Fig. 5
![Fig. 5](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs12893-021-01259-3/MediaObjects/12893_2021_1259_Fig5_HTML.png)
a The schematic diagram of fixed point and roll angle. b The schematic diagram of the fixed point and pitch angle. The fixed point acts as a fulcrum rather than the incision in the abdomen. The instrument moves around the fixed point in a roll–pitch–roll way. The course of the surgical procedure can be regarded as the combination of roll angle and pitch angle in the tridimensional workspace of the mechanism