Long-Life Fiber-Optic Pressure Sensors for Harsh Environment Applications

Wlodarczyk Marek T., Poorman Tom, Arnold Jacob, Coleman Terry

Optrand, Inc.

Plymouth, Michigan 48170, USA

ABSTRACT: Optrand has developed a family of fiber-optic pressure sensors targeting monitoring and control applications that are characterized by long service lifetime, large number of pressure cycles, temperatures in excess of 150^oC, high levels of EMI, or the presence of explosive or corrosive media. In a robust and low cost design Optrand sensors utilize the principle of light intensity changes, transmitted by two optical fibers, upon reflection from a specially shaped, metal diaphragm deflecting under the effect of pressure. The non-contact detection principle combined with the diaphragm design optimized for infinite fatigue life results in sensor’s extraordinary lifetime. The sensor’s signal conditioner, permanently attached to the sensing fibers, contains one LED source and one Silicon PIN photodiode detector. Optrand patented auto-referencing function compensates for the effects of fiber bending, fiber-to-optoelectronics coupling changes, sensor thermal drift, as well as temperature and aging effects of LEDs and photodiodes. By varying the thickness of a specially shaped high-strength Inconel diaphragm, the maximum stresses are limited to the levels guarantying infinite theoretical fatigue-life. Optrand offers sensors as small as 1.7mm in diameter for pressures ranging from 0-7 bar to 0-2,000 bar. Both dynamic and static sensors are available covering frequency ranges of 0.01 Hz to 30kHz and 0 Hz to 15kHz, respectively. Under constant temperature Optrand sensors can offer linearity and hysteresis as low as 0.05% with a typical value of ±0.25%. For combustion pressure detection, when mounted in an engine head, Optrand sensors have demonstrated combined linearity, hysteresis, and thermal shock of as low as ±0.5%, matching the performance of research-grade water-cooled piezoelectric devices. Due to their miniature sizes and high temperature and EMI resistance Optrand sensors can be uniquely fitted into pressure sensing spark plugs, glow plugs, or fuel injectors. So far the sensors have demonstrated over 12000 hours and 500-Million pressure-cycle lifetime in stationary natural gas-burning engines. Some key non-combustion applications include pressure monitoring in natural gas pipeline compressors, plastic melt pressures, and pressure control in a pharmaceutical plant. So far 750 Million and 2.45 billion pressure cycles have been demonstrated in those applications, respectively.

INTRODUCTION: Fiber-optic sensors offer unique benefits in harsh environment applications characterized by high temperatures (over 150^oC), high levels of electromagnetic interference (EMI), chemical attack, or in the presence of explosive materials [1]. They can be of very small size, sensitive, and located hundreds of meters from a measurement site. An example of an extreme environment application where an optical fiber sensor may be the only solution for a reliable and durable device is pressure measurement of a combustion engine cylinder. In-cylinder mounted pressure sensors may be subjected to instantaneous gas temperatures of 1,500^oC and continuous temperatures up to 300^oC, and pressures up to 300bar. For monitoring and control applications the sensor has to function reliably over up to 10 years or hundreds of millions pressure cycles. For automotive applications the sensor has to meet stringent cost targets as low as $10.

To date fiber optic pressure sensors (FOPS) have not fulfilled their promise for widespread use due to a combination of technical, practical, and cost limitations. From the technical standpoint, the majority of reported FOPS are typically convoluted in the signal conditioner area due to the use of bulky, fragile, and dissimilar optical components and materials such as fused fiber optic couplers, electro-optic modulators, and component receptacles [2]-[5]. Many fiber optic sensors, such as those based on the interferometric principle, use laser diodes, single mode fibers, waveguide modulators, or ferrite isolators. Typically the sensing fiber connects to its signal conditioner via an optical connector leading to high cost and performance limitations associated with fiber optic connectors. Sensor designs that do not use optical connectors are either limited in their temperature ratings to 70^oC, due to the use of laser diodes or other photonic devices, or are bulky and not robust due to the use of discrete opto-electronic components. From the cost standpoint, fiber-optic sensors suffer from high component and manufacturing costs, typically one to two orders of magnitude more than their electronic counterparts. While some of these cost disadvantages are related to lower production volumes, it is primarily the construction, materials, components, and fabrication techniques of present-day FOPS that make these devices very expensive.

In a long service life, robust and low cost design Optrand sensors alleviate the limitations of other fiber optic pressure sensors. This paper describes the sensor operating principle, design, packaging and mounting issues, and performance in a number of monitoring and control applications including natural gas engines and compressors, gasoline and diesel engines and fuel systems, and plastics molding and injection equipment.

SENSOR DESCRIPTION: As shown in Fig. 1, the fiber optic sensor developed by Optrand consists of three basic components: a sensing head with a metal diaphragm directly exposed to pressure, a cable containing two multimode fibers, and an opto-electronic signal conditioner containing all optical and electronic components. To avoid the “real-world” problems

associated with optical connectors the signal conditioner (“smart” connector) is permanently attached to the sensor cable. Fig. 2 shows a photograph of a typical sensor.

The sensor response to pressure results from the displacement of a diaphragm that in turn changes optical signal transmitted from the sending to the receiving fiber upon reflection from the diaphragm. Over a large displacement range light intensity collected by the receiving fiber may either decrease or increase with increasing diaphragm to fiber distance, as shown in Fig.3. In a pressure sensor, the diaphragm displacement is quite small, typically around 20 microns. For a given diaphragm diameter, shape, and thickness, and therefore full scale deflection at maximum pressure, the sensor response (e.g. light modulation, nominal signal level, and linearity) can be significantly changed by changing the fiber core and cladding sizes, Numerical Aperture, separation between fibers, and fiber to diaphragm distance at zero pressure [6]. Fig. 4 demonstrates an experimental sensor response at constant temperature compared to a research grade reference transducer. Note excellent response linearity exceeding typical values of many laboratory devices.

The diaphragm is one of the most critical sensor elements. In a low-cost and durable design, Optrand sensor’s diaphragm is made of a nickel-based alloy and is shaped during the coining process. The diaphragm is laser welded to a metal housing containing a fiber holding ferrule and two fibers bonded inside the ferrule, as schematically shown in Fig. 5. Please note that the dimensions shown in Fig. 5 are not to scale.

Fig. 5. Sensor head design

At present the diaphragm diameters range from 1.7mm to 8 mm covering the pressure range from 7 bar (100psi) to 2000 bar (30,000psi). Small diaphragm diameters create a significant design challenge due to the simultaneous requirement of large deflection (for high signal to noise ratio) and low stresses required for infinite lifetime. In some natural gas compressor applications the sensor has to function reliably over billions of pressure cycles. The diaphragm reflectivity must also remain nearly unchanged over the sensor lifetime. To ensure durable operation, the present sensor uses an Optrand patented sculptured, hat-shape diaphragm with varying thickness across its diameter [6]. A high strength alloy (Inconel) has been used as a diaphragm material. This material and design have been selected so that the peak stresses of the diaphragm are below the level guaranteeing an infinite lifetime. Other benefits of the present construction include excellent linearity of the pressure response and reduced sensitivity to direct flame and hot combustion gas effects.

The opto-electronic conditioner contains low-cost and reliable PIN Si photodiode and near infrared LED and a small in size all analog electronic circuitry. Two input electrical leads are for power supply and ground while one output pin is for pressure output and the other for sensor fault diagnostics. The electronic circuitry controls light intensity, amplifies and filters photodiode signal, and provides the auto referencing function. This Optrand patented technique regulates LED light intensity in response to any undesirable environmental conditions that may alter minimum detected light intensity [6]. Baseline light intensity in fiber optic sensors may vary due to optical link transmission fluctuations resulting from connector mechanical and thermal instabilities, fiber bending, light source or detector temperature dependence, or aging over time. The auto-referencing approach not only corrects for offset drift but sensor gain error as well. A side benefit of the technique, not possible with other combustion pressure sensors, is the availability of sensor health monitoring output. By continuously monitoring the LED current level or its rate of change, one can identify potential sensor failure before it occurs. This ability is particularly important in control applications where sensor failure may cause malfunction or even failure of the controlled device.

SENSOR PACKAGES: Covering a wide range of applications, Optrand offers sensor packages that can accommodate different sizes and shapes, mounting techniques and locations. In particular, due to their miniature sizes and high temperature and EMI resistance, Optrand sensors can be uniquely fitted into pressure sensing spark plugs, glow plugs, or fuel injectors. Such multifunctional devices allow non-invasive combustion cylinder pressure detection without need for engine head drilling or modifications. For spark ignited engines Optrand offers sensors mounted in a modified production spark plug, the PSIplug, as shown in Fig. 6. For reciprocating engines and compressors two types of mountings are typically used in the industry, one in which the sensor is installed inside the cylinder head and the other when an indicator valve is employed. Mounting of a dynamic pressure sensor in the cylinder head is preferred resulting in high-pressure data fidelity (lack of channel resonances), accuracy (reduced thermal transients due to head water-cooling), and sensor durability (reduced sensor temperature). For such application Optrand offers pencil shaped packages as small as 5mm in diameter and as long as 30 centimeters. For applications where EMI/RFI is not a concern, the signal conditioner may be attached to the end of a pencil-like shaft, as shown in Fig. 7. For safety, the sensor signal conditioner may be mounted inside an explosion proof enclosure. A package like this is already in use to monitor and control large reciprocating compressors.

Another important parameter that is monitored in various types of engines is fuel pressure. For this application Optrand offers stand-alone sensors as well sensors integrated with fuel injectors [7]. Such a “smart” injector, called PSIjet^TM, can be fitted in addition with a combustion cylinder pressure sensor, for optimum performance, reliability, and low-cost. The resulting device does not need to be individually balanced, as currently done, so its price can be significantly lower. Differences caused by manufacturing variability, aging, pressure line fluctuations, or fuel quality can be compensated for by using a closed-loop control of fuel timing, duration, and pressure.

SENSOR SPECIFICATIONS AND PERFORMANCE: The basic specifications of the pressure sensors currently offered by Optrand for control and monitoring applications of industrial engines and machinery are summarized below:

Pressure range:	0-7 bar (100psi) through 0-2,000 bar (30,000psi)
Overpressure	x2, x1.5 of pressure range
Continuous sensor temperature range:	-40 to 300^oC (570^oF)
Intermittent sensor temperature range:	Combustion
Cable temperature	-40 to 200^oC (390^oF)
Signal conditioner temperature:	-40 to 65^oC (150^oF); model AutoPSI–S –40 to 125^oC (260^oF); model AutoPSI–HT
Frequency response:	0.01 Hz to 30 kHz (models AutoPSI–S, AutoPSI-TC) 0 Hz to 5kHz (model AutoPSI–DC)
Linearity & hysteresis – non-combustion	+/-0.25 to +/-0.5% Full Scale Output
Linearity & hysteresis – combustion	+/-1 to +/-2% Full Scale Output
Temperature coefficient of sensitivity:	+0.03%/^oC (model AutoPSI–S) +/- 0.005%/^oC (model AutoPSI–TC)
Signal to Noise Ratio	2000:1 @ 15 kHz
Sensor output:	0.5-5 V
Sensor diagnostics output	0 – 3.6V
Guaranteed service life time	500 Million cycles, 3 years (indicator valve mounted) 1 billion cycles, 5 years (head mounted)

Below we present the series of sensor performance data obtained in various monitoring and control applications including passenger cars, natural gas engines and compressors, and marine diesel engines.

Fig. 8 demonstrates the comparison data obtained with a miniature Optrand sensor in a single cylinder diesel engine (Yanmar). A water-cooled, head-mounted research-grade piezoelectric transducer (Kistler 6061) was used as a reference.

Fig 8. Single cylinder diesel engine test data comparison between AutoPSI-S sensor and piezoelectric reference transducer

The data presented in Fig. 8 were obtained with a sensor designed for nominal 3000-psi (200 bar) pressure range. The left side vertical axis is for both optical and reference transducers while the right side axis shows the difference between the sensor readings. The measurement and reference traces are normalized so their peak-to-peak values are equalized. Compared to the full-scale output of approximately 650psi, +/-0.23% accuracy was recorded, including linearity, hysteresis, repeatability, and thermal shock.

The data obtained on a large-bore natural gas engine is shown in Fig.9 comparing the performance of Optrand sensor against an air-cooled strain gauge transducer (Enspec). Both sensors were mounted in a Kiene valve, external to the engine head. Fig. 4. Note excellent linearity, hysteresis, and thermal shock performance of +/-1% of Optrand uncooled sensor.

Fig. 9. Engine test comparison between Optrand and reference sensors on a natural gas engine

Fig. 10 shows the data obtained on a large, high-speed (1,500-RPM) compressor. The sensor package used in this application is shown in Fig. 7. The sensor was mounted inside an existing indicator channel with the diaphragm flush mounted with the compressor cylinder wall. Please note an excellent fidelity of pressure data free of periodic perturbations associated with sensors mounted at the end of an indication channel.

Fig. 10. Pencil-like sensor performance in a compressor

Finally, Fig. 11 demonstrates a comparison between Optrand and Kistler sensors performance in a diesel engine of a naval ship. Both sensors were installed in a Kiene valve of the engine indicator port.

Fig. 11 Optrand vs. Kistler sensor (Model 7613) comparison on a naval ship engine

During the last 12 months several hundred sensors have been subjected to endurance and calibration stability tests in natural gas and diesel engines as well as gas compressors and fuel injection pumps. With the exception of some sensors that were damaged by incorrect handling and a few defective sensors, all the sensors have demonstrated durability exceeding 500 Million pressure cycles or 12,000 hours. In compressor and fuel injection endurance tests the sensors have already demonstrated over 1 Billion pressure cycles service lifetime and target 3-5 Billion cycles.

In addition to the endurance tests, during the last year tens of sensors have been subjected to long-term calibration stability tests. Periodically, every few to several months, Optrand sensors were re-calibrated using air or water-cooled reference transducers (strain gauges or piezoelectric sensors). During a 6 to 12 month period the sensors demonstrated excellent calibration stability (compared to the initial values), ranging from a non-detectable to +/-0.1% change in the sensor sensitivity value.

SUMMARY AND CONCLUSIONS: In a robust, durable, and low-cost design Optrand fiber optic pressure sensors operate on the principle of light reflection from a metal diaphragm flexing under the effect of pressure. When optimized for high linearity, optical signal level, and modulation, the sensor demonstrates accuracy comparable to that of a laboratory-grade piezoelectric transducer. For combustion engine applications the sensor as small as 1.7 mm in diameter can be either directly inserted into an engine head or integrated with a spark plug, glow plug, or fuel injector. At constant temperature the sensor accuracy is typically +/- 0.25%; under combustion conditions the combined sensor’s hysteresis, non-linearity, and thermal shock effects result in pressure reading accuracy of +/-1% to +/-2% full-scale output. Currently Optrand offers four types of sensors: AutoPSI-S, AutoPSI-TC, AutoPSI-HT, and AutoPSI-DC. The originally developed “-S” sensor provides the most economical solution for dynamic pressure measurement. The AutoPSI-TC sensor offers temperature-compensated operation matching the performance of water-cooled piezoelectric sensors without water or air-cooling and at fraction of cost. The AutoPSI-HT sensor comes with a signal conditioner rated for –40°C to 125°C. Finally, the AutoPSI-DC sensor offers the capability of continuous static pressure detection at 300°C. Currently the AutoPSI-S and AutoPSI-HT sensors are guaranteed for 500-Million pressure cycles or three years under combustion engine conditions and for indicator valve mounting. For head-mounted combustion applications the warranty is extended to unprecedented 1 Billion pressure cycles. In compressor or fuel injection applications, currently the sensor service lifetime is guaranteed for 1 Billion cycles and it is expected shortly to increase to 3 billions. To date, hundreds of Kiene valve mounted sensors have demonstrated the lifetime of at least 12,000 hours and over 500 Million pressure cycles. The sensors have also demonstrated an excellent calibration stability, better that 0.1% over a 6 month period, respectively.

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