Taking shape

You can’t get round it there are some things that just really difficult to measure. Aspects of bending, displacement, flow, vibration, pressure, angle and force, for example, can all equate to serious headaches for the design engineer, especially where there are considerations of cost, environment, associated signal processing equipment, etc.

Well, now there just might be an alternative to the aspirin bottle. Because now you can get Shape Sensors, which translate curvature into an electrical signal through the use of light and optical fibres. The sensors are low cost, safe, reliable, and easy to use.

Shape Sensors use tight loops of optical fibre that have been treated on one side to lose light, this loss being proportional to bending. The loss of intensity as the light travels through the fibres and round bends is used to determine the shape.

The lost light is contained in absorptive layers that prevent interaction of light with the environment. For standard sensors, the treated zone is only 2mm long, very near the end of a tight loop of fibre. Placement at this loop permits sensing at the end of a structure, and works with the loop geometry to produce a larger throughput and modulation than otherwise possible. Modulation is more than 3000 times greater than for bending of untreated fibres.

Shape Sensors are bipolar. When straight, their throughput is at the middle of the linear range. Bends to the ‘left’ increase the throughput, while bends to the ‘right’ decrease it. In this way, Shape Sensors can be used to make one-degree-of-freedom measurements such as displacement, curvature, flow or acceleration. For up to six degrees of freedom, an array of paired Shape Sensors can be formed to create Shape Tape.

Shape Sensors use optical bridge technology to obtain stable, compensated measurements. Two loops treated on opposite sides are arranged so light throughput increases in one and decreases in the other, for the same bend. The light throughputs are detected and amplified, then analogue sums and differences are taken. The difference is the signal output the sum is held constant in a control loop.

APPLICATIONS

Although a cantilever beam with an applied point or distributed load deflects in a non linear shape, the displacement of any point along the cantilever varies linearly with applied force. Also, the curvature of any section of the beam inboard of the applied force changes linearly with force or displacement, and varies linearly with distance along the beam. Similarly, all points along the beam move linearly with the displacement of any other point.

Shape Sensors with integral cantilever, or those mounted onto a cantilever, can be used directly without further processing to make linear measurements of displacement, force, acceleration, or flow. All these measurements deflect the cantilever in a linear fashion, leading to linear measurements proportional to force or displacement (flow force is proportional to the square of velocity, whilst inertial force is proportional to acceleration). Sensitivity can be high: beams 0.127mm thick and 6.35mm wide offer a response better that 0.2V/gf. Cantilever beams can be used in many applications to replace LVDT technology. Because of the cantilever, its full scale electrical range can be used to measure a wide variety of displacement ranges. Moreover, it is a complete sensor with high level DC output, intrinsically sealed, at a low cost.

Other examples of applications for Shape Sensors include attaching a float to measure liquid level, diaphragm capsules to measure pressure, a cam to measure rotational position, or a magnet to measure magnetic field or current flow.

Of course, not all motions are confined to one plane, and that is where Shape Tape comes in. The Tape is made in a lamination procedure similar to that for basic sensors. An optical multiplex box is used to illuminate groups of sensor loops and measure their return light intensity. Groups of eight signals at a time are sent to an A/D card in a PC. The PC takes sums and differences of signals from paired loops to obtain separate bend and twist components. This information is used to form a real time 3D model of all the portions of the tape, which is accurate as long as the portions of tape between sensor pairs form approximately circular arcs. Its shape is determined by a simple application of limit calculus that does not require knowledge of stiffness or solution of any beam equations.

Shape Tape can be attached in a draped configuration between any two or more objects (such as a person’s torso and upper arm) and will report the relative 6DOF position and orientation of the two. The power of Shape Tape derives from its ‘knowledge’ of its own location when not in contact with the surface. Thus it can be used to track arm segments even though it travels in a loose, arbitrary curve over the joints and segments.

Shape Tape can also be used as a 6DOF input device for a computer (eg for 3D CAD input), by forming it into a U shape and manipulating the end of the U with the hand and fingers, or by attaching it to the operator’s arm and using the entire arm as an input device. In dynamic mode, Shape Tape can be used to scan surfaces, collecting shape profiles relative to a fixed location.

Other applications for Shape Tape include structural analysis (eg in response to wind or earth quakes), crash testing (to assess the shape of the buckling floor section or the dummy’s spine), or accident investigation.