Out of Water Vel Sensor

The 'Gravity' Microwave Sensor. A male pin has been soldered onto the op-amp, allowing us to probe the raw velocity data

We are developing a new velocity sensor which we hope will be able to detect the velocity of flowing water when placed outside of a flowing water body. The motivation for this is to ease the installation process, as this would allow the sensors to be installed without the removal of drain covers. The sensor should be capable of penetrating concrete drain covers and detecting the flow of water underneath.

21st January 2020

Microwave Doppler shift motion detectors can be sourced very affordably from the internet. Microwaves will readily penetrate thick concrete, but can also be easily blocked using a faraday cage to remove external noise. Our goal is to modify one of these sensors to detect not just motion, but the velocity of motion as well. We are currently using the Gravity Digital Microwave Sensor, which uses a 10.525 GHz microwave source.

The sensor sends out a signal and uses the phase difference between the outgoing and incoming waves due to doppler shift as a measure of velocity. hen a threshold velocity is observed, which is not useful for our application. The sensor outputs a digital high/low signal when a threshold velocity is observed. Our first task is to backtrack through the circuitry of the sensor, to probe the raw measured signal which measures the amplitude of the observed velocity.

By investigating the schematics for the sensor, we found that probing the pin 1 output of the LMV358 op-amp gave us a signal whose amplitude correlates well with velocity. We intend to find a calibration curve which will allow us to relate the amplitude of this signal to velocity.

The data taken below is a measure of the voltage output of the probed velocity sensor data over number of measurements. The measurement frequency is set to 5kHz. While no velocity is observed, the signal floats at around 208 which corresponds to approximately 1V. When agitated, the sensor outputs readings between 0 and 1023, (0-5V). Due to the signal floating at 208, we intend to use the time-averaged absolute difference between the current reading and 208 as a measure of velocity amplitude. Our next task will be remove the noise in the signal. The voltage output reading is clearly bounded by a reading of 1023 (5V). This will create some uncertainty when measuring very high velocities which we will also need to somehow avoid.

22nd January 2020

A low pass filter was integrated into the circuit to remove the high-frequency noise present in the data taking. Further removal of noise was accomplished by taking the weighted sum of the past 40 measurements (with higher priority to the latest measurements). Testing with flowing water will indicate whether or not this has removed enough noise to produce a meaningful calibration curve. We are also investigating toggling the sensitivity of the sensor, and choosing an appropriate material for the faraday cage which will block out external noise.

4th February 2020

As there is still too much noise present in the velocity sensor’s readings, we smoothed out the readings by averaging over a longer interval. Frequent velocity measurements are not required anyway.

The sensor was used to attempt to measure the speed of flowing water in a flume. The sensor was wrapped up in aluminium foil, leaving only one side of the sensor open, which was pointed at the flowing water. Raw sensor data from pin 1 was taken alongside actual water speed measurements using a FloMate probe. Post-processing the raw sensor data shows that unfortunately, no correlation appears between the sensor velocity and FloMate probe.

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  Velocity measured by sensor. Amplitude of measurements was taken to correlate with velocity, which was used in this graph

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  Velocity measured by FloMate

The strength of the signal from the sensor was quite small, so there was clearly a lot of noise that could have affected measurements. There are several ideas for improving measurements. We will investigate increasing the sensitivity of the sensor (although this could increase noise as well). While using the averaging technique on the signal from pin 1 was useful for measuring the movement of solids, it did not work as well with water. Perhaps we may use an alternative form of measurement by using the frequency of the signal from the sensor's OUTPUT pin as a measure of velocity. We wish to also investigate using alternative breadboards for the sensor. We intend to continue using the HB100 microwave sensor for now, but it may be preferable to use alternative wiring for amplifying the signal from the HB100 sensor. Different breadboards might work better for detecting velocities.

5th February 2020

Experimentation was made with detecting the flow of water running through a 90mm diameter PVC pipe. Water flowed from a tap down through the PVC pipe, which was inclined at a variable angle. The following graph shows the relative frequency of motion detections made by the sensor. The horizontal scale is in units of time (each unit is around 0.5 seconds), and depicts various different setup arrangements. Throughout the last three measurements, the sensor was placed directly next to the pipe and was pointed at the pipe. For the first measurement, the sensor was placed roughly 25cm away from the pipe. When there was no flowing water in the pipe, the sensor consistently read a 0 frequency of motion detections

These measurements give us a qualitative understanding of the sensor’s abilities, in particular, that being any more than a mere 25cm away from the pipe gives us no data. The sensitivity was increased to account for this, but it was found that even with slightly increased sensitivity, the sensor was picking up ambient movements. Measurements were also unchanged by turning the flow of water on/off.

We also see that changing the flow of water leaves the detection frequency virtually unchanged, even though the flow of water was changed quite substantially. By carefully toggling the flow of water to be very small, we saw a decrease in the detection frequency, but even this effect was subtle and outweighed by a large amount of noise present in the signal.

Alternatively, using the pin 1 output of the sensor seemed to give a slightly more consistent correlation between average voltage and the angle of the pipe. With increased distance from the pipe, the average voltage signal strength decreased, but still gave consistently good readings at 25cm away from the pipe.

A problem was encountered with the sensor overheating after a long period of intermittent measurements. At this point, the sensor started giving bogus results. I.e. Giving high frequency detection readings when completely alone in a room. The issue is suspected to be either due to reflections from a large amount of aluminium foil covering the sensor (which was used to block ambient noise), or perhaps from some overheating fault.

7th February 2020

Further experimentation was made with shielding the sensor, which seems to have been causing issues. The sensor was wrapped entirely in aluminium foil and packaged together. The readings from the sensor, contrary to expectations fluctuated and depicted a non-zero motion detection frequency. Even a thin sheet of aluminium reflects microwaves very well, so this result was surprising. After carefully re-packaging the sensor and weighing it down tightly to remove any kind of movement of aluminium foil, the sensor read a consistent zero reading. For this reason, it is thought that even small movements of the aluminium foil un-crinkling could cause significant spikes in the sensor's readings and that if aluminium foil is used for shielding, it will have to be very carefully packaged to avoid any slight movement of the foil. Other thoughts include the possibility of grounding the foil, which in its ungrounded state could yield some unexpected effects. This requires more investigation.

We intend to investigate an alternative circuit for using the HB100 sensor. The Gravity Microwave Sensor is hardwired as a motion detector for human and other physical objects, not water velocity. We'll need a more versatile platform to experiment with in order to develop a sensor for measuring water velocity. For this reason, we will be building a custom circuit.