In a previous work, we proposed a general approach for synthesizing a maximally permissive Petri net controller as a set of control places to connect to a given plant Petri net model. The resulting net has the maximum behavior w.r.t. forbidden state specifications. However, Petri net controller does not always exist. Such situations are geometrically caracterized and a solution is proposed in this paper for forbidden state problems for which no Petri net controller exists. It is shown that a maximally permissive control policy may be achieved by accounting of the actions of several partial Petri net controllers. Each of them prevents the reachability of at least one forbidden state transition and allows any marking in the desired behavior. The design procedure of partial PN controllers detailed here is based on the approach we previously proposed. It is applied to an example.