Author: Yurish, Sergey Y
Date published: March 1, 2012
(ProQuest: ... denotes formulae omitted.)
Optoelectronic sensors are widely used in various applications such as medical, automobile, environmental, bio-chemical, etc. Many of them are based on integrated light-to-frequency converters, which convert a light intensity to quasi-digital (frequency or duty-cycle) format for direct connection to a microcontroller, DSP or interfacing with a PC. In comparison with analog output (voltage or current), the frequency signal as an informative parameter of sensor's output has a lot of advantages, namely: a high noise immunity, high reference accuracy, wide dynamic range, multiparametricity, simplicity of coding, multiplexing, interfacing and integration, etc.
According to the IC Insights report, '2011 Optoelectronics, Sensors, and Discretes', the optoelectronics market will grow 1 1 % to $ 26.4 billion in 201 1, with sensor/actuator sales increasing 15 % to $ 8.5 bn and discretes rising 8 % to $ 23.4 bn this year .
Modern light-to-frequency converters  have a broad frequency range: from part of Hz to 1.6 MHz (Table 1) . Nevertheless a simple frequency-to-digital conversion (based on classical methods for frequency measurements) can be performed by any low-cost microcontroller, a wide dynamic frequency range of such converters brings as usually, many design problems. In order to get reasonable or high metrological performances of designed optical sensor systems, the frequency-to-digital conversion should be based on advanced methods for frequency measurements. Such methods must have a constant quantization relative error in a whole broad frequency range, scalable resolution, non-redundant conversion time and a possibility to measure frequency, which exceeds a reference frequency: f^sub x^ > f^sub 0^ in order to design a sensor systems with a reasonable power consumption.
Existing on the modern sensor market digital light sensors with embedded ADCs as usually have a slow conversion time, for example, the embedded 16-bit ADC of light sensor from Intersil (ISL29015) has the integration time 45-90 ms ; the ADC from Maxim MAX9635 has the conversion time 97-107 ms . Such sensors can be used for proximity or ambient light applications, but it can not be used for light sensing applications, in which a conversion speed is a critical parameter.
The main aim of these research and development was to propose a universal design solution for all existing light-to-frequency converters in order to eliminate all mentioned above design problems, and introduce intelligent and smart features for various sensor systems, which can be realized in different technologies: hybrid, standard CMOS technology, System-on-Chip (SoC) or/and System-in-Package (SiP).
This paper is the extension version of paper presented at SENSORDEVICES conference , and divided into four main parts. The first part describes a design approach for various optical sensor systems based on a light-to-frequency converter (LFC) and Universal Sensors and Transducers Interface circuit (USTI). The description includes the system design in term of OEM hardware and software. The second part devotes to experimental investigation of designed optoelectronic sensor system prototype based on the light-to-frequency converter S9705 from Hamamatsu . The third part includes an experimental determination of main metrological performances of designed sensor system. The last part of the paper provides conclusions and future research directions.
2. Smart Sensor System Design
2.1. Universal Sensors and Transducers Interface
The proposed solution is based on the developed by the author USTI integrated circuit. In comparison with the developed earlier and introduced on the modern market in 2004 and 2007 Series of Universal Frequency-to-Digital Converters UFDC-I and UFDC-IM- 16 respectively [8, 9] this new IC has extended frequency range up to 9 MHz without prescaling and 144 MHz with prescaling, reduced relative error up to ±0.0005 %, increased functionality and decreased conversion time. It is based on the patented modified method of the dependent count for quick and precision measurement of frequency and period of electrical signals . This 2-channel IC has three popular serial interfaces: RS232, 12C and SPI, which are widely used in various sensor systems. It contains of three main blocks: measuring unit, communication unit and time-to-digital converter (TDC). Only one external component - a 20 MHz quartz crystal oscillator should be used as a reference. The measuring unit releases 2-channel measurements of various frequency-time parameters of electrical signals with programmable relative error form ± 1 % to ± 0.0005 %: frequency, period, duty-cycle, phase shift, time intervals, duty-off factor, pulse number, frequency (period) deviation, frequencies or periods ratios and differences, etc. The communication unit supports three popular serial interfaces, such as RS232 (master and slave communication modes with programmable baud rate), SPI and I2C (slave communication mode). The TDC is used in parameter-to-digital converter for a direct interfacing of capacitive, resistive and bridge sensing elements to USTI.
The USTI can work in RS232 master communication mode. In this mode, neither microcontroller nor PC or DAQ system are necessary to control this IC. It will continuously generate measuring results on its output.
2.2. Sensing Element
The S9705 is a CMOS photo IC combining a current-to-frequency converter and photodiode and outputs an oscillating frequency (duty ratio 50 %) proportional to input light intensity incident in the photodiode .
The CMOS level digital output allows direct connection to the USTI. The sensing element has a wide dynamic range, spectral response (see Table 1), and light intensity can be easy measured by the USTI. The light-to-frequency converter S9705 and USTI are shown in Fig. 1, and circuit diagram of optoelectronic sensor system example based on these components is shown in Fig. 2.
Other optical sensors, for example, colour sensor TCS230 from TAOS (USA)  or reflective colour sensor OPB780 from OPTEK Technology  can also be interfaced by the same manner. Two frequency output sensors can be connected to the USTI at the same time.
A software example for the RS232 interfacing slave connection mode for two optical sensors (light sensor S9705 and colour sensor OPB780) is shown in Fig. 3. The command ?02' sets the relative error for frequency-to-digital conversion  and should be use only once. The relative error must be in ten times less (or at the least, in 5 times less) than the sensor's error in order to be neglected. Appropriate command 'M' sets the frequency measurement mode in the 1st and 2nd channels. The command 'S' starts measurement in appropriate channel. The command 'C checks the measurements status and returns the value 'b' if the measurement in progress or the value 'r' if the measuring results is ready. The last command 'R' reads results. The use of 'C command is very important at low frequencies measurements. In opposite side, there is a risk to get a previous result instead of the new one.
Any terminal software can be used with the USTI in RS232 slave communication mode (for example, Terminal V 1.9b Window ). The following options should be selected for this software: appropriate number of serial port, baud rate - 2400; data bits - 8; Parity - none; Stop Bits - 1; Handshaking- none. For data acquisition, the Lab View software or similar can be easily used.
2.3. Conversion Time
The conversion rate of USTI is determined by the method of frequency measurement  and can be calculated according the following equation:
where N^sub δ^ =1/δ is the number proportional to the required programmable relative error d; T^sub x^=1/f^sub x^ is the period of converted frequency, f^sub 0^=625 kHz is the internal reference frequency of USTI.
A measurement time T^sub meas^ for the USTI includes three main components: conversion rate (t^sub com^), communication (t^sub comm^) time and calculations (tcaic) time:
T^sub means^ = t^sub conv^ + t^sub comm^ + t^sub calc^ , (2)
All these components can be calculated by the same way as was described in . For example, the communication time for a slave communication mode (RS232 interface) can be calculated according to the following equation:
t^sub comm^=10 * n* t^sub bit^, (3)
where t^sub bit^ is the time for one bit transmitting; ? is the number of bytes (n=13... 24 for ASCII format). The communication time for SPI interface should be calculated as:
where fscix is the serial clock frequency, which should be chosen for the USTI in the range from 100 to 500 kHz; n=12... 13 is the number of bytes. The number n is dependent on measurement result format: BCD (n=13) or binary (n=12). The communication standard mode's speed for the I^sup 2^C interface can be determined according to the same equation (4), where instead of f^sub SCLK^ the serial clock frequency fsc should be used, which equals to 100 kHz for the USTI; n=12... 13 is the number of bytes for measurement result: BCD (n=13) or binary (n=12). The calculation time depends on operands and is as usually t^sub calc^ ~ 3.6 ms.
Due to non-redundant conversion time for the modified method of the dependent count  it is possible to obtain the conversion time, less than in digital output optical sensors mentioned above. The same is also true for the design approach, when analog light sensor (with voltage output), voltage-tofrequency converter and USTI are used to build a sensor system. The conversion time can be decreased in 3-10 times in comparison with existing standard integrated digital sensors, mentioned above [4, 5].
2.4. Intelligent Features
One of the intelligent functions of modern sensor system is so-called self-identification. The USTI can contain a Transducer Electronic Data Sheet (TEDS) according to the IEEE 1451 standard in its memory. A possible TEDS for optical sensor system is shown in Table 2. This TEDS must also contain a value of programmable relative error for the frequency-to-digital conversion (USTI relative error).
The USTI supports three functions of smart transducers: high accurate frequency (time)-to-digital conversion, TEDS storage in the flash memory and communications.
3. Experimental Results
The aim of experimental investigation was to determine main metrological performances of the designed optical sensor system based on the light-to-frequency converter S9705 (Hamamatsu, Japan) and USTI IC. The measuring set-up is shown in Fig. 4.
Preliminarily, the USTI has been calibrated at laboratory temperature range (+25.3 0C to +26.4 0C) in order to eliminate additional systematic error due to quartz oscillator trimming inaccuracy (calibration tolerance) and a short term temperature instability . The USTI has been connected to a PC, where terminal software Terminal v1.9b was running.
The light-to-frequency converter S9705 has mounted on a LED evaluation board together with a white light diode, the light intensity of which was set-up with the help of current source (Promax FAC 363B) and changing by a potentiometer with 25 µA step. The current through this diode was measuring by an amperemeter (Fig. 4).
The circuit diagram and photo of the LED evaluation board are shown in Figs. 5 and 6 respectively.
The output of LFC was directly connected to the USCI. The frequency counter Agilent 53 132 A was used for frequency measurements in parallel with the USTI, and digital oscilloscope - for wave form visualization at LFCs output/USTI input. The frequency measurements have made for minimal and maximal possible frequencies of LED evaluation board: 5 Hz and 462 kHz respectively for both cases: without and with Schmidt trigger (74HC 14D). Oscillograms of investigated sensor's output signals are shown in Figs. 7-12.
The dependence of LFCs output frequency on current through the white light diode on the LED evaluation board is shown in Fig. 13.
Each of investigated frequencies where measured 60 times and classical statistics was used for results processing. Measuring results for maximal and minimal frequencies for both: without and with Schmidt trigger are shown in Fig. 14 and 15.
The χ^sup 2^ test for goodness of fit test was applied to investigate the significance of the differences between observed data in the histograms and the theoretical frequency distribution for data from the Gaussian distribution law.
The number of equidistant classes was calculated according to the following equation:
k =1.9 × N^sup 0.4^, (5)
where N is the number of measurements.
At probability P =91 %, and 6 equidistant classes k=6, the hypothesis of Gaussian distribution law can be accepted for all sets of measurement data. The statistical characteristics are adduced in Table 3 and 4.
As it is shown from the tables, the Schmidt trigger does not increased the accuracy of frequency-todigital conversion.
The described smart sensors and intelligent sensor systems with self-adaptation can be also realized based on the Series of Universal Frequency-to-Digital Converters (UFDC-I and UFDC- IM- 16) [3, 16-18].
The proposed design approach can be used for development of various low-cost optoelectronic sensor systems , for example, a sensor system for automatic paper type and thickness detection; noncontact, short distance measuring system and different light-to-digital converters for various applications.
The proposed design approach for optoelectronic sensor systems based on the USTI IC gives a unique opportunity to create various OEM sensor systems with high metrological performances including intelligent feature such as self-identification. Taking into account, that many semiconductor sensors and the USTI IC are made according to CMOS standard technological processes, different sensor systems and digital sensors can be realized in various existing technologies: hybrid, system-in-chip and systemin-package.
Since 201 1 the USTI is available on the modern market from Technology Assistance BCNA 2010 S. L., Spain .
This research and development was funded by the European Commission in the frame of Marie Curie Chairs Excellence (EXC) project MEXT-CT-2005-023991 Smart Sensors Systems Design (SMARTSES) and supported by International Frequency Sensor Association (IFSA).
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12 Sergey Y. YURISH
1 Technology Assistance BCNA 2010, S. L.
2 International Frequency Sensor Association (LFSA)
Pare UPC-PMT, Edificio RDIT-K2M
C/ Esteve Tetradas, 1, 08860 Castelldefels, Barcelona, Spain
Tel: +34 696067716
Received: 15 November 201 1 /Accepted: 20 December 201 1 /Published: 12 March 2012