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.