Omnidirectional range measurement?

Hi guys,

I would like to pose a question out to you gurus;

My problem is I am desiging a machine that needs to know the distance of a wireless controler/transmitter from its reciver. The machine is a peice of constuction equipment and we do not want the operator to be able to operate the machine if it comes with in 3-4 feet of him/transmitter. The machine will have a lot of vibration. We cannot use traditonal prox. sensors because the mahine will need to operate next to things like trees. Any thoughts?

The Dude.

Measuring Proximity based on RF signal strength is not very reliable. I would be looking for solutions involving ultrasonics...

I agree with Mike ML that an ultrasonic approach may be the least complex method.

You could have the first unit transmit an ultrasonic pulse, and the second unit transmit a pulse (at a different frequency to differentiate it from a reflection off another object) when it detects the first pulse. The round-trip time it takes between the transmit pulse and the first unit receiving the return pulse from the second unit can be converted to distance based upon the speed of sound of about 1ms/ft. Thus if the time is less than 8ms, then the operator is less than 4 feet from the equipment.

In theory you could do the same thing with light or radio frequencies, but the time interval is much more difficult to measure, since the speed of light is about 1ns/ft.