FIBER-OPTIC PRESSURE SENSORS WITH 0.01% ACCURACY

Jerome M. Paros
President
Paroscientific, Inc.
4500 148th Ave. N.E.
Redmond, WA 98052

Fiber-Optics, Pressure Instrumentation, Pressure Sensors, Smart Sensors & Transmitters

Pressure transducers with 0.0001% resolution and 0.01% accuracy have been developed which combine the high performance of inherently digital resonant sensors with the optical isolation, noise immunity, intrinsic safety, and long line driving capability of fiber-optic technology.

The widespread use and continuing trend toward digital information and control systems, together with the need for more accurate pressure instrumentation, prompted the development of digital pressure transducers capable of precision measurements in pressure ranges from fractions of an atmosphere to thousands of atmospheres.

High precision digital pressure sensors were developed by Paroscientific, Inc. over the last several decades.  These sensors operate on the principle of changing the resonant frequency of load-sensitive quartz crystals with pressure-induced stress.  Quartz crystal temperature sensors are incorporated as an integral part of the pressure transducers for purposes of thermal compensation.  Frequency signals from the quartz crystals are counted and linearized through microprocessor-based electronics to provide two-way communications and control in a number of digital formats.1,2 

The aforementioned quartz crystal pressure transducers have a resolution of a few parts per billion and have been used to determine the performance of high precision, primary standard dead weight testers.3 

There are some limitations to the use of these electrically powered/electrical output sensors in high noise environments and hazardous areas.  Thus the development effort focused on combining the precision of the quartz resonator sensors with the total optical isolation of  fiber-optic technology.

Proprietary force-sensing and temperature sensing elements have been fabricated from quartz crystals .  These elements, whose change in resonant frequency is related to the measurand, have remarkable repeatability, low hysteresis, low power consumption, insensitivity to environmental errors, and excellent long term stability.1,2

FIGURE 1:  SINGLE TINE FORCE-SENSITIVE QUARTZ RESONATOR

A single tine force-sensitive quartz resonator is shown in Figure 1.  The resonant frequency of the central vibrating beam is a function of the input force applied between the two mounting surfaces.  Isolating springs and overhanging isolator masses are integrally machined from a single blank of quartz and this isolation system acts as a low-pass mechanical filter to reduce energy losses to the mounting pads.  Thus the reactive forces and moments generated by the vibrating beam are balanced, resulting in a high Q (low energy loss) resonance.  The beam may be driven piezoelectrically at its resonant frequency by applying voltage to deposited electrodes through a self-tuning oscillator circuit.1

A dual tine force-sensitive resonator is shown in Figure 2.  Conceptually, it consists of two identical tuning forks joined at the middle.  Very little energy is lost through the mounting pads since the two tines vibrate 180 degrees out of phase to cancel the reactive forces and moments.  The resonant frequency of vibration is a function of the tine dimensions, composition, and the tension or compressional loads to be measured.  The double-ended tuning fork resonators may be fabricated using photolithographic, chemical milling, and drive-electrode deposition techniques developed by the watch crystal industry.2

FIGURE 3:  TEMPERATURE-SENSITIVE QUARTZ RESONATOR

The force-sensitive quartz resonators are fabricated with crystallographic orientations and dimensions that optimize their stress-sensing characteristics while minimizing their sensitivity to temperature-induced errors.  The small residual thermal sensitivity of the pressure sensing crystal and mechanism is compensated using a quartz resonator whose frequency output is a function of temperature alone.  A separate oscillator is used to drive and detect the resonant frequency of the dual torsionally vibrating tines of the temperature sensor shown in Figure 3.  Calibration consists of measuring the frequency outputs of the pressure and temperature sensing crystals and deriving coefficients used in a characteristic equation to provide fully thermally compensated, linearized outputs.2

FIGURE 4:  PRESSURE TRANSDUCER MECHANISMS

Bellows or Bourdon tubes are used to convert applied pressures to forces on the quartz resonators as illustrated in Figure 4.  Thus pressure acts on the effective area of the bellows to generate a force and torque about the pivot and compressively stress the resonator.  The change in frequency of the quartz crystal oscillator is a measure of the applied pressure.  Similarly, pressure applied to the Bourdon tube generates an uncoiling force which applies tension to the quartz crystal to increase its resonant frequency.  Temperature sensitive crystals are used for thermal compensation.  The mechanisms are acceleration compensated with balance weights to reduce the effects of shock and vibration.  The transducers are hermetically sealed and evacuated to eliminate air damping and maximize the Q of the resonators.  The internal vacuum also serves an excellent reference for the absolute pressure transducer configurations.  Scaling of the bellows and Bourdon tube mechanisms allows the design and production of numerous full scale ranges from 0.1 MPa (15 psi) to 276 MPa (40,000 psi).  Microprocessor-based electronics include:  counter-timer circuitry to measure transducer frequency or period outputs, storage of the linearization and thermal compensation algorithm, calibration coefficients, and command/control software to process the outputs in a variety of digital formats.2

Figure 5. Fiber -Optic Block Diagram

The resonant sensors may be located in a hazardous area or an environment with high levels of electromagnetic interference.  The requirement of optical isolation means that only light power and light signals can cross the local interface between the safe and hazardous areas.  It is also desirable to minimize the optical power levels required for excitation and transmission for safety reasons and long distance propagation.  Thus optical power is supplied by a laser diode or LED in the safe zone via an optical fiber to the remote resonant sensors in the hazardous or electrically noisy area.  The delivered optical power is used to excite the quartz crystal pressure and temperature sensors into oscillation and their resonant frequencies are detected by light modulation or motion detection techniques.  Periodic optical pulses representative of the crystal resonant frequencies are then transmitted via optical fiber to a receiver/demodulator in the safe area.  The light power for excitation and the returning pressure and temperature light pulses can all be transmitted on a single 100 micron glass fiber for over a kilometer.  The block diagram for receiving, demodulating, and processing the signals is shown in Figure 6.

The optical pressure and temperature pulses are received, demodulated, and converted into electrical signals whose periods are measured by gating a high frequency clock and performing a period average.  The microprocessor performs the linearization and temperature compensation tasks using equations stored in EPROM.  The two way RS-232  bus allows the user to select resolution, update time, engineering units, baud rate, pressure adders, pressure multipliers, and a variety of commands such as single readings, continuous updates, etc.  Up to 98 transmitters can be connected in a single serial loop.

Figure 7. Total Static Error Band

Figure 7 shows the total static error band for a 0.1 MPa (15 psia) fiber-optic pressure transducer compared to a primary dead-weight standard.  The error band, including hysteresis, non-repeatability, and non-conformance, is less than 0.003% Full Scale.  Data are taken at 15 points spanning the F.S. range of the transducer.  Repeat points are also shown at the midpoint and starting point.  Comparable performance has been achieved for F.S. pressure ranges up to 276 MPa (40,000 psi).5

High accuracy fiber-optic pressure transducers have been developed by applying optical technology to resonator-based sensors.  Performance is comparable to the primary pressure standards, even in hazardous locations, with high levels of electromagnetic interference and under other difficult environmental conditions.  These transducers are available in a broad range of pressures and can be used in such diverse fields as process control, aerospace, oceanography, meteorology, hydrology, energy exploration, and laboratory instrumentation.

[1.]       Paros, Jerome M., "Precision Digital Pressure Transducers", ISA Transactions, Vol. 12, No. 2, pp. 173-179, 1973.

[2.]       Busse, D.W., "Quartz Transducers for Precision Under Pressure", Mechanical Engineering, Vol. 109, No. 5, pp. 52-56, May, 1987.

[3.]       Wearn, R.B., Paros, J.M., "Measurements of Dead Weight Tester Performance Using High Resolution Quartz Crystal Pressure Transducers", Presented at:  ISA Aerospace Industries and Test Measurements Divisions 34th International Instrumentation Symposium, Albuquerque, N.M., May 2-5,  1988.

[4.]       Paros, J.M., "Fiber-Optic Resonator Pressure Transducers", Measurements & Control, Issue 154, pp. 144-148, Sept., 1992.

©2007 Paroscientific, Inc.