US3187844A - Hydraulic elevator control - Google Patents

Description

June 8, 1965 Filed Sept. 6, 1961 FLUID CHAMBER J. F. M NAIR EI'AL HYDRAULIC ELEVATOR CONTROL 4 Sheets-Sheet 1 LEVELING VALVES um i \66 T0 FLUID SINK sYmHRo ELECTRICAL CONTROL MEANS 26 I UP-VALVE/ VALVE MEANS HYDRAULIC CONTROL gm \28 -C|RCUIT DOWN VALVE \24 I 70/ MEANS SELECTOR 74 INVENTORS A JAMES F. MAC NAIR BY PETER J. SCULLY ATTORNEY June 8, 1965 Filed Sept. 6, 1961 J. F. M NAIR EIAL HYDRAULIC ELEVATOR CONTROL 4 Sheets-Sheet 2 IE 7 I, a T

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1:33 mfg [:lg mfg 1:1 mfg [:2 g [mfg x g T] 72 INVENTORS JAMES F. MAC NAIR [F BY PETER J. SCULLY H m AW M ATTORNEY June 8, 1965 J. F. M NAIR EI'AL HYDRAULIC ELEVATOR CONTROL 4 Sheets-Sheet 3 Filed Sept 6. 1961 220V 60 CPS 1 U MM S A v U E T.. Ul B Du 5 DM 8 1. U 2 U D s U YA ;V X D l s w L U .l X M U w L L V F A D U. S s U U L1 F XDZ DSA INVENTORS F. MAC NAIR J. SCULLY JA MES PETER ATTORNEY DAL LJ United States Patent York Filed Sept. 6, 1961, Ser. No. 136,275 11 Claims. (Cl. 1137-69) This invention relates in general to hydraulic elevator controls and in particular to an automatic speed control system which provides overspeed protection, speed equalization, and smooth acceleration and deceleration in hydraulic elevator installations. The invention can be utilized in any hydraulic elevator system, but it is particularly useful in automatic elevator control systems such as described, for example, in US. Patent Number 2,913,- 070, which was issued on November 17, 1959, to Magnus N. Nyberg for an Automatic Elevator Control.

Hydraulic elevator systems were superseded forty years ago by electric elevator systems, which are much smoother in operation and much easier to control than their hydraulic counterparts. Before this happened, however, many large buildings were erected all over the World with hydraulic elevator systems, and a large number of these hydraulic elevator systems are still in operation today. In New York City alone there are approximately 300 buildings which use hydraulic elevator systems. As it happens, the structural requirements for hydraulic elevators diifer so radically from those for electric elevators that these installations cannot be changed without major structural alterations in the entire building. In most cases the necessary structural alterations are out of the question, for one reason or another, so that these hydraulic elevator systems will stay in service until their buildings are torn down and rebuilt. Therefore any improvements or modernization of the elevator service in these buildings must be made by improving the hydraulic elevator systems therein rather than replacing them with electric systems.

One such improvement is described in the above noted US. Patent Number 2,913,070, which discloses an automatic electric control circuit that replaces the cumbersome, slow acting, mechanical control systems formerly used in hydraulic elevator systems. This automatic control system obviates the requirement for skilled operators, and provides a completely automatic control system, which can be operated by the passengers if desired. But although this automatic control system was a vast improvement over manual control systems, it was by no means perfect. It delivered the passengers to their selected floors, but it gave them a rough, jarring ride in transit, particularly in the deceleration period when the elevator was approaching the selected floor. Furthermore, the system did not provide any means for equalizing elevator speed under different load conditions, whereby a heavily loaded car would move up like a snail and down like a rock while a lightly loaded car would do the opposite. In addition, the system did not provide for overspeed protection and the elevator cars were therefore liable to build up so much speed as to frighten the passengers, to give them a severe jolt when the car stopped, and to overshoot the selected stop.

This invention is addressed to the problems which were unsolved in these prior art improvementsthe problems of 1) equalizing speed under varying load conditions, (2) providing smooth, effective overspeed protection, and (3) providing smooth acceleration and deceleration. Many different methods have been used in the past in an attempt to solve these problems, but none of these prior art methods have proved workable in practice. For example, some prior art systems have tried to equalize speed by counting the number of passengers entering a car and then selecting a hydraulic drive setting based on the number of persons in the car multiplied by their hypothetical average weight. The passengers were counted by a photocell counter circuit in the car, and the hydraulic drive setting was selected in a variable hydraulic drive circuit which could be adjusted in predetermined increments in response to a signal from the photocell counter. This system, however, was an approximation at best, and in practice it was subject to grave defects. In the first place, it was relatively complex in structure and its complexity encouraged circuit failure or mis-operation. In the second place, errors were cumulative in the circuit. If the counter added a count, as counters sometimes do, these erroneous counts would accumulate from one trip to the next until the counter would indicate a full condition on an empty car. This, of course, produced a worse ride than an unequalized drive system. And finally, the counter method of equalization was not useful in solving the other two problems-smoothing deceleration and providing overspeed protection.

The deceleration problem has, in the prior art, been approached by a speed increment system similar to those used in the prior art speed equalizing systems. The speed of the car was decreased by timed increments instead of by a sudden switch from top speed to approach speed. This method, however, was also relatively complex in structure and relatively unreliable in operation. In practice it multiplied the number of jolts instead of producing a smooth transition from high speed to approach speed, and it complicated the problem of reaching the correct approach speed to achieve exact leveling without overshoot. Since the reduction of speed was timed in accordance with a hypothetical average car speed, the deceleration was too fast on slow moving cars and too slow on fast moving cars. Furthermore, this speed increment system made no contribution to overspeed protection.

The prior art approach to overspeed protection was based on a governor which was coupled between the elevator drive shaft and the hydraulic valves which controlled the movement of the drive shaft.

of the drive shaft increased the governor would close the p h draulic valves via a mechanical linkage and as the s eed of the drive shaft decreased the governor would open the hydraulic valves. This method, however, was unsatisfactory in practice because it takes much more force to close a hydraulic valve than it does to open the valve.

Therefore when the governor cut in the car would slow 7 down with a jolt and then accelerate slowly until the governor cut .in again, whereupon the car would slow down with another jolt. This produced a very unpleasant sensation, particularly on long rides in a heavily loaded car.

Other methods of solving the problems of speed equalization, smooth deceleration, and overspeed protection As the speed rnatic elevator control system of that type.

and 2 the structural elements numbered ltl through 68 have been proposed in the past, but in general they have all suffered from the above noted disadvantages-Le. they have applied to only one of the three problems instead of to all three, they have been unduly complex in nature and unreliable in operation, they have been based on hypothetical quantities, and they have not provided an adequate solution to their particular one of the three problems let alone to all three at once. In addition to these disadvantages, the prior artspeed control systems have suifered from a more serious handicap in that their action was based on factors which vary from elevator to elevator, whereby each speed control system had to be tailor made for its particular application.

, Accordingly, one object of this invention is to prov-ide a hydraulic elevator control system which provides overspeed protection, speed equalization, and smooth deceleration in hydraulic elevator installations.

Another object of this invention is to provide an automatic speed control system which simultaneously provides overspeed protection, speed equalization, and smooth deceleration in hydraulic elevator installations.

A further object of this invention is to provide an automatic speed control system of the above noted type which is simpler in structure and more reliable in operation than those heretofore known in the art.

An additional object of this invention is to provide an automatic speed control system of the above noted type which is smoother in operation and more effective than those heretofore known in the art.

Another object of this invention is to provide an automatic speed control system of the above noted type which can be applied to widely varying elevator installations without any significant changes in the automatic speed control circuit or in the elevator system.

A further object of this invention is to provide an automatic speed control system of the above noted type which is less expensive to manufacture, easier to install, and easier to maintain than those heretofore known in the art.

Other objects and advantages of this invention will be apparent to those skilled in the art from the following description of one specific embodiment, as illustrated in the attached drawings, in which:

MG. 1 is a block diagram of one illustrative hydraulic elevator and automatic elevator control system incorporating the automatic speed control of this invention;

FIG. 2 is an elevation section of one illustrative selector switch rneans and hydraulic control means for the elevator system of FIG. 1;

FIG. 3 is a schematic diagram showing one illustrative arrangement of brushes and brush wipers for the selector switch means shown in FIG. 2;

FIG. 4a is a schematic diagram of one portion of an illustrative circuit arrangement for the valve control circuit of PEG. 1;

FIG. 4b is a schematic diagram of the remaining portions of the circuit shown in PEG. 4a; and

FIG. 5 is a schematic diagram of the speed correction circuit.

Although the speed control system of this invention can be used with any hydraulic elevator, it is particularly useful in connection with hydraulic elevators containing automatic control systems, such as disclosed in the above noted U.S. Patent Number 2,913,070, and this invention will therefore bedescribed in connection with an autoln FEGS. 1

are identical with the structural elements disclosed in FIGS. 1 and 2 of the above noted US. Patent Number 2,913,070, except for valves 24c, 24d, 26c, and 26d, and the structural elements numbered ill through 92 and 24c, 24d, 26c, and 26d correspond to the novel structure of this invention. e

The prior art portions of FIG. 1 show a hydraulic elevator system in which an elevator car N is positioned up and down an elevator shaft by means of a hydraulic ram'lz. As the elevator car ltl moves from floor to floor a synchro generator 14, which is coupled to a counterweight pulley shaft 16, feeds back signals to synchro motor 18 indicating the actual instantaneous elevation of the elevator car. When an operator wishes to move the elevator to a different floor, he depresses the push button associated with the desired floor on a iloor selector panel 2%, which generates a signal indicating the selected floor. This signal is coupled to electrical control means 22. At the same time, synchro motor 13 transmits a signal to electrical control means 22 indicating the present level of the elevator car llt These two signals are compared in the electrical control means 22 and if the selected floor is below the present position of the elevator car, the down valve means 24. will be energized, causing the hydraulic control means 2% to bleed fluid from the hydraulic ram 12, thus moving the car downward. When the elevator car reaches the selected floor, the electrical control means 22 de-energizes the down valve means 24- and the hydraulic ram stops. If, on the other hand, the selected floor is above the present level of the elevator car, the electrical control means will energize the up valve means 26. l-lydraulic control means 238 will then feed fluid into by draulic ram 12, and the elevator car will start on an upward motion; This upward motion continues until the selected floor and the indicated floor are the same. At this time the up valve means 26 is de-energized and the hydraulic ram stops.

in accordance with the novel features of this invention the speed of the elevator car ltl is controlled by means of a valve control circuit ill which is coupled between electrical control means 22 and valve means 24 and 26. Valve control circuit 7d receives a signal indicating the elevator cars actual speed of movement from a tachometer 72 which is also coupled to counterweight pulley shaft 16. Valve control circuit 7d also receives signals indicating the actual state of hydraulic control means 2% from selector switch means 74 which is mechanically coupled to, the movable part of hydraulic control means Valve control circuit 7t? also receives signals from electric control means 22, which selects the direction movement and the starting and stopping times as in the prior art. Valve control circuit 7d is adapted to correlate 7 these various input signals in such manner as to provide equalization of speed under varying loads, smooth acceleration and deceleration, and overspeed protection as will be described in greater detail below.

PlGQZ shows one illustrative hydraulic control means, up and down valve means, and selector switch means which can be used in the elevator system illustrated in FIG. 1. The structural elements numbered 112 through 63 are identical with the structural elements disclosed and described in the above noted US. Patent Number 2,913,079, except for valves 24c, 24d, Zdc, and 26d and the structural elements numbered 74 through 92 and 1234a, Ede and 26d are the novel structure of this invention. .The numbers on the prior art elements in FIG. 2 are the same as tho'seused in the above noted patent and the function of these prior art elements is identical in this invention whereby the description given on columns 2 and 3 of the above noted patent will be valid except for the modifications noted below. In the prior art portions of the structure, hydraulic fluid is driven I into hydraulic ram i2 when valve stem is moved downward from the neutral position shown in 2', and hydraulic fluid is bled from hydraulic ram 12 when valve stem M is moved upward from its neutral position.

The position of valve stem 4% is controlled by solenoid operated valves il lb, 26a, andildb, which operate in CODJURCUOH with a pilot valve 32 to control the fluid pressure in fluid chamber 36. In the upward position of valve stem id, pilot valve 32 is positioned so as to open a iiuid conduction path between Valve ports 52 and].

. 5 56 thereof, which connects conduit 57 to conduit 68. In the downward position of valve stem at), pilot valve 32 is positioned so as to open a fluid conduction path between valve ports 52 and 54 thereof. This connects conduit 57 to conduit 53. In the neutral position of valve stem 4%), pilot valve 32 is also positioned in a neutral position, i.e. a position in which valve port 52 is closed so that conduit 57 is not coupled to either of the other two conduits. When valves 24a, 24b, 26a, and 26b are de-energized, valve stem 46 will be driven to its neutral position by the action of pilot valve 32, which will couple the fluid source to fluid chamber 3-6 if valve stem 49 is above its neutral position and which will couple the fluid sink to fluid chamber 36 if valve stem dil is below its neutral position.

To raise elevator car 1%, the valve stem 4% of the hydraulic valve control means is moved to its downward position by energizing solenoid valve 26a and 26!). Valve 26a introduces fluid from the fluid source into fluid chamber 36 and thus drives valve stem 40 downward. (The fluid sink is isolated from conduit 58 by valve 2612, which is closed in the energized state thereof.) When solenoid valve 26a and 26b are de-energized, valve 26b couples the fluid sink to fluid chamber 36 via conduit 58, valve ports 54 and 52, and conduit 57, thus moving valve stem 40 upward to its neutral position. To lower elevator car 19, solenoid valves 24a and Zeb are energized, which couples the fluid sink to fluid chamber 36 through valve 24a, thus driving valve stem 46 upward. When solenoids 24a and 2412 are de-energized, the fluid source is coupled to fluid chamber 36 via conduit 68, valve ports 56 and 52, and conduit 57.

It can be seen, then, that the up and down valve means operate in pairs, the normally closed member of the pair acting to couple either the fluid source or the fluid sink to fluid chamber 36 to initiate the upward or downward movement of valve stem i0, and the normally open member of the pair acting to reverse the coupling and re-center valve stem 4-!) when the valves are de energized. in this prior art arrangement, the hydraulic control means is driven either to its full open or full closed position by the up and down valve means, and is held in a neutral position in the absence of a signal to either the up or down valve means. In accordance with this invention, however, novel structure is added to partially open and partially close the hydraulic valve means to achieve slow acceleration or deceleration and also to move valve stem 49 upwardly or downwardly to compensate for overspeed or to decelerate the elevator in the approached zone. This is done quite simply by adding small solenoid valves 24c, 24d, 25c, and 26a in parallel with the prior art control valve and by coupling selector switch 74 to valve stem 46 to determine the actual position thereof. In this particular embodiment of the invention, the selector switch comprises a stationary plate 76 containing brushes and brush wipers and a movable plate 78 containing brushes and brush Wipers which engage the brushes and brush wipers on plate 76. The brushes and brush wipers are indicated schematically in FIG. 2 by the tooth-like projections on the surface of plates 76 and 78. Movable plate 73 is coupled via pulleys 8i) and S2-to the top of valve stem 40 and therefore moves up and down in synchronism with the movement of valve stem 40. The brushes and brush wipers on plates 76 and '78 are adapted to provide signals which indicate the approximate speed of the elevator car and these signals are used to control valve control circuit 79 to approximate the desired speed conditions.

In accordance with this invention valves 26a and 2512 are only energized during the high acceleration portion of the upward or downward travel. As soon as valve stem 46 reaches a position indicating that top speed is being approached, valve 26a (or 24:!) is cut out or" the circuit and is replaced by smaller valve 26c (or Ma) til for the duration of the upward or downward travel. Since valves 24c and 260 are smaller than valves 24:: and 26a, the acceleration will continue at a much slower rate when top speed is approached. If the top speed is exceeded during the upward or downward travel, valves 2 5d or are opened, by valve control circuit 70, until the speed of elevator car 1% drops back to its rated top speed. The circuit that opens valves 24d and 25d is responsive to tachometer 72, which measures the actual speed of elevator car 16. When elevator car It is near its selected fioor, the valves are de-energized and normally open valves 24b or 2615 return valve stem 49 toward its neutral position until the desired approach speed is reached. If the elevator car does not come down to its proper approach speed, valves 24d or 26d will be opened again to decelerate down to the proper speed so that the elevator car will not overshoot the selected floor. If the elevator car is traveling at or slightly below its rated top speed it will decelerate smoothly down to the proper approach speed without any opening of valves 24in or And if the elevator car reaches the proper approach speed before it comes into the leveling zone,

which extends one foot above and below the desired level, the elevator car will come to rest at the desired level without any overshoot.

it should be noted here that the tachometer means could be used by itself to perform the above noted functions, but that the combination of a tachometer with a selector switch is preferable because it provides a significant simplification in the valve control circuit. The position of the brushes on the selector switch does not indicate the exact speed or" the elevator car, since the exact speed varies as a function of load, but the position of the brushes will indicate the approximate speed within a relatively large but predetermined tolerance. Therefore the valve stem of the hydraulic control means can be positioned to an approximately correct position by a simple switching circuit which is responsive to the selector switch, and any errors due to a change in the load condition can be corrected in an equally simple correction circuit which is responsive to the tachometer. This combination provides the desirable objectives of smooth acceleration and deceleration, speed equalization, and overspeed protection without the drawbacks of complex and unreliable control circuits. It should be understood, however, that the selector switch and its associated circuits could be used independently of the tachometer and its associated circuits if desired, and that the tachometer and its associated circuits could also be used independently of the selector switch and its associated circuits. In its basic form, this invention comprises (1) a novel approximate speed control device comprising the selector switch and its associated structural elements, (2) a novel speed correction device comprising the tachometer and its associated structural elements, and (3) a novel automatic speed control system comprising the selector switch, the tachometer, and their associated structural elements.

7 FIG. 3 shows one illustrative layout for the brushes and brush wipers of selector switch 74 in this particular embodiment of the invention. There are two symmetrical sets of brushes on movable plate 8; one set corresponding to the up direction of elevator travel, and the other set corresponding to the down direction of elevator travel. The up brushes are indicated by the letter U in their designation while the down brushes are indicated by the letter D in their designation. Stationary plate 76 contains a single set of brushes and brush wipers which are adapted to engage the up brushes of movable plate 73 when the elevator is moving upward and to engage the down brushes of movable plate 78 when the elevator is moving downward. The brushes and their corresponding wipers are adapted to make and break contact at predetermined approximate speeds. The approximate speed is, of course, directly related to the position of the hydraulic valve means, although the exact relationship might difler in aromas some elevator installations. This relationship, however, can be easily determined by well known prior art techniques so that the brushes can be set to make or break at any desired approximate speed. 7

FIGS. 4a and 4b show one illustrative circuit which can be used to embody valve control circuit 7% with the particular selector switch layout shown in FIG. 3. in the circuit of PEG. 4a, the relay contacts marked UM, BM, SP, LV, and APP are actuated by relays in electrical control means 22 or in the elevator shaft. All of the other contacts are actuated by relays shown in PEG. 412. Electrical control circuit 22 can be any suitable prior art elevator control circuit which provides the following signal inputs to toe valve control circuit of this invention: (1) an up signal indicating that the elevator car is to begin moving upward (this signal actuates all of the UM contacts shown in FIG. 4a); (2) a down signal indicating that the elevator car is to begin moving downward (this signal actuates all of the DM contacts shown in FIG. 4a); a high speed signal indicating the elevator car is to make a long run (this signal actuates the Si contacts shown in EEG. 4a); a low speed signal indicating that the elevator car is to make a short run (thissignal momentarily actuates the SP contacts shown in MG. 4); a high speed approach zone signal indicating that the elevator car has reached the approach zone for its selected floor under high speed conditions (this signal de-actuates the SP contacts shown in FIG. 4a); a low speed approach zone signal indicatin g that the elevator car has reached the approach zone for its selected floor under low speed conditions (this signal actuates the AP? contacts shown in FIG. 4a); and a leveling zone signal indicating that the elevator car has reached the leveling zone for its selected floor (this signal actuates the LV contacts shown in FlG. 4a). in addition to these input signals from electrical control means 22, the valve control circuit of FIG. 4b also receives an input signal from tachometer T2 and several inputs from selector switch '74, whose individual brushes and brush wipers are identified by the same designations used in FIG. 3. The relay contacts and brushes are separated from their solenoids in FIG. 4a and FIG. 4b for clarity of illustration, but it will be understoodby those skilled in the art that contacts with the same designation are actuated simultaneously by the solenoid having the corresponding designation.

in describing the overall operation of the valve control circuit, it will be useful to first describe the speed correction portions of the circuit and then to present some exemplary sequences of operation for the circuit as a whole. Referring to FIG. 5, tachometer is coupled to relay meters 84, $6, and M, which can comprise any suitable prior art relay meter such as the Simpson model 29X voltmeter or the like. Each relay meter has a coil, which is indicated in FIG. 5 by the resistor within the box denoting the relay meter, and a relay contact circuit comprising the needle of the meter and two stationary contacts which can be adjusted to contact the needle of the meter at any predetermined point in its positive or negative direction of travel. The needle of the meter rests in the center position in the absence of a voltage input and travels either to the right or to the left when a voltage is applied thereto, depending upon the polarity of the voltage. The defied tien of the needle, of course, is proportional to the magnitude of the applied voltage. The coils of all three relay meters are coupled in parallel to tachometer '72, which can be anysuitable tachometer generator that produces a DC. voltage output proportional to its speed of rotation. T he scales of the relay meters are preferably calibrated in arms of speed rather than voltage to facilitate adjustment of the stationary contacts to intercept the needle at any d sired speed.

'l'n this particular embodimentof the invention, relay meter is adapted to close its contacts when the elevator reaches its normal running speed; relay meter 36 is adapted to close its contacts Whenthe elevator reaches its normal approach speed; and relay meter 34 is adapted to close its contacts when the'elevator exceeds its maxivmum speed. The specific values of these speeds will, of

course, depend upon the particular elevator installation for which the circuit is designed, so that the exact speed values cannot be specified in general. For purposes of explanation, however, it will be assumed that relay meter 88 is set to close at a speed of 460 feet per minute, and that relay meter 36 is set to close at feet per minute, and that relay metertld is set to close at 440 feet per minute. The operation or" the relay meters and their associated circuits can be best described by running through atypical operating cycle thereof. Assume that an up signal or a down signal is received from electrical control means 22 to initiate an upward or a downward movement of the elevator. The up or down signal actuates all of the UM or DM contacts in the valve control circuit, whereby voltage is applied to rectifier all by Way of contacts UM or DM thereby developing a positive DC. voltage on the needle of each relay meter and a negative DC. voltage on the solenoid circuits coupled to the stationary contacts of the relay meters. Other UM or DM contacts actuate the up or down valve means, and the elevator starts to move in the specified direction. When the elevator begins its motion, tachometer '72 starts to devedop a voltage output proportional to the instantaneous speed of the elevator. As the speed of the elevator rises, the output voltage of tachometer '72 rises unti the elevator reaches its normal running speed of 400 feet per minute, at which time the contacts of relay meter its close. (Relay meter 86, which closes at 110 feet per minute, is initially removed from the circuit by normally open contacts VSU and VSD which do not close until the elevator is decelerating to its approach speed.) When the contacts of relay meter 88 close, solenoid TAO?) is energized via diode D1, normally closed contacts TACD and normally closed contacts TAC3 As soon as solenoid TACS is actuated it shuts itself oil by opening normally closed contacts TAC3 A resistor Rl and capacitor Cl are coupled in parallel with solenoid TAC3 to hold the solenoid energized momentarily after its circuit is broken by the opening of contacts TAC3 Capacitor Cl. stores electrical energy while contacts TAC3 are closed and discharges its energy as soon as they open. The time constant of this RC circuit is chosen to hold solenoid TACS energized long enough to allow solenoid TAC4 to be energized by normally open contacts TAC3 After energizing solenoid TACd, solenoid TACEl is dc-energized by the discharge of capacitor Cit while solenoid TAC l latches itself closed by means of contacts TACdg and also opens the circuit to-the coil of relay meter and to the solenoid of TAC3 through normally closed contacts TACd and TAC4 The diode Dll in series with solenoid TAG? protects relay meter 83 and rectifier t ll from the inductivekickback voltage which arises when the current through solenoid TAC3 is interrupted. Resistor R1 and capacitor C1 further reduce the inductive kickback by smoothing the transiston between the energized and deenergized states of the solenoid. In summary, the closure of relay meter 83 acts to energize solenoid TACd, which indicates that the elevator has reached its normal running speed, and to remove relay meter 88 from the circuit for the duration of the elevator run to protect its contacts from being overloaded. The actuation of solenoid TACd switches other circuits that terminate motion in the hydraulic control means to theoretically hold the elevator at its normal running speed.

In certain circumstances, however, the elevator may continue to accelerate after having reached its normal running speed until it exceeds its maximum speed of 440 feet per minute This occurs, for example, in a heavily loaded car which is moving downwardly. When the maximum speed is reached, the contacts of relay meter 34 will close, which will momentarily energize solenoid TAC through diode D2 and normally closed contacts TAC Before solenoid TAC has time to energize, however, the

Q voltage applied through diode D2 will be coupled directly to capacitor C2 through normally closed contacts TAC This will charge capacitor C2 up to the full value of the DC. output voltage of rectifier 9%. When the contacts of solenoid TAC close, however, the solenoid circuit is immediately broken by the opening of contacts TAC and capacitor C2 is coupled to resistors R2 and R3 by the opening of contacts TAC2. Capacitor C2 then discharges through resistors R2 and R3 to hold solenoid TAC in the energized conditions for some predetermined length of time which is controlled by the setting of R3. When capacitorCZ has discharged, solenoid TAC will be deenergized, but as soon as it returns to the de-energized state it will be immediately re-energized if the contacts of relay meter 84 are stillclosed. (Diode D2 protects relay meter hdandrectifier 9% from inductive kickback.) Thus solenoid TAC will produce periodic pulses as lon as the contacts of relay meter 84 remain closed. The time duration of these pulses is adjustable within a reasonable range by the setting of variable 'resistorRB. This pulsation is transferred to solenoid TAC2 by means of contacts TAC so that solenoid TAC2 will also produce periodic pulses of a predetermined time duration as long as the contacts of relay meter 84 remain closed. (Resistor R4 protects the contacts of relay meter 84 when they open.) The pulsations of TAC2 are coupled to the elevator speed reduction valve described previously, and each pulse serves to reduce the speed of the elevator by a speed increment which is proportional to the duration of the pulse. Thus the pulses will reduce the elevators speed by small increments until it falls below the rated top speed of 440 feet per minute at which time the contacts of relay meter 84 Will open again and the pulsations will cease.

The frequency of the above described pulsations is determined jointly by the response time of relays TAC and TAC2 and the pulse duration selected by the setting of adjustable resistor R3. It is not possible to specify the exact frequency or pulse duration of the circuit, however, because the best frequency and pulse duration will be determined by the particular hydraulic valve means used in connection With the circuit and the particular elevator system with which the elevator is used. It should be noted, however, that a pulsating overspeed protection circuit is not essential to the basic form of this invention. The pulsating circuit of this particular embodiment is used to protect the meter relay contacts from being welded by inductive kickback current when relay TAC is de-energized. With the pulsating meter circuit, the relay meter contacts only act to make the TAC relay circuit, which then breaks itself. This protects the relay meter contacts from the inductive kickback current. if a buffer amplifier is coupled between the meter relay contacts and the TAC relay, however, relay TAC could be energized continuously as long as the meter relay contacts remain closed. The overspeed circuit would op-' crate in the manner described above except that the elevator would be decelerated continuously until it drops below its maximum speed instead of being decelerated in pulses. The above described overspeed circuits can be mechanized by many other suitable circuit arrangements which will be apparent to those skilled in the art. For

example, the relay meters could be replaced by electronic threshold circuits of one type or another, and the relays could be replaced by flip-flops if desired.

Due to the action of the above described overspeed circuit, the elevator will be traveling somewhere between its normal running speed of 400 feet per minute and its maximum speed of 440 feet per minute when it enters the approach zone for its selected floor. In this particular example the approach zone extends for approximately 15 feet above and below the selected floor level. When the elevator enters the approach zone for its selected floor, a set of approach contacts close in the elevator shaft and a decelerate signal is generated by electrical control means 22 to initiate deceleration to the approach speed of feet per minute. In the valve control circuit this decelerate signal returns all of the SP contacts to their normal condition, which begins a smooth deceleration process in the hydraulic control means. At the time the elevator should have theoretically reached its approach speed, the VSU or VSD relay is energized, which closes the circuit to the coil of relay meter 86 through normally closed contacts SP and normally open contacts VSU or VSD If the elevator is going faster than its desired approach speed, the contacts of relay meter 86 will close and the elevator will be pulsed down to its approach speed by the same pulser circuit which was used to limit the elevators maximum speed. The elevator will then come into its leveling zone at the correct approach speed so that it will come to rest at its selected floor level without overshoot. The leveling zone in this particular example extends approximately one foot above and below the selected iioor level. When the elevator comes into the leveling zone the UM or DM and VSU or VSD contacts return to their normal condition and solenoid TACd is de-energized. This returns the valve control circuit to its original condition and the hydraulic control means then returns to its neutral position, thus smoothly decelerating the elevator from its approach speed to a stop at the selected floor.

The other portions of the valve control circuit can also be best explained by running through some illustrative el vator operating cycle. Assume, for example, that the elevator receives a signal from electrical control circuit 22 indicating that the elevator car is to move upward on a long run (two or more floors). This signal actuates all of the UM and SP contacts shown in the circuitry of FIG. 4a. The first effect of these closures is to apply power to all of the solenoid circuits through normally open contacts UM and to energize solenoids UF, UFA, and USA through their respective UM and SP contacts. These solenoids in turn energize control valves 26a, 26b, and Zea, which start the elevator moving upward at a relatively high rate of acceleration. In this particular embodiment of the invention, the CU contacts of selector switch '74 break when the elevator reaches a speed of approximately 300 feet per minute, thereby dc energizing solenoid UFA and dropping out the large valve 26a (FIG. 2), thus reducing the elevators rate of accelera tion. The elevator then continues to accelerate toward the normal running speed but at a slower rate which is determined by the size of small valve 250. When the elevators speed reaches approximately 360 feet per minute, contacts DU of selector switch 74 make up, thereby energizing solenoid UP which latches itself closed via normally open contacts UP Solenoid UP provides an extra measure of overspeed protection for the circuit. If, for example, a short were to exist around the CU brushes of selector switch 7 5, the UFA solenoid would not drop out when it was supposed to and the elevator would continue to increase its speed at a high rate of acceleration. This possibility is precluded by the opening of normally closed contacts UP which will dc-energize the UFA solenoid even if the CU brushes are shorted. Solenoid UP also energizes solenoid HS, which latches itself closed through normally open contacts H8 Solenoid HS acts to open up the circuit to the single floor (slow speed) solenoids X, XU, and XD so that they will be removed from the circuit in a high speed run. TheX, XU, and XD solenoids are only used on single floor runs, which will be described later.

After the DU contactshave made up, the elevator continues to accelerate at a low rate of acceleration until it reaches it normal running speed of 400 feet per minute at which time solenoid TACd will be energized by relay meter 88 as described previously. When solenoid TAC4 is energized, solenoid UAL will be energized via normally open contacts TAC4 and UM and this will deenergize solenoid USA by opening normally closed contacts UAL As a safety measure, normally closed consci a with solenoid UFA to the CU brushes should tacts UAL are added in de-energize this solenoid in case be short circuited and the DU brushes or the UP solenoid in position so as to terminate all acceleration due to movement of the hydraulic control means. Under ideal conditions this would hold the elevator at its normal running. speed of 400 feet per minute.

But suppose, for one reason or another, that the ole vator continues to accelerate after it has reached its normal running speed and that its speed exceeds the maximum speed of 440 feet per minute. This can happen when an empty car is moved upwardly, or when a fully loaded car is moved downwardly, or when leakage develops in normally closed valves or 26a. in this case the pulsating overspeed circuit will be actuated and solenoid TACZ will pulsate as long as the overspeed condition persists. The pulsation of solenoid TACZ will actuate solenoid USD via normally opening contacts TAC2 and the pulsations of solenoid USD will pulsate the hydraulic decelerating valve 2nd by means of normally open contacts USD and USD The elevator will then be decelerated by predetermined increments until it has fallen below the maximum speed, at which time the pulsations will terminate. The overspeed circuit will cut in as often as necessary to hold the elevator below its maximum speed during the run.

When the elevator reaches the approach zone for its selected floor, all of the SF contacts will return to their normal condition, in response to a signal from electrical control means 22, and solenoid UP will be ole-energized, thereby de-energizing control valve 26b. When control valve 26b is de-energized the elevator starts to decelerate 7 down toward its approach speed, and the deceleration process is stopped by re-energizing solenoid UF when the approximate approach speed is reached. Solenoid UP is re-e nergized through normally open contacts VSU which are actuated by the closure of the EU contacts on selector switch means '74. The EU contacts are set to make at approximately 280 feet per minute, but since the hydraulic valve means is in motion when the EU contacts make, and since a certain amount of time is required to close the contacts of solenoid VSU, the elevator speed should under normal conditions be decelerated down to about 110 feet per minute when the UP solenoid is re-energized. The UP solenoid re-energizes hydraulic control valve 26b to terminate deceleration of the elevator. If the elevator speed is above 110 feet per minute when the deceleration period is terminated, however, the elevator will be pulsed down to the correct approach speed by means of solenoid USD and solenoid TACZ, which will be pulsed by relay meter as until the elevator drops below the correct approach speed.

When the elevator comes into its leveling zone, solenoid 22-, and a level sensor in the elevator shaft. The level sensor can comprise any suitable prior art device, as for example the level sensor described in said U. S. Patent Number 2,913,070. The leveling valves can comprise any suitable prior art valve arrangement for adding a small amount of fluid to or bleeding a small amount of fluid from hydraulic ram 12 when hydraulic control means 23 is in its neutral position. (it will be noted, in FIG. 2, that conduit )2 is coupled directly to hydraulic ram 12 in the neutral position of valve stem id.) The leveling valve control circuit can comprise any suitable circuit adapted to switch conduit 92 to the fluid source when the elevator car is below its desired level, as indi cated by the output of the level sensor, and to switch conduit @2 to the fluid sink when the elevator car is above its desired level. The exact details of the level sensor, leveling valves, and leveling valve control circuit will not be disclosed in. this document, since this invention is concerned with speed control rather than with leveling, but leveling systems of this type are well known to those skilled in the art and any suitable prior art leveling systems can be used in connection with this invention.

For a high speed run in the down direction, the above described cycle of operation is identical except for direction. Motion in the downward direction is controlled by down direction solenoids VSD, DP, DSA, BSD, DPA and DAL. Each of these down direction relays is analogous to the correspondingup direction relay described above and each connected in an identical contact circuit to duplicate the above described sequence of operation for long runs in the down direction. The relay meter circuits, of course operate the same for either the upward or downward direction of travel.

On short runs in the up or down direction, the operation of the up and down relays is controlled by the XU, XD, and X relays shown in FIG. 4b. As mentioned earlier, a hort run is initiated by momentarily closing the 8? contacts and permanently closing either the UM or DM contacts depending on whether the short run is in the up or down direction. When the SP contacts are momentarily closed, solenoids XU and X1) will be energized by normally closed contacts X H8 and SP as soon as the SP contacts are returned to their normal position. If the short run is to be in the up direction, solenoid XU will energize solenoids UP, USA, and UFA through normally open contacts XU XU and XU These solenoids will in turn energize hydraulic valve 26a, 26b, and 26c, which will accelerate the elevator car upward at a fast rate of acceleration. When the speed of the elevator reaches approximately 150 feet per minute, contact PU of selector switch 74 breaks and de-energizes solenoid UFA, hereby dropping out the fast acceleration hydraulic control valve 26a. The upward acceleration then continues at a slower rate until the elevator reaches approximately 250 feet per minute, at which time contacts EU of selector switch 74 breaks and de-energizes solenoid USA, which drops out hydraulic control valve 260 and stops all acceleration in the hydraulic control means. The elevator then travels at approximately 250 feet per minute until it reaches the slow speed approach zone for the selected floor, which is indicated by the closure of a set of contacts APP which are actuated by vanes in the elevator shaft.

The closure of contacts APP energizes solenoid X, which its correct approach speed before it reaches the leveling circuit can comprise a set of leveling valves coupled to conduit 92 of hydraulic control means 28 (PEG. .2), a

leveling valve control circuit in electrical control means In most cases, however, itwill be latches itself closed through normally open contacts X When solenoid X is energized it breaks normally closed contacts X thereby tie-energizing solenoids XU and dropping out all of the hydraulic control valves in the hydraulic control means. The elevator then decelerates down towards its approach speed, and the deceleration is arrested at the approximate approach speed by the EU contacts of the selector switch'rneans and solenoid VSU, which operates on the slow speed run just as it did on a high speed run. If the speed of the elevator is above the desired approach speed when solenoid VSU is energized, the speed willbe pulsed down by solenoid TACZ,

which will be apparent to those skilled in the art.

herein as the selector switch means.

just as it was in a high speed approach. When the elevator enters its leveling zone, which is the same on slow speeds as it is on high speeds, the VSU solenoid is de-energized by contact LV and the elevator decelerates to stop at the selected floor level. The UM contacts then drop, thus returning the valve control circuit to its original condition, and the leveling circuit takes over to level the elevator. It will be noted that deceleration relay USD is removed from the circuit by contacts XU.; during the accelerate and constant speed portions of the short run. This is necessary to prevent USD from being energized through contacts SP which are only opened momentarily in the short run. Relay USD is, however, connected back into the circuit in the deceleration portion of the run, which is initiated by de-energizing relay XU.

The above described sequence of operation is repeated for short runs in down direction by means of the X relays and the down direction relays. On high speed runs in either direction the X relays are removed from the circuit by normally closed contacts H8 which open at a speed of approximately 360 feet per minute in this particular embodiment. During the short run, of course, the elevator does not have time to reach 360 feet per minute and contacts H3 consequently remain closed for the duration of the short run.

From the above described sequence of operations it will be apparent that the valve control circuit of this invention utilizes the signals from the selector switch '74 to control the approximate speed conditions of the elevator, and that the relay meter circuits are used to make adjustments if the approximate speeds are not within predetermined limits of the actual speed of the elevator car as measured by tachometer 72. It will also be noted that smooth accelerations and decelerations are achieved in part by utilizing the natural inertia of the hydraulic valve means, which take time to move from one position to another. The rate of acceleration set by the mechanical inertia of the hydraulic valve means can be increased artificially if desired by coupling a dash pot or the like to the stem 40 of the hydraulic valve means. It will further be apparent that approximate overspeed protection is provided by the selector switch circuit per se, and that this overspeed protection is augmented by exact overspeed protection from the tachometer and relay meter circuits. Therefore, it will be clear that the automatic speed control system of this invention provides smooth acceleration and deceleration, equalization of speed under varying load conditions, and overspeed protection for hydraulic elevator systems.

Although this invention has been described and illustrated with reference to a specific embodiment thereof, it should be understood that the invention is by no means limited to specific structure disclosed herein, since many modifications can be made in that structure without departing from the basic teaching of this invention. For example, although it is preferable to integrate the approximate speed and exact speed control means, as disclosed herein, it is not necessary to do so in every embodiment of the invention. The approximate speed control means and exact speed control means can be used independently of each other if desired by making circuit alterations Furthermore, it is not necessary to use the exact circuits shown herein to generate the deceleration pulses, or to use the particular brush and Wiper structure Shown A rotary selector switch can be used if desired, and the pulsating relay circuits could be replaced by continuously energized circuits Without altering the basic operation of the overspeed circuit. These and many other modifications of the disclosed structure will be apparent to those skilled in the art, and this invention includes all modifications falling within the scope of the following claims.

We claim:

1. A hydraulic elevator speed control system for use in combination with a hydraulic elevator system containing an elevator car, a hydraulic ram adapted to move said car up and down, and hydraulic valve means cou pled to said hydraulic ram to control the operation thereof, said hydraulic valve means containing a movable member adapted to regulate the flow of hydraulic fluid therethrough, said speed control system comprising tachometer means coupled to said elevator to measure the speed of movement thereof, said tachometer means being adapted to produce an output signal proportional to the speed of said elevator car, threshold circuit means coupled to said tachometer means, said threshold circuit means having an oil state and an on state and being adapted to switch from its off state to its on state whenever the output signal of said tachometer exceeds a predetermined value and being adapted to switch from its on state to its 0d state whenever the outputsignal of said tachometer drops below said predetermined value and valve control means coupled between said threshold circuit means and said movable member of said hydraulic valve means, said valve control means being adapted to move said movable member in response to the on state of said threshold circuit means in such direction as to reduce the flow of hydraulic fluid through said hydraulic valve means by a predetermined amount, thereby reducing the speed of said elevator car down to a speed corresponding to said predetermined value of said tachometer output signal.

2. A hydraulic elevator speed control as defined in claim 1 and also including selector switch means coupled to said movable member of said hydraulic valve means, said selector switch means being adapted to generate output signals in response to predetermined positions of said movable member, said selector switch means being coupled to said valve control means, and said valve control means being responsive to the output signals of said selector switch to retard movement of said movable member beyond a predetermined position thereof.

3. The combination defined in claim 2 wherein said threshold circuit comprises a relay meter and a relay multivibrator circuit coupled to said relay meter, and wherein said valve control circuit includes a pulser valve adapted to move said movable member when actuated, and wherein said output pulses from said relay multivibrator serve to actuate said pulser valve.

4. The combination defined in claim 3 wherein said selector switch means comprises an electrical switch having a fixed switch section and a movable switch section and switch contacts communicating thereinbetween, said movable switch section being coupled to said movable member of said hydraulic valve means, and said contacts being adapted to close and to open at predetermined positions of said movable switch section.

5. A hydraulic elevator control system for use in combination with a hydraulic elevator system containing an elevator car and a hydraulic ram adapted to move said car up and down, said elevator control system comprising a source of hydraulic fluid under pressure, a hydraulic fluid sink, a hydraulic control valve coupled between said hydraulic ram and said source of hydraulic fluid and said hydraulic fluid sink, said hydraulic control valve having a movable member therein adapted to control the flow of iydraulic fluid therethrough, said movable member being movable in an up direction in which said hydraulic ram is coupled to said hydraulic fluid source and a down direction in which said hydraulic ram is coupled to said fluid sink and having a neutral position in which fluid flow is blocked by said hydraulic control valve, the rate of fluid flow through said hydraulic control valve being proportional to the displacement of said movable member from said neutral position thereof, up valve means coupled to said movable member of said hydraulic control valve, said up valve means being operable when energized in a first mode of operation to move said movable memher in the up direction thereof, and being operable when coupled to said relay meter.

. answers.-

energized in a second mode of operation to hold said movable member fixed in position, and being operable when de-energized to return said movable member to the neutral position thereof, said up valve means containing a first pulser valve adapted to move said movable member toward the neutral position thereof when energized, down valve means coupled to said movable member of said hydraulic control valve, said down valve means being oper able when energized in a first mode of operation to move said movable member in the down direction thereof, and being operable when energized in a second mode of operation to hold said movable met iber fixed in position, and being operable when tie-energized to return said movable member to the neutral position thereof, said down valve means containing a second pulscr valve adapted to move said movable member toward the neutral position thereof when energized, valve control means coupled to said valve means and said down valve means, said valve control means bieng adapted to energize said up valve means in its first mode of operation to move said elevator car upward and being adapted to de-energize said up valve means to terminate said upward movement of said car, said valve control means being adapted to energize said down valve means in its first mode of operation to move said elevator car downward and being adapted to de-* energize said down valve means to terminate said downward movement or" said car, selector switch means coupled between said movable member of said hydraulic control valve and said valve control means, said selector switch means being responsive to the position of said movable member and being operable to generate output signals at predetermined positions in tl e up and down direction of said movable member, said predetermined positions corresponding to approximate elevator car speeds in the up and down direction of movement thereof, said 'valve control means being operable to switch said up and down valve means from their first to their second mode of operation in respo se to said output signals from said determined level and being adapted to switch from its I on state to its off state when the output signal of said tachometer drops below said predetermined level, a pulser circuit coupled to said threshold circuit, said pulser circuit being adapted to produce periodic output pulses in response to the on state of said threshold circuit, said pulser'circuit being coupled to said valve control means, and said val e control means being adapted to energize said first and second pulser valves in response to the output pulses of said pulser circuit.

6. The combination defined in claim 5 wherein said up valve means and down valve means are adapted to be energized in a plurality of sub-modes of operation in the first mode of operation thereof, each sub-mode of operation corresponding to a different speed of movement of said movable member in the upward and downward direction thereof, and wherein said valve control means is adapted to switch said up and down valve means from one sub-mode of operation to another in response to 7 output signals from said selector switch means.

'7. The combination defined in claim 6 wherein said threshold circuit comprises a relay meter, and wherein said pulser circuit comprises a relay multivibrator circuit 3. The combination defined in claim 7 wherein said selector switch means comprises an electrical switch having a fixed switch section and a movable switch section and switch contacts communicating thereinbetween, said movable switch section being coupled to said movable member of said hydraulic valve means, and said contacts being adapted to close and to open at predetermined positions of said movable switch section.

9. The combination defined in claim 3 wherein said hydraulic control valve comprises a hollow valve casing having a movable valve stem mounted therewithin, said valve casing having a first, second, third, and fourth openings formed therein, said first opening being adapted to be coupled to said hydraulic ram, said second opening being adapted to be coupled to said hydraulic fiuid source, said third opening being adapted to be coupled to said fiuid sink, and said fourth opening being adapted to be coupled to said up and down valve means, a first piston member rigidly attached to said valve stem within said valve casing, said first piston member being adapted to couple said first opening to said second opening when moved in a first direction and being adapted to couple said first opening to said third opening when moved in the other direction and having a neutral position in which said first opening is disconnected from all of said other openings, the degree of coupling between said first and second and first and third openings being proportional to the displacement of said first piston member from the eutral position thereof, a second piston member rigidly attached to said valve stem within said valve casing, said second piston member having a first face which is coupled to said second opening in all positions of said valve stem and a second face which is coupled to said fourth opening in all positions of said valve stem, said first face having a smal er surface area than said second face so as to develop a net force on said second piston member when the fluid pressure at said second and fourth openings is equal, and wherein said up and down valve means is adapted to couple said fourth opening to said hydraulic fluid source to move said valve stem in one direction and to couple said fourth opening to said hydraulic fluid sink to move said valve stem in the other direction and to close said fourth opening to hold said valve stem in position.

ill. The combination defined in claim 9 wherein said up and down valve means comprises a pilot valve having a second hollow valve casing and a second movable valve stem mounted therewithin, said second hollow valve casinghaving first, second, and third openings formed therewithin, a third piston member rigidly attached to said second valve stem, said third piston member being adapted to couple said first opening to said second opening when rnoved in a first directionand being adapted to couple said first opening to said third opening when moved in the other direction and having a neutral position in which said first opening is disconnected from said second and third openings, said second valve stem being coupled to said first mentioned valve stem to follow the movement thereof and being aligned with said first mentioned valve stem so as to be in its neutral position when said first mentioned valve stem is in its neutralposition, said first opening of said pilot valve being coupled to said fourth opening of said hydraulic control valve, said second opening of said pilot valve being coupled to said fluid source through a first normally open valve, said third opening of said pilot valve being coupled to said fluid sink through a second normally open valve, said fourth opening of said hydraulic control valve being coupled to said fluid source via a first normally closed valve and being coupled to said fiuid sink via a second normally closed valve, a first normally closed pulser valve coupled in parallel with said first normally open valve, a second normally closed pulser valve coupled in parallel with said second nor mally open valve/ l. The combination definedin claim ill wherein said up valve means is energized in its first mode of operation by simultaneously energizing one of said normally open and one of said normally closed valves, and wherem said 5 energized in its first mode of operation by simultaneously energizing the other of said normally open and the other of said normally closed valves, and wherein said down valve means is switched to its second mode of operation by de-energizing said other normally closed valve, and wherein said down valve means is de-energized by deenergizing said other normally open and normally closed valves.

References Cited by the Examiner UNITED STATES PATENTS 2,913,070 11/59 Nyberg l8729 3,056,469 10/62 Wilson 187-29 3,105,573 10/63 Leveski 187-29 ORIS L. RADER, Primary Examiner. MILTON O. HIRSHFIELD, Examiner.

Claims (1)

1. A HYDRAULIC ELEVATOR SPEED CONTROL SYSTEM FOR USE IN COMBINATION WITH A HYDRAULIC ELEVATOR SYSTEM CONTAINING AN ELEVATOR CAR, A HYDRAULIC RAM ADAPTED TO MOVE SAID CAR UP AND DOWN, AND HYDRAULIC VALVE MEANS COUPLED TO SAID HYDRAULIC RAM TO CONTROL THE OPERATION THEREOF, SAID HYDRAULIC VALVE MEANS CONTAINING A MOVABLE MEMBER ADAPTED TO REGULATE THE FLOW OF HYDRAULIC FLUID THERETHROUGH, SAID SPEED CONTROL SYSTEM COMPRISING TACHOMETER MEANS COUPLED TO SAID ELEVATOR TO MEASURE THE SPEED MOVEMENT THEREOF, SAID TACHOMETER MEANS BEING ADAPTED TO PRODUCE AN OUTPUT SIGNAL PROPORTIONAL TO THE SPEED OF SAID ELEVATOR CAR, THRESHOLD CIRCUIT MEANS COUPLED TO SAID TACHOMETER MEANS, SAID THRESHOLD CIRCUIT MEANS HAVING AN "OFF" STATE AND AN "ON" STATE AND BEING ADAPTED TO SWITCH FROM ITS "OFF" STATE TO ITS "ON" STATE WHENEVER THE OUTPUT SIGNAL OF SAID TACHOMETER EXCEEDS A PREDETERMINED VALUE AND BEING ADAPTED TO SWITCH FROM ITS "ON" STATE TO ITS "OFF" STATE WHENEVER THE OUTPUT SIGNAL OF SAID TACHOMETER DROPS BELOW SAID TACHOMETER EXCEEDS AND VALUE CONTROL MEANS COUPLED BETWEEN SAID THERSHOLD CIRCUIT MEANS AND SAID MOVABLE MEMBER OF SAID HYDRAULIC VALVE MEANS, SAID VALVE CONTROL MEANS BEING ADAPTED TO MOVE SAID MOVABLE MEMBER IN RESPONSE TO THE "ON" STATE OF SAID THRESHOLD CIRCUIT MEANS IN SUCH DIRECTION AS TO REDUCE THE FLOW OF HYDRAULIC FLUID THROUGH SAID HYDRAULIC VALVE MEANS BY A PREDETERMINED AMOUNT, THEREBY REDUCING THE SPEED OF SAID ELEVATOR CAR DOWN TO A SPEED CORRESPONDING TO SAID PREDETERMINED VALUE OF SAID TACHOMETER OUTPUT SIGNAL.

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