No Oil Droplets Formation in Thermopile Flow Sensors

Used in Internal Combustion Engines

Tu XianZheng

Hot-film air mass sensors are commonly used for measuring an air-mass flow, which include a resistor for heating and one or two other resistors for temperature sensing. It has been reported that when hot film air mass flow sensors are used directly in the intake tract of the internal combustion engine or in a bypass channel to the intake tract of the internal combustion engine, oil may deposit on the sensor chips and, in particular, on the sensor diaphragms during operation or shortly after the internal combustion engine has been shut off. This oil deposit may result in undesirable effects on the measuring signal of the sensor chip, in particular because an oil film affects the thermal conductivity of the sensor chip surface, which results in corruption of the measuring signals or a signal drift.

As well known, condensation can produce water droplets on the outside of soda cans or glasses of cold water. When warm air hits the cold surface, it reaches its dew point and condensed. As result droplets of water leave on the glass or can.

It is the same that when a thermal air mass flow sensor is operated at the border region of the heated measuring areas oil accumulates and over time results in oil droplets. The air flow drives the oil droplets on the surface up to the boundary of the heated measuring area, at which a stronger temperature gradient appears. The strong temperature gradient exerts a force opposite to the force exerted by the air flow. Oil droplets thus accumulate on the boundary line, which, when they reach a certain size, may be entrained again by the air flow to then contaminate the surface of the measuring area. In addition to the oil droplets, other contaminants also reach the surface of the measuring area due to this effect.

How to solve this problem? Back to the above mentioned condensation. Condensation of water occurs when water vapor within the air cools enough in order to change into the liquid state. A good example of condensation often occurs in the home during winter time, when water droplets form on the surfaces of cold windows. If open the window to let the cold air enter the room there will no any water droplets form on the surface of the window. So the only way to solve the problem is to reduce the operation temperature of the sensor. This can be done using a thermopile air mass flow sensor instead of a hot film air mass flow sensor.

A thermopile is an electronic device that converts thermal energy into electrical energy.

It does not respond to absolute temperature, but generates an output voltage proportional to a local temperature difference or temperature gradient. A thermopile air mass flow sensor is constructed with a heater for heating and several thermocouples for temperature sensing. The thermopiles are in series and so the output voltage due to temperature change is summed and increased over that of a single thermocouple.

With this advantage, POSIFA has developed thermopile air mass flow sensors with two group thermopiles positioned at the two side of the resistor heater and each group consisting of 40 thermopiles. Since the summed output of each group is so great that the operation temperature of the heater can be reduced as low as no condensation to take place. For example, the operation temperature can be set 10 to 20 degree Celsius higher than the air temperature. In this case condensation of oil vapor in the air is almost impossible.

Single Silicon Wafer Micromachined Thermal Conduction Sensor 

US Patent Application (20150097260)

Xiang Zheng Tu

A single silicon wafer micromachined thermal conduction sensor is described. The sensor consists of a heat transfer cavity with a flat bottom and an arbitrary plane shape, which is created in a silicon substrate. A heated resistor with a temperature dependence resistance is deposed on a thin film bridge, which is the top of the cavity. A heat sink is the flat bottom of the cavity and parallel to the bridge completely. The heat transfer from the heated resistor to the heat sink is modulated by the change of the thermal conductivity of the gas or gas mixture filled in the cavity. This change can be measured to determine the composition concentration of the gas mixture or the pressure of the air in a vacuum system.

Sigma-Delta Conversion Liquid Flow Switch Circuit

 Xiang Zheng Tu

The present circuit comprises all sigma-delta modulator necessary elements. Among them are an integrator, a comparator, a 1-bit DAC, and a summing junction. The integrator function is acted by a capacitor. The comparator plays as a 1-bite quantizer, which is built in a microcontroller. The 1-bite DAC is a PWM outputted from the microcontroller, which is resulted by filtering and emerging of the output of the comparator. The summing junction combines the input signal and the PWM output, which is carried out by a thermal flow sensor.

The thermal flow sensor is micromachined to have a resistor heater and one or two thermopiles on a suspended bridge created in a silicon substrate. The heater is located at the central region of the bridge, where the temperature is raised above the environment temperature by heating the heater. The hot and cold junctions of the thermopile are located near the heater and the surrounding silicon frame, respectively. The temperature difference between the central region and the sounding frame is measured by the thermopile utilizing the Seebeck effect in which a thermal electromotive force is generated in proportion to the temperature different.

It should be noted that the thermal flow sensor is an inherent low pass filter. When the heater is heated by a frequency change of the PWM output, the thermopile will take some time to respond. A model of the response of the sensor can be based on a simple heat transfer analysis. The rate at which the sensor exchanges heat with its environment must equal the rate of change of the internal energy of the sensor. Since in a fluid, the dominant mechanism of heat exchange is convection (neglecting conduction and radiation), the energy balance is

hA(T∞ − T) = mc(dT/dt) ,    (1)

where h is the convection coefficient, A is the surface area of the sensor, T is the temperature, m is the bridge mass, and c is the bridge heat capacity.

Writing equation (1) in the form

τ (dT/dt) + T = T∞ ,         (2)

where the time constant is τ = mc/hA, which is the response of the sensor reaching 63.2% of its final value. It has been obtained that the time constant of the sensor is about 1ms or the cut-off frequency of the sensor is about 1kHz.

In operation of the present circuit, the capacitor integrates an input signal, so its output passes a threshold voltage established with the comparator and the voltage reference. The input signal is provided by the thermopile of the thermal flow sensor with its heater by applying the PWM outputted from the microcontroller, which is added a negative feedback signal from the output of the comparator. The added signal is converted by the capacitor to a voltage that is presented to one of the two inputs of a comparator. When the voltage passes the reference voltage of the comparator the output of the comparator toggles between high and low. The output is fed back to the input of the sensor via the PWM. Additionally, the output of the comparator is fed forward to the digital filter of the microcontroller. With time, the output of the digital filter provides a bit stream result.

With the present circuit, the thermal flow sensor can be operated in a constant temperature mode. The reference voltage expresses the constant temperature that the heater is expected to be heated. In order to do this, the PWM should be adjusted carefully so that the thermopile produces the output exactly equal to the reference voltage. Since the performance of the sensors may have dispensability, zero offset is needed before fluid flow measurement. A bit stream produced in the zero offset process represents no fluid flow. When a fluid flow is conducted the temperature of the heater is reduced due to the fluid flow cooling effect. The PWM needs to be adjusted again as to get the temperature of the heater back to the original value.

The present circuit is an inexpensive and high resolution solution to the thermal flow sensors for countless applications. In the circuit, the conversion from analog to digital is performed with the internal-voltage reference, comparator, and two counters in the microcontroller. These internal-microcontroller analog peripherals, along with a thermal flow sensor, are used to complete the implementation of a first-order modulator. This modulator is then combined with an output-digital filter, which also is implemented in the microcontroller unit, to complete the circuit. Consequently, the only components external to the microcontroller are a thermal flow sensor, several resistors, and a capacitor.

Electronic Cigarette with Thermal Flow Sensor Based Controller

United States Patent Application (20150173419)

Xiang Zheng Tu

Electronic cigarette emits doses of vaporized nicotine that are inhaled. It has been said to be an alternative for tobacco smokers who want to avoid inhaling smoke.

Tobacco smoke contains over 4,000 different chemicals, many of which are hazardous for human health. Death directly related to the use of tobacco is estimated to be at least 5 million people annually. If every tobacco user smoked one pack a day, there would be a total of 1.3 billion packs of cigarettes smoked each day, emitting a large amount of harmful tar, CO and other more than 400gas contents to homes and offices, causing significant second-hand smoking damages to human health.

In order to overcome these problems, people have invented many new technologies and products, such as nicotine patches, nicotine gum, etc. Recently, several new inventions have been made, including the following U.S. Pat. Nos. 5,060,671; 5,591,368; 5,750,964; 5,988,176; 6,026,820 and 6,040,560 disclose electrical electronic cigarettes and methods for manufacturing an electronic cigarette, which patents are incorporated here by reference.

The electronic cigarettes currently are available on the market. Most electronic cigarettes take an overall cylindrical shape although a wide array of shapes can be found; box, pipe styles etc. Most are made to look like the common tobacco cigarette. Common components include a liquid delivery and container system, an atomizer, and a power source. Many electronic cigarettes are composed of streamlined replaceable parts, while disposable devices combine all components into a single part that is discarded when its liquid is depleted.

These cigarette substitutes cannot satisfy habitual smoking actions of a smoker, such as an immediacy response, a desired level of delivery, together with a desired resistance to draw and consistency from puff to puff and from cigarette to cigarette. It is desirable for an electronic cigarette to deliver smoke in a manner that meets the smoker experiences with more traditional cigarettes so that it can be widely accepted as effective substitutes for quitting smoking. 

An objective of the present invention is to provide a thermal flow sensor based electronic cigarette that overcomes the above-mentioned disadvantages and provides a cigarette that looks like a normal cigarette and smokes like a normal cigarette. The thermal flow sensor based controller comprises a housing; a battery, a controller assembly consisting of a thermal flow sensor and an application-specific integrated circuit (ASIC) which is disposed in the housing and connected with the battery and the thermal flow sensor electrically; an air inlet for allowing air to enter into the housing, a mouthpiece for allowing user to suck on the housing; a fluid reservoir; an atomizer consisting of a coil heater, wherein the coil heater is arranged on the outside of an atomizer; at least a light emitting diode; and a display.

The thermal flow sensor is fabricated using Micro-Electro-Mechanical Systems (MEMS) technologies.

In a first embodiment the thermal flow sensor composes of a resistive heater and a thermopile, wherein the thermocouples of the thermopile are perpendicular to the resistive heater and the hot contacts of the thermopile and the resistive heater lie on a stack layer consisting of a porous silicon layer and an empty gap, which recessed in a silicon substrate and provides local thermal isolation from the silicon substrate and the cold contacts of the thermopile lie on the bulk portion of the silicon substrate.

In a second embodiment the thermal flow sensor composes of two parallel resistive heaters and two thermopile, wherein the thermopiles dispose on two opposite sides of the resistive heaters respectively and the thermocouples of the two thermopiles are perpendicular to the resistive heaters and the hot contacts of the thermopiles and the resistive heaters lie on a stack layer consisting of a porous silicon layer and an empty gap, which are recessed in a silicon substrate and provides local thermal isolation from the silicon substrate and the cold contacts of the thermopiles lie on the bulk portion of the silicon substrate.

The thermal flow sensor is installed in the housing with its longitudinal direction perpendicular to the resistive heater(s) so that when there is no air flow through the housing, the temperature profile around the resistive heater(s) is symmetric and when an air flow is produced by a smoker inhalation, the temperature profile will shift from the up flow direction to the down flow direction, which represents the temperature change coursed by the air flow and can be detected by the thermopile(s) of the sensor so that an electrical signal is generated which represents the rate of the air flow.

[0012] An advantage of the present invention is that the thermal flow sensor based controller is able immediately to response to the air flow caused by a smoker inhalation or is able to response in about 5 ms to the air flow caused by a smoker inhalation.

Another advantage of the present invention is that the thermal flow sensor based controller can be operated in pulse heating mode in which the power consumption can be as low as in the range of 0.01 to 10 mw in which the low power consumption can be used in sleep mode and the high power consumption can be used in normal working mode.

Another advantage of the present invention is that the thermal flow sensor based controller has high dynamic range and can measure air volume flow rate from 0.01 to 100 liter/min so that the airway for air flow caused by a smoker inhalation can be configured without any constriction to provide a flow resistance which makes the smoker feel like to smoke a real tobacco cigarette.

Still another advantage of the present invention is that the thermal flow sensor based controller can be configured to: receive the output voltage representing the air flow rate from the amplifier which is produced by a smoker inhalation, determine a heating current that is used to heat the coil heater of the atomizer, and deliver an amount of the fluid vapor generated by the heating the coil heater of the atomizer which is wanted by the smoker regardless of a hard inhalation or a weak inhalation and a longer inhalation or a short inhalation.

Still another advantage of the present invention is that the thermal flow sensor based controller can be configured to: receive the output voltage representing the air flow rate from the amplifier which is produced by a smoker inhalation, determine a drive current that is used to drive the light emitting diodes, and deliver the drive current to the light emitting diodes so that the light emitted by the light emitting diodes can be gradually bright or gradually faded or flashing or intermittent.

Still another advantage of the present invention is that the thermal flow sensor based controller can be configured to: receive the output voltage representing the air flow rate from the amplifier which is produced by a smoker inhalation, calculates the amount of nicotine evaporated in each inhalation and over period time, and displays the total amount of nicotine in a over period time which is inhaled by the smoker.

Still another advantage of the present invention is that the thermal flow sensor based controller can be configured to receive the output voltage representing the air flow rate from the amplifier which is produced by an accident event such as mechanical vibration or temperature changes, and determine no heating current to heat the coil heater of the atomizer since there is no real smoker inhalation to take place. 

Wireless Fluid Flow Sensing Circuit Using Zero offset Thermal Flow Sensor

Xiang Zheng Tu

It should be pointed that the above circuit comprises a negative feed-back loop consisting of a heater, one or two thermopiles, both are integrated on a thermal insulating bridge, an amplifier, and a microcontroller. The heater and the thermopiles are the elements of a thermal flow sensor and they work in conjunction for sensing fluid flows. The amplifier is built in the microcontroller. A reference voltage provided by the microcontroller is sent to the amplifier. When the heater is heated by a starting PWM voltage provided by the microcontroller the thermopiles convert the temperature difference between the bridge of and the environment into a voltage sending to the amplifier for suppressing the reference voltage.

If the flow is zero, the output voltage of the thermopiles and the reference voltage should be equal. This can be realized in the following way. The different voltage between the reference voltage and the output voltage of the thermopiles is first amplified. Then the output voltage of the amplifier sends to the microcontroller for A/D conversion and digital processing. As a result, the starting PWM voltage is modified so as to make the output voltage of the thermopiles eventually reach the reference voltage. The digital number used to produce new PWM voltage represents a chosen temperature for the operation of the thermal flow sensor. It also indicates the flow velocity being zero.

When a flow is applied, the output voltage of the thermopiles may be higher or lower than the reference voltage. Another new digital number is obtained by amplification of the amplifier and digital processing of the microcontroller. So the heater is heated by another new PWM voltage driving the thermopiles to generate an output voltage equaling to the reference voltage. At the same time the sensor is operated back to the chosen temperature. The new output digital number of the microcontroller expresses the applied flow velocity.

With the negative feed-back loop, the operation of the sensor can be maintained at a constant temperature above that of the flow. So zero-offset can be realized without need for low-offset amplification. It also enables the sensor to have faster response since the temperature of the sensor is no longer modified by the flow. A further advantage is that small thermal asymmetries introduced during the sensor fabrication process can be automatically compensated. 

As shown in the above circuit, CC2540 combines a RF transceiver with an industry-standard enhanced 8051 MCU. It is suitable for wireless sensor modules where very low power consumption is required. A main problem with its MCU is lack of PWM output. Fortunately, it has two general-purpose timers, which can be used for creating a PWM interrupt generator. 

MEMS Thermal Boundary Layer Flow Sensors

Xiang zheng Tu

A thermal boundary layer flow sensor is a heated micromachined sensor resided on a side wall of a tube, which is used to measure the velocity of a fluid flowing through the tube. It can be described more detail by referencing the above picture.

The picture shows a fluid such as air flowing through a tube, for example, a manifold of a mass flow meter. A MEMS sensor is resided on the side wall of the tube. The sensor consists of a thermal insulating bridge, a resistor heater, and two thermopiles. The bridge is heated up to a temperature higher than the fluid temperature by applying a current passing through the heater. So not only a velocity boundary layer is formed on the wall, but also a thermal boundary layer is formed on the surface of the sensor.

Newton’s Law of cooling states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings, which. can be expressed by the equation:

                                                        q_s = h (T_s -T)                                    (1)

where q_s is the heat flux from the bridge of the sensor, h is the convection coefficient, and Ts and T∞ are the temperatures of the bridge and the fluid, respectively.

About heat transfer close to walls in laminar flows, André Lévêque introduced the very reasonable assumption that for fluid flows of large Prandtl number, the temperature transition from surface to free stream takes place across a very thin region close to the surface. Therefore, in this region the change in velocity can be considered linear with normal distance from the surface.

According to André Lévêque’s assumption, the velocity of the fluid is zero at the wall and the velocity profile is approximated as being linear very close to the wall. As a result, the heat transfer from the surface of the bridge to the flow stream adjacent to the surface is by pure conduction. Thus, the convection coefficient can be expressed as:

                             h = q_s / ((T_s -T) = -k (ծT / ծY)_y=0 ((T_s - T)                (2)

where k is the thermal conductivity coefficient of the fluid.

Energy equation for flow over an isothermal flat plate has been solved by Blasius for numerous values of Prandtl number. For Pr>0.6, the nondimensional temperature gradient at the surface was found to be expressed as:

                           k (ծT / ծY)_y=0 = 0.332 Pr ^1/3 ((T_s -T)   (u /x)^1/2           (3)

Substituting this relation into equation (2) leads to

                                     h = 0.332 Pr ^1/3 k (u /x) ^1/2                                   (4)

That is, the convection coefficient h or the heat flux q_s is proportional to (u) ^1/2. It can be concluded that the heat lose or the temperature change of the heated bride of the sensor is caused by the flows of the fluid. The velocity of the flows can be deduced by measuring the temperature change of the bridge.

In order to measure the change of the temperature of the bridge two thermopiles are arranged on the two opposite sides of the heater of the sensor. They are composed of several thermocouples connected in series. Each thermocouple consists of an aluminum stripe and a polysilicon stripe, which are connected together. The Seebeck effect drives the two different stripes to generate a voltage related to a temperature difference.

Generally, MEMS thermal boundary flow sensors are miniaturized to allow maximal spatial resolution, minimal power consumption, highly dynamic response, and negligible flow interference. Particularly, the rugged design of the sensors minimizes the disturbance to the flow stream and provides an accurate reading of both smooth and turbulent flows. With such excellent performance the sensors are especially favorable for air mass flow meter applications. 

Pointer Devices Using Thermal Motion Sensors

Xiang Zheng Tu

 

Pointer devices are used for tracking movements of objects. They can be categorized as electromagnetic, acoustic, image-based, inertial systems, and optical types. The most common pointer device is the computer mouse detecting two-dimensional motion relative to a surface. Other applications include motion capture when producing animation in computer games, video production, movie production and virtual environments.

The present pointer is a new type of pointer devices. Its working principle is based on a thermal motion sensor. The thermal motion sensor comprises a suspended bridge created in a silicon substrate, a resistive heater, and two thermopiles both are formed on the surface of the bridge. It is easy to understand when the sensor moves the heat of the bridge generated by the heated resistor will transfer away by flowing air. The corresponding change of the output voltage of the thermopiles can be calibrated to the moving velocity of the sensor.

The laws of physics teaches that the temperature field generated by a moving heat source is asymmetry and able to be measured. In steady state, the vertical cross-sectional temperature field is a sequence of symmetry concentric circles each representing an isotherm on the lateral plane. When the heat source moves the vertical cross-sectional temperature field will be skewed towards down motion direction. The skewed lateral cross-sectional temperature field consists of a contracted half plane and an expended half plane both are divided by a line perpendicular to the motion direction. If a temperature sensor array is placed on the plane around the heat source, all isotherms can be reconstructed. A lot of useful information including the direction and velocity of the moving heat source can be extracted form the reconstructed plane isotherms.

Similarly, the pointers based on thermal motion sensors can be used to control movements of objects, people, or body parts. As an example, the above picture shows a hand-held pointer guiding an unmanned air vehicle. The hand-held pointer consists of three thermal motion sensor modules each locates on an axis of Cartesian coordinates and measures the velocity component of this axis, respectively. After conditioning and processing the measured signals send by wireless to a ground communication station for further processing. Then the processed signal transfers to the unmanned air vehicle so that the vehicle is able to coordinate its movement according to the hand-held pointer. 

The pointers using the thermal motion sensors can be explored wide range of applications for their small size, short response time, low power consumption, higher sensitivity to low velocity, and low cost. Among them is patient activity monitoring such as Parkinson’s disease assessment. The assessments often entail one to three silicon accelerometers, each designed to detect even slight motion in a single axis. Three of these devices can be mounted orthogonally to provide an accurate description of movement in three directions. Actually the thermal motion sensors are more suitable for such application because it directly measures velocity of the motion without integration of acceleration measured using the accelerometers.