# Dynamic measurement of robot's distance from fixed point

Hello,

I am interested in wirelessly and dynamically/continuously measuring the distance between my slowly moving robot and a fixed point. The robot will act in a space which will never be more than 5ft from the fixed point. Is this possible??

Thank you very much.

Tobee11

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Ultrasonic is perfect for this. The sensors typically used on robots at LMR might not be suitable - they're good at detecting a large flat surface, but not a small object.

You might consider an untrasonic transmitter as your fised point. It would send out a ping. Your "robot" would contain a U/S receiver. When it received the ping, it would transmit a ping back to the "fixed point". The fixed point receiver would pick it up and calculate the travel time for the conversation.

This gives me an idea. You could have two fixed points. One transmitting a single ping and the other transmitting a double ping. The robot would echo them both. Using this technique, you could establish the rbotos position from two fixed points and thereby trianglate its position in 2D space.

An alternative I've seen used is a ring of (appx 12) I/R transceivers around the top of your robot. They all fire in a cyclic sequence. Two fixed points with transcievers detect when the signal is strongest and therefore the angle between the bot and the fixed points. From this the distance can be calcualted.

This post is a little disjointed, but if it doesn't give you something to work on, ask me. I know what I mean!

While reading the above posts I was thinking about an idea similar to yours BOA.

With the fixed position, you could have the ping sensor and an ir emitter(array)/detector.

On the mobile unit, you could use just the ir emitter/detector

Both units could keep track of each others location based on the ir emmision as a fixed point to align to.

The bot would also have a small flat surface that would be perpendiculer to the ir transmission.I would guess it would have to be flat black so as not to bounce the fixed point ir signal(false readings). It wouldneed to be big enough so that the pinger could get a reading off of it.....maybe a 2x2 piece of plastic or something....

Not sure if you'd need an emmiter array as to keep better track of bot/fixed point but it might help.

Long, long, long ago before infra-red was invented and just after sound was discovered (both events happened in the 1960's), ultrasonic was used in TV remote controls. They only had limited numbers of buttons. This may have been because the device was only really capable of sending or receiving 3 or 4 distinct frequencies, but I think it's a feature which could be used here.

(NB "gravity" was rumoured to have been invented shortly before, but gravity turned out to be a myth: the reality being that the world sucks.)

I knew of TV-remotes that actually used dual-tones (in ultra sound).. I am petty sure!
Perhaps a ring of sensors on the fixed point can tell you how far the robot is at any given time. The 5 feet range may be a bit much for the typical Sharp IRs, so a ring of sonars could be good. Just difficult if there are any other obstacles in between. It might be done with as few as 3 or 4 sonars, depending on the type of sensor, and the size of the robot being detected. Alternatively you could try something really strange that I'd thought of : direct a single sonar up (or down) into an omni-directional reflector, which sends the pulses out radially in all directions, and would catch returns to have them be detected. It would seem this would only work with single transducer sensors, like the Maxbotix ones or the Sensecomp devices, if it worked at all.

if it only needs to sense the distance from a central point, you could use the easy radio device

http://letsmakerobots.com/node/77 and use the analogue signal strength output to gauge distance...

not so good for 2D locations though.

Or if you wanted to keep it super simple just tether the bot to the central point.  On the bot side attatch the tether to a spring loaded rotating drum with a pot attatched.  Use the pot to determine how many times the drum has wound out and multiply that by the circumference.