如图，点$P$是$\angle \mathit{AOC}$的角平分线上一点，$\mathit{PD}\perp \mathit{OA}$，垂足为点$D$，且$\mathit{PD}=3$，点$M$是射线$\mathit{OC}$上一动点，则$\mathit{PM}$的最小值为　　．