- Email: [email protected]
A FMS physical simulator
WORK IN PROGRESS Technical Notes
B. Khoshnevis, Ph.D. A. Kiran, Ph.D. Department of Industrial and Systems Engineering University of Southern California Los Angeles. CA
A FMS Physical Simulator An integrated scale model of a flexible production line and an automated storage and retrieval system is described. The scale model is developed for the Department of Industrial and Systems Engineering at the University of Southern California. The model has been used for educational purposes as well as for physical simulation of n e w scheduling and control algorithms.
Introduction In-house design and development of CAM equipment and mock-up const ruction of large CA M systems seem to be attractive alternatives that provide the closest student involvement and serve as useful tools for modern manufacturing education. With the relevance of this fact in mind and with the help of students of the ISE Department at USC, the authors have developed a computerized flexible manufacturing model composed of an automated warehouse and an automated shop floor material handling system that serves two production work stations. Each of the components of this integrated system is described in subsequent sections of this technical note.
implementation of inventory policies, and warehouse database design concepts. In addition, the warehouse has been interfaced with an automated material handling system that transfers the warehouse contents to a production floor, and returns the workin-process, or the finished parts back to the warehouse. The physical structure of the warehouse is comprised of three parallel aisles each servicing two storage bay's, and a back aisle servicing a back wall storage bay'. A front aisle is also included to provide for shorter travelling in transactions dealing with the front parts of the storage bay's. This aisle is also the channel for interfacing the warehouse with an outside material handling system (Figure 1). The top view of the warehouse structure together with the material handling system and the work stations layout are shown in Figure 2.
The Automated Warehouse Today', many giant warehouses that handle large volumes of items are benefiting from the productivity increases offered by computer automation. More recently, the automated warehouse has become an integrated component of flexible manufacturing systems. In this latter setting, an automated warehouse coupled with an automated shop floor material handling system can serve as an on-line storage/retrieval system for incoming materials, work-in-process and finished parts, and manufacturing tools. Increased speed of storage and retrieval, organized bookkeeping with well structured databases, inventory control, integration with the shop floor computer controlled material handling systems, ease of implementation of scientifically designed pick and place algorithms, minimal requirements of lighting and air conditioning, elimination of on-the-job injuries, and reduction of pilferage are some benefits that automated warehouses bring. Because of the large size of a full scale automated warehouse, it is not possible to use a real automated warehouse as a teaching tool in a university laboratory. A physical simulator is the only alternative for providing a hands-on approach to teaching the basic concepts involved. For this purpose, a model automated warehouse has been developed at USC which incorporates man,,' realistic features. The model uses a palletized storage and has a sufficiently high number of storage aisles, bay's, levels, and bins. The system is driven by a microcomputer and is capable of demonstrating various aspects of the automated warehouses such as computer interface and control, optimization algorithms,
65
Figure 1 The Automated Warehouse Simulator System
This particular configuration for the warehouse structure is chosen because it provides challenging control problems in the software design, as there are alternative paths from one bin location to the other. This allows for implementation of a shortest path finder which first eliminates the infeasible paths. Each storage bin is designed to center and maintain alignment of one storage pallet.
Journal of Manufacturing Systems Volume 5 No. I
technical n o t e s I
_1
Say.
I
AISLE 3 l [ v '~ mf
i
"- Work - " Stations
I
i--
i
AISLE 2 Bay Bay C B AISLE I
I
! ,
Track
Bay E Bay D
Relay
f
BayA
q
Cart ]
~ Warehouse
Vcc
Figure 2
To Computer
Top View of the integrated System
Figure 4
A storage pallet consists of a wooden rectangle supported by two wooden triangle prisms use as legs (Figure 3). Four degrees of freedom are required for storage and retrieval of the pallets within the warehouse structure. These provide for the forward/backward, left/right, up/down and rotational movement of the forklift to access the bins in all required directions. For this purpose, the lifting mechanism is mounted on a cart that can travel back and forth on top of a bridge crane assembly. The bridge crane itself is positioned over two mounted rails, one on the top front and the other on the top back of the structure (precision steel shafts and linear bearings with recirculating balls are used for all linear movements). The forklift is mounted on a vertical guide pipe that can rotate in both directions. For the three linear moves, precision chain-sprocket assemblies are used which convert the rotary movement of the motor shafts into linear motions. The drive mechanism uses 12 volt DC motors with gearheads. The motor control is performed using the circuit shown in Figure 4. Here, transistor Tt is used to turn the motor on, or off, and transistor T2 is used to drive the relay which changes the polarity of the power applied to the DC motor. Changing the power polarity changes the direction of rotation of the motor. Motor speed is controlled by subsequently turning the motor on and off under the software command. The ratio of the duration of the on time to the off time determines the effective speed.
Motor Control Circuit
The sensing of the position of the forklift is made possible by mounting microswitches that are momentarily closed by the screw heads properly positioned along the rails over which the crane and cart travel, and along the vertical guide that holds the fork. Four rotational positions are also sensed using four switches installed on the cart and around the guide pipe. A screw head mounted on the guide pipe can activate these switches. Also, the shaft of each motor is equipped with an optical encoder that generates information about the length of the distance travelled after the sensing of a screw head. This allows for deceleration before stopping at the last switch count. Altogether there are eight output lines used: two for each motor (one for direction and the other for o n / o f f a n d speed control). Five input lines are used, one connected to each switch used for each direction and one connected to the optical shaft encoders. An apple computer with a 6522 programmable I/O chip mounted on an add-on board is used to provide the necessary I/O operations. The control software is written in BASIC supported by several assembly subroutines. The BASIC portion makes necessary calculations to determine the number of switch signals required along each direction of movement and sequences of optimum forklift movement directions for travelling between any two given bin locations through the shortest feasible path. The assembly programs are used for real-time applications such as polling for detecting the switch closures, and for signal timing, acceleration, and deceleration of the motors to avoid jerking and overshooting. The warehouse simulator systems described above can be an ideal device for experimentation with various item picking and rearranging algorithms. The control software and the item picking algorithm can be interfaced with an inventory control software that in turn interfaces with a database reflecting the updated status of the warehouse contents. As mentioned earlier, the system can also serve as a component of a model FMS, using an automated material handing system such as the one described below.
Figure 3 Storage Bin and Pallet Designs
66
Journal qf Manulacturing 5~vstems Volume5 No. 1
technical n o t e s
Microswitches are mounted under the track and in front of each work station and the warehouse delivery points for exact positioning of each pallet in front of tile work stations and the warehouse forklift. I O commands for the control of the three motors (two l o a d e r unloaders and the electric car) and the sensing of the six switches are performed by a microcomputer that is linked to the computer controlling the warehouse. This allows for simultaneous and synchronized operation of the two systems, The interface board is designed using two 6522 PIO chips which are arranged to create two parallel 16-bit output (16-bits per processor). These outputs are further broken down to two 8-bit sections each of which is separately accessible from the keyboard of a computer. Thus, the board has a total capacity of generating a 32-bit parallet output. The board is also capable of simultaneously receiving input and sending output data. The software is controlled by a menu driven BASIC program. At a cold start the program will display the following to locate the initial position of the cart:
The Physical Simulator The warehouse simulator is interfaced with a simulated, small scale automated material handling system which integrates the automated warehouse with two work stations. The automated material handling system (Figure 5) is composed of the engine car of a toy electric train which is reconfigured to carry two warehouse pallets (Figure 6).
WHERE IS THE START POINT OF THE CART ] 2 3 4
WAREHOUSE A ' WAREHOUSE 'B' WORKSTATION 'A' WORKSTATION '8'
YOUR CHOICE (1-4) ? (1) 2
Figure 5 SET CART AT WAREHOUSE '8"
The Automated Shop Floor Material Handling System
IF OK THEN HIT SPACE BAR
The car is DC powered through the tracks that connect the delivery point in front of the warehouse to two work stations. At each work station there is a pallet l o a d e r / u n l o a d e r that slides the pallet from the top of the engine car to the top of a small table located at the work station. The reverse pallet transfer is also possible. The sliding of the pallets at each station is done by a small DC motor that provides a linear move to an overhead fork assembly through a sprocket-chain mechanism. The circuit shown in Figure 4 is also used for all motor controls in this system. The layout of this system is shown in Figure 2.
I f the start point mismatches the position of the cart, an error message will be issued, and a new choice should be made thereafter. This part is extremely helpful in familiarizing the students with the system and the names used to identify the different locations. When everything is in order, the computer will load a main program and run the program. Now the screen will display the main menu as follows: LAYOUT
! COMMANDS ![1] GOTO WHSE A ' ..... B - - - ![2] GOTO WHSE 'B' ! [3] GOTO WKST A ' I [4] GOTO WKST 'B' B ! [5] WKST 'A" < - - P A R T - WHSE ! [6] WKST 'A' --.:~CART A ! [7] WKST B"--~--PART ! [8] WKST 'B" --:;-CART ! [A] WRITE TO DISK SPEED 20 ! [8] DELETE FILE .................................. ![C] MODIFY FILE FILE ! [D] EXECUTE FILE CMD WAIT(S) PALLET ! [E] SET CART SPEED 1> ! IF] END OF JOB WKST A ......
2>
3-" 4..-
5> 62-
7:' 9:>
Figure 6 The Cart Moving Two Pallets
67
YOUR CHOICE ? _ _
Journal ok/"Manutacturing SvMe'm$ Volume 5 No. 1
technical notes
The purpose of this technical note has been to describe an integrated scale model of a flexible production line and automated storage and retrieval system developed for the Department of Industrial and Systems Engineering at the University of Southern California. The scale model has been developed to be used as a teaching tool to demonstrate the integration issues as well as computer interfaces encountered in flexible automation. Besides educational purposes, the scale model described above is currently being used for physical simulation of scheduling and control algorithms.
There are three sections in the main menu: the layout section, the file section and the command section. The layout section shows the track layout, the locations of the load, unload stations and the workstations and the actual cart locations (i.e., the movements of the cart can be followed on the screen). The file section keeps the records according to the instructions set to move the cart, load, and unload. Each file is allowed to store up to 99 instructions. The file section displays 10 instructions at a time. The command section displays available command options. The user can select from the list of commands by pressing the appropriate key or by shifting the cursor to the appropriate command and pressing the return key. Integration of this setting with robotized work stations and networking of the computers in control of the subsystems result in a realistic FMS environment capable of demonstrating the principle problems involved. The system can also provide a research tool for experimenting with various control and scheduling software. Addition of miniature robots to the work stations to perform simple assembly operations on the items retrieved from the warehouse results in a realistic integrated FMS that provides for a range of related experimentations. The system exposes the students to the hardware, software, and interfacing techniques involved in a complex computer controlled manufacturing environment.
A cknowledgement
The development of the F M S physical simulator has been partially supported by the Faculty Research and Innovation Fund of the University of Southern California.
Bibliography Bed,xorth, D. and Sobczak. J., "'Students Learn on A S / R S Model". Journal of Industrial Engineering. December 1976. Groover, M.. Computer-Aided Design and Manufacturing. Prentice-Hall, Inc.. 1984. Khoshnevis. B.. "'Computer-Aided Production Sxstems Laboratory at USC", Proceedings of the Annual International Industrial Engineering Conference, Los Angeles. pp, 488-494, May 1985. Khoshnevis, B, and Rogers. R.. "'An Automated Warehouse Simulator S~stem". Proceedings of the F(/?h htternational Conference on Automation in Wareho,tsing. December 1983. Kiran, A.S. and Tansel. B.C., "'A Framework for Flexible Manufacturing S 3 stems", Proceedings o[ the ,4nnual biternational btdustrial Engineering Cot~ference. Los .Angeles, pp. 446-450, May 1985. Rank 3. P.G., The Design and Operation of F3IS, NorthHolland Publishing Co., 1983.
Summary Scale models are ideal tools for educational purposes. They are also efficient and inexpensive ways to analyze and test actual systems prior to their construction. Time, effort and money can be saved by first working out the bugs on a scale model in order to maximize start up efficiency of the actual systems. In addition, scale models can be used as on-line simulators to test feasibility questions and performance of the actual systems under different operating rules and policies.
68
B. Khoshnevis, Ph.D. A. Kiran, Ph.D. Department of Industrial and Systems Engineering University of Southern California Los Angeles. CA
A FMS Physical Simulator An integrated scale model of a flexible production line and an automated storage and retrieval system is described. The scale model is developed for the Department of Industrial and Systems Engineering at the University of Southern California. The model has been used for educational purposes as well as for physical simulation of n e w scheduling and control algorithms.
Introduction In-house design and development of CAM equipment and mock-up const ruction of large CA M systems seem to be attractive alternatives that provide the closest student involvement and serve as useful tools for modern manufacturing education. With the relevance of this fact in mind and with the help of students of the ISE Department at USC, the authors have developed a computerized flexible manufacturing model composed of an automated warehouse and an automated shop floor material handling system that serves two production work stations. Each of the components of this integrated system is described in subsequent sections of this technical note.
implementation of inventory policies, and warehouse database design concepts. In addition, the warehouse has been interfaced with an automated material handling system that transfers the warehouse contents to a production floor, and returns the workin-process, or the finished parts back to the warehouse. The physical structure of the warehouse is comprised of three parallel aisles each servicing two storage bay's, and a back aisle servicing a back wall storage bay'. A front aisle is also included to provide for shorter travelling in transactions dealing with the front parts of the storage bay's. This aisle is also the channel for interfacing the warehouse with an outside material handling system (Figure 1). The top view of the warehouse structure together with the material handling system and the work stations layout are shown in Figure 2.
The Automated Warehouse Today', many giant warehouses that handle large volumes of items are benefiting from the productivity increases offered by computer automation. More recently, the automated warehouse has become an integrated component of flexible manufacturing systems. In this latter setting, an automated warehouse coupled with an automated shop floor material handling system can serve as an on-line storage/retrieval system for incoming materials, work-in-process and finished parts, and manufacturing tools. Increased speed of storage and retrieval, organized bookkeeping with well structured databases, inventory control, integration with the shop floor computer controlled material handling systems, ease of implementation of scientifically designed pick and place algorithms, minimal requirements of lighting and air conditioning, elimination of on-the-job injuries, and reduction of pilferage are some benefits that automated warehouses bring. Because of the large size of a full scale automated warehouse, it is not possible to use a real automated warehouse as a teaching tool in a university laboratory. A physical simulator is the only alternative for providing a hands-on approach to teaching the basic concepts involved. For this purpose, a model automated warehouse has been developed at USC which incorporates man,,' realistic features. The model uses a palletized storage and has a sufficiently high number of storage aisles, bay's, levels, and bins. The system is driven by a microcomputer and is capable of demonstrating various aspects of the automated warehouses such as computer interface and control, optimization algorithms,
65
Figure 1 The Automated Warehouse Simulator System
This particular configuration for the warehouse structure is chosen because it provides challenging control problems in the software design, as there are alternative paths from one bin location to the other. This allows for implementation of a shortest path finder which first eliminates the infeasible paths. Each storage bin is designed to center and maintain alignment of one storage pallet.
Journal of Manufacturing Systems Volume 5 No. I
technical n o t e s I
_1
Say.
I
AISLE 3 l [ v '~ mf
i
"- Work - " Stations
I
i--
i
AISLE 2 Bay Bay C B AISLE I
I
! ,
Track
Bay E Bay D
Relay
f
BayA
q
Cart ]
~ Warehouse
Vcc
Figure 2
To Computer
Top View of the integrated System
Figure 4
A storage pallet consists of a wooden rectangle supported by two wooden triangle prisms use as legs (Figure 3). Four degrees of freedom are required for storage and retrieval of the pallets within the warehouse structure. These provide for the forward/backward, left/right, up/down and rotational movement of the forklift to access the bins in all required directions. For this purpose, the lifting mechanism is mounted on a cart that can travel back and forth on top of a bridge crane assembly. The bridge crane itself is positioned over two mounted rails, one on the top front and the other on the top back of the structure (precision steel shafts and linear bearings with recirculating balls are used for all linear movements). The forklift is mounted on a vertical guide pipe that can rotate in both directions. For the three linear moves, precision chain-sprocket assemblies are used which convert the rotary movement of the motor shafts into linear motions. The drive mechanism uses 12 volt DC motors with gearheads. The motor control is performed using the circuit shown in Figure 4. Here, transistor Tt is used to turn the motor on, or off, and transistor T2 is used to drive the relay which changes the polarity of the power applied to the DC motor. Changing the power polarity changes the direction of rotation of the motor. Motor speed is controlled by subsequently turning the motor on and off under the software command. The ratio of the duration of the on time to the off time determines the effective speed.
Motor Control Circuit
The sensing of the position of the forklift is made possible by mounting microswitches that are momentarily closed by the screw heads properly positioned along the rails over which the crane and cart travel, and along the vertical guide that holds the fork. Four rotational positions are also sensed using four switches installed on the cart and around the guide pipe. A screw head mounted on the guide pipe can activate these switches. Also, the shaft of each motor is equipped with an optical encoder that generates information about the length of the distance travelled after the sensing of a screw head. This allows for deceleration before stopping at the last switch count. Altogether there are eight output lines used: two for each motor (one for direction and the other for o n / o f f a n d speed control). Five input lines are used, one connected to each switch used for each direction and one connected to the optical shaft encoders. An apple computer with a 6522 programmable I/O chip mounted on an add-on board is used to provide the necessary I/O operations. The control software is written in BASIC supported by several assembly subroutines. The BASIC portion makes necessary calculations to determine the number of switch signals required along each direction of movement and sequences of optimum forklift movement directions for travelling between any two given bin locations through the shortest feasible path. The assembly programs are used for real-time applications such as polling for detecting the switch closures, and for signal timing, acceleration, and deceleration of the motors to avoid jerking and overshooting. The warehouse simulator systems described above can be an ideal device for experimentation with various item picking and rearranging algorithms. The control software and the item picking algorithm can be interfaced with an inventory control software that in turn interfaces with a database reflecting the updated status of the warehouse contents. As mentioned earlier, the system can also serve as a component of a model FMS, using an automated material handing system such as the one described below.
Figure 3 Storage Bin and Pallet Designs
66
Journal qf Manulacturing 5~vstems Volume5 No. 1
technical n o t e s
Microswitches are mounted under the track and in front of each work station and the warehouse delivery points for exact positioning of each pallet in front of tile work stations and the warehouse forklift. I O commands for the control of the three motors (two l o a d e r unloaders and the electric car) and the sensing of the six switches are performed by a microcomputer that is linked to the computer controlling the warehouse. This allows for simultaneous and synchronized operation of the two systems, The interface board is designed using two 6522 PIO chips which are arranged to create two parallel 16-bit output (16-bits per processor). These outputs are further broken down to two 8-bit sections each of which is separately accessible from the keyboard of a computer. Thus, the board has a total capacity of generating a 32-bit parallet output. The board is also capable of simultaneously receiving input and sending output data. The software is controlled by a menu driven BASIC program. At a cold start the program will display the following to locate the initial position of the cart:
The Physical Simulator The warehouse simulator is interfaced with a simulated, small scale automated material handling system which integrates the automated warehouse with two work stations. The automated material handling system (Figure 5) is composed of the engine car of a toy electric train which is reconfigured to carry two warehouse pallets (Figure 6).
WHERE IS THE START POINT OF THE CART ] 2 3 4
WAREHOUSE A ' WAREHOUSE 'B' WORKSTATION 'A' WORKSTATION '8'
YOUR CHOICE (1-4) ? (1) 2
Figure 5 SET CART AT WAREHOUSE '8"
The Automated Shop Floor Material Handling System
IF OK THEN HIT SPACE BAR
The car is DC powered through the tracks that connect the delivery point in front of the warehouse to two work stations. At each work station there is a pallet l o a d e r / u n l o a d e r that slides the pallet from the top of the engine car to the top of a small table located at the work station. The reverse pallet transfer is also possible. The sliding of the pallets at each station is done by a small DC motor that provides a linear move to an overhead fork assembly through a sprocket-chain mechanism. The circuit shown in Figure 4 is also used for all motor controls in this system. The layout of this system is shown in Figure 2.
I f the start point mismatches the position of the cart, an error message will be issued, and a new choice should be made thereafter. This part is extremely helpful in familiarizing the students with the system and the names used to identify the different locations. When everything is in order, the computer will load a main program and run the program. Now the screen will display the main menu as follows: LAYOUT
! COMMANDS ![1] GOTO WHSE A ' ..... B - - - ![2] GOTO WHSE 'B' ! [3] GOTO WKST A ' I [4] GOTO WKST 'B' B ! [5] WKST 'A" < - - P A R T - WHSE ! [6] WKST 'A' --.:~CART A ! [7] WKST B"--~--PART ! [8] WKST 'B" --:;-CART ! [A] WRITE TO DISK SPEED 20 ! [8] DELETE FILE .................................. ![C] MODIFY FILE FILE ! [D] EXECUTE FILE CMD WAIT(S) PALLET ! [E] SET CART SPEED 1> ! IF] END OF JOB WKST A ......
2>
3-" 4..-
5> 62-
7:' 9:>
Figure 6 The Cart Moving Two Pallets
67
YOUR CHOICE ? _ _
Journal ok/"Manutacturing SvMe'm$ Volume 5 No. 1
technical notes
The purpose of this technical note has been to describe an integrated scale model of a flexible production line and automated storage and retrieval system developed for the Department of Industrial and Systems Engineering at the University of Southern California. The scale model has been developed to be used as a teaching tool to demonstrate the integration issues as well as computer interfaces encountered in flexible automation. Besides educational purposes, the scale model described above is currently being used for physical simulation of scheduling and control algorithms.
There are three sections in the main menu: the layout section, the file section and the command section. The layout section shows the track layout, the locations of the load, unload stations and the workstations and the actual cart locations (i.e., the movements of the cart can be followed on the screen). The file section keeps the records according to the instructions set to move the cart, load, and unload. Each file is allowed to store up to 99 instructions. The file section displays 10 instructions at a time. The command section displays available command options. The user can select from the list of commands by pressing the appropriate key or by shifting the cursor to the appropriate command and pressing the return key. Integration of this setting with robotized work stations and networking of the computers in control of the subsystems result in a realistic FMS environment capable of demonstrating the principle problems involved. The system can also provide a research tool for experimenting with various control and scheduling software. Addition of miniature robots to the work stations to perform simple assembly operations on the items retrieved from the warehouse results in a realistic integrated FMS that provides for a range of related experimentations. The system exposes the students to the hardware, software, and interfacing techniques involved in a complex computer controlled manufacturing environment.
A cknowledgement
The development of the F M S physical simulator has been partially supported by the Faculty Research and Innovation Fund of the University of Southern California.
Bibliography Bed,xorth, D. and Sobczak. J., "'Students Learn on A S / R S Model". Journal of Industrial Engineering. December 1976. Groover, M.. Computer-Aided Design and Manufacturing. Prentice-Hall, Inc.. 1984. Khoshnevis. B.. "'Computer-Aided Production Sxstems Laboratory at USC", Proceedings of the Annual International Industrial Engineering Conference, Los Angeles. pp, 488-494, May 1985. Khoshnevis, B, and Rogers. R.. "'An Automated Warehouse Simulator S~stem". Proceedings of the F(/?h htternational Conference on Automation in Wareho,tsing. December 1983. Kiran, A.S. and Tansel. B.C., "'A Framework for Flexible Manufacturing S 3 stems", Proceedings o[ the ,4nnual biternational btdustrial Engineering Cot~ference. Los .Angeles, pp. 446-450, May 1985. Rank 3. P.G., The Design and Operation of F3IS, NorthHolland Publishing Co., 1983.
Summary Scale models are ideal tools for educational purposes. They are also efficient and inexpensive ways to analyze and test actual systems prior to their construction. Time, effort and money can be saved by first working out the bugs on a scale model in order to maximize start up efficiency of the actual systems. In addition, scale models can be used as on-line simulators to test feasibility questions and performance of the actual systems under different operating rules and policies.
68