The centre of gravity of the scanning arm changes its position in a 3D space while scanning and hence the moments exerted by gravity on the wall-climbing vehicle vary in magnitude. This causes a variable loading on and instability and vibration of the vehicle frame, which in turn affects the scanning of the sensors and leads to unwanted changes of the stand-off or contact pressure between the sensor and the surface under inspection. As a result, signal output of the sensors becomes unstable and poor.

An original model is established to emulate the kinematics of the centre of gravity of the arm, from which values of the maximum rotating moment and the maximum overturning moment on the climbing vehicle are obtained. Effects of the changing moments on the dynamic and static performances of the system are analysed. Optimum performance criteria of the system are obtained. For example, from the kinematics equations, it is possible to elucidate the scanning geometry that produces minimum temporal changes of position of the centre of gravity. Also, from the moment analysis, the payload and the travelling speeds of the vehicle can be optimised to the maximum values consistent with maintenance of statutory safety factors, thus minimising inspection times.

The analysis is illustrated by experiments on a prototype system with optimised performance. Some indicative trial results are given using ultrasonic wet and dry contact techniques applied to a simulated wall of a large ferrous oil storage tank containing corrosion thinning defects. The derived kinematics equations will be of general interest to engineers involved in robotic NDT. REFERENCES

1. S. Chen, T. Sattar, A. Khalid and B. Bridge, "Climbing Robots-off the Shelf", Industrial Robot, Vol. 21, No. 5, pp. 27-30, 1994.
2. A. Khalid, S. Chen, T. Sattar, and B. Bridge, "Development of a Climbing Robot for the Deployment of NDT sensors in Hazardous Environments", the British Institute of NDT Journal, INSIGHT, Vol. 36, No. 12, pp. 943-948, 1994.
3. T. Sattar, S. Chen, A. Khalid and B. Bridge, "On the Design of Multi-task, Compact, Climbing Robotic NDT Systems for the Remote Operation on Large Surface and in Hazardous Environments", submitted to Non-destructive Testing and Evaluation, 1995.