Note: Descriptions are shown in the official language in which they were submitted.
21 68l 67
Emeraency brakina of mine hoist
TECHNICAL FIELD
Ore, coal, etc. which are recovered in mines are
transported up to the surface with the aid of so-called
mine hoists. The mine hoists are also used for transport of
personnel to the various mine adits. The mining often
occurs at very large depths which may amount to several
thousand metres. Various faults may arise which makes it
possible to have access to an emergency braking system to
be able to stop the mine hoist as quickly as possible. The
present invention suggests a method and a device for such
emergency braking
BACKGROUND ART, THE PROBLEM
To place the invention in its proper context, a brief
description of mine hoists and the mechanical braking
systems which are used according to the state of the art as
holding brakes, that is, for securing the rope drum of the
mine hoist during a standstill, will first be given.
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 (or
conveyance) is going 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.
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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.
Independently of which electric drive system is used for
driving the rope drums, the operation consist of an exter-
nal speed control with an internal current and torque con-
trol of the motor. Examples of such controls are described,
inter alia, in an article entitled ~Control Systems for
Mechanical Brakes for Emergency Stops", published in
connection with MINE HOISTING 93, Second International
Conference, 28-30 June 1993, pp 2.3.1-2.3.6, The Royal
School of Mines, London. To obtain smooth starting and
stopping cycles, ~S~-shaped reference ~ignals are used for
the speed control and ramp functions for the torque con-
trol.
During a standstill, the mine hoist is secured with the aid
of various electrically/hydraulically/pneumatically con-
trolled mechanical braking systems, which are applied to
the rope drum.
A number of different, serious accidents have occurred in
connection with faults in the mechanical braking systems.
If a fault occurs in the mechanical braking system with an
empty skip at the very bottom of the shaft and a filled
skip at the very top of the shaft and with an empty skip at
the very bottom of the shaft and the counterweight at the
very top of the shaft, respectively, this may result in the
empty skip being driven upwards to excess speed and
crashing against an upper stop.
Another serious fault scenario is a hoist which, during
transport of personnel, gets out of control and crashes
against the bottom of the shaft.
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A drum hoist with double rope drums is described, inter
alia, in the AsB Pamphlet 3ASM01C200, 1993-06, Ass Mine
Hoist. This shows, among other things, a mechanical brake
where the side members of the rope drum are each provided
with a 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. It is further clear that a rope
tension measuring device may be applied to each rope.
In an article entitled ~Mine hoist braking system",
published in CIM sulletin~ October 1986, pp. 50-60, both
various mechanical drum braking systems and disc brakes are
described. An additional type of drum brake with V-shaped
brake shoes is described in US 4 977 982.
Characteristic of modern mechanical braking systems is that
they are mechanically prestressed with springs, preferably
of the Belleville type springs. During normal operation of
the mine hoist, the spring force of the prestressed springs
is counteracted by hydraulically or pneumatically con-
trolled pistons such that the brakes are lifted. sy varying
the pressure in the pistons, the braking power can thus be
influenced in accordance with the desired braking effort.
The risk of a fault arising in the normal speed control or
in the mechanical braking systems or their operation has
resulted in the development of a number of emergency
braking systems for mine hoists. These emergency braking
systems normally enter into operation when the speed of the
hoist exceeds predetermined maximum speeds in relation to
the position of the skip, or if the acceleration of the
hoist exceeds the maximally allowed values.
A few systems for emergency braking are described in an
article entitled ~Emergency braking systems for mine
elevators~, published in connection with a conference in
Phoenix, USA, February 24-27, 1992, by the Society of
Petroleum Engineers of AIME, pp. 325-336.
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In one of the systems which are described, a so-called
~passive dynamic braking' is used, which assumes that the
existing drive system of the mine hoist is used for the
braking. This system assumes that the driving is performed
with a dc motor. The passive dynamic braking is performed
by connecting a resistive resistor across the rotor termi-
nal of the motor. Such braking cannot stop the mine hoist
but may limit the speed in both directions.
A similar system, which, however, is not mentioned in the
above article, is to use regenerative braking by feeding
braking effort back to the network.
Further, the above article describes a newly developed rope
braking system which consists of friction linings pressed
against the ropes.
Most emergency braking systems, however, utilize as execu-
ting objects the mechanical brakes which are always inclu-
ded in a mine hoist and which enter into operation when themine hoist, with the aid of the electric drive system, has
stopped. In connection with starting the mine hoist again,
the brakes are lifted allowing the rope drum to rotate
freely. This thus means that there is an electric control
system which activates the brake for full braking effort
when the mine hoist stops, for example for filling the skip
with ore, and lifts the brake, respectively, allowing the
rope drum of the mine hoist to rotate freely.
As mentioned above, the emergency braking system is to
enter into operation if the speed of the hoist exceeds
predetermined maximum speeds or if the acceleration of the
mine hoist exceeds maximally allowed values. If maximum
braking effort should be applied directly in case of excess
speed, this would have very serious consequences, partly
because of the strong deceleration, partly because such a
procedure would initiate very strong vertical oscillations
in the skip. This, in turn, could also entail a rope rup-
ture or rope slipping in a friction hoist. It is therefore
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necessary to control the braking power during an emergency
stop in such a way that the deceleration of the mine hoist
does not exceed the values which are allowed from the point
of view of safety.
In an article in ASEA JOURNAL, 1978, Volume Sl, Number 6,
pp. 139-142, entitled ~Single-drum hoist with electroni-
cally controlled disc brakes~, it is described how an
emergency braking system for a mine hoist with disc brakes
can operate. To have redundancy, two parallel braking
systems are always used, each having a set of mechanical
brakes, in this case of disc brakes, a control system, a
hydraulic system with an oil pump, a pressure accumulator,
valves, etc. Each one of the two braking systems is
sufficient to be able to brake the mine hoist with a good
margin. The speed of the mine hoist is measured conven-
tionally by means of a tachometer. A measure of the dece-
leration is obtained by deriving this signal. The task of
the control system is to compare this signal with a
reference signal corresponding to the desired maximum
deceleration. The controller of the control system then
influences the valves of the hydraulic system in such a way
that the desired braking effort is obtained and maintained.
During emergency braking, the hydraulic pump motor is dis-
connected and the necessary hydraulic pressure is obtained
via a pressure accumulator. The valves which control the
oil flow from the pressure accumulator are adapted such
that the braking effort can only increase.
To reduce the risk of rope slipping, vertical oscillations
of the skip, etc., when starting an emergency braking
cycle, the braking effort is nowadays normally increased
linearly from zero up to a value corresponding to the
maximally allowed deceleration. This is achieved by
allowing the deceleration reference to pass through a ramp
function which, when a maximum deceleration reference is
obtained, changes into a constant deceleration reference.
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`
A publication from the Linkoping University, with reference
LiTH-ISY-EX-1422, ~Studium av lastpendlingar vid styrning
av gruvspel~ A study of load oscillations during control
of mine hoists"), analyzes the oscillation problems which
may arise in the skips of a mine hoist. The analysis is
based on studies of a mathematical state model of a mine
hoist, taking into consideration the fact that the rope
mass is distributed. Because of the distributed rope mass,
several resonance frequencies caused by wave propagation
will arise in the ropes. The analysis shows that the two
lowest frequencies are the dominant ones and those which
cause the large oscillations which arise especially during
emergency braking.
A very important reason for minimizing oscillations in the
ropes is that the oscillations and the stresses in the
ropes thus occurring may have a very significant influence
on the service life of the ropes.
The publication also suggests how the mechanical brakes
should be applied to prevent, as far as possible, the
occurrence of oscillations in the ropes. When applying the
brakes, the braking torque should increase according to a
ramp up to half the maximum braking torque which can be
applied at the actual load to obtain the desired decele-
ration in a time corresponding to the period T2 for the
highest of the frequencies, that is, f2. Then, the braking
torque is to be kept constant for a time corresponding to
the difference between half the period T1 for the lowest of
the frequencies, that is, f1 and the period T2. Then the
braking torque shall again increase to the current maximum
torque according to a ramp in a time which also corresponds
to the period T2 for the highest of the frequencies, that
is, f2-
The determination of the periods is carried out, forexample, with the aid of a mathematical expression for the
resonance frequencies in a simple model of a mine hoist
with the rope mass concentrated to the skips and the rope
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drum. sy allowing the model to take into consideration
current operating data, the load, the position of the skips
in the mine shaft, etc., the resonance frequencies which
will arise during a possible emergency stop at different
depths in the shaft, as well as the speed of the skips, can
be determined.
This means that the above-mentioned dominating resonance
frequencies may be determined, which in turn allows a
possibility of applying the proper ramp functions for the
emergency braking independently of where in the mine shaft
the skips are located when the mine hoist is to be emer-
gency-stopped.
As a loaded skip is being moved from the loading level to
the surface and the counterweight or the other skip is
being lowered from the surface and downwards in the mine
shaft, respectively, the frequencies f1 and f2 will be
changed. This means, for example, that when half the period
T1 is less than T2, the second ramp in the emergency
braking process according to the above will start before
the first one has finished.
For an emergency braking to function satisfactorily, there
must be an interaction between the control system of the
braking system and the speed and torque control of the mine `
hoist. This applies both to the application of the emergen-
cy braking process and at the end of the emergency braking
process, that is, when the mine hoist stops. When applying
the braking process, the drive motor has either a driving
or a braking torque which has to be disconnected. A problem
in connection with the emergency braking is also that rope
oscillations will inevitably be initiated when the mine
hoist stops and the deceleration instantaneously approaches
zero. The above-mentioned LiTH publication suggests no
methods for reducing these oscillations. The present inven-
tion relates to a method and a device which control the
interplay between the speed control and the torque control
to reduce the risk of rope oscillations during emergency
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braking including measures to reduce the risk of oscilla-
tions when the mine hoist stops and holding brakes are
engaged.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures la, lb, lc, ld, le show an emergency braking cycle
when the motor of the mine hoist develops a driving torque.
Figures 2a, 2b, 2c, 2d, 2e show an emergency braking cycle
when the motor of the mine hoist develops a braking torque.
Figure 3 is a control diagram showing the principle of
application of the mechanical brakes during emergency
braking.
Figure 4 shows an embodiment of an emergency braking system
according to the invention
SUMMARY OF THE INVENTION
The principle of an emergency braking cycle according to
the invention will be described with reference to Figures
la, lb, lc, ld, le and 2a, 2b, 2c, 2d, 2e and relates to a
drum hoist.
Figures la, lb, lc, ld describe an emergency braking cycle
at the start of a hoisting cycle when a load is hoisted
from the bottom of the mine shaft towards the surface, that
is, when the motor drive according to Figure le needs to
develop a driving, that is, a positive, torque.
As the loaded skip is being moved upwards, the need of a
driving motor torque will be reduced because the empty
descending skip including the rope weight reduces the
unbalance.
When the loaded skip approaches the surface, the empty skip
will approach the bottom of the shaft and then the sum
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weight o~ the skip and the weight of the ropes of this skip
will be greater than the loaded skip, and the motor drive
according to Figure 2e must then develop a braking, that
is, a negative, torque. An emergency braking during this
stage of the hoisting cycle will be described in Figures
2a, 2b, 2c, 2d.
If an loaded skip is to be moved upwards in the shaft and
an empty skip at the same time is to be moved downwards in
the shaft, there is thus an unbalance on the shaft of the
rope drum and the motor which tends to counteract the up-
ward movement. This means that the load exerts a torque on
the shaft which corresponds to a negative torque equal to
the load torque. To drive the load upwards, the motor
therefore has to develop a driving corresponding positive
torque ML, the magnitude of which is determined by the
actual load and the position in the shaft; see Figure la
for the time up to to that is, when the emergency stop
cycle is to start. The torque in the shaft will thus for
the period up to to be practically zero at constant speed.
The application of the braking torque is now performed
according to the described principle with two ramp func-
tions, whose ramp times are determined by the dominating
resonance frequencies for the ropes of the two skips, see
Figure lb for the time up to t3. The determination of the
respective periods Tl and T2 is performed in the same way
as previously described.
To achieve the expected braking cycle, an interaction
between the speed and torque control of the motor is
required. By reducing the motor torque to zero with the
same ramp functions as the braking torque is applied for
the period up to t3, that is, up to a maximum current
braking torque, the torque in the shaft will be the sum of
the loading torque and the braking torque, see Figure lc.
The application of the braking torque means that the torque
in the shaft becomes the sum of the motor torque and the
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braking torque, that is, after the maximum braking torque
has been attained and the motor is completely reduced, the
shaft torque will amount to (-Mg - ML).
During the time after the maximum braking torque has been
applied, that is, after the time t3 and until the speed
approaches zero, there will be constant deceleration. The
control system of the braking system continuously senses
the speed of the rope drum. When the speed approaches zero,
according to the invention the braking torque is to be
reduced down to zero according to Figure lb for a time
interval t4 to ts. This means that in the meantime the
torque in the shaft is changed to -ML, see Figure lc. When
the hoist has stopped and the braking torque is zero, full
application of the brakes is made, which then directly
compensates for the unbalance with a positive torque ML,
see Figures lb and lc.
The reduction of the speed nltr of the rope drum from
current speed to zero during an emergency stop cycle
according to the above is clear from Figure ld. During the
time to to t3, the deceleration increases from zero to the
constant deceleration which applies to the time t3 to t4.
During the time interval t4 to ts, the braking torque
decreases to zero and the deceleration decreases to the
level corresponding to the current unbalance.
An emergency braking cycle, when the motor has to develop a
negative torque, is clear from Figures 2a, 2b, 2c, 2d, as
mentioned above. The driving torque of the load is balanced
up to emergency stop by a corresponding braking torque from
the motor according to Figure 2a, and the torque in the
shaft is thus practically zero according to Figure 2c, in
the same way as for the load case mentioned above.
The application of the mechanical brake is also here per-
formed in the same way via two ramp functions, whose ramp
times are determined by the current resonance frequencies
and their respective periods. Via the speed and torque
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control, the braking torque of the motor is reduced to zero
according to the same ramp functions; see Figure 2a. The
braking torque of the mechanical brake must now be applied
to such a magnitude that it takes care of the loading
torque as well as the necessary torque for the allowed
deceleration. After the time t3, there is constant dece-
leration, as shown, with a shaft torque equal to the diffe-
rence between the braking torque and the loading torque;
see Figure 2c.
By sensing when the speed approaches zero in the same way
as described above, it is possible also for this operating
case, according to the invention, to decrease the braking
torque of the mechanical brake down to a value correspon-
ding to the torque of the load during a time interval t4 to
ts, whereby the shaft torque decreases to zero; see Figures
2b and 2c. When the hoist has stopped, full application of
the brakes is made, which then directly compensates for the
unbalance with a negative torque -ML. This allows an
oscillation-free stop cycle since both the shaft torque and
the deceleration may be reduced to zero.
The reduction of the speed nltr of the rope drum during
this emergency stop cycle is clear from Figure 2d.
In addition to the fact that the braking system consists of
the described ramp functions, the invention includes the
feature that the control system can damp the vertical
oscillations of the skips which may arise in connection
with the emergency stop. This is done by providing the
control system with the same oscillation-damping methods as
are described in the Swedish patent application filed con-
currently herewith. This means the utilization of an esti-
mator in the form of a Kalman filter based on a state model
of a mine hoist. In this way, estimated values of the speed
of the skips may be obtained, whereby torque-damping
measures may be taken.
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12
As will have been clear from the description of the prin-
ciple of the invention according to the above, a well
developed interplay between the control systems for the
mechanical brake and the drive system is required. This
interplay will be described in greater detail in the
description of the preferred embodiments of the invention.
In this connection, a relatively detailed description of
the control system for the mechanical braking system will
also be given, containing, inter alia, a description of how
the dominating resonance frequencies are continuously
determined.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 3 is a control diagram showing the principle of the
application of the mechanical brakes during an emergency
stop. The reason that the control diagram is described as
showing the principle is that, according to current
technique, such a control function is always implemented in
a calculating means, preferably in a computer. The division
into the units which are shown in the diagram according to
Figure 3 should therefore be interpreted more as a func-
tional description of how a control diagram for an emer-
gency stop functions.
A plurality of alternative embodiments of the emergency
stop function are included within the scope of the inven-
tion. The embodiment which is to be used may be dependent
on the current mine hoist, the load situation, the position
of the skip in the shaft, etc. The control diagram for the
principle shown in Figure 3, however, describes a preferred
embodiment. The functional units which are included in this
diagram may, however, intervene in various ways in the
embodiment under discussion.
As will have been clear from the description of the inven-
tion, during the emergency braking there is an interaction
between the electric drive system of the mine hoist and the
braking system. During normal operation, the drive system 1
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13
is given its control functions via a control system which
may, for example, be designed as described in the above-
mentioned Swedish patent application.
Further, it is assumed that such sensing members are
available which may supply a signal that an ~Emergency
stop" of the mine hoist is to be initiated. As is clear
from Figure 3, such a signal initiates the performance of a
plurality of parallel and simultaneous actions. One of
these actions is to disconnect the control system of the
normal operation and to prepare for reduction.
Figure 3 further shows that a functional unit 2, referred
to as ~srake application generator~, is included. In a
preferred embodiment, this unit functions as a ramp func-
tion generator and in that case a program for continuous
determination of the two dominating resonance frequencies
for the ropes is implemented in the unit. This unit is
continuously fed with operating information in the form of
the current load, the position of the skips, etc. sased on
these two frequencies and the corresponding periods as well
as the current loading torque, those ramp functions can be
formed which are needed as control signal Mg to the mecha-
nical braking system 3 corresponding to the braking torques
according to Figures lb and 2b. The same ramp function is
used also via a reduction unit 4 for reducing the drive
system 1.
To be able to distinguish between the drive system and the
mechanical braking system, Figure 3 shows the electrically
driven motor 5, the brake disc 6, and the mechanical brake
7. Otherwise, the figure also shows the rope drums 8 and 9,
the ropes 10 and 11, the skips 12 and 13, and the measuring
devices 14 and 15 for determining the load in the ropes.
Figure 3 shows that the braking system is an internal open
system which functions as executing object in a speed-
controlled emergency braking system for the rope drums. For
this speed control, an emergency stop reference generator
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14
16 is required which, when an emergency stop is to take
place, delivers a reference to the speed control. This
reference is dependent on several factors, such as the
current speed of the rope drums, the maximally allowed
deceleration, the ramp function from the brake application
generator, which per se contains information about the
current load and the position of the skips in the mine
shaft, etc.
The speed control comprises, in the usual way, a speed
controller 17 which, in turn, delivers a torque reference
Mn to a Torque reference generator 18. It is in the nature
of things that also during a controlled emergency stop,
vertical oscillations may arise in the skips. To reduce the
risk of such oscillations, an emergency stop also includes
the same oscillation-damping functions as those described
in the above-mentioned Swedish patent application. For this
purpose, an estimator 19 is used in the form of a Kalman
filter based on a state model of a mine hoist. The estima-
tor is fed with information about the load S1 and S2 in thetwo ropes 10 and 11 as well as information about the
current speed of the rope drums and may deliver to the
torque reference generator signals which correspond to
estimated values Z1 and Z2 of the speed of the skips. To
the torque reference generator there is also supplied
information about the current speed nltr of the rope drums.
By supplying the torque reference generator 18 for the
current mine hoist with the weighting factors P1 and P2,
which are determined depending on the current operating
situation, that is, end position, central position,
acceleration/deceleration, etc., a torque reference is
formed
Mrer MM + Pl (nlrr Zl ) + P2 (nlfr Z2 )
As will be clear from Figure 3, the control signal MB of
the brake application generator to the braking system
21 681 67
passes through a summator 20. In a preferred embodiment,
the braking torque contribution from the speed control,
that is, Mref according to the above, is intended, via a
contact 21 and the summator 20, immediately after the ramp
function has been discharged, to be switched in as an
additional signal to the control signal for the mechanical
brake to reduce the influence of possible vertical oscilla-
tions of the skips.
In an alternative embodiment, a simple time constant func-
tion is formed in the brake application generator as con-
trol signal to the mechanical braking system:
M = Mg(l - e~t/T)
; where the time constant T is determined on the basis of
current load data, the position of the skips in the mine
hoist, etc. The other part of the emergency braking system
will then be as is shown in Figure 3.
In additional embodiments, independently of which function
is formed in the brake application generator, the Mref
signal may be constantly connected to the braking system.
As mentioned in the summary of the invention, a reduction
of the braking effort of the mechanical brake will occur
when the speed approaches zero. This is performed by
changing the speed reference from the emergency stop
reference generator in such a way that the control signal
Mref of the mechanical brake from the speed control reduces
the mechanical brake.
From a purely practical point of view, the functional units
which are described above will be implemented as programs
in a superordinate calculating member, suitably in the form
of a computer. An embodiment of the invention could, there-
fore, be described starting from Figure 4. Implemented in
the calculating member 22 are thus programs for the n-
emergency stop reference generator 16, the n-emergency stop
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16
controller 17, the torque reference generator 18, the
estimator 19, the summator 20, the contact 21, the brake
application generator 2, and the reduction unit 4.
The input signals to the calculating member consist of the
emergency stop signal, the load signals from the rope ten-
sion measurement, the speed of the rope drums, and contin-
uous operating information and the control system for nor-
mal operation. The output signals from the calculating mem-
ber consist of control signals to the drive system 1 andthe mechanical braking system 3.
