Electrochemical force sensor

Abstract: A gas or fluid pressure or force sensor is disclosed consisting of a fluid-tight housing containing a supply of electrochemically active gas and an electrolytic membrane disposed to divide the housing into two chambers (12,14). The electrolytic membrane (16) has one or more pervious electrodes disposed on each side of and in contact with the membrane. A flexible bellows or diaphragm (22) is disposed to form one wall of the fluid-tight housing and provide the contacting surface for transferring a force or pressure to the sensor. A controller is connected to display measurements and to permit automatic sensor calibration, differential pressure-induced diffusion compensation, temperature compensation and sensor chamber pressure control. The disclosed force sensor is useful for application to robotic manipulators and the basic device is useful for measurement of the gas partial-pressure of a selected gas.

Description: This invention involves generally devices for sensing and effecting force and pressure and more particularly involves a pressure transducer that reversibly converts mechanical force or pressure into an electrical voltage by means of an electrochemical process. Several methods for measuring force and pressure are known in the art. Except for gravity column measurements, such methods generally use a pressure transducer to convert applied pressure to an electrical voltage. Pressure transducers generally rely on an intermediate physical displacement induced by the applied pressure to generate an electrical voltage. The transduction principles or means of transforming a change in physical displacement into a change in electrical voltage or current circuit are as varied as the many physical relationships known to modern science. However, the transduction techniques in general use are limited to a relative few. These are essentially those which involve variations in resistance, inductance, and reluctance. Useful data are obtained by supplying such transducers with an input voltage and developing an output voltage as a function of the changes in displacement produced as an intermediate result of the application of the pressure or force to be measured. Of the remaining transduction principles, the piezoelectric and vibrating-wire types are most commonly used. The piezoelectric transducer uses a crystalline material that produces an output voltage proportional to the rate of change in strain over time. Because the output voltage is generated only under dynamic conditions, the piezoelectric device cannot be used for static or steady-state pressure or force measurements. It is normally used as a miniature instrument intended to measure vibration or dynamic pressure. The vibrating-wire transducer comprises a fine wire supported so that its tension and strain varies according to the applied pressure or force. When this wire lies between the poles of a permanent magnet, it vibrates at its natural frequency upon application of an oscillating voltage. The vibrating-wire transducer output is an oscillating voltage that changes in frequency according to changes in the wire tension induced by the applied pressure or force. Each of these transducer principles requires a displacement to generate a voltage output from the sensor. Pressure measurements are achieved by summing applied pressure over a flexible material that in turn is permitted limited travel. The flexible member may be a diaphragm, capsule, bellows or several variations of bourdon tubes. Each of these provides a limited displacement under applied pressure. Absolute pressures are generally determined by sealing either the capsule or bellows assembly, or the instrument chamber, at a reference pressure usually close to zero. Gauge pressure (relative to ambient atmospheric pressure) is generally measured by connecting the reference side of the instrument to the ambient condition. Differential pressure can be measured by permitting the two pressures in question access to opposite sides of the sensing member. Each of the above-mentioned transduction techniques is subject to several limitations. Except for the piezoelectric transducer, all such methods require a separate voltage source. The piezoelectric transducer requires no separate voltage source, but it is not useful for static pressure measurement. Again except for the piezoelectric transducer, none of the above transduction techniques are amenable to self-calibration. Although a piezoelectric transducer can be self-calibrated by means of the reciprocity principle, this method is useful only for dynamic (sinusoidally-varying) pressures. A simple, self-calibrating pressure sensor that can be calibrated to accurately sense the absolute value of static pressure without reference to a calibration standard is unknown in the art. In addition to the simple measurement of force or pressure, two other related sensing requirements exist in the art. First, there is often a requirement for measurement of the partial-pressure of a gas in a mixture of gases. For instance, the direct sensing of the partial-pressure of oxygen (O 2 ) is required in the arts of medicine, biology, automotive engineering, chemistry, petroleum engineering, and others. Secondly, in the emerging robotics arts there is a well-known requirement for a transducer that combines the capacity to sense force or pressure with the capacity to effect a desired force or pressure. Such a device may be considered a force or pressure sensor/effector and is useful for robotic manipulators. The above-mentioned force transduction techniques cannot meet either of these important requirements because the transduction phenomenon is either nonreversible or (in the case of the piezoelectric transducer) the transduction phenomenon does not exist under static conditions. Another method for measuring the partial pressure of oxygen was demonstrated by R. Richter at Jet Propulsion Laboratory in 1984, as reported in Richter, R., "Measuring Absolute Oxygen Pressure, "NASA Tech Brief 8 , No. 3, Item #97. Richter did his work with a high temperature (>1000°c) zirconia-electrolyte fuel cell, using it to measure the partial-pressure of oxygen. Richter's method is not useful for measuring the partial-pressure of oxygen. Richter's method is not useful for measuring the partial-pressure of other electrochemically active gases (eg. H 2 , Cl 2 , et al) at ambient temperature. An electrochemical cell is typically formed by positioning an electrolytic membrane between and in contact with a cathode and an anode. Although such a cell can either generate electricity (as a fuel cell) or do mechanical work (as a motor), it has not heretofore been used as a transducer for the sensing or effecting of force and pressure. When the cell is configured as a fuel cell to generate electricity, a fuel gas such as hydrogen is supplied to the anode and a gaseous oxidant such as oxygen is supplied to the cathode, as disclosed by Wentworth in U.S. Patent 3,418,168. When the cell is configured as a motor to produce mechanical energy, an electrical voltage is applied across the anode and cathode and an electrochemically active gas capable of entering into an oxidation/reduction reaction is supplied to the anode, as disclosed by Maget in U.S. Patent 4,402, 817. Neither of these patents teaches or suggests the use of an electrochemical cell as a force or pressure sensor. Such an application is briefly disclosed by the inventor in copending patent application 07,563,050 entitled "Efficient Electrochemical Motor" filed concurrently herewith. UK Patent Application No. 2051374 relates to a pressure transducer suitable for measuring pressures at high temperatures and comprises a solid non-porous ionically-conductive member separating a first material, which is subjected to the pressure to be measured, from a second material which is a chemical element in fluid form whose ions can pass through the conductive member. The electric potential developed across the member is a measure of the pressure applied. European Patent Application No. 0209 644 relates to a drug dispenser driven by an electrochemical cell. An electrochemical pump is coupled to a bladder which can be actuated to dispense a drug at an accurate rate. According to the invention there is provided a single partial pressure gas sensor comprising: a gas-tight housing, having two ends and a first wall at a first end; a rigidly-supported electrolytic membrane having two sides disposed to form a second wall at a second end of said gas-tight housing and defining therein a storage chamber for holding a supply of electrochemically active gas; at least one pervious electrode disposed on each said side of and in contact with said electrolytic membrane; said at least one electrode on the first said side of said electrolytic membrane being separated from one another by spaces and said at least one electrode on the second side of said electrolytic membrane being disposed to form, with said at least one electrode on said first side, at least one electrode pair separated by said electrolytic membrane; said gas being electrochemically reversibly active so as to enter into an anodic reaction at one side of said at least one electrode pair where the gas molecules are converted to ions transportable through said electrolytic membrane and a cathodic reaction at the opposite side of said at least one electrode pair where said ions are reconverted to gas molecules; and measurement means for measuring the voltage between said at least one electrode pair. An advantage of the present invention is that it provides a direct partial-pressure gas sensor for several electochemically active gases such as oxygen (O 2 ), hydrogen (H 2 ), and several of the halogens. Because the electrolytic membrane will transport only molecules of the electrochemically active gas involved in the ionic reaction at the membrane surface, the present invention is uniquely suited to the measurement of partial-pressure of selected electrochemically active gases such as hydrogen, oxygen, and the halogens. In fact, if one surface of the electrolytic membrane is exposed to the gas under measurement, the electrolytic membrane output voltage will always be directly related to the ratio of partial-pressures across the electrolytic membrane and not to the ratio of gas mixture pressures. Another advantage of the present invention is its suitability for miniaturization. The output voltage from the electrolytic membrane is not related to the membrane area. Thus, for instance, membranes with areas of less than 2 mm 2 can function effectively as pressure sensors for use within catheters for measurement of internal blood and tissue pressures. Because the typical pressure sensor transducer known in the art relies on some physical displacement to generate the transducer output signal, these sensors often comprise a number of moving parts, thereby limiting the practicality of miniaturization. Finally, the present invention offers the additional advantages of high efficiency, low power requirements, and flexible geometry. All of these advantages together with the absence of moving parts result in a low production cost for the preferred embodiments of the present invention. The present invention is also ideally suited for application as an internal pressure sensor in an electrochemical motor as disclosed in copending application 07/563,050 by the inventor, entitled "Efficient Electrochemical Motor," filed concurrently herewith. The foregoing, together with other features and advantages of the present invention, will become more apparent when referring to the following specifications, claims and accompanying drawings. For a more complete understanding of the present invention, reference is now made to the following detailed description of the embodiments illustrated in the accompanying drawings, wherein: Figure 1, comprising Figures 1A and 1B, is a schematic illustration of two different versions of a single-chamber electrochemical partial-pressure sensor; The illustrated embodiment of interest is the partial-pressure gas sensor illustrated in Figure 1. Figure 1A shows a gas sensor 26 having reference chamber 12 and an electrochemical cell comprising membrane 16, electrodes 18 and 20, and rigid supports 17 and 19 exposed to the gas pressure to be measured. Reference chamber 12 contains an electrochemically active gas, which can be oxygen. If electrode 20 is exposed to an air mixture, the oxidation-reduction process occurring across the electrochemical cell will induce a voltage between electrodes 18 and 20 which is related to the ratio of the electrochemically active gas pressure in reference chamber 12 to the electrochemically active gas pressure in air mixture 28. Thus, it is seen that the embodiment illustrated in Figure 1A acts to measure the partial pressure of the electrochemically active gas component of air mixture 28. The desired component, whether hydrogen, oxygen or a selected halogen, is established by the chemical properties of the electrochemically active gas contained in reference chamber 12. Referring to Figure 1B, a similar partial-pressure gas sensor embodiment is illustrated. However, a semi-pervious membrane 30 is added to cover the electrochemical cell assembly to prevent evaporation of the moisture content of ionomer 16. Membrane 30 is selected to be pervious to the electrochemical gas to be measured while also being impervious to moisture. In Figure 1A, if chamber 12 is filled with oxygen or air, sensor 26 will function as an oxygen sensor. To avoid contamination or drying of ionomer 16, protective film 30 can be placed over electrode 20 as shown in Figure 1B. Film 30 can be a silicon sheet or other similar semi-pervious material. Obviously, other embodiments and modifications of the present invention will occur readily to those of ordinary skill in the relevant arts in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such obvious embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.

Claims: A single chamber partial pressure gas sensor (10) comprising: a gas-tight housing, having two ends and a first wall at a first end; a rigidly-supported electrolytic membrane (16) having two sides disposed to form a second wall at a second end of said gas-tight housing and defining therein a storage chamber (12) for holding a supply of electrochemically active gas; at least one pervious electrode (18, 20) disposed on each said side of and in contact with said electrolytic membrane (16); each electrode (18) on the first said side of said electrolytic membrane (16) being separated from any other electrode on the first side by spaces and said at least one electrode (20) on the second side of said electrolytic membrane (16) being disposed to form, with said at least one electrode (18) on said first side, at least one electrode pair (18, 20) separated by said electrolytic membrane (16); said gas being electrochemically reversibly active so as to enter into an anodic reaction at one side of said at least one electrode pair (18, 20) where the gas molecules are converted to ions transportable through said electrolytic membrane (16) and a cathodic reaction at the opposite side of said at least one electrode pair (18, 20) where said ions are reconverted to gas molecules; and measurement means (24) for measuring the voltage between said at least one electrode pair (18, 20). The single chamber partial pressure gas sensor (10) of Claim 1 further comprising a semi-pervious covering (30) over the portion of said electrolytic membrane (16) disposed outside of said gas-tight housing, said semi-pervious covering (30) being impervious to water vapour and pervious to said electrochemically active gas. The single chamber partial pressure gas sensor (10) of Claim 2 wherein said electrochemically active gas is oxygen (O 2 ). The single chamber partial pressure gas sensor (10) of Claim 2 wherein said electrochemically active gas is hydrogen (H 2 ).

Inventors: MAGET HENRI J R, MAGET, HENRI J. R.
Applicants: BAXTER INT, BAXTER INTERNATIONAL INC.
Application year: 1991
Main classification: G01L9
IPC codes: G01L11, G01N27, G01L9, G01L1