What is the Benefit of Drift Analysis?
Drift Analysis allows the engineer to evaluate the impacts of temporary over-Takt cycle time work conditions on mixed-model production assembly lines with fixed TAKT times during portions of the day.
Many mixed production assembly lines can experience significant fluctuations in station cycle time based upon the specific work content required for each configuration (a variant of model and options) of product being produced. In order to maximize productivity, those companies often select a line rate (Takt Time) which is less than the peak cycle time of their highest work content product. Alternatively, some companies struggle with product quality or cycle time repeatability issues which also create cycle times in stations that exceed the line Takt.
Figure 1: Multi-unit drift chart showing interdependent delays causing a substantial increase in the cycle time of selected stations at a specific time in the shift.
When a station’s cycle time exceeds the line Takt, it reduces the productivity of the entire assembly line. As such a 15-second delay in one station can result in a 15-second delay in all stations and therefore a reduction in the line throughput for the shift. Of course in a mixed model production line, it is also possible for a station cycle time to be less than Takt and thus absorb some of the impacts of the prior over-Takt cycle time issue. For this reason, some companies choose to alternate high and low cycle time products to provide a type of insurance (buffer) against over-Takt cycle time conditions.
Other companies choose to group together high and low cycle time units and thus run the line at different Takt rates for different times of the day. Either way, with the combination of Proplanner’s Sequence Planner and Drift Analysis modules, engineers can quickly evaluate the impacts of different unit sequences at different Takt times on a day or shift basis. The goal is to determine the ideal unit production sequence and commensurate Takt to ensure the maximum line productivity for any given day, production demand, and line staffing plan.
Figure 2: Sequence Planner interface shown by time of the day with shift breaks grayed out. The top chart shows the cycle time of each unit (green or blue unit) at each station relative to the Takt Time (red line).
Proplanner’s Drift Analysis module is a feature of the Sequence Planner module and is closely related to the Assembly Line Balancing module. As such, a user begins their analysis by first loading a production schedule for a future date range and selecting the set of units and their sequence for evaluation. At this time, the Sequence and Drift analysis features are enabled for unit-based as opposed to batch-based production analysis. As such, any batch of identical units would be evaluated on a Unit-specific basis.
The order selection window makes it easy to Include or Exclude single or multiple Units from the production build forecast into sequence planner for analysis. Once imported, the sequence planner quickly shows the production for the day with time-stamped rows for each station shown as columns. Unit sequence numbers (serial numbers) are then shown in the cells with their cycle times at those stations which are color-coded according to if (and by how much) they exceed the currently defined line Takt.
Using Sequence Planner, engineers can simply drag and drop units to different times of the day to establish a production sequence for subsequent review by the Drift Analysis module.
The Drift Analysis module has three primary windows. By default the Gantt chart is in the top left, the Drift chart is in the top right and the Data table comprises the lower half. Each window pane is individually selectable and resizable, so the engineer can create any layout they wish, or detach and drag these windows to multiple computer displays. Drift analysis studies at companies with short Takt times and many workstations can result in very large data sets. Proplanner can easily handle this complexity but multiple large computer displays make the job much easier for the engineer to see.
Figure 3: Default Drift Analysis view showing the Gantt and Drift charts as well as the Drift Data Table.
Drift analysis studies are generated from production forecast build (order) lists. These are essentially future customer orders (currently single units) with their applicable Model and Option attributes so that Assembly Planner can determine the cycle time in each workstation for that specific unit being produced on a specific day (i.e. effectivity of process is taken into consideration).
The Drift Table shows the model of the unit along with the cycle time required in each workstation. Based on the currently specified Takt time and unit sequence, the Drift module computes when each unit will start and end in each station and how late (if late) a unit is, as well as, the amount of time that this Unit was blocked from progressing into this station. These data fields (columns) are selectable by the user and can be easily exported to applications such as Excel.
Figure 4: Close up of the Drift Data Table with all data fields enabled.
In the most simple case of one Unit progressing through a set of stations on the assembly line Figure 5 shows how a Unit with a cycle time exceeding Takt in one station causes it to enter the next station late. Even if the cycle time of the Unit in that next station is below Takt, it can still leave that station late. Only a series of successive under-Takt cycle times will allow the unit to eventually catch back up.
Unfortunately, a unit that spends too much time in a station, restricts the next unit from entering that station and thus a Blocking condition will persist for multiple Units which is most easily seen in the Gantt Chart view (Figure 7).
Figure 5: Drift chart of a single unit which begins on time in the first station but enters successive stations late due to early station cycle times exceeding Takt.
Proplanner’s Drift Analysis module allows engineers to view many Units through many Stations all at the same time so that the impact of one Unit’s late departures can be evaluated for the Units which must follow it. Multi-Unit with Multi-Station Drift charts can look someone complicated at first (Figure 6), but a trained engineer can quickly Zoom in on the portion of greatest interest and develop a solution to minimize throughput effects.
Figure 6: As additional Units and Stations are added to the Drift Study, the charts can become substantially more cluttered. As such, Proplanner’s interface includes sophisticated Zoom, Pan and Filter capabilities.
If a Unit with relatively low cycle times follows a Unit with may station over-cycle times, it is possible for the line to partially recover and stay on target to meet the daily production goals (as shown in the Figure 7 Gantt chart).
Figure 7: Gnatt Chart
The Gantt chart is designed to assist the engineer in evaluating the specific Blocking, Empty and Break conditions which can impact throughput. While the Drift Chart quickly shows how over-Takt cycle times impact successive units in successive stations, the Gantt chart shows the various types of blocking and empty conditions as well as the timing and impact of scheduled production breaks. Users can Zoom, Pan, Scale and Filter these charts in order to pinpoint specific impacts of production sequence or Takt time.
Figure 8: Zoomed Gantt chart showing two successive units progressing through two stations. The Y-axis rows are successive stations and the X-axis columns are time throughout the day. The gray bars denote planned breaks.
Watch our latest YouTube video over Drift Analysis below.
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