# Aerial Tele-Manipulation with Passive Tool via Parallel Position/Force Control +
## 5 Experimental Results
### Discussion  
The experimental results validated the capability and efficacy of the aerial manipulator with a passive tool in free flight pose tracking and in contact force tracking while keeping the contact and maintaining the flight stability. After performing several experiments to investigate the properties of the system and the controller, the following results from interpreting observations were obtained. In the free flight phase, the autonomous control of pose is not very precise, and there are position/velocity errors that prevent the autonomous controller from doing very fine tasks, even in the case of knowing the environment perfectly.  
This happens due to the existence of aerodynamics disturbances and unmodeled dynamics that are not incorporated in the controller design. However, a human teleoperated aerial system is capable of doing such tasks, thanks to superior human learning and sensory motor capabilities. A similar observation has been made in [8].  
The contact establishing phase (docking) involves bouncing; this is a natural behavior because the reaction force of the contact is directly applied to the robot CoM that is a floating (hovering) object, and this results in jumping back; unlike grounded manipulators this force is not transmitted to the ground. To mitigate this effect, velocity slowing-down policy and physical shock absorber on the end-effector surface is used, that results in a smooth transition and reduces the bouncing effect significantly. Moreover, deploying \(f_{PD}\) in the haptic feedback prevents the operator from loosely grasping the master robot, which could increase the effect. Note that in establishing a contact with non-zero reaching velocity, having a small amount of bouncing is commonly acceptable, even for grounded robotic arm manipulators [33].  
Contact maintenance function, in conjunction with appropriate gain tunings (to avoid overshoots as mentioned in the stability analysis section) plays an important role; without respecting these constraints the end-effector could slide on the surface. Therefore, using this function is necessary if the task involves keeping the contact, unlike the hybrid position-force controllers such as in [2,24,34] in which it is desirable to slide over the surface.  
The force error constituted by a non-zero-mean part (DC) and a high frequency signal (AC). The DC part is the effect of the not-force-controlled axes of motion (due to the underactuation and uni-lateral contact), and the AC part is because of the propeller rotational (aero)dynamics. The DC part can be decreased by choosing appropriately large gains, while increasing these gains increases the AC part. Therefore, care is to be taken in tuning the controller to achieve an acceptable trade-off depending on the task.  
The detachment phase also showed some bouncing effects, which is due to the time it takes to deplete the integral term, and imprecise free flight position control in the vicinity of the object. This effect is mitigated by deploying the switch function (7), and by bounding the integral term and choosing a small coefficient for it while choosing appropriately big proportional and derivative terms. A similar phenomenon is also observed in the case of grounded manipulators controlled by the parallel position/force control method in [35] and referred to as the sticky-effect.  
The proposed controller can also be utilized in redundant omni-directional or partially omni-directional aerial manipulators, such as [1,8], by keeping their position controllers and providing it with a reference based on the presented force controller that considers contact maintenance and deploying orientation to generate the desired force.