Page 528 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

19.6 Hydraulic versus electrical power 525
FIGURE 19.23
Position controller.
19.5.3 Force feedback control
(Courtesy Schilling Robotics.)
The manipulators designed for position control and force feedback control are essentially the same design as both require sensing of the position of the manipulator joints. The difference is in the design of the master controller. For a position-controlled manipulator, the master controller directs the manipulator’s joints to move in response to movement of the replica controller, which is known as “unilateral control.” When the master controller is stationary, so is the manipulator. Movement of the manipulator, due to some external force, does not cause the replica controller to move in response.
In the case of force feedback, small electric actuators built into the joints of the master control- ler cause the controller to be moved in response to forces sensed by the manipulator (Figure 19.24). This bilateral control allows the operator, who is holding the master controller, to feel any forces that the manipulator is experiencing. This tactile feedback can be a valuable sensory input during complex or delicate tasks. Also, the operator has the ability to adjust the level of the force being fed back through the replica controller, thus reducing any physical fatigue that might be experi- enced in longer operations. As in the position-controlled manipulators, today’s systems that are operating in the field are highly reliable, even if a bit more complex.
The choice of rate, position, or force feedback control is task-dependent and should be taken into consideration in the early planning stages.
19.6 Hydraulic versus electrical power
Since most of the heavy-duty manipulators on work-class vehicles have access to abundant hydrau- lic power, their choice over an all-electric system is a given. The use of hydraulic power allows the manipulator to deliver large forces in a reasonably sized package.
For the smaller OCROVs (which are nearly all electrically powered), the choice of an electri- cally operated manipulator is also logical since the forces needed for most tasks required of those vehicles are not as large. If electrically operated arms could be made with lift-to-weight ratios and