INTELLIGENT DIGITAL PRESSURE TRANSMITTERS FOR AEROSPACE APPLICATIONS

Donald W. Busse - Dr. Richard B. Wearn, Jr.

In the earlier days of system engineering, many systems consisted of a grouping or collection of existing sensors and actuators to perform a specific task to sense, anticipate or react to some control function.  With the advent of practical size computers, systems increased in size and complexity integrating more tasks, functions, and capability into one system, but most sensors changed very little except in size and accuracy.  The smart part of the system remained in the computer and sensors were required to do little more than sense and report, usually in some time honored analog format that had to be converted eventually to a digital format for use by the computer. 

More recently, the advent of the microchip has contributed to a profusion of “intelligent” devices and applications from automobile engine ignition fuel control systems and talking toys to sophisticated smart terminal and peripherals for large network computer systems.  Smart equipment and hardware for integrated systems is becoming more the standard rather than the exception.  The time has come for intelligence in peripheral system sensors of all types, particularly in aerospace systems, which frequently require the ultimate in accuracy, speed, and applicability. 

The Paroscientific, Inc. Digiquartzâ Intelligent Transmitter consists of a unique vibrating quartz crystal pressure transducer and a digital interface board in an integral package.  Commands and data requests are sent via an RS-232 or RS-485 channel and the transmitter returns data via the same two-way bus.  Digital outputs are provided directly in engineering units with typical accuracy of 0.01% over a wide temperature range.  Output pressure is fully thermally compensated using an internally mounted quartz crystal specifically designed to provide a temperature signal.  All transmitters are programmed with calibration coefficients for full plug-in interchangeability.  The Intelligent Transmitter can be operated either as a stand-alone standard output pressure sensor with display, or as a fully integrated addressable computer controlled system component. 

The pressure sensor device for the Intelligent Transmitter is the Paroscientific, Inc. Digiquartzâ pressure transducer.  These transducers use crystalline quartz as the key sensing elements for both pressure and temperature because of its inherent stability and precision characteristics.  The pressure sensing element is a quartz beam, which changes frequency under an axial load.  Applied pressure is converted to a measurable force by means of a miniature electroformed bellows.  This force is transmitted to the quartz crystal resonator by means of a pivotal lever arm as shown in Figure 1.  The transferred force acts on the quartz beam to give a controlled, repeatable, and stable change in the resonator’s natural frequency, which is measured as the transducer output.  The load dependent frequency characteristic of the quartz crystal beam can be characterized by a simple mathematical model to yield highly precise measurements of pressure and pressure related parameters.  The output is a square wave frequency, which can be readily interfaced with air data systems. 

The quartz crystal resonator operates in an ultrahigh vacuum in order to eliminate mass loading and damping effects and to ensure the excellent stability characteristics.  The bellows arrangement isolates the quartz element from the pressure media and eliminates density and humidity effects.  In the absolute pressure transducers, the vacuum in which the resonator operates also serves as the reference vacuum.  With  pressure, P,  applied to the bellows, and with the high reference vacuum internal to the housing, the bellows exerts a force proportional to the product of the pressure applied times the bellows effective area. 

Figure 1:  Absolute Pressure Transducer

Differential pressure measurements can be obtained by providing a second bellows on the opposite side of the pivotal lever arm as shown by the dashed lines in Figure 1. 

Each pressure transducer also contains a second quartz crystal, which is designed specifically for its temperature/frequency characteristics.  This second quartz crystal is mounted in close thermal proximity to the pressure sensing crystal.  The oscillating frequency of this temperature crystal varies in a known and measurable manner with respect to temperature.  Thus, the outputs of the two crystals provide information on both pressure and temperature.  Since both of the crystals are made out of crystalline quartz, the characteristics of one ( the temperature crystal) can be used to compensate for the inherent temperature characteristics of the other ( the pressure crystal), achieving optimum thermal compensation.

The Digiquartzâ Intelligent Transmitter consists of a standard Digiquartzâ pressure transducer in any pressure range from 15 psia to 40,000 psia and a digital interface board in an integral package.  The digital board has a microprocessor-controlled counter and RS232 port.  The microprocessor operating program is stored in permanent memory (EPROM) and user controllable parameters are stored in user writable memory (EEPROM).  The user interacts with the transmitter via the two-way RS232 interface.  These transmitters output fully temperature corrected pressure information on a two way addressable RS-232 bus that can be interfaced to a computer or stand-alone readout display. 

The digital interface board contains a precision clock, microprocessor for computation, logic circuitry for input-output control and EPROM and EEPROM memory for storing the operating program and calibration coefficients.  Figure 3 shows a block diagram of the interface board circuitry and processing method. 

The RS232 interface allows complete remote configuration and control of all operating parameters of the Intelligent Transmitter, including resolution, sample rate, integration time and baud rate.  Resolution is programmable from 0.05 to 100 parts per million depending on system requirements. Baud rate is user selectable from 300 to 19,200.  Pressure data is available in eight different selectable standard engineering units.  Up to 15 data readings per second can be obtained with normal sampling commands.  More than 100 samples per second can be obtained with special burst sampling commands. 

Transmitters are available in full-scale absolute pressure ranges from 15 psia to 40,000 psia; full-scale gauge pressure ranges from 15 to 200 psig and full scale differential ranges of 3, 6, and 18 psid. 

The Intelligent Transmitters can be operated as a single standard output device or as an addressable chain of up to 98 transmitters on one RS232 port.  Each transmitter can be addressed individually or globally.  High baud rates allow multiple transmitter operation on one RS232 port without data backup or scrambling.  Power supply requirements are 5 to 25 volts dc with current drain from 10 to 24 milliamperes depending upon the chosen sample rates.

The Digiquartz® Intelligent Transmitter can operate on any standard RS232 bus such as a PC port.  Up to 98 individual Intelligent transmitters can be chained off one computer 2 way port to form a system of mixed pressure measurement functions and ranges as might be required by a fully integrated Aerospace system which could monitor or control airspeed, altitude, fuel pressure, engine pressure ratio, hydraulic pressure, etc.

Each Intelligent Transmitter can be addressed individually or collectively for separate or simultaneous pressure measurements.  Since each transmitter is individually addressable its operation can be controlled separately to change pressure units, resolution, or offset as required by the system computer. 

Simultaneous pressure measurements can be made by a group of transmitters for later individual or sequential readout.  This is frequently useful for strobing pressure measurements on a system such as a wind tunnel during rapidly changing or fluctuating conditions. 

If a single stand-alone pressure measurement is required for, say, data recording or visual observation, the transmitter can be preprogrammed to output pressure information at a given resolution in chosen engineering units. It can then be operated as an independent sensor feeding information to a printer, processor, or a visual display such as the Model 710 Display shown in Figure 4. 

Figure 4:  Intelligent Transmitter With Model 710 Visual Display

The Model 710 is a six digit LCD display in a standard DIN size panel mount enclosure.  It displays pressure data from any Digiquartzâ Intelligent Transmitter.  The display operates either as a stand-alone device with a transmitter or with a computer on line.  For stand-alone operation, the display shows pressure whenever power is applied to the transmitter. 

When a computer is on-line, the Model 710 can also be used to display other data calculated in the computer (filtered data values, time, temperature, error messages, etc.)

Command format and a list of available commands are available in the Intelligent Instruments Operations and Programming Manual.                           

  CHARACTERISTICS AND PERFORMANCE

Because the Digiquartzâ pressure transducer uses a quartz resonator to provide high constraint to the pressure generator element (bellows or Bourdon tube) there is very little mechanical motion under load (100 microinches or less).  This results in superior repeatability and minimal hysteresis. 

The crystals themselves produce the kind of stability characteristic of any quality quartz resonator.  In addition, the lack of extraneous noise on the output signals due to the exceptionally high Q offers, unparalleled resolution.  The use of a frequency output quartz temperature sensor for temperature compensation yields an achievable accuracy of 0.01% full scale over the entire operating temperature range.

 Thus the Intelligent Transmitter is capable of primary standard performance even under difficult system or field conditions.  The standard digital output in  number of selectable engineering units on a high speed addressable data bus allows plug-in compatibility with most computer based systems.  The transmitters are field interchangeable without recalibration or reprogramming.

For performance characteristics and models of the Digiquartz® Intelligent Transmitters, check our the transmitters section of this web site.

The Digiquartzâ Intelligent Transmitters present a powerful solution to many pressure instrumentation needs.  They combine the precise performance of the Paroscientific transducers with the speed, versatility and convenience of a dedicated internal microprocessor.  The RS-232 interface allows complete remote configuration and control of all operating parameters, including resolution, sample rate, integration time, and baud rate.   The Intelligent Transmitter provides unparalleled accuracy across a wide temperature range in a package that ensures operation under adverse conditions of vibration and shock. 

As a stand alone sensor or as a string of system sensors the transmitters offer versatility in operation and diversity in measurement, whether absolute or gauge, high or low pressure.  The transmitters lend themselves well to system applications for both monitoring or control allowing various pressure ranges to be interfaced on one system port.  An aircraft system might require two pressure ranges for static and dynamic pressure and additional ranges for fuel pressure, engine pressure ratio, etc.  All could be accommodated on one data link.  Calibration and Metrology laboratories or test facilities will find the transmitter useful, not only as a transfer standard but as an intelligent monitor of other equipment.  For instance the unprecedented resolution of the transmitter enables it to make measurements of better than 1 ppm on primary pressure standards allowing personnel to study weight wobble or settling times during actual operation. 

The engineers and scientists who are concerned with the ultimate in speed, precision, and adaptability will undoubtedly conceive of new and intriguing uses for these pressure transducers and transmitters.  The versatility and broad applicability of these sensors should provide plug-in solutions to a wide variety of pressure measurement problems. 

©2007 Paroscientific, Inc.