def positive_x_constraint(x, constants, variables):
    """ Returns the constraints that specify the model parameters are positive.

    :param np.ndarray x: Parameters of updated Voce-Chaboche model.
    :param dict constants: Defines the constants for the constraint.
    :param dict variables: Defines constraint values that depend on x.
    :return float: Value of the constraint in standard form.

    Notes:
        - The constants dictionary must contain the entry 'min_x': min_val, where min_val >= 0 is the minimum value that
        any x[i] should be able to take (e.g., min_val = 0 specifies that all x[i] should be positive).
    """
    min_val = constants['min_x']
    g = [-xi + min_val for xi in x]
    return g