Method and system for detecting unbalance of threshing cylinder in free sliding descending process

According to the method and system for detecting the unbalance of the threshing cylinder in the free sliding descending process, power transmission is cut off, vibration coupling between the threshing cylinder and an engine is decoupled, and interference of the engine on online dynamic balance detection of the threshing cylinder is fundamentally solved; meanwhile, sine and cosine signals consistent with the rotating speed of the roller are constructed through the rotating speed signals, cross-correlation operation is conducted on the sine and cosine signals and the modal components, characteristic values based on cross-correlation operation are obtained, the characteristic values serve as fitness functions, variational modal decomposition parameters are optimized by combining a parameter optimization algorithm, and the characteristic values can serve as indexes for screening the modal components; the recognition effect on the variable-speed unbalanced vibration signals is improved, the detection precision of