"The Rubbery Ruler"

An elegant, economical answer to the requirements of displacement measurement
Developed by The School of Physics, The University of Melbourne

The Rubbery Ruler is a wide range, conformable, capacitive displacement transducer. In its basic form the Rubbery Ruler consists of a bifilar helix of insulated conductive wires embedded in a tube of elastomeric material (Fig. 1).

This combination results in an extraordinary long range of elongation. The range can be more than double its initial length. The flexibility permitted by the design allows the transducer to conform to the contour of objects to which it is attached (e.g. chest, joint, heart, fruit, mechanical structure) whilst monitoring accurately their dimensional changes (Fig. 2).

The transducer is basically a spring -- elongation and compliance are determined by the elastic properties of the elastomeric covering and the double helix core dimensions. The Rubbery Ruler is fully selaed and can be operated in harsh environmental conditions.

Electrically the Rubbery Ruler can be described as a distributed capacitor. The two wires of the double helix core constitute the electrodes of a variable gap capacitor. As the Rubbery Ruler is stretched, the two wires of the core separate in a uniform, reversible fashion controlled by the elastomeric covering. Standard or propietary signal processing techniques are used to convert capacitance changes into a frequency or current or voltage proportional to elongation.

Rubbery Rulers can be made in just about any shape and size, depending on the desired application. Elongation can be up to 250% with an accuracy of 0.1%.

The European Space Agency is using Rubbery Rulers in the Analog Biomedical Recorder Space Suit to provide a database of human movements in space.

Specific applications of Rubbery Ruler technology

Biomedical and health care

The Rubbery Ruler can be easily configured to monitor flexions, joint movements and expansion and contraction of body parts. Unlike the more expensive inductive transducers, they are insensitive to electro-magnetic interference and require simple conditioning electronics. Some specific applications are:

Monitors for breathing, plethismography, joint movements
Miniature sensors for "in vivo" applications
Monitors and alarms (e.g. backbone injury prevention)
Kinesiology
Nocturnal penile tumescence monitors for impotence clinics (as a replacement for mercury transducers)

Robotics

Input transducers for remote manipulators and for "virtual reality" environments and games

Silviculture

Rubbery Rulers are used as dendrometers in orchards and forests

Geomechanics

Monitoring the movements of foundations
Monitoring movements in mining operations
Monitoring the movements of ground faults

Aerospace

Rubbery Rulers have an electrically balanced structure with inherently little electro-magnetic leakage. They can be constructed with a high capacitance per unit length, have high noise immunity and very high sensitivity. They are of very low weight.

Engineering

Rubbery Rulers are conformable (in fact they can be wrapped around objects), self-aligning and are insensitive to vibration. They are sealed and the elastomer (silicone, flurosilicone, etc.) can be selected to be impervious to corrosive fluids. Rubbery Rulers are being used successfully in an aluminium smelting plant to servo position the carbon electrodes in electrolytic cells. They can operate in the presence of very large DC magnetic fields, in a highly corrosive atmosphere and in high temperatures. Other applications include: