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Academic Open Internet Journal |
Volume 15, 2005 |
INTELLIGENT UNIVERSAL SENSOR INTERFACE
DEVELOPMENT USING MICROCONTROLLER
A.V. Mancharkar And
S.H.Behere*
Lecturer,
Dept.of Electronic Science,
E-mail : mancharkar_av@rediffmail.com
*Professor, Dept.of Physics,
ABSTRACT
Intelligent sensor interface is a rapidly
expanding area of research, which will inevitably bring extraordinary changes
in the current real life problems. It is implemented with a number of
high-performance front-end circuits for various types of sensors. Sensing
elements can be directly connected without the need for extra circuits. The
commercially available sensor interface allows a limited
The interface is
divided into two parts a base unit and a sensor specific plug-in module. The
base unit allows only one module to be plugged in. Each module consists of a
signal conditioner for a specific sensor and the user has to adjust the DIPswitches present on the module to indicate the type of
sensor and range of measurement. The base unit consists of microcontroller (for
monitoring and control) and other circuits. The microcontroller software
identifies the connected sensor type, measurement range and sets amplifier
gains accordingly to provide a fixed range of voltage 0-5V. The base unit
converts this voltage into fixed 4-20 mA current and
parallel 12 bit digital data. The microcontroller sends this digital data to a
host PC using RS 232C interface for further processing. The interface provides
connection of 8 different types of sensors and provides four types of outputs.
These are among the major tasks of an intelligent sensor interface.
Keywords : Intelligent sensor interface,
Base unit, Plug-in module, Microcontroller.
1.
Introduction
The
need for sensors has grown rapidly in recent years for industrial control,
automation and consumer applications. In modern measurement and control system,
a great number of sensors collect the information about the process available
in the system being monitored. They usually provide analogue information on the
system being monitored through signal conditioning circuits connected to a
processor. The processor interprets the information, makes appropriate
decisions most likely in conjunction with higher-level control, and implements
those decisions via actuators [1].
In
1982, Ko and Fung introduced the term “intelligent transducer” [2]. An intelligent
or smart transducer is the integration of an analog or
digital sensor or actuator element, a processing unit, and a communication
interface. In case of a sensor, the smart transducer transforms the raw sensor
signal to a standardized digital representation, checks and calibrates the
signal, and transmits this digital signal to its users via a standardized
communication protocol [3]. In case of an actuator, the smart transducer
accepts standardized commands and transforms these into control signals for the
actuator. In many cases, the smart transducer is able to locally verify the
control action and provide a feedback at the transducer interface. With the
advent of modern microcontrollers it became possible to built low-cost smart
transducers by using commercial-off-the-shelf microcontrollers that provide a
standard communication interface, such as a UART. Thus, the usage of smart
transducers can become a cost decreasing factor for building embedded control
systems [4]. A Universal Transducer Interface (UTI), presented by Meijer, Frank and Goes in 1997, is a low-cost
high-performance interface between sensing elements and microcontrollers or DSPs. This interface, which is read out by a
microcontroller, services the following sensor elements: capacitors, platinum
resistors, thermistors, resistive bridges and
potentiometers. The circuit has been fabricated in a 0.7 mm
CMOS process [5]. The universal sensor interface chip (USIC) represents a
complete signal processing capability for data acquisition systems designed to
support a wide range of sensor applications. Offers high performance with
flexibility and requires only a small number of external components for many
applications [6]. A smart sensor module includes a microcontroller and signal
conditioning circuit [7]. The signal conditioning integrated circuit is mixed
signal application specific integrated circuit (ASIC) incorporating an
instrumentation amplifier, offset correction, sensitivity correction, low pas
filtering, an ADC, temperature sensor, digital I/O interface, crystal oscillator. The smart sensor module allows very
precious calibration of sensor, signal processing capabilities, decision-making
capability and serial interface capabilities [8]. In 2002, Xiujun Li; Frank, van de Goes; Meijer;
Rolf de Boer presented universal transducer interface allowing capacitive
sensors, RTDs, thermistors,
resistive bridges and potentiometers. Examples of smart sensor systems,
using the UTI and the popular microcontrollers, Intel 87C51FX and PIC16C73,
have been presented [9]. The UTI developed by Smartec
provides interfacing for five sensors [10].
The
universal sensor interface chips, smart sensor modules are available in the
market but they are very costly. The objective of this paper is to give a brief
overview on principles, communications, and configuration aspects for a
microcontroller based intelligent universal sensor interface.
Sensors work on variation of some electrical parameter
such as resistance, capacitance, voltage, current, etc. according to the
physical variable. The magnitude of change, ranges of operation and the
relationship differs from sensor to sensor [11]. This fact forces to have an
interface circuit, which accepts sensors on one side and giving a fixed range of
electrical output as a voltage or current on the other one. The commercially
available sensor interface cards allow connection of only 4-5 limited number of
sensors at input and provide only 2-3 types of outputs [10,12,13,14,15]. So there is a need to have a facility of
connection of more number of sensors at input, as shown in fig.1, and five different
types of outputs viz. (1) Digital parallel port (2) RS- 232C protocol (3) GPIB
(4) Analog 0-5 V and (5) 4-20 mA. In this system the micro-controller selects
the analog signal conditioning circuit, sets amplifier gain,
ranges, etc. The interface provides
the facility of connection of Thermocouples, Thermistors, RTDs, LVDTs, Strain gauges, LDRs,
Fiber Optic Displacement Sensor, and Capacitive Sensor on the front panel and
four different types of outputs are simultaneously available on the rear
panel to make the system universal one. The development and
implementation of universal sensor interface system is discussed in this paper.
2. A Universal Sensor Interface
The intelligent universal sensor
interface system, shown in Fig.1, allows connection of 8 different sensors to
the front panel. No external signal conditioning is required. All required
excitations and linearization are provided on board. On the front panel of this
system the user has to connect the sensor (e.g. RTD) and set the position of
DIP switches present on the module depending upon the type of sensor (e.g. Pt
100) and range of measurement (e.g. 0-100°C). The
back panel consists of different connectors for various forms of outputs. The
details of this interface are given in the following section.
2.1 Modular Designing
The signals produced by the sensors are required to be
conditioned by signal conditioner for user-friendly access. At the output of
signal conditioner, the analog data is generally acquired and converted to digital form for the
purposes of processing, transmission, display and storage [16]. Various
approaches of signal conditioning depend upon the sensor used. The complete system,
as shown in Fig.2, is divided into two parts a base unit and a sensor
specific signal-conditioning module. In this system every module consists of the signal
conditioning circuit and DIPswitches. The user must
set these switches before use to indicate the type of sensor and the range of
measurement. The same module is used for various types of sensors and range of
measurements. At a time only one module can be plugged in the base unit. For
eight sensors there are eight different modules.
The
base unit consists of a microcontroller, programmable gain amplifier (PGA),
ADC, V to I converter, RS 232C interface and a power supply. When the
signal-conditioning module is plugged into the base unit, the microcontroller
software identifies it and reads position of DIPswitches
and sets the gain of PGA accordingly. The output of PGA ,0-5V,
is converted into current 4-20 mA by V to I converter
and also into digital form by a 12bit ADC. The microcontroller software is
stored in the internal EEPROM and it continuously acquires the output of ADC
using parallel ports and then sends it to a PC for further processing The
output analog voltage, current, a raw 12 bit digital data, RS 232C serial port
are provided on the rear panel of the instrument, as shown in fig.4. The instrument
case has been designed as an economical packaging solution for bench-top
equipment.
Fig.2 |
Fig.3 |
2.2 Testing Procedure and observations
While testing the signal conditioning modules for
different sensors initially only one module was independently tested for only
one type and range of measurement by applying the simulated inputs. The zero
and span adjustments were carried out so as to produce the fixed range of 0-1V
output. Following section gives testing procedure for RTD sensor as an
illustration.
The resistance of Pt-100, for a=0.00385,
at 0°C
and 100°C
is 100 and 138.5 W
respectively [17]. The signal-conditioning circuit for Pt 100 was tested for 0-100°C
range. A fixed resistor of 100W was connected at the position of sensor input and
PT1, as shown in Fig.3, was adjusted to make output 0V. A fixed resistor of 100W
was then replaced by 138.5W,
adjusted using trim pot of 470W and PT2 was adjusted to make output 1.000 V. The
simulated resistance at the input was varied by using trim pot and
corresponding outputs were noted. For other types of RTDs
user has to select the resistor in the bridge circuit using the link. It was
observed that as input simulated resistance changes from 100 to 138.5W,
the output of the card varies from 0 to 1.000 V and this change is linear.
After plugging the module in the base unit the gain of
PGA was adjusted manually to provide a fixed range of output 0-5V. This output
was applied to V to I converter to provide a fixed range of current 4-20 mA. The same output was also applied to ADC whose digital
output changes from 000H to FFFH. For the same module the simulated inputs were
applied for all remaining types and ranges and every time the gain of PGA was
adjusted manually to provide same 0-5V range of output. This procedure was
repeated for all eight modules. The microcontroller software was then stored in
the internal EEPROM of microcontroller 89C51. After testing complete instrument,
when a particular module was plugged in the base unit it was observed that as
the simulated input changes over a selected measurement range, the instrument
provides all the outputs simultaneously. It was observed, as shown in Fig.5, that as the simulated input increases the output
increases from 0-5 V.
3. Results and Discussion
Intelligent sensor interface is a rapidly expanding area of research, which will inevitably bring extraordinary changes in the current medical , industrial, avionics, consumer products, space research, manufacturing, and many other real life problems. Many scientists and engineers who are confronted with real life sensor problems agree that solution will only be found when an intelligent universal sensor interface, with standard outputs, can be produced. It. The UTI chips are available in the market and are costly and include high performance front-end circuits for resistive and capacitive sensors only
Fig.4 |
Fig.5 |
For practical utility of any sensor it is important to
tailor its performance according to the need of the application. The present system
provides interface for commonly used 8 different sensors and four types of outputs,
which are useful in both the scientific research and industrial processes interacting
basically with control systems, process instrumentation, etc.
Present
paper particularly deals with the development of a versatile intelligent instrument
that reduces the burden of the system designer. The user has to only set the position
of DIP switches present on the sensor specific signal-conditioning module and
then plug the module in the base unit. The microcontroller controls the overall
operation of the instrument intelligently. Depending upon the need one can use
the required outputs, which are provided. If user wants to use the sensor, whose
interface is not provided in this system, then he has to only make a signal-conditioning
card for that sensor. The present system is slow in acquiring number of samples
per second as compared to commercial available interfaces but using faster ADC
and increasing the number of sensors connected to the system can improve performance.
The GPIB interface can also be provided.
4.
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