Doppler MIcrowave

I see a bunch of kits out there that are microwave motion detectors. But what I really want is a speed/direction measurement. Is it that much harder to pick out the specific frequency that is being received rather than if it's just altered or not?

I am unsure if it is a technical or commercial reason why there are no simple doppler speed kits.

THanks.

Jaycar sell a speedgun kit, is that what you want?.

Somewhat. It's a bit big and I'm not sure if enough information is given about it to hack it for a robot. More importantly, I don't think it works at the ridiculously slow speeds I'm interested in (I should have checked that our for radar speed guns first I guess).

I'm more interested in speeds like up to 1~2ft/s-10m/s. THat's a bit slow for a radar gun right?

Anyways, I found an ultrasonic transformer I can use to try the same thing in ultrasonics like I was trying before. Except I'm terrible at receiver circuits that are capable of dissecting the incoming signal (much better at drivers, but terrible at receivers).

Try a mattel hot wheels radar gun, there are a few hacks you can do and at the moment they are quite cheap. At those speeds you are more likely to need a distance measurement than a speed measurement as the doppler shift stays the same in RF, ultrasonics or audible. Try http://nerdipedia.com/tiki-index.php?page=Sonar+CE&PHPSESSID=d56850b944a6fb8c2b668fdbef5b3f6efor a FM chirp audible sonar, they have source code.

What speeds are you looking at 1-2ft/sec =1-2MPH while 10m/sec =36KmPH, I dont have a conversion calc handy but that would be around 20MPH

Yeah, I was wanting a doppler more for measuring ground speed than object detection (but I suppose that if I can figure out a way to decently pick out fixed points I can do distance measurements for speed).

My speeds are walking speeds.

The mattel hot wheel radar would probably do OK in that situation as it is only a very waek radar, and therefore low return sig. The freq difference is only about 7.7Hz per mile per hour and as you are only going 1-4 MPH not a great difference. Take the audio output amplify it, then run thru a comparator/schmidt trigger and any microcontroller should be able to keep up with that low data rate.

I've seen a few of the mattel radars in the discount bins, I really couldn';t see why they thought that a radar gun would make a good kids toy (toy of the year 2006???). I might buy another before they dissappear completely.

dknguyen said:

Yeah, I was wanting a doppler more for measuring ground speed than object detection (but I suppose that if I can figure out a way to decently pick out fixed points I can do distance measurements for speed).

Click to expand...

You may perhaps have noticed that cars don't measure speed in that way?, why not just measure the rotational speed of a wheel? - easier, simpler, and more accurate.

THere's too much slip and bumpiness with this terrain to accurately determine sleep. I am measuring wheel rotations too of course (since it's sooo easy), but I'm trying to find something better to augment it.

Since the original post though, I've revisited ultrasonics again instead. THe only problem I really have with that right now is finding a suitable transformer (or figuring out how and where to wind one myself that will give me the frequency response I want)

dknguyen said:

THere's too much slip and bumpiness with this terrain to accurately determine sleep. I am measuring wheel rotations too of course (since it's sooo easy), but I'm trying to find something better to augment it.

Since the original post though, I've revisited ultrasonics again instead. THe only problem I really have with that right now is finding a suitable transformer (or figuring out how and where to wind one myself that will give me the frequency response I want)

Click to expand...

What distance are you trying to achieve?, it would be EXTREMELY unusual to require a transformer for such purposes.

I would also suggest that any such radar scheme is going to be no where near as accurate or useful as a wheel type measurement.

10m is the range capable of the transducers. The transformer is mainly so I can basically work with <12V but end up with a 400Vpp signal to drive the transducer with (the other problem was I couldn't find 200V tolerant transistors capable of switching fast enough to produce a 100kHz waveform). But somebody in the other thread has introduced me to some high voltage transistors capable of switching at the required speeds so I can use a high-voltage H-bridge to directly drive the transducer.

https://www.electro-tech-online.com/threads/transformer-saturation.34848/

Don't ultrasonic tape measures give 10m range?, and presumably no transformers in them? - or at least not 200V worth!.

I'm not sure how they do it if they are using electrostatic transducers. I think they use a tiny flyback transformer. THe problem with the flyback transformer provided by the transducer's manufacturer is that it's duty cycle was too low (only 5 readings per second) and it's designed to work at the transducer's 50kHz optimal frequency. The transducer can work up to 100kHz though. COnsidering that I needed something that could allow many more readings per second and allow me to adjust the beamwidth by varying the frequency between 50kHz and 100kHz.

But if they used the cheaper, simpler, less sensitive, "single frequency" piezo transducers, then you don't really need anything special since they are driven by 12-20V or so. THey only work up to 6m for regualr sized objects in "regular" conditions. THey can work up to ~10m for large objects. Now that I write this out, they probably do just use piezo transducers.

But I'm using the more sentive electrostatic ones that allow multi-frequency operation in a single transducer (like chirps! but I'm probably just varying the frequency to adjust beamwidth for now since I can't figure out an efficient method to record the entire incoming signal to dissect it without some fast ADCs and a whoooole lot of memory and processing).

RIght now, I am going to use zero cross detection to measure the fundamental of the incoming signal (since the incoming signal is also high voltage so I'm just voltage clamping the echo and measuring the zero crosses). Assuming the harmonics aren't completely distorting the waveform and causing zero crosses at points other than the fundamental's zero crosses, it should work (right?) THe outgoing signal is basically a square wave so I don't see why the echo should be any worse as far as harmonics distorting the zero crossings goes. In fact, I would think it would be better since the transducer does dampen some of the higher frequency harmonics.

I'd add an inductor to try and filter out the harmonics before the drive signal reaches the transducer so that it is truly tone driven, but because the transducer is capacitive and because I want it to operate from 50kHz-100kHz, I haven't been able to figure out an LC filter that can do that, nor have I been able to figure out how to use LC passive components to filter the signal to the transducer since it always introduces it's own capacitance into the filter. I suppose...I could...just...use a simple RC lowpass filter eh? With a small resistor to minimize losses and a filter capacitor much much larger than the capacitance of the transducer.