Why not use a spreadsheet to size and optimize a workcell? Well, because “It doesn’t work!” Product mix changes, variability and resource reassignment make calculations difficult and unreliable at best. On the other hand, workcell simulation allows accurate prediction of a workcells capability and optimization a reality. Yes but, simulation models take too long and cost too much to build. Not anymore. New workcell model objects allow a novice to create a simulation of a detailed, 8 station, workcell in about 20 minutes. Cycle time and utilization calculations are automatically provided.
“These objects were designed in coordination with the number one supplier of fittings, valves and manifolds used in critical corrosive applications,” said Scott Baird. “With over 20 manufacturing facilities and hundreds of workcells of all sizes and shapes, these model objects have been refined. All workcell model objects have been reworked ‘with a little help from our friend’.”
Each object is made to duplicate common functions found in a typical workcell. There are object for machines, parts washing, assembly, testing and inspection. The objects take into account, human interaction, equipment failure, batching, inspection and scrap and much more. You just place the objects, make simple connections and fill in the details in a spreadsheet. The model objects can handle everything from cells dedicated to specific part numbers to cells with over a thousand part numbers. Additional model objects have even been created for flexible cells. Continue reading to see how a simulated workcell is created…
How is a workcell simulation model created?
- Place the simulation work cell components.
Design the workcell using common components. Machining, cleaning, assembly, test and inspection stations are included with multiple orientations. This allows you to design clockwise or counterclockwise workcell orientations.
The suffix LR provides a “left to right” object orientation, while a RL provides a “right to left” orientation, etc. Any of the model objects can be viewed and run independently by selecting the Open button. The model object can be inserted into a model by selecting the Insert button.Order of placement is important, because common attributes for each station (i.e. Setup, Load Time, Run Time, Unload Time) will be incremented and added for each new object placement. The spreadsheet used to control schedule and timing will have a set of entries for “station one” that correspond to the attributes mentioned above. The point is, it will be easier pre plan rather than haphazardly entering a model object and then having to remove attributes, variables and scenario paramters that will go unused. If you place model objects in the wrong order, then the entries in the spreadsheet controlling time will be out of order. Placing workstations out of order is not a showstopper, just confusing and a little inconvenient for later analysis. A simple workcell simulation example follows:
- Add a data collection device. This model object is titled “Workcell Stats Collector 1 Piece Flow.” This is used to collect stats on the performance of the workcell.
- Add the arrivals Model object. This model object is titled “Workcell Arrivals 1 Piece Flow – LR” This allows rapid changes to the model to be made with a spreadsheet.You will need to replace the “InQ Station1” with the in queue from the arrivals model object, but certain elements contain logic, so routes can’t be randomly deleted and have the model work correctly. Instructions are provided below, so that the correct items are retained. Move the origin of the Route shown below:
Then Delete “InQ Station1” and move “InQ Stationa1” into its place as show below:
- Connect model objects.
Use the connector line tool to route between model objects and connect the additional objects as indicated.
The required connections are shown in red.
- Change times to reflect moves between stations.
- Assign the workers to their stations.
Use scenarios parameters to specify the worker assignments.
Remove unused workers from the layout.
- Link the cell to the arrivals spreadsheet. The purpose of linking a spreadsheet is to: allow a centralized location for making changes to the simulated workcell; provide a mechanism for scheduling multiple part numbers into the cell without having to change the simulation model; offer a method for making rapid changes to a complex system. There are several steps in linking a spreadsheet. Instructions are provided below:
- Export Data –
Select only arrival information for export. This will create an Excel spreadsheet that will be used to transfer information into ProcessModel. Identify the “key” number from the “Scheduled Arrivals” tab. This will be a unique number for your model. Write down the number then close the spreadsheet.
- Open the Excel spreadsheet linked to the model. This spreadsheet is found by right-mouse clicking on the Excel icon on the model layout.
Enable the macros to run by selecting the Enable Content button, then select reset.
Enter the “Key” from the last step.
- Link to the export sheet. Since all three of these file (simulation model, export sheet workcell arrival sheet) work together, you will need to point out the location of the export spreadsheet to the arrivals spreadsheet. This is accomplished by selecting the update button and finding the export spreadsheet. The export spreadsheet will have the same name as your model with the word “Data” appended. This file will be found in the same directory as the simulation model.
When updating the model, the export sheet is opened, updated, saved and closed. Then the model is opened and the export sheet is imported into the model. The newly imported information is saved in ProcessModel.Now the model is linked. Any changes made to the spreadsheet will be transferred to the simulation model with subsequent updates.Other changes to the model will not be overwritten. This allows batching before an operation, reassignment of resources, etc. without overwriting your work when an update is performed.
- Export Data –
- Change the schedule and operational characteristics of the workcell. Headings of columns with a suffix of 1, are linked to station 1 while headings of columns with a suffix of 2, are linked to station 2, etc. If the times are changed in this spreadsheet and the import button is selected then the model is updated.
- Below are a few of the items that could be changed
- Each row represents a new order usually denoted with a change in a_Name (part number)
- Time – The day and time this order will start
- Quantity – the number of batches of a particular part. For example a single part number may have several batches for reporting purposes.
- a_Name – an integer value used to describe each unique part. If your part numbers have alpha or special characters, create a conversion table to associate your part number with an integer value. Use only the integer value in this table.
- a_Batch_Size — The number of parts in a batch, this is used to collect batch cycletime.
- Setup time for each station. This will be used when the part name changes.
- Processing time for each station. These can be integer values or distributions to include variability.
- Load and unload time. Used as needed to represent load and unload times. If there is no Load or Unload time then set this attribute to zero (0).
Update the model with the newly added information.
- Below are a few of the items that could be changed
- Run the model and see the simulated performance of the workcell.
- Test workcell changes to meet the goals of your production system. For example, some of the things you can easily change are listed below:
- Test worker assignment modifications. This is easily accomplished by changing the scenario parameters.
- Add duplicate equipment or “Change out” machines and see the effect. The simple method of adding an additional unit of equipment can be accomplished by:
- Changing the capacity of the process station.
- Changing the Quantity of the blue resource above the station.
- Change the setup time on the “Workcell Arrivals” spreadsheet to reflect the time for the additional equipment.
- Show the impact of better maintenance. Using the scenario parameters, make adjustments to the breakdown or repair characteristics and see the impact on productivity.
- Change station batching. Altering the station batching is often used to reduce the human touches for something that is better accomplished in a batch. For example ultrasonic cleaning. To change from one piece flow to batching six items before running a cleaning cycle, the following steps are needed.
- Change the quantity to “wait” for in the input queue from one to six.
- Add batching at the “Load Station” and set the “quantity” to six.
- Add unbatching to the unload station.
- …and many more things can be changed… all within minutes.
A cell might be dedicated or could have many different part numbers each with unique characteristics. Scheduling could be updated and tested within minutes by changing the arrivals spreadsheet and pressing the “update” button.
In addition to making quick changes, this spreadsheet makes it possible to easily incorporate 1000 unique part numbers (or more) in a single workcell.
You provide the ideas and let the workcell model objects provide the detailed modeling to make your workcells perform to their potential. Workcell model objects are free to all users with ProcessModel purchases, maintenance and support, and upgrades.