FMCWRadar
| Project FMCWRadar | |
|---|---|
| Experimenting with inexpensive FMCW radar modules | |
| Status | Initializing |
| Contact | bertrik |
| Last Update | 2020-05-07 |
Goals
This project is about experimenting with inexpensive FM-CW radar modules as can be found on AliExpress:
- gaining experience with the hardware
- gaining experience on what kind of compensation/calibration is needed
- apply it in a fun project, e.g. pedestrian speed indicator, or detect bats with it
Perhaps start a cool collection of inexpensive "software-defined" radar projects?
The current generation of inexpensive radar modules (around E40,-) has these typical features:
- operates on 24 GHz
- has quadrature outputs (I and Q) so it can not just detect movement (through Doppler) but also distinguish direction
- has a modulation input that allows a subtle change of the radar operating frequency
- often combined with a cortex M0 or M3/M4 microcontroller,
- pre-configured with firmware to do detection of object speed and direction
Theory
An FM-CW radar is a few steps more advanced than the very basic Doppler radars:
- I and Q outputs as described above
- Modulation input, that allows you to quickly sweep the radar frequency
The basic idea behind an FM-CW radar is that the frequency is sweeped, at some continuous rate. A signal that is reflected by an object in the view of the radar, will arrive back at the radar with some delay. Because of the delay, the outgoing signal will have already changed in frequency compared to the incoming reflected frequency. At the radar, the delayed incoming signal is "mixed" with the outgoing signal, resulting in a low-frequency I+Q output of the difference frequency. The difference frequency at the output of the radar is therefore linearly related to the time delay, so also linearly related to the object distance.
Challenges
- Do the available radar modules actually use the FM modulation input of the radar?
- How is the FM modulation input wired to the CPU? The STM32 processors typically used dont't have a DAC output!
- How to cope with real-world inaccuracies:
- I-Q outputs are not exactly 90 degrees apart
- I-Q outputs have an amplitude inbalance
- I-Q outputs have a bias
- dynamic range: this needs to be very high, since the radar equation has a 4th-power dependency on distance
- Get some rough idea of
- inaccuracies a described above
- Frequency change per m/s
- Typical modulation index: MHz / V on the modulation input, and the corresponding sweep rate
- Choose signal processing properties
- choose a appropriate sample rate
- can we calculate an FFT fast enough, fixed point or floating point
- what properties should the FFT have, obviously complex->complex, window function?
- can we stack multiple FFTs for increased sensitivity?
Buying an FM-CW radar
Acconeer radars:
- https://www.acconeer.com/products has a list of smart radar modules
- https://learn.sparkfun.com/tutorials/getting-started-with-the-a111-pulsed-radar-sensor/all
AliExpress 24 GHz radar with FMCW, no CPU:
- Yh-24g04, a 24 GHz quadrature doppler radar (no CPU), has modulation input, for about E16,-
- YH-24G01 (no CPU), for about E15,-
AliExpress 24 GHz radar, DM-series:
- DM-39, a 24 GHz quadrature doppler radar with CPU for about E32,-. The page shows a SRK1101 radar, with I/Q outputs and tune input. Mentions Cortex M0.
- DM-19, a 24 GHz quadrature doppler radar with CPU for about E48,-. Appears similar in possibilities to DM-39. Mentions Cortex M3/M4 processor.
- another DM-19, 24 GHz quadrature doppler radar, about E40,-
- DM-19 / DB-16 another FMCW radar model, about E39,-
AliExpress 24 GHz radar, FM-series:
- FM-42, a 24 GHz FMCW radar with CPU for about USD107,-
- FM-49 another FMCW radar module.
AliExpress 24 GHz radar, other:
- 182MOD, a module outputting speed for about USD28,- appears to use a 5-pin radar (Vcc,Gnd,I,Q,tune?)
- USRR187 mentions FMCW, has CPU (UART output), for about E38,-
Analog front-end for many of the radar modules above seems to be SRK1101A.