topic Re: Shift plannification in Mathematical Optimization, Discrete-Event Simulation, and OR

Shift plannification

ThomasMuraille — Tue, 02 Apr 2019 11:26:43 GMT

Hi SAS Community !

For my final year work in operations management, I work with SAS Simulation Studio 15.1 to model the flow of an assembly workshop. My problem concerns the management of the operators' work schedule. (Note : annotations and ressources types are in french).

The work consists in assembling a kit of parts (“PIECES” resource) on a shaft (“ARBRE” resource), passing through three assembly stations in this order:

Primary assembly (MP) – operating time 31.8min - capacity 3 operators.
Final assembly (MF) - operating time 46.8min - capacity 4 operators.
Crating (MC) - operating time 22.2min - capacity 2 operators.

An operator (“OPS MONTAGE” resource) assembles these two resources. All these resources are supported by the entity named “OF”. At the end of the operations, the OPS MONTAGE resource returns to the beginning of the model to be used again. The rest is destroyed and counted in order to have the number of modules produced.

The simulation time is 64,800min, which corresponds to 10 weeks of work. A workweek of 6,480min takes place according to this program:

Day	1st shift	2nd shift	Night
Monday	480	480	480
Tuesday	420	420	600
Wednesday	420	420	600
Thursday	420	420	480
Friday	420	420	0
Saturday	0	0	0
Sunday	0	0	0
Total	2160	2160	2160
Ops scheduled	6	6	5

6 operators are required for the first and second shifts, but only 5 during the night.

So I programmed a schedule for the control of shifts :

Despite many attempts to program the agenda, each time, my operators are still 6 when they must be 5. In the example below, I must have 5 operators between time 2280 and 2760. However, at time 2328 I still have 6 operators.

I think it is because the “operator resource” must return to the "resource pool block” before being impacted by the agenda. I have already tried to make several “exits” between the assembly stations but without success. To summarize, my question is as follows:

How can we get the right number of operators, exactly when we need them?

In practice, if an operator does not have time to complete the next operation, he should not start it. Ideally, the model should anticipate the shift by preventing entities from going into the next blocks if there is not enough time to perform the next operation before the shift takes place.

I look forward to receiving appropriate assistance and thank you for your consideration.

Re: Shift plannification

nmok — Tue, 02 Apr 2019 17:35:33 GMT

Hello, thanks for your question.

It seems that you wish to be able to adjust the capacity of a mobile resource (Operator) while the operator is in service in one of the 3 Server blocks (MP, MC or MF). In order to enable immediate schedule adjustment for a resource entity held in a holding block (i.e. a Queue, Delay, or Server),

You need to connect the "OutResource" port of the Server to the InEntity port of the Resource Pool. This enables the seized Operator entity whose resource units are to be adjusted to flow back to the Resource Pool at the instant when the schedule change goes into effect.
You also need to connect the "OutPreempt" port of the holding block to a block downstream to enable the entity which had seized the resource (also referred to as the controlling entity) to flow out of the holding block. What this port is connected to depends on what you wish to do with this controlling entity. If it still needs to finish service then it needs to be routed back to the service queue. If it needs to be destroyed, it needs to be sent to a Disposer block.
Connections described in steps 1 and 2 need to be carried out for each of the Server blocks in your model (MF, MP and MC).

Information about schedule based preemption can be found in the Overview for Queue, Delay or Server. For example, the information in the Server block Overview is present in the last paragraph of the section named "Holding Block Preemption". It can be found in the online simulation Studio documentation at:

https://go.documentation.sas.com/?docsetId=simsug&docsetTarget=simsug_app1_sect029.htm&docsetVersion=15.1&locale=en

The above approach will ensure that you have the correct number of Operators exactly when you need them. However, it does not give you precise control over which Operator's schedule is adjusted. If you want more precise control, you would need to use the Resource Schedule Entity. You can look at the example model docSchedEntity to get a basic idea about how to use this feature. Description for the example model can be found at the following link in the documentation:

https://go.documentation.sas.com/?docsetId=simsug&docsetTarget=simsug_app7_sect020.htm&docsetVersion=15.1&locale=en

If you need additional assistance, we could organize a Webex session with you to understand your modeling needs in greater detail. Please feel free to reach out to SAS Tech Support in that regard so that they would be able to address your question (and any other concerns). Link to SAS Tech Support:

https://support.sas.com/en/technical-support/contact-sas.html

Re: Shift plannification

ThomasMuraille — Wed, 03 Apr 2019 10:09:19 GMT

Thank you very much !

With your answers, I am unlocked and I can go further for other applications.

Thank you again.

Re: Shift plannification

ThomasMuraille — Wed, 03 Apr 2019 14:00:35 GMT

Hi, it's me again,

I have another question concerning the same model.

Is there any way to save the "time progression" of my entity into a server ?

For example, my entity stay 10min into MP instead of 31,8 when she's preempted. Is it possible to continue the assembly with another resource during 21,8 min insted of restart it for 31,8 min?

Thank you

Re: Shift plannification

nmok — Wed, 03 Apr 2019 14:30:06 GMT

Yes, it is certainly possible to keep track of how much time is left to complete service. Instead of reading the service time from a Number holder, you can assign the service time as an entity attribute and read it in at the time of service. That way, you can modify the value of the attribute if the entity is preempted to calculate the balance of the time left to complete service.

Hope this Helps.

Re: Shift plannification

ThomasMuraille — Wed, 03 Apr 2019 15:37:04 GMT

Nmok,

I don't see how I can do that... Is it possible to have a little example with some screenshoots ?

I don't know how to make a server reading an attribute, neither how to create the balance of time when the entity is preempted. Do I use the "Time now" block ?

Your help is precious for me, I hope we'll find a solution to make this final work a real sucess !

Thank you again

Re: Shift plannification

nmok — Wed, 03 Apr 2019 18:27:44 GMT

Sure I will get back to you in a day or two.

Re: Shift plannification

nmok — Sun, 07 Apr 2019 19:50:29 GMT

Hello Thomas,

Attached is an example project created using Simulation Studio 15.1 which would address your issue. Just unzip the contents and run the project in Simulation Studio 15.1. I hope the comments in the model will help you understand the flow. Let me know if you have any questions.

Regards

Nmok

Re: Shift plannification

ThomasMuraille — Tue, 09 Apr 2019 14:01:17 GMT

Hi nmok,

Thank you again, your example helped me a lot !

I just have a last (I hope so) problem.

I try to put a probability of non-attendance on my operator resources. I want to put it at each shift between the time they are created and when they go into the model (to make an assembly). The goal is to have 5% chance of absenteeism on each operator for each shift.

I tried with and without attributes but in vain. I have always the same problem : it works for the first shift, but for the second, there is only one operator into the model to continue the previous assembly.

Do you have an idea or an easy way to create this probability of absenteeism on the ops ressource ?

Regards,

Thomas Muraille

Re: Shift plannification

nmok — Tue, 09 Apr 2019 14:11:44 GMT

Hello Thomas,

This can be modeled independently of scheduling and I can create a simple model to demonstrate that as well. A few questions from my side before I do that:

Shift change happens at specific times in the simulation, correct? (for example every 8 hours?)
So you are trying to model a situation where the schedule requires an operator to come back to work but with 5% probability, he\ she will not show up? And also if an operator is working 2 shifts in a row, then he\ she might choose not to continue working the second shift with 5% probability?

Re: Shift plannification

ThomasMuraille — Tue, 09 Apr 2019 17:55:56 GMT

Sorry I wasn't very clear in the last message. Here my answers :

1) Yes it's exactly like that. Every 8 hours (480 min into my model). Practically, I use 479 min with 6 ops and then 1 min to 0, because it help me to highlight the shift. But whatever...

2) No, none operators have to make 2 shifts in a row. I will explain the situation :

First, 6 operators take one drive shaft and spare parts to start assembly. Into the model it's 1 OPS MONTAGE + 1 ARBRE + 1 PIECES on 1 entity. They work during 480min on 3 differents servers with differents capacities. If an operator has finished his work during the shift of 480min, he starts another one (new one or ongoing one with no operator to make it, you'll see after)

At time 480, everybody have to stop the assembly and 6 others operators have to continue it during afternoon. For the night, we do the same thing but we have only 5 operators. So there is one "onging construction" with no one to work on it. The first operator who finished his work takes this ongoing one.

All this situation is done and work very well on my model, but I have to add one more situation on it : absenteeism.

So, at time 480, I have to allocate the 6 new operators on the 6 "ongoing constructions". But sometimes, someone can't makes his shift. So before allocate the operator on the "ongoing construction", I want to put a 5% chance he won't do his job (so he just does nothing).

Practically, I use 6 ressources and it's only the number of resources in uses that change. Because the goal is just to see how many constructions I can make during one week.

I hope you understand beter.

Re: Shift plannification

nmok — Mon, 15 Apr 2019 12:54:46 GMT

Hello Thomas, my apologies for the delay. I shall get back to you with a solution in a few days.

nmok

Re: Shift plannification

nmok — Mon, 15 Apr 2019 14:34:17 GMT

Hello Thomas, Can you give me the information about the arrival rate for "OF" entity, and the starting inventory values for "Arbre" and "Pieces" entities in your model?

Re: Shift plannification

ThomasMuraille — Mon, 15 Apr 2019 19:58:55 GMT

Hello,

No problem for the delay, thank you for coming back !

It's a push flow, so there is always an "OF" entity ready. I can't tell you the real number of "Arbres" and "Pieces" but you can put 100 Arbres and 200 Pieces to start.

Re: Shift plannification

ThomasMuraille — Sat, 20 Apr 2019 09:24:28 GMT

Hi nmok,

I solved my problem yesterday.

Here we have the OPS MONTAGE generator and assignation. When I start the simulation, 6 operators go into the resource pool without merging. With the "Présence" switch, I put 10% of probability to each operator to go into the delay for all the duration of his shift (which symbolizes an absence). The queue block is a storage before the assignation for the job. This is a non-blocking one.

When the shift is over, all operators are pushed by the OutPreempted or OutResource port and return into the resource pool. But if an operator has finised his job during his shift, he has to start another one without having a chance to be absent. So he goes directly into the queue block instead of the resource pool.

Re: Shift plannification

nmok — Sat, 20 Apr 2019 14:05:53 GMT

Hello Thomas,

The approach that you are using might not give you correct results.

I am working on your solution. It involves the use of Resource Schedule Entities. Also, it seems that you want to model 6 operators for shift 1, 6 for shift 2, and 5 for shift 3. So you have a total of 17 operators and each of them needs their own separate schedule (which for all operators in same shift is the same but still implemented separately for each operator).

I shall update you with the solution by Monday or Tuesday at the latest.

Stay tuned.

Re: Shift plannification

nmok — Mon, 22 Apr 2019 15:08:55 GMT

Hello Thomas, I have attached the solution model for your reference. the zip file contains the Simulation Studio project as well as a word document containing detailed explanation. the same explanation is also given below.

Hope this is helpful.

Regards

nmok

MODEL SNAPSHOT

MODEL EXPLANATION

Modeling Creation of OF entity, Shaft entities and Piece entities (see Fig. 1 below):
1. Creating OF Entities:
  - OF entities are created at a constant rate with an interarrival time of 120 minutes between 2 entities.
  - OF entities are assigned an MP_Time, MF_Time, and MC_Time attribute.
  - After assigning attributes, the OF entities wait in a queue until they can seize 1 Shaft entity, 1 Piece entity and 1 Operator entity. This queue (and all other queues in the model are nonblocking queues. Please read the section on nonblocking queues in the Simulation Studio documentation for more details)
  - OF entity will only go into Seize block if all 3 resources (Shaft, Piece, and Operator) are available.
2. Creating Shaft Entities and Piece Entities:
  - A batch of 2000 entities of type Shaft and Piece respectively, is created at time 0. This is the initial inventory for Shafts and Pieces at the start of the simulation.
  - Shaft and Piece entities are held in the respective resource pools until they are seized by the OF entity.

*RankValues will be explained in a later section

Figure 1

Figure 2

Modeling Creation of Operator Entities and Operator Schedule Entities (see Fig. 2. above):
1. Creating Operator entities:
  - Operator entities for different shifts are created in separate entity generators. Each Operator is assigned a “Shift” attribute in the entity generator during creation, as shown in the screenshot below
  - The number of Operator entities to create for shift X is controlled using the NumShiftXOps Number Holders.
  - Each Operator entity in the same shift is assigned a unique Operator number. Thus, across all shifts, each Operator entity has a unique combination for Shift+OperatorNo combination (Shift=2, Operator=1 or Shift=3, Operator=4, etc.).
2. Creating Operator Schedule Entities:
  - Operator Schedule entities for different shifts are created in separate entity generators. Each Operator is assigned a “Shift” attribute in the entity generator during creation, as shown in the screenshot below using the rectangle highlighted in orange.
  - Some default attributes are also assigned specific values as shown in the screenshot using the rectangles highlighted in blue.
  - Lastly, some additional attributes are defined on the Operator Schedule entity which are highlighted in green.
  - After creating the Operator Schedule entity, it is assigned an “Operator” attribute which corresponds to the OperatorNo attribute on the Operator entity.
  - Each Operator Schedule Entity is also assigned a TargetSearchRule attribute from a String Holder as shown in the screenshot below. For example, the screenshot shows that one of the Operator Schedule entities was assigned an attribute value of “Entity Type == Operator; Attribute Rule == Shift == 1 && OperatorNo == 6”. This means that this particular Operator Schedule Entity will be used to control the schedule of Operator No. 6 in Shift No. 1.

Operator Entity Flow:
1. All Operators from all shifts are held in a single resource pool (labeled Operator Pool) as shown in Fig. 3. Below. The Operators are seized from this resource pool by the OF entity and released when the assembly is complete. The operators are also released when there is a schedule change. In either case, Operator entities are always routed back to the Operator Pool.

Figure 3

Implementing Operator Schedules:
1. The scheduling of Operators is managed by the logic implemented in the compound block labeled “Implement Schedule”. All Operator Schedule entities enter the Resource Scheduler block at time 0.
2. Each Operator Schedule entity pulls its schedule from the correct ShiftXAgenda block depending on its “Agenda” attribute value.
3. In each ShiftXAgenda block, the schedule has been defined for Shift X for 1 operator. The total duration adds up to the total no. of hours in a week. Let’s take a look at the screenshot below. It shows the schedule of a Shift 2 operator as defined in the Shift2Agenda block.
  - The duration values add up to 10080 minutes, which is the total no. of minutes (working and nonworking) in a week.
  - The Value column shows the quantity of each Operator during each duration. Since each Operator Schedule entity implements schedule for an individual Operator entity, (based on TargetSearchRule attribute), the Resource Units specified in the Shift2Agenda block can either be 1 (i.e. Operator is available) or 0 (i.e. Operator is unavailable).
4. The Start Time value for the Schedule as specified in the “StartTime” attribute for Operator Schedule entity is 0. We need to run the simulation for 10 weeks. Hence we need to repeat this schedule specified in the Resource Agenda every week. We have specified the “ToRepeat” attribute for all Operator Schedule Entities as “true”. So, after 10080 minutes of simulation time, the Operator Schedule entity exitas the Resource Scheduler block. We now need to specify the StartTime for the next schedule (which is the current time i.e. 10080) and send the Operator Schedule entity back to the Resource Scheduler block and implement the schedule for the next week of simulation. This process is repeated for 10 weeks of the simulation for every Operator Schedule entity.
5. Operator Resource Utilization Statistics can be viewed using the Resource Stats Collector block, as shown in Fig. 3.
6. The Primary Assembly, Final Assembly, and Crating operations are carried out as shown in Fig. 4.
  - If there is a change in schedule while an OF entity is in Server, the associated Operator entity is preempted and sent back to Operator Pool. The OF entity then recalculates the remaining assembly\ crating time and waits in Pending_queue for the Operator from the next shift. It then seizes the Operator from the next shift and goes back into the Server.

Figure 4

Block Ranking:

The RankValue dialog is used to control the precedence of events in case there is a tieduring the simulation model run. For example, after a shift change, there could be an OF entity waiting to finish assembly in the Pending_queue and there could also be another OF entity waiting in the In_OF_Q. so which one gets the available Operator from the next shift? To manage such ties, Simulation Studio has a Block ranking dialog which can be used to give higher priority to certain blocks such as entity generators, Queues, Servers, etc.

In this model, we have used the block ranking dialog to:

Give higher priority to pending queues than the In_OF_Q (they have been assigned a higher RankValue =10 compared to RankValue=5 for In_OF_Q).
Specify order in which entities are created at time 0:
1. Operator entities are created first (RankValue=16).
2. Operator Schedule entities are created second (RankValue=14).
3. Resource Scheduler starts implementing schedule for all Operator entities at time 0 (RankValue=12).

Blocks with higher RankValue get higher priority. In item ii. above, we need to create Operator entities before we can create their Operator Schedule Entities. And Operator Schedule entities need to be created, assigned attributes, and routed to the Resource Scheduler block BEFORE the Resource Scheduler can start implementing the schedules. Hence, this hierarchy is important.

All other blocks in Simulation Studio have a default RankValue of 0 (except all Queue blocks which have a default RankValue of 1).

The RankValue can be specified in the Block Ranking Dialog. The Block Rankin gDialog can be viewed by right clicking the model name and selecting “Block Ranking” option. The Block Ranking Dialog appears as shown below:

Using Factors to Control Inputs

Factors have been defined in the experiment window to control various inputs to the simulation model, namely:

Initial Shaft Inventory
Initial Piece Inventory
MP_Time
MF_Time
MC_Time
# of Shift 1 Operators
# of Shift 2 Operators
# of Shift 3 Operators
MP server Capacity
MF Server Capacity
MC Server Capacity
OF Interarrival time

Lastly, the Operator is released and the OF entity leaves with 1 Shaft and 1 Piece entity, as shown in Fig. 5 below:

Figure 5

Re: Shift plannification

ThomasMuraille — Wed, 24 Apr 2019 13:28:58 GMT

Thank you nmok !

This is really clear and precise. Thanks a lot for all the work you have done for me !