La familia y la discapacidad

In this thesis, compliant force control was chosen as an appropriate method to automate the inspection of sub-sea welds. This choice is justified in this section.

What is meant by compliant force control may be explained by examining the simplified physical system illustrated in figure 3.6. This shows how, as a probe moves across a surface (3.6a), it meets a sudden change in surface height (3.6b). This ‘disturbance’ to its trajectory will compress the compliant device (here shown as a simple spring) and transmit a force to the force sensor. The force magnitude is then used to adjust the manipulator trajectory such that the force returns to its original level (3.6c).

The key point of compliant force control is that a large change in surface height can happen rapidly without damaging the probe or force transducer as the motion is absorbed by the compliance. This has the effect of moderating the force change that takes place.

In brief, we are using force feedback to modify the manipulator trajectory, and a passive compliance to allow for rapid changes in surface height. The justification for this approach is:

1. As was mentioned in chapter one, sub-sea inspection is a slow task. Whether carried out by divers or ROV’s, the process is slow at every stage from deployment, through inspection to completion. Hence, the speed with which the inspection itself is carried out has a relatively small impact on the overall time taken. Compliant force control, therefore, may be used even though it results in slower manipulator motions than a stiffer, higher bandwidth system would give.

compliant device (spring) force sensor probe- manipulator manipulator irregularity in surface mampulator normal forces

FIG. 3.6 - SIM PLIFIED S C H EM A TIC OF THE M ECH A N IC S OF C O M PLIA N T FORCE CONTROL

It has the advantage that the manipulator may be less precise and operate at a lower bandwidth saving considerable expense.

2. The accuracy with which the structure geometry is known will be severely limited due to structure mis-modelling. This may arise from the control system having a simplified model of the true environment and will cause errors as a result of:

• Non-registration. The known relative positions of the ROV and structure are incorrect,

• Incorrect data. The engineering drawings of the structure contain nominal dimensions. The true structure will differ slightly from these in both size and geometry.

• Noise. The structure surface will not be smooth mainly due to weld spatter, marine growth and corrosion.

These inaccuracies may be taken up by the movement of a compliant device. Non­ registration and incorrect data will not matter once contact has been made as force control will ensure the correct stand-off between manipulator and work piece. The noise will matter. Marine growth will have been removed prior to inspection but weld spatter may result in disturbances to the probe position of up to 4mm.

3. As discussed in section 3.3, there will be ROV motion relative to the structure. These motions will increase the problem of non-registration and make it a dynamic problem. These motions need to be compensated for and are up to 15mm (see chapter 5). With a compliant device a slower response time may be accommodated. 4. Manipulator imprecision. The manipulator wül not go precisely where it is asked

and so the position errors may be taken up by the compliant device. The repeatabüity is 1mm (see Appendix B).

The resulting errors are cumulative with noise adding 4mm, ROV motion adding up to 15mm and manipulator imprecision adding 1mm. This gives potential errors of 20mm and so, for the real sub-sea application, a compliant device would need travel of up to ±20mm with contact forces o f up to ICON max. This will result iu constrained direction stififiiesses of 5000 N/m.

An additional advantage of using a compliant device between the probe and the manipulator arm is that it simplifies tele-operation of the arm. When the manipulator

position is being directly controlled by a sur&ce operator using camera views, the deformation of the compliant device gives the operator good visual cues as to the contact made. It is normally very hard to judge the nature of a contact just from camera views as the view points tend to be fixed and the visibility poor.

3.6. Conclusions

The key conclusions from this chapter are:

1. There is considerable hardware in existence to carry out sub-sea inspection work. 2. A specific problem that is identified as requiring a solution is that of running a probe

along a weld toe at the junction of two cylinders when the angle between the cylinders is 30°.

3. Compliant force control is chosen as an appropriate solution to the problem because inspection speed is not critical, model errors are expected and it is known that the docked ROV is unlikely to be a rigid platform.

CHAPTER 4