Measuring Height from Suspended Object

I have a suspended object that is raised via a chain hoist, either indoors or outdoors. The object can be anywhere from 5' to 50' high, depending on the situation. The top of it can be as much as 75' off the ground, and the bottom can be up to 25' off the ground. It will only travel vertically at slow speeds, with some swaying as it is lifted.

I currently have 2 slave devices, one at the top of the object and one at the bottom, which are connected to a master device via RS485 communication. The master device is handheld and near the ground. The slaves each contain an arduino pro mini and a 3 axis accelerometer. These currently measure tilt angle and send the data to the master device.

In addition to the tilt angle, I also need an inexpensive method for measuring the distance from the top of the object to the ground below with accuracy of ~1cm.

I've thought of many options, but can't determine which would be the best for this application, given my budget (must be $50 or less, or it's not worth it). I don't have access to the hoist mechanism, so measuring the length of chain via a rotary encoder won't work, unfortunately. IR and ultrasonic don't have the range I need. Altimeters and gps don't get the accuracy I need. Lasers seem to be the way to go, but my budget obviously limits me there, and I get nervous about swaying lasers pointing at the ground where people might be. I'm considering hacking a laser distance meter, but wanted to see if anyone had any alternative ideas or suggestions.

Thanks!

50' == 50 feet? (sorry I'm metric)

Yep! Those are feet. :slight_smile:

Altimeters can be surprisingly good. The cheap ones these days get 1 inch accuracy and 1cm (mixing units) is doable without too much expense. The closer you are to sea level, you will get more resolution.

1cm over 50ft is a big request. Why so fine? Infrared and ultrasonic will cost big bucks to cover that distance. What about a mix of sensors? One type for close-up fine maneuvering and a different one for long range?

I don't have access to the hoist mechanism, so measuring the length of chain via a rotary encoder won't work, unfortunately.

Have the chain rotate a sprocket with a rotary encoder attached.

zoomkat:
Have the chain rotate a sprocket with a rotary encoder attached.

Thanks! That was actually one of my first thoughts. Unfortunately, when I said "hoist mechanism", I was including the chain and pulleys (sorry, should have been more clear). I am not able to mounts a sensor anywhere other than the actual object itself.

MorganS:
Altimeters can be surprisingly good. The cheap ones these days get 1 inch accuracy and 1cm (mixing units) is doable without too much expense. The closer you are to sea level, you will get more resolution.

That's really great info, thanks! I've been going by datasheets, and wondered if/how altitude affected accuracy. This sucker will be lugged all over, and the altitude of the location can vary. When you say "the closer you are to sea level", how "close" are we talking in order to maintain decent accuracy? What kind of variations can I expect between a location in the mountains vs a location at sea level? What altimeter would you recommend for my kind of application? What about if it was indoors...like in a large warehouse or arena?

MorganS:
1cm over 50ft is a big request. Why so fine? Infrared and ultrasonic will cost big bucks to cover that distance. What about a mix of sensors? One type for close-up fine maneuvering and a different one for long range?

Technically it's 75 feet with 1 cm accuracy...so it's an even bigger request. :wink: It's equipment that must be at a certain height and angle in order to operate at maximum. How would you implement a mixed sensor approach in something like that?