Note: Descriptions are shown in the official language in which they were submitted.
k -,~ ~1 ;, ~"~
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Oscillation-damDina control of motor torque for mine hoist
TECHNICAL FIELD
Ore which is recovered in mines is transported up to the
surface with the aid of so-called mine hoists. The ore
mining often occurs at very large depths which may amount
to several thousand metres. Because of the long ropes which
are then needed between the rope drum and the skip (or
conveyance) of the mine hoist, problems with oscillations
of the skip in the vertical direction often arise. This is
due to the ropes behaving as springs. The present invention
suggests a method and a device for preventing the
occurrence of such oscillations.
BRIEF DESCRIPTION OE THE DRAWINGS
Figure 1 shows speed control of a mine hoist according to
the state of art.
Figures 2 and 3 show ramp functions for reducing the risk
of oscillations of the skip during start, stop, etc.
Figure 4 shows a physical model of a mine hoist.
Figure 5 shows a control system for a mine hoist according
to the invention
Figure 6 shows an embodiment of a control system for a mine
hoist according to the invention
BACKGROUND ART, THE PROBLEM
To place the invention in its proper context, a brief
description of mine hoists and the problems which may arise
both during normal hoist movement and during starting and
normal retardation, as well as during emergency braking of
the load, will first be given.
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There are two types of mine hoists which are usually
referred to as drum hoists and friction hoists.
Drum hoists comprise (a) mine hoists with a rope drum where
the rope is wound onto the drum when the skip goes up, and
(b) mine hoists with double rope drums with one skip each
and where the ropes are also wound onto the rope drum and
are so arranged that, when one of the skips is furthest
down in the shaft, the other skip is furthest up in the
shaft.
In friction hoists, one or more ropes in the form of steel
wires are suspended freely in separate grooves over the
rope drum. From the rope ends on one side, the skip in
which the ore is loaded is suspended. From the rope ends on
the other side, another skip or counterweight is suspended.
This means that the only thing which prevents the ropes
from slipping or sliding over the drum is the friction
between ropes and drum grooves. To keep the total suspended
rope mass on both sides of the rope drum constant, balance
ropes are arranged between the under sides of the skip and
the counterweight.
To be able to secure the mine hoist during a standstill,
that is, to so-called holding braking, various electri-
cally/hydraulically/pneumatically controlled mechanical
brake systems are used, which are applied to the rope drum
or to the ropes themselves. These mechanical brakes are
also used for emergency braking of the mine hoist. In a
~() commonly used mechanical brake system, the side members of
the rope drum are each provided with an annular brake disc
and the braking is performed with the aid of hydraulic disc
brakes with brake blocks on both sides of the brake disc.
In another commonly used brake system, the braking is
performed with the aid of drum brakes.
A drum hoist with double rope drums is described, inter
alia, in the ABB Pamphlet 3ASMOlC200, 1993-06, ABB Mine
Hoist. As is clear from the pamphlet, the drive system may
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consist of an a.c. or a d.c. drive system. The drive
systems are designed to minimize dynamic strains in the
ropes by giving soft changes both as regards speed and
motor torque. Further, it is clear from this pamphlet that
rope tension meters may be applied to each rope. Rope
tension measurement may, of course, also be used in
friction hoists. This permits a possibility of continuously
monitoring the rope tension, among other things to check
slackening of some rope. In addition, the rope tension
measurement is used during starting to influence the motor
torque to counter the unbalance which prevails between the
rope tension on the skip side and on the counterweight
side. This means that, when the brakes release after the
skip has been filled with ore, the drive motor may be given
such a torque that the load does not drop but is, instead,
given a smooth acceleration with a reduced risk of
introducing oscillations in the ropes.
The problems with vertical oscillations of the skip may
arise both in a drum hoist and in a friction hoist. The
risk of oscillations arising increases with the length of
the ropes, that is, when the depth of the mine shaft
increases. It is primarily during loading/unloading of a
load, during starting/stopping of a driving cycle and
during emergency braking that such oscillations may arise.
Independently of which electric drive system is used for
driving the rope drums, the operation comprises an external
speed control with an internal current or torque control of
the motor. A typical scheme for such control with a speed
controller 1, which compares the desired speed of the rope
drum, nref~ with the actual speed of the rope drum nltr,
and which delivers a signal MM corresponding to the
reference of the torque control, a torque controller 2 and
a drive system 3 according to the prior art is shown in
Figure 1. Example of such controls are described, inter
alia, in an article entitled ~Control Systems for Mechani-
cal Brakes for Emergency Stops", published in connection
with MINE HOISTING 93, Second International Conference, 28-
21 681 71
30 June 1993, pp 2.3.1-2.3.6, The Royal School of Mines,
London. To avoid oscillations in the ropes in connection
with starting, stopping and emergency braking of the load,
"S"-shaped reference signals for the speed control both
during a starting and a stopping cycle are also used in
accordance with the prior art. A reference procedure for
the speed control for a stop is clear from Figure 2. Figure
3 shows a corresponding retardation reference for the
torque control for a stopping cycle. The retardation begins
with a linearly increasing ramp when then changes into a
constant retardation and which, at the end of the retarda-
tion process, changes in a ramp decreasing linearly towards
zero.
In spite of the above-mentioned measured with a smooth
start/stop etc., it has been difficult to prevent the
occurrence of oscillations in the ropes. It has therefore
been necessary, inter alia, to have such a smooth start and
retardation that it has had an injurious effect on the
duration of the running cycle. In addition, especially in
case of deep mine shafts and correspondingly long ropes,
disturbances of various kinds during transport of a load at
a constant speed may also introduce oscillations. In drum
hoists with two rope drums, oscillations in the skip on one
of the rope drums may initiate oscillations of the skip on
the other rope drum. Therefore, there is a considerable
need of a method which, independently of the stage in the
running cycle, as soon as a tendency of oscillations
arises, can initiate measures to prevent a development of
the oscillation tendencies.
As will have been clear from the above, these oscillations
may arise because the ropes behave as springs. Equations
for the behaviour of suspended springs belong to classical
mechanics. Because of the length of the ropes, however,
when carefully studying these oscillations, distributed
rope masses should be taken into consideration. As a
technical base for describing the invention, a differential
equation system for an aggregated model will be briefly
21 681 71
s
described, wherein the rope is allowed to be approximated
by point masses separated by massless springs and with the
rope weight included in the skip and the counterweight.
Such a model is shown in Figure 4 which relates to a
friction hoist. The equation system is based on the
following parameters:
Fk1,Fk2 - spring forces on skip and counterweight
11,12 - actual rope length
v1,v2 - speed of skip and counterweight
u - applied torque, input signal to the system
m1,m2 - mass of skip including load and rope mass on skip
side and mass of counterweight including rope mass on
counterweight side
f1,f2 - viscous damping in ropes
k1,k2 - spring constants of ropes
r - radius of rope drum
c - bearing friction
J - moment of inertia for motor rotor with shaft and rope
drum
~ - angular velocity of rope drum
The differential equations will then be as follows:
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v, =--(f~(r~ - v, ) + Fk, - mlg)
m,
V2 =--( f 2 (ro - V2 ) + Fk2 - m2g)
Fk, = k, (r~ - v, )
Fk2--k2 (r~)--v2 )
~ = J (u - CCI~ - fir(r~ - vl) - f2r(r~ - V2) - rFkl - rFk2)
It is part of the state of the art to transfer such a model
to a state model wherein
Xl = Vt
X2 = V2
X3 = Fk~
X4 = Fk2
x5 = ~,
are allowed to be the states in the model. This gives the
system on a state form according to
x(t) = A-x(t) + B-u(t)
y(t) = C-x(t) + D-u(t)
with system matrices defined on the basis of the described
differential equations, for example where
- f' O1 o fi r
ml ml ml
O - f.'O 1 f2 r
A = m, m, m,
-k, on () kl r
f, r f, r r r c+f, r~+ f, r~
J J J J J
21 681 71
To be able to handle and to apply measures which prevent
the occurrence of such oscillations, respectively, it would
be desirable to have continuous access to the speed of the
skip. However, this speed is not directly measurable or
definable. If, on the other hand, the speed of the skip
were known, it should be possible, by determining the
difference between the speed of the rope drum and the speed
of the skip, to apply stabilizing measures. However, the
differential equations for a mine hoist described above
give no direct indication as to how this problem should be
solved.
SUMMARY OF THE INVENTION, ADVANTAGES
As mentioned above, the rope tension may be determined with
the aid of tensile force-measuring devices in each rope.
With access to the rope tension, a mathematical state model
of a mine hoist, for example according to the following
equations (1), (2) och (3), may be utilized, according to
the invention, for estimating the speed of the skip
x(t) = A-x(t) + B-u(t) +N-el (1)
y(t) = C-x(t) + e2 (2)
z(t) = H-x(t) (3)
Here, x(t) represents a state vector for the mine hoist,
x(t) the derivative-of the state vector, y(t) the available
measurement signal from the rope tension, and z(t) the
desired speed of the skip. In this equation system, A, s,
N, C and H are matrices which may be determined starting
from the data of the mine hoist in question, for example
from the differential equation system described. Further,
el and e2 represent disturbances with an intensity Rl and
R2, respectively, and with a cross spectrum equal to R12.
2 1 68 1 7 1
Based on such a state model, the following Kalman filter
may be defined
x(t) = A-x~t) ~ B-u(t) + K(y(t) - C-x(t)) (4)
z(t) = H-x(t) (5)
where x(t) is the estimated value of x(t), x(t) is the
estimated value of x(t), u(t) is the applied torque, and
z(t) is the estimated speed of the skip. K is an
amplification according to:
K = (p cT + N R12) ~
Generally, according to known linear algebra, a matrix with
an exponent ~T~ is a transponant of the respective matrix.
P is the posive semidefinite solution of the generally
known matrix equation of a Kalman filter, that is
A-P + P-AT + N-Rl-NT-(P-CT + N-R12)- ~ (P-CT + N-R12)T =0
The state vector x(t) mentioned above may be adapted to the
mine hoist used in such a way that it may describe
different types of mine hoists. It may, for example,
describe a mine hoist with two rope drums, whereby the
estimated speeds z(t)1 and z(t)2, respectively, of the two
skips may be obtained.
With the scope of the invention, the Kalman filter may
contain a less aggregated state model than that described
by the equations (1), (2) och (3), for example a more
advanced state model with several point masses.
Figure 5 shows a diagram, according to the invention, of
the principle of the speed control of a mine hoist, compri-
sing a drum hoist with two rope drums. The control circuit
comprises, in conventional manner and in accordance with
Figure 1, a speed controller 1 which compares the desired
speed of the rope drum, nref~ with the actual speed of the
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rope drum, nltr, a torql~e controller 2 with feedback of the
actual motor torque M, and a drive system 3. The speed con-
troller may be designed in different ways and with diffe-
rent characteristics depending on the static and dynamic
requirements which are imposed on the speed control. Here,
as in Figure 1, it is assumed that the output signal of the
speed controller is equal to MM. It is further assumed that
either nref or the speed controller may comprise the "S"-
shaped limits and ramp functions which have been described
above to reduce the risk of oscillations and which are part
of the prior art.
Figure 5 also shows the two rope drums 4 and 5 with their
respective ropes 6 and 7 as well as the skips 8 and 9. The
rope tension Sl in the rope 6 is measured in a measuring
device 10 and the corresponding rope tension S2 in the rope
7 is measured with the measuring device 11.
What is new for the speed control according to the inven-
tion is that signals from the rope tension measurement and
the actual speed nltr of the rope drum are utilized as
input signals to an estimator 12 in the form of a Kalman
filter based on some state model of a mine hoist, for
example according to the above-mentioned model according to
equations tl), (2) and (3) and which, according to the
above reasoning, may continuously deliver signals which
correspond to the estimated values of the speeds of the two
skips, that is, Zl and Z2, respectively.
~() Further, as will be clear from Figure S, according to the
invention, a torque reference generator 13 is used which
delivers the desired torque reference Mref to the torque
control of the drive system. Input signals to the torque
generator consist of the output signal MM of the speed
controller according to the above, the estimated values of
the speeds Zl and Z2 of the skips, and the actual speed
nltr of the rope drum. The torque reference is now formed
according to:
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1"
M",f = MM + Pl (n~,r--Z, ) + P2 (nllr--Z2)
The torque reference generator 13 comprises, for the mine
hoist in question, the weighting factors P1 and P2 which
are determined depending on the actual operating situation,
that is, end position, centre position,
acceleration/retardation, etc.
By imparting to the drive system 4 a torque reference
according to the above, the torque control included in the
drive system, superimposed on the torque required via the
speed controller, will have a torque addition which acts in
such a way that, as soon as differences arise between the
speed of the rope drum and the estimated speed of the
skips, the tendencies to oscillations are counteracted and
reduced after a short building-up process. The torque addi-
tion will thus enter into operation indpendently of whether
the oscillation tendencies are introduced because of
loading/unloading of a load or during start/stop of a
2() running cycle.
The advantages of this system, which significantly reduce
the dynamic stresses which arise when the skip oscillates,
in addition to giving a reduced rope oscillation, are also
a reduced risk of rope slipping in a friction hoist.
Another and very important advantage with a reduced dynamic
load is that the safety factor for the rope may be lowered,
which permits the payload of the mine hoist to be
increased. This is particularly important at very large
~() hoist depths.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 5 shows the speed controller 1, the torque con-
troller 2, the estimator 12, as well as the torque
reference generator 13 in the form of blocks. However, it
is self-evident that state models and Kalman filters with
current technique are implemented in the form of programs
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Il
in a calculating means, preferably in a computer. The same
also applies to the controllers which are used in this type
of control equipment. The hardware which is used consists
of the power converter of the drive system, the motor, the
rope drum, the skips, etc.
An embodiment of the invention could, therefore, be descri-
bed based on Figure 6. Implemented in the calculating means
14 are thus programs for reference functions in the form of
10 U S n - and ramp functions, the n-controller 1, the Mref-
generator 13, the torque controller 2, and the estimator
12. Signals to the computer are obtained from the requested
maximum speed nref~ the actual speed of the rope drum nltr,
the torque feedback M, and the signals SI and S2 from the
rope tension measuring devices 10 and 11. The output signal
from the computer will then be the torque reference Mref to
the drive system 3. Otherwise, Figure 6 shows the rope
drums 4 and 5, the ropes 6 and 7, and the skips 8 and 9.
The description of the invention and Figure 5 relate to
friction hoists with two rope drums and two skips, respec-
tively. The principle of the control according to the
invention may, of course, be applied also to friction
hoists with one rope drum and one skip, and also to drum
hoists.
From a purely practical point of view, the estimator is
adapted such that, if the rope tension signals disappear
because of an interruption or otherwise, or if the estima-
tor quite obviously deliver incorrect signals, the estima-
tor will deliver signals which are estimated to correspond
to the speed of the skip/skips.
