Recently I was asked to make a proximity alarm for a job site vehicle with no rear visibility.
I'm thinking, since other radios will be in use on the site, I would use 2.4ghz signals. Now, I know that cell phones and other devices use this range as well, so here's my plan:
I'm thinking I'll set up the transmitters to send out a looped signal. I was thinking if I had the pedestrians wear a small transmitter and mount a receiver on the vehicle set to sound an alarm in the cab at a specific signal strength, this would do it. The whole purpose is to set off an alarm when one of the workers gets too close. If I can encode the signal and get the receiver to respond to the signal strength of only that encoded signal, I can do it. Alarm should go off at 60 feet. Also, it if works, they may put the receivers on other vehicles. To start with, I'm going to proof-of-concept with one transmitter and receiver.
I'm pretty well in the dark on this, so any advice would be very helpful.
I tend to write confusing, so I'll break it down.
Multiple transmitters, same frequency
Single receiver
Signal strength based proximity alarm
Preferably encoded to limit interference
Once again, thanks in advance for help!
Just don't think about using the NRF24L01. The range is terrible. Find some other radio.
It won't even cover 60 feet?1?
If that's the case, I'll find another transmitter. Still wondering about the whole project, though...
Signal strength is an extremely unreliable indicator of distance, no matter what radio you use. It is certainly not acceptable when personal safety is at stake. Antenna orientation and the presence of nearby reflecting and absorbing surfaces have far more influence on signal strength than does simple distance.
The standard approach to get around "no rear visibility" for vehicles is closed circuit TV cameras and displays. You can get them quite cheaply.
I've seen posts (Gil Scargill) that said the NRF24L01 was useless beyond 10 ft.
There's a version of it that has a long antenna but I have no info on the range for that model.
60 feet? How fast is this thing moving (and backwards too?) that you need to detect objects at 60 feet?
You could try ultrasonic detectors. Or a rear-facing camera with a small lcd screen on the dashboard?
Pete
Yes, ultrasonic transducers are common too. Check ebay for "backup radar alarms" -- for US$ 30 you get four rear mount transducers and a bright display with distance readout for the driver.
I think a backup camera would be more reliable. There are many small digital cameras available now. I use 5.8 Ghz video transmitter for my quadcopter . The transmitter and receiver can be bought as a pair. Any display that accepts composite video (the yellow RCA connectors that are usually used with the RED & WHT connectors for VCRs and such will work. A small 5"x7" portable TV with a 12V dc pwr input and the yellow composite video input would work perfect. Put the camera (any camera with NTSC video out) like a GOPRO or any other small camera plugged into the video transmitter on the outside of the vehicle and the video receiver on the inside plugged into the tv and you would be able to see behind you. You would need to mount the camera on the top center rear of the vehicle.
They're not moving super fast, maybe 10 to 15 miles an hour, but we're also talking about machines that are sometimes 30+ tons and 30-40 feet long. Keep in mind, the man who's feeding me this data has been working in and around this equipment for 50+ years. I'm taking his word that if he wants a 60 foot buffer, there's a reason.
Okay. Just got him on the phone while writing this post... He gave me his exact list of reasons:
He told me backup cameras aren't an option, as the operators would have to look away to see them.
Ultrasonics and IR get too much interference(already been tried).
Doesn't want to warn operators of objects, specifically PEOPLE.
Now, my plan(I think I said this in the original post) is to have the transmitters on the workers, and the receiver on the machine. The goal is to warn the driver that there is someone within 60 feet of the machine. I'm told(not being an operator myself) that they have to focus on their load or what's in front of them, almost like Target Fixation.
So, putting transmitters on the workers, if I can find one with a range of 60ft or more, can I get one receiver to respond to multiple transmitters, and separately, fine-tune the necessary signal strength to cause the alarm to go off at somewhere within the threshold I've been given for safe distance(40-60ft)?
How about a loud beeper that is sounded at the rear when reverse gear is selected. Then people in the danger area will be warned to move out of the way.
I suspect ultrasonic detection is more suited to detecting fixed objects to prevent a collision and material damage rather than personal injury.
And, unfortunately, there seems to be plenty of academic evidence to support the notion that drivers subconsciously take more risks when there is a safety device installed.
Of the things mentioned so far my preference would be for a good colour TV system PROVIDED that the absence of a person visible on the screen really means that you can reverse with impunity. I would ALSO have the loud beeper.
...R
This sounds like a safety-critical application and that makes it extremely unsuitable for development by a novice.
I'm not up to speed on techniques for keeping people out of danger areas but I've never heard of anything like this approach being used - and I suspect that's because it is a bad idea. Using active transponders carried by each person does not sound at all reliable and has horrible failure modes - if your transponder fails there is no way for anyone to know that the safety system is no longer working.
If I were you I would look for far simpler solutions, such as a fence, or adopt working practices that prevent people from being able to get into dangerous positions.
raschemmel:
Just don't think about using the NRF24L01. The range is terrible. Find some other radio.
As with all RF communications the antenna and receiver sensitivity are major factors in the range.
If you use a module with Receiver Preamp and external antenna the range would probably be OK with the small nRF24L01 modules as the remotes. Like one of THESE:
Lot of nRF24L01 info on the ArduinoInfo.Info WIKI HERE:
DISCLAIMER: Mentioned stuff from my own shop...!
The detection range of an RF signal is massively variable which makes it unsuitable to determine when somebody has crossed a threshold. Either it will allow people well inside the threshold range without triggering, or it will trigger falsely on people outside the threshold who happen to have a better signal. IMO using simple radio signal strength for range finding is a non-starter.