Figure 3 The combination of the calls
Figure 4 The cancellation of up calls
In Figure4, the auxiliary relay M4.0 is the up moving flag of the elevator. When the
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current direction of the elevator is up, M4.0’s contacts are closed; on the contrary, when the current direction of the elevator is down, M4.0’s contacts are opened. M0.1 to M0.7 denotes the car-calls’ stopping request signal of floor 2 to floor 8 respectively.
This program has two functions:
(1) Make the elevator response the normal down hall-calls when it is moving down, and when a down hall-call is served, its registration is canceled.
(2) When the elevator is moving up, the corresponding floor’s down hall-call it passing by is not served and the registration is remained.
The cancellation of down hall-calls is reversed with up hall-calls. 3.4 Elevator’s direction
The elevator may be moving up or down, depending on the combination of hall-calls and car-calls. The following ladder diagram in Fig.5 illustrates that the elevator will move up.
Figure 5 Up moving of the elevator
Figure5 shows that when the calls corresponding floor is higher than the elevator’s current location, the elevator will go up. Here the auxiliary relay M4.0 is used as the up-moving flag. When the elevator is moving up, the up-moving lamp is illuminated, so
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the M4.0 is connected on. When the elevator arrives the top floor, the up-moving lamp is off and the timer starts. After 0.2s, the M4.0 is disconnected, the up-moving display is off. Here we used M4.0 to replace Q3.1 which can ensure the cancellation’s reliability. 3.5 Elevator’s floor-stopping
Figure6 shows the ladder diagram of the elevator’s floor-stopping function.
As showed in Figure6, M6.4 is the flag of floor-stopping signal. M6.6 is the floor-stopping signal sent by the driver. M7.0 is the fire signal sent by the fire switch. And M6.7 is the forced speed changing signal. When either of these contacts act, the system should send out the floor-stopping signal.
Figure 6 The elevator’s floor-stopping
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4. Minimum waiting time algorithm
In traffic of elevator systems, there are two types of control task usually. The one is the basic control function to command each elevator to move up or down, to stop or start and to open and close the door. The other is the control of a group of elevators.
The main requirements of a group control system in serving both, car and hall calls, should be: to provide even service to every floor in a building; to minimize the time spent by passengers waiting for service; to minimize the time spent by passengers to move from one floor to another; to serve as many passengers as possible in a given time[1].
There are many dispatching algorithms for elevator’s group control. Such as Nearest-neighbor Algorithm[2], which the elevator always serve the closet request next; Zoning Algorithm[3] which by analyzing the traffic of elevator system with unequal floor and population demand to dispatch the elevator; and Odd-even rule, which an elevator only serves the odd floor and the other only serves the even floor.
The Nearest-neighbor Algorithm minimizes the length of the elevator’s empty move to the next request. It usually has very small average waiting times, but individual waiting times can become quite large[2]. The Zoning Algorithm usually used in buildings which has heavy traffic situations, such as the office building at lunch time.
Compared to the office building and shopping mall, the traffic flow of residential buil dings is relatively low and even in every floor. Secondly, people usually think of elevators as purely functional objects and the experience of riding an elevator is time waited for most of them. Furthermore, there exist immense problems when attempting to satisfy all requirements.
Considering all of the reasons above, we adopted the “minimum waiting time” algorithm to realize the 2 elevators’ parallel running[4]. 4.1 Evaluation function
The goal of the “minimum waiting time” algorithm is to predict the each elevator’s response time according to all calls, and select the elevator which has the shortest response time to serve.
When there is a call, the system calculates out the function values of each elevator according the evaluation function showed in (1) and (2):
J(*)=Min[J(1),J(2),…,J(n)] (1) J(i)=Tr(i)+KTd(i)+KTo(i) i=1,2,...,n (2)
J(i) is the evaluation index of each elevator; Tr(i) denotes the time of the elevator directly moving to the destination corresponding the latest call from its current floor; To(i) denotes the additional acceleration and deceleration time of a floor-stop of the elevator; Td(i) denotes the average time of the passenger boarding and alighting the elevator; and K is the sum of hall-calls and car-calls. But when a hall-call and a car
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call corresponds the same floor, the K is only calculated one time. 4.2 Calculation of minimum waiting time
In equation (2), K is a certain value, To and Td can be obtained by means of statistics. Tr = T*L, where T denotes the average time of the elevator passing by one floor; L denotes the desired floors of the elevator from current floor to the hall-call floor.
In order to calculate the L value, we defined the 2 elevators are A and B respectively; YA,YB denotes the current floor of elevator A and B respectively. H is the corresponding key value when a hall-call button is pressed, and H=floor number of the hall-call.
We defined 4 tables for the PLC realization: up hall-call registration table, down hall-call registration table, car-call registration table of A and car-call registration table of B. When a certain call button is pressed, its floor value is recorded in corresponding table.
Here we take elevator A as an example. First, define the variable MA, MB and MW. Where MA, MB denotes the extreme value of car-calls with same direction of A or B’s movement respectively.
When elevator A is up-moving, set MA is equal to the maximum value in car-call registration table A; when elevator A is down-moving, set MA is equal to the minimum value in car-call registration table A.
MW denotes the extreme value of hall-calls with same direction of A’s movement.
When elevator A is up-moving and up-hall-call value≥YA, set MW=0; otherwise, set MW is equal to the minimum value in up-hall-call registration table A. When elevator A is down-moving and up-hall-call value≤YA, set MW=0; otherwise, set MW is equal to the maximum value in down-hall-call registration table A .
Thus, we can determine the L value according to YA, H, MA and MW. There are 3 situations:
(1) When the hall-call’s direction is opposite to elevator A’s movement:
L=|YA-MA|+|MA-H| (3)
(2) When the hall-call’s direction is same as elevator A’s movement and it is in the front of elevator A:
L=|YA-H| (4)
(3) When the hall-call’s direction is same as the elevator A’s movement and it is in the back of elevator A:
L=|YA-MA |+|MA-MW|+|H-MW| (5)
So the i-th floor’s minimum waiting time can be calculated by (6) as follows:
Time(i)=TL(i)+KTd(i)+KTo(i) i=1,2,...,n (6)
When the calls change during the operation of elevators, the system calculates the minimum waiting time of each elevator. Then it allocates the current call to the elevator which has small value. When the each elevator has the same value, then the current call is prior to elevator A.
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