GB2126375A - Lift system - Google Patents

Abstract

A lift system having an overspeed monitoring function for monitoring terminal stops includes an electromechanical transducer (52) arranged to provide a first signal (ECI) related to desired or nominal speed as a function of the distance of the car from the terminal floor. The first signal is processed (86) to have substantially the same magnitude and slope as a second signal (TACI) produced by a tachometer (70) during a normal terminal slowdown. The first and second signals are compared (82) to provide a deviation or error signal which in turn is compared with a reference (92) selected according to the maximum allowable speed error. Should this maximum allowable speed error be reached, the overspeed function provides a signal which may be used to initiate an emergency terminal stop. <IMAGE>

Description

SPECIFICATION Elevator system The invention relates in general to elevator systems, and more specifically to an improved speed monitoring apparatus for elevator systems.

Speed monitoring and limiting devices adjacent to the terminals or travel limits of an elevator car may monitor the floor selector. If the floor selector is not operating in a manner which will produce a normal slowdown, an auxiliary speed pattern is produced for controlling terminal slowdown. In a prior art arrangement for monitoring an electromechanical floor selector, a long cam is disposed adjacent each terminal. The cam opens a series of switches mounted on the elevator car, one after another, as the car approaches a terminal floor. If the floor selector is operating properly, for each cam operated "switch opening" in the hoistway there will be a "switch closing" on the floor selector carriage. If this fails to occur, an auxiliary speed pattern is provided, such as by an electromechanical transducer activated by the same long cam.

Speed monitoring and limiting devices adjacent to the terminals may monitor the speed pattern generator as the elevator car approaches a terminal floor.

Aterminal slowdown pattern is provided in place of the normal deceleration pattern when a malfunction is detected, to decelerate the car into the terminal floor. Modification of the speed pattern generator signal, however, will not cause the car to decelerate if there is a problem in the drive system. Also, the speed pattern generator may be functioning correctly, but because of a problem in the drive system, the car may not be decelerating along a desired trajectory as it approaches a terminal floor. Such a system takes no action and may allow the car to approach the terminal at an excessive speed.

A speed monitoring system which monitors car speed as a function of car position can provide a high degree of protection against approaching a terminal at an excessive speed. U.K. Letters Patent No. 1,436,741, which is assigned to the same assignee as the present application, discloses such a system which continuously monitors the car speed as a function of car position, as the car approaches each terminal floor. In this arrangement, closely spaced markers mounted in the hoistway adjacent each terminal cooperate with a sensor disposed on the car to provide a continuous speed error signal which is used in a reference circuit to detect overspeed. The speed error signal is also used in a circuit which generates an auxiliary slowdown pattern. The auxiliary slowdown pattern is substituted for the normal speed pattern when overspeed is detected.

If the problem is not in the speed pattern circuits, but in the drive, generation of an auxiliary speed pattern will not be effective. Thus, this arrangement is used with a low inertia, fast acting car speed sensor switch as a backup, such as the speed sensor disclosed in U.K. Letters Patent No. 1,468,037, which is assigned to the same assignee as the present application. If the car speed is excessive at the car position relative to the terminal monitored by this speed sensing switch, the car is forced to make an emergency stop.

U.K. Letters Patent 1,561,536, which is assigned to the same assignee as the present application, discloses a discrete car speed monitoring system, as opposed to the continuous car speed monitoring system of the first mentioned U.K. Patent. This discrete monitoring system monitors car speed as a function of car position at a plurality of discrete speed checkpoints in the hoistway. The car speed is compared with two reference speeds at most car position checkpoints. If the car speed exceeds the lower but not the upper reference speed, the system attempts to decelerate the car by employing an auxiliary terminal slowdown velocity pattern. If the car speed exceeds the upper reference speed at any checkpoint, the car is forced to make an emergency stop.

The chief object of the present invention is to provide an elevator system which achieves the effectiveness of the discrete car speed monitoring system, without the time consuming adjustment of all of the discrete checkpoints in the hoistway, and without reducing the number of checkpoints, since a tradeoff is reached between nuisance tripping and late detection of overspeed.

With this object in view of the invention resides in an elevator system, in a building having a plurality of floors, including upper and lower terminal floors, and a hoistway, an elevator car mounted in said hoistway, motive means for effecting movement of said elevator car, control means for said motive means which controls the slowdown and stopping of said elevator car at a terminal floor, with said terminal slowdown of the elevator car normally defining a predetermined velocity curve having a predetermined slope, first and second continuous marker means in said hoistway adjacent to each terminal floor defining upper and lower terminal zones, respectively, and the distance-to-go from any point thereof to the associated terminal floor, electromechanical transducer means on the elevator car responsive to the marker means of each terminal zone for providing a first signal which decreases substantially linearly as a function of the distance of the elevator car from the associated terminal floor, means providing a second signal responsive to the actual speed of the elevator car, at least as the elevator car approaches a terminal floor in a terminal zone, means for adjusting the slope and magnitude of the first signal to correspond to the predetermined velocity curve and its predetermined slope of a normal terminal slowdown, means for providing a deviation signal having a magnitude responsive to any deviation between the first and second signals, means providing a reference signal indicative of the maximum allowable deviation, and comparator means for comparing said deviation and reference signals, and for providing a predetermined signal when the actual speed deviation exceeds the maximum allowable speed deviation.

The invention will become more readily apparent from the following exemplary description, taken in connection with the accompanying drawings, in which: Figure 1 is a partially schematic and partially block diagram of an elevator system constructed according to the teachings of the invention; Figure2 is a partially schematic and partially diagrammatic view of an electromechanical transducer arrangement which may be used to provide an electrical signal having a magnitude representative of distance from a terminal floor; Figure 3 is a schematic diagram of the functions shown in block form in Figure 1; Figure 4 is a graph which illustrates the operation of the elevator system during a normal slowdown and stop art a terminal floor; and Figures 5A and 5B are graphs which illustrates the operation of the elevator system during two different overspeed conditions as the elevator car approaches a terminal floor in a terminal slowdown zone.

Briefly, the present disclosure reveals an improved elevator system having a speed monitoring arrangement which continuously monitors car speed as a function of car position, or distance-to-go, from a terminal floor that the elevator car is approaching.

The speed monitoring arrangement utilizes an elec tromechanicaltransducer having an electrical output responsive to the position of a lever arm, which in turn is operated by a cam in the hoistway as the elevator car approaches each terminal floor. The same transducer used in the prior art to provide a substitute speed pattern for terminal slowdown may conveniently be used.

The distance-to-go output (ECI) of the electromechanical transducer is processed such that it has substantially the same slope and magnitude as a signal generated in response to actual car speed as the elevator car makes a normal terminal stop. This signal, for example, may be provided by the output (TACI) of a tachometer disposed to be responsive to the speed of the elevator car. The actual speed TACI of the elevator car as it approaches a terminal floor is continuously compared with the nominal or desired speed ECI at each instantaneous location of the elevator car relative to the terminal floor, to provide a deviation or error signal. The polarity and magnitude of the deviation signal is compared with a reference value selected to be indicative of the maximum allowable speed error before modification of the operation of the elevator system is required.

For example, the magnitude of the reference may be chosen to correspond to a car speed of about 80 FPM. Should the deviation reach 80 FPM, having a polarity indicating the actual car speed is exceeding the desired car speed by 80 FPM, a signal is produced which modifies the operation of the elevator system. For example, the modification may be an emergency terminal stop in which the drive power is removed from the drive motor, dynamic braking resistors are inserted into the armature circuit of the drive motor, and the mechanical friction brake is deenergized to initiate mechanical braking. Thus, in effect, the elevator system includes a improved solid state speed switch arrangement mechanically related to car position.The speed switch arrangement continuously monitors car speed versus car position as the elevator car approaches a terminal floor, providing a signal when the actual car speed exceeds the normal or desired speed for the car position by a predetermined value.

The present invention may be incorporated into any elevator system, but it will be assumed shown in U.K. Letters Patent Nos. 1,431,832 and 1,436,743.

Certain signals and circuitry used in the present application and not shown in the hereinbefore mentioned patents, are shown in U.K. Letters Patent Nos. 1,554,934 and 2,067,366. These patents, which are assigned to the same assignee as the present application, should be consulted for a more complete understanding thereof.

Referring now to the drawings, and to Figure 1 in particular, there is shown an elevator system 10 having an elevator car 12 mounted in a hoistway 14 for movement relative to a structure 16 having a plurality of landings. Only the bottom terminal floor 18, the top terminal floor 20, and one intermediate floor 22 are shown in Figure 1. The elevator car 12 is supported by a plurality of wire ropes 26 and a counterweight 27. The ropes 26 are reeved over a traction sheave 28 mounted on the shaft 30 of a drive motor 32, such as a direct current motor as used in the Ward-Leonard drive system. A source of electrical energy for motor 32, which includes drive control 34 and an alternating potential source 36, is connected to motor 32 via a contactor having an electromagnetic coil 7C and contacts 7C-2.An electromechanical brake having a coil BK is disposed to apply a brake force to sheave 28 when it is deenergized, and to lift the brake when coil BK is energized.

A pulse wheel 38 is disposed to be driven by movement of sheave 28, or from a governor sheave (not shown). A pickup 40 is disposed to detect movement of the elevator car 12 through the effect of circumferentially-spaced openings in the pulse wheel 38. The openings are spaced to provide a distance pulse for each standard increment of travel of the car, such as a pulse for each 0.25 inch of car travel. Pickup 40, which may be of any suitable type, such as optical or magnetic, provides the distance pulses for the car controller, shown generally at 42.

Car controller 42 includes a floor selector which maintains the car hatch position in response to the pulses.

Calls for elevator service are registered by car call pushbuttons 44 in the car 12, and by hall call pushbuttons in the hallways of the various floors of the building, such as the up pushbutton 46 at the bottom terminal 18, the down pushbutton 48 at the top terminal, and the up and down pushbuttons 50 located at each intermediate landing. As described in detail in U.K. Patent 1,436,743, car controller 42 includes a floor selector which processes the distance pulses from detector 40 to develop information concerning the position of car 12 in the hoistway 14, and the floor selector also directs these processed distance pulses to a speed pattern generator which generates a speed reference signal for drive control 34, which in turn provides the drive voltage for drive motor 32. The floor selector keeps track of the elevator car 12, the calls for service for the car, it provides the request to accelerate signal to the speed pattern generator, and it provides the deceler ation signal for the speed pattern generator at the precise time required for the car to decelerate according to a predetermined deceleration pattern and stop at a predetermined floor for which a call for service has been registered. The floor selector also provides signals for controlling such auxiliary devices as the door operator and hall lanterns and it controls the resetting of the car call and hall call controls when a car or hall call has been serviced.

According to the teachings of the invention, elevator system 10 includes an electromechanical transducer means 52 mounted on elevator car 12 which provides a continuous signal ECI of reducing magntidue as car 12 approaches either terminal floor 18 or 20 in its associated terminal slowdown zone.

Transducer means 52 may be the electromechanical transducer shown in U.S. Patent No. 3,207,265, which is used in that particular patent to provide a substitute speed pattern. Transducer 52, alternatively, may be the transducer shown in detail in Figure 2, or any other suitable electromechanical transducer may be used which can be mechanically related to the distance-to-go from the elevator car to the terminal floor that it is approaching. The invention recognizes that the electromechanical devices heretofore used to provide a substitute speed pattern, may also function to provide a distance-to-go signal, and that this distance-to-go signal may be further processed and utilized in an accurate solid state speed switch arrangement.

The mechanical tie between the position of the car 12 relative to a terminal floor may be provided by continuous marker means in the form of cams 54 and 56 disposed adjacent to the bottom and top terminal floors 18 and 20, respectively, and a cam follower on transducer 52. Cams 54 and 56 define terminal slowdown zones, and also, by their slope, each point on each cam is uniquely related to the distance of the point from the associated terminal floor. Transducer 52 may include an actuating arm 58 connected to a lever 60 having a roller 61 disposed to contact the cams at the start of each terminal slowdown zone. The cames 54 and 56 are sloped to rotate actuator arm 58 about its axis in direct proportion to the distance of the elevator car 12 from a terminal floor.

Cams 62 and 63 in the hoistway 14 and switches 64 and 66 on the elevator car 12, or vice versa, are disposed to provide signals PBOT and PTOP which are high or true when the elevator car 12 is in the bottom and top terminal zones, respectively. A terminal slowdown zone is defined as that distance over which the elevator car 12 must change from contract speed and decelerate to a stop at the associated terminal floor according to a predetermined deceleration schedule. Thus, when car 12 is located in the lower terminal zone, signal PBOT will be high, and signal PTOP will be low. When car 12 is located in the upper terminal zone, signal PTOP will be high and signal PBOTwill be low. Both signals will be low when the elevator car 12 is between the two terinal slowdown zones.

Suitable means, such as tachometer 70, may be disposed to provide a continuous signal having a magnitude responsive to actual car speed. As shown in Figure 1, tachometer 70 may be rim driven from a surface of the traction sleeve 28, or governor driven, in order to provide a relatively noise-free d.c. signal.

The solid state speed switch function will first be described from a functional viewpoint, utilizing the block diagram portion of Figure 1, and then a preferred embodiment will be described in detail with reference to the remaining Figures. The output of tachometer 70, referenced TACI, is filtered in a low-pass waveform filter 72 to remove spikes and other high frequency noise, and the conditioned signal is applied to an absolute value circuit 74to provide a positive polarity signal regardless of the travel direction of the elevator car 12. Terminal zone logic 76 is responsive to the car travel direction, represented by signal 1 RL and 2RL, which indicate up and down travel, respectively, when true or high, and the terminal zone car position signals LTOP and LBOT, which are logic level signals from an interface 78, corresponding to the power level signals PTOP and PBOT, respectively.Signals 1RL and 2RLare logic level signals responsive to the up and down travel direction relays 1 R and 2R, respectively, shown in U.K. Patent 1,554,934. A high or true enable signal EN is provided when the elevator car is between the terminal zones, and when it is in a terminal zone approaching a terminal floor. Signal EN is low when the car 12 is in a terminal zone and traveling away from the associated terminal floor.

The output of absolute value circuit 74, and the enable signal EN, are both applied to an enable function 80, which provides an output signal responsive to TACI when enabled by signal EN. The output of enable function 80 is applied to a deviation or error detector 82.

The output ECI of the electromechanical transducer 52 is applied to a low-pass waveform filter 84. The magnitude of the conditioned signal represents the distance of the elevator car 12 from a terminal floor when the elevator car is in a terminal slowdown zone. The signal is then processed in function 86 to adjust its slope and magnitude to correspond to that defined by signal TACI when the elevator car is making a normal stop at a terminal floor.

As the output of transducer 52 reaches the lower end of its output range, it may become non-linear, and it is thus clamped in function 88 when it drops to a predetermined value. If there has been no overspeed detected by the time the elevator car reaches the position at which the signal is clamped, it is known that the car is making a normal terminal slowdown and the clamping of the nominal speed signal is of no effect.

The processed signals ECI and TACI are compared in deviation detector 82, and any difference is applied to a comparator function 90. A reference function 92 provides a reference magnitude for comparator 90 indicative of the maximum allowable deviation when the actual car speed TACI is exceeding the nominal speed ECI. If the deviation reaches the reference magnitude, the output of comparator 90 changes, which change may be used to energize, or deenergize, as desired, a relay ETS. The contacts of relay ETS are connected to modification means 94. Modification means 94 may include the contacts of relay ETS connected to initiate a terminal emergency stop. For example, a contact may be connected to deenergize contactor 7C, which removes drive power from the drive motor 32 and connects dynamic braking resistors in the armature circuit, as shown in U.K.Patent 2,067,366. A contact would also be connected to deenergize the brake coil BK, to initiate mechanical braking.

Figure 2 is a schematic diagram of an electromechanical transducer which may be used for transducer 52 shown in Figure 1. In general, transducer 52 includes a transformer 94 in which the coupling between the primary and secondary windings is continuously adjustable in responsive to the rotation of actuator arm 58. Transformer 94 includes an E-type magnetic core having an E-shaped section 96 defining an inner and two outer legs, and an I-shaped section 98, with each section being formed of a plurality of metallic, magnetic laminations stacked together. The primary or input windings 100 may be disposed about the inner leg, and the secondary or output windings 102 and 104 are disposed about the outer legs. Actuator arm 58 may be attached to I-section 98, with I section 98 being rotatable about the axis of actuator arm 58.The closely adjacent portions of the E and I sections are configured such that as I-section 98 is rotated from its normal position of providing maximum flux transfer, to a position of minimum flux transfer, the output voltage appearing across the secondary windings 102 and 104 is substantially linearly related to the angular position of shaft 58. In order to adjust the magnitude of the output voltage, the primary windings 100 may be connected to a source 106 of alternating potential via an adjustable voltage autotransformer 107.In order to provide a unidirectional output voltage, the output voltage appearing across the secondary windings 102 and 104 is rectified by a rectifier bridge 108, the rectified waveform is filtered by an RC filter circuit 110 to remove the ripple, and the resulting unidirectional signal is signal ECI, suitable for use by the solid state speed switch function.

Figure 3 is a schematic diagram ofthe solid state speed switch function shown in block form in Figure 1. The signal TACI from tachometer 70 is first applied to a function 72, which may include an operational amplifier (op amp) 112 connected as a low pass filter. An adjustable resistor 114 provides magnitude adjustment. Since the tachometer voltage TACI has a positive polarity when the elevator car is moving in one direction, and a negative polarity when it is moving in the opposite direction, the filtered signal TACI is applied to an absolute value function 74.

Function 74 may include op amps 116 and 118 connected as a precision rectifier and as a summing amplifier, respectively, to cause signal TACI to have a positive polarity at the output of function 74.

Logic 76 may include four dual NAND gates 120, 122, 124 and 126 with NAND gate 126 connected as an inverter gate. Signals LTOP and 2RL are applied to inputs of NAND gate 120, and signal LBOT and 1RL are applied to inputs of NAND gate 122. The output of NAND gates 120 and 122 are connected to inputs of NAND gate 124. The output of NAND gate 124 is inverted by NAND gate 126. If the car 12 is in the upper terminal zone, traveling in the upward direction, the up travel signal 1 RL and the upper terminal zone signal LTOP will both be a logic one, but signals 2RL and LBOT are both a logic zero, causing gates 120 and 122 to both apply a logic one to NAND gate 124. NAND gate 124 thus outputs a logic zero, which is inverted to a logic one by gate 126. Thus, signal EN is high, which is its "enabling" state.A similar result occurs when car 12 is in the lower terminal slowdown zone, traveling in the downward direction. If the elevator car 12 is in the top terminal zone traveling down, gate 120 will output a logic zero, and if the car is in the bottom terminal zone traveling up, gate 122 will output a logic zero. Thus, in either case, the output of gate 124 will be high, and the output of gate 126 will be low, which is the disabling level of signal EN.

The enable function 80 may include an op amp 128 connected as a clamp and an op amp 130 connected as a non-inverting voltage follower. Op amp 128 has its inverting input connected to the output of absolute value function 74, and its non-inverting input connected to receive signal EN. The non-inverting input of op amp 130 is connected to the inverting input of op amp 128. A diode 132 has its anode electrode connected to the inverting input of op amp 128, and its cathode electrode connected to its output. When the enable signal EN is high, the output of op amp 128 is high and the output of follower 130 follows the output of the absolute value circuit 74. When the enable signal EN is low, the output of op amp 128 is low, which clamps the voltage applied to op amp 130 to substantially zero.

Thus, the output of op amp 130 has a positive polarity, following the absolute value of signal TACI, when signal EN is high, and it has a substantially zero output when signal EN is low.

Signal ECI from the electromechanical transducer 52 is applied to signal conditioning function 84, which may include an op amp 134 connected as a low pass filter. The conditioned signal is applied to processing function 86, which may include an op amp 136 connected as an inverting amplifier. Means is provided for selectively biasing the signal applied to the inverting input, such as an adjustable resistor 138 having its control arm connected to the inverting input of op amp 136 and its end connected to opposite polarity unidirectional sources. Op amp 136 is also connected such that its gain may be adjustably selected, and thus the desired adjustment of the slope of signal ECI, such as by an adjustable resistor 140 in the feedback loop. Adjustable resistors 138 and 140 are adjusted to match the magnitude and slope of signal TACI, when signal TACI is responding to a normal terminal slowdown.

Since signal ECI becomes non-linear as it approaches zero, its magnitude is clamped once its magnitude is reduced to a predetermined value close to zero. This clamping is performed by function 88, which may include an op amp 142 connected as a clamp, and an op amp 144 connected as a noninverting voltage follower. The output of function 86 is applied to the inverting input of op amp 142, and its non-inverting input is connected to an adjustable reference, which includes an adjustable resistor 146 and a negative source of unidirectional potential.

Adjustable resistor 146 is set to select the negative voltage at which it is desired to clamp signal ECI.

When signal ECI drops to the value selected by the reference, the output of op amp 142 will switch from high to low, to clamp the voltage applied to follower 144.

As shown in the graph set forth in Figure 4, when the elevator car 12 is making a normal terminal slowdown, signals TACI' and ECI' will have substantially equal, but opposite polarity, magnitudes at each incremental "distance-to-go" to the terminal floor. Thus, the difference or deviation between the opposite polarity signals may be obtained by summing function 82, which includes an op amp 148 connected as an inverting summing amplifier. If the actual car speed, represented by TACI', exceeds the nominal car speed, represented by ECI', the output of op amp 148 will be negative. If the signal TACI' is less than signal ECI', its output will be positive. Thus, the solid state speed switch function is completed by a comparator function 90, which may include an op amp 150, and a reference 92.Reference 92 may include an adjustable resistor 152 connected to a source of negative unidirectional potential. The reference 92 is connected to the inverting input of op amp 150, and the output of op amp 148 is connected to the non-inverting input of op amp 150. The output of op amp 150 may be connected to one side of the operating coil of an electromechanical relay ETS, which has its other side connected to a positive source of unidirectional potential. If the deviation does not reach the negative value selected by reference 92, the output of op amp 150 will be high and relay ETS will not be energized. If the deviation reaches the preset reference level, which is selected to represent a predetermined speed error, such as 80 FPM, the output of op amp 150 will switch low and energize relay ETS. Its contacts may be used to initiate an emergency terminal stop, as hereinbefore described.

Figure 4 also illustrates that when the transducer output ECI' is clamped, the deviation goes positive, i.e., away from the maximum deviation reference.

The deviation would also be positive should the enable signal EN go low to force the signal TACI' to zero.

Figure 5A is a graph which illustrates a first example of detection of overspeed by the solid state speed switch function. In this example, the elevator car does not start to decelerate at the proper time.

Thus, signal TACI' will stay constant while signal ECI' starts to reduce. When the difference between their magnitudes, i.e., the deviation output of function 82, reaches the maximum deviation reference at point 154, an emergency terminal stop will be initiated. In another example, setforth in Figure 5B, the elevator car started to decelerate at the proper time, but along a different-than-normal slope in which the car deceleration rate is too low. The two signals will thus deviate more slowly than in the first example, but when the deviation reaches the maximum deviation reference at 156, an emergency terminal stop will be initiated.

In summary, there has been disclosed an im proved elevator system having a solid state speed switch function which continuously monitors ter minal slowdown as a function of the distance of the elevator car from the terminal floor. Should the car speed deviate from the desired speed by a predetermined amount at any distance-to-go point as the terminal floor is being approached, the overspeed condition is immediately detected and a signal is promptly issued which initiates the desired modification of the operation of the elevator system.

Claims (6)

1. An elevator system, in a building having a plurality of floors, including upper and lower terminal floors, and a hoistway, an elevator car mounted in said hoistway, motive means for effective movement of said elevator car, control means for said motive means which controls the slowdown and stopping of said elevator car at a terminal floor, with said terminal slowdown of the elevator car normally defining a predetermined velocity curve having a predetermined slope, first and second continuous marker means in said hoistway adjacent to each terminal floor defining upper and lower terminal zones, respectively, and the distance-to-go from any point thereof to the associated terminal floor, electromechanical transducer means on the elevator car responsive to the marker means of each terminal zone for providing a first signal which decreases substantially linearly as a function of the distance of the elevator car from the associated terminal floor, means providing a second signal responsive to the actual speed of the elevator car, at least as the elevator car approaches a terminal floor in a terminal zone, means for adjusting the slope and magnitude of the first signal to correspond to the predetermined velocity curve and its predetermined slope of a normal terminal slowdown, means for providing a deviation signal having a magnitude responsive to any deviation between the first and second signals, means providing a reference signal indicative of the maximum allowable deviation, and comparator means for comparing said deviation and reference signals, and for providing a predetermined signal when the actual speed deviation exceeds the maximum allowable speed deviation.

2. The elevator system as claimed in claim 1 including modification means responsive to the comparator means for modifying the operation of the elevator system when the comparator means provides the predetermined signal.

3. The elevator system as claimed in claim 1 or 2 including means for clamping the first signal when it is reduced to a predetermined magnitude.

4. The elevator system as claimed in claim 1 or 2 including means for providing signals indicative of the travel direction of the elevator car and the presence of the elevator car in each terminal zone, and means responsive to said signals for providing an enabling signal which enables the predetermined signal to be produced by the comparator means only when the elevator car is approaching a terminal floor in a terminal zone.

5. The elevator system as claimed in claim 1 or 4 wherein the electromechanical transducer means includes a transformer having input and output windings, and means for varying the coupling between said input and output windings in response to the position of the elevator car relative to the marker means.

6. An elevator system, substantially as hereinbe fore described with reference to and as illustrated in the accompanying drawings.