Abstract:
A strain gage sensor having improved operational aspects and lower production costs. The strain gage sensor includes two active resistors, two passive resistors, and a structure for minimizing strain experienced by the two passive resistors. The active and passive resistors are attached in a Wheatstone bridge configuration and are Piezoresistors. The active resistors and the strain minimizing structures are mounted on the backing plate. The backing plate or the strain minimizing structure includes alumina ceramic substrate. A voltage to current converter circuit is attached to the active and passive resistors and mounted to the backing plate. Multiple Wheatstone bridge circuits or multiple Wheatstone bridge circuits with voltage to current converter circuits are manufactured on a single backing plate and separated prior to use.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to force sensors and, more particularly, to strain gage sensors.  
       BACKGROUND OF THE INVENTION  
       [0002]     Metal foil strain gage sensors, when applied to an object, can adequately measure straining forces on the object by determining how much the resistance of the resistors change within the metal foil strain gage sensor. A typical metal foil strain gage sensor includes four active resistors and thus four hard wire connections to the resistors. Because of the numerous wires extending from the metal foil strain gage sensor and its low signal to noise ratio, this type of sensor is susceptible to electromagnetic interference (EMI). Alternative strain gage technologies (thick film and semi-conductor) supply a higher signal to noise ratio, but have not traditionally been used due to their inability to measure axial and transverse strains with four active resistors.  
         [0003]     Therefore, there exists a need for a strain gage sensor that is less susceptible to EMI/EMC while having an increased signal to noise ratio and an increased ability to measure multi-directional straining forces on an object.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention is a strain gage sensor having improved operational aspects and lower production costs. The strain gage sensor includes two active resistors, two passive resistors, and a structure for minimizing strain experienced by the two passive resistors.  
         [0005]     The active and passive resistors are attached in a Wheatstone bridge configuration and are Piezoresistors. The active resistors and the strain minimizing structures are mounted on the backing plate.  
         [0006]     In accordance with further aspects of the invention, the backing plate or the stress minimizing structure includes aluminum ceramic substrate.  
         [0007]     In accordance with other aspects of the invention, a voltage to current converter circuit is attached to the active and passive resistors and mounted to the backing plate.  
         [0008]     In accordance with still further aspects of the invention, multiple Wheatstone bridge circuits or multiple Wheatstone bridge circuits with voltage to current converter circuits are manufactured on a single backing plate and separated prior to use. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0010]      FIG. 1  illustrates a block diagram of a system formed in accordance with the present invention;  
         [0011]      FIG. 2  is a circuit diagram of components of the system shown in  FIG. 1 ;  
         [0012]      FIG. 3  illustrates a side view of a thick film strain sensor formed in accordance with the present invention;  
         [0013]      FIG. 4  illustrates a process for creating the thick film strain sensor shown in  FIG. 3 ; and  
         [0014]      FIG. 5  is a perspective view of a plurality of components formed in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     The present invention is a strain sensing system, such as the system  20  shown in  FIG. 1 , for supplying greater strain sensitivity than traditional metal foil strain gages with the ability to sense bi-directional strains while being produced by a more cost effective manufacturing method. The system  20  includes a strain sensor  24  in electrical communication with a sensing system  28 . The strain sensor  24  is coupled to an object for sensing straining forces experienced by the object. The strain sensor  24  produces an electrical signal that is used by the sensing system  28  for identifying the strain force on the object and presenting the identified force to an observer.  
         [0016]     As shown in  FIG. 2 , the strain sensor  24  includes a thick film strain sensor  40  that is in electrical communication with a signal conditioning circuit  44 . The sensing system  28  includes a voltage source  50  and a sensing resistor  52  coupled across two leads from the signal conditioning circuit  44 . In another embodiment the sensing system  28  includes circuitry or computer components for analyzing the voltage drop across the sensing resistor  52  to determine a strain force applied to an object with the attached thick film strain sensor  40 . The sensing system  28  may also include components for storing the determined strain force or presenting the determined strain force to an operator.  
         [0017]     The thick film strain sensor  40  includes a Wheatstone bridge circuit  58  having a first passive resistor  60  coupled at a first end to one end of a first active resistor  62  (connection point  76 ) and at a second end to a first end of a second active resistor  64  (connection point  70 ). The Wheatstone bridge circuit  58  also includes a second passive resistor  66  attached between second ends of the first and second active resistor  62  and  64  at connection points  74  and  72 , respectively. The connection points  70 - 76  are electrically coupled to the signal conditioning circuit  44 .  
         [0018]     The signal conditioning circuit  44  includes first and second low pass filters  80  and  82 , an instrumentation amplifier  84 , a voltage to current converter  86 , a voltage regulator  88 , and an electro magnetic interference (EMI) filter  90 . The connection point  76  is electrically coupled to the first low pass filter  80  and the connection point  72  is electrically coupled to the second low pass filter  82 . The first and second low pass filters  80  and  82  output to the instrumentation amplifier  84 . The instrumentation amplifier  84  takes the difference between results of the first and second low pass filters  80  and  82 , amplifies the difference and outputs the amplified difference to the voltage to current converter  86 . The voltage to current converter  86  converts the amplified voltage differential into a current and supplies the converted current through a resistor  94  to the voltage source  50  of the sensing system  28 . The connection point  76  of the Wheatstone bridge circuit  58  is electrically connected to ground. The sensing resistor  52  of the sensing system  28  is electrically coupled to an input of the EMI filter  90 . An output of the EMI filter  90  is electrically coupled to an input of the voltage regulator  88  and the output of the voltage regulator  88  is electrically coupled to the connection point  70  of the Wheatstone bridge circuit  58 .  
         [0019]     As the object that the thick film strain sensor  40  is attached to experiences a straining force, the active resistors  62  and  64  change in resistance, while the passive resistors do not. The voltage corresponding to the first active resistor  62  is filtered by the first low passive filter  80  in order to screen noise and EMI. The second low pass filter  82  performs the same task as the first low pass filter  80  but performs it on the voltage corresponding to the second active resistor  64 . The instrumentation amplifier  84  takes the difference of the filtered voltages relative to the first and second active resistor  62  and  64  and amplifies that difference. The voltage to current converter  86  converts the amplified voltage differential into a current value that is passed through the resistor  94  to the sensing system  28 . The sensing system  28  produces a sensing current based on the converted current received from the voltage to current converter  86  and produces a voltage drop across the sensing resistor  52  which thus becomes a sensing voltage value. The sensing voltage value is analyzed to determine the strain force experienced by the Wheatstone bridge circuit  58 .  
         [0020]     The voltage source  50  supplies a DC voltage to the EMI filter  90  based on the sensing current. The EMI filter  90  removes anomalies from the received voltage that might be produced by the voltage source  50  and sends the result to the voltage regulator  88 . The voltage regulator  88  outputs a regulated voltage to the thick film strain sensor  40  at the connection point  70  of the Wheatstone bridge circuit  58 .  
         [0021]     As shown in  FIG. 3 , a cross-sectional view of a thick film strain sensor  100  mounted onto a structure  102  is shown. The thick film strain sensor  100  includes a sensor backing plate  108  that is a treated specialty steel backing plate, or an alumina ceramic substrate. Two active thin film Piezoresistors  110  are deposited onto the sensor backing plate  108 . A resistor carrier material  114 , such as an alumina carrier, is also attached to the sensor backing plate  108 . Two passive thick film Piezoresistors  116  are deposited onto the resistor carrier material  114 . The resistor carrier material  114  isolates the passive Piezoresistors  116  and virtually eliminates any change in their resistance due to a stress applied to the structure  102 . In one embodiment, each passive resistor  116  is deposited onto a separate resistor carrier material  114  or onto a single piece of carrier material  114 . The Piezoresistors  116  may be mounted onto other stress minimizing structures. The sensor backing plate  108  is attached to the structure  102  with a strain gage adhesive.  
         [0022]     The resistor carrier material  114  is thermally conductive thus allowing both the active and passive resistors to operate at the same temperature and do not experience thermal gradients that would deteriorate accuracy. In order to ensure accurate thermal performance, thermal compensation between the active and passive resistors  110  and  116 , the active and passive resistors  110  and  116  are doped to different temperature coefficient of resistance (TCR) values. The different TCR values are to ensure that thermal growth and shrinkage of the structure  102  does not produce an induced strain as sensed by the sensing resistor. Almost every type of structure  102  grows or shrinks with temperature, and that dimensional change appears to the active resistors to be the same as growth or compression from a load being applied to the structure  102 . By keeping the passive Piezoresistors  116  at a different TCR than the active Piezoresistors  110 , the passive Piezoresistors  116  can change as much as the active Piezoresistors  110  change from their own TCRs plus the induced strain of the structure  102  being measured. Equation 1 shows this: 
 
Thermal Differential Output=(active( TCR )+induced strain}−passive( TCR )≅0 volts   (1) 
 
         [0023]     As stress is applied to the structure  102  and thus to the sensor backing plate  108 , the active Piezoresistors  110  change in resistance causing a voltage differential between the active Piezoresistors  110 . The voltage differential is directly proportional to the force applied on the structure  102 .  
         [0024]      FIG. 4  illustrates an example process  160  for generating the strain sensor  100 . At block  166 , a dielectric material is applied and baked to the sensor backing plate  108 . The dielectric is necessary for steel backing materials, but not for alumina backings. At a block  170 , the active thick film Piezoresistors  110  are applied and baked onto the sensor backing plate  108 . At a block  172 , two passive thick film Piezoresistors  116  are applied and baked to resistor carrier material  114 . At a block  174 , the active and passive Piezoresistors  110  and  116  are electrically connected to circuitry on the sensor backing plate  108 . The electrical connection of the passive resistors also mechanical attaches the carriers to the backing material.  
         [0025]     The process  160  preferably uses a ceramic-on-metal process for fabricating the thick film Piezoresistors  110  and  116  onto the backing plate  108 . An example ceramic-on-metal process is performed by Honeywell, Inc. At a block  176 , the sensor backing plate  108  is trimmed to the desired size using either laser scribing, photo chemical etching, or a similar process. The goal is to fabricate the backing to the proper dimensions.  
         [0026]     In one embodiment, a glass material is applied to the strain sensor  100  for environmentally protecting the sensor  100 .  
         [0027]      FIG. 5  illustrates a plurality of sensors  190  that are formed on a single piece of backing plate  192 , thereby allowing the sensors  190  to be manufactured at a low cost. Each sensor  190  is removed from the array prior to bonding to a structure that is to be measured for stress or strain.  
         [0028]     In another embodiment, the circuitry of the signal conditioning circuit  44  is applied to the same sensor backing plate that includes the thick film strain sensor  40 . By placing the circuitry of the thick film resistors  40  and the signal conditioning circuit  44 , only the two leads of the signal conditioning circuit  44  are exposed.  
         [0029]     Further circuitry can be included to add a wireless transponder capability. The addition of wireless transponder circuitry to the backing would eliminate the need for any wires to be connected to the sensor. The sensor would receive its power from a radiated signal and would in turn transmit a radiated signal containing the strain information. This circuitry would be different than the one depicted in  FIG. 2 .  
         [0030]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, steps in the process  160  may be performed in various order without departing from the scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.