Single inverted pendulum is a non-linear, unstable system.

倒立摆是非线性、不稳定的系统。

Open-loop unstable system with time delay is difficult to control using routine control methods.

开环不稳定时滞系统是工业现场中常见的一类被控过程，这类系统控制难度高，控制精度难以保证。