Friday, October 21, 2016

Precise Water Delivery System for Coffee Machines
Xiang Zheng Tu

 
The above figure shows a precise water delivery system for coffee machines, which is provided by POSIFA Microsystems Company. The system comprises a cold water inlet, a water filter, a solenoid valve, a thermal flow sensor module, a one-way valve and a water heater. The cold water may be supplied by running water or a water container in which water flow is driven by water self weight. The cold water enters the inlet and then successively passes through the solenoid valve, the thermal flow sensor module, the one-way valve. Finally the cold water is heated in the water heater and the hot water is ready to flow into a spray head, and onto a ground coffee, which is contained in a brew basket mounted below the spray head.

On or off of the cold water flow is realized by the solenoid valve. The valve is controlled by an electric current through a solenoid. If the valve is open when the solenoid is not energized, then the valve is termed normally open (N.O.). Similarly, if the valve is closed when the solenoid is not energized, then the valve is termed normally closed. The solenoid is controlled by a switched circuit. A digital output is connected to the base of a transistor which controls the current to a normally open relay. When the relay coil is energized, it closes the contacts, which allows current from the DC supply to flow through the solenoid. When the solenoid coil is energized, the valve opens, allowing cold water to flow through the valve.


The digital output is send by the thermal flow sensor module. The thermal flow senor module measures the flow rate of the cold water and calculates an amount of cold water flowing through in a certain time by integration, which should equal to a desired amount of cups of finished coffee to be made. When the corresponding digital output matches the number of cups, the control circuit is open to the solenoid valve thus closing the valve and stopping the flow of cold water. As seen in the above figure, there is another similar circuit for controlling the water heater. Likewise, at the same time, another digital output is sent to the contactor of the circuit for shutting down the power supply to the water heater.


The thermal flow sensors of the module rely on the ability of fluid flows to affect thermal phenomenon by way of heat transfer that, in turn, is transduced into a varying electrical signal capturing the sensor response to flow change. The sensors are thermally isolated so only heat transfer due to flow can occur. Other heat transfer pathways such as through substrate or electrical leads result in thermal losses that degrade sensor performance and are minimized in the device design. The thermal flow sensors measure mass flow rate and response is independent upon a constant fluid temperature. 

Wednesday, October 12, 2016

Humidity Compensated MEMS Air Mass Flow Meters
Xiang Zheng Tu

Mass flow rate of air entering a fuel-injected internal combustion engine is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine. Air mass flow rate varies with the ambient absolute humidity, which means that a mass flow sensor should be in injunction with a humidity sensor for determining the quantity of intake air in each cylinder. That is hwy POSIFA Microsystems Company provides humidity compensated MEMS air mass flow meters.

The humidity compensated MEMS air mass flow meter is located ahead of a throttle body. After an air filter, the meter utilizes a MEMS thermal conductivity sensor measures the absolute humidity of air entering the throttle body. Then the entered air passes through a MEMS thermal mass flow sensor, which is incorporated in the same body and is used to measures the air mass flow rate. A microcontroller of the meter processes the data collected by the two sensors and provides a humidity compensated air (or dry air flow rate) to ECU.  

The combustion of gasoline or octane in pure oxygen follows this reaction:

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O                                                 (1)

This is so-called “on ratio” or “stoiechiometric” combustion. Molecular weights of the above reagents are C8H18 = 114, O2 = 32, CO2 = 44, H2O = 18. The ratio of mass of oxygen to mass of octane is 25 x 32 / mol / 2 x 114 /mol = 3.51, which means that 1 kg of octane reacts with 3.51 kg of oxygen to produce 3.09 kg of carbon dioxide and 1.42 kg of water.

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.
So in dry air the reaction is expressed as:

2 C8H18 + 25 (O2 + 3.7 N2) → 16 CO2 + 18 H2O                                (2)

The ratio of mass of air to mass of octane is 12.99. Therefore for octane, the dry air–octane mixture is 12.99 i.e. for every one gram of octane 12.99 grams of air is required.

Combustion process never runs stoichiometric. It always incorporates a modest amount of excess air - 10 to 20% more than needed to burn the gasoline completely.
If insufficient amount of air is supplied to engine, unburned fuel, soot, smoke, and carbon monoxide are exhausted from the engine. The results are heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion.

Like all thermal mass flow meters, humidity affects their output. Sine water vapor is added to the dry air, the total mass is increased and both the overall thermal conductivity and overall viscosity change. To correct the mass flow readings of the meters what percentage of the water vapor should be know.

The thermal flow sensor of the humidity Compensated MEMS air mass flow meter consists of a thermal insulating base, a resistive heater and two thermopiles. The heater is structured as a long stripe extending from one side of the base to the opposite side and the hot junctions of the two thermopiles are arranged along the two opposite sides of the heater respectively. The cold junctions of the two thermopiles are arranged along the two opposite edges of the base. As air flows by the sensor, molecules of the flowing air transport heat away from the sensor, the sensor cools, and energy is lost, which is governed by the equation as

Qt = ΔT [ k + 2 (k Vv ρ π d Vavg)1/2 ]                                                  (3)

Where:
qt = rate of heat loss per unit time
ΔT = mean temperature elevation of the thermal insulating base
d = width of the resistive heater
k = thermal conductivity of the air passing through the sensor
Cv = specific heat of the air passing through the sensoe
Ρ = density of the air passing through the sensor
Vavg = average velocity of the air passing through the sensor

In this equation, ρ, Vavg, qt, and ΔT are the unknowns, because they change with time while the other variables are known. However, qt and ΔT can be obtained through measuring devices, leaving in the product of ρ and Vavg and cross section area of the pipe.

The thermal conductivity sensor the humidity Compensated MEMS air mass flow meter is the same as the thermal flow sensor except that the thermal insulating base is replaced by a plate suspending over a cavity. The cavity is filled with a measured humidity air and by conduction the humidity air transports heat away from the sensor.

According to the Wassiljewa Equation the thermal conductivity of a humidity air can be expressed as:

kh = xd kd / (xd Ad + xw Aw) + xw kw / (xd Ad + xw Aw)                          (4)

Where:
x= mole concentration of dry air
xw = mole concentration of water vapor
kh = thermal conductivity of humidity air
kd = thermal conductivity of dry air
kw = thermal conductivity of water vapor
Ad, Aw = constants to be specified
It could be convenient to use linear least squares method for anglicizing the measurement data of the thermal conductivity sensor. The regression model can be expressed as:

Vout = β xd + βw xw                                                                                (5)

xd + xw = 1                                                                                             (6)

Where Vout is the output of the thermal conductivity sensor, β and βw are constants to be specified by experiments. 

Assuming:
mh = mass flow rate of humidity air which is measured by the thermal flow sensor
mw = mass flow rate of water vapor which is calculated using absolute humidity measured by the thermal conductivity sensor
The following expression can be established:

md = mh – mw                                                                                                         (7)

Where md = mass flow rate of dry air which is required by all combustion engines.

In conclusion, POSIFA Microsystems Company provides humidity compensated MEMS air mass flow meters which combine air flow rate and absolute air humidity measurements and directly output dry air flow rate without adding temperature and pressure measurements.

Wednesday, October 5, 2016


Natural Gas Calorific Meter with Two MEMS Sensors
 Xiang Zheng Tu

As shown in the above figure, a MEMS natural gas calorific meter mainly comprises a MEMS thermal flow sensor and a MEMS thermal conductivity sensor. The thermal flow sensor measures natural gas mass flow rate. Natural gas is a naturally occurring gas mixture, consisting of methane, ethane, propane and nitrogen. The thermal conductivity sensor measures the mole percentage of methane, ethane, propane and nitrogen in the natural gas flow. With the measured mass flow rate and each composition mole percentage the calorific flow rate and total calorific value displayed on the smart phone in the above figure can be calculated.

With the state-of-the-art electronics for the signal process, MEMS natural gas calorific meters have extended dynamic range, enhanced data safety and are easy for network and remote data transmission. They have automatic temperature and pressure compensation and directly provide calorific value. As the MEMS sensor is miniature, the sensor assembly including the electronic control board can be designed into a compact form that is substantially smaller than the mechanical counterpart. This could benefit for the reduction of the cost not only in manufacture but for overall gas distribution management.

The composition of natural gas can be determined based on the fact that the temperature curve of the thermal conductivity coefficient is unique for each natural gas mixture, but highly correlated. A multiple linear regression can be used to model the relationship between two or more explanatory variables and a response variable by fitting a linear equation to observed data. The model can be expressed as:

Y = β0 + β1 xEthane + β2 xPropone 3 xNitrigen                                                               (1)

xMethane + xEthane + xPropone  + xNitrigen= 1                                                                  (2)

Where Y is the sensor signal output, β0, β1, β2, β3 are the parameters of the regression equation, and xMethane,  xEthane,  xPropone ,  xNitrigen are each component mole fraction of natural gas.

These equations describe how the mean response Y changes with the explanatory variables. The observed values for Y vary about their means y and are assumed to have the same standard deviation σ. The fitted values estimate the parameters β0, β1, β2, β3 of the regression equations. Since the observed values for y vary about their means Y, the multiple regression models include a term for this variation. In words, the model is expressed as DATA = FIT + RESIDUAL, where the "FIT" term represents the expression β0 + β1 xEthane + β2 xPropone 3 xNitrigen . The "RESIDUAL" term represents the deviations of the observed values y from their means Y, which are normally distributed with mean 0 and variance σ. The notation for the model deviations is ɛ.

The thermal conductivity sensor is excited using three voltage steps V1, V2, V3, resulting in three different operation temperatures. Each operation temperature or driving voltage results a multiple linear regression as follows:

Yv1 = β0v1 + β1v1 xEthane + β2v1 xPropone 3v1 xNitrigen                                                  (3)

Yv2 = β0v2 + β1v2 xEthane + β2v2 xPropone 3v2 xNitrigen                                                 (4)

Yv3 = β0v3 + β1v3 xEthane + β2v3 xPropone 3v3 xNitrigen                                                  (5)

With measured Yv1, Yv2, Yv3, and estimated parameters β0v1……β3v3, the composition mole percentage of a natural gas coming from different sources can be determined in this way: First the sensor is excited by the three voltage steps V1, V2, V3 and each results a sensor signal outputs;
Then the three equations can be obtained for each exciting voltage;
Finally the equations are solved to find each mole fraction of the measured natural gas.

The calorific value of natural gas can be further calculated using the above measured data as flows:

Calorific flow rate =
( GMethane) (HVMethane) + (ṁG ethane2) (HVEthane) + ( GPropone) (HVPropone)          (6)                                  

GMethane = MWMethne  xMethane / (MWMethne  xMethane +MWEthane  xEthane
+MWPropone  xPropone +MWNitrigen  xNtrigen  )                                                                 (7)

GMethane = MWEthane  xEthane/ (MWMethne  xMethane +MWEthane  xEthane
+MWPropone  xPropone +MWNitrigen  xNtrigen  )                                                                (8)

GMethane = MWMethne  xMethane / (MWMethne  xMethane +MWEthane  xEthane
+MWPropone  xPropone +MWNitrigen  xNtrigen  )                                                                (9)

Where: = mass flow rate measured by the thermal mass flow sensor, in b/min
HVn = heating value of gas component n, in BTU/SCF
xn = mole fraction of gas component n. The table below contains the list of individual component LHV & HHV.
MW = Molecular weight of gas component n.


     
The higher heating value (HHV) refers to a condition in which the water is condensed out of the combustion products. The higher heating value includes the sensible heat and latent heat of vaporization especially for water. In other words, HHV assumes all the water component is in liquid state at the end of combustion.


The lower heating value (LHV), on the other hand refers to the condition in which water in the final combustion products remains as vapor (or steam); i.e. the steam is not condensed into liquid water and thus the latent heat is not accounted for.  The LHV assumes that the latent heat of vaporization of water in the fuel and the reaction products is not recovered.