These axes are jointly driven by three actuators. One can observe that by moving joint 4 moves motors 4, 5 & 6 together. Moving joint 5, moves motors 5 and 6, whereas moving joint 6 moves just the actuator 6.
Please see paper published at iNaCoMM 2013 at
http://www.inacomm2013.ammindia.org/P...