In another post I ask about an ultrasonic radial beacon finder: Does it exist: 360 degree ultrasonic receiver/beacon combo
Assuming it exists or I can hack one of those together, would it be possible to have the beacon emit multiple frequencies to detect the distance from the base(robot)?
Let me explain better. As the receiver spins in a circle it listens for specific frequencies. Theoretically, the beacon could output 2 frequencies that have different maximum distances.
When the receiver detects the longer range frequency, if it doesn't detect the shorter range one, then the object must be outside the short-range radius.
Ideally, I would like to be able to have multiple radii in 5 foot increments up to 20 feet(ish). The small radii are why I ruled out RF.
So now that my use-case is explained: Do different frequencies have different ranges?
Edit: To clarify Use-case, both the robot and the beacon will be mobile so fixed base stations are not an option.
Best Answer
The sensitivity of ultrasonic transducers does vary over frequency. The following example is from a Kobitone 255-400PT16-ROX datasheet. It would also be fair to assume it would fall further outside the normal operating frequencies that are shown below.
However in my opinion using it do determine distance isn't likely to be a good solution. When operating outside normal ranges you may find differences in performance between parts but for experimental / hobbyist purposes I see no harm in trying it as a simple solution.
Another way you could go about it is to change the amplitude of the signal at the ultrasonic emitter end and encode it with a digital signal that indicates the transmission level used. That does get quite a bit more complex though. If you wanted to take a look at going that way perhaps the VirtualWire project would be worth a look at for encoding and decoding the digital signal.