Why can't radar cruise control detect stationary vehicles?

If I am not mistaken, radar-based system can only detect metal objects.

 

I would think it would base the distance calculations based on a reference point (vehicle moving in fron of you). Sudden changes in that reference point would indicate a vehicle braking suddenly.
A stationary car wouldnt ever create that "moving" reference point.
If it did, you the emergency breaking would activate every time you drove past a tree or like.
( my uneducated opinion anyways)

 

It might slow you down when passing parked cars on the side of the road if it did.
It is designed on the premise that there will be a driver at the controls.

 

I used to play with radar modules back in the mid-70s, so I am definitely making this up as I go along and am quite happy to be shot down in flames. I am too lazy to work out the maths, please forgive.

As noted above, the cars' radar system doesn't really want to detect stationery objects. You don't want the car to drop into panic mode everytime it passes a rubbish-bin, sees an oncoming vehicle in a different lane or pylons associated with overhead bridges.

As you may know, a radar module transmits a radio-beam. When this beam strikes an object, it is reflected back to the module. The frequency of the returned beam is changed. This is known as the doppler effect. The amount of frequency change is a function of the differences in speed between the transmitter and the object reflecting the beam.

The amplitude of the returned signal depends on a whole slew of variables (including the material of the object reflecting the signal) but is definitely an inverse function of the distance between the objects. Double the distance and the amplitude will drop by a factor of 4. Triple the distance and it drops by a factor of 9.

The modules I worked with combined the transmitted frequency and the returned signal, and what was actually output was the difference between the Tx and Rx frequencies. Say the radar sends out a 10,700,000,000 Hz signal and the returned signal is 10,700,006,000 Hz. The difference is 6KHz and this would be the output from the module. This was done in those days because working at extremely high frequencies ( ) in the GHz range was tricky.

Multiple reflections (due to multiple objects all having different relative speeds) result in a whole symphony of output signals.

Deciphering this cacophony way back when would have been fairly difficult. Nowadays, thanks to magic of silicon, it seems a whole lot simpler. ( I dips me lid to the programmers. )

I am guessing that the prius builds a 'map' of surrounding objects (much like those green screens seen in old-time Navy movies). Possibly 5, 10 or more new maps are created every second. Subsequent maps are compared to earlier ones and the stationery objects identified. This is 'relatively' simple to do as the vehicle speed is known.

Removing the clutter produced by stationery objects reduces the number of objects left for further processing.

As the the prius gets closer to the stationery object, the amplitude of the returned signal increases. Once a certain threshold has been exceeded, the 'stationery' object is brought back into the computation set.

While I have been speaking in terms of absolute values, it is more likely that the computations and alert thresholds are based on the rate of change of the readings.

Thus a car travelling in front of you at a constant speed, will produce a rate of change of zero.

A stationery object will appear in the new map, in the 'same place' as the original map once vehicle speed and the time between readings has been factored in. Once again a rate of change of zero.

A vehicle in front of you that is slowing, will be in a slightly different place on the map than expected. The rate of change in this case is non-zero and the signal amplitude would be increasing because the vehicle is getting closer to you.

An oncoming vehicle in your lane (ie head-on crash impending) would have a dramatically increasing rate of change of the returned frequency and the amplitude of the returned signal would also be increasing even more dramatically.

I have ignored changes of phase in the returned signal but I am guessing that this is how the prius determines that the reflection is coming from a vehicle in a different lane.

While probably wrong in pretty much every respect, this is how I visualise the system working.

Cheers
Warwick

 

Boiling down all that was said above and in previous posts, the short answer is that the radar system deliberately ignores stationary objects, such as trees, signs, and bins. This means it will also ignore stationary cars.

Tom

 

Definitely see mine reacting to cars stopped at lights ahead of me when in cruise control. Have not had the courage to take it beyond the rapid slowing that precedes beeping and then full on braking.

Certainly have seen it able to igore cars in adjoining lanes until the road starts to curve a lot and momentarily I see the car in the adjoining lane being picked up.

One situation that gets a bit scary is when a car cuts in front of you and is travelling faster than you. The car then starts to speed up as it sees the distance as increasing so tries to speed up to the set speed. Sometimes you get the acceleration and then sharper braking as the car in front then has to brake,or when the car in front continues on into the next lane and the car suddenly re aquires the original car in my lane.

David

 

I use mine a lot and over the last 10K miles I have only had it slow for a vehicle in the adjacent lane one time. In that instance it was a very large truck with a shinny aluminum trailer and I think there was a little curve to the road. Overall the DRCC system seems to work great.

I have resisted the impulse to test mine on a stationary object such as a parked car or a metal building. I have this terrible vision of me trying to untangle myself from the airbag while saying "Honest officer, I haven't had a thing to drink I was just testing my cruise control."

 

Have you' all thought about testing against a cardboard 'target'? To replicate reflectivity, a layer of paper towels soaked in salt water, possibly in plastic baggies, should work 'good enough.' In mythbusters, they'd use a butchered hog but that can be a little expensive and messy ... Ok, maybe brisket but do report how well the Prius tenderizer works. <grins>

In particular, I would be interested in how it works on the following silhouettes:

• cat sized
• dog sized
• child sized
• adult sized
• deer sized
• moose sized

Given how easily it can be misinterpreted, I would recommend testing in obscure parking lots. It is science but you never know who might want to report a cardboard 'hit and run.'

Bob Wilson