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Fixed Location Service Time

Overview​

The Fixed Location Service Time constraint allows you to model a static time duration required at a specific location, independent of the number or size of orders being processed. This is particularly useful for modeling activities such as:

  • Vehicle docking and undocking.
  • Security checkpoints.
  • Administration or paperwork processing.

Unlike standard service times which might scale with the number of items (e.g., 5 seconds per parcel), a fixed service time adds a constant overhead to the stop.

When to Use​

Use this constraint when:

  • Docking is required: Drivers must spend time backing into a bay before loading/unloading starts.
  • Security procedures are mandatory: A fixed time is spent clearing security regardless of the delivery size.
  • Administrative tasks: Drivers must sign in/out or process paperwork at a reception desk.

Do not use this constraint for:

  • Per-item handling time (use standard service time or Dynamic Service Time).
  • Variable delays based on traffic (use travel time matrices).

Implementation​

This constraint is implemented using the Compound Zone mechanism, specifically by leveraging the exit_time parameter. By treating a single location (OperationsLocation) as a compound zone, you can effectively attach a fixed duration to it.

See Compound Zones and Operations Locations for configuration details.

While both enter_time and exit_time can technically be used, exit_time is the recommended approach.

  • Using exit_time: The time is added after service completion, just before departure.

    • Benefit: It does not affect the scheduled_ts (arrival time) of the node. The vehicle arrives, performs the service (loading/unloading), and then spends the fixed time (e.g., undocking).
    • Result: Accurate scheduling of the arrival and service start times.

  • Using enter_time: The time is added before arrival.

    • Drawback: This shifts the scheduled_ts later, effectively incorrectly calculating when the vehicle arrives at the dock. This can lead to issues with tight time windows.
Recommendation

Always prioritize using exit_time for fixed location service overheads to maintain accurate arrival timestamps.

Example: Warehouse Docking​

Imagine a warehouse requires a fixed 15-minute (900 seconds) period for a truck to undock and clear usage after loading.

Configuration​

Set the exit_time to 900 seconds in the compound zone or operations location settings.

Schedule Impact​

The following diagram illustrates why exit_time is preferred over enter_time.