Thermal Anemometer Design for automotive HVAC - ?
Posted: Sun Dec 09, 2007 11:49 pm
Hi, it's nice to finally post here, as I have spent much time on this site(N&V), running the updates every month prior to the new remodel, I love the site, and look forward to spending some time in here.
Here’s a hobby project that I’m working on, and I’m hoping that I can get some help on this forum.
Background:
The project requires an air velocity measurement; sensed in an automotive HVAC system, which by design has no portion long enough to use a differential pressure/pitot tube measurement method. The airflow was originally going to be measured at one speed and interpolated based on fan supply voltage. The system airflow resistance, however, is very dynamic (varies with temperature door position, and mode). Also, some other variables in the control scheme require using an accurate air flow rate to deduce some heat transfer coefficients, used in an overall heat balance control method. Taking all of this into account, I’ve decided measured air velocity is required(fan curves for these blowers are virtually non-existant).
Looking at the normally used anemometers, I have narrowed my choices to turbine, and heated wire/thermistor (or equivalent). I prefer the thermistor solution, as it introduces less flow restriction to the system, has no moving parts that may require periodic replacement/ maintenance.
There are two likely places in the flow-path that this sensor will be placed, either prior to, or after the evaporator. Before the evaporator will provide a more constant air temperature (varies with cabin temperature in recirc mode or with ambient temperature), placement after the evaporator will introduce large, rapid temperature changes as the compressor clutch engages and disengages, but will provide the added functionality of sensing temperature in a location that is not currently measured, and would be a useful parameter to have. I’m not sure if the effect of the evaporator cooling the air during compressor operation will degrade the airflow indication. Will it?
This is a shared project that involves an analog input/digital input and output device, connected via usb to a computer. Control is done with the computer software, which means:
Sensor output has to be 0Vdc-5Vdc.
The output of the anemometer does not need to be calibrated for cross sectional flow area and temperature, this will be done by adjusting constants in the software (part of an overall project directive to allow for use in many types of vehicles). Velocity is all that is required.
Expected air flow rates are somewhat unknown, but the suspected range is 0-500CFM. Cross sectional area range is expected to be 20 insq. to 200 insq.
Questions I do not yet know the answer to:
What exactly is the signal output of the thermal anemometer most proportional to? Mass flow or velocity past the heated sensing thermistor?
Do I need to adjust the output to get velocity, so that it can be used to obtain total airflow as mentioned above?
I’ve seen the circuit referenced at this site:
http://electronicdesign.com/Files/29/1978/Figure_01.gif
http://electronicdesign.com/Articles/In ... cleID=1978
-How are R6 and R7 ‘thermally coupled’?
-Is this PIC design overkill, considering the signal can be adjusted in software?
-Would the standard hot-wire method be better in this case? Where can I find an example?
Would ‘hacking’ an automotive MAF found on ebay (as long as it’s not a vortex pulse counter type) yield acceptable results?
What do I need to do to obtain linear output of the device? Noting that automotive MAF sensors, using the same technology have curves similar to this:
http://www.autoshop101.com/forms/h34.pdf
Am I not asking other important questions?
Any help would be much appreciated, I’m sorry for asking such complicated questions about such a simple concept, I just want to be sure I’m doing things right.
How would you do it?
Also with respect to placing the thermistors in a tube? Do they both go in the tube, where only one is exposed (with holes) to direct airflow?
Is there a good schematic for a simple one of these somewhere?
Here’s a hobby project that I’m working on, and I’m hoping that I can get some help on this forum.
Background:
The project requires an air velocity measurement; sensed in an automotive HVAC system, which by design has no portion long enough to use a differential pressure/pitot tube measurement method. The airflow was originally going to be measured at one speed and interpolated based on fan supply voltage. The system airflow resistance, however, is very dynamic (varies with temperature door position, and mode). Also, some other variables in the control scheme require using an accurate air flow rate to deduce some heat transfer coefficients, used in an overall heat balance control method. Taking all of this into account, I’ve decided measured air velocity is required(fan curves for these blowers are virtually non-existant).
Looking at the normally used anemometers, I have narrowed my choices to turbine, and heated wire/thermistor (or equivalent). I prefer the thermistor solution, as it introduces less flow restriction to the system, has no moving parts that may require periodic replacement/ maintenance.
There are two likely places in the flow-path that this sensor will be placed, either prior to, or after the evaporator. Before the evaporator will provide a more constant air temperature (varies with cabin temperature in recirc mode or with ambient temperature), placement after the evaporator will introduce large, rapid temperature changes as the compressor clutch engages and disengages, but will provide the added functionality of sensing temperature in a location that is not currently measured, and would be a useful parameter to have. I’m not sure if the effect of the evaporator cooling the air during compressor operation will degrade the airflow indication. Will it?
This is a shared project that involves an analog input/digital input and output device, connected via usb to a computer. Control is done with the computer software, which means:
Sensor output has to be 0Vdc-5Vdc.
The output of the anemometer does not need to be calibrated for cross sectional flow area and temperature, this will be done by adjusting constants in the software (part of an overall project directive to allow for use in many types of vehicles). Velocity is all that is required.
Expected air flow rates are somewhat unknown, but the suspected range is 0-500CFM. Cross sectional area range is expected to be 20 insq. to 200 insq.
Questions I do not yet know the answer to:
What exactly is the signal output of the thermal anemometer most proportional to? Mass flow or velocity past the heated sensing thermistor?
Do I need to adjust the output to get velocity, so that it can be used to obtain total airflow as mentioned above?
I’ve seen the circuit referenced at this site:
http://electronicdesign.com/Files/29/1978/Figure_01.gif
http://electronicdesign.com/Articles/In ... cleID=1978
-How are R6 and R7 ‘thermally coupled’?
-Is this PIC design overkill, considering the signal can be adjusted in software?
-Would the standard hot-wire method be better in this case? Where can I find an example?
Would ‘hacking’ an automotive MAF found on ebay (as long as it’s not a vortex pulse counter type) yield acceptable results?
What do I need to do to obtain linear output of the device? Noting that automotive MAF sensors, using the same technology have curves similar to this:
http://www.autoshop101.com/forms/h34.pdf
Am I not asking other important questions?
Any help would be much appreciated, I’m sorry for asking such complicated questions about such a simple concept, I just want to be sure I’m doing things right.
How would you do it?
Also with respect to placing the thermistors in a tube? Do they both go in the tube, where only one is exposed (with holes) to direct airflow?
Is there a good schematic for a simple one of these somewhere?