IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 47, NO. 2, APRIL 1998 489 Microcontroller-Based Performance Enhancement of an Optical Fiber Level Transducer Giovanni Betta, Associate Member, IEEE, Antonio Pietrosanto, and Antonio Scaglione Abstract— A proposed digital microcontrolled transducer for liquid-level measurement is based on two optical fibers from which the cladding was removed in suitably spaced zones at known distances. A 1-m full-scale, 6.25-mm resolution prototype was made and tested. The adopted hardware and software strate- gies assure a low-cost, high-static, and dynamic performance, good reliability, and the possibility of remote control. These features were confirmed by the experimental tests carried out on gasoline. Index Terms—Liquid level measurement, microcontroller, opti- cal fiber transducer, petroleum industry, sensor characterization. I. INTRODUCTION A LARGE number of physical principles have been adapted to produce liquid-level sensors. Generally, the choice of the most suitable level sensor for a specific application is based on the following requirements: 1) measurement range, 2) resolution, 3) accuracy, 4) the characteristics of the liquid, and 5) those of the environment [1], [2]. A great deal of interest is currently being focused on the last two requirements and, in particular, on developing reliable and safe sensors for hazardous or explosive liquids and environments [3], [4]. The use of optical fiber technology [5], [6] is particularly appropriate for this purpose. Several optical- fiber liquid-level sensors have been developed during the past few years [7]–[9]. Most of these sensors are either level controllers or have limited range and accuracy. Furthermore, it seems to be difficult to ensure that robustness, reliability, and low cost are all characteristics of the same sensor [5]. The authors have already proposed both a continuous level measurement technique [10], [11] and digital level-transducer [12]. The latter is based on the observation that the power propagating along an optical fiber is attenuated if part of its cladding is removed and if the external surrounding medium has a refractive index greater than that of the core. Conse- quently, the sensing element consists of a fiber that extends over the whole depth of the tank and whose cladding has been removed in equally spaced zones. Every time the liquid reaches or leaves one of these zones, the output power increases or decreases depending on the direction of the change of the liquid level. The liquid measurement is then Manuscript received June 30, 1997; revised November 1, 1998. G. Betta is with the Department of Industrial Engineering, University of Cassino, Cassino 03043 Italy (e-mail: betta@ing.unicas.it). A. Pietrosanto and A. Scaglione are with the Department of Information Engineering and Electrical Engineering, University of Salerno, Fisciano, Italy (e-mail: pietrosa@diiie.unisa.it). Publisher Item Identifier S 0018-9456(98)09846-5. carried out by a discrete-component analog signal-conditioning circuit, which sums the up and down output power variations, each of which is counted separately. This prototype showed itself to have good accuracy and an acceptable dynamic performance. The transducer resolution was equal to 25 mm, i.e., one-fortieth of full-scale. The reliability needs improving, since eventually counting errors affect all the subsequent measurement, thanks to the incremental measurement principle of the transducer. An improvement in resolution can be obtained only by increasing the number of unclad zones per meter. But, having fixed both the input power and the width of the unclad zones, the higher the number of unclad zones per meter, the smaller the output power changes when the liquid level nears full-scale. Hence, if the resolution has to be improved, the sensitivity of the signal conditioning hardware must be increased, to always allow useful output power variations to be distinguished from noise. As far as transducer reliability is concerned, it could be increased by means of periodic calibration checks to detect coarse counting errors. These checks only can be made if an absolute level reference, supported by “intelligent” signal conditioning hardware, is available. To overcome these limi- tation and, thus, to better meet requirements 2) and 3), a new transducer is proposed. The proposed prototype is described in detail and its performance is reported with reference to characterization tests carried out on gasoline. II. THE MICROCONTROLLER-BASED OPTICAL FIBER LEVEL TRANSDUCER The new transducer is characterized by the following: • a double-fiber sensing element; • a microcontroller-based conditioning circuit; • suitably designed software. A. Sensing Element The numerical simulation, based on the theoretical analy- sis [12]–[15], showed that, for the chosen power source (5 mW laser diodes TOLD 9201 by Toshiba) and conditioning hardware and for a 0.50-mm zone width, only 60 zones would give voltage variations significantly greater than the noise level (about 10 mV rms). Hence, to increase the number of unclad zones, a variable width strategy was introduced. In particular, the lower 50 unclad zones were made narrower (0.25 mm) than the remaining 30 (0.50 mm), thus obtaining a total of 80 unclad zones. The width-changing point was experimentally 0018–9456/98$10.00  1998 IEEE