samples in 
RAM by executing instructions out of EPROM. Filter parameters are stored 
in EEPROM or EPROM for use in the lag compensation filters and the AGC 
logic. The resulting magnet PWM commands are sent to the magnet driver 
circuits. 
FIG. 5 illustrates a simplified software flow diagram for the dual force 
loop controller. The calculations are executed sequentially at the 
indicated rate. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof it will be understood by those skilled in the 
art that the foregoing and various other changes, omissions and deviations 
in the form and detail thereof may be made therein without departing from 
the spirit and scope of this invention.