GB2153465A

GB2153465A - Emergency stop control apparatus for elevator

Info

Publication number: GB2153465A
Application number: GB08500633A
Authority: GB
Inventors: Hisakatsu Kiwaki; Seiya Shima; Katsu Komuro; Toshiro Narita; Korefumi Tashiro; Noboru Arahori; Sadao Hokari
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1984-01-11
Filing date: 1985-01-10
Publication date: 1985-08-21
Also published as: JPH0729746B2; GB8500633D0; KR930000422B1; JPS60148879A; GB2153465B; KR850005366A

Abstract

An elevator system has a cage 6, the movement of which is controlled by a motor 1. A friction brake device 7 is provided to brake the cage 6 if any abnormality occurs, normal movement being braked by stopping the motor 1. In the present invention a brake control circuit 8 receives a signal from 60 representing the load of the elevator and the braking force of the brake device 7 is controlled, either stepwise or continuously, in dependence on the load to brake the elevator. The brake control circuit 8 is designed to apply the brake device 7 during any abnormality in the movement of the elevator, but also due to any abnormality in the control circuit 8 itself. <IMAGE>

Description

SPECIFICATION Emergency stop control apparatus for elevator The present invention relates to an emergency stop apparatus for an elevator, and more particularly to an emergency stop control apparatus well suited for lowering the inertia of an elevator system.

In general, the construction of the mechanical system of an elevator system is such that, as shown in Figure 1, a reduction gear 2 and a sheave 3 are coupled to a driving motor 1, while a counterweight 5 and a cage 6 are suspended through a rope 4 which is wound round a sheave 3. In order to brake the cage 6 and hold it at rest, a friction brake device 7 is disposed. It is usually composed of a brake coil 7C, a brake shoe 7S and a brake drum 7D. The replacement of the components 7S and 7D with a disc brake is also known.

In such construction, when any abnormality has occurred in the elevator during the running of the cage 6 under the drive of the motor 1, the driving force of the motor 1 is cut off, and the brake device 7 is actuated thereby to stop the cage 6 in emergency. That is, the friction brake device 7 is also utilized as a protection device for safely stopping the cage 6 even when an elevator control apparatus is abnormal by way of example.

Accordingly, even in an elevator which adopts a system wherein during a decelerating stop operation in a normal condition, the braking force of the friction brake device 7 is controlled thereby to produce a braking torque orto alleviate a floor arrival shock, the friction brake device 7 is unconditionally actuated in the abnormal condition, whereby priority is given to the safety.

It is known that, when the moment of inertia of the mechanical system shown in Figure 1 is rendered small, the capacity and power consumption of the motor 1 can be reduced.

Therefore, it is recently considered to save electric power by rendering the moment of inertia still smaller.

It has been found, however, that the following problems are posed anew as the inertia is lowered. As stated before, in the abnormal condition of the elevator, the safety ought to be secured first by actuating the friction brake device 7. However, when the moment of inertia is small, the cage 6 stops suddenly. When a deceleration on this occasion is great, passengers undergo a great shock dangerously. It is accordingly considered to weaken the braking force of the friction brake device 7. In this case, however, a deceleration distance becomes long. Therefore, when an abnormality has occurred during descent of the cage 6, when the cage is at full capacity thereof or during ascent with few passengers by way of example, the car may not stop at a suitable floor, and a downward dash or upward dash arises, which might cause a serious accident.

In addition, the friction brake device 7 requires at least a sufficient braking force to hold the cage 6 at rest if passengers in excess of a fixed capacity enter the cage 6, and weakening the braking force may be limited from this standpoint.

There are therefore limits to the reduction of the moment of inertia.

The present invention seeks to overcome this problem by providing an emergency stop control apparatus including means for detecting the load of the elevator and means to control the braking force of a friction brake in dependence upon the load detected by the load detection means. The elevator load may be determined from the magnitude of the load on the shaft of the motor which drives the elevator. The present invention permits the moment of inertia of the elevator system to be reduced, whilst maintaining safety in emergencies.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa view of the basic construction of a conventional elevator mechanical system; Figure 2 is a diagram for explaining a method of setting a friction brake torque for an elevator; Figures 3, 4 and 5 are explanatory diagrams each showing a method of setting a friction brake torque according to the present invention; Figure 6 and Figures 7(a) and 7KbJ are explanatory diagrams of friction brake torque characteristics according to the present invention; Figure 8 is a diagram of an embodiment of an elevator emergency stop control apparatus according to the present invention; Figure 9 is a diagram of an embodiment of a decision circuit in Figure 8;; Figure 10 is a time chart for explaining operations in Figures 8 and 9; Figure ills a diagram of an embodiment of a load detector according to the present invention; and Figure 12 is a diagram of another embodiment of the elevator emergency stop control apparatus of the present invention.

Here, the relationship between the braking force of a friction brake device (hereinafter, simply termed "braking force") and the moment of inertia will be first elucidated.

Figure 2 shows the relation between the load factor in a cage in an ascent operation and the deceleration a of the cage in a braking operation.

The deceleration a in the braking operation is evaluated by: α = K. TB + TLx (m/52) (1) J Here, K; coefficient determined by the reduction ratio of a reduction gear and the diameter of a sheave (m), TB; braking tor TLx; load torque (N.m), TLX = K (WLX - WC) (N.m) WLx; load factor in the cage (kg-f), Wc; weight of a counterweight (kg-f), J; moment of inertia of an elevator system (kg-m2).

As referred to before, there are given the upper limit value αUL of an acceleration inflicting no injury on passengers in the braking operation and the lower limit value αDL of the acceleration for preventing upward and downward dashes at terminal floors. Accordingly, the deceleration a of the cage needs to be set at aUL (y a DL irrespective of the load factor in the cage and the running direction of the cage. In the ascent operation, the deceleration needs to be set at a characteristic indicated at L1 in the figure as obtained with Eq.

(1).

The gradient k, of this characteristic L1 has the following relation to the gradient ko of a characteristic LO which connects the lower limit RDL and upper limit auL of the deceleration: k1 # ko Further, the deceleration (xNl of the characteristic L1 at no load NL needs to be: αN1 eeDL Here, k, and eeN1 are evaluated as follows: k1 = 2#K#1LF (2) J#WLF K = K T5 - TLo (m/s) (3) J TLF; unbalance torque at the full load (N-m), TLO; unbalance torque at no load (N-m), WOLF; unbalance load at the full load (kg).

It is accordingly necessary to set the moment of inertia J with Eq. (2) and to set the braking torque TB with Eq. (3).

The moment of inertia J which is evaluated from the above relations becomes very great ordinarily, and the moment of inertia 3 - 6 times greater than that of a rectilinear system which is evaluated from the cage and counterweight weights needs to be afforded to a rotating system on the shaft of a motor by, for example, enlarging a brake drum etc. in excess offunctions.

In consequence, the torque for driving the elevator becomes great, and energy to be consumed by the motor increases naturally.

Next, the fundamental concept of the present invention will be explained. Figure 3 illustrates the relationship between the load factor in the ascent operation and the deceleration similarly to Figure 2.

Firstly, there will be stated a case where J2 to which the moment of inertia can be functionally minimized has the following relation to the foregoing characteristic LO: JO=kJ#2 J2 Assuming that the braking torque TBO required in the case of the characteristic LO be constant, a characteristic L2 in the figure in which the deceleration at no load NL is aN2 = kJ a DL and which has a gradient k2 = kJ-ko is established in the case of the aforementioned moment of inertia J2. This characteristic exceeds the upper limit value αUL in substantially the whole region.

Therefore, the braking torque needs to be weakened so that the deceleration may equalize to at least the lower limit (XDL The braking torque TB3 at this time becomes: awN3aJ2 TB3 = + TLO K where aN3 2 UDL.

A characteristic to produce the braking torque TB3 becomes as indicated at L3 in the figure. Here, merely by weakening the braking torque, the characteristic L3 has the same gradient as that of the characteristic L2, So its part indicated by oblique lines Olin the figure exceeds the upper limit value au.

Therefore, by further weakening the braking torque, a deceleration aH4 at a balanced load HL is set so as to be equal to or greater than the lower limit ADL, or a deceleration aF4 at the full load FL is set so as to be equal to or less than the upper limit auL. Letting TB4 denote the braking torque on this occasion, the following holds for aH4 ' tXDL TB4 = OLH4'J2 (where TB3 > TB4) Also in this case, naturally a part indicated by oblique lines (2) in the figure becomes less than the lower limit value aoL.

During the ascent operation, therefore, the braking torque is switchedly set at TB3 (characteristic ) for a region from the no load point NLto the balanced load HL and at TB4 for a region at and beyond the balanced load HL, whereby a characteristic Lup indicated by a solid line in Figure 4 can be obtained. In the descent operation, the load torque is reverse to that of the ascent operation with the balanced load HL as a boundary, so that the characteristics L3 and L4 in the ascent operation become characteristics L3, and L4, symmetric about the balanced load as shown in Figure 4.

Accordingly, the braking torque is switchedly set at TB4 for the region from the no load point NL to the balanced load HL and at TB3 for the region at and beyond the balanced load HL, whereby a characteristic LDN indicated by a dotted line in the figure can be obtained.

In view of the above, subject to kJ s 2, the deceleration X can be put into A DL a OLUL by switching the braking torque in two or more stages.

Next, there will be explained a case where the relation of Jo/J2 = kJ > 2 is held.

In this case, likewise to the case of kJ s 2, a characteristic is endowed with a gradient which is kJ times greater than the gradient ko of the characteristic LO. The braking torque is set in three or more stages for 2 < kJ < 3 and in four or more stages for 3 < kJ s 4, and they are switched according to the load factor. Then, decelerations within the upper limit auL and the lower limit aDL can be produced.

As an example, the relations in the case of kJ = 3 are illustrated in Figure 5.

The braking torques are respectively set at TBoi, TBo2 and TBc3 So as to establish illustrated characteristics Lol, L02 and L03, and they may be switched according to load conditions.

The brake is intended, not only to stop the elevator during the running thereof, but also to hold the cage of the elevator at rest. Usually, a torque for holding the cage at rest needs to have at least a magnitude enough to hold the state in which the cage carries 180 - 200% of the rated load of the elevator.

Letting TBS denote the rest or standstill holding torque, when the maximum value of the braking torque set as described before is less than TBS, for example, when the set maximum value TB2 is TBS < TB2, the braking torque is further constructed so that the standstill holding torque TBS can be generated.

To sum up, when the gradient at a low inertia is kJ times the gradient ko which is evaluated from the upper limit ocuL and lower limit RDL of the deceleration, the braking torque is set at, at least, a value obtained by adding l to the integral part of the gradient kJ. Further, when the maximum value of the set values of the torque is less than the torque for holding the elevator at rest, the braking torque is set so as to also include the holding torque.

In the above, the principle of the present invention has been explained by exemplifying in order to facilitate understanding, the case of evaluating the load torque lLx with reference to the load factor in the cage and from the relation with the counterweight weight. However, the above method is not restrictive, but the load torque lLx can also be detected by, for example, detecting directly a load torque acting on the rotary shaft of the motor during running or detecting the magnitude of a motor current. The braking force may well be controlled according to the load torque thus detected.

That is, the present invention consists in controlling the braking force according to the load viewed from the rotary shaft of the motor, namely, the load of the elevator, and it is not restricted by the method of detecting the load.

Now, one embodiment of the present invention in the case of performing the torque switching illustrated in Figure 4 will be described.

Figure 6 illustrates the magnitudes of the braking torque required for the construction of Figure 4 (two-stage change-over). In the figure, torques T2, T1 and To have relations: T2 > T1 > To = O T2 corresponds to TB3 mentioned in Figure 4, and T1 to TB4. The braking torque To (= 0) corresponds to the running operation of the elevator.

The relations of the torques T1 and T2 in abnormal conditions are illustrated in Figures 7(a) and 7(b). As seen from Figure 4, the torque T1 needs to be applied during the ascent operation under at least the balanced load HL orthe descent operation under at most the balanced load HL, and the torque T2 during the ascent operation under at most the balanced load HL or the descent operation under at least the balanced load HL.

Figure 6 shows a brake control circuit for controlling the torque. Usually, the brake has a braking spring, the force of which is used for setting the maximum braking force. At this time, the magnitude of the torque is determined by current which is caused to flow through the electromagnetic coil 7C of the brake. More specifically, when the current flowing through the coil 7C is great, a torque cancelling the braking spring force is generated.

The brake control circuit 8 is composed of an A.C. power source AC, a rectifier 81 for converting the alternating current, resistors 82 - 84 for suppressing the current to flow through the coil 7C, and contacts R10, and R201 which are the contracts a of relays to be described later.

With the above circuit, when the contact R201 is open, the circuit is broken, so that a torque based on the braking spring force, the torque T2 here, is generated. Under the closed state of the contact R201, when the resistor 82 is short-circuited by the contact Ri 01, the braking torque is set at T0 = 0, and when the resistor 82 is inserted, the torque T2 is generated. The contacts are switched by a decision circuit 9 which receives signals from an abnormality detection unit E, a running direction detection unit DE, a load detection unit WD for deciding the load factor on the basis of a load detector 60 mounted, e.g., under the floor of the cage, and a zero velocity detection unit VD for detecting the zero velocity on the basis of a signal from a velocity detector 10.

The decision circuit 9 will now be explained with reference to a relay sequence shown in Figure 9.

In Figure 9, R10 denotes the relay which turns on from start to stop in the normal operation, RiOT a relay which turns off with a delay of a time tD with respect to the relay R10, R20 the relay which adjusts the braking torque, and RE a relay which turns off when the abnormality detection unit E has detected an abnormality.

RE, denotes the normally open contact of the relay RE; R102, R103 and the symbol R10, indicated in Figure 8 denote the normally open contacts of the relay R10; RioT1 denotes the normally open contact of the time relay RiOT; and the symbol R201 in Figure 8 denotes the normally open contact of the relay R20. RPa denotes the normally open contact of a relay which turns off at the stop level of the cage, RHF1 the normally open contact of a relay which turns on when the output of the load detection unit WD is not less than the balanced load, RNH, the normally open contact of a relay which turns on when the output of the unit WD is not greater than the balanced load HL, RUP1 the normally open contact of a relay which turns on when the running direction detection unit DE indicates the upward or ascent operation, RDN1 the normally open contact of a relay which turns on when the unit DE indicates the downward or descent operation, and RV1 the normally open contact of a relay which turns off when the zero velocity detection unit VD indicates that the velocity is substantially zero.

The time charts of the above embodiment are shown in Figure 10 with solid lines corresponding to the normal condition and broken lines to the abnormal condition.

First, the case of the normal operation will be described. In Figure 9, it is assumed that the abnormality detecting relay RE be turned on owing to the abnormal operation, with its contact RE, being closed, and that the stop level detecting relay contact RP1 be also closed by a positional signal. At the time of starting, accordingly, the relays R10, RiOT and R20 are brought into the 'on' states by operating a start button SB, and these states are held by the contact Ri 02.

Thus, in the brake control circuit 8 in Figure 8, the contacts Ri Oi and R201 are closed so that the brake falls into the released state, and the motor 1 is started so that the cage 6 begins to run. When the stop floor level has been reached, the stop level detecting relay contact RP1 is opened, and the zero velocity detecting relay contact RV1 is opened by the output of the zero velocity detection unit VD because the velocity is in the substantially zero state. Accordingly, the relay R10 is turned off by the contact RP1, and the relay R20 by the contact RV1. Therefore, current is cut off by the contact R201 in the brake circuit of Figure 8, and the brake applies the maximum set torque T2.

That is, as shown at (A') in Figure 10, the braking to torque during the normal operation becomes To (= 0) at time to (at the time of start) and T2 (maximum set value) at time t3 (at the time of stop).

Next, the emergency stop at the occurence of an abnormality will be described. Let's consider a case where the abnormality has occurred attire to in Figure 10 during the running of the elevator. Upon the occurrence of the abnormality, the abnormality detection unit B operates to turn off its detecting relay RE in the sequence of Figure 9. The relay R10 is turned off by the contact RE1, and the time relay R1 OTis turned off with the delay of the time interval tD. At this time, the brake torque adjusting relay R20 is turned off by the contact R103 subject to the condition that the path between Q and Ob in the figure is open.Thus, as in the normal operation, the brake circuit 8 in Figure 8 is cut off by the contact R201, and the maximum set braking torque T2 is applied as shown at (B') in Figure 10. The open circuit condition of the path between (i) and corresponds to the ascent operation with at most the balanced load HL or the descent operation with at least the balanced load HL. In this case, accordingly, the cage can be stopped in emergency under the decelerations indicated by the solid line part of the characteristic L3 and dotted line part of the characteristic L3, in Figure 4, namely, the decelerations a within the range of auL > a > aDL.

Next, subject to the condition that the path between (i) and ) in Figure 9 is closed, the adjusting relay R20 is in the 'on' state even when the contact R103 has opened, because a path extending along contact RiOT1 contact RV1 - Q - is in the closed state. On this occasion, the relay R10 is 'off' as described before. In the brake circuit 8, accordingly, the contact R201 is closed and the contact R101 is open, resulting in the state in which the resistor 82 is inserted.That is, the braking torque is applied with the magnitude 11. Thereafter, when the time relay RiOT has turned off upon lapse of the time interval tD or when the velocity has become null owing to the application of the braking torque, the contact RiOT1 of the relay RiOT or the contact RV, for the velocity detection is opened, whereby the relay R20 turns off as shown at (C) in Figure 10. Accordingly, the brake circuit is cut off by the contact R201 of the relay R20, so that the maximum set value T2 is applied as the braking torque. Thus, the braking torque condition in this case becomes as shown at (C') in Figure 10.

The closed circuit condition of the path between (i) and (6) corresponds to the ascent operation (the contact RUP1 being closed) with at least the balanced load HL (the contact RHF, being closed) or the descent operation (the contact RDN1 being closed) with at most the balanced load HL (the contact RNH1 being closed). The cage stops under the decelerations indicated by the solid line part of the characteristic L4 and the dotted line part of the characteristic L4' shown in Figure 4.

Owing to the above construction, even when the inertia is low, the stop shock at the time of the emergency stop can be moderated. Conversely, the inertial can be rendered still lower.

Meanwhile, the elevator is provided for safety's sake with forced deceleration switches which serve to apply braking when the stop levels of the uppermost floor and lowermost floor have been exceeded, and final limit switches FLS which operate when the elevator is running yet, to the end of preventing the upward and downward dashes. By way of example, the contact FLS1 of the final limit switch FLS is connected to a power source line as shown in Figure 9 so as to apply the set maximum torque upon the operation of the switch FLS, whereby the safety can be given priority.

In a case where the load detector 60 has operated erroneously, for example, a case where it has decided the load factor as being at least the balanced load in spite of no load, thereby allowing the ascent operation to proceed, the braking torque becomes smaller than a required magnitude.

To the end of preventing this drawback, it is considered to afford a high reliability by the use of an interlock circuit or the like, and also to construct a load detecting portion as shown in Figure 11 in order to attain a double failsafe structure.

Figure ii shows one embodiment for constructing a failsafe load detection device in such a way that a frequency corresponding to a load is detected and is used as a load signal.

Referring to the figure, numeral 61 indicates the floor surface of the cage 6, numeral 62 the outer frame of the cage, numeral 63 an elastic member of rubber, a spring or the like which is inserted between the floor surface 61 and the outer frame 62 and which contracts according to the load, and numeral 64 an adapter plate which is mounted on the floor surface 61 and moves in association with the load and in which holes H1 and H2 of different positions are provided. Numeral 65 indicates an adapter plate which is mounted on the outer frame 62 and which supports two light emitting diodes P1 and P2, and letter Pa frequency generator which applies pulse voltages of frequencies f, and f2 to the respective light emitting diodes P1 and P2.

Further, symbol PR denotes a receiver which detects the signals of the light emitting diodes P1 and P2 through the respective holes H1 and H2 of the adapter plate 64, and symbol fD a frequency decision unit which decides the frequency of the frequency signal detected by the receiver PR.

Here, the hole H1 is worked into dimensions suited to pass the signal from the light emitting diode P1 at the positions of the load factor from no load to the balanced load, and the hole H2 into dimensions suited to pass the signal of the light emitting diode P2 at the positions of the load factor not less than the balanced load. The light emitting diode P1 generates the signal of the frequency f, produced by the frequency generator P, and likewise the light emitting diode P2 generates the signal of the frequency f2 (f1 *f2).

In the above construction, the frequency decision unit fD has a frequency band width A f. Upon detecting the signal: fq - At < fa < f, + +Af it supplies the decision circuit 9 in Figure 8 with a signal indicative of the load factor from no load to at most the balanced load, and upon detecting the signal: f2 - at < < f,f, + Af it supplies the same with a signal indicative of the load factor of at least the balanced load. Then, the decision circuit 9 controls the braking torque as in the foregoing description on the basis of these signals.When the signal from the frequency decision unit fD has disappeared on account of a frequency outside the specified ranges of the damage of the light emitting diode byway of example, the load signal contacts RHN1 and RHF1 in Figure 9 are opened. Accordingly, in the absence of the signal, including the cases of service interruption etc., the control apparatus can be actuated so as to open the contacts RHN1 and RH Fl, so that the braking torque control of the present embodiment can be rendered perfectly failsafe.

In the above, the case of controlling the braking force in two stages has been stated. Also in case of increasing the number of stages more, the braking force can be controlled on the basis of the foregoing operating principle of the present invention by detecting the load factor in accordance with the number of stages.

Next, another embodiment of the brake control circuit 8 is shown in Figure 12. Referring to the figure, the points of difference from the arrangement of Figure 8 will be explained. The control circuit 8 is composed of a magnetic multivibrator 85 for controlling a D.C. power source DC in accordance with a frequency signal input fs, a transformer 86 and a rectifier circuit 87. The frequency signal s is a frequency command for providing the braking torque illustrated in Figure 4from the magnetic multivibrator 85, and it is delivered by a magnetic multi-control circuit ii in accordance with the signals from the decision circuit 9.

Thus, when the magnetic multivibrator 85 has any trouble or when the output of the magnetic multicontrol circuit 11 continues to be provided or is null, no current flows through the brake coil 7C, and the braking torque becomes the maximum magnitude. That is, the control apparatus can be rendered failsafe.

While, in the above, the practicable examples have been mentioned and explained, the present invention is not restricted thereto. By way of example, shocks at abnormal stops can be rendered constant in such a way that the load factor is continuously detected by employing a differential transformer etc. as the load detector 60, while a signal circuit indicated by a dotted line in Figure 12 is constructed using a microcomputer, so as to generate a frequency signal fas which continuously controls the magnetic multivibrator. In the microcomputer implementation, a more perfect failsafe structure is provided by adopting the frequency signals elucidated in Figure 11 forthe load detecting portion. Besides, while in the foregoing embodiment the braking torque afterthe stop of the elvator has been set at the maximum, the braking torque may well be controlled according to the load so that the cage can be held at rest even in the course of the stopping operation. Further, while the friction brake device illustrated has been of the brake drum type, the apparatus can be similarly constructed even with a disc brake.

In accordance with the present invention, a frictional braking force at the time of an emergency stop is set according to the load of an elevator, so that the moment of inertia can be more reduced while the safety is maintained by preventing a rapid stop or an upward or downward dash in an abnormal condition. With the reduction of the moment of inertia, accordingly, it can be expected to save power consumption more.

Claims

1. An elevator system having an elevator cage, a motor for driving the cage, a friction brake device for braking the cage to an emergency stop in an abnormality of the elevator system, and an emergency stop control apparatus having means to detect the load of the elevator and means to control the braking force of the friction brake device in dependence upon the load detected by the load detection means during an emergency stop.

2. A system according to claim 1, wherein the means to control the braking force is adapted to decrease the braking force with increase in elevator load.

3. A system according to claim 1 or claim 2, wherein the load detection means is adapted to detect the load from the weight of the cage and the direction of running of the elevator.

4. A system according to any one of claims 1 to 3, wherein the means to control the braking force is adapted to select one of a plurality of predetermined braking forces in dependence on the elevator load, and the force selected is applied by the friction brake means.

5. A system according to any one of claims 1 to 3, wherein the means to control the braking force is adapted to control the friction brake device to generate the maximum possible braking force a predetermined time after detection of the abnormality of the elevator or after the velocity of the cage has been reduced to a predetermined velocity.

6. A system according to claim 5, wherein the friction brake device is an electromagnetic brake, and the elapsing of the predetermined time or the attainment of the predetermined velocity causes the means for controlling the braking force to stop supplying a voltage to the electromagnetic brake, whereupon the electromagnetic brake applied its maximum force.

7. A system according to any one of the preceding claims, wherein the friction brake device is adapted to generate its maximum force in the event of a defect in the load detection means and1or the means for controlling the braking force.

8. A system according to claim 7, wherein the load detection means is adapted to generate an output signal in the absence of a defect, and the means for controlling the braking force is adapted to cause the friction brake menas to apply its maximum force in the absence of that output signal.

9. A system according to claim 7 or claim 8, wherein the means for controlling the braking force is adapted to generate an output signal in the absence of a defect, and the friction brake means is adapted to apply its maximum braking force in the absence of the output signal from the means for controlling the braking force.

10. A system according to any one of claims 7 to 9, wherein the means for controlling the braking force includes a D.C. power source, a multivibrator for converting the D.C. power into an output of A.C. power on the basis of a frequency command dependent on the elevator load, and a transformer, the primary winding of the transformer being connected to the output of the multivibrator and the secondary winding of which is connected to the friction brake device, the friction brake device applying a braking force in dependence upon the voltage across the secondary winding.

11. An elevator system substantially as herein described with reference to and as illustrated in Figures 8 to 11 or Figure 12 of the accompanying drawings.