Building a sensor to measure how tall the grass is

@farmeralan

Wow, I grew up on a farm finishing off beef cattle for market. From what I remember by the time you've added enough cement and razor wire to "cow-proof" this sensor, it won't even be able to see the grass 🙂

Since the grass won't be very dense, will you get enough return from it to calculate the height ? Especially if active grazing is happening, you may sense the height of your cattle.

I'm thinking that you'd have to have your sensor(s) arranged vertically and do the sensing horizontally. That is, is there any return signal across from this height.

I'm looking forward to seeing your mechanical drawing.

PS - what power source(s) will be available 

I think the rcwl0516 is your best bet. Mount it on a pole away from all the hazards. If you can then mount a horizontal pole in order to get the sensor pointed straight down will give the best results I think. You will have to calibrate the sensor over a fairly long time. What I am suggesting is to graph the sensor output against the measured grass height over time to see if the results are at least repeatable for a given reading/grass height. I worry that water content in the grass may throw it off, but maybe someone knows of a way to measure that and provide an adjustment.

I wonder if straight down is the best, maybe a 30 degree, or 45 degree angle will give a better result, just guessing. Or the inverse of 30.

Please keep us informed, this is a very interesting challenge.

This is my original sketch. Sorry it is so rough.  Because of the harsh conditions (cows are hard on stuff) I planned to bury the mechanicals and have the sensor rise up to measure the grass height.  If I build it out of heavy pvc and use a threaded rod with a known pitch I should be able to protect things from the weather and move things a known distance easily.  I have this summer to trial things, so I need to get prototyping soon.  I know the range on the rcwl0516 is adjustable.  Not sure if I need it spinning to pick up the difference between grass and everything else. Still working out the actual mechanicals.  Input is welcome on all aspects.

@farmeralan 

My first suggestion would be to put a light sensor in the "lid" and only extend the probe at night to reduce the possibility of damage. That may limit the types of sensor that you can use but should prolong it's useful life 🙂

Are you planning to use the threaded rod to raise the sensor ?

What power source will you be using inside the tube ?

@farmeralan Spinning??? So now you need a commutator. Just use 4 sensors arranged at 90 degrees apart. I doubt this will work though, even if the mechanical stuff all works, what will you be measuring? I don't know what the propagation path looks like but just looking at mine it may be as much as 90 degrees or as little as 30 degrees. I think you need to connect one and see what the readings are while moving a piece of turf up and down the distance you expect it to change. Lot's of challenges.

@will Mud will defeat the light sensor. Lot's of power needed.

Posted by: @zander

@will Mud will defeat the light sensor. Lot's of power needed.

It won't be mud that covers it:)

How about an RTC, they're low power and probably more reliable. Maybe even the Arduino millis() function would be close enough to just trigger it at night for a couple of months. Just test it and whenever it gets greater than a set value, add the total range and wait for the next crossing 🙂

Not sure what'll happen if one of the lazy buggers lies down on it though 🙂

I have to say that it seems counter-intuitive to measure the height of the grass while there are animals actively feeding on it.

@farmeralan

Surely it is density of grass, not simply its height, that provides forage production?

Usually this would be done manually. You throw a square randomly onto the paddock. Cut the grass within the square and then place it in a drying oven for enough time to remove the water content. You then weigh it. I used to do this for the ag dept. It also involved separating one type of grass from another.

There is the rising plate meter which squashes a selection of pasture beneath a weighted disc and measures the compressed height. The higher the plate sits from the ground, the taller and denser the pasture. To automate it I guess you just lower it to make a reading and lift it to let it continue to grow.

I think you can also use satellite imagery to calculate the height, density and health of vegetation.

Right now I have a small vegetable farm.

Define "small" and what types of vegetables?

@will Agreed, this is a super interesting challenge. Once I am finished my current project I might board up a sensor and see how it performs. I still think a sensor mounted above the ground high enough to not get damaged and measuring the return at a known angle might work, but the conditions re what walked, laid on the grass will really affect the readings. The reason I like my idea is by exposing the sensor to a wider scan area it may help average the differences between trampled and not.

Posted by: @zander

@will Agreed, this is a super interesting challenge.

They're the best kind 🙂

Posted by: @robotbuilder

@farmeralan

Surely it is density of grass, not simply its height, that provides forage production?

Also the type of grass (and whatever else is mixed in with it (clover, for instance).

@robotbuilder Well that makes a lot of sense but I do not understand why you would dry the grass, isn't the water an integral part of the product?

@robotbuilder right now I have 5 acres of intensive vegetables for a CSA and 20 acres of pasture with a few cows, meat chickens, and various other critters.  In the past I managed a large cattle and hay opperation in WY. 

I agree that there are other ways to measure forage, and those manual methods will be used to give us forage density and quality information.  What this sensor is for is to look at growth rate so we can see if the changes made to the soil microbiome change the productivity of a plot and to help in fine tuning the grazing rotation plan.

Ive been doing some testing with the rcwl-0516.  There seems to be a cycle time of about 4 seconds built into it.  Once it has triggered it will not detect motion during that 4 second time.  Not sure if there is a way to change this.  I know these have been used for collision avoidance in some projects.  Has this been an issue for anyone?  Maybe I just got a bum batch.