Ultrasonic Anemometer Part 3 – Amplifier

Now that we’ve made some initial design choices about how the signals will be routed in the design, it’s time to start nailing down some of the finer details. But first, before getting into too much detail about the receiver design, let’s take a moment to survey the signals that we intend to receive using an oscilloscope:

Ultrasonic Anemometer Waveform 40kHz
Test waveform recorded using test setup

We can see both the transmitted and received signals using the simple test setup shown in the photograph below. In the final design the transducers will be much farther apart in order to minimize airflow disturbance.

40kHz ultrasonic transducers in a desktop vise.
Test Setup – Two transducers in bench vise

The received signal will be much weaker in the final design due to the increased spacing between transducers. Amplification will obviously be necessary. This leads us to the design of the first stage of the receiver.

The Amplifier

The incoming signal from the transducer will need to be amplified to account for the losses incurred during the waves journey through the air as well as the losses incurred in the transducers and multiplexer. Based on some rough oscilloscope measurements, we will need around 35dB of gain to amplify the 50mV signal that will reach the amplifier to get a 3V signal that we can then send on to the two processing stages. In order to make the circuit more immune to self-oscillation and to increase flexibility I will split the amplifier into two stages.

For the initial amplifier stage I have chosen to use a multi-feedback (MFB) band-pass topology. This topology will use an op-amp to both amplify and filter the incoming signal.  The gain of the first stage will be fixed at around 20dB. I’d like to have a Q of approximately 5. This is a trade-off between amplifier ringing and filtering ability. The transducers themselves have inherent filtering ability since they are tuned, thus, anything above 5 does not seem necessary at this point.

Amplifier Schematic
2-Stage Bandpass Amplifier Schematic

For the second stage I have chosen a basic inverting topology, whose gain could be easily adjustable by changing a resistor or using a potentiometer. I have drawn the second stage with about 20dB of gain. This will likely be adjusted later, but should be close enough for now. If the system ends up being too sensitive simply varying the output drive a few Hertz away from the resonant point will reduce the sensitivity without the need for any adjustments to the circuit. The output drive frequency is very easily modified in micro-controller firmware, and may end up as part of a calibration/initialization routine in the future.

Amplifier SPICE Simulation
Amplifier SPICE Simulation

It’s not yet clear to me weather or not separate bias circuits will be necessary, so I’ve included two at this point. The SPICE simulation doesn’t seems to show any adverse affects, but because there is a signal path to the bias circuit in the first stage, I’ve chosen to have another isolated bias circuit in the second stage just to be sure. I’ll find out later if this was over-kill (a colleague and I have a running joke that ‘over-kill’ is often ‘just-the-right-amount-of-kill’). The SPICE simulation shows results as expected, with just under 40dB of total system gain, and a nice peak at around 40kHz.

With a solid design in place for the amplifier I will move on to the envelope detector design process.

 

Ultrasonic Anemometer Part 2 – Signal Path

Since we’ve established the methodology behind how we will approach making these wind speed measurements, it’s time to begin the design. The design of this system was inspired by several other individuals who have also designed and built their own ultrasonic anemometers. Here you will find their own reports documenting their efforts:

The Signal Path

The first thing I chose to tackle was designing the overall flow of signals. Because the design has four transducers I needed either four transmitters and receivers, or, a way to select the appropriate signals and route them to one transmitter and receiver. In order to minimize cost and size I chose to build one receiver and transmitter and use a multiplexer to route the signals to the correct destination.

Ultrasonic Anemometer
Signal path for ultrasonic anemometer

The transmitter in this case is simple. I determined that the impedance of the transducers was approximately 680 Ohms at 40kHz. This allows me to drive them directly from a pin of the micro-controller at either 3.3 or 5 volts without cause for concern since most micro-controllers can source and sink around 20mA, or more. Additionally, the multiplexer will have some resistance, which should also protect the micro-controller. Because the transducers are tuned circuit driving them with a square wave directly from the micro-controller seems like it should be acceptable at this point. The transducer will filter out most of the harmonics in the square wave. I will re-evaluate this later in the design process.

Side note: In order to determine the impedance of the ultrasonic transducer I connected a variable resistor in series with the transducer. I then applied a voltage with a signal generator set to 40kHz. Next, I adjusted the variable resistor until the voltage amplitude was equal across the resistor and the transducer. At that point the impedance of the resistor and transducer were equal. Using a simple multi-meter I measured the DC resistance of the variable resistor which yielded a result of 680 Ohms. While this doesn’t provide the whole story regarding the impedance of the transducer, it does give enough information to move forward with the initial design. Plus, it’s very quick and easy to do with only basic equipment.

Designing the receiver in this system was the most difficult part of the hardware design process. I will start to cover that process in the next entry in this series.

Ultrasonic Anemometer Part 1 – Math Concepts

Index


With the first stage of the weather station project nearing completion I’ve decided to explore building some additional sensors to collect more readings. One of the things that I’ve been very interested in measuring is wind speed and direction. A device that measures wind speed is called an anemometer. Wind direction is usually measured with a wind vane.

There are a few problems with traditional designs of these instruments, especially with anemometers. One of the biggest problems is that they usually have a minimum wind speed threshold. If the wind is below this threshold the device will not be able to measure it. This is usually because the wind is not strong enough to spin a propeller of some kind. To overcome the problem of low speed wind measurements I’ve decided to go with a solid-state (no moving parts) alternative. This alternative involves measuring the difference in <a href="https://en have a peek at this web-site.wikipedia.org/wiki/Time_of_flight” target=”_blank”>time-of-flight (TOF) of a sound wave across a known distance.

The overall approach to measuring the wind speed using this method will be:

  1. Create a pulse of sound at point A
  2. Measure the time it takes to arrive at point B
  3. Create a pulse of sound at point B
  4. Measure the time it takes to arrive at point A
  5. Use the difference in the two times to determine wind speed

This method will determine the wind speed in along one axis. In order to get wind direction you must measure the wind speed along another axis, then use trigonometry to calculate overall speed and direction. To make these calculation easy we will measure the wind speed in the North-South direction and the East-West direction. In doing so the equation for overall speed is

Total Speed = SquareRoot(VNS2 + VEW2

where VNS is the wind speed in the North-South direction and VEW is the speed in the East-West direction. The formula is a direct application of the Pythagorean theorem. The equation for overall direction is

Angle = tan-1(VNS/VEW)

where tan-1 is the inverse tangent function. These equations are relatively simple because of the fact that we intend to make our wind speed measurements exactly 90 degrees apart. It would still be possible to calculate wind speed and angle if we had chosen another angel, but the math would not be as simple as this.

Now that the theory is out of the way, we will begin to actually implement this method in part 2 using some cool electronics and sensors.