use \hspace ,eg., \hspace{-.1cm} before and after summation to stop violation of margin。
比如下面一段公式代码
1 \begin{align}
2 \label{sys_1}
3 {\Delta{\ddot {\theta}}}_{i}(t) & = ( -\dfrac{1}{T_{pi}} -\dfrac{k_{pi}}{T_{pi}R_{i}}) {\Delta{\dot {\theta}}}_{i} - \dfrac{k_{pi}}{T_{pi}}2\pi T_{i}\sum\limits_{j\in \mathcal{N}_{i\sigma}(t)}{({\Delta\theta}_{i} - {\Delta\theta}_{j})} \nonumber \\
4 & - \dfrac{k_{pi}}{T_{pi}}\big(\Delta P_{di} + \Delta E_{i\sigma}(t)\big) + \dfrac{k_{pi}}{T_{pi}}u_{i\sigma}(t)
5 \end{align}效果如下图：
显然，我们看到summation符号和前面以及后面的表达式之间都空了一段距离，这是不应该的。然后我们在summation符号得前面和后面使用\hspace{-.3cm}之后:
\begin{align}
\label{sys_1}
{\Delta{\ddot {\theta}}}_{i}(t) & = ( -\dfrac{}{T_{pi}} -\dfrac{k_{pi}}{T_{pi}R_{i}}) {\Delta{\dot {\theta}}}_{i} - \dfrac{k_{pi}}{T_{pi}}\pi T_{i}\sum\limits_{j\in \mathcal{N}_{i\sigma}(t)}\hspace{-.3cm}{({\Delta\theta}_{i} - {\Delta\theta}_{j})} \nonumber \\
& - \dfrac{k_{pi}}{T_{pi}}\big(\Delta P_{di} + \Delta E_{i\sigma}(t)\big) + \dfrac{k_{pi}}{T_{pi}}u_{i\sigma}(t)
\end{align}效果如下：
可以看到，summation符号前后的距离都被消除了。Oh yeah~