HK1147235B - Multiple car hoistway including car separation control - Google Patents

Description

Multi-car hoistway including car separation controller

Technical Field

The present invention relates to elevator control systems. More particularly, the present invention relates to controlling the distance between a front elevator car and a rear elevator car traveling in the same direction in an elevator hoistway.

Background

Elevator systems are designed to minimize the required number of elevator hoistways employed in the elevator system while also maximizing the effectiveness of the needs for passenger and cargo transport within the building. Solutions aimed at reducing the number of hoistways and improving service have included: higher elevator travel speed, shorter door opening and closing times, more advanced control systems, faster elevators, building partitions, etc. However, in buildings with multiple floors, these measures may lead to discomfort when the elevator accelerates, inconvenience when the door is closed quickly, or frustration due to the use of complex systems, where passengers may have to switch between elevator cars one or more times to reach the target floor.

One way to improve passenger transport efficiency while minimizing the number of elevator hoistways is to incorporate into each hoistway a plurality of independently controllable elevator cars, each capable of servicing most or all of the floors in the building. In such systems, each elevator car must be separated from the other elevator cars by a certain distance for safe operation of the elevator cars. When two or more elevator cars are traveling in the same direction in the hoistway, it becomes important to assign operating times for each elevator car for stopping and non-stopping stations to avoid interference between elevator cars.

From the above, the present invention aims to solve the following problems: a sufficient and suitable separation distance is ensured between elevator cars traveling in the same direction in the hoistway.

Disclosure of Invention

The present invention relates to maintaining a separation distance between a front elevator car and a rear elevator car traveling in the same direction in an elevator hoistway. The shortest stopping distance of the front elevator car and the normal stopping distance of the rear elevator car are determined. The separation distance is controlled such that the difference between the normal stopping distance of the rear elevator car and the shortest stopping distance of the front elevator car is greater than or equal to a threshold distance. In other words, the separation distance is controlled such that the final shortest stopping position of the front car (which is the position at which the front car will stop in an emergency stopping condition) will be separated from the final normal stopping position of the rear car (which is the position at which the rear car will stop in a normal stopping condition) by at least a threshold distance.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.

Drawings

These and other features, aspects, and advantages of the present invention will become apparent from the following description, appended claims, and the exemplary embodiments shown in the drawings, which are briefly described below.

Fig. 1 is a schematic view of an embodiment of an elevator system including a plurality of independently controllable elevator cars operable to travel in the same direction in a hoistway.

FIG. 2 is a graph of time as a function of time showing: (a) a normal operating position and an emergency stop position of the front elevator car; and (b) a normal run position and a normal stop position of a rear elevator car traveling in the same direction as the front elevator car in the hoistway of fig. 1.

Detailed Description

The same or similar reference numbers are used throughout the drawings to refer to the same or like parts.

Fig. 1 is a schematic view of an elevator system 10, the elevator system 10 including a first elevator car 12 and a second elevator car 14 disposed vertically relative to each other in a hoistway 16. In this example, hoistway 16 is located in a building having 30 floors including floors L1-L30 and is configured to allow first elevator car 12 and second elevator car 14 to service passenger demand on most or all of the floors. Controller 18 is connected to a first elevator mechanism 20 and a second elevator mechanism 22. First elevator mechanism 20 includes mechanical components for operating first elevator car 12 and second elevator mechanism 22 includes mechanical components for operating second elevator car 14.

Elevator cars 12 and 14 are independently controlled by controller 18 (via elevator machines 20 and 22, respectively) based on service demands received at call devices at floors L1-L30. Controller 18 receives service requests from passengers at floors L1-L30 and controls elevator cars 12 and 14 to efficiently and safely transport the passengers to their respective destination floors. Controller 18 monitors and controls the position, speed, and acceleration (which may be positive or negative) of each elevator car 12 and 14 as elevator cars 12 and 14 are serviced for passenger requests. In some embodiments, controller 18 determines the position and speed of elevator cars 12 and 14 based on data provided to controller 18 by position and speed sensors in elevator machines 20 and 22, respectively.

The hoistway 16 may be configured such that: elevator car 12 serves all floors except the highest floor, which is inaccessible to elevator car 12 due to the presence of elevator car 14; and the hoistway 16 may be configured such that: elevator car 14 serves all floors except the lowest floor which is inaccessible to elevator car 14 due to the presence of elevator car 12. Alternatively, hoistway 16 may include a parking area below floor L1 such that elevator car 12 may be temporarily parked to allow elevator car 14 to service requests for floor L1. Similarly, hoistway 16 may include a parking area above floor L30 such that elevator car 14 may be temporarily parked to allow elevator car 12 to reach floor L30. It should be noted that although 30 floors L1-L30 are shown, the elevator system 10 may be adapted for use in a building that includes any number of floors. Further, although two vertically disposed elevator cars 12 and 14 are shown, the hoistway 16 may include any number of elevator cars operable to service most or all floors in a building.

When service demands require elevator cars 12 and 14 to travel in the same direction in hoistway 16, controller 18 controls the distance between elevator cars 12 and 14 to ensure that: if a sudden stop (e.g., an emergency stop) occurs in the front of the two cars, the rear of the two cars can stop at a substantially normal (i.e., controlled) rate. A "normal" stopping rate (and "under normal stopping conditions") should be understood to refer to a controlled rate at which the car is slowed and stopped at a given travel speed. Thus, the rear car may not stop in the vicinity of the elevator landing, since a "normal" stop may start at any time due to a corresponding emergency stop.

For example, if elevator car 12 at floor L13 is assigned to service a passenger request at floor L17 and elevator car 14 at floor L16 is assigned to service a passenger request at floor L20, both elevator cars move upward in hoistway 16 to service their respective needs. In this example, elevator car 14 is a front car and elevator car 12 is a rear car. Controller 18 controls elevator mechanism 20 to ensure that: at any time, if front car 14 suddenly stops under abnormal (e.g., emergency) braking conditions, rear car 12 will be able to stop under normal stopping conditions and thereafter be at least a minimum or threshold distance from front elevator car 14.

To determine the appropriate separation between elevator cars 12 and 14, controller 18 considers various parameters that make up each elevator car motion profile (profile). The parameter whose position varies over time over the course is referred to as the "motion profile" of the elevator car. For example, controller 18 may set a motion profile for each elevator car 12 and 14 that relates the maximum acceleration, maximum steady state speed, maximum deceleration, direction (up or down), and rate of acceleration (third time derivative of position) for each elevator car under normal operating conditions.

Since the speed, direction, acceleration, etc. of each car 12, 14 will change during its trajectory, the separation distance d between cars 12 and 14_sepAlso necessarily varies, i.e. the separation distance d_sepIs a dynamic value. The controller 18 determines the shortest stopping distance d of the front car by continuously (or periodically)_sslAnd the normal stopping distance d of the rear car_nstControlling separation distance d between elevator cars 12 and 14 traveling in the same direction_sep. In the above example, elevator car 14 is the front car. Shortest stopping distance d_sslIs the distance that front elevator car 14 will come to a stop when front car 14 slows at maximum deceleration. For example, when emergency braking is employed in an emergency condition, front elevator car 14 may slow down at a maximum deceleration. Shortest stopping distance d_sslAt least as a function of the speed, direction, acceleration, and rate of change of acceleration of elevator car 14, and the load in elevator car 14. For example, controller 18 may determine the speed, direction, acceleration, and load of front elevator car 14 based on data provided by sensors associated with front elevator car 14 and/or elevator machine 22. In the above example, elevator car 12 is the rear car. Rear endNormal stopping distance d of elevator car 12_nslCan be determined based on the motion profile of rear elevator car 12 stored in controller 18 and the speed, direction, acceleration, and load of rear elevator car 12. Note that the normal stopping distance d_nstUnder normal operating conditions need not be a function of the deceleration of trailing elevator car 12, but can be a function of any deceleration in trailing elevator car 12 that maintains minimum passenger comfort.

As previously described, controller 18 continuously (or periodically) determines the normal stopping distance d of trailing elevator car 12 based on the measured load and motion parameters (e.g., speed, direction, deceleration, and rate of acceleration change) of each elevator car 12 and 14_nstShortest stopping distance d from front elevator car 14_ssl. These successive (or periodic) determinations may be calculated using models that utilize simulations, numerical methods, analytical equations, or the like based on the motion profiles of elevator cars 12 and 14. Controller 18 may also compare the measured load and motion parameters of each elevator car 12 and 14 to data stored in a look-up table or the like to determine an immediate normal stopping distance d_nstAnd the shortest stopping distance d_ssl. In any event, as the speed, direction, acceleration, and load of each elevator car 12 and 14 change over time, the normal stopping distance d of the trailing elevator car 12 is determined in real time_nslShortest stopping distance d from front elevator car 14_ssl. Thus, when both elevator cars 12 and 14 are traveling at full speed, the separation distance maintained between elevator cars 12 and 14 is greater than the separation distance maintained between elevator cars 12 and 14 when the cars are just beginning to move or are nearly stopped under normal stopping conditions.

Controller 18 ensures separation distance d between cars 12 and 14_sepAt any time such that: if front car 14 is forced to stop under emergency braking conditions, rear car 12 will be able to stop under normal stopping conditions and thereby cause the distance formed between cars 12 and 14 to be greater than or equal to threshold distance d_thresh. In some embodiments, the threshold distance is about 1 or 2 floors or floor heights;in other embodiments, the threshold distance may be significantly less than 1 floor (such that each car may receive passengers on adjacent floors simultaneously) or greater than 2 floors. Threshold distance d_threshA safety margin may also be included to accommodate measurement errors that may occur in determining stopping distances for elevator cars 12 and 14. In any event, the controller 18 ensures that the following inequality is satisfied when both cars are stopped under normal stopping conditions:

d_sep＝|y_l-y_t|≥d_thresh (1)

wherein, y_lIs the resting position of the front elevator car (elevator car 14 in the example provided), y_tIs the resting position of the rear elevator car (elevator car 12 in the example provided).

To satisfy inequality (1), controller 18 also continuously (or periodically) determines the required normal stopping distance d for trailing elevator car 12 when elevator cars 12 and 14 are both moving in the same direction_nstThe shortest stopping distance d required from front elevator car 14_ssl. In particular, controller 18 controls rear elevator car 12 to ensure that: if front elevator car 14 stops at maximum deceleration, rear elevator car 12 can stop at normal deceleration and remain separated from front elevator car 14 by threshold distance d_thresh. Thus, the separation distance d_sepIs dynamic in that it is time-varying and is constantly (or periodically) determined by controller 18 during operation of trailing elevator car 12.

To understand d_sepDynamic nature of (1), assuming T_startAnd T_endAre the start time and the end time of the run of the rear elevator car 12. Suppose x_l(T) is the position of the front car at time T, x_t(T) is the position of the rear car at time T. Since the parameters (e.g., speed, acceleration, etc.) on which the stopping distance is based also vary with time, the shortest stopping distance d of the front car_ssl(T) is also a function of time. For similar reasons, the normal stopping distance d_nst(T) is also free from any timeAnd (4) change. Thus, the controller 18 ensures that for T_start≤T≤T_endIn terms of:

d_sep(T)＝|(x_l(T)+d_ssl(T))-(x_t(T)+d_nst(T))|≥d_thresh (2)

it is important to note that: d_sepVaries as a function of time, and d_threshIs a constant.

According to d_sepIf front elevator car 12 stops at a maximum deceleration, then rear elevator car 12 may stop at any location in hoistway 16 in accordance with normal deceleration parameters such that the final stopping position of rear elevator car 12 is separated from the final stopping position of front elevator car 14 by at least a threshold distance d_thresh. By controlling the separation distance d_sepSuch that stopping of the rear elevator car 12 is achieved in accordance with normal deceleration parameters, substantially avoiding, if not completely avoiding, any negative consequences on ride quality of the rear elevator car 12 other than an accidental stop.

If at any time controller 18 determines the actual distance d between cars 12 and 14_actLess than the separation distance d required at that time_sepAnd elevator cars 12 and 14 travel in the same direction in hoistway 16, controller 18 may reduce the speed of rear elevator car 12 to achieve the desired separation distance d_sep. By reducing the speed of rear car 12, the actual distance d between front car 14 and rear car 12_actIncrease and normal stopping distance d of rear elevator car 12_nstAnd decreases. Alternatively, controller 18 may stop rear elevator car 12 in accordance with normal deceleration parameters, and only then rear elevator car 12 may service its original destination without violating separation distance d again_sepThe rear elevator car 12 is restarted.

In some embodiments, controller 18 may delay the activation of rear elevator car 12 until the distance between rear elevator car 12 and front elevator car 14 is large enough that rear elevator car 12 begins moving up to rear car 12 satisfies inequality (2) at the time of the next destination. By doing so, controller 18 may not need to make frequent adjustments to continuously satisfy inequality (2) during operation of elevator car 12. In particular, in one embodiment, a method is used to determine whether a delayed start elevator car is needed. This method uses a model of the predicted motion trajectory of each car to ensure that the condition of equation (2) is met during the following car and the preceding car travel in the same direction. Let theta_l(T) is that T is more than or equal to 0 and less than or equal to T_lPredicted position of the time-front car over time T with a predicted motion trajectory model, wherein the car starts moving from its original floor at time 0 and at time T_lThen to its destination floor; let theta_t(T) is that T is more than or equal to 0 and less than or equal to T_tPredicted position of the rear car with a predicted motion trajectory model over time T, wherein the car starts moving from its original floor at time 0 and runs at time T_tAnd then to its destination floor. Assume that at a particular time, rear elevator car 12 is stopped at a floor and is ready to begin its trip to its destination floor, while front elevator car 14 has traveled T from its original floor to its destination floor_runTime unit, where 0 ≦ T_run≤T_l. In this case, controller 18 may allow rear elevator 12 to start operating only when the following conditions are satisfied:

|(θ_l(T+T_run)+π_ssl(T+T_run))-(θ_t(T)+π_nst(T)|≥d_thresh (3)

wherein T is more than or equal to 0 and less than or equal to min { T_t，T_l-T_run}；

π_nst(T) is the predicted normal stopping distance of the rear car at time T; and

π_ssl(T) is the predicted shortest stopping distance of the front car at time T.

It should be noted that since the front car has already traveled T_runTime units, so that the only time for both cars to operate is at time 0 and (a) and (b)In the middle of the minimum value of (a), wherein (a) the rear car running time T_t(ii) a (b) Remaining time T of operation of front car_l-T_run. If equation (3) is satisfied, then the rear elevator car 12 can begin operation without delay. However, if equation (3) is not satisfied, then the rear elevator car 12 may wait for a time interval and recalculate whether the condition is satisfied (at that time T)_runWill have increased). Alternatively, it is possible to do so by finding the minimum T that satisfies the following equation_delay≧ 0 to determine the required delay:

|(θ_l(T+T_run+T_delay)+π_ssl(T+T_run+T_delay))-(θ_t(T)+π_nst(T)|≥d_thresh (4)

wherein T is more than or equal to 0 and less than or equal to min { T_t，T_l-T_run-T_delay}，

Note that with respect to θ_l(T)、π_ssl(T)、θ_t(T) and π_nstThe predicted motion trajectory model of (T) may be calculated in the form of a simulation model, a numerical model, or an analytical formula.

In another embodiment, lower elevator car 12 is directed to move upward if upper car 14 is stationary, and if the distance between upper car 14 and the destination to which lower elevator car 12 is to be moved upward is less than threshold distance d_threshController 18 may delay the upward movement of lower car 12 toward its destination until upper car 14 can move upward a sufficient distance to satisfy inequality (2). Of course, the upward movement of upper car 14 may also be performed simultaneously with the upward movement of lower car 12 toward its destination. However, if upper car 14 is not ready to move up at the appropriate time (e.g., due to passenger load/unload delay), then this possible violation d is addressed_threshAnother way of solving the problem of (2) is to conditionally stop the lower elevator car 12 with the controller 18 at a position that satisfies inequality (2).

In a further embodiment, if cars 12 and 14 are in the same direction in hoistway 16Travels forward and is separated by an actual distance that is greater than the desired separation distance d_sepIs much larger and if front car 14 makes an emergency stop, controller 18 may choose to stop rear car 12 in one of three ways. First, the controller can immediately stop rear car 12 under normal stopping conditions. Second, controller 18 may allow rear car 12 to continue traveling until the actual distance between cars 12 and 14 equals separation distance d_sepAt this point, controller 18 may stop rear car 12 under normal stopping conditions. Third, the controller may continue to move the rear car 12 a predetermined distance, at which point, when stopping under normal stopping conditions is initiated, the car 12 will end up having the car 12 located adjacent to the hoistway doors for that particular floor so that passengers in the rear car 12 can exit the car 12 in the normal manner.

It should be noted that while the foregoing examples relate to a situation where both elevator cars 12 and 14 are traveling upward, similar algorithms may also be applied to elevator system 10 if both elevator cars 12 and 14 are traveling downward to service a request. In this case, elevator car 12 would be the front car and elevator car 14 would be the rear car.

Fig. 2 is a position X of front elevator car 14 traveling in the same direction in hoistway 16_lAnd position X of rear elevator car 12_tGraph of time function of. Specifically, line 30 is the time-varying position X of rear elevator car 12 traveling under normal operating conditions_tLine 32 is the position X of front elevator car 14 traveling under normal operating conditions as a function of time according to the motion profile of front elevator car 12 stored in controller 18_l. Line 34 shows the stopping position Y of front elevator car 14 as a function of time at maximum deceleration (e.g., when emergency braking is applied)_l(T). In other words, if the front elevator car 14 stopped at the maximum deceleration at any time depicted in line 32, the front elevator car 14 will stop at the corresponding location depicted on line 34 (i.e., X)_l+d_ssl) The corresponding position on line 34 is plotted on line 32 directly above the time at which it begins to stop at maximum deceleration, i.e., although the front car 14Stopping (position on line 34) at a time after the time (on line 32) at which stopping at maximum deceleration begins, although for ease of viewing the stopping position (on line 34) is shown at the same time. Line 36 shows the stopping position Y of the rear elevator car 12 over time under normal deceleration conditions in accordance with the motion profile of the rear elevator car 12 stored in the controller 18_t(T). In other words, if the rear elevator car 12 stops under normal deceleration conditions at any time depicted in the line 30, the rear elevator car 12 will stop at the corresponding location depicted on the line 36 (i.e., X)_t+d_nst) The corresponding position on line 36 is drawn directly above the time at which the normal deceleration stop is initiated on line 30, i.e., although the rear car 12 is stopped (position on line 36) at a time after the time at which the normal deceleration stop is initiated (on line 30), the stop position (on line 36) is shown at the same time for ease of viewing.

To ensure that elevator cars 12 and 14 travel a separation distance d from their start_sepSeparate, elevator car 14 begins moving upward at time 0s, as shown by line 32; while elevator car 12 remains in its home position as shown by line 30. The time period during which elevator car 12 remains in its home position is denoted delay time t_delay. In the illustrated embodiment, the delay time t_delayApproximately 3.72 seconds. When the delay time t has elapsed_delayAt this point, controller 18 causes elevator car 12 to begin moving upward. In some embodiments, the delay time t_delayIs set such that: inequality (2) is satisfied from when rear elevator car 12 starts moving upward until all service requests in the upward direction of rear elevator car 12 are satisfied. In other words, the delay time t_delayCan be set such that: during operation of trailing elevator car 12, controller 18 need not make frequent adjustments to continuously satisfy inequality (4). In other embodiments, t_delayMay be greater than necessary to provide a safe time buffer into elevator system 10 that may take into account when determining separation distance d_sepAny error in time. By allowing rear elevator car 12 to follow front elevator car 14 as close as possibleWhile at the same time ensuring d_sepSo that rear car 12 can always stop under normal deceleration conditions, the dispatch performance of elevator system 10 is improved by taking into account safety and ride quality.

In another embodiment of the invention, if cars 12 and 14 are intended to move in the same direction but are separated by an actual distance that is greater than separation distance d_sepMuch larger, rear car 12 movement may be indicated before front car 14 movement is indicated. In this manner, the time delay of front car 14 is substantially a negative time delay. Of course, for whatever reason, if front car 14 does not begin moving as planned and the actual distance between cars 12 and 14 becomes equal to separation distance d_sepController 18 may instruct rear car 12 to make a conditional stop under normal stopping conditions. Similarly, if the destination of rear car 12 conflicts with the current position of front car 14, the controller may instruct rear car 12 to make a conditional stop under normal stopping conditions until front car 14 begins to move away from rear car 12, thereby enabling rear car 12 to reach its destination.

Controller 18 detects the separation between elevator car 12 and elevator car 14 to ensure that the distance between the normal stopping position of trailing elevator car 12 depicted on line 36 and the shortest stopping position of leading elevator car 14 depicted on line 34 always remains equal to or greater than threshold distance d_thresh. For example, at about time 12.5s, stopping location 38 of trailing elevator car 12 at a normal deceleration condition (about floor 16) is a programmed threshold distance d from stopping location 40 of leading elevator car 14 at a maximum deceleration condition (about floor 17)_thresh。

The present invention relates to maintaining a separation distance between a front elevator car and a rear elevator car traveling in the same direction in an elevator hoistway. The shortest stopping distance of the front elevator car and the normal stopping distance of the rear elevator car are continuously (or periodically) determined. The separation distance is controlled such that the difference between the normal stopping distance of the rear elevator car and the shortest stopping distance of the front elevator car is greater than or equal to a threshold distance at any time. By controlling the separation distance of adjacent elevator cars traveling in the same direction, mutual interference between adjacent cars can be avoided even when an emergency situation occurs in the front car. In addition, if the front car needs to make a sudden emergency stop, the rear car can be stopped according to normal deceleration parameters, thereby minimizing the impact on the riding quality of the rear car. At the same time, the dispatch performance of the elevator system is improved by taking into account safety and ride quality by allowing the rear car to follow the front car as close as possible while ensuring the separation distance so that the rear car can always stop under normal deceleration conditions. The above-discussion is intended to be merely illustrative of the present invention and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Thus, while the present invention has been described in particular detail with reference to specific exemplary embodiments thereof, it should be appreciated that numerous modifications and changes may be made to the embodiments without departing from the broader scope of the invention as set forth in the claims that follow.

The specification and drawings are accordingly to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. In light of the above disclosure of the invention, those skilled in the art will recognize that other embodiments and modifications are possible within the scope of the invention. Accordingly, all modifications attainable by one versed in the art from the present disclosure within the scope of the present invention are to be included as further embodiments of the present invention. The scope of the invention is defined by the appended claims.

Claims (19)

1. A method for maintaining a separation distance between a front elevator car and a rear elevator car traveling in the same direction in an elevator hoistway, the method comprising the steps of:

a) determining a shortest stopping distance of the front elevator car and a normal stopping distance of the rear elevator car; and

b) controlling a separation distance between the front elevator car and the rear elevator car such that a difference between a normal stopping distance of the rear elevator car and a shortest stopping distance of the front elevator car is greater than or equal to a threshold distance.

2. The method of claim 1, further comprising:

c) iteratively repeating steps a) and b) as the front and/or rear elevator cars travel in the hoistway.

3. The method of claim 1, wherein before the front and rear elevator cars begin traveling in the same direction in the hoistway, the controlling step comprises:

delaying starting the rear elevator car until a distance between the front elevator car and the rear elevator car is such that: the front and rear elevator cars are separated by at least the separation distance when the front and rear elevator cars travel in the same direction in the hoistway.

4. The method of claim 3, wherein delaying the starting of the rear elevator car comprises:

determining a period of time 0 < T during which the rear car will move_tPredicted position θ of the rear car_tAnd an expected normal stopping distance of pi_nst；

Determining a period of time 0 < T during which the front car will move_lPredicted position θ of the front car_lAnd predicted shortest stopping distance pi_ssl(ii) a And

calculating whether the following conditions are satisfied:

|(θ_l(T+T_run)+π_ssl(T+T_run))-(θ_t(T)+π_nst(T)|≥d_thresh，

wherein T is more than or equal to 0 and less than or equal to min { T_t，T_l-T_run}，T_runIs the time that the front car has traveled, d_threshIs the threshold distance, and wherein 0 ≦ T_run≤T_l。

5. The method of claim 1, wherein if a difference between a normal stopping distance of the rear elevator car and a shortest stopping distance of the front elevator car is less than the threshold distance, the controlling of the rear elevator car comprises:

a) reducing the speed of the rear elevator car; or

b) The rear car is stopped.

6. The method of claim 1, wherein determining the shortest stopping distance of the front elevator car comprises:

measuring at least one parameter of the front elevator car, the at least one parameter selected from the group consisting of speed, direction, acceleration, load, and rate of acceleration change of the front elevator car.

7. The method of claim 6, wherein the step of determining the shortest stopping distance of the front elevator car further comprises:

calculating a stopping distance of the front elevator car at a maximum deceleration based on at least one measured parameter of the front elevator car.

8. The method of claim 1, wherein the shortest stopping distance of the front car is the stopping distance during an emergency condition.

9. The method of claim 1, wherein determining the normal stopping distance of the rear elevator car comprises:

measuring at least one parameter of the rear elevator car, the at least one parameter selected from the group consisting of speed, direction, acceleration, load, and rate of acceleration change of the rear elevator car.

10. The method of claim 9, wherein the step of determining the normal stopping distance of the rear elevator car further comprises:

calculating a stopping distance of the rear elevator car at a controlled deceleration based on at least one measured parameter of the rear elevator car.

11. The method of claim 1, wherein the threshold distance is about at least one floor.

12. An elevator system comprising:

a hoistway;

first and second elevator cars in the hoistway; and

a controller configured to: a) operating the first and second elevator cars, wherein when the first and second elevator cars are operating in the same direction in the elevator hoistway, one of the first and second elevator cars is a front elevator car and the other of the first and second elevator cars is a rear elevator car; and b) maintaining a separation distance between the first and second elevator cars such that a difference between a normal stopping distance of the rear elevator car and a shortest stopping distance of the front elevator car is greater than or equal to a threshold distance.

13. The elevator system of claim 12, wherein the normal stopping distance of the rear elevator car is a function of at least one parameter of the rear elevator car selected from the group consisting of speed, direction, acceleration, load, and rate of change of acceleration of the rear elevator car under normal operating conditions.

14. The elevator system of claim 12, wherein the shortest stopping distance of the front elevator car is a function of at least one parameter of the front elevator car selected from the group consisting of speed, direction, acceleration, load, and rate of change of acceleration of the front elevator car under emergency operating conditions.

15. The elevator system of claim 12, wherein the shortest stopping distance is a stopping distance during an emergency condition.

16. The elevator system of claim 12, wherein the controller is further configured to delay activating the rear elevator car until a distance between the front elevator car and the rear elevator car is such that: the front and rear elevator cars remain separated by at least the separation distance while the front and rear elevator cars travel in the same direction in the hoistway.

17. The elevator system of claim 16, wherein the controller is configured to delay the activation of the rear elevator car by:

determining a period of time 0 < T during which the rear car will move_tPredicted position θ of the rear car_tAnd an expected normal stopping distance of pi_nst；

Determining a period of time 0 < T during which the front car will move_lPredicted position θ of the front car_lAnd predicted shortest stopping distance pi_ssl(ii) a And

calculating whether the following conditions are satisfied:

|(θ_l(T+T_run)+π_ssl(T+T_run))-(θ_t(T)+π_nst(T)|≥d_thresh，

wherein T is more than or equal to 0 and less than or equal to min { T_t，T_l-T_run}，T_runIs the time that the front car has traveled, d_threshIs the threshold distance, and wherein 0 ≦ T_run≤T_l。

18. The elevator system of claim 12, wherein the threshold distance is at least about one floor.

19. The elevator system of claim 12, wherein if a difference between a normal stopping distance of the rear elevator car and a shortest stopping distance of the front elevator car is less than a threshold distance, the controller is configured to:

a) reducing the speed of the rear elevator car; or

b) The rear car is stopped.