OPTICAL SENSORS-CONSTRUCTION ALTERNATIVES Mariana ENACHE, Cristina ŢUINEA BOBE Universitatea Valahia Târgovişte, Facultatea Ştiinta si Ingineria Materialelor, B-dul Regele Carol I, Nr.2, 0200, Târgovişte, Tel./fax 0245/206106, E-mail: tuinea_cristina@yahoo.com Optic sensors are used to measure the position and displacement data. Their main advantages are their simple structure and the relatively long operating distances. A position optical sensor has usually at least three components: a light source, a detector, light guiding devices, lens, mirrors, optic fiber, etc. 1. INTRODUCTION The increasing need in advanced robotics for sensory systems having a complexity similar to the one of the human beings, leads to a new approach to sensor technology. The advantages of fiber optic sensor technology are well known and they include impressive precision, electromagnetic interference immunity and chemical compatibility with a wide range of environmental media. Fiber optic sensor technology can have important applications in robotics where its performances are needed. Silicon micro machining is the only technology that can offer this kind of performance. After electrical sensors, optic sensors are used the most to measure the position and displacement data. Their main advantages are their simple structure and the relatively long operating distances. Optic sensors respond to electromagnetic radiation, including the radiation with high levels of energy like X-rays, gamma rays, visible light, UV, infrared light and heat. These sensors are designed in order to respond only to a certain frequency of radiation. The selection of the spectrum can be realized using a monochromatic filter. The performances of an optic sensor are measured by its sensitivity, which is a ratio between the output signal and the intensity of the radiation. When a sensor is characterized, other parameters are taken into consideration, such as detection D, which measures the time of reaction and the level of noise in the system, and the efficiency of quantum, which is defined by the number of current carriers (electrons or holes) that are actually created through impact of a photon. A position optical sensor has usually at least three components: a light source, a detector, light guiding devices, lens, mirrors, optic fiber, etc. 2. FIBER OPTIC SENSORS BASIC PRINCIPLES The electro-optical sensorial system of the robot detects electro-magnetic radiation in the visible spectrum, which in most cases means converting light (e-m waves) into an electric variable parameter, such as electric current, electric potential, electric impedance or electric charge. Fiber optic sensors can be classified as intrinsic (Fig.1) devices and extrinsic devices (Fig.2). Intrinsic sensors have a simpler mechanical structure but they have the disadvantage of responding to environmental stimuli, as they are extremely sensitive devices. Extrinsic sensors have a complex mechanical structure, but they are relatively simple to arrange so that most of the optic modulation occurs in the area of interest. In principle, the detection of modular radiation is a direct process so it should be stressed that optic detectors are able to respond instantaneously to external intensity of the light. Most of the fiber optic sensors impose analog modulation of the optical carrier. That is why the ability to solve variable possible values of the parameters is given not only by the process of modulation itself, but also by the characteristic of an optical carrier. The light, in most of the photodetection forms, manifest itself as a quantum phenomenon, so that the detection process is done according to quantum particle statistics.
Mariana ENACHE, Cristina ŢUINEA BOBE 364 Therefore the optical carrier shows power fluctuations that are determined by Poissons statistics of the photons forming the light beam. If the light beam is formed by N photons per second, then it will be an intrinsic level of noise of N photons per second imposed by this process. This process limits the resolution of the optical detection. Fig.1. Intrinsec sensors Fig.2. Extrinsec sensors An important feature of fiber optic sensors is the wide range of optical parameters that can be measured. The optic systems can be implemented using intensity modulation (through a switch, for example), state of polarization (using the modulation of the birefringent crystals), optical phase (by measurements which interact with one arm of the fiber optic interferomenter), optical frequency ( through Doppler effect), wave range distribution(by using rotating prisms or environmentally sensitive filters) and forms of modulation waves (which involves vibration measurements). All these schemes are illustrated in Fig. 3. These modulation mechanisms interact differently, but it should be mentioned that in the end they all detect optical intensity. Most of the fiber optic sensors have 1 mw of optical power. The resolution is another characteristic of the fiber optic sensors for which they are preferred to other types of trasductors. The design of the sensors in order to realize this dynamic range can be exploited by a single sensitive device ( cell ) or by the elements of an array of cells.
365 Optical sensors-construction alternatives Fig.3. Schematic of basic optical modulation techniques It is possible that this last configuration to be the most effective for robotics. Fiber optic techniques are used for monitoring most of the measurements. 3. GYROSCOPE The fiber optic gyroscope uses the Sagnac effect in the measurement of the inertial rotation. Fig. 4 shows the scheme of the gyroscope. Its principle can be viewed from the point of view of an imaginary person sitting on the beam splitter (or fiber optic directional coupler) input into the loop.
Mariana ENACHE, Cristina ŢUINEA BOBE 366 Fig.4. The simplest low noise gyroscope showing the use of lock-in amplifier detection The clear light injected in the loop will exit the counter propagating direction sooner than the light propagating with rotation. This difference of time is measured as a difference of optical phase and depends linearly of rotation rate.an important research effort was laid upon studying the gyroscope, as it is the most complex fiber optic sensor. The gyroscope can operate in high vibration media and can be configured in a large variety of forms and dimensions. In robotics, the fiber optic gyroscope is considered to be a perfect instrument for coordinate calibration. The most useful application could be the angular transducer for monitoring the relative position of the robot arm (Fig.5). 4. TEMPERATURE DETECTION USING FIBER OPTICS Many fiber optic sensors are nowadays used for detecting temperature. In most of the cases, the functioning principle is based on the changes in the chromatic properties of the different materials due to temperature change. The example presented includes the exploit of the rare fluorescence spectra of the earth and the changes in the band gap of the semiconductors, inducing changes in fluorescence or in absorption spectrum. Temperature detectors are made up by an optic source with a short wave length that excites a material; then the fluorescence spectrum is observed and measured using a spectrometer. The principle of such a device, which is based on fluorescence variation of a semiconductor, is shown in Fig 6. This device has the following advantage: the excitation spectrum and the fluorescence are both in infrared light so that they both can be transmitted without a substantial attenuation. The gap band of a semiconductor varies by an order of 2 mv per degree Celsius temperature change, so that the range 50 degrees Celsius +20 degrees Celsius can be expected a bandgap change of order 100mV. Therefore, the shift in fluorescent wavelength is of order 10%over the temperature range of interest. The major advantages of the system are the small size and the perfect immunity to electric interference. Further research approaches the construction of minute optically read transducers which would perform tactile thermal monitoring. Fig.5. Possible type of fibre gyroscopes in robot arms
367 Optical sensors-construction alternatives Fig.6. Optoelectromic system of fiber-optic temperaatur sensor. 5. LIMIT STOPS The simplest fiber optic sensor is the limit switch, which function in an analogue way to a microelectric switch. Such devices are commercially available. The principle of this device is shown in Fig. 7. Fig.7. Schematic of fibre optic limit switch 6. SILICON MICROTRANSDUCERS AND THE FIBER OPTICS In the context of fiber optic sensors, silicon micromachining offers the potential of highly precise mechanical fabrication in very small structures. Fig. 8 shows such an example, the scheme of a device consisting of a silica bridge over an anisotropically etched orifice in a silicon substrate. This bridge has a characteristic self-resonant frequency at a fundamental frequency of 270KHZ with observable overtones to 1MHZ. These frequencies can be directly excited using the conversion of optical energy to mechanic energy through heating on a metallic evaporated stratum on the silicon substrate. The resonance frequency is a function of temperature (by almost 0.1% per degree). The optical power levels returned from the sensor are comparable with the incident power levels, resulting that the integration times are shorter( of the order of microseconds) and therefore fast thermal changes can be monitored. 7. PROXIMITY SENSORS A proximity sensor is the Doppler probe, as shown in Fig. 8.
Mariana ENACHE, Cristina ŢUINEA BOBE 368 Fig.8 Schematic diagram of a fibre optic Doppler anenometer This probe is designed to monitor the movements of microscopic bodies in fluids. In robotic systems, such detectors can be useful when determining the rate of approach of a hand to an object. This technique is completely analogous to the one using ultrasonic devices. Slow motion approaches can be monitored with a high precision. Also, this technique allows us to monitor easily speed varying from micrometers to meters per second. The basis of concepts of modulated position probe can be modified with a higher resolution than the one realized by the photon device, and can approach nanometer levels. For tactile purposes, surface imaging and related tasks, a resolution of 1micron from arranged structure would be adequate. Such an array can be configured as in Fig 9. It could consist of 10-100 tactile pixels per square mm. Fig.9. Schematic diagram of fibre optic tactile sensor 8. CONCLUSION The commercial exploitation of fiber optic sensors has increased dramatically in recent years. The application area for fiber optic sensors is now well identified and center on traditional applications in high radiation environments and intrinsic safety, but recent commercial applications have considerably expanded this scope. The value potential of sensor array and sensor nets has also been only recently exploited. They offer a very high packing density obtainable by recent advances in photolithographical silicon micromachining. The promise of significant developments is high and the contributions to advancing the science of robotics will be substantial. REFERENCES 1. C MENADIER, C KISSINGER AND H. ADKINS, The Fotonic Sensor, Instruments and Control Systems, 1967. 2. B. CULSHAW, Optical fibre and signal processing, Peter Perignus, Stevenage 1984 3. B. CULSHAW, Fibre optic sensors in advanced robotic systems, Edited by P.Dario,1988 4. S. FATIKOV, U. REMBOLD, Tehnologia microsistemelor şi robotică, Editura Tehnică, 1999