Snake-arm inspection

OCRobotics has just completed a two-year contract for the UK Ministry of Defence (MoD) which called for the company to build a 2.5m long snake-arm robot that could carry a 25kg payload and be used on a remote vehicle.

Where a rigid-link robot is restricted by the "elbows" in its arms, a snake-arm can follow its nose to reach through small gaps and around narrowly spaced obstacles. This makes it ideal for getting into awkward spaces.

The OCRobotics snake-arm robot that was developed is a bit like the human spine: it is comprised of a large number of vertebrae. The arm is ‘tendon’ driven with wires terminating at various points along its length. By controlling the length of each wire, the curvature and plane of curvature of each segment can be independently controlled. Software calculates the necessary lengths of all the wires to produce the desired shape.

The user can operate the robot in three modes: ‘joystick path following’, ‘cartesian tip motion’ and ‘joint mode’.

In the joystick path following mode, the operator drives the tip of the snake-arm using the view from a tip camera while the computer controls the rest of the device to follow the tip, thus avoiding any obstacles. Path following is achieved by the coordinated motion of the linear axis and the snake-arm axes. Stable motion is required for advancing and retracting.

Once the arm is deployed the operator then needs to move in the Cartesian work space. Obviously this motion has to be achieved whilst also making sure that the rest of the arm does not hit any the obstacles previously avoided. It also means that the return path has to be modified for the arm to retract.

In the joint mode, the operator is able to move individual joints independently of each other. This can be used for ‘in-situ’ revision of the path or to control the final segment and wrist to optimise the tip camera view. Again, the software deals with the implications for retracting from the work site and switching between these different modes of operation.

The arm itself is the cheapest part of the system, meaning that if it is damaged during operation it can be replaced at relatively little cost. OCRobotics has designed the system so that the arm can be detached remotely allowing the robot to discard its arm, for example, if it gets trapped or wedged, and return to the operator so that a new arm can be attached. Arms can be quickly hot-swapped if a different tool or arm size is required.

The arm works mainly in a horizontal plane and, being flexible, gravity has a considerable effect on it. For this reason it was necessary to develop a method of measuring the actual shape of the arm.

OCRobotics’ software compares the desired shape to the actual shape and compensates for externally applied forces. This also allows the arm to be controllable when it picks up a payload.

The company has successfully demonstrated the capability of the arm to reach into a car through an open window. The arm reached through the driver’s window to the back seat of the car where an object was viewed through the tip-mounted camera.

The MoD contract also stated that the arm needed to be strong enough to tow a car. OCRobotics arm met that requirement because the snake-arm is inherently axially strong because of the high loading ability of the wires running along its length.