1. ECE5320 Mechatronics Assignment#01: Literature Survey on Sensors and Actuators Topic: Hot Wire Anemometer
Prepared by:
Michael Wood
Dept. of Electrical and Computer Engineering
Utah State University
; T: (435) ;
F: (435) (ECE Dept.)
2/02/09

2. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Outline
Reference list
To explore further
Major applications
Limitations
Illustration of Hot Wire Probe
Pros and Cons
Constant Temperature Hot Wire Anemometer
Wheatstone Bridge Configuration
Using current to find flow rate
Calibration
Good to Know
Probe Cost Examples
Major Specifications
Where to buy
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

3. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Reference list
Zou Yue
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. To explore further (survival pointers of web references etc)
Virginia Tech Department of Aerospace and Ocean Engineering, Aerospace Engineering Lab Notes: Hot Wire and Hot Film Anemometry,
Perry, A., Hot Wire Anemometry, Clarendon Press, Oxford, 1982.
Payne, S., Unsteady Loss in a High Pressure Turbine Stage, Chapter 4: Hot Wire Anemometry, DPhil thesis, University of Oxford,
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Major applications
Measuring velocity of fluids
Aerodynamics – lift, drag
Combustion – IC, gas turbine engines
Meteorology
Fires and fire safety
Ocean currents
Turbulence
Ordinary measurement tools, i.e. HVAC probes
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

6. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Limitations
Very low speed (air flow speed lower than 1 m/s)
Highly turbulent and possibly reversed flow
Non-isothermal flows
Multi-phase flows
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

7. Illustration of Hot Wire Probe
Picture from
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

8. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Pros and Cons
 • Pros:  
- Excellent spatial resolution.  
High frequency response, > 10 kHz (up to 400 kHz).   
• Cons:  
Fragile, can be used only in clean gas flows.  
Needs to be recalibrated frequently due to dust accumulation (unless the flow is very clean).  
High cost.
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

9. Constant Temperature Hot Wire Anemometer
Most Common type of Hot Wire Anemometer
Accurate over a large range of fluid velocities (very slow to very fast fluid speeds)
Placed in a Wheatstone bridge configuration to assure accurate data
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

10. Wheatstone bridge configuration
R1 & R2 are known
R3 is variable
Probe represented by Rw
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

11. How Wheatstone bridge Works
The equation
balances wheatstone circuit
Making error voltage=0
Rw is a function of temperature
As the air speed around the probe changes Rw changes
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

12. How Wheatstone bridge Works
The Wheatstone bridge must
be calibrated
R3 is adjusted until the bridge
is in equilibrium
After calibration a change in
Velocity changes Rw and creates an error voltage
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

13. Using current to find flow rate
The error voltage inputted into the op-amp causes the op-amp to produce a feedback
The feedback current balances the Wheatstone bridge
This feedback current is measured
and used to calculate fluid flow
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

14. Using current to find flow rate
Constants used in the derivation I = current through the hotwire Rw = resistance of the hot wire Rg = resistance of the wire at gas temperature E2 =square of the output of the hot wire anemometer bridge U = calibration velocity N = exponent (usually close to 0.5) As = surface area of exposed wire
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

15. Using current to find flow rate
HG - heat generated
HT - heat transfer
HA - heat absorbed (assumed to = zero)
HG = HT = (I is measured)
Rw is the resistance at temperature qw and is found with the equation
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

16. Using current to find flow rate
C = temperature coefficient of resistivity Qo = initial wire temperature Ro = resistance at qo disregard high order and using boundary conditions Ro = Rg and qo = qg yields Dq = (Rw-Rg)/RoC
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

17. Using current to find flow rate
Dq = (Rw-Rg)/RoC
Rg = wire resistance when wire temp = fluid temp
Dq = difference between wire temp and fluid temp
Now we use the emperical heat transfer equation
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

18. Using current to find flow rate
More necessary constants
h = convective heat transfer coefficient
d = characteristic length (wire diameter)
k = fluid thermal conductivity
m = dynamic viscosity of the gas
r = gas density
cp = specific heat of gas at a constant temp
U = velocity of the flow
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

19. Using current to find flow rate
Assuming convection only HT=hAsDq = Where X and Y come from R = Rw/Rg (Kings Law)
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

20. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Sensor Calibration
To calibrate the sensor I2 is plotted vs
A best fit algorithm is used to find A and B of the equation
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

21. Measurement Errors to be accounted for during system calibration
1. Calibration measurement errors: Errors in measuring the calibration flow parameters and hot wire voltages.
2. Calibration equation errors: Errors due to the fitting of a calibration equation, as well as the solution of the calibration equation and lookup table.
3. Calibration drift errors: Errors caused by variations in calibration over time and due to switching the feedback circuitry on and off, as well as by probe contamination.
4. Approximation errors: Errors caused by assumptions about the flow field that are used to solve the calibration equations.
5. High frequency errors: Errors caused by the change in hot wire behavior at high frequency.
6. Spatial resolution errors: Errors caused by spatial averaging of the flow field.
7. Disturbance errors: Errors caused by the probe interfering with the flow field.
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

22. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Good to Know
Anemometer wires are usually made of platinum or tungsten and is 4 ~ 10 µm in diameter and 1 mm in length.
Typical commercially available hot-wire anemometers have a flat frequency response (< 3 dB) up to 17 kHz at the average velocity of 9.1 m/s , 30 kHz at 30.5 m/s , or 50 kHz at 91 m/s .
The small fragile wire is suitable only for clean gas flow.
In liquid flow or rugged gas flow,
a platinum hot-film coated on a
25 ~ 150 mm diameter quartz fiber
or hollow glass tube can be used instead.
quartz fiber or glass tube
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

23. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Probe Cost Examples
Single Sensor Probes Double Sensor Probes
Triple Sensor Probe
Note:
Eur = 1.29 US Dollars
(according to Google Currency Conversion)
examples from
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

24. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Major Specifications
Technical data for miniature wire sensors
Medium Air
Sensor material Platinum-plated tungsten
Sensor dimensions 5 µm dia, 1.25 mm long
Sensor resistance R20 (approx) W
Temperature coefficient of resistance
(TCR) a 20 (approx.) %/°C
Max. sensor temperature °C
Max. ambient temperature °C
Max. ambient pressure Depends on the type of mounting
Min. velocity m/s *
Max. velocity m/s
Frequency limit fcpo (CCA mode, 0 m/s) 90 Hz
Frequency limit fmax (CTA mode) kHz
This example of Specification is for a miniature wire sensor from
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ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

25. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators
Where to buy
4/17/2017
ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators