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Measurand patented SHAPE SENSORS™ use optical fibers that have been treated on one side to lose light proportional to bending. The lost light is contained in absorptive layers that prevent interaction of light with the environment. For standard sensors, the treated zone is only 2 mm long, very near the end of a tight loop of fiber. Placement at the 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 fibers.

SHAPE SENSORS™ have another useful characteristic: they are bipolar. When straight, their throughput is at the middle of linear range. Bends to the "left" increase the throughput; bends to the "right" decrease it.

SHAPE SENSORS™ are not strain gauges. They are equally effective in the neutral axis of a beam, where there is no strain, or glued informally to the surface of a bending object, bypassing the need for careful attachment associated with strain gauges.

SHAPE SENSORS™ are used to make one degree of freedom (1DOF) measurements, such as displacement, curvature, force, flow, or acceleration. SHAPE TAPE™ is an array of paired SHAPE SENSORS™ that makes six degree of freedom (6DOF) measurements with a minimum number of sensors. 

SHAPE TAPE™ uses paired loops to sense twist and bend along a ribbon substrate. In this case, sums and differences correspond to twist and bend, and there is no control loop used in ordinary applications.

Single DOF SHAPE SENSOR™ measurements

Curvature sensing is very basic to single or multiple degree of freedom (DOF) measurements. In fact, equations for strain are derived from the geometry of curved bodies. Although a cantilever beam with an applied point or distributed load deflects in a nonlinear 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.

Either the Measurand S220 mounted on a cantilever beam, or the S210 with integral cantilever beam, can be used directly without further processing, to make very linear measurements of displacement, force, acceleration, or flow. All of these measurands 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; inertial force is proportional to acceleration). Sensitivity can be high: S210 beams are 0.127 mm thick, 6.35 mm wide, with a response of better than 0.2 V/gf at 35 mm from the sensor body.

The S210 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 low cost.

SHAPE SENSORS™ may also be mounted to other curving structures. A useful structure is a flexible beam mounted at its ends between two surfaces. Examples include beams on human arms, automobile control arms, or roof trusses undergoing angular deflections. If properly mounted, the beam will curve linearly with the angular deflections. Large ranges (± 90 degrees or more) are possible with long beams; short ranges (a few degrees) are possible with shorter beams. The attached SHAPE SENSOR ™ is used to sample the curvature within the linear range of the sensor.

Other applications include attaching a float to measure liquid level, Bourdon tubes or diaphragm capsules to measure pressure, a cam to measure rotational position (to simulate a linear, noise free potentiometer), or a magnet to measure magnetic field strength or current flow (ac or dc).

Measurand also can provide twist sensors that are the converse of its bend sensors: twist sensors measure twist but are immune to axial bending; bend sensors measure axial bending but are immune to twist. SHAPE TAPE™ measures both.
 
 

Multiple DOF SHAPE TAPE™ measurements

Not all motions are confined to a plane or can be sampled easily by a one degree of freedom (DOF) sensor. For this reason, Measurand has developed SHAPE TAPE™, an array of loops paired to sense twist and bend along a ribbon. Because the ribbon can only twist and bend, SHAPE TAPE™ "knows where it is." (SHAPE TAPE™ is produced under license from the Canadian Space Agency).

SHAPE TAPE™ is made in a lamination procedure similar to that for SHAPE SENSORS™, resulting in a lightweight, flexible structure that can be used to measure position and orientation in six degrees of freedom. An interface box is used to illuminate the loops, detect return light, and take sums and differences to separate bend and twist components. The signals are digitized and passed along to the serial port of a PC, which forms a model of the position of all portions of the SHAPE TAPE™, based on the samples taken by the sensor pairs. As long as the portions of tape between sensor pairs form circular arcs, the model will be accurate. 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 shape between any two or more objects (e.g. a person's torso and upper arm) and will report the relative six DOF position and orientation of the two (e.g. upper arm motions relative to torso). The power of SHAPE TAPE™ derives from its "knowledge" of its own location when not in contact with a surface. Thus, it can be used to track arm segments even though it travels in a loose, arbitrary curve over the joints between segments.

SHAPE TAPE™ can also be used as a six DOF input device for a computer (e.g. 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 the end to the back of the hand, or attached to the operator's arm to use the entire arm as an input device. When hand operated, SHAPE TAPE™ is referenced to a fixed location (e.g. a desk stand); when attached to the entire arm, it would normally be referenced to the torso. In dynamic mode, SHAPE TAPE™ can be used to scan surfaces, collecting shape profiles relative to a fixed location, to build up a surface model.  It can also be used with elastomers to form multidimensional input devices, such as distributed musical keyboards, contoured instrument panels, or for impact location and measurement. In these forms its bend and twist properties can be locally mapped to trigger or modulate various functions. For instance, twist measurement allows a single SHAPE TAPE™ to sense whether pressure application is to the left or right of the tape (this could be the basis of radio input functions built into the contoured dashboard of a car, along with lighting controls; or for implementation of full steering column functions from torque through horn and turn signals, all from the same sensor tape.  SHAPE TAPE™ is also the material of choice for measuring structural deflections where cameras do not have access, or in conjunction with camera views. Applications include structural analysis (e.g. response to wind, driving loads, earthquakes, or frost), furniture shape resulting from seating loads (of obvious use in furniture design and airbag deployment), crash testing (e.g. the shape of a buckling section of floor), accident investigation (correlation of SHAPE TAPE™ measurements of shapes and locations of objects or victims with photographic records), or archaeological records.