Sequencing - Standard

Position

The task of operation sequence optimization takes place in the scheduling process in-between the Solving External Supplies and Solving Bottlenecks sub-processes.

Results

Proper use of Sequencing - Basic Methods leads to these results.

• Primary result

  The understanding of the basic methods of sequencing gives the opportunity to gain benefits from the sequencing tools provided by the planning application.

• Usage
  • Possibility to improve delivery capability
  • Possibility to obtain a production schedule where reduction of lead times and increase of resource utilization has reached an optimum balance
  • Increase production on bottleneck resources through minimization of setup times
  • Reduce production costs related to material waste or setup time
  • Obtain higher quality piecework from operators
• Changes in M3 BE

  The process of optimizing the sequence of operations has great effect on the delivery times for the affected manufacturing orders (MO). Changes made to any MO are included when exporting the schedule to M3 BE.

Prerequisites

Before Sequencing - Basic Methods is used, you must meet these prerequisites: You must acquire a fundamental understanding of Planning and Scheduling processes and techniques.

Basic methods of sequencing

Sequencing is determined to reschedule operations in such a way that their sequence is optimized towards minimization of setup times and maximization of resource utilization.

Sequencing is conducted in two different ways: manual sequence optimization and automatic sequence optimization. The manual form is conducted entirely by the user while the automatic form is conducted by the planning application according to parameter settings made by the user. Manual sequencing is based on one simple method while automatic sequencing is based on several different methods that span from simple to advanced.

Manual sequencing

Conducting sequencing manually means that the user directly changes the sequence of operations. This is done in the Scheduling Board component where you can manually shift operations, alter the operation status and check potential results. In the Scheduling Board, it is also possible to calculate the accumulated setup time for a selected number of operations. You can then compare the setup time to the new accumulated setup time after the operations have been optimized manually.

When an operation sequence is satisfactory, you can apply sequence status 17 (sequence locked) to the operations. Sequence status 17 retains the manually optimized operation sequence during the next scheduling process.

All adopted changes is taken into consideration in the following Automatic Simulation.

Automatic sequencing

Automatic sequencing is controlled by settings in the Sequence Optimization Controls component where sequencing steps are defined. These types of sequencing steps are available:

• Standard

  This step type is used to define the order in which operations is optimized and planned based upon the user's experiences.

• Minimize setup time

  This step type is used to determine in which sequence the operations is optimized and planned focused on minimizing setup times.

• Oven

  Steps of the oven type are used to conduct sequence optimization on work center resources defined as an oven, that is, in a ceramic oven where a group of operations are taken in and out simultaneously.

• Cyclic

  The cyclic step type is used to minimize setup times according to a repeated profile pattern. For example, a cyclic step type can be set up to raise and lower the shade of colors according to a profile pattern (for a molding machine).

• Batch-sizing

  This step is used to adjust batch sizes of orders according to the resource capacities. For example, if the fixed batch size is set to 100, the batch-sizing function collects operations with equal grouping part of the co-sorting key until a quantity as close to 100 as possible is reached. For example, if two operations of 50 and 40 are collected into a sequencing group the original order quantity is 90, but the produced batch size is 100. This means that a surplus of 10 is to be distributed among the parent orders.

  The descriptions and examples in this concept document are based upon the Standard step type.

• Forward order scheduling

  There is a sixth sequencing step type which stands aside the five others: Forward order scheduling. This sequencing step type is different in that way it conducts a forward scheduling step with no sequence optimization included. For example, when two bottleneck work centers have sequence optimized operations, and in-between there is a work center that is not a bottleneck, then it is appropriate to define a forward order scheduling step in-between the two sequence optimization steps. This provides a correct plan, because it ensures that the second sequence optimization step is based upon calculation of new setup times, which again is based upon operation start and end times after the first sequence optimization.

  Automatic sequencing is conducted in the Automatic Simulation component after locking the operation sequence and before forward scheduling. These settings control the automatic sequencing process:

  • Sequencing control
  • Sequencing horizon (default or defined per sequencing step)
  • Operation start time control
  • Whether or not scheduling is continued at same secondary resource within each new sequence group
  • Whether or not operations are allowed to overlap one another during sequencing

  After sequencing the resource and the order of individual operations is locked according to sequencing status 15 (applied in automatic sequence optimization) and sequencing status 17 (according to manual sequence optimization).

Basic elements in sequencing

These are the brief description of the basic elements used in order to make automatic sequence optimization of operations possible.

Co-sorting key

The co-sorting key is used for grouping and sorting operations and is the key to control sequencing. First part of the co-sorting key (grouping part) is used to collect operations with similar properties (size, color, etc.) into sequence groups. The second part of the co-sorting key (sequencing part) is used to sort or group the operations within each sequence group.

Setup time matrix

Each record in the setup time matrix specifies the setup time between two operations on a work center resource. The standard setup times are retrieved from these resources. The earliest start time for the operation is used when searching for setup times in the matrix.

Sequencing status

Sequencing status indicates the current status for sequence optimized operations. The status can be Active (status 10), Sequenced (status 15), Sequence locked (status 17) or Locked (status 20). The Scheduled resource field shows the specific resource on which the operation is locked.

Sequencing horizon

Sequencing horizon determines the number of working days used for the operation sequence optimization. All operations, with a scheduled end time that is earlier than the calculated horizon, are candidates for being sequence optimized. The sequencing horizon is defined by a default value (set in the Initial Step during Automatic Simulation) or a specific value (the Redefine sequencing horizon field) defined for each sequencing step. If a specific value is defined on a sequencing step, the default value is overruled.

Sequencing priority

The sequencing priority controls the sequence in which the resources including sequenced operations are scheduled. The sequencing priority should be defined to reflect the normal production flow. When the sequence optimization is done the sequenced operations is assigned sequencing status 15 and these operations is scheduled during the Initial Step in Automatic Simulation. Operations assigned to the resource with the lowest sequencing priority are scheduled first, and operations assigned to the resource with the highest sequencing priority are rescheduled last.

Balancing lead time and resource utilization

The main benefit of sequencing is an improved delivery capability together with an efficient balance between lead time and capacity utilization in concrete situations. Such a situation could be, for example, that the normal production capacity and delivery capability changes with seasonal demand and variations.

Sequencing involves creating a balance between lead times and resource utilization. Even though sequencing gives significant advantages in attaining an optimal schedule, special attention must be paid to possible undesirable consequences. Contingent on the circumstances, these consequences could include:

• Prolongation of lead time. If sequencing is applied to large time horizons, then single operations have the possibility to be moved far ahead in time. This may result in longer lead times on particular orders.
• Orders produced too early or too late. Related to the possible prolongation of lead times (or the movement of entire orders), single orders may be produced earlier than required (increasing stock expenses) or later than required (decreasing delivery capability).
• Inappropriate flow between resources. When optimizing the utilization of one resource this might create a situation where the utilization of another resource is decreased. Also, the optimization of operation sequence on one resource could make it more difficult to optimize resources that follow it. For example, operations scheduled on different work centers may overlap even though they belong to the same order. This may result in starting a demanding operation before the supplying operation is finished.

The balancing act of optimizing lead time and resource utilization at the same time is illustrated by an example in the next section.

In addition to balancing lead time and resource utilization, it should be secured that operations occurring prior to the bottleneck are scheduled before the bottleneck itself is prioritized and scheduled. This is especially important if these operations control the earliest start time on the bottleneck and in this way also control the time frame for optimization on the bottleneck.

Balancing lead time and resource utilization example

This example is based on a Standard sequencing step. Three orders (A, B and C) each having three operations (10, 20 and 30) are scheduled on three different resources (1, 2 and 3). After a normal forward order scheduling the operations are scheduled from the earliest possible start time and forwards. In this case all orders can be delivered on time, but it also entails gaps in the schedule for the resources 2 and 3.

The coloring of the operations indicates the way in which they can be sequence optimized, that is, operations with the same color is collected in order to save setup time. After sequencing is conducted, operations belonging to orders A and C are now scheduled after each other. This saves 2 hours of setup on each resource. Another benefit is that the gaps is removed from the schedule, which means the resource utilization is increased. But there should also be paid attention to the changes for order B. The lead time for order B is increased and it is no longer delivered on time.

How a schedule is sequence optimized (Standard)

When the Sequence Optimization Controls component is activated, its reference point is taken from the current active schedule that is created by forward scheduling. See Automatic Simulation. In this schedule, the order of operations is controlled based on their priorities and available capacity. During sequencing the order of operations is changed in such a way that setup times are minimized and production thereby increases. The sequencing function includes these activities:

• Build sequence groups
• Optimize operations within sequence groups
• Calculate setup times

The descriptions in the following subsections are entirely based upon the definition of a sequencing step of Standard type.

Build sequence groups

An operation can be part of a sequence group if it:

• Has the same grouping part of the co-sorting key
• Has the relevant resource in its own work center
• Has the forced resource on a resource that has to be optimized

Sequencing collects groups of operations with duplicate grouping parts of the co-sorting key in those cases where changes to the co-sorting key involve increased setup times. Scheduling operations in a group right after each other reduces setup time.

Sequencing also takes note of the maximum size of each group, the order of operations within a group and the treatment of possible gaps in the schedule.

Optimize operations within sequence groups

Sequence groups are collected based on the grouping part of the co-sorting key. Next they can be sorted or grouped on the sequencing part of the co-sorting key. Choosing when best to sort or group on the sequence part of the co-sorting key depend on the relevant scheduling situation. The difference between sorting and grouping on the sequencing part of the co-sorting key is illustrated in this example:

Forward scheduling could appear as follows for a given sequence of operations collected using the grouping part of the co-sorting key. The letters represent the arrangement of the orders co-sorting keys.

By grouping instead of sorting on the sequencing part of the co-sorting key, you can sometimes avoid (as in this example) pulling some operations too far forward which causes delay of other operations.

Calculate setup times

On the work center resource is defined the way in which setup time is calculated and whether setup time is obtained from the operation, the resource or a combination thereof. Both standard and reduced setup times can be assigned to each respective operation.

Setup time for current operations is always calculated as it relates to the previous operation. In addition, calculation of setup time is carried out on operations, which fit into these categories:

• The operation is the first in a sequence group.
• The operation within a sequence group is in progress (operation status 50) or locked (sequencing status 20).
• The operation is within the locked sequence horizon and is in progress (operation status 50) or locked (sequencing status 20).

Thus, locked operations are taken into account when calculating setup time and changes in subsequent scheduling are prevented. In this way all operations are planned with correct setup times.

Special conditions related to sequencing

Even though significant advantage using sequencing is attained, there are circumstances where this optimization may not be beneficial. These circumstances are listed below:

• Situations where sequencing should be avoided
• Situations where sequencing demands special attention
• Special considerations regarding lead time

Situations where sequencing should be avoided

Sequencing should be avoided in these situations:

• The possibility of reducing setup times on the resource is minimal or non existent.
• Clarification of the best order of a series of operations takes place as an integrated part of the production process. For example, it is done by the operators.
• There is no buffer time in the production process. Buffer time should be at least the same as the sequencing horizon.
• A bottleneck resource is defined as a fill-up resource.

Situations where sequencing demands special attention

These are some tips that may help when parameters for sequencing are defined.

• Large number of operations with equal grouping part of the co-sorting key.

  If there are many operations with the same grouping part of the co-sorting key, the orders with the last sequence optimized operation have prolonged lead times. You can limit this effect by defining realistic values for the parameters: Maximum time allowed for grouping and Maximum operation time within group.

• Work centers with multiple resources.

  If you sequence optimize a work center and it contains more than one resource, all operations in a sequence group is scheduled on that resource and the operation that comes first has the earliest start time.

• Sequencing horizon must be defined.

  The most common reason operations fail to be sequence optimized is that the sequencing horizon (represented in days) has been omitted. This parameter is defined in the Initial Step during Automatic Simulation and calculated based on the schedule start time.

• Operations depending on order delivery date.

  It needs careful attention when sequence optimization is conducted on a resource where one or more operations depend on an order delivery date. The delivery date establishes boundaries for the earliest start date for the first operation in the order, which can result in gaps in the use of the resource. You can limit this problem by setting the parameters Gap control and Maximum gaps allowed within groups.

• Sequencing on more than one resource at a time.

  If sequence optimization is conducted on more than one resource at a time, a sequencing step with forward scheduling should be placed between each work center optimization. Moreover, if an order uses more than one work center, it is also necessary to define a sequencing step between optimizations of each work center.

• Operations without operation time.

  If you have operations without operation time (time = 0), sequencing can give an erroneous picture. This is because these operations are placed in the schedule at the schedule start, even though their respective resources are not available at that point. The sequence horizon is calculated based on operation end time, which in this case is the schedule start time.

Special considerations regarding lead time

Special attention is given to changes in lead time if:

• Scheduling is conducted using pull-up after sequencing. See Pull-up parameters.
• Material synchronization is conducted between two sequence optimizations on two different levels in the network, such as finished goods and intermediate goods. See Material synchronization.

Advanced sequencing methods

Although the basic methods of sequencing (Standard sequencing step, forward scheduling and manual optimization) involve a number of potential improvements towards an optimized schedule, they are not always sufficient to solve any kind of sequencing problem. The planning application provides several advanced sequencing methods, which are described in separate concept documents.

• Batch-size adjustments

  Used for adjusting batch sizes automatically when optimizing operation sequence on resources that produces orders in fixed batch sizes.

• Cyclic sequence optimization

  Used for optimizing resources where the sequence of operations follows a certain pattern that is repeated.

• Oven scheduling

  Used for optimizing resources that can handle several operations at the same time.

Parameters in Sequencing - Standard

Step ID

The Step ID controls the order in which the Planning application carries out different steps during sequence optimization. The step ID must be larger than zero. The sequencing step with the lowest step ID is scheduled first.

Enable

This field is used to enable or disable the step. If a sequence step should not be executed temporarily, then you can disable it, which keeps the definition if the sequence again later is enabled. A scenario is that different sequence steps and logic is used in high season and low season, then a high season sequencing step is disabled in low season. It is easier to enable it again for the next high season.

Description

You can add a description of the step in this field. You can type any letters or figures.

Work center

You can define the work center in this field, which you want the sequence steps to apply, that is, the work center where you want to optimize the order of the operations. A sequence group is initiated by finding the operation that is not yet scheduled, has the earliest forward scheduled time point in the original schedule, and scheduled on one of the resources within the given resource group. This operation is the initiating operation, which is used to find other similar operations in the sequence group. The resource where the initiating operation can have the earliest start time is then selected as the resource where the sequence group is planned on.

Grouping part of co-sorting key

The grouping part of the co-sorting key is used to group operations. All operations in a sequence group must have identical grouping parts of the co-sorting key controlled by the value of the initiating operation. The program finds all operation with planned start which are inside a horizon from the initiating operation.

You can construct the co-sorting key by the automatic function Co-sorting key generator and contains the elements which controls the sequencing.

Maximum time allowed for grouping

Grouping horizon is calculated as the maximum time allowed for grouping added to the planned of the initiating operation. All operations with planned start inside this horizon are added to the sequence group if they also have identical grouping part of co-sorting key as the initiating operation.

Maximum time allowed for grouping according to delay calendar

Respecting delay calendar in maximum time allowed for grouping considers weekend, holidays, and other none-production days defined in delay calendar. If horizon is 4 days, then grouping from a Monday or Friday will give a different sequence group unless delay calendar is considered.

Maximum Operation Time Within Group

The maximum total operation time within a sequence group is specified in hours. Every operation added to a sequence group, adds to the overall operation time.

You can use this function to avoid large sequence groups. For example, there is one primary grouping part of co-sorting key (product type), which takes 80% of production volume, and grouping on week level would take the first 4 days of the week and other product types are produced end of week with possible late deliveries. Limiting maximum group would break the main product type into several sequence groups during the week and allow other product types, which must be grouped on week level, to be produce earlier in the week.

Maximum resource time within group

The maximum total resource time within a sequence group is specified in hours. Every operation added to a sequence group, adds to the overall resource time.

Using maximum resource time instead of maximum operation time, handles two scenarios. If a resource can change in capacity, for example, in high season we increase from 6 persons in assembly 8 persons, then resource time considers the changing capacity level. Another scenario is the operations using different work centers, which have different efficiency on the work center resource relation defined in PDS011. Maximum resource time is independent of the work center resource relation the different operations have.

Redefine sequencing horizon

Automatic simulation defines the sequence horizon, which is the number of working days you can use to define the Default sequencing horizon of the operation sequence optimization. You can specify a value between 0 to 999.

This horizon is calculated by adding the schedule start date to the number of working days specified here. The non-working days are included based on the delay calendar. You can sequence optimize all operations with a planned end earlier than this horizon.

This parameter allows the user to redefine the sequence horizon for each sequence step. This is useful if some sequence steps for certain resource should have one sequence horizon and other sequence steps for other resource should have another sequence horizon.

Group Operations with Same Schedule Number

Select the parameter to group operations with same schedule number. If one operation is included in a sequence group, then other operations which have the same schedule number are also included in the same sequence group. You can set the schedule number in M3BE. This option allows SWB to respect schedule number in sequence optimization.

Break group if causing delay

If sequencing causes delay for an operation, then that operation and the remaining of operations in the sequence group is not planned. Instead, a new initiating operation is identified and a new sequence groups is found and planned.

Sequencing part of co-sorting Key

This parameter is the first character in the sequencing part of the operation's co-sorting key. The sequencing part of the co-sorting key is used to sort operations within the group. You can also collect operations with identical sequencing parts of the co-sorting key in groups within individual sequence groups.

Sorting and Grouping

This field defines whether operations with equivalent sequencing part of co-sorting key are sorted or grouped. The field is used when setting up a standard sequencing step.

The field has these options:

• Sorting. You can conduct sorting by using a variety of different sorting options. Sorting is only conducted if sequence optimization is enabled. This is enabled in the initial step in the Preparation and Sequencing panel.
• Grouping. By grouping instead of sorting on the sequencing part of the co-sorting key, you can in some cases avoid that some operations are pulled too far ahead, thereby causing delays in other operations. By grouping, the operations are always sorted on planned start, before grouping is conducted.

Sorting Options

This field is used to set the sorting method for operations within a sequencing group when setting up a sequencing step of the type Standard. When sorting operations within a sequence group, these options are available:

• Standard. The operations within the sequence group are sorted according to parameter settings in the Preparation and Sequencing panel. The planned start and latest start times are not taken into account.
• Planned start. The operations within the sequence group are sorted as in Standard. Afterward, all operations with equivalent sequencing part of the co-sorting key are sorted based on the forward scheduled start (planned start) with respect to finite capacity on the work center resources.
• Latest start. The operations within the sequence group are sorted as in Standard. Afterward, all operations with equivalent sequencing part of the co-sorting key are sorted by backward scheduled start (latest start) based on infinite capacity.

Only Use Resources Common to Step

The Work center defined on a Sequencing step is only used as an initial selection criterion, when collecting operations for sequencing. During the sequencing itself, the Work center defined on the operations is used instead, that is, you can plan the selected operations on all resources in the operation Work center. If this Work center and the Work center defined on the Sequencing step only share some of the resources, then you can plan operations on resources outside the Sequencing step Work center.

To avoid this situation the parameter, only use resources common to the step and you can set on the Sequencing step to guarantee that the operations are planned only on resources that are common to both the Work center defined on the operations and the one defined on the Sequencing step.

Using this parameter, it is also possible to sequence on a specific resource. To do this, you must create a Work center for each of the resources that are included in the operation Work center and then create a sequencing step for each of these Work centers with the new parameter enabled.

Gap Control

Gaps in sequencing are normally investigated only as they occur within a sequence group and not as they occur between the first operation in a group and the previous operation. Gaps which occur between sequence groups are problematic on the bottleneck. Gap control helps to ensure that you can avoid many kinds of gaps. These options are available:

• ' ': All gaps accepted. All gaps are accepted
• S: Remove operation from group. All operations creating unacceptable gaps are removed from the sequence group and treated in the next sequence group
• P: Postpone operation until later. This parameter is depending on these Sequence Optimization Types:
  • Sequence Optimization Type = Standard: All operations creating unacceptable gaps are postponed until the gap between the current and the previous operation is acceptable. If you cannot do this, the operation is removed from the group and treated in the next. If the parameter Group operations with same schedule number is also selected, Gap control respects the grouping, that is, if one operation is skipped due to gap control, then all other operations with the same schedule number are also skipped.
    Note: You can skip an operation temporarily then plan all operations with the same schedule number according to gap control. You can take the skipped operation into the plan again. If you cannot plan according to gap control, all the operations in the group are skipped.
  • Sequence Optimization Type = Minimize setup time and Multi level: Since setup time increases with the size of the gap, the operation causing a gap is assigned a lower priority.
  • Sequence Optimization Type = Oven, Cyclic, and Oven repetition: All gaps are accepted, that is, the sequence group is scheduled with the same start time as the latest operation.
• Y: Create new group. Scheduling on the current sequence group is stopped and a new sequence group is created. This option is not available when using the sequence optimization type Multi level.

Maximum Gaps Allowed Within Groups

This option assigns a maximum size that gaps between two operations within a sequence group. The size is given in centihours. If a gap exceeds this size, the parameter gap control controls the subsequent handling.