Proximity Sensors?

[Nyx]

As some of you know, I'm working on a "big" robot using a wheelchair base. At least 3 people mentioned the risk that the robot could run into objects/people and damage them. Airman mentioned the use of a remote kill switch. I happen to have a little remote control rig that I can use for this. However, I'd also like a more autonomous safety, such as proximity sensors.

What I'm not sure about is what to use for this purpose. I bought a couple of atmega8-16PU chips, so I figure I could use IR rangefinders... However, those seem to only give you a distance reading within some range... Such as 10" to 40". In this case, if my robot is initialized when it's already close to an object/person, the sensors would not tell me anything (or am I wrong there?). They might also be ineffective if my 300 pound robot is moving fast, and does not have time to completely stop within a 30" range.

What would be some smart and cheap options to look into?

[Brandon121233]

Wide range sonar can work from about 6 inches to 20 feet so that gives you a lot more room to play with. Try maxbotix EZ line og Ultrasonic sensors

[SixRingz]

Sensors are of course great but what happens if the sensor somehow fails to "see" a person? Since they are (mostly) software controlled there can be errors hard to expect. (Of course none of us here would program a somehow errorenous software for their robot  never happened and never will!  ) But what if the batteries of your controller starts to run out and you get unexpected behaviour (restarts etc)? Maybe the wheelchair would run amok making Bad Things(tm) happen and you voted #1 bad guy of the neighbourhood...
What I'm trying to say is: if it CRUCIAL with 110 percent safety, I would go for some type of mechanical switch that instantly kills motor power in case of an impact. Maybe you could use a "hoola hoop" hung around the chair with some switch connected to it... oh well, I'm rambling....

[airman00]

lol on chives ( my wheelchair motor butler robot) I have a big kill switch on the back

it ran me over multiple times (before putting on sonar) and so I installed kill switches

Bumper switches would be a good backup sensor idea.

[Nyx]

Anyone knows what's the cheapest (and easiest to interface) sonar you an find?

[JesseWelling]

Depends.

Maxbotix EZ series use PWM, Analog, and inverted TTL serial.
Devantech uses I2C, and timing PWM.

Easy would be Analog but that's prone to noise, Accurate transfer of data would be I2C or Serial. If you were using a micro that didn't have either of those, there is allways timing the PWM... but I don't recommend that...

If it were me on my robot, I'd use Serial or I2C.

[airman00]

Parallax PING ultrasonic sensor is really easy to use . It has only 3 pins . PWR , GND , and signal. The signal pin is used to trigger the sonar as well as read the distance of the sonar.

[Brandon121233]

it only has a 3 meter range though

[airman00]

They might also be ineffective if my 300 pound robot is moving fast, and does not have time to completely stop within a 30" range.

       
3 meters = 118.110236 inches

thats more than 3 times the amount where it "might not have enough time to stop"

also, nyx there is something called a brake , lol

and brandon ,

nice to see you back on the forum , you sure do love the maxsonar 

[Brandon121233]

I stick with what works

[ALZ]

Hi:

"However, those seem to only give you a distance reading within some range... Such as 10" to 40". In this case, if my robot is initialized when it's already close to an object/person, the sensors would not tell me anything (or am I wrong there?). "

People mount the I.R. sensor back on the base so that it see objects that are close. So in this case you would mount it about 10" back.

"300 pound robot" My God what are you feeding that thing !  If you are just using two wheelchair motors, that robot is not going to be moving that fast.  I see no reason it can not brake within 30" unless the base is too small and it tips.