SHOP FLOOR CONTROL Shop floor control is concerned with the release of production orders to the factory, monitoring and controlling the progress of the orders through the various work centers, Acquiring current information on the status of the orders. A typical shop floor control system Consists of three phases: (1) Order release, (2) Order scheduling, (3) Order progress. In today's implementation of shop floor control, these Phases are executed by a combination of computer and human resources, with a growing Proportion accomplished by computer automated methods.
Order Release "The order release phase of shop floor control provides the documentation needed to process a production order through the factory. The collection of document is sometimes called the shop packet.
It consists of: (1) The route sheet, which documents the process plan for the item to be produced: (2) Material requisitions [0 draw the necessary raw materials from inventory: (3) Job cards or other means to report direct labor time devoted to the order and to indicate progress of the order through the factory; (4) Move tickets to authorize the material handling personnel to transport parts between work centers in the factory if this kind of authorization is required; and (5) Parts list, if required for assembly jobs. In rho operation of a conventional factory, which relies heavily on manual labor, These are paper documents that move with the production order and are used to track its progress through the shop. In a modern factory, automated identification and data capture technologies are used to monitor the status of production orders, Thus rendering the paper documents (or at least some of them) unnecessary The order release module is driven by two inputs, The first is the authorization to produce that derives from the master schedule. This authorization proceeds through MRP which generates work orders with scheduling information. The second input to the order release module is the engineering and manufacturing data base which provides the product structure and process planning information needed to prepare the various documents that accompany the order through the shop.
Order Scheduling The order scheduling module follows directly from the order release module and assigns the production orders to the various work centers in the plant. In effect, order scheduling executes the dispatching function in PPC. The order scheduling module prepares a dispatch list, which indicates which production orders should be accomplished at the various work centers. It also provides information about relative priorities of the different jobs, for example, by showing due dates for each job. In current shop floor control practice, the dispatch list guides the shop foreman in making Work assignments and allocating resources to different jobs so that the master schedule can best be achieved. The order scheduling module in shop floor control is intended to solve two problems in production control: (1) Machine loading and (2) Job sequencing. To schedule a given set of production orders or jobs in the factory, the orders must first be assigned to work centers. Allocating orders to work centers is referred to as machine loading. The term shop loading is also used, which refers to the loading of all machines in the plant. Since the total number of production orders usually exceeds the number of work centers, each work center will have a queue of orders waiting to be processed.
The remaining question is: In what sequence should these jobs be processed? Answering this question is the problem in job sequencing. Job sequencing involves determining the sequence in which the jobs will be processed through a given work center. To determine this sequence, priorities are established among the jobs in the queue, and the jobs are processed in the order of their relative priorities, Priority control is a term used in production control to denote the function that maintains the appropriate priority levels for the various production orders in the shop. Priority control information is an important input in the order scheduling module. Some of the dispatching rules used to establish priorities for production orders in the plant include: • First-come-first serve. Jobs are processed in the order in which they arrive at the machine. One might argue that this rule is the most fair. • Earliest due date. Orders with earlier due dales are given higher priorities. • Shortest processing time. Orders with shorter processing times are given higher priorities. • Least slack time. Slack lime is defined as the difference between the time remaining until due date and the process time remaining. Orders with the least slack in their schedule are given higher priorities. • Critical ratio. The critical ratio is defined as the ratio of the time remaining until due date divided by the process time remaining. Orders with the lowest critical ratio are given higher priorities. When an order is completed at one work center, it enters the queue at the next machine in its process routing. That is, the order becomes part of the machine loading for the next work center, and priority control is utilized to determine the sequence of processing among the jobs at that machine. The relative priorities of the different orders may change over time. Reasons behind these changes include: (1) Lower or higher than expected demand for certain products, (2) Equipment breakdowns that cause delays in production, (3) Cancellation of an order by a customer, and (4) Defective raw materials that delay an order. The priority control function reviews the relative priorities of the orders and adjusts the dispatch list accordingly.
Order Progress The order progress module in shop floor control Monitors the status of the various orders in the plant, WIP, and other characteristics that indicate the progress and performance of production. The function of the order progress module is to provide information that is useful in managing the factory based on data collected from the factory. The information presented to production management is often summarized in the form of reports, such as the following: • Work order status reports. These reports indicate the status of production orders.
Typical information in the report includes the current work center where each order is located, processing hours remaining before completion of each order, Whether the job is on-time or behind schedule, and priority level. • Progress reports. A progress report is used to report performance of the shop during a certain time period (e.g., week or month in the master schedule). it Provides information on how many orders were completed during the period, how many orders should have been completed during the period but were not, and so forth.
Exception reports. An exception report indicates the deviations from the production schedule (e.g. overdue jobs). and similar exception information
The above reports are useful to production management in making the decisions about allocation of resources, authorization of the overtime hours, and other capacity issues, and in identifying areas of problems in the plant that adversely affect the implementation of the master production schedule.
Manual (Clerical) Data Input Techniques. Manually oriented techniques of factory data collection are those in which the production workers must read from and fill out paper forms indicating order progress data. The forms are subsequently turned in and compiled, using a combination of clerical and computerized methods. The paper forms include: • Job traveler. This is a log sheet that travels with the shop packet through the factory. Workers", ho spend time on the order are required 10 record their times on the log sheet along with other data such as the date, piece counts, defects. and so forth. The job traveler becomes the chronological record of the processing of the order. The problem with this method is its inherent incompatibility with the principles of real-time data collection. Since the job traveler moves with the order, it is not readily available for compiling current order progress. • Employee time sheets. In the typical operation of this method. a daily time sheet is prepared for each worker, and the worker must fill out the form to indicate work that he/she accomplished during the day. Typical data entered on the form include order number, operation number on the route sheet, number of pieces completed during the day, and time spent. Some of these data are taken from information contained in the documents traveling with the order (e.g., typical documents traveling with the order include one or more engineering drawings and route sheets). The time sheet is turned in daily, and order progress information is compiled (usually by clerical staff). • Operation tear strips. With this technique, the traveling documents include a set of preprinted tear strips that can be easily separated from the shop packet. The preprinted data on each tear strip includes order number and route sheet details. When a worker finishes an operation or at the end of the shift, one of the tear strips is torn off. Piece count and time data are recorded by the worker. and the form is turned in to report order progress. • Prepunched cards. This is essentially the same technique as the tear strip method. except that prepunched computer cards are included with the shop packet instead of tear strips. The prepunched cards contain the same type of order data, and the workers must write the same kind of production data onto the card, the
difference in the use of pre punched cards is that in compiling the daily order progress, mechanized data processing procedures can be used to record some of the data.
Factory Data Collection System There are a variety of techniques used to collect data from the factory floor. These techniques range from clerical methods which require workers to fill out paper forms that and later compiled, to fully automated methods, that require no human participation. The factory data collection system (FDC system} consists of the various paper documents, terminals, and automated devices located throughout the plant for collecting data on shop floor operations. Plus the means for compiling and processing the data. The factory data collection system serves as an input to the order progress module in shop Floor control. It is also an input to priority control, which affects order scheduling. Examples of the types of data on factory operations collected by the FDC system Include: piece counts completed at a certain work center. Direct labor time expended on each order. Parts that are scrapped. Parts requiring rework, and equipment downtime. The data collection system can also include the time clocks used by employees to punch in and out of work. The ultimate purpose of the factory data collection system is twofold(1) To supply status and performance data to the shop floor control system and (2) to provide current information to production foremen, plant management. and production control personnel. To accomplish this purpose. the factory data collection system must input data to the plant computer system. In current ClM technology. this is done using an on-line mode, in which the data arc entered directly into the plant computer system and are immediately available to the order progress module, The advantage of on-line data collection is that the data file representing the status of the shop can be kept current at all times. As changes in order progress are reported. these changes are immediately incorporated into the shop status file. Personnel with a need to know can access this status in real-time and be confident that they have the most upto-date information on which to base any decisions. Even though a modern FDC system is largely computerized. paper documents are still used in factory operations, and our coverage includes both manual (clerical) and automated systems.
AIDC TECHNOLOGIES I. Optical. Most of these technologies use high-contrast graphical symbols that can be interpreted by an optical scanner. They include linear (one-dimensional) and two dimensional bar codes, optical character recognition, and machine vision. 2. Magnetic. Which encode data magnetically, similar to recording tape. The two important techniques in this category are (a) magnetic stripe, widely used in plastic credit cards and bank access cards. and (b) magnetic ink character recognition. Widely used in the banking industry for check processing, 3. Electromagnetic. The important ADC technology in this group is radio frequency identification (RFlD). 4. Smart curd. This term refers to small plastic cards (the size of a credit card) imbedded 'With microchips capable of containing large amounts of information. Other terms used for this technology include chip card and integrated circuit card. 5. Touch techniques. such as touch screens and button memory. 6. Biometric. These technologies are utilized to identify humans or to interpret vocal commands of humans. They include voice recognition, fingerprint analysis, and retinal eye scans
BAR CODE TECHNOLOGY Bar codes divide into two basic types: (1) Linear, in which the encoded data are read using a linear sweep of the scanner, and (2) Two-dimensional, in which the encoded data must be read in both directions.
The advantages of the automatic data collection methods are:
(i) The accuracy of data collected increases (ii) The time required by the workers to make the data entry can be reduced.
The basic elements in data collection systems are: (i) Machine readable media (ii) Terminal configuration
(iii) Software for data collection.
