EDN Access — 09.26.96 Transistor and FVCs make linear anemomete
Transistor and FVCs make linear anemometer
W Stephen Woodward, University of North Carolina, Chapel Hill, NC
A previous Design Idea presented a simple flow-to-frequency transducer that easily fits into mP-based measurement systems (Reference 1). You can accumulate and linearize its frequency output by using software without the need for ancillary circuitry. In other applications, however, you might prefer a linearized analog output. The circuit in Figure 1 combines the hot-transistor-sensor idea (Reference 1) with an unusual square-of-frequency-to-voltage converter to provide the analog output.
The basic operation of this front-end sensor is identical to that of the earlier circuit. Q1 is the self-heated airflow sensor; Q2 senses ambient temperature. Zero-adjust resistor R2 sets the zero-airflow quiescent bias currents for Q1 and Q2. With proper adjustment of R2, the temperature rise of Q1 from its collector power dissipation reduces its VBE (at a rate of 2 mV/8C) to just slightly below the sum of Q2's VBE plus the drop across R1. Comparator IC1A then has a noninverting input whose voltage is slightly less positive than the inverting input. Its output, therefore, goes low, holding C1 discharged and multivibrator IC1B reset, with its output high. This condition produces a 0V output in switch S3 and, consequently, the output of IC2.
When airflow impinges on Q1 and Q2, the resulting increase in the cooling rate tends to reduce Q1's temperature, causing its VBE to rise relative to that of Q2. This disparity reverses the relationship between IC1's inputs and releases the reset on C1, thereby causing IC1B to oscillate and cycle S2. Thus, a feedback loop arises that acts to maintain a constant temperature differential between Q1 and Q2. The average duty cycle at IC1B's output is proportional to the extra power required to regulate Q1's temperature and is, therefore, proportional to the square root of air speed. This proportionality stems from the physics that govern heat transfer from a warm object to a free-moving air stream.
IC1B sets S1's duty cycle, causing the average voltage at the input of the IC1 lowpass filter to be 0.0025 times IC1B's operating frequency. In other words, S1 and IC1 combine to form a conventional frequency-to-voltage converter (FVC), operating on IC1B's output frequency and using the 5V supply as the voltage reference. The 0 to 2.5V (as IC1B's frequency goes from 0 to 1 kHz) output of this first FVC serves as the voltage reference for a second FVC consisting of S3 and IC2. Here, the output undergoes multiplication by IC1B's duty cycle a second time and by IC2's gain (4), producing the voltage 5×106(f1B)2 at IC2's output.
IC2's output is a filtered, 0 to 5V signal proportional to the square of the heat applied to Q1 and is, therefore, linearly proportional to airflow. Through appropriate adjustment of R3, you can obtain any desired full-scale flow rate, from less than two to more than 60 knots (nautical mph). The response to changes in air speed is fast (approximately 2 sec), due to the constant-temperature operation of Q1. Epoxy-based paint does a good job of weatherproofing the transistors for outdoor use. Of course, you must keep Q1 sheltered and dry for rainy-day operation. (DI #1927)
1. Woodward, W Stephen, "Self-heated transistor digitizes airflow," EDN, March 14, 1996, pg 86.EDN Magazine. EDN is a registered trademark of Reed Properties Inc, used under license.