The invention relates to a measuring device for measuring a physical quantity such as a pressure and/or a force. More particularly, the invention relates to a piezo-resistive measuring device.
A measuring device of the above type is known from EP1790964A1. The pressure-measuring device comprises a circular sensing structure and strain gauges attached to the sensing structure. The circular sensing structure comprises a membrane section which is deflected by pressure variations of the fluid acting on the circular sensing structure. The strain gauges measure the pressure dependent strain at a surface of the membrane section. A first strain gauge is configured to measure radial strain in a first surface area of the membrane section. A second strain gauge is configured to measure radial strain in a second surface area of the membrane section. An increase in pressure acting on the sensing structure results in shrinking (=negative strain) of the first surface area measured by the first strain gauge and stretching (=positive strain) of the second surface area measured by the second strain gauge. The first and second strain gauges are integrated in a sensing electrical element. Two of such sensing electrical elements are attached to the circular sensing structure. One sensing element could be used in a half Wheatstone bridge. Two sensing electrical elements each comprising a pair of strain gauges could be used in a full Wheatstone bridge.
The strain gauges are made of silicon and have a resistance which has a relationship with the strain measured in the surface. The costs of the sensing electrical elements could be reduced by reducing the size of the sensing electrical elements. Reducing the size means that the radial distance between the two strain gauges decreases and the difference in radial strain in the surface area below both strain gauges would decrease. This reduces the sensitivity of the sensing electrical element.
Furthermore, the resistance value of piezo-resistive strain gauges is very temperature dependent. A temperature difference of 0.2° C. between the two resistors of a sensing element already results in an error of 1% full-scale. In current designs, the temperature difference can be up to 2° C. which results in very large errors. By reducing the radial distance between the two strain gauges the temperature difference reduces and so the error. However, this is at costs of reduced sensitivity since the difference in radial strain under pressure between both resistors would reduce.
It is an object of the present invention to provide an improved measuring device for measuring a physical quantity which overcomes at least one of the disadvantages mentioned above. A physical quantity could be pressure, force or a combination of pressure and force. Another object of the invention to provide a measuring device which is at least one of: reliable, cheaper to manufacture, long lasting and/or robust to harsh pressure media, withstanding the high temperature and vibration typical of an internal combustion engine.
According to a first aspect of the invention, this object is achieved by a measuring device having the features of claim 1. Advantageous embodiments and further ways of carrying out the invention may be attained by the measures mentioned in the dependent claims.
A measuring device according to the invention is characterized in that the first strain gauge measures radial strain in the membrane section and the second strain gauge measures tangential strain in the membrane section.
The invention is based on the insight that when a force acting on the circular sensing structure increases there are areas on the circular sensing structure which stretch (=positive strain) and there are areas on the circular sensing structure which shrink (=negative strain). The force could be in the form of a pressure acting on the circular sensing structure. It has been found that the strain in radial direction might be different from the strain in tangential direction. This insight increases the degrees of freedom to design the circular sensing structure and to position the strain gauges on the circular sensing structure such that one strain gauge measures positive strain, i.e. stretch, and the other strain gauge measures negative strain, i.e. shrink, when the force increases.
In an embodiment, a resistance change in the first strain gauge due to a predefined increase in force is defined by the equation:
ΔR1=GF1×ε−×R0
wherein GF1 is the Gauge Factor, ε− is negative strain in the first surface and R0 is the unstrained resistance of the strain gauge. A resistance change in the second strain gauge due to the predefined increase in force is defined by the equation
ΔR2=GF2×ε+×R0
wherein GF2 is the Gauge Factor, ε+ is positive strain in the second surface and R0 is the unstrained resistance of the strain gauge. The first strain gauge and the second strain gauge have the following mutual relationship:
GF
1×ε−=GF2×ε+.
These features allow providing a pair of strain gauges which provide a comparable change in resistance whereas the strain in the surface might differ. This improves the accuracy of the electrical signal derive from the resistance values of the pair of strain gauges.
In an embodiment, the first strain gauge and the second strain gauge have a similar distance to a cylinder axis of the circular sensing-structure. This is possible on surface areas on the membrane section where the strain in radial direction is opposite to the strain in tangential direction. Due to the circular structure, this allows to attach two strain gauges at the same distance from the centre axis of the circular structure one measuring in radial direction and another in tangential direction. This further allows minimizing the distance between the two strain gauges, which reduces possible temperature difference between the two strain gauges without decreasing the sensitivity of the strain gauges.
In an embodiment, in use the membrane structure comprises radially a temperature gradient and the first and second strain gauge have an average temperature which differs less than 0.2° C. from each other. This feature allows reducing thermal-shock effects in the electrical output signal below 1% full scale.
In an embodiment, the first strain gauge and the second strain gauge have a midpoint, the midpoint of the first and second strain gauge having a similar distance to a cylinder axis of the circular sensing-structure. This feature reduces the temperature difference between the two strain gauges.
In an embodiment, the first and second strain gauges are integrated in one sensing element. This reduces the temperature difference between the two strain gauges further.
In a further embodiment, the sensing electrical element comprises a first bond path, a second bond path and a third bond path. The second bond path is located between the first bond path and the third bond path. A first part of the first strain gauge is located between the first bond path and the second bond path, a second part of the first strain gauge is located between the second bond path and the third bond path, the second strain gauge is located adjacent a side of the first, second and third bond path. These features allow providing a sensing electrical element with reduced size which could be wire bonded with the same wire bond technology as used before.
In an embodiment, the device further comprises a third strain gauge and a fourth strain gauge. The third strain gauge is configured to measure radial strain in the membrane section and the fourth strain gauge is configured to measure tangential strain in the membrane section. These features enable to improve the sensitivity of the device. In a further embodiment, the first, second, third and fourth strain gauges are integrated in one sensing element. This feature allows reducing the manufacturing costs without concessions with respect to temperature sensitivity and signal quality.
In an embodiment, the circular sensing structure comprises an outer section and an inner section. The circular sensing structure allows the inner section to move relatively to the outer section along the cylinder axis of the circular sensing structure by deformation of the membrane section. It has been found that this type of sensing structures has a membrane with a surface having strain in radial direction which is opposite to the strain in tangential direction. The same applies when the inner section comprises a through hole. The ability to have smaller sensing electrical elements allows reducing the size of the circular sensing structure. This increases the applicability of the measuring device.
Other features and advantages will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, various features of embodiments.
These and other aspects, properties and advantages will be explained hereinafter based on the following description with reference to the drawings, wherein like reference numerals denote like or comparable parts, and in which:
The strain gauges 120 are configured to measure strain in the surface of the membrane section 102A. The strain gauges could be in the form of piezo-resistive elements. This type of strain gauges has a higher Gauge Factor (GF) than metal strain gauges. However, the idea of the invention could also be applied in pressure-measuring devices or force-measuring devices with metal strain gauges.
The working principle of the prior art gauge is that both strain gauges of the sensing electrical element measure radial strain but at two different radiuses. In
It can further be seen that it is possible to measure both positive strain and negative strain with comparable values at a position with a radius of 1.7 mm. This could be done by measuring radial strain and tangential strain.
The sensing electrical element 107 further comprises a first bond path 107A, a second bond path 107B and a third bond path 107C. The second bond path is located between the first bond path and the third bond path. The first part 103A of the first strain gauge is located between the first bond path and the second bond path. The second part 103B of the first strain gauge is located between the second bond path and the third bond path. The second strain gauge 104 is located adjacent a side of the first, second and third bond path. The sensing electrical element 107 has a length of 0.52 mm and a width of 0.46 mm. The bond paths 107A, 107B and 107C have corresponding size and location as the bond paths of the prior art sensing electrical element 50 shown in
In
The smaller sensing electrical element allows for measurement in almost one point and has comparable signal amplitudes because of same amplitudes for both the negative radial strain and positive tangential strain. Furthermore, in the small gage design non-linearity is zero because the Wheatstone bridge remains balanced when a pressure, a force or a combination of a pressure and force is acting on the circular sensing structure.
The circular sensing structure 102 comprises an outer section 102C and an inner section 102D. The circular sensing structure allows the inner section 102D to move relatively to the outer section 102C along the cylinder axis 102B of the circular sensing structure 102 by deformation of the membrane section 102A when a force is acting on the inner section. A flexible membrane 113 is attached to the outer section 102 and the inner section 102D. The flexible membrane 113 forms a sealing which protects the membrane section 102A against the harsh environment of the combustion gasses. A pressure acting on the flexible membrane 113 and the inner section 102D is converted to a force which is transported via the inner section 102D to the membrane section 102A as a result of which the membrane section 102A deforms.
The inner section 102D could comprise an axial passage for positioning a rod-like element in the inner section. In this way, a second function could be added to the pressure-measuring device. Examples of a second function are: glow plug, temperature sensor.
It should be noted that the strain gauges in sensing electrical elements have substantially the same Gauge Factor. The Gauge factor (GF) or strain factor of a strain gauge is the ratio of relative change in electrical resistance to the mechanical strain ε, which is the relative change in length. As a consequence the Wheatstone bridge is balanced if the mechanical strain ε in the surface below the first strain gauge and the second strain gauge is similar in amplitude but opposite in sign.
A resistance change in the first strain gauge due to a predefined increase in pressure, force or combination of pressure and force is defined by the equation:
ΔR1=GF1×ε−×R0
wherein GF1 is the Gauge Factor, ε− is negative strain in the first surface and R0 is the unstrained resistance of the strain gauge. A resistance change in the second strain gauge due to the predefined increase in pressure, force or combination of pressure and force is defined by the equation
ΔR2=GF2×ε30 ×R0
wherein GF2 is the Gauge Factor, ε+ is positive strain in the second surface and R0 is the unstrained resistance of the strain gauge.
If there is no area on the surface of the membrane available to attach a sensing electrical element for which holds εradial=−εtangential it is possible to adapt the Gauge Factor of the strain gauges such that the first strain gauge and the second strain gauge have the following mutual relationship: GF1×ε−=GF2×ε+. In that case the Wheatstone bridge is again balanced.
By using the sensing elements shown in
The embodiments shown above all relate to a measuring device measuring the physical quantity pressure. In the measuring device shown in
While the invention has been described in terms of several embodiments, it is contemplated that alternatives, modifications, permutations and equivalents thereof will become apparent to those skilled in the art upon reading the specification and upon study of the drawings. The invention is not limited to the illustrated embodiments. Changes can be made without departing from the idea of the invention.
Number | Date | Country | Kind |
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12193715.5 | Nov 2012 | EP | regional |