Constraints

1. Hard

Hard constraints represent rules and limitations of the real world that the resulting solution has to respect. For that reason, the Field Service Model prefers leaving visits unassigned to breaking hard constraints.

1.1. Vehicle shift end time (hard)

A vehicle shift may end either by:

The former is more appropriate for vehicles that end their shifts in employer’s depot or office. The latter is suitable for vehicle shifts that end at employee’s home and the travel from the last visit home is considered outside the working hours.

Make sure to define at least one of these hard limits, otherwise the vehicle shift effectively never ends.

1.2. Require skills

Both vehicle shifts and visits define skills. You can use skills to model a skill or trade with its level of proficiency (optional) and its service duration multiplier (optional). The constraint makes sure visits are assigned to vehicle shifts that match their required skills and their levels. Triggers for any visit assigned to a vehicle shift with at least a single missing skill (or its insufficient level) required by the visit.

A vehicle shift can also define temporary skills that apply only in the specified time interval.

1.3. Require tags

Both vehicle shifts and visits define tags. You can use tags to model skills, regions, contractors, and similar. The constraint makes sure visits are assigned to vehicle shifts that match their required tags. Triggers for any visit assigned to a vehicle shift with at least a single missing tag required by the visit.

A vehicle shift can also define temporary tags that apply only in the specified time interval.

1.4. Require service maximum start time

Vehicles have to fulfill the service within the customer visit’s availability window. The constraint triggers when the vehicle shift’s start service time exceeds the visit’s maximum start time.

1.5. Require service maximum end time

Vehicles have to fulfill the service within the customer visit’s availability window. The constraint triggers when the vehicle shift’s departure time exceeds the visit’s maximum end time.

1.6. Maximum travel time per visit hard limit

Vehicle shift can define a maximum travel time spent per a single visit. This limit may result in visits in distant locations being unassigned. The constraint triggers when the vehicle shift’s travel time exceeds the limit.

1.7. Require visit dependency delay

Visits may depend on each other with a minimal required delay between them. For instance, between two consecutive maintenance tasks, there needs to be a technological pause. Triggers if a visit starts before the minimal delay after the departure time from the defined preceding visit.

1.8. Balance movable and non-movable visits

Often it is important to spread movable visits over the planning windows to keep free time for optimizing non-movable visits and placing sudden emergencies.

The movable occupation ratio threshold defines the capacity for movable visits in the following way:

If the vehicle shift occupancy grows above the threshold, the constraint penalizes the solution score for the exceeding travel and service duration of movable visits assigned to this vehicle shift.

2. Soft

Soft constraints represent optimization goals and additional requirements on the quality of feasible solutions.

2.1. Vehicle shift end time (soft)

Similarly to Vehicle shift end time (hard), there are also soft limits of the vehicle shift end:

If you use these soft limits, make sure to set them to earlier time than the corresponding hard limits.

2.2. Minimize travel time

Minimizes the total travel time over all vehicles. Triggers for every visit assigned to a vehicle shift. This constraint represents an optimization goal to reduce the travel time and is typically not used in combination with the Minimize travel distance constraint.

2.3. Minimize travel distance

Minimizes the total travel distance over all vehicles. Triggers for every visit assigned to a vehicle shift. This constraint represents an optimization goal to reduce the travel distance and, by that, also the mileage driven together with a fuel consumption. It is typically not used in combination with the Minimize travel time constraint.

2.4. Prefer visits scheduled to the earliest day

Schedules movable visits to the earliest possible day to reduce the risk of not being able to assign them later. The constraint triggers for every movable visit that is scheduled to a later date that its minimum start time.

2.5. Prefer scheduling optional visits

Assigns as many optional visits as possible without breaking hard constraints and without making a mandatory visit unassigned. Optional visits with approaching deadlines come first. Applies to any unassigned optional visit with its maximum end time outside the current planning window.

2.6. Minimize the visit completion risk

If a vehicle crew gets behind the schedule during the day, there is a risk the last one or two visits might not get finished. As a result, these visits have to be rescheduled to some other day.

This constraint penalizes any visit with its departure time exceeding the visitCompletionRiskMinimalTimeToShiftEnd based on the visit priority. To define the minimal priority level to consider for the risk evaluation, use the visitCompletionRiskMinimalPriority.

That way, high-priority visits are more likely to be scheduled earlier in the day, while low-priority visits are likely to move towards the vehicle shift end.

2.7. Minimize scheduling vehicles with unnecessary skill levels

Minimizes the sum of all unnecessary skill levels (provided skill level minus required skill level) over all assigned visits. The penalty is proportional to both the skill level difference and the visit effective service duration (the time of a more skilled technician is more expensive).

In the case of not specifying any skill levels, the provided skill levels will default to 1 and required skill levels will default to null (meaning the skill requirement does not care about skill level at all), therefore no optimization will take place. If this is the case, you might even want to turn off this constraint for performance reasons by setting its weight to 0.

Triggers for every visit assigned to a vehicle shift.

This constraint represents an optimization goal to reduce over-qualification: scheduling highly qualified vehicles/technicians to simple tasks.

3. Constraint configuration

Constraint configuration contains weights of soft constraints together with additional constraint parameters. Every constraint defines a match weight; a basic score impact for every constraint match. The overall score impact of a constraint match is calculated by multiplying the match weight by the constraint weight.

The default value of a constraint weight is 1. To make the score impact of a constraint ten times bigger, set the related constraint weight to 10. To effectively turn a constraint off, set the related constraint weight to 0.