In Stage 3, we study in detail the operation of the system using simulation. We will implement lean manufacturing to maintain a smooth process flow, reduce the flexibility to ease inspection and prevent traffic jams, but not reduce flexibility too much that it impacts the efficiency of the system.
Flexibility Plan Tab
Project Strategy and Horizon
At the top of the flexibility plan tab, the choice of manufacturing strategy from Stage 2 is shown. Each strategy has a separate flexibility plan, but we suggest you leave the manufacturing strategy unchanged during Stage 3. If you are considering changing the manufacturing strategy, return to Stage 2. Also at the top of the flexibility plan tab, the horizon can be edited which is the number of hours/day and total days. The overall horizon (yearly, biannually, monthly, etc.) can be changed in Stage 1.
Inventory Management Card
The Inventory Management card allows you to create, edit, and delete kanbans. A kanban (also sometimes called a fixture or tombstone) is a container for parts. At the beginning of the part flow, a kanban is assigned from the available kanbans, the part is placed inside the kanban, and the kanban travels with the part throughout the entire manufacturing process.
Lean Manufacturing is the idea that by reducing in-process inventory, the system experiences higher efficiency, better throughput, and achieves lower cost. The tactic to eliminate batching and implement lean manufacturing is to use kanbans. By limiting the number of kanbans, the work-in-process is limited and kanbans flow through the entire process before being reused for a new part.
There is typically three ways of implementing kanbans in the flexibility plan:
Each kanban or fixture can be entered as a separate kanban with quantity 1. This is typical for automated handing systems which move around pallets. This technique can also be used to study the specific kanban implementation in detail for a manual handling system, allowing specific assignments of parts to kanbans.
A kanban is created for each family of parts (might even be only a single family so a single kanban type). The number of kanbans is then tweaked to limit in-process inventory; large enough to not impact efficiency and small enough to promote lean manfuacturing.
A single kanban (typically called "Universal") with a very large count can be used to effectively ignore kanbans. For almost all systems, a lean manufacturing approach will be better but occasionally kanbans are unneeded complexity and in-process inventory can be limited via bounded queues at each station (entered in the Stations card).
In addition to limiting the quantity of each individual kanban, the simulation can limit the overall number of active kanbans. This is useful to reduce buffering, ease traffic problems for the automation system, and streamline throughput.
The Labor card allows you to create, edit, and delete labor teams. A labor team is a group of operators who work together; any operator in the team can perform any task assigned to the team.
There are typically two ways of implementing labor in the flexibility plan:
For each operator, create a "team" with the number of operators equal to 1. This is typical for manual handling systems, where each operator is assigned to a small number of machines.
Use a single team with a larger number of operators. This is typical in automated handling systems, where there is usually only a single load/unload workcenter that requires labor.
Of course, each system is different so the above are only suggestions. Any combination of labor into teams that makes sense can be used; the main restriction is that each station can be assigned to only a single team (the assignment happens in the Stations card).
The Stations card allows you to edit the workcenters. For each workcenter, you can edit:
Station Count Each station in a workcenter is assumed to be identical and the part will visit one of the stations (the specific assignment can be edited in the Parts card). The overall station count can be edited here, but drastic changes to the station quantities should be done by returning to Stage 2.
Efficiency % During the simulation, each workcenter will be limited to the given percentage of utilization. This efficiency is used to model things like downtime, setup, tool changes, labor breaks, and so on. In the past we modeled all of these situations individually, but in our experience on a range of systems over several decades, just combining all these station problems to a single number produces simulations that give great predictions. In our experience, about 60-65% utilization is correct for manual handing systems, 80-85% for automated handling systems, and about 90% for robotic loading. Only the bottleneck station must have a non-100% efficiency, so typically you only need to enter an efficiency on a single workcenter. The efficiency can be entered only at non-labor workstations.
Labor Team Each station that requires labor can be assigned a team. When a part is located at the station, an operator from the assigned team must be present. If no operator is available, the simulation will cause the part to wait for an operator before beginning.
Station Queue Sizes (Labor-limited process and machine-limited manual handling only) For labor-limited and machine-limited manual handing system manufacturing strategies, the Stations card allows you to enter queue sizes for input and output. When a kanban arrives at the station but the station is busy, it is placed in the input queue. When a part completes at the station, the simulation attempts to directly move it to the next station in the flow. If the next station has a full input queue, the part will be placed into the output queue to wait. By editing queue sizes, in-process inventory can be limited. In our experience, lean manufacturing using kanbans is a much better way of limiting in-process inventory, so typically we leave the queue sizes at a large number such as 100 to effectively ignore queues. If you are editing queue sizes, you must be careful because a single incorrectly sized queue can have a drastic impact on the entire system.
The Parts card allows you to edit the part flexibility by assigning kanbans and restricting stations.
Priority The simulation uses the priority to decide which parts to manufacture first. Lower numbers are higher priority so will be manufactured first. The priority is not a hard limit; parts with multiple priorities might run simultaneously if there are available stations and kanbans.
Parts/Kanban The simulation tracks each kanban as it moves through the flow, and will use the Parts/Kanban to decide how many parts are completed during a single pass through the stations. The simulation assumes that the flow times from Stage 1 are the dwell times of a single part. For example, if during Stage 1 you entered 30 minutes as the dwell time and entered 2 parts/kanban, the simulation will assume that it takes 30 minutes to machine a part and will therefore cause the kanban to dwell at the machine for an hour.
Kanbans Clicking on the Kanbans column opens a dialog which allows you to assign kanbans to parts. A part must have at least one assigned kanban to be produced. If the part is assigned more than one kanban, the simulation will choose an available kanban for each pass through the stations. Thus each time a part passes through the flow route it might use a different kanban. By assigning more kanbans or assigning a single kanban type that has multiple kanbans, flexibility is introduced.
When a part has multiple sequences, the simulation assumes that if the sequences are assigned the same kanban the sequences are all loaded simultaneously on the kanban. For example, consider a part ABC with two sequences: ABC-1 and ABC-2. If ABC-1 and ABC-2 are assigned the same kanban, the simulation will assume the kanban has locations to store both ABC-1 and ABC-2. On the very first cycle, the kanban will contain only ABC-1 and dwell at each workcenter according to the times for ABC-1. Thereafter, on each pass through the workcenters, the kanban will contain both ABC-1 and ABC-2. The kanban will dwell at each workcenter the sum of the times for ABC-1 and ABC-2; thus if it take 45 minutes to machine ABC-1 and 15 minutes for ABC-2, the workcenter will dwell at the machine for an hour before moving to the next workcenter. The final cycle will contain only ABC-2 to finish the planned number of parts.
Alternatively, if sequences receive different kanbans, the simulation will treat the sequences separately. The simulation keeps track of how many parts have completed each sequence and will only schedule a kanban for a sequence if there is a completed earlier sequence. For example, again consider a part ABC with two sequences: ABC-1 and ABC-2. If ABC-1 is assigned Kanban1 and ABC-2 is assigned Kanban2, the simulation will first schedule Kanban1 containing ABC-1 to pass through all the workcenters. The simulation will maintain a count of the number of parts to have completed ABC-1, and only schedule Kanban2 containing an ABC-2 when this count is larger than zero.
Stations to Visit Clicking on the Stations to Visit column opens a dialog allowing you to restrict which stations a part visits. Restricting stations reduces flexibility.
Flexibility (via multiple kanbans or stations) is important for the system to adapt to daily fluctuations in demand; we suggest around 20% of the work be flexible. That is, at least 20% of the total manufacturing time should be used by parts which have a choice between kanbans (either multiple kanban types or a single kanban type with a number of kanbans larger than 1) and a choice between multiple stations. The remainder of the work can be assigned a specific kanban and specific machine to ease inspections, quality control, and proveouts. Note that this is 20% of the daily demand, so if you have a highly variable daily or weekly demand, each combination should have 20% of the work flexible. You need to make sure that you don't get a situation where none of your flexible parts are actually scheduled. For that reason, with a high variety of work we just suggest making everything flexible and using a tracking system (such as FMS Insight) which attaches barcodes to parts to implement quality control. At the inspection stand, the barcode can be used to determine which kanban and which station the part actually visited.
Layout Card (automated handling systems only)
In the Layout card, you enter data about the automated handling system. The layout is an abstract diagram which consists of several components: a track on which the automated vehicle travels, a location for each station, and a buffer stand.
Selecting Components Components such as tracks, stations, and buffer stands can be selected by clicking on the them. When selected, they are drawn in green and properties about them are shown in the sidebar on the right. A component can be de-selected by clicking on the background.
Zoom and Pan The layout diagram can be zoomed using the buttons at the bottom-right corner of the layout card. The layout can be panned by clicking, holding, and dragging the background. At the moment, the only way to reset the zoom and panning is by refreshing the page.
Track The thick black line is the vehicle track. It can be selected by clicking on it. While selected, the following operations and data can be entered about the track.
Move Action The track can be moved by clicking, holding, and dragging the track.
Resize Action The track can be extended by clicking, holding, and dragging one of the circles at the endpoints of the track.
Number of cart stopping points: The entire length of the track is divided into the given number of stopping points. A stopping point is a place where the vehicle can potentially stop to transfer a kanban/fixture to a station or to the buffer. The potential stopping points are equally spaced along the track. In our experience, even if the actual automated handling system might not have equally spaced stopping points, the layout works fine as an ideal diagram and there is negligible impact on the simulation.
Length of track: This is the total length of the track used by the simulation. This quantity can be in any units (feet, meters, etc.), because the simulation ultimately only cares about the time it takes the cart to move between stopping points. The length of the track determines the distance between stopping points and the speed of the cart then determines how much time the cart takes, so as long as the units of the length of the track and the cart speeds match, the simulation will produce good results.
Speed of cart when loaded : This is the speed of the cart when carrying a kanban. The value should be given in units of distance/minute, where distance is the same unit used for the length of the track. For example, if the length of the track is in meters, the cart speed should be given in meters/minute.
Speed of cart when empty: This is the speed of the cart when no kanban is present on the cart.
Stations Each station must be added to the layout diagram and is drawn as a colored rectangle. If no component is currently selected, the stations which are not yet present into the diagram are shown in the right sidebar.
Adding a station: To add a station, click on it in the left sidebar, click somewhere on the diagram to place the station, and then connect the station to the track by clicking on a black dot on the track.
Moving a station: Once selected, a station can be moved by clicking, holding, and dragging the station.
Station connection: The station is connected to a cart stopping point on the track. This is the stopping point that the cart visits to drop off and pick up kanbans at the station. More than one station can be connected to the same stopping point. When initially adding the station, you will be prompted to select a cart stopping point (drawn as a black dot on the track). If you did not establish the connection while adding the station, when you select the station by clicking on it you will again be prompted to select a connection point. To change the connected stopping point, select the station and then click the button "Reconnect Station" shown in the sidebar on the right.
Connection Type: The simulation supports several techniques for transferring kanbans between the cart and the station.
Rotary Pallet: The most common technique for machine-tools, a rotary pallet consists of a table with locations for two pallets. One of the table locations is inside the machine and one of the table locations is available for the cart to pickup or dropoff pallets. Once the machine cycle completes, the table rotates so that the pallet that was inside the machine becomes available for pickup, and the pallet that was just dropped off by the cart rotates into the machine.
Input/Output Queues: The station has input and output queues separate from the actual station worktable. The cart drops off a kanban at the input queue, the station transfers the kanban from the input queue into the worktable for the station, and once the station dwell time is finished, the station transfers the kanban to the output queue. When this connection type is selected, the size of the input and output queues can be specified.
Cart Table: The cart itself acts as the worktable for the station. The cart will stay at the station for the entire operation time.
No Queue: The cart transfers a kanban directly onto and off of the station workcenter with no queuing. The simulation will not attempt to bring a kanban to the station unless the station worktable is empty. (This is typical for example at load/unload stations.)
Transfer times: For each connection type besides cart table, the right sidebar contains settings for the time it takes the kanban to move between various locations at the station.
Buffer When the queue at a station is full, the cart will instead transfer a kanban to a buffer location. The kanban will wait at the buffer stand until the destination station becomes available. Every layout must include a buffer stand, even if it is unlikely to be used. In our abstract diagram, we add a single buffer stand and the simulation assumes it is big enough to hold all kanban. (Automated handling systems typically have multiple buffer stands, one per kanban, but the approximation of a single large buffer stand still produces good simulation results.)
Adding the buffer stand: If all stations have been added and nothing is selected, the right sidebar will show that the buffer is missing. To add the buffer, click on it in the right sidebar, click on the layout to place it into the diagram, and finally click on a track point to connect the buffer to the track.
Moving the buffer stand: Just like stations, the buffer can be moved by selecting it and then clicking, holding, and dragging the station.
Reconnecting the buffer stand: To change the track point to which the buffer is connected, select the buffer and click the button in the right sidebar "Reconnect Station".
Transfer time: When selected, you can enter the time it takes a kanban to transfer between the cart and the buffer.
Invalid Layout If no component is selected, the right sidebar will display any problems with the layout. Problems consist of missing stations, missing buffers, and stations not connected to the track. If the layout is valid, the text "Layout is valid" will appear in the right sidebar. In the case of an invalid layout, the simulation will ignore the automated handling system completely and instead run a simulation with a manual handling system. A warning will be shown on the projected operations tab in this case.
The Projected Operation tab shows the results of the simulation. The "Start Forecast" button at the top of the tab will start the simulation. The simulation runs in the cloud and for most flexibility plans the simulation completes in a couple of seconds (for some complex projects the simulation takes longer to run). Once the simulation completes, the results will appear on the Projected Operation tab. Also, the "Start Forecast" button will disappear until a change in the projcet is made. If there are problems with the flexibility plan (such as missing kanbans or a bad layout), a message will appear at the top of the tab. Note that the simulation will still run, but the results might not be meaningful because of the identified problems.
The Stations card shows the statistics of each station from the simulation.
Block % A station is blocked if the cycle completes but there is nowhere to transfer the kanban to move it out of the station worktable. For example, this typically happens at load stations in automated handling systems (using the No Queue connection type); when the load completes but the cart has not yet picked up the kanban, the station is blocked. It can also happen if the output queue is full. For example, a machine with a Rotary Pallet connection type can become blocked if the cycle completes and the other location on the rotary pallet contains a kanban that has not yet been picked up by the cart.
Typically, we want to make sure the Block % is zero or as close to zero as possible. If the Block % is non-zero at the bottleneck station, there are several possible fixes. First, the cart could be too busy to keep up with the kanbans. The overall number of active kanbans could be limited so that the cart focuses on moving fewer numbers of kanbans. Alternatively, multiple parts might be placed in a kanban or fixture to lengthen the cycle time. Finally, the number of output queue positions could be increased. Increasing the output queue size should be a last resort, since if the system is producing faster than the transportation system can keep up, the queue will eventually fill up and then still block the station.
Utilization Graph The utilization graph plots the utilization percentage over the entire simulation. The x-axis is the time, with zero the start of the simulation and extending to the total project horizon. The y-axis is the station utilization percentage, which is the percentage of time the station is busy. The y-axis ranges from 0% to 100%.
- The horizontal black line is the target rate calculated using Little's Law. It represents the percent utilization that is required to finish all production.
- The green line represents the utilization for the station during the simulation.
- When you mouse over the graph, a tooltip will popup which shows (x,y) coordinate of the point under the mouse, so shows the time offset and the simulated station utilization at the point under the mouse.
The Labor card shows the statistics of each operator from the simulation.
- Utilization Graph The utilization graph plots the utilization percentage over the entire simulation. The x-axis is
the time, with zero the start of the simulation and extending to the total project horizon. The y-axis is
the labor utilization percentage, which is the percentage of time the operator is busy. The y-axis ranges from 0% to 100%.
- The horizontal black line is the target rate calculated using Little's Law. It represents the percent utilization that is required to finish all production.
- The green line represents the utilization for the operator during the simulation.
- When you mouse over the graph, a tooltip will popup which shows (x,y) coordinate of the point under the mouse, so shows the time offset and the simulated operator utilization at the point under the mouse.
The Inventory card shows the statistics of each kanban from the simulation.
Average in Use The average number of this specific kanban that are in use during the simulation.
Maximum in Use The maximum number of this specific kanban that are in active use at any point in time during the simulation.
Average Cycle Time (min) The cycle time for a kanban is the time from when the kanban is loaded with a part and starts the first operation until the final operation in the part flow completes. The average cycle time is then the average of the cycle times over all cycles of the kanban. The average cycle time is a key metric to watch for lean manufacturing. When the average cycle time is large (much larger than the sum of operation times), kanbans are queuing and material is accumulating inside the system (this should also be reflected in the average number of kanbans in use).
The Transportation card shows the statistics of the cart in the automated handling system from the simulation.
Move % The percentage of time the cart is moving when it is empty.
Shuttle % The percentage of time the cart is moving when it is carrying a kanban.
Idle % The percentage of time the cart is idle.
Number of Assignments An assignment is the cart visiting a station to pick up a kanban, traveling to the destination station or buffer stand, and dropping off the kanban. The number of assignments is a key metric to understand how lean the system runs. The more material that accumulates in the system increases how often the cart needs the buffer stand. Ideally, the buffer stand is rarely used and most of the time kanbans move directly from station to station. Excessive buffer use, caused by extra in-process material, will increase the number of assignments because there will one assignment from station to buffer, and a second assignment from buffer to destination station.
Average Assign Time The average time in minutes it takes for a single assignment (picking up a kanban, moving to the destination, and dropping it off). Typically in a real system the average assignment time is between 45 seconds and 1 minute, so typically we just adjust the length of the track so that the average assignment time is around a minute.
The Parts card shows the statistics of the parts during the simualtion.
Completed Quantity The completed quantity shows how many parts were produced by the simulation, the total part demand, and the percentage of completed parts. For example, "745 / 1,255 (59%)" means that we wanted to produce 1,255 parts but at the time the simulation ended, we only produced 745 parts. Thus only 59% of the required parts were actually produced.
Production Rate Graph The production rate graph plots the part production rate over the entire simulation. The x-axis is the time, with zero the start of the simulation and extending to the total project horizon. The y-axis is the part production rate in units of parts/hour.
- The horizontal black line is the target rate, which is simply the total part demand divided by the total horizon. For example, if
the part demand is 1000 parts and the horizon is 365 days at 24 hours per day, the target rate is
1000 / (365*24) ≈ 0.114 parts/hour.
- The green line represents the parts/hour produced by the simulation over time.
- When you mouse over the graph, a tooltip will popup which shows (x,y) coordinate of the point under the mouse, so shows the time offset and the simulated parts/hour at the point under the mouse.
- The horizontal black line is the target rate, which is simply the total part demand divided by the total horizon. For example, if the part demand is 1000 parts and the horizon is 365 days at 24 hours per day, the target rate is
The Animation tab at the top of the Flexibility Plan page shows an animation of the system's operation (automated handling system strategies only). The animation is built by running the simulation with the current flexibility plan.
Generate Animation Events Button: When the flexibility plan changes, the simulation must be re-run to regenerate the events for the animation. The simulation runs in the cloud to generate the animation events; typically the simulation takes a few seconds.
Current Time The current animation time is shown at the top of the sidebar on the right. The animation time corresponds to the snapshot of the simulation shown in the diagram.
Simulation minutes per wall clock second While the animation is playing, for every second that passes SeedTactics will advance the animation time by this number of minutes. Reducing this number thus causes the animation to slow down and increasing this number causes the animation to speed up.
Pause/Play Button: Once the animation events have been generated, the animation initially starts paused at 0 time. By clicking Play the animation will start advancing at the rate given by the Simulation minutes per wall clock seecond setting. Once playing, the animation can be paused by clicking on the Pause button.
Reset Animation Button: While paused, the animation can be reset back to time 0. Once the animation ends, it can also be reset back to time 0.
Station Details: By mousing over a station in the diagram, details about the kanbans and parts at the station are shown in the sidebar on the right.
Station Status Icons: Each station in the diagram shows status icons. There is an icon for when an operator is present at the station, an icon for the station status (busy, waiting, idle, etc.), and icons for each kanban present at the station. By mousing over the station, details about the station and the icons are shown in the sidebar on the right.
Zoom and Pan The animation diagram can be zoomed using the buttons at the bottom-right corner of the sidebar on the right. The diagram can be panned by clicking, holding, and dragging the background. At the moment, the only way to reset the zoom and panning is by refreshing the page.