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AS/RS Real Time Simulation and Control Integrated System

Research

Xu Xusong, Xiong Hongbin Department of Management science and Engineering

Economics and Management School of Wuhan University,

Wuhan, P.R. China, 430072

Abstract Usually, AS/RS simulation and control system are separate. In our research, we have found

the way to build a real time AS/RS simulation and control integrated system based on the AS/RS control

system structure. The integrated system can be developed just using the AS/RS control system

programming language such as C++ 6.0.

In this paper, we focus on the integration of AS/RS simulation and control system, it describes

some of the key features and the more significant methods utilized in the building of simulation models

by using the virtual reality approach (involve main control module, ASRSVehicle model, conveyor

model ,trolley cart model, flow route define, object arrival/exit time model, etc). An actual Case study about an automobile parts factory AS/RS project is presented in this paper,

compared the simulation results with the model’s built by flexsim 3.0 and actual operation data over 10

months, we found the real time AS/RS simulation and control integrated system is demonstrated feasible,

flexible and comprehensive. It provides a more flexible and less expensive method for AS/RS

simulation and implement , it help to evaluate and improve the design of AS/RS control system, perform

testing and commissioning in early design phase under a virtual reality environment.

Key words AS/RS, Simulation, Virtual Reality Technology, Control system Integration, Case study

1 Introduction AS/RS (Automated Storage and Retrieval System) is an important key part of modern logistic

system, along with the consideration of limited space, high labor costs, requirements of flexibility,

expandability, quality, reliability, management control, AS/RS application increase rapidly throughout

the world in recent years, especially in China.

AS/RS system is very complex, it consists of lots of mechanical equipments, automation control

equipments, computer systems, and full testing on-site is difficult, expensive, time consuming. On the

other hand, AS/RS system cost huge, especially the initial investment cost. There are high risks

associated with implementation of such a complex costly system. Either the investor or the engineering

company needs a system could provide excellent overall system visibility, effective in communicating concepts and design, reduce design and implementation time, significantly improve the design quality.

Simulation and emulation system has been proven to be an accurate predictor and valuable tool for

use in design and implement of AS/RS Project, it can provide an overall view that is often impossible in

the real situation. It is a powerful analysis tool to study the many design and operating alternatives.

We did the literature review to establish the justification and validity of our research. Usually,

AS/RS simulation and control system are separate. Many papers and literature are found about the real

time AS/RS simulation system and control system, but few about the integration. There are also many

simulation software such as Automod, Arena, Flexsim that can build very complex logistic simulation

system, However, they are mainly used for design verification, alternatives comparison, system

diagnosis, although Automod provides emulation with MCM(OPC), it’s still difficult and inefficient to

integrate with the real time control system. AS/RS control system is designed to run by connecting to the devices such as ASRSVehicle, PLC, when finishing simulation, still need to rebuild the AS/RS control

system again, and the actual AS/RS control system control logic may be different from the prevenient

simulation system’s. How to integrate the simulation and control system together is our objective.

While considering solutions to these problems, the practicality of developing an AS/RS simulation

and control integrated system was raised. Unlike most AS/RS simulation studies, we developed a new

approach to the design of AS/RS real time control system, simulation system, and their integration. The

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integrated system is built just base on the AS/RS control system structure by directly using the control

system programming language - C++ 6.0.

This paper is divided into five sections: Following the introduction, Section 2, basic concept of

AS/RS and simulation. Section 3, details about the design of the integrated AS/RS simulation and

control system. Section 4, Case study about an automobile parts factory AS/RS project. Section 5,

Conclusions and future applications and work.

2 Basic Concept of AS/RS and Simulation 2.1 AS/RS System Overview

According to the definition of The Material Handling Industry of America, AS/RS is a storage

system that uses fixed-path storage and a retrieval machine running on one or more rails between fixed

arrays of storage racks.

The AS/RS has following major components: storage rack, ASRSVehicle, transport devices

(conveyor, AGV, trolley cart), assistant devices(barcode scanner, weight scale), PLC control system,

computer integrated planning and control system. It is designed to do storing, retrieving, stock checking,

picking, container distribution and collection automatically,

2.2 AS/RS Computer Integrated Planning and Control Software System Of all the systems, computer integrated planning and control system is the most important. It connect to

ERP/MRP and PLC control system, fetch working commands then dispatch ASRSVehicle, PLC system (control conveyor system, trolley cart) and assistant devices(barcode scanner, weight scale)working. When

finishing store/retrieve, results will be transferred to ERP/MRP.

It is made up of following modules and sub-systems (each containing related subroutines): � Main control module

� ASRSVehicle control module

� PLC control module

� Assistant devices control modules � System visual monitor and display module

By using timing and muti-thread technology, it connect to AS/RS database server, according to the

working commands, dispatch the relative devices working through the communication between IPC and

connected devices. The structure of AS/RS control system is illustrated in Figure 1.

Figure 1: The structure of AS/RS control system

2.3 AS/RS Simulation

Simulation is the imitation of a system, based on knowledge or assumptions about the behavior of

parts of that system, with the purpose of obtaining insight in the behavior of the whole system. The following are general 5 steps for a logistic system simulation:

� Prepare for simulation(problem description, information collection)

� Define the objective system and it’s alternatives

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� Build the computer simulation models

� Run simulation alternative experiments

� Analyze and verify the simulation outputs and alternatives

For an AS/RS system, it is designed to do storing/retrieving the right products in the right places at

the right time at the lowest cost. AS/RS Simulation system can help to reach the goal, with simulation,

numerous options were tested that are currently carried out either on-site or at test sites without

optimizing layouts and operating procedures and incurring large capital costs or disrupting operations.

3 Real time AS/RS Simulation and Control Integrated System Design 3.1 System design overview

We design the realtime AS/RS control and simulation integrated system by modifying the

prevenient main control module and adding the simulation modules. includes following modules : � M1:Main planning and control module(including the interface with AS/RS data process system)

� M2:ASRSVehicle control module

� M3:PLC control module

� M4:Assistant devices control modules(barcode scanner, weight scale)

� M5:System visual display and monitor module

� M6:Virtual ASRSVehicle simulation control modules

� M7:Virtual PLC simulation control modules

� M8:Simulation parameters setup module

� M9:Simulation data analysis module All the above modules work together and are managed by the main planning and control module.

An interface between control module and simulation module is constructed, a simulation switch flag is

designed, when it switch to Y, the system works in simulation mode, the actual devices (ASRSVehicle,

PLC) stop working, the virtual devices (virtual ASRSVehicle, virtual PLC) will start to work. When the

flag switch to N, the system works in actual control system mode, the control system will connect to

actual devices, simulation modules stop working. Figure 2 shows the process flow and structure of the

new AS/RS control and simulation integrated system.

Figure 2 Process flow and structure of new AS/RS control and simulation integrated system.

There are many issues that could be addressed regarding the design of AS/RS integrated control

and simulation system, following are some of the important items.

3.2 Main Planning and Control Module Design It’s designed for control the AS/RS actual/virtual devices control and working order planning, by

using Virtual reality, database and timer technology, for every a time cycle, repeat the following works:

� Actual/virtual devices control

The module keeps reading devices (PLC, weight scale, barcode scanner) status from the memory

array updated by the Actual /virtual devices threads/ timers. If condition is OK and exists command in

working task queue table, it will send commands to the actual/virtual devices.

� ASRSVechiles dispatch

The module keeps reading ASRSVechile status code from the memory array updated by the Actual

/virtual ASRSVechile threads/ timers. If inspecting the ASRSVechile’s idle and exists command in

working task queue table, It will send a command (store, retrieve, location to location, station to station)

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to ASRSVechile, the ASRSVechile will start to work. For the reasonable dispatch, the module is

designed to do storing/retrieving by turns: first do storing then do retrieving in next working cycle.

� Update database tables

The module is designed to update the relative ASRS database table when device’s status changed.

� Devices error and exception handle

The module is designed to identify and record the device’s exception occurrence and recovery in

AS/RS database during the system running, such as ASRSVechile encode error, motor overload error.

3.3 Actual Devices (ASRSVehicle, PLC) Control Modules Design

The modules only work in control system mode, by using thread and task queue technology,

through the communication of RS232/RS422 port between IPC and device, it keep scanning devices

status, fetching command from working queue table, sending commands to devices.

3.4 Simulation Model and System Design AS/RS is a typical discrete system of queues, processors and event. In order to simply design the

simulation system, only the simulation of ASRSVehicle, PLC control system (roll conveyor, trolley cart)

are considered, other devices, such as weight scale, barcode scanner will be ignored. The key features

and main modules and methods utilized in the building of simulation system are summarized as below:

3.4.1Simulation clock, Simulation time/ real time rate factor, Simulation time pushing ruler

For the simulation timer pushing, a globe simulation clock(Cm) and the ASRSVechile and PLC

simulation clock (Cv ,Cp) are designed, they keep on working until simulation end.

In order to accelerate simulation, a time rate factor (time_rate) is designed, initially, it’s set as 1, if

it is set to 10, means 1 minute in simulation system is equal to 10 minutes in reality time.

Assuming the timer interval of clock Cm, Cv, Cp is Tm, Tv, Tp, so the corresponding minimum

time step is: Tm/time_rate, Tv/time_rate, Tp/time_rate. For every a time cycle, time will be increased in a step, all the objects in AS/RS system is moving or waiting according to the corresponding clocks.

3.4.2 Mobile Object Movement Simulation Model

There are many mobile objects in AS/RS system, such as ASRSVehicle, trolley cart, roll conveyor,

transporter. In order to simulate the object movement, following basic theories are proposed:

� 2-Stage Movement Trip Theory

Usually, a mobile object could move in both horizontal and vertical direction, for either direction,

it’s movement trip can be divided into 2 stage: first, from the current position move to the first

destination position. Second, from the first destination position move to the last destination position.

Figure 3 shows the illustration of 2-stage travel trip theory for ASRSVehicle. This is example of a

storage procedure, in the first stage trip, the ASRSVehicle travel from the current position(Pc) to the

store-station(P1) to fetch the object, then move to the destination position (P2) to put the object into the

assigned rack no. Therefore, the complete 2-Stage Movement Trips are: Pc�P1, P1�P2.

Figure 3 Illustration of 2-stage travel trip theory for ASRSVehicle.

� Characteristic Velocity-Time Curve of AS/RS Mobile Object

In order to estimate the separate and total travel time for the 2-Stage trip mentioned above, a typical

velocity profiles are proposed as shown in Figure4. The area of triangle OAD represents the Critical

Distance(CD) of a mobile object. If the single trip distance is larger than CD, it will be considered as a

normal trip, like the trapezia OABC. If the distance is less than CD, just like the triangle OEF, it will be

regarded as a short trip, during the trip, the velocity can never reach the constant velocity.

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Figure 4 Characteristic velocity-time curve of AS/RS mobile object

At the assigned design velocity and acceleration, the critical distance (CD) can be calculated.

Therefore, once the distance of a single trip is confirmed, the type of trip (normal or short trip) can be

confirmed, and the maximum velocity and the total travel time can be calculated, then the movement

position Pi at time Ti during the trip can also be estimated.

For example, assuming the trip distance is S(m), velocity is V (m/s), acceleration and deceleration

is A and B (m/s2), the critical distance can be obtained as CD=

A

V

2

2

+B

V

2

2

,compared the S with CD

if it is a normal trip, the total travel time for this trip is : T=V

S+2

V× (A

1+B

1).

3.4.3 Virtual ASRSVehicle Simulation Model

For the ASRSVehicle simulation model design, following elements are considered:

� Dispatching Rule

Based on the AS/RS control system and timing technology, the dispatch of virtual ASRSvechile is

same as the actual ASRSvechile control system, it is arranged by the main planning and control module,

depends on the data in the working command queue table.

� Movement Simulation: Travel distance, Travel time, Position

ASRSVehicle can work for different types such as: store in, retrieve, station to station, location to location. During a working cycle trip, the actions can be separated into following steps:

Walk(Tw1) � Fork fetch goods(Tf1) �Walk(Tw2) � Fork put goods(Tf2).

It moves simultaneously in both horizontal and vertical directions, moves along floor-mounted rails

parallel to the storage racks for horizontal trips, and along a mast for the vertical part of the trip.

According to the 2-stage trip movement theory and velocity-time curve mentioned above, by the

positions (Pc ,P1, P2) of the trip, the every single stage trip(trip 1, trip 2) distances in both horizontal

and vertical directions be calculated first, then at the assumed ASRSvechile velocity, acceleration, the

corresponding travel time (Tw1h, Tw1v), (Tw1h, Tw1v) in horizontal and vertical direction in trip 1,2

can be calculated too. Assuming the load/unload time and final travel time in trip 1, trip 2 is (Tf1,Tf2 ),

(Tw1,Tw2). The total time (Ti) consumed in the whole working period can be calculated as following:

Ti=Tw1+Tf1+Tw2+Tf2 Tw1=Max(Tw1h, Tw1v) , Tw2=Max(Tw2h, Tw2v)

During the travel trips, according to the 2-stage trip movement theory and velocity-time curve and

the simulation clock mentioned above, for every a cycle timer step i, the corresponding position (Pxi,

Pyi) of ASRSVehicles in both horizontal and vertical directions can be calculated too.

� Simulation Statistical Data Analysis

Assuming total simulation time is TT, there are M ASRSVehicle s in the system, total N working

task need to be executed. When simulation finished, the statistical data and analysis for ASRSVehicle i

(i＝1…M) can be obtained as followes:

Work starting and finishing time queue : BT(i,1),ET(i,1) …BT (i,j), ET (i,j)…BT (i,N), ET (i,N)

Total travel_load time(s): ∑ST1(i,j)

Total travel_empty time(s): ∑ST2(i,j)

Total work time(s): ∑ST(i,j)= ∑ST1(i,j)+ ∑ST2(i,j)

Total idle time(s)： TT-∑ST(i,j)

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Utilization rate(%)： ∑ST(i,j)/ TT × 100

3.4.4 Virtual PLC Simulation Model Similar to the actual PLC system, a simulation cycle timer is designed by using timing technology,

for every a cycle time, the designed PLC timer will keep scanning the status of virtual devices and

simulating the actions of actual devices.

� Object Flow Path Definition

A database table is designed to record the definition of flow path. For usual conveyor stations, the

flow path can be defined by assign the next destination station of current one, but for the bidirectional

mobile devices such as trolley cart and lifter, both in/out flow path and ports must be defined.

� Conveyor System Simulation Model

When an object enter a station of the AS/RS conveyor system, the relative clock and timer start

working, its next destination station and stand move time can be obtained from the flow path definition

table. For every a cycle time step, the waiting time increase, when the lapsed time is over the stand time and next station is spare, the object will move to the next station, otherwise the object will stay till

its movement condition is ok. By this way, the object movement on conveyor system can be simulated.

� Trolley Cart Simulation Model

The trolley cart’s simulation mode is very similar to ASRSVehicle’s, but the dispatching logic and

control must be redesigned like a real PLC system’s: when a trolley cart is idle, there may be several

objects in different stations are waiting for it, a reasonable dispatching rule must be considered first,

once it is decided, similar to ASRSVehicle simulation mode, the corresponding travel path, distance,

total time consumed, positions at every time step can be calculated and simulated.

� Simulation Statistical Data Analysis

Assuming simulation total time is TT, there are M PLC control devices in the AS/RS system, total

N work task need to be executed. When simulation finished, the object arrival/leave time queue for

device i (i＝1…M) can be obtained:

BT(i,1),ET(i,1) …BT (i,j), ET (i,j)…BT (i,N), ET (i,N) By this queue, the following statistical data and analysis for station i can be obtained as followes:

Stay time queue： ET (i,1)-BT(i,1) ，ET (i,2)-BT(i,2) ……ET (i,N)-BT(i,N) Idle time queue: BT(i,2)- ET (i,1)，BT(i,3)- ET (i,2)……BT(i,N)- ET (i, N-1) Total stay time queue： ∑(ET (i,j)-BT(i,j))

Total idle time queue： ∑(BT(i ,j)- ET (i,j-1))

Device idle rate(%)： ∑(BT(i ,j)- ET (i,j-1))/ TT × 100

4 Case Study The integrated simulation and control system and models described above have been used in a

number of actual AS/RS applications. In this paper, a case study about an automobile parts factory

AS/RS project is presented to demonstrate the feasibility and validity of the integrated system.

4.1 Case Description

In this case, the AS/RS is designed for both use of sem-finished goods and finished goods. It is

built in a workshop of the factory. Not only for traditional warehouse use, the AS/RS is designed for the

packing sem-finished goods supply by directly connecting to the packing machines.

There are 2 ASRSVehicles, 2 PLC, 3 trolley carts in the system, the rack specification are: 4 row x

62 bay x 16 level=3968 cells, use tank as container, the max weight is 1000 kg. The system are designed

to work 8 hours/day, planning to finish incoming 200 tanks, outgoing 200 tanks, picking and stock

check 50 tanks, packing line supply 56 tanks. It requires the ASRSVehicle finish store/retrieve 70

tanks/hour. Figure 5 shows the AS/RS realtime control and simulation system layout display page:

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Figure 5 Example AS/RS realtime control and simulation system layout and visual display page

The main system work types and flow route of this AS/RS case are described as followes:

� Storage: 3 route, station 1->cart 21->rack, station 1->cart 54->rack, station44->cart 54->rack. � Retrieve:3 route, rack->cart 21->station 8, rack->cart54->station 27, rack->cart54->station 48.

� Packing supply: rack->cart54->station40->cart55->packing machines (56-62)-> station 29,64.

� Picking, Stock check: rack -> cart 21-> station 9,10,19,20 working ->cart 21->rack.

The AS/RS simulation and control integrated system was built in Jul, 2005, the actual AS/RS was

installed and started running in Sep, 2005. Following are some of simulation experiments of the time.

4.2 Simulation Experiments and Results Analysis

4.2.1 Simulation Parameters Setup

In this case, the basic simulation experiment parameters are as followes:

� Rack specification: 4 row x 62 bay x 16 level= 3968 cells, rack unit size: 865Wx925Dx875H.

� ASRSVehicle: travel speed 2m/s, lift speed: 0.5m/s, acceleration: 0.3m/s2, load time: 15s.

� Trolley cart(21,54, 55): travel speed 0.8m/s, acceleration: 0.2 m/s2, load/unload time: 8s.

� Roll conveyor: travel speed 0.3m/s.

4.2.2 Simulation Experiments

� ASRSVehicle Limit Output Test:

In order to measure the maximum capacity of ASRSVehicle, a simulation model with only one

ASRSVehicle and corresponding 2 racks is built, at the assumed devices(ASRSVehicle, conveyor, rack) specification and parameters, by randomly assigning the store/retrieve rack no, after for about one

hour’s simulation running, simulation output data are collected and the results are shown in Table 1:

Table 1. ASRSVehicle limit output test results (1 hour)

Working type In Out total Utilization (%)

Only store in 45 45 99.99

Only retrieve out 41 41 99.99

Both In/out 28 28 56 89.00

� Continuous Store/Retrieve Flow Route Time Test:

In this simulation experiment, for some continuous assigned working tasks, we could calculate the

times consumed on the ASRSVehicle, conveyor, trolley cart. Some of the results are shown in Table 2:

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Table2. Continuous s store/retrieve flow route time consumed test results

Time consumed(s) Seq

Type

Flow route conveyor Trolley cart ASRSVehicle Total

1. In 1 -> 12 -> 21-> 0303606 42 16 76 134

2. Out 0100108-> 32 -> 54 -> 27 38 18 60 116

3. In 24 -> 54 -> 31-> 0204407 42 27 72 141

4. Out 0404407 -> 13 -> 21 -> 8 59 16 74 149

5. In 1 -> 16 -> 21-> 0203206 62 18 52 132

� Integrated Emulation Test:

In the AS/RS daily operation, storing, retrieving, supplying packing line, picking, stock checking

are dispatched by the main planning and control module, and can be done in the same time. In order to simulate this situation and test the system performance (throughput, bottleneck, equipment utilization ),

about 100 records are prepared in working queue table for integrated experiment. After running for

about 1 hour, simulation finished. Table 3 shows some of the sum-up results.

Table 3. Integrated emulation test sum-up results

Stay time (s) Device In Out total

Min Average Max

Utilization (%)

ASRSVehicle1 26 29 55 19.77 37.19 48.88 96.30

ASRSVehicle2 24 26 50 26.19 38.50 47.87 93.21

Trolley cart21 43 - 43 10.08 15.18 20.52 49.88

Trolley cart54 68 - 68 12.98 21.90 33.27 81.91

Trolley cart55 14 - 14 17.69 33.09 43.17 33.75

Station 1(in) 15 - 15 60.00 96.87 393.69 96.25

Station 24(in) 15 - 15 65.98 161.61 462.90 87.47

Station 46(in) 15 - 15 68.64 175.39 482.14 91.68

Station 8(out) 14 14 34.04 103.38 125.12 86.61

Station 27(out) 14 14 50.00 95.00 135.00 61.66

Station 48(out) 13 13 51.37 139.89 242.34 34.22

Remark: the time interval for store, retrieve, pick, packing supply is 180s, 180s, 720s, 500s.

4.2.2 Simulation Results Analysis

By the above simulation experiments and output data, following analysis conclusions are obtained:

� The ASRSVehicle’s maximum output for store/retrieve is about 112 tanks/hour, this is also

the limit output of AS/RS. Compared with the system design requirement (70 tanks/hour),

only need the ASRSVehicle’s utilization over 63%, the design requirement will be met.

� In order to test the limit output of the system, in the above integrated simulation experiment,

enough data were prepared for the system, the ASRSVehicles cost about 60 minutes finish

105 tanks in/out, including (store in, retrieve, delivery out, supply packing line, picking etc),

the results also verified the design validation. On the other hand, all the ASRSVehicle’s utilization are over 90% (the usual suggested utilization is about 70%-80%). Compared with

conveyor and trolley cart system’s utilization, the ASRSVehicle maybe the bottleneck for the

future system output improvement.

� For the flexibility and convenience of connecting to workshops and the manufacture flow, the

AS/RS store in/out stations are arranged in both origin and opposite side of the rack. During

simulation, we found this layout design will reduce the efficiency of ASRSVehicle about 20%

than the usual mode because of the ASRSVehicle’s travel-empty time were increased.

� If one ASRSVehicle have to handle several stations in the same time just like this case, the

dispatch for ASRSVehicle storage must be considered very carefully, otherwise it will keep

storing just on one side while the other side’s object keep on waiting. By simulation, we found

the bug mentioned above in control system design and correct it.

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Compared with the simulation model built by Flexsim3.0, we found the two simulation results are

very similar. After the AS/RS system installed, about 10 months daily operation actual data are collected,

we do the comparison again, and also found the integrated simulation and control system is

demonstrated feasible, flexible and comprehensive.

5 Conclusions and Future Work This paper presents the concept, modeling method of building AS/RS real time simulation and

control integrated system based on the AS/RS control System structure. By just using the AS/RS control

software system programming language, such as C++, Delphi, Vb, the integrated control and simulation

system share the same logic, can be developed more quickly, and is more easier to modify, much less

costly to run, easier to explain to the customers.

Several experiments and projects have been carried out for the design and simulation, testing of

new AS/RS, and the diagnosis of the existing AS/RS systems. All the results show that the above

integrated system is indeed feasible and provide many benefits.

As future study, many promising research directions have been identified and presented, mainly for

simulation modules. The system visual display mode will be changed form from 2D to 3D style. To

reduce the time drastically for simulation modeling, more stand components will be added into the

simulation module such as processor, combiner, separator, etc. We are also considering the possibility of

designing a simple visual assistant constructor for object flow path definition just by mouse drag. Other improvement and modification such as device’s MTBF, MTTR, object arrival/exit time distribution,

data statistical analysis will be considered in the next version.

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