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. 

Vacuum-Cavity-Insulated Thermal Flow Sensors

US20140069185 (US Patent Application)

Xiang Zheng Tu

This invention relates to a micromachined thermal flow sensor. Particularly, the invention relates to a micromachined vacuum-cavity-insulated flow sensor utilizing a rigid porous silicon membrane as its suspending structure and a micromachined vacuum-cavity as its thermal insulation structure. 

The vacuum-cavity-insulated thermal flow sensor comprising: a single crystal silicon substrate; a porous silicon wall with numerous vacuum-pores therein which is created in said silicon substrate; a porous silicon membrane with numerous vacuum-pores therein which is created in said silicon substrate, surrounded and supported by said porous silicon wall; a cavity with a vacuum-space therein which is created beneath said porous silicon membrane and surrounded by said porous silicon wall; a dielectric layer deposited on the surface of said silicon substrate which includes the surface of said porous silicon wall and said porous silicon membrane; a heating element disposed laterally passing through at a middle of said porous silicon membrane; two temperature sensing elements disposed parallel with said heating element at two opposite sides thereof; and three pairs of metal conducting strips with three pairs of metal bonding pads disposed opposite to two sides of said porous silicon membrane for respectively connecting said heating element and said temperature sensing elements to an external circuit therefore. 

The vacuum-cavity-insulated thermal flow sensor has the following features: 

• One feature of the vacuum-cavity-insulated flow sensor provided by the present invention is to create a vacuum-cavity in a silicon substrate so that there is no vertical thermal conduction from a heating element on a suspended porous silicon membrane over the vacuum-cavity to the silicon substrate.
• Another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the suspended porous silicon membrane is supported by a same thick porous silicon wall so that the detachment of the suspended porous silicon membrane from the silicon substrate is prevented.
• Still another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the suspended porous silicon membrane is thicker and more rigid so that it is able to withstand at least 7 atm.
• Still another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the process for fabricating the sensor is simple and compatible with a CMOS process except anodization in HF solution.
• Still another feature of the vacuum-cavity-insulated flow sensor provided by the present invention is that the vacuum and sealing of a cavity created in the sensor is automatically realized during a dielectric film deposition process without an additional specific processing step.

Wireless Infusion System with Liquid Flow Rate Self-Measuring Module

Xiang Zheng Tu

   

A large volume infusion can be done using gravity driving method. In this method, a spit need inserts into an infusion bottle containing a drug liquid. The spit need has two side channels, one channel connecting the bottle to atmosphere and the other connecting the bottle to the infusion line. In operation, air enters into the bottle and drives the liquid into the infusion line. The driving force is provided by the height of the liquid of the bottle. Although this method is manual and labor intensive, it offers some significant advantages. First, the use of gravity for a driving force is energy efficient. Second, the force is low, so the dangers of large volume infusions can be avoided. Third, the gravity infusions are allowed to use a low cost and readily available pressure cuff. Forth, a gravity administration is not capable of infusing much air into the infusion tube line, because the driving force goes to zero as the liquid empties. 

In order to keep the advantages of the gravity driving method and reduce its labor independence, a liquid flow rate self measuring module has been developed by POSIFA.

The module comprises a house with an inlet and an outlet tubes, a print circuit board, a thermal flow sensor, an amplifier, a microcontroller, a wireless communication IC, and a lithium battery. When air flows through the house the thermal flow sensor will measure the flow rate of the air and produce a corresponding electronic signal. After conditioning and processing, the signal will be further send to a wireless infusion monitor.

The thermal flow sensor is fabricated using MEMS technology based on silicon micromachining processes. It is well known that silicon and other materials adapted for MEMS are bio-compatibility. Many MEMS devices are implantable for medical applications. For example, MEMS implantable pressure sensors have been used for continuous monitoring in Glaucoma patients. MEMS accelerometers have been used in defibrillators and pacemakers. An implantable drug delivery system based on MEMS technology also has been designed as a platform for treatment in ambulatory emergency care.  Generally, the thermal flow sensor based liquid flow rate self measuring module can be not used only for gravity driving infusion, but also for pump driving infusion. It means that the thermal flow sensor is allowed to measure drug liquid flow rate without worry about cross contamination.

As shown in the above picture, a bottle filled with a drug liquid is hung up and inserted with a split needle. The module is installed on the air suction side of the split needle. The liquid flow out side of the split needle is connected to an infusion line. When the liquid flows into the infusion line, a similar volume air will flow into the bottle. According the laws of physics, the flow rate of the liquid is equal to the flow rate of the air. So the module has a liquid flow rate self-measuring function.

In the picture, the wireless infusion monitor locates at the central region of an infusion room. It receives the signals from all modules arranged in the room and then displays graphically them. Through the monitor, a nurse in charge of the infusion room can see the liquid volume, battery level and all the other infusion parameters online and can be instantly notified for clinical issues. Physicians can supervise the infusion progress and therapy related events like the requested and given bolus. The physicians also can view the patient’s infusion history, make decision on the evolution of the therapy, and change protocol online. Indeed with the module everything can be done in real time through the web.

Smart Phone Compatible Thermal Conductivity

Carbon Dioxide and Humidity Sensors

              

                        

A smart phone sensor has built-in more and more sensors to gather all kinds of data on who we are, what we are doing, and the world around us. Any sensors used in smart phones should be miniaturization, low power consumption, fast response, high accuracy, high reliability, and low cost. Micromachined thermal conductivity sensors provided by POSIFA can meet all these requirements. It has been used for hydrogen, vacuum, and methane measurement, and exhibited very high performance.

The device is fabricated with CMOS compatible micromachining processes. The sensor structure consists of a hot plate suspending over a cavity. A platinum resistor is located on the hot plate, which acts as both a heater and a temperature sensor. The sensor chip is installed in a metal casing using a high thermal conductivity epoxy adhesive. A metal mesh disposed on the metal casing provides gas exchange with the surrounding atmosphere. A photo graph of the encapsulated sensor can be seen in the above figure.

A thermal conductivity sensor measurement circuit is shown in the following figure. The circuit consists of two thermal conductivity sensors: a measurement sensor and a reference sensor, in which the measurement sensor is used for measuring the mixture gas containing air, carbon dioxide, and water vapor (humidity), and the reference sensor is sealed and used for canceling the signal generated by the pure air and the temperature of the surrounding atmosphere. The heaters of the sensors are heated by two DAC respectively, which are provided by a precision analog microcontroller such as ADuC7019/20 or the like. The differential voltage of the two sensor output is amplified by a chopper stabilized zero-drift operational amplifier, such as TP5554-TR. When zeroing, the measurement sensor is immersed in a reference air, the output of the two sensors will be the same and the output of the amplifier will be zero. When the measurement sensor is immersed in the mixture air containing carbon dioxide and water vapor, the output of the amplifier will correlate to the concentrations of the carbon dioxide and water vapor.

     

The working principle of the thermal conductivity sensor is easy to understand. It senses change of the thermal conductivity of a mixture air consisting of air, carbon dioxide, and water vapor, and compares it to the thermal conductivity of reference pure air. Since both carbon dioxide and water vapor have a thermal conductivity less than the thermal conductivity of a pure air, the thermal conductivity of the mixture air will be low containing carbon dioxide and a detectable signal will be produced.

According to Wassiljewa equation and empirical measurements, at fixed temperature t₁, a linear equation can be used for calibration and calculation of the concentration of each composition gas of a mixture air, which can shown as the follows:

y(t₁) = a(t₁)_air+a(t₁)_carN_car+a(t₁)_waterN_water.           (1)

In this equation (1), y is the output of the sensor response; a(t₁)_air is a bias term, N_car and N_water are the carbon dioxide and water vapor concentrations, and a(t₁)_car and a(t₁)_water are the coefficients sensitive to carbon dioxide and water vapor concentrations. The output is expressed in the unit of volt, and so are the coefficients, a(t₁)_car and a(t₁)_water, while the concentrations N_car and N_water are expressed as molar fractions. The equation (1) has two degrees of freedom, so N_car + N_water + N_air = 1, where N_air is the air concentration.

The coefficients a(t₁)_air, a(t₁)_car, a(t₁)_water can be obtained by measurement of several precision calibration sample mixtures. The data collected by the sensors can be processed using partial least squares (PLS). It has been shown that PLS can produce proper coefficient.

It should be noticed that N_car and N_water are two independent variables. It is impossible to find two variables by solving a single linear equation. Another similar linear equation (2) at fixed temperature t₂ needs to be built as follows:

y(t₂) = a(t₂)_air+a(t₂)_carN_car+a(t₂)_waterN_water.        (2)

In this equation the coefficients a(t₂)_air, a(t₂)_car, a(t₂)_water are different from the above similar coefficients, while N_car and N_water are the same. Based on the measured data collected by the thermal conductivity sensors, both N_car and N_water can be calculated by solving the equation (1) and equation (2).