CA2168167A1 - Emergency braking of mine hoist - Google Patents

Abstract

A method and a device during an emergency stop of a mine hoist for preventing the occurrence of vertical oscilla-tions which may arise in the skips (12, 13) of the mine hoist, and wherein the mine hoist is driven by means of an electric drive system (1) and wherein the mine hoist has a mechanical braking system (3) which, in case of emergency stop, is applied via a brake application generator (2) of a control signal which is adapted such that the risk of oscillation when applying the brake is minimized, and via a control signal which is generated by the speed control of the mine hoist, and wherein the control functions are implemented in the form of programs in a calculating member (22) .

Description

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.

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.

`- 21 681 67 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.

21 68l 67 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 `- 2168167 s 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.

`

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 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 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 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 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 `- 21 681 67 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.

`~ 21 681 67 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 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 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 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 ` 2168167 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.

Claims (6)

1. A method during an emergency stop of a mine hoist for preventing the occurrence of vertical oscillations which may arise in the skips (12, 13) of the mine hoist, and wherein the mine hoist is driven by means of an electric drive system (1) which is included in a speed control of the rope drums (8, 9) of the mine hoist, and wherein the skips are suspended from ropes (10, 11) between the rope drums and the skips, and wherein the ropes are provided with rope tension measuring devices (14, 15) for determi-ning the rope tension S1 and S2 in the ropes, and wherein the mine hoist also comprises a mechanical braking system (3), the method being characterized in that, when an emergency stop is called for, there is generated in a brake applica-tion generator (2), which is provided with current informa-tion about the load and the position of the skips in the shaft of the mine hoist, a control signal (Mg) to the mechanical braking system for increasing the mechanical braking torque from zero to a value corresponding to the maximum braking torque at the actual load, and that the control signal (Mg) generated by the brake application generator is also used for reducing the drive system, via a reduction unit (4), concurrently with the mechanical braking torque being increased, and that there are formed in a generator (16) a speed emer-gency stop reference (nref), which is based on the output signal of the brake application generator, as well as a signal (nltr) corresponding to the current speed of the rope drum, and that the speed emergency stop reference is compared with a signal (nltr) corresponding to the current speed of the rope drum in a speed controller (17), whose output signal (Mn) is passed to a torque reference generator (18) together with a signal (nltr) corresponding to the current speed of the rope drum, as well as estimated value ?1 and ?2 of the speed of the skips which are obtained via an estimator (19) in the form of a Kalman filter based on a state model of a mine hoist, which is supplied with the signals S1 and S2 from the rope tension measuring devices and a signal nltr corresponding to the speed of the rope drum, and that in the torque reference generator with available input signals and with given weighting factors P1 and P2, there is formed a signal corresponding to a torque reference Mref = MM + P1(nltr-?1) + P2(nltr-?2) which torque reference signal is used as additional control signal to the mechanical braking system, and that when the speed of the rope drum approaches zero, the braking effort of the mechanical brake is reduced via the speed control.

2. A method during emergency stop of a mine hoist according to claim 1, which is characterized in that the control signal (MB) from the brake application generator (2) is formed by determining, with a simple model of a mine hoist, the two dominating resonance frequencies of the ropes of the mine hoist, and that the control signal from the start is to increase according to a ramp up to a control signal which corresponds to half the maximum braking torque which is to be applied at the current load in a time correspon-ding to the period of the highest of the two dominating frequencies, whereafter the control signal is to be main-tained constant for a time corresponding to the difference between half of the period of the lowest of the two domina-ting frequencies and the period of the highest of the two dominating frequencies, whereafter the control signal is again to increase according to a ramp to a signal corre-sponding to the maximum torque of the current load in a time corresponding to the period of the highest of the two frequencies.

3. A method during emergency stop of a mine hoist according to claim 1, which is characterized in that the control signal (Mg) from the brake application generator (2) con-sists of a signal which, from zero, increases as a simple time constant function to a signal level which corresponds to the maximum braking torque at the current load and wherein the time constant is determined on the basis of the current load data.

4. A method during emergency stop of a mine hoist according to claim 1, which is characterized in that the additional control signal (Mref) to the mechanical braking system is switched in only after the control signal (MB) from the brake application generator has reached a signal level corresponding to the maximum braking torque for the current load.

5. A method during emergency stop of a mine hoist according to claim 1, which is characterized in that the additional control signal (Mref) to the mechanical braking system is switched in immediately after an emergency stop has been called for.

6. A device for carrying out the method according to claim 1 during an emergency stop of a mine hoist for preventing the occurrence of vertical oscillations which may arise in the skips (12, 13) of the mine hoist, and wherein the mine hoist is driven by means of an electric drive system (1) which is included in a speed control of the speed (nltr) of the rope drums (8, 9) of the mine hoist, and wherein the skips are suspended from ropes (10, 11) between the rope drums and the skips, and wherein the ropes are provided with rope tension measuring devices (14, 15) for determi-ning the rope tension S1 and S2 in the ropes, and wherein the mine hoist also comprises a mechanical braking system (3), the device being characterized in that a calculating mem-ber (22) incorporates programs for an emergency stop reference generator (16) for forming an emergency stop reference (nltr) for the speed control, an emergency stop controller (17) for the speed control with an output signal (Mn), an estimator (19) for estimating the speeds z1 and z2 of the skips, a torque reference generator (18) for forming a control signal (Mref) to the mechanical braking system, a brake application generator (2) for forming a control signal (Mg) to the mechanical braking system, a reduction unit (4) for reducing the drive system, a summator (20) for summing the control signals (Mg) and (Mref), a contact (21) for connecting the control signal (Mg) to the summator, and that the control signal Mref = Mm + P1(n?-?1) + P2(n?-?2) with given weighting factors P1 and P2, and that the input signals of the calculating member con-sists of an emergency stop signal, the rope tensions S1 and S2, the speed (nltr) of the rope drums, operating informa-tion about the load of the mine hoist, the position of the skips, and information from the control system of the speed control during normal operation, and that the output signals of the calculating member con-sists of a reduction signal to the drive system as well as control signals to the mechanical braking system.