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by John Ball
Sequenced Delivery System, a refinement of:
Just-in-Time (JIT) Inventory Control
Real-Time Storage and Retrieval
Radio Frequency (R.F.) Controlled Terminals
Event Driven Programming
Electronic Data Interchange (EDI)
Inter-Process Communication
Ever wonder how an automobile factory can build vehicles of various colors and models one after the other while keeping almost no inventory of parts?
Can Radio Frequency (RF) terminals be incorporated into your information systems?
This article is a case study describing a CA-Clipper solution to a mission critical application in an automotive manufacturing environment. The information system brings together TELXON radio frequency (R.F.) controlled terminals, electronic data interchange (E.D.I.) and application software written in CA-Clipper.
The Scenario:
The company, a plant in the Magna International group, is a large auto parts manufacturer under contract to deliver various models and colors of parts to a Chrysler automobile assembly plant. The parts must be delivered in the exact sequence in which the vehicles are being built. In a sequenced just-in-lime (J.I.T.) manufacturing environment only the parts needed for a specific vehicle are shipped.
Since the manufacturer produces parts in a batch process, they must balance the random shipping requirements against the need to produce the parts in batches. The company must constantly monitor future demand for the various parts in specified colours, build appropriate batches of those parts, store in a racking system, then retrieve the parts one at a time carefully following the sequence in which the customer demands the parts.
The customer requires that the right part be delivered with only a 4 hour lead time. The margin for error is small since picking takes I hour and shipping takes another hour. Failure to ship the right part on time can result in a shutdown of the automobile assembly plant. If this happens, the manufacturer is subject to huge per hour charges for its customer's downtime.
Managing inventory is crucial to the success of this enterprise. The company must keep as little inventory on hand as possible, but always be able to cover the demands of the customer. Since inventory is turning every 2-3 days, inventory accuracy and part quality are of paramount significance to maintain a high level of customer satisfaction.
Technology Employed
Since system up-time is critical a RAID level 5 disk system was added to the Novell 3.11 network. The database is managed by the CA-Clipper programming language. Plant personnel update the database with forklift mounted R.F. terminals and hand-held R.F. terminals. Several vendors offer suitable R.F. equipment. After a selection process, TELXON R.F. equipment was chosen. These R.F. terminals are equipped with either laser guns or wands to read bar coded pan numbers and rack labels. Although the R.F. devices are programmable, the system handles all look-ups and screen displays from the host workstation. This means that more data is transmitted to the R.F. devices, but provides the benefit that the application software can be changed or ported easily to other R.F. equipment. All R.F. I/O is handled in one module; thus any change to the physical characteristics of the R.F. devices is isolated to this one module. One network workstation has an R.F. base station connected through a serial port. This base station receives and transmits data to and from the CA- Clipper database software. (See Figure below)
R.F. Units connected to a network
How R.F. Works (A Programmer's Perspective)
Communication with the R.F. terminals (TELXON calls them Portable Tele Computers (PTC» is accomplished through a CA-Clipper compatible library. The purpose of the library is to open a serial connection to the TELXON base station, send messages to the base station, receive messages from the base station, and close the connection. The TELXON supplied library works flawlessly.
Transmit from Base Station
A transmission to an R.F. device begins when the application software directs a message to a particular R.F. unit. This message is sent to the R.F. base station where it is packaged up for transmission.
Each R.F. computer has a unique ID. Although the transmission is broadcast to all R.F. devices, the signal is only acted upon when the R.F. computer's ID is referenced. The R.F. unit decodes the message into a screen display and possibly requests some data input. The data input can be entered through the keyboard or through a bar code reader.
Receive by Base Station
The usual way for an R.F. computer to convey data to the host is for the operator to press the 'SEND' key. This sends the R.F computer's ID and the data back to the base station. The base station syntactically checks for a good transmission then forwards the data to the application software on a workstation.
Event Driven Programming
The system interactively checks each data input against the database for validity in real-time interactive mode (these are not block mode transmissions). There are many R.F. devices communicating through one R.F. base station. All of the remote R.F. devices are handled by single threaded, single tasking application logic residing on one workstation. The software sees a stream of inbound transmissions coming from various R.F. devices at random. Since a complete transaction can require several verification steps to complete, the system must do a form of multitasking to keep track of what stage of which type of transaction the R.F. device currently being serviced was doing last. A form of context switching is employed to restore and save key elements of each transaction across terminal calls.
Implementation
Sequenced Delivery System
The sequenced delivery system consists of the physical parts storage (a flow-thru racking system), the logical inventory system (a database on a PC network) and facilities to update and interrogate the database in real- time.
This system reflects an integrated customer-supplier parts delivery system. It is decidedly customer-driven since the manufacturing, delivery and payment events are triggered by E.D.I. transmissions from the customer. Each of the four E.D.I. transmission types are discussed below.
1.2 Week Forecast -A 2 week E.D.I. forecast from Chrysler lets the scheduler develop a rough-cut plan for a rolling 2 week period. This information is also provided to the Purchasing Department to drive component procurement.
2. 48 Hour Notice -This E.D.I. data is a vehicle-by-vehicle list of automobiles to be produced in the next 48 hours including the VIN (Vehicle Identification Number). Since vehicles can be shuffled or removed from the assembly line, this is not a sequenced list of vehicles. The production planner nets the requirements from this list against on-hand balances, and adjusts the production plan to ensure that all parts required will be available for shipment.
3. Ship Trigger -This is a vehicle-by-vehicle demand to ship the part. From this data a truckload of parts is accumulated (96 parts) into a single order. The order is picked, a Bill of Lading generated, and the truck leaves for the Chrysler assembly plant. Keeping the right quantity of good quality parts is crucial to the success of this program.
4. Payment- From a trigger point on the Chrysler assembly line, vehicle data is automatically captured and daily payment data is transmitted back to the manufacturer. This information is used to close the order-ship-payment loop. On a part-by-part basis, the Accounts Receivable Dept. updates shipping data with payment data. Automatically matching payments to shipments is essential in this kind of a high volume situation.
Inter Process Communication
Two important processes must run to complete each delivery:
1. the R.F. order picking program, and
2. the background process (daemon) that prints the Bill of Lading, moves orders to a shipment file, retrieves the next set of orders from the EDI data, and prints a pick list and other documentation.
Since the R.F. picking program and this daemon process are mutually exclusive operations (no picks can occur when orders are being generated and vice versa) a simple semaphore system prevents a collision between the two programs. When the R.F. has completely picked a truckload of orders, it turns itself off preventing further picking. When the entire order is filled, the daemon is turned on. The daemon performs its various chores, then. turns itself off and turns on the R.F. pick so that the next truckload of orders can be picked. The daemon continually monitors the semaphore, only proceeding when appropriate.
Operation of The Racking System
The Sequenced Delivery System controls the inventory and shipping of finished goods. A flow-thru racking system is in place to act as a buffer between the large batch production runs, and the pseudo-random part requirements from Chrysler. Parts are added to the racks under R.F. control from the database application which directs the forklift operator to store parts in a specific rack location. (See Figure 2 )
As parts are picked from the lower racks, the R.F. system directs the forklift trucks to move parts from higher locations to the lower picking lanes.
The R.F. functionality has 5 main features outlined below.
I. Add -Pallets of finished goods are received by scanning the part number. The system directs the forklift to put stock away in either the flow-thru locations, the push-back locations, or static racks.
2. Move -Since parts can only be picked from flow-thru lanes, parts previously added to the push-back or static locations need to be moved to the flow-thru locations. This move is triggered either when the lane is empty, or a location has fallen below the minimum threshold quantity. The Move and Add routines can be used to enforce a first-in first-out (FIFO) inventory flow.
3. Pick -For each part required, the Pick routine will direct pickers to the flow-thru location containing that part. Since the Add, Move and Pick routines are operating in real-time, the instant that parts have been moved to flow-thru lanes, the system can direct pickers to use that part to fill an order.
4. Rejects -Parts discovered to be defective (hopefully a rare event) can be rejected through the R.F. terminals.
5. Inquiry -The R.F. devices can be used to inquire into the current status of inventory. By entering a rack location, the part number, description and quantity-on-hand are displayed. By entering a part number, the locations and quantities of that part are displayed.
THE FINISHED GOODS RACKING SYSTEM
Figure 2-Cross Section of the Racking System
A. Batches of parts on pallets are received from Production. Through R.F. communication, the computer directs the forklift operator to the correct put-away location.
B. Since parts can only be picked from the Flow-Thru lanes, the forklift operator must both "ADD" new parts from production and "MOVE" parts from the Push-Back lanes and Static racks into the Flow-Thru lanes.
C. Based on live shipping requirements, the R.F. transmits information to the parts pickers, indicating which of the 174 Flow-Thru lanes contain the part to be picked next.
D. A truckload of parts is picked, verified for accuracy and shipped to the automobile assembly location
Constant availability of the system (about 20 hours a day sometimes 7 days a week) is crucial to the success of this venture. All of the functionality of the R.F. system is duplicated in the networked workstations. If the R.F. fails, the network becomes the backup system with some loss of the real- time responsiveness which is available through the R.F. devices.
Conclusion
The sequenced delivery system is a response to the customer's JIT inventory requirements, using EDI data to manage a real- time mission critical inventory storage and retrieval system, augmented with bar code readers, and embodying elements of event driven programming and inter-process communication.
Glossary
Sequenced Delivery System
Parts are delivered to the customer in the exact sequence that they will be used in his facility .E.g. : A vehicle manufacturer can build many models and colors. The supplier must deliver the right parts in the exact order that the vehicles are assembled.
Electronic Data Interchange (EDI)
Electronic data interchange refers to the computer to computer exchange of business documents between trading partners.
Flow-Thru Racking System
A flow-thru racking system is a rack equipped with rollers to allow pallets added on one side to flow to the other side (a queue). Push-back racks are slanted in the opposite direction (a stack). The system described also uses static racks for low volume parts and for overflow parts.
John Ball is a Senior Consultant with Strawberry Computing Corp., a Toronto area software company. John holds a M.Sc. degree in Computer Science and has over 20 years experience in systems management, software development and computer education. John can be reached at (416)948-6327.
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