Technical Field

The disclosure relates generally to sensors, and more particularly, to flow sensors that are configured to sense the flow of a fluid in a flow channel.

Background

Flow sensors are used to sense fluid flow, and in some cases, provide flow signals that can be used for strumentation and/or control. Flow sensors are used in a wide variety of applications including industrial applications, medical applications, engine control applications, military applications, aeronautical applications, to name just a few.

Summary

The disclosure relates generally to sensors, and more particularly, to flow sensors. Traditional flow sensors include an upstream resistive sensor element, a downstream resistive sensor element and an intervening heater resistive element. To help reduce the size and/or cost of such flow sensor, it is contemplated that the heater resistor may be eliminated. When so provided, the space required for the heater resistive element, as well as the corresponding heater control circuit, may be eliminated. This can reduce the cost, size and complexity of the flow sensor.

In one example, a flow sensor may be provided that has an upstream self heating sensor element and a downstream self heating sensor element, with no intervening heater element. In some cases, the upstream resistive element and the downstream resistive element are operatively connected in a bridge circuit. The bridge circuit may be configured to supply a current to each of the upstream resistive element and the downstream resistive element that causes resistive heating such that both the upstream resistive element and the downstream resistive element are heated above the ambient temperature of the fluid flowing through a flow channel. When fluid flow is present in a flow channel, the fluid flow causes the temperature of the upstream resistive element to be lower than the temperature of the downstream resistive element. The difference in temperature causes an imbalance in the bridge circuit that is related to the flow rate of the fluid flowing though the flow channel. The above summary is not intended to describe each and every disclosed illustrative example or even' implementation of the disclosure. The Description that follows more particularly exemplifies various illustrative embodiments. Brief Descripti on of the Fi gures

The following description should be read with reference to the drawings. The drawings, which are not necessarily to scale, depict selected illustrative embodiments and are not intended to limit the scope of the disclosure. The disclosure may be more compl etel y understood in consideration of the following description of vario us illustrative embodiments in connection with the accompanying drawings, in which:

Figure 1 schematic cross-sectional view of an example flow sensing device; Figure 2 is a schematic circuit diagram of an example prior art flow sensor; Figure 3 is a top view of an example prior art flow sensor die;

Figure 4 is a schematic circuit diagram of an illustrative flow sensor with one or more self heating resistive elements;

Figure 5 is a top vie w of an illustrative flow sensor die wit one or more self heating resistive elements;

Figure 6 is a chart showing sensitivity versus flow rate of a prior art flow sensor die such as shown in Figure 3 at various heater voltages; and

Figure 7 is a chart showing sensitivity versus flow rate of a flow sensor die with one or more self heating resistive elements such as shown in Figure 5 at various bridge supply voltages.

Description

The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings which are not necessarily to scale, depict selected illustrative embodiments and are not intended to limit the scope of the disclosure. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art wil l recognize that many of the examples provided have suitable alternatives that may be utilized.

Figure 1 is a schematic cross-sectional view of an example flow sensing device 100. The illustrative flow sensing device 100 includes a flow sensing device body 102 that defines a flow channel 104 having first end 106 and a second end 108. A fluid may flow through the flow ^r channel 104 from for example the first end 106 to the second end 108 and past a flow sensor 110. The flow sensor 110 may sense the flow of the fluid passing over the flow ^r sensor 110, and provide one or more output signals indicative of that flow. In some cases, the flow sensor 110 may provide one or more output signals that identity the flow rate of the fluid passing over the flow sensor 110.

While not required, the flow sensor 110 may include a flow senor die that is mounted to a substrate 112, The substrate 112 may be mounted in the flow sensing device body 102. In some cases, some of the support circuitry for the flow sensor die may be located on the substrate 1 12 and/or may be located outside of the flow sensing device 100 altogether (e.g. located in a device that uses the output of the flow sensing device 100). Figure 1 shows one example configuration of a flow sensing device. It should be recognized that such flow sensor devices can and do assume a wide variety of different configurations, depending on the application.

Figure 2 is a schematic circuit diagram of an example prior art flow sensor 200. The example flow sensor 200 includes two upstream resistive elements RUl and RU2 and two downstream resistive elements RDl and RD2 connected in a full Wheatstone bridge configuration. The two upstream resistive elements RU l and RU2 are positioned upstream of the two downstream resistive elements RD 1 and RD2 within a flow channel, as best shown in Figure 3. In the example shown, RUl is connected between nodes L and A, RU2 is connected between nodes B and K, RDl is connected between nodes G and F, and RD2 is connected between nodes E and H. A differential output of the bridge is taken betw ^r een nodes Vn 202 and Vp 204. During use, a supply voltage, such as 2.4 volts, is provided to nodes E and B, and ground is connected to nodes A and F, either directly or through a resistor Rl .

The example flow sensor 200 of Figure 2 also includes a heater resistor Rh. Heater resistor Rh is connected between nodes C and D as shown. The heater resistor Rh is physically positioned between the upstream resistive elements RUl and RU2 and the downstream resistive elements RDl and RD2, as best shown in Figure 3. The heater resistor Rh is heated by a heater control circuit 206. The heater resistor Rh typically has a resistance that is significantly lower than the nominal resistance of the resistive elements RUl , RU2, RDl and RD2, such as 200 ohms. Resistive elements RUl, RU2, RDl and RD2 may have a nominal resistance of, for example, 2.5 K ohms). When no flow is present, the heater resistor Rh heats the fluid in the flow channel, which through conduction and convection, evenly heats the resistive elements RUl , RU2, RDl and RD2. Since all of the resistive elements RU l, RU2, RD l and RD2 are heated evenly, the bridge circuit remains in balance. However, when flow ^r is present, the upstream resistive elements RUl and RU2 are lowered in temperature relative to the downstream resistive elements RDl and RD2. As the flow rate of the fluid in the flow channel increases, the difference in temperature between the upuStream resistive elements RUl and RU2 and the downstream resistive elements RDl and RD2 increases. This difference in temperature causes the downstream resistive elements RD l and RD2 is have a higher resistance than the upstream resistive elements RUl and RU2 (assuming a positive temperature coefficient), thereby causing the bridge to become imbaianced. This imbalance produces a differential output signal between Vp 204 and Vn 202 that increases with flow rate and is monotonic with flow rate, in some cases, a sensing circuit (not shown) may receive Vp 204 and Vn 202, and may perform some compensation and/or

linearization before pro viding a flow ^r sensor output signal, if desired.

The example flow sensor 200 also includes a temperature reference resistor Rr. Temperature referenced resistor Rr is connected between nodes I and J. The reference resistor Rr may have a nominal resistance of, say, 4 K ohms. The heater control circ uit 206 controls the tem perature of the heater resistor Rh to be above a reference (or ambient) temperature of the fluid sensed by reference resistor Rr. In most cases, it is desirable to heat the heater resistor Rh some amount (e.g. 200 degrees F) abo ve the ambient temperature of the fluid in the flow channel to increase the signai-to-noise ratio of the flow sensor.

Figure 3 is a top view of an example prior art flow sensor die 300. The flow sensor die has an etched cavity 302 that extends under a membrane 304. The etched cavity 302 helps to thermally isolate the membrane 304 from the substrate 308 of the flow sensor die 300. The example flow sensor die 300 includes a slit 310 through the membrane 304 that extends transverse across the membrane 304. During use, the flow sensor die 300 is positioned in a flow channel.

To help explain the operation of the flow sensor die 300, it is assumed that fluid flows over the flow sensor die 300 in the direction indicated by arrow 312. When so provided, the two upstream resistive elements RUl and RU2 are positioned on the membrane 304 upstream of the slit 310, and the two downstream resistive elements RDl and RD2 are positioned on the membrane 304 downstream of the slit 310. The heater resistor Rh is positioned between the upstream resistive elements RU1 and RU2 and the downstream resistive elements RDl and RD2. In the example shown, the heater resistor Rh includes two legs connected in series, with one leg positioned on either side of the slit 310. The example flow sensor die 300 is one possible layout of the schematic circuit diagram shown in Figure 2, with the corresponding nodes indicated (A-L). The example flow sensor die 300 does not include the heater control circuit 206, the connection between nodes H-L, the connection between nodes K-G, the connection between nodes E-B, or the connection between A-F. This example flow sensor die 300 is considered a test die, and these connections are intended to be made external to the flow sensor die 300. However, they could be made on the flow sensor die 300 if desired.

To help reduce the size and/or cost of the prior art flow sensor die 300 discussed above, it is contemplated that the heater resistor Rh may be eliminated. When so provided, the space required for the heater resistor Rh, as well as the heater control circuit 306, may be eliminated. Figure 4 is a schematic circuit diagram of an illustrative flow sensor 400 with this modification. The flow sensor 400 eliminates the heater resistor Rh and the corresponding heater control circuit discussed above. In order to provide the necessary heat to make the flow measurement, it is contemplated that one or more of the resistive elements RU1, RU2, RD l and RD2 may be self heating. That is, one or more resistive elements RU1, RU2, RDl and RD2 may not only heat the fluid but also sense the temperature of the fluid. In one example, all of the resistive elements RU1 , RU2, RDl and RD2 are self heating (i.e. heat and sense). In other instances, only one upstream resistive element RU1 or RU2 may be self heating, both upstream resisti ve elements R.U1 and RIJ2 may be self heating, only one upstream resistive element RU1 or RU2 and only one downstream resistive element RD l or RD2 may be self heating, or any other combination of resistive elements may ^¬ be self heating so long as at least one upstream resistive element is self heating. In some cases, only one upstream resistive element and only one downstream resistive element is provided, rather than two.

In the example shown, the illustrative flow sensor 400 includes two upstream resistive elements RU1 and RU2 and two downstream resistive elements RDl and RD2 connected in a full Wheatstone bridge configuration. It is contemplated, however, that only one upstream resistive element RXJ1 and one downstream resistive element RD2 may be provided, which in some cases, can he connected in a half- bridge or other configuration. In the example shown in Figure 4, the two upstream resistive elements RUl and RU2 are positioned upstream of the two downstream resistive elements RD1 and RD2 within a flow channel, as best shown in Figure 5. RUl is connected between nodes L and A, RU2 is connected between nodes B and , RD1 is connected between nodes G and F, and RD2 is connected between nodes E and H. A differenti al output of the bridge is taken between nodes Vn 402 and Vp 404, During use, a supply voltage, such as 2.4 volts, may be provided to nodes E and B, and ground may be connected to nodes A and F, either directly or through a resistor R 1.

In most cases, resistive elements RUl, RU2, RD1 and RD2 have substantially the same temperature coefficient (positive or negative). Substantially the same here means plus or minus ten (10) percent. In some cases, resistive elements RUl, RU2, RD1 and RD2 have temperature coefficients that are within 1 percent or l ess of each other. Also, resistive elements RUl, RU2, RD1 and RD2 may have substantially the same nominal resistance, such as about 500 ohms. In some cases, resistive elements RUl, RU2, RD 1 and RD2 may have nominal resistance valves that are within twenty (20) percent, ten (10) percent, five (5) percent, or one (1) percent or less of each other. In some cases, the resistive elements RUl , RU2, RD 1 and RD2 may be formed from a common set of one or more layers. Notably, in Figure 5, the two upstream resistive elements RUl and RU2 and the two downstream resistive elements RD1 and RD2 are not separated by an intervening heater resistor Rh, and in particular, a heater resistor Rh that has a significantly lower resistance than the resistance of the resistive elements. Significantly less means at least twenty (20) percent less.

For discussion purposes, it is assumed that all of the resistive elements RUl , RU2, RD1 and RD2 are self heating. When no flow is present, the resistive elements RUl, RU2, RD1 and RD2 heat the fluid in the flow channel, which through conduction and convection, evenly heats the resistive elements RUl, RU2, RD1 and RD2. Since all. of the resistive elements RUl , RU2, RD! and RD2 are heated evenly, the bridge circuit remains in balance. However, when flow is present, the upstream resistive elements RUl and RU2 are lowered in temperature relative to the

downstream resistive elements RD! and RD2. As the flow rate of the fluid in the flow channel increases, the difference in temperature between the upstream resistive elements RUl and R.U2 and the downstream resistive elements RD1 and RD2 increases. This difference in temperature causes the downstream resistive elements RDl and RD2 is have a higher resistance than the upstream resistive elements RU1 and RU2 (assuming a positive temperature coefficient), thereby causing the bridge to become imbalanced. This imbalance produces a differentia] output signal between Vp 404 and Vn 402 that increases with flow rate and is monotonic with flow rate. In some cases, a sensing circuit (not shown) may receive Vp 404 and Vn 402, and may perform some compensation and/or linearization before providing a flow sensor output signal, if desired.

Figure 5 is a top view of an illustrative flow sensor die 500. The illustrative flow sensor die has an etched cavity 502 that extends under a membrane 504. The etched cavity 502 helps to thermally isolate the membrane 504 from the substrate 508 of the flow sensor die 500. The illustrative flow sensor die 500 includes a slit 510 that extends transverse across the membrane 304, but this is not required. During use, the illustrative flow sensor die 500 is positioned in a flow channel.

To help explain the operation of the flow sensor die 500, it is assumed that fluid flows over the flow sensor die 500 in the direction indicated by arrow 512.

When so provided, the two upstream resistive elements RU 1 and RU2 are positioned on the membrane 504 upstream of the slit 510, and the two downstream resistive elements RDl and RD2 are positioned on the membrane 504 downstream of the slit 510. Note, there is no separate heater resistor Rh positioned between the upstream resistive elements RU1 and RU2 and the downstream resistive elements RDl and RD2. The illustrative flow sensor die 500 shown in Figure 5 is one possible layout of the schematic circuit diagram shown in Figure 4, with the corresponding nodes indicated (A-B, E-H and K-L). The illustrative flow sensor die 500 also does not include heater control circuitry.

The illustrative flow sensor die 500 does not include the connection between nodes H-L, the connection between nodes -G, the connection between nodes E~B, or the connection between A-F. This flow sensor die 500 is considered a test die, and these connections are intended to be made external to the flow sensor die 300 itself. In some cases, these connections may be made on the flow sensor die 500. To further reduce the size of the membrane 504, and thus the flow sensor die 500, it is contemplated that the two upstream resistive elements RU 1 and RU2 may be moved closer to the two downstream resistive elements RDl and RD2 that is shown in Figure Figure 6 is a chart showing sensitivity (differential bridge output) versus flow rate for a prior art flow sensor die such as that shown in Figure 3 at various heater voltages. The bridge voltage was at 2.4 volts. As can be seen, the sensitivity at heater voltages of 1.5-2.0 volts produces a sensitivity (differential bridge output) in the range of about 96-134 mv at flow rate of about 200. Figure 7 is a chart showing sensitivity (differential bridge output) versus flow rate of a flow sensor die with four self heating resistive elements RU1, RU2, RD1 and RD2 such as shown in Figure 5 at various bridge supply voltages. As can be seen, a similar sensitivity (differential bridge output) can be achieved to that shown in Figure 6 by increasing the bridge voltage (VDD) to about 10-14 volts. Notably, the chart shown in Figure 7 assumes that the resistance of the resistive elements RU1, RU2, RD1 and RD2 is the same as the resistance of the resistive elements RU1, RU2, RD1 and RD2 for the chart of Figure 6 (2.4 K ohms). To reduce the bridge voltage that is required in Figure 7, it is contemplated that the resistance of the resistive elements RU1, RU2, RD1 and RD2 of Figure 4-5 may be reduced, such to 500 ohms (e.g. 300-900 ohms). This may allow each of the resistive elements RU 1, RU2, RDl and RD2 to produce a similar amount of heat but at a lower bridge voltage. It is believed that this should result in a similar sensitivity (differential bridge output) to that shown in Figure 6 and at a similar bridge voltage (e.g. 2.4 volts).

The disclosure should not be considered limited to the particular examples described above. Various modifications, equivalent processes, as well as numerous structures to which the disclosure can be applicable will be readily apparent to those of skill in the art upon review of the instant specification.