Product Manager, Canpolar East Inc.

ABSTRACT

Cavity sensor technology, utilizing silicone or urethane foam for deformation sensing, has potential for seat occupant sensing via placement in cushioning material. Originally developed for space robotics touch sensors, the technology can also be integrated into energy absorption bumpers for sensing crush zone deformation and to determine crash severity. A look at the principles, performance characteristics, and a new product now being licensed.

INTRODUCTION

Increasing machine 'intelligence' has created new demand for sensing technologies to better inform machines about their interactions with the surroundings. Autonomous mobile machines need sensors which are closely analogous to the human senses of vision and touch. Automated vision systems have been under intensive development for several decades and have become quite sophisticated. By contrast tactile sensing technologies are rare and are relatively primitive.

There is a growing need for kinesthetic and tactile sensors in machines which become involved in deliberate or accidental contact with other objects or systems. Safety is a particular concern for mobile robots.

The Canadian Space Agency (CSA) is the developer and supplier of the robotic manipulator systems used on the Shuttle and on the International Space Station now under construction. In orbital operations an unintended contact or collision can have catastrophic consequences. In 1995 the Canadian Space Agency contracted a consortium led by Canpolar East to develop a tactile sensing technology for robotic manipulator contact and collision sensing. A new optical 'cavity sensor' was developed. This technology is now undergoing functional testing at the CSA in the form of a robotic collision detection sleeve. The patented technology has been licenced by Canpolar East and is being applied in a number of terrestrial markets under the trade name KINOTEX^TM. The technology has potential for application in automotive occupant safety systems.

TECHNOLOGY PRINCIPLES

Many force sensing technologies have been adapted for 'tactile sensing'. These include piezo devices, force sensitive resistors, a variety of intrinsic and extrinsic fiber optic sensors, micro-machined devices, and capacitive devices.

Electro-mechanical sensors are well understood, deliver excellent sensitivity or precision and can be built up into tactile sensing arrays. However, only a few of these technologies are suited for use in direct exposure environments such as the primary contact surface of a robotic manipulator. Few are mechanically compliant, many are fragile and are vulnerable to overpressure damage, and most are too expensive for use in a wear environment. Almost all electro-mechanical sensors are vulnerable to environmental influences such as EMI, and corrosion.

Optical devices have significant advantages in respect to EMI and corrosion. A number of fiber optic force/displacement sensors have been successfully developed [2,4,6]. A common extrinsic sensor configuration is a diaphragm type sensor in which diaphragm displacement is optically sensed [5]. Intrinsic fiber optic sensors include microbend devices and ablated cladding sensors such as the SHAPE sensor supplied by Measurand Inc.[7].The new cavity sensor has many of the advantages of these fiber optic devices.

The KINOTEX^TM pressure/displacement sensor depicted in Figure 1 utilizes a novel sensing principle based on deformation of an 'optical integrating cavity'. The functional performance of Kinotex sensors is comparable to other force or displacement sensors, its primary advantage is simplicity & robustness. The sensor can be fabricated from common polymer foam materials such as silicone or urethane. Deformation of the foam generates a change in optical properties proportional to the extent of deformation. A simple optical transducer can sense the change.

Figure 1. KINOTEX Sensor

The KINOTEX^TM transducer operates by detecting a change in energy intensity in and around an illuminated integrating cavity. Deformation of the integrating cavity by an external influence such as pressure results in a localized change to the illumination energy intensity. This change is measured. The information can be used to infer the state of deformation.

Some KINOTEX^TM sensors use a real physical cavity but most do not. The integrating cavity is normally a virtual space in an isotropic scattering medium. Dimensions of the virtual cavity are defined by the energy scattering properties of the medium. The principles of operation are as set out below.

If an illumination source is embedded in an isotropic scattering medium, the source will be surrounded by a halo of scattered energy (see Figure 2). The intensity of scattered energy at any given distance from the source is a function of the scattering and absorbing characteristics of the medium. The parameter of interest is the characteristic scattering length (C_SL). The C_SL is a mean free path dimension. Radiating energy is scattered as

E_s = E_o [1 - e ^-t/CSL ]

Where E_s is the scattered energy and L is the distance traveled. The term t/C_SL is a normalized dimension, the optical thickness or optical depth.

The medium illustrated in Figure 2 is transparent and compressible. It contains embedded incompressible scatterers. If the medium is compressed, the optical thickness of the medium, t/C_SL, will be unchanged while the thickness, t, and the characteristic length, C_SL, are proportionately reduced. The change in C_SL can be measured internally.

Figure 2. KINOTEX Principles

Figure 3 illustrates a robust method for measuring the C_SL of the medium. An illumination source with controlled energy output is embedded in the compressible scattering medium. An energy detector is embedded adjacent to the illumination source but in such a way that it cannot receive any energy radiating directly from the source. The detector will see only scattered energy. If the dimensions of the source and of the detector apertures are small compared to the C_SL of the medium, the detector will see the integrated intensity of scattered energy in its vicinity.

Figure 3. KINOTEX Principles

Integration takes place over a volume around the detector with a radius equivalent to about one characteristic length. The C_SL of the medium defines a virtual integrating cavity around a source or around a detector. If the C_SL is reduced the integrating cavity shrinks. As the integrating cavity surrounding an energy source shrinks, the intensity of scattered energy inside the cavity increases. If an energy detector is located 'inside' this cavity, it will register an increase in integrated energy intensity. If an energy detector is distant ( > 1 C_SL) from the source, it will register a decrease in integrated scattered energy intensity. An appropriately spaced array of sources and detectors in a compressible isotropic scattering medium can provide information as to the location, extent and direction of a compression event.

Figure 4 shows a number of practical 'half space' sensor configurations.

Figure 4. KINOTEX Sensor Configurations

Figure 5 shows response curves for 'full space' KINOTEX^TM sensors assembled from off-the-shelf silicone foams. If the elastic properties of the foam material are known, the force can be inferred. Models for signal vs force/displacement have been developed [3]. Figure 6 shows modeled and actual load response for one of these foams.

Figure 5. SIGNAL VS. DEFLECTION

Figure 6. SIGNAL vs. LOAD

The sensors can resolve relative changes as small as 0.1% of total thickness but do not deliver high absolute accuracies because of the visco-elastic properties of the foam materials. Typical absolute accuracies are in the order of +/- 10%.

Preliminary testing of frequency response indicates that displacement events on a millisecond time scale can be resolved. Frequency response characteristics are determined by the scattering mediums density and stiffness as well as the interstitial fluid viscosity & compressibility.

To date investigation of environmental influences has been limited. Preliminary thermal testing suggests a modest temperature dependence (See Figure 7).

Figure 7. COMPRESSION VS. RESPONSE

APPLICATIONS

KINOTEX^TM has evident use in a wide variety of applications such as robotics, security, computer and game input devices, sports, toys, medical and automotive. In automotive applications it can potentially be used for control surfaces, occupant sensing, ride sensing, and crash sensing. Of these two stand out as being of immediate interest.

CRUSH ZONE SENSING

As a crush zone intrusion sensor, KINOTEX^TM has the potential to provide more essential crash information than any other existing crash sensing technology. The Kinotex technology can be integrated with the energy absorbing foam used in side panels and in bumpers, measuring the intrusion rate and location of impact directly and relaying the information to the air bag computer for improved air bag deployment decisions. Few of the sensor technologies now in use or being considered for use are competent to discriminate intrusion events in this way. Figure 8 shows the impact event response of a KINOTEX^TM prototype device.

Figure 8. AOS Impact Test

SEAT OCCUPANT SENSING

KINOTEX^TM sensors can be integrated with automotive seat cushioning. Deformation of the seat will provide a pressure pattern. Signals can be used to infer occupant weight and position. Kinotex provides a functionality and performance similar to the resistive force sensing technology that has been reported by Billen et al [1]. The possibility of integrating the sensors within the cushioning material is an advantage, leading to possible reductions in inventory and assembly cost. Figure 9 shows the contact image of a 185 lb male seated on a 12 mm thick KINOTEX^TM pad.

Figure 9. Seat Contact Image

DEMONSTRATION PRODUCT

One of the KINOTEX^TM licensees, Tactex Controls Inc., is developing 'touch pad' devices for computer input and music markets. The touch pad device illustrated in Figure 10 embodies many of the functional characteristics which are of interest in automotive applications.

CONCLUSIONS

The principles of operation and performance characteristics of a new cavity sensor technology have been presented. This simple and robust technology holds promise for automotive sensing and control applications

References:

1. Billen, K., Federspiel, L., Schockmel, P., Serban, B., and Sherrill, W.,"Occupant classification system for smart restraint systems", SAE Conference, Detroit, USA, Paper Number 1999-01-0761, 1999.
2. Caldwell, D.G., and Gosney, C., "Multimodal tactile sensing and feedback (teletaction) for enhanced telemanipulator control",Proc. of the 1993 IEEE/RSJ Internat. Conf onIntelligent Robots and Systems, Yokohama, Japan, July 26-30, pp. 1487-1494, 1993.
3. "Kinesthetic Textiles, Phase II - Part 2, Final Report", Canpolar East Inc., Vol I-II, pp.37-69, 1999. Proprietary.
4. Jia, Yun-De, and Li, Ke-Jie, "A high resolution and high compliance tactile sensing array", Chinese Science Bulletin, Vol. 39, No. 15, pp. 1249-1252, 1994.
5. Philips, G., "Method and apparatus for determining the size of defects in rolling element bearings with high frequency capability",US Patent 4,870,271, 1989.
6. Porter, J.H., "Differential fiber optic proximity sensor", US Patent 4,358,960, 1982.
7. "SHAPE SENSOR^TM and SHAPE TAPE^TM Technology", Measurand Inc., www.measurand.com/techbackground.html.