Preactor 400 APS Production Scheduling Software

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In particular, Preactor 400 APS has additional scheduling techniques (event and resource focused), which allows you to select and create your own production scheduling rules, and it has features for adding material constraints based on the Bill of Materials supplied to it either from your ERP/MRP system or through the Preactor BOM Exploder in Preactor 500 APS. Traditionally in production scheduling terms, the availability of advanced production scheduling rules and the ability to have material constraints at the raw, intermediate and finished product levels, is described as Advanced Planning and Scheduling (APS). Production Scheduling Algorithms All the production scheduling tools in the Preactor family can use the sequential or job at a time scheduling method. Here, each job or order, is loaded in sequence, dependent on due date, priority etc and the operations are loaded forward, backward or bi-directionally. Additionally, Preactor 400 APS allows you to use the parallel loading method, where, as each resource becomes free, operations are selected from the queues of work waiting to be processed using industry standard dispatching rules. For example, you may want to minimize setup times by selecting jobs in color sequence.

This shows the schedule generated using without Preferred Sequence Rule.	This shows the same orders scheduled with Preferred Sequence Rule.

A look-ahead period can be defined to decide which jobs should be pulled forward to, say, minimize setups, when generating the production schedule. In-built Scheduling Rules The in-built scheduling rules are:- Preferred Sequence (by job attribute, setup time, process time, critical ratio) Minimize Work In Process (WIP) Forwards Minimize WIP Backwards Selective Bottleneck (TOC) Dynamic Bottleneck Minimize Overall Setup Campaigning The preferred sequence rule is data driven. You can select the criteria by which the preferred sequence is created. Critical ratio is used, for example, to minimize late jobs by dynamically changing the priority of jobs based on a comparison of remaining process time with time to due date. The minimize WIP rules are sequential rules that endeavour to minimize the make-span for each job. Min WIP forward loads each job forward from the current time, locks the last operation and backward sequences from there. Min WIP backward loads each job from its due date, locks the first operation and forward sequences from there. The Selective Bottleneck rule is rule is based on the Theory of Constraints (TOC) philosophy. It works by the user selecting the 'Bottleneck Resource' or 'Bottleneck Resource Group'. Each order is then backward scheduled from the Due Date (less Delivery Buffer). Any operations loaded onto a bottleneck resource are offset by the Bottleneck Buffer time (defined in the Resource data table for each resource) which is designed to give some 'slack' such that any delays to operations before the bottleneck resource will not result in it being 'starved' of work. Preactor then detects whether any operations in that job must start before current time. If so then these operations are re-scheduled forwards using up some or the entire bottleneck buffer. If this is consumed then some or the entire delivery buffer may also be used up and the 'At Risk' or 'Late' flag is set. The Dynamic Bottleneck rule is an improvement over the classical selective bottleneck scheduling where the bottleneck will 'wander' depending on the current product mix that is scheduled. Rather than pre-processing your orders to determine a single bottleneck, the Dynamic Bottleneck Rule calculates the bottleneck individually for each job. Operations 'upstream' of the bottleneck are then backward sequenced inserting a resource buffer like the Selective Bottleneck rule. The Minimize Overall Setup rule is similar in some respects to the Preferred Sequence rule. However it is focussed purely on minimizing the setup or changeover time on resources. In the Preferred Sequence rule, as each resource becomes idle it selects the next operation to load based on the preferred sequence criteria in the resource database without any consideration of other resources that could also be used. Thus, provided one or more operations can be run on the resource and they lay within the 'Look Ahead Window' one of them will be scheduled based on the preferred sequence. In the Minimize Overall Setup rule, consideration is made across all resources able to run the operation even if they are still busy. The rule does not use the preferred sequence criteria in the resource database. It does however use the 'Look Ahead Window' to decide whether an operation should be available to be scheduled in the same way as the Preferred Sequence rule. The Campaigning rule is used when a production schedule is to be created in waves and uses a PESP script. When the rule runs, a dialog box asking the user to enter a Reference Date, Campaign Period and Number of Campaigns appears. The first pass of the rule locates all orders where the reference time entered is greater or equal to the due date of the order, these orders are then forward scheduled. The Reference Date is then incremented by the Campaign Period, the Number of Campaigns decremented. The second pass of the rule will again locate all orders where the reference time entered is greater or equal to the due date of the order. The number of passes of the rule will be the same as the number entered into the Number of Campaigns field. Creating Unique Production Scheduling Rules Unique production scheduling rules can be created in Preactor 400 APS. There are two methods to do this. Using standard actions in the Preactor Event Script Processor (PESP) a series of actions are selected from a library provided to process jobs in a series of scheduling passes. For example the actions could be... Highlight all jobs with attribute Customer = ABC Forward schedule Highlight all jobs with Order Status = Suggested Backward Schedule Schedule Remaining Jobs forward by due date Preactor 400 APS also allows you to create your own production scheduling rules using the Preactor Open Planning Board, and its extensive library of ActiveX methods. In this way almost any scheduling logic can be created. Material Pegging Preactor is designed for linking with your MRP/ERP software as well as other packages such as data collection systems, spreadsheets etc. Typically your orders along with their process route information are downloaded into Preactor, scheduling is carried out and the results are passed back to the host. For many applications this approach is adequate but it fails to overcome one of the basic weaknesses of most ERP/MRP systems in the way they aggregate materials. Preactor 400 APS has additional functionality for dealing with material and shop order pegging. It will automatically take these shop orders and link (peg) them together. Thus materials from one shop order may be used by one or more other orders. If the first order is delayed you will want all downstream orders to be delayed accordingly, or, perhaps, you may want the materials re-allocated. Preactor 400 APS uses SMC (Static Material Control) to create the links (pegs) during the download of orders to Preactor from your ERP/MRP system.
	Schematic of a typical Preactor linked with an ERP/MRP system.

The MRP Weakness The weakness of MRP in dealing with materials is illustrated in this simple bike making example. Bikes are assembled from a frame, wheels and a saddle (bought out). The weakness of MRP in dealing with materials is illustrated in this simple bike making example. Bikes are assembled from a frame, wheels and a saddle (bought out). When MRP is run, sales orders for bikes from many different customers, are converted into works or shop orders and are offset in time e.g. the start time for WO3 for assembly is offset by the expected finish time of the component parts.



If, say, WO1 is delayed then the schedule is invalid because WO3 (assembly) may not be able to start at the expected time. Another problem is that WO 1 may include frames for a number of assembly orders so that the overall impact on deliveries may be even more complex. How SMC Tackles the Problem SMC uses the BOM data from your ERP/MRP system along with your shop orders and optionally purchase and sales orders. It then allocates the materials between your orders. In our simple example where materials have been allocated to WO 3 from each of WO 1, WO 2 and PO 1, Preactor creates the links between them. These links are used by Preactor when your orders are scheduled, so not only are the machines, labor and tools taken into account, but also the availability of the materials.

	If WO 1 is delayed then Preactor can delay the other orders that depend on it. You may choose to run the SMC materials allocation as part of the download from MRP or run it independently from within Preactor.

	How SMC works A flow chart showing how materials are allocated or pegged between orders is shown below. SMC is an integral part of Preactor 400 APS. SMC first creates a transaction log for each material, listing when they will become available. The orders are then processed by earliest start date and the materials are allocated (pegged).

The usage of materials in the transaction log is updated and the next order dealt with until all orders have been processed.

Sequencing with Material Links
The links created by Preactor can be displayed in a variety of ways. Links that are within an order are shown as solid flow lines between bars on the Sequence Overview. Material Control links (pegs) between orders are shown as dotted arrows.

	Dynamic Material Control The Preactor 400 APS SMC feature can deal with the linking (pegging) of orders received from your ERP/MRP system. By providing these links Preactor APS can take into account both resource and material availability when generating your schedule. The allocation of materials produced by one works order to another consuming order is similar in concept to "parts pegging". This works well in many scheduling environments. However, pegging of materials can be a problem for some processes

where changes in demand, delay in supply, or variability in process yields can mean unacceptably long waiting times. In the food industry for example there may be a maximum time that a batch can remain in a mixer or storage tank, while finished products may have specific, well defined shelf lives. Similar concerns may be important where the value of materials is high and WIP holding cost is a significant item in the cost of production. In these environments, SMC may not work well and an alternative method, Dynamic Material Control (DMC)is available. Food Processing Example To illustrate how DMC works, consider a factory making yogurt. Here a fruit yogurt product is made from some fruit additive, base yogurt and pots. Two products are being processed, one a fruit yogurt, made from the strawberry fruit and a batch of plain low fat base material, and the other a plain yogurt that does not require any fruit.

Production Schedule produced in Preactor APS using standard SMC Preactor 400 APS's SMC feature has allocated the first batch of low fat base to the strawberry product, however as the illustration shows, due to a delay in fruit production, the schedule has become invalid and must be repaired.

After schedule repair the packing of the strawberry product has been delayed to take account of the fruit production problem.


When a repair/re-schedule takes place Preactor 400 APS uses the links produced by the SMC module to ensure that the correct sequencing is retained but the delay in packing the strawberry low-fat product means that the shelf life of the first batch of incubated base yogurt has been exceeded. The DMC functionality provides Preactor 400 APS with the capability to reallocate the materials as the production schedule is built. In this case the low-fat plain yogurt is packed using the first batch of base material while the fruit yogurt is packed later and uses the second batch of base material. SMC v DMC The key difference between SMC and DMC is that DMC allocates materials dynamically while your production schedule is being built, whereas SMC is a pre-process which is carried out prior to scheduling. As in our yogurt example above, DMC allows you to re-allocate materials from one (late finishing) batch to another. DMC is aimed at applications where product shelf life, variable process yields and/or the cost of work-in progress are significant problems. The effect of DMC is to reallocate materials destined for a delayed order to another order that has not been delayed, so meeting shelf life constraints and/or reducing work in progress costs.



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