6. Conclusions
In summary, great achievements for the control of manipulators by means of neural networks have been gained in the last two decades. However, there are still many new problems to be solved. All these future developments will accompany the development of the advanced manufacture and material for various kinds of robot manipulators as well as the mathematical theory for constructing and developing neural networks. Keeping in mind, different kinds of neural networks have their own feasible ranges, and one cannot expect that only a few existing results on neural networks can tackle all the control problems existing in different manipulators with different tasks. Every class of neural networks, for example, feedforward neural networks, recurrent neural networks, dual neural networks as well as their modifications, has their own advantages, which has considered different tradeoffs between computational complexity and efficiency for the control of robot manipulators. Finally, two possible future research directions on control of robot manipulators using neural networks are pointed out, which may open a door to the research on this topic.