Assignment problem with variable tasks to be done

Question

I'm dealing with a kind of assignment problem, in which I have a set of tasks $t$ to be executed by machines $w$, but these tasks depend on the variatns $v$ of components $m$ being selected, which is stated by decision variable $Y_{mv}$.

I would like to ensure that all selected components will satisfy a set of requirements when creating a given product composed of many components. Besides that, I would like to ensure that all the selected components' tasks will be fulfilled somewhere in a process plan.

Considering the following parameters:

Let $F = 1, 3, 4,...$ be the set of requirements to be satisfied.

Let $M = 1, 2, 3,...$ be the set of components that can appear in a product.

Let $V_{M} = 1, 2, 3, ...$ be the set of variants $v$ of a given component $m$ in $M$

Let $T = 1, 2, 3, ...$ be the set of available tasks

Let $R_{mvt}=1$ a parameter that states if variant $v$ of component $m$ requires task $t$, $0$ otherwise

Let $S_{mvf}=1$ a parameter that states if variant $v$ of component $m$ satisfies the requirement $f$, $0$ otherwise

Considering the following decision variables:

$X_{twj}=1$ if task $t$ is performed by machine $w$ in process plan position $j$, $0$ otherwise

$Y_{mv}=1$ if variant $v$ of component $m$ is selected in the product, $0$ otherwise

My objectives are:

• Ensuring that, besides all available tasks, only the tasks required by the selected variants $v$ of components $m$ will be performed (first equation)
• Ensuring that all requirements will be satisfied by the selected variants $v$ of components $m$ (second equation)

$$ \begin{align} \sum_{m} \sum_{v} Y_{mv}*S_{mvf} &= 1 & \forall f in F\\ \sum_{m} \sum_{v} Y_{mv}*R_{mvt} &= \sum_{t} \sum_{w} \sum_{j}X_{twj} \\ \end{align} $$

How to state a constraint to ensure that only the required tasks will be executed?

EDIT: I am using cplex, and when I run the code using the constraints above, it does not return any result (model is infeasible as suggested by @Alex Fleischer).

AttributeError: 'NoneType' object has no attribute 'get_objective_value'

I suppose that the constraints are not well established, but I still have not identified where the problem is. Could someone help with this trouble?

Answer 1

I guess your model is not feasible.

Let me use the tiny zoo example:

from docplex.mp.model import Model

mdl = Model(name='buses')
nbbus40 = mdl.integer_var(name='nbBus40')
nbbus30 = mdl.integer_var(name='nbBus30')
mdl.add_constraint(nbbus40*40 + nbbus30*30 >= 300, 'kids')

mdl.minimize(nbbus40*500 + nbbus30*400)

mdl.solve(log_output=True,)

print("solution is empty : ",mdl.solution.is_empty())

print("obj : ",mdl.solution.get_objective_value())

for v in mdl.iter_integer_vars():
    print(v," = ",v.solution_value)

gives

solution is empty :  False
obj :  3800.0
nbBus40  =  6.0
nbBus30  =  2.0

But if add a constraint that I should not use more than 4 buses in total

mdl.add_constraint(nbbus40 + nbbus30 <= 4, 'nbbus')

then I get your error

    print("solution is empty : ",mdl.solution.is_empty())
AttributeError: 'NoneType' object has no attribute 'is_empty'

You should check why your model is not fasible