Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L1/00—Measuring force or stress, in general
  • G01L1/18—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L1/00—Measuring force or stress, in general
  • G01L1/20—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress
  • G01L1/22—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges
  • G01L1/2206—Special supports with preselected places to mount the resistance strain gauges; Mounting of supports
  • G01L1/2231—Special supports with preselected places to mount the resistance strain gauges; Mounting of supports the supports being disc- or ring-shaped, adapted for measuring a force along a single direction
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L27/00—Testing or calibrating of apparatus for measuring fluid pressure
  • G01L27/002—Calibrating, i.e. establishing true relation between transducer output value and value to be measured, zeroing, linearising or span error determination
  • G01L27/005—Apparatus for calibrating pressure sensors

Definitions

• the invention is concerned with a force sensor comprising at least one sensing element to which a force to be measured is applied whereupon the sensing element changes its electrical properties, typically its electrical resistance, in response t said force.
• a voltage is applied to th sensing element such that an electrical current is flowing through the sensing element. Due to the dependency of the electrical resistance (impedance) from the applied force, the measured electrical current is a function of the applied force and the measurement of the current is indirectly also a measurement of the force.
• Force sensors which use a so-called sandwich design (DE-A-28 54 080) .
• sandwich design a non-conductive elastic elastomer containing conductive carbon fibers is used as the sensing element. This sensing element is sandwiched between contact elements so that the force is applied to the flat layer of elastic elastomer in a direction which is perpendicular to the plane of the layer.
• the electrical electrodes are in contact with opposing surfaces of the elastomer layer and when the elastomer layer is compressed by the force, the electrical impedance of the layer is reduced because the conductive carbon fibers are oriented in a direction perpendicular to the plane of the elastomer layer a it is believed that during the compression more and more carbo fibers conduct the current between the electrodes such that t resistance is reduced as a function of the compressive force.
• an elastic elastomer containing conductive carbon fibers which are oriented across the elastomer layer, i.e. in the direction of the applied force, there is no other way than measuring the current flow in the direction of the applied force.
• the manufacture of such a sensor structure using an elastomer does not involve technological problems regarding short circuits because the elastomer has a typical thickness in the millimeter range.
• thickfilm technology involves a printing process.
• the thickness of a thickfilm resistive layer is dependent upon the print thickness of the thickfilm resistive ink, which in itself is dependent upon the mesh size of the print screen frame, and additionally dependent upon the number of print applications.
• junction diffusion Another major problem with thickfilm technology in connection with force sensors is caused by junction diffusion.
• junction diffusion When firin two or more different paste materials which have a similar firing temperature and/or melting point, it should be seen tha the junction areas between the two adjoining pastes will reflo and mix together (diffuse) to some extent. This reflow does no pose a major problem if the thickfilm resistor has a thickness in excess of 1000 ⁇ m between the sandwiching contacts.
• the resistive thickfilm layer has a thickness of only 30 to 80 ⁇ m, for example, this will cause problems regarding partial or complete short circuits, where the diffusion (penetration ratio) of the conductor paste into the resistor paste is large.
• electrical short circuits between the electrodes can also be the result of organic compounds within the thickfilm paste materials. During the firing and/or drying process such organic compounds can produce gases wich can percolate upwards through the paste carrying with them conducti particles and thereby causing a short circuit across the sensing element. Because of these numerous problems related to short circuits in thickfilm technology the prior art measured the current flow through the body of the thickfilm layer and not through its relatively small extension in the direction of the applied force.
• the technical problem underlying the present invention is to provide a force sensor with high measuring sensitivity, high reliability, and insensitivity to the location of the applied force (or pressure) .
• the sensor shall also be relatively compact.
• the invention solves this problem by combining thickfilm technology for the sensing element (which is known as such) with a current flow essentially in the direction of the applied force.
• the resistive layer the sensing element
• thickfilm technology is used for the resistive layer (the sensing element) wherein a change in the applied force in a direction perpendicular to the plane of the sensing element layer results in a change of the corresponding component of resistance in the sensing element (i.e. the component perpendicular to the plane of the layer) , and hence in the current flowing in the direction of the force.
• This change of resistance/current can be directly measured by contact elements sandwiching the thickfilm resistive layer, wherein these contacts are at both sides of the resistive layer.
• the resistance changes in the direction of the force are greater than in a direction perpendicular to the force.
• the measurement signal is also relatively independent from the position where the force is applied. Preferred embodiments of the present invention are described i the dependent claims.
• the present invention includes also a method of manufacturing force sensor of the type described above.
• the force sensor is provided with a bridge circuit which is known per se in connection with such sensors (DE-A-41 11 148) .
• a bridge circuit e.g. a Wheatstone bridge
• the additional bridge components are at least partly printed together with th other components of the sensor onto the same support substrate. This results in improved temperature characteristics.
• Fig. 1 is a section of a first embodiment of a force sensor
• fig. 2 is a view onto the force sensor according to fig. 1 in the direction of the force, partly scraped;
• fig. 3 is an electrical equivalent circuit of a force senso according to fig. 1 and 2;
• fig. 4 is a sectional view of a second embodiment of a forc sensor
• fig. 5 is a plan view of a force sensor according to fig. 4;
• fig. 6 is an equivalent circuit of a force sensor according to fig. 4 and 5;
• fig. 7 is a sectional view of a third embodiment of a force sensor
• fig. 8 is a plan view of a force sensor according to fig. 7;
• fig. 9 is an equivalent circuit of a force sensor according to fig. 7 and 8;
• fig. 10 is a sectional view of a fourth embodiment of a force sensor
• fig. 11 is a plan view of a force sensor according to fig. 10;
• fig. 12 is an equivalent circuit of a force sensor according to fig. 10 and 11;
• fig. 13 is a sectional view of a fifth embodiment of a force sensor
• fig. 14 is a plan view of a force sensor according to fig. 13;
• fig. 15 is an equivalent circuit of a force sensor according to fig. 13 and 14;
• fig. 16 is a sectional view of a sixth embodiment of a force sensor
• fig. 17 is a plan view, partly scraped, of a force sensor according to fig. 16;
• fig. 18 is an equivalent circuit of a force sensor according to fig. 16 and 17;
• fig. 19 is a plan view of a force sensor with a bridge circuit
• fig. 20 is a plan view of a force sensor with a bridge circuit and a dummy sensor.
• fig. 21 is another embodiment of a force sensor with a bridge circuit, where the a dummy sensor and the sensor are integrated.
• a support substrate 10 is provided which is made of e.g. a ceramic material like A1_0 (alumina) .
• a lower contact element 12 is arranged on the support substrate 10 .
• the lower contact element is made of a conductive material.
• a sensing element 14 is arranged above the lower contact element 12 .
• the sensing element 14 is realized by a thickfilm resistor layer or a series of thickfilm resistor layers.
• an upper contact element 16 is arranged on top of the sensing element 14 arranged.
• the force F to be measured is indicated by an arrow in the figures, i.e. the force F is applied in a direction perpendicular to the main plane of the flat sandwich structure.
• the force F can be applied to the force sensor by means of a stamp 18.
• An upper connecting track 20 is brought down from the level of the upper contact element 16 to the level of the support substrate 10 as is indicated in fig. 1 and 2.
• a lower contacting track 22 provides an electrical terminal for the lower contact element 12. As is shown in fig. 1 and 2, the sensing element 14 overlaps the lower contact element 12 with the exception of the lower connecting track 22.
• the upper contact element 16 is in alignment with the lower contact element 12, as is also shown in fig. 1 and 2. Further, the upper contact element has at least approximately the same dimensions as the lower contact element.
• the material used for the track 22 and the corresponding contact element 12 is the same in this embodiment.
• the sensing element 14 is provided by a thickfilm resistive layer which is printed on top of the lower contact element 12 (see below) .
• the upper contact element 16 is isolated from the lower contact element 12 by means of the overlapping sensing element 14 (of resistive material) .
• the upper connecting track 20 has an offset angle to the lower connecting track 22 in order to avoid a short circuit between the two tracks.
• Fig. 3 shows an equivalent circuit of the above described force sensor.
• the sensing element 14 i.e. the resistive thickfilm layer
• the current i between the upper and lower connecting tracks 20, 22 changes its magnitude accordingly (if a voltage is applied to the connecting tracks, of course) .
• the direction of the current i is indicated by an arrow.
• Fig. 4 to 6 show another embodiment of a force sensor where the lower contact element is subdivided into at least two distinct flat elements. With each subdivided area of the two lower contact elements 12, 12a is connected a lower contact track 22, 22a (fig. 5). In the embodiment of fig. 4 to 6, there is no connecting track to the upper contact element 16. This embodiment offers the benefit of having connection tracks only on the lower suppert substrate 10.
• the lower contact members 12, 12a are separated by a gap 24.
• the main conducting path (current flow) of this force sensor is from the first lower contact element 12 through the area of the sensing element 14 located above this contact element to the upper contact element 16, then along the upper contact element 16 and then through a second area of the sensing element 14 to the second lower contact element 12a. This means, that the current flows at least over a part of its path in parallel to the direction of the force F (always indicated by the fat arrow in the drawings) .
• a second minor current path exists which is from the first lower contact element 12 through the resistive material (which can be the same as in the sensing element 14) which is arranged in the gap 24 to the second lower contact element 12a.
• the effect of this gap is small but can be further reduced by either increasing the size of the gap 24 and/or by omitting the resistive material from the gap.
• the current flows from the lower connecting track 22 to the other lower connecting track 22a and is indicative of the magnitude of the applied force F.
• Fig. 7 to 9 show an embodiment which corresponds to the embodiment of fig. 4 to 6 with the exception of an additional connecting track 20 for the upper contact element 16 which provides multiple connecting possibilites.
• Fig. 10 to 12 show another embodiment of a force sensor in which the order of the printing process is modified such that the form of the upper and lower contact elements 12, 16 and the upper and lower connecting tracks 20, 22 are printed on opposite sides of the sensing element 14 as compared to the embodiments shown in fig. 4 to 9.
• the embodiment shown in fig. 13 to 15 corresponds to the embodiment of fig. 10 to 12 with exception to the additional connecting track 22 for the lower contact element 12, however, two different forces F and F are applied as shown in fig. 13 and 15 and the two forces F and F can be measured individuall by means of the changes of the resistivity (impedance) of the respective areas of the sensing element 14 which are subjected to the forces F. and F_ , respectively.
• This also applies to figures 7 to 9.
• Fig. 16 to 18 show a further embodiment of a force sensor in which the sensor structure has a through going hole 26 within the area of the different layers of which the sensor structure is composed.
• the hole 26 which is shown in fig. 16 and 17 has a cylindrical form, but can have an other form as well. For example, an open slot can be formed in the support substrate and all other elements above this.
• the hole 26 can be used to insert a mechanical element by which the support substrate 10 and the elements 12, 14 and 16 are subjected to a compressing force which provides a certain bias before the force F to be measured is applied.
• the material of the support substrate 10 can be a non-con ⁇ ductive material, e.g. 96 % aluminium oxide, but can also be a conducting material, for example a metal structure with part of the outer surface being non-conductive, e.g. oxide coated or glass film coated.
• a fired thickfilm resistive material is used as the sensing element 14, wherein the electrical signal (current) relating to the applied force (or pressure) is in the same direction as the applied force.
• the sensing element 14 has an amorphous structure containing conducting metal atoms fixed in a glass body, wherein the impedance of the sensing element 14 is dependent upon the separation of the outer orbitals of the conductive atoms. This separation and hence the impedance of the sensing element 14 can be changed by applying a force. The absolute magnitude of deformation of the thickness of the resistive layers by the applied force is measured in microns.
• the support substrate 10 is mounted so as to allow a bending of the sensor structure when under load (strain gauge mode) , the output impedance change of the resistive layer of the sensing element 14 will be greater, but the force will be limited to the bending strength of the support substrate 10.
• a selection of the operating modes can be performed by the configuration of the housing (not shown) of the sensor.
• the sensor shown in the above described embodiments can be modified by using a plastically deforming medium between the uppermost surface of the sensor shown in the drawings, i.e. the upper surface of the upper contact element 16, and the surface of the mechanical part by which the force is applied, e.g. the stamp 18.
• This plastically deforming medium compensates for any non-flat parts of the surfaces.
• the plastically deformable medium can be preloaded during the manufacturing process of the force sensor so as to adapt its surfaces to those of the sensor and the other mechanical parts (e.g. the stamp 18) by which the force is applied.
• the preload force can be in excess of the normal working load of the sensor.
• the properties of the individual layers must be considered with respect to the other layers and the bonding agents and/or the relative melting and firing points of the material layers should be selected properly.
• mixing of technologies can be advantageous, for example, one or more of the contact elements 12, 16 can be realized by thinfilm technique.
• the embodiments of force sensors shown in fig. 1 to 17 can be manufactured as follows.
• the lower contact element 12 and, if required, the lower connecting track 22 can use the same material.
• the preferred material for these conducting parts is one which uses the oxide bonding mechanism to bond to the ceramic substrate 10, that is the conductor paste undergoes a chemical reaction during the firing process (around 850°C) which is irreversible during further refiring.
• the lower contact element 12 and the connecting track 22 are printed onto the support substrate 10 and are then dried and fired as per the manufacturers instructions.
• a paste is selected from a family of pastes which are designed to fire at a peak temperature of 850°C.
• the bonding mechanism of such pastes is the so-called frit bonding.
• the paste contains small glass particles (frits) which melt during the firing process, the melting temperature of the frits being below that of the peak firing temperature of the resistor paste. Therefore, on cooling the structure is held together by these solidified glass particles. This bond is dependent upon temperature when the glass frits are reheated to a temperature above its melting point.
• the bonding mechanism will dissolve, thereby allowing particles previously fixed in the glass structure to move freely.
• the resistor can be printed with a minimum of two resistor layers whereafter each print of resistive ink is allowed to dry, e.g. in an oven at 150°C for approximately 15 minutes. This time can be adjusted depending upon the volume of ink material and the solvent content of the resistor paste.
• the dried layers of ink are fired onto the lower contact element 12 and the support substrate 10.
• the resistive layers of the sensing element 14 can be fired additionally in at least one intermediate state before the final print thickness is arrived at.
• the same conductor material can be used, e.g. a paste material that bonds by the use of low melting point glass frits.
• the low melting point frits melt and fire at a temperature lower than the melting point of the glass frits contained within the underlying resistive layers of the sensing element 14.
• the paste for manufacturing the upper contact elements 16 is printed onto the fired resistor material of the sensing element 14 and then allowed to dry in an oven and is then fired as per the manufacturers instructions onto the sensing element 14.
• the fired thickness of the resistive layer of the sensing element 14 is preferably in the range of 5 to 100 ⁇ m, most preferably 5 to 50 ⁇ m.
• Fig. 19 to 21 show a modification and special use of a force sensor described above by including a bridge circuit.
• the voltage supply to the bridge circuit is applied across the input terminals 38, 38a.
• the impedance of resistor 34 can be adjusted, e.g. by laser calibration or similar abrasive techniques to achieve the required sensitivity of the bridge.
• the bridge components, in particular resistors 30, 32, 34 and 36, including at least one additional calibration resistor 34 can be included onto the support substrate 10 together with the sensor structure 28.
• Reference numeral 28 in fig. 19 to 21 indicates e.g. a force sensor according to one of the fig. 1 to 18.
• the resistors of the bridge, the connecting tracks (lines) and, if required, resistors associated to a signal amplifier can be processed during the printing and firing processes of the sensor structure 28, wherein the firing of the resistors can coincide with the final firing of the thickfilm resistor of the sensing element 14.
• the resistor 32 and the sensor structure 28 use different resistance paste materials and also very different forms of construction. This results in that the two components have normally different temperature coeffecients of resistance. The difference between the two materials helps to dictate how stable the final output signal will be in relation to the temperature.
• Fig. 20 shows a design with improved properties with respect to the problem of temperature compensation.
• the resistor 32 of fig. 19 is replaced by a resistor 40 which is a near copy of the sensor 28 with respect to the area and the thickness of the resistive material of the sensing element 14 (between the lower contact element and the upper contact element) .
• the paste materials used for the resistance layers of the dummy sensor 40 and the sensor 28 are the same.
• the individual layers which make up the sensor 28 and the dummy sensor 40 should be printed using the same screen print frame or the same pass of the screen print applicator so as to equalize any thickness differences or material differences in both of the printed parts.
• any difference in the temperature coefficients of the resistors 30 and 36 of fig. 19 determines the stability of the final output signal with respect to temperature. Therefore, using the same paste material for the resistors 30 and 36 will improve the temperature compensation.
• the resistors 30 and 36 can be printed with either different thicknesses of the same resistive ink, or with differing aspect ratios (length/width) of the resistors, again using the same paste material, such that the location of the calibration resistor will be given by the ratios of the resistive components within the complete bridge.
• the selected calibration resistor (selected by the printing process) is then calibrated, for example, by laser calibration until such point as the required bridge output voltage is reached between TP1 and TP2.
• dummy sensor 40 and the sensor 28 can be so brought together to for one combined sensor/dummy sensor, which offers benefits of reduced size.
• Fig. 21 shows an application of such an integration where the area of the dummy sensor 40a has no force applied to it.
• This figure shows a sensor design after the type seen in fig. 14, but can alternatively use a design after the type seen in fig. 8.
• the first screen is used for manufacturing the lower contact element on the support substrate.
• the screen is made from
• the second screen is used for manufacturing the resistors of the bridge.
• This screen is made from stainless steel and has a mesh densitity of 200/Inch 2. The angle of the mesh is 45°.
• the third screen is used for manufacturing the resistive layer of the sensing element.
• This screen is also made from stainless steel and has a mesh densitiy of 200/Inch 2. The mesh angle is
• the fourth screen is used for manufacturing the upper contact element. It is made from stainless steel and has a mesh density of 325/Inch 2. The mesh angle is 45°.
• Thickness after firing 10 ⁇ m
• the aforementioned technology can be used in conjunction with Hybrid technology and/or ASIC technology to include an integrated signal amplifier with the corresponding circuitry being built onto the same support substrate 10 as the bridge and the sensor.
• the outer surface of the complete sensor assembly (if required including the bridge and the amplifier) can be either partially or completely hermetically sealed by a film acting as a protecting or isolating coat.
• a sensor of the type described above can installed into a braking system of a vehicle.
• the signal corresponding to the measured force can be used as an input signal to an eletronic control circuitry for indicating the force by which the driver wishes to brake.
• such a force sensor can be used in a vehicle for controlling the intensity of brake lights such that the intensity is increased in dependency upon the magnitude of the measured force, thereby giving an improved visual indication to drivers of other following vehicles.
• Such a force sensor can be used to control vehicle hazard warning lights under certain strong braking requests of the driver.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Force Measurement Appropriate To Specific Purposes (AREA)
• Steering Control In Accordance With Driving Conditions (AREA)
• Air Bags (AREA)
• Push-Button Switches (AREA)
• Pressure Sensors (AREA)
• Measurement Of Force In General (AREA)
• Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)

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• 1993
  • 1993-01-15 DE DE4300995A patent/DE4300995C2/de not_active Revoked
• 1994
  • 1994-01-10 ZA ZA94133A patent/ZA94133B/xx unknown
  • 1994-01-11 BR BR9405809A patent/BR9405809A/pt not_active Application Discontinuation
  • 1994-01-11 CZ CZ951802A patent/CZ180295A3/cs unknown
  • 1994-01-11 WO PCT/EP1994/000060 patent/WO1994016300A1/en not_active Application Discontinuation
  • 1994-01-11 EP EP94905030A patent/EP0679248A1/en not_active Ceased
  • 1994-01-11 AU AU58820/94A patent/AU687030B2/en not_active Expired - Fee Related
  • 1994-01-11 KR KR1019950702781A patent/KR960700443A/ko not_active Application Discontinuation
  • 1994-01-11 JP JP6515685A patent/JPH08505471A/ja active Pending
  • 1994-01-12 TW TW083100198A patent/TW286358B/zh active
  • 1994-01-15 CN CN94100722A patent/CN1092165A/zh active Pending

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