GB2138566A

GB2138566A - Thermal mass flow sensor for fluids

Info

Publication number: GB2138566A
Application number: GB08310273A
Authority: GB
Inventors: David William Satchell
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-04-15
Filing date: 1983-04-15
Publication date: 1984-10-24
Also published as: GB2138566B; GB8310273D0

Abstract

A mass flow sensor operates by measuring the cooling effect of a flowing fluid on a body 2. The body is made from etched silicon with integral p-n diode junctions, transistors or resistors as thermal sensors 8, 9 and a diffused resistor 10 as a heater. A feedback control circuit increases or decreases the heater current to maintain a predetermined temperature differential between the sensors 8 and 9 in a balanced bridge network. The thin silicon body gives fast response and the feedback control circuit gives high accuracy suitable for monitoring the fuel/air mixture in a carburettor. <IMAGE>

Description

SPECIFICATION Thermal mass flow sensors This invention relates to thermal mass flow sensors.

Thermal mass flow sensors are known which can be used in a system to measure gases on the basis of mass flow. The sensor is located in a field flow path and produces an output signal which is dependent on flow rate and which can be used to compute flow rate and for indicating, recording or control purposes.

One known device is the Brooks Mass Meter made by Emerson Electric UK Ltd of Stockport in Cheshire. The operating principle is thermodynamic. A precision power supply directs constant heat to the midpoint of a sensor tube carrying the gas. On the same tube, equidistant upstream and downstream of the constant heat input are resistance temperature measuring elements. With no flow the heat reaching each temperature element is the same. With increasing flow the flowing stream carries an increasing amount of heat towards the downstream element and away from the upstream element. An increasing temperature difference developes between the two elements, and this difference is proportional to the amount of gas flowing or the mass flow rate. A bridge circuit interprets the temperature difference and an amplifier provides an output to an indicating meter.

The sensor comprises a sensor tube through which the gas flows and around which the heater is wound in the form of a wire and the temperature sensors are wound in the form of resistance wires which provide a significant resistance change with temperature.

This is an expensive device to manufacture.

Another mass flow meter is discussed in an IBM Technical Disclosure Bulletin Vol. 21 No. 8 January 1979, which refers to the device first discussed. In a measuring chamber the tube and the three coils are replced by a silicon (Si) strip in which the three coils are embedded by the planar diffusion of a resistor structure and by a suitably shaped base plate defining the cross-section of the flow channel. For scale adjustments, the base plate is made to be readily replaceable. The stripshaped silicon chip has three rectangular meander resistors. The silicon chip may be provided with a protective coating of SiO2, for example.

Use of the semiconductor strip instead of a metal tube eliminates corrosion of the tube and contamination of the gas, as well as slow thermal response, thus permitting automatic gas flow control. Volumes of as low as 0.5 ml per minute are stated to be measurable.

It is an object of the present invention to provide a cheaper and more effective mass flow sensor.

According to the present invention there is provided a mass flow sensor for fluids which operates by measuring the cooling effect of a flowing fluid on a body, wherein the body comprises an etched silicon structure having at least one limb extending across a gap in the structure and having a thermal sensing element formed in the limb, the structure having a heater for heating the limb, wherein the thermal sensing element is connected to a control circuit the mass flow can be sensed or determined in dependence upon the cooling effect of the fluid on the thermal sensing element.

Preferably the sensor comprises a second limb extending across the gap and spaced from the first limb in the direction of fluid flow, the second limb carrying a second thermal sensing element, wherein when the second thermal sensing element is connected to the control circuit the mass flow can be sensed or determined in dependence upon a comparison with the cooling effect of the fluid in each thermal sensing element.

This enables ambient conditions to be compensated. Preferably then the heater and the first thermal sensing element are located near one another on the same limb. In this arrangement the control circuit comprises an electrical bridge including the or each thermal sensing element connected in respective arms of the bridge. The control circuit can very preferably have a feedback control loop which acts to increase or decrease a current supplied to the heater such as to maintain a predetermined temperature differential between the two thermal sensing elements.

Preferably the or each gap in the body is empty apart from the fluid being sensed. However it could be filled with a solid heat insulating material.

Preferably one or both limbs have a thickness of the order of 0.1 mm. Also each sensing element can using semiconductor processing techniques on the silicon, be formed by a p-n junction diode, preferably operated in reverse bias mode by the control circuit.

According to another aspect of the invention there is provided a mass flow sensor for fluids which operates by measuring the cooling effect of the flowing fluid on a body, wherein the body comprises an etched silicon structure having formed therein two thermal sensing elements and an electric heater, and comprising a control circuit having an electrical bridge with the elements connected in respective arms of the bridge and a feedback loop which maintains a predetermined temperature differential between the thermal sensing elements by increasing and decreasing the heater current in dependence upon fluid flow.

In order that the invention can be clearly understood, reference will now be made to the accompanying drawings in which: Fig. 1 shows somewhat schematically a thermal mass flow sensor according to an embodiment of the invention, and Fig. 2 is a block circuit diagram of a system incorporating the sensor of Fig. 1.

Referring to Fig. 1 there is indicated in dashed line a portion of a tube 1 through which a fluid (gas or liquid) will flow in the direction of the arrow A. A laminar body 2 is formed from a single piece of single-crystai silicon by selective etching.

As shown in the figures, holes 3, 4 and 5 are etched in the silicon so as to ieave gaps between two limbs 6 and 7 which bridge the flow. Thermal sensors 8 and 9, such as diodes, transistors or resistors, but in this embodiment p-n junction diodes, are formed in the limbs by normal semiconductor techniques of diffusion or implantation. One limb 6 also has a diffused resistor 10 formed by similar techniques, which is used to heat the limb a few degrees above ambient. By means of a feedback control circuit such as shown in Fig. 2 and including an operational amplifier 11, a current is driven through the resistor 10 for heating purposes, the feedback loop acting to stabilise the temperature differential between the two thermal sensor elements 8 and 9.The magnitude of this current is proportional to the heat loss and therefore to the rate of mass flow of the fluid. A current limiting device 1 2 protects against overheating in case of failure.

The sensors (diodes) 8 and 9 together with the resistors 13 and 12 form a balanced bridge in the quiescent state. As the fluid starts to flow more heat is lost from sensor 9 and the heater. The feedback loop in the control circuit increases the current i to increase the heat provided by the heater 10 so as to tend to maintain the temperature differential between the diode 9 and the diode 8 and thus restore the balance of the bridge.

The sensor element 8 acts as an ambient temperature compensator.

The gap 4 between limbs 6 and 7 can be filled with a heat insulating material such as plastics or expoxy resin. The outside limbs can be much thicker than the active limbs 6 and 7.

For example, the iimbs 6 and 7 can be as thin as 0.1 mm. The gap can be of the order of 1.0 mm and the outside limbs can be say 0.5 mm thick for added strength. The thinner the limbs 6 and 7 become the less will be their thermal inertia, giving a sensor which responds rapidly to change of flow.

The gap would be of the order of 1 mm but could be as small as 10 microns provided the fluid being measured is not likely to foul the gap and cause permanent bridging by foreign matter.

Preferably the diodes 8 and 9 are reverse biassed as shown, so that they provide an exponential response giving good sensitivity, yet the feedback loop in the control circuit maintains a linear overall response.

In an alternative embodiment the etched single crystal material can have three limbs, the first having the element 8, the second having the heater 10 and the third having the element 9, with the limb spacings and thicknesses much as described above with reference to Fig. 1. Instead of a feedback control loop the control circuit could operate in open fashion like the one which is used with the prior art device made by Emerson and discussed in the preamble.

The sensor described is suitable for use in monitoring the fuel/air mixture in an automotive carburettor.

Claims

1. A mass flow sensor for fluids which operates by measuring the cooling effect of a flowing fluid on a body, wherein the body comprises an etched silicon structure having at least one limb extending across a gap in the structure and having a thermal sensing element formed in the limb, the structure having a heater for heating the limb, wherein when the thermal sensing element is connected to a control circuit the mass flow can be sensed or determined in dependence upon the cooling effect of the fluid on the thermal sensing element.

2. A mass flow sensor as claimed in claim 1 comprising a second limb extending across the gap and spaced from the first limb in the direction of fluid flow, the second limb carrying a second thermal sensing element, wherein when the second thermal sensing element is connected to the control circuit the mass flow can be sensed or determined in dependence upon a comparison of the cooling effect of the fluid on each thermal sensing element.

3. A sensor as claimed in claim 2, wherein the heater and the first thermal sensing element are located near one another on the same limb.

4. A sensor as claimed in any preceding claim, wherein the or each gap is empty apart from the fluid being sensed.

5. A sensor as claimed in any of claims 1 to 3, wherein the or each gap is filled with a solid heat insulating material.

6. A sensor as claimed in claim 2 or any claim appended thereto, wherein the gap between the two limbs lies in the range 10 microns to 1 mm.

7. A sensor as claimed in any preceding claim, wherein the body is encapsulated in a heat insulating material.

8. A sensor as claimed in any preceding claim, wherein one or both limbs have a thickness of the order of 0.1 mm.

9. A sensor as claimed in any preceding claim, wherein the or each thermal sensing element is a p-n junction diode.

10. A sensor as claimed in claim 9, wherein the or each diode is operated in reverse bias mode.

11. A sensor as claimed in any preceding claim, in combination with the control circuit comprising an electrical bridge including the or each thermal sensing element connected in respective arms of the bridge.

12. A sensor as claimed in claim 1 wherein the control circuit has a feedback control loop and the body has the two thermal sensing elements, and the feedback control loop acts to increase or decrease a current supplied to the heater such as to maintain a predetermined temperature differential between the two thermal sensing elements.

1 3. A mass flow sensor for fluids which operates by measuring the cooling effect of the flowing fluid on a body, wherein the body comprises an etched silicon structure having formed therein two thermal sensing elements and an electric heater, and comprising a control circuit having an electrical bridge with the elements connected in respective arms of the bridge and a feedback loop which maintains a predetermined temperature differential between the thermal sensing elements by increasing and decreasing the heater current in dependence upon fluid flow.

14. A mass flow sensor for fluids substantially as hereinbefore described with reference to and as illustrated in Fig. 1 of the accompanying drawings.

1 5. A mass flow sensor for fluids as claimed in any preceding claim, in combination with a control circuit substantially as hereinbefore described with reference to and as iliustrated in Fig. 2. of the accompanying drawings.