# Position sensing

#### cmartinez

Joined Jan 17, 2007
6,594

#### geoffers

Joined Oct 25, 2010
308
Thanks again everyone,
Fierce Guinea pigs got it! There's some great suggestions now, I like the inductive idea and the laser range finder, both involve minimum moving parts.

Who would of thought you could get a laser range finder on one small surface Mount chip amazing and so cheap.
I think I will get a module and see how I go with that, probably going to be a one off...

Budget, around £100, cheaper if I can
Cheers Geoff

#### shortbus

Joined Sep 30, 2009
7,078
We're all shooting in the dark over this. If the TS would just tell us what he's trying to measure better ideas could be given. I'm assuming it's some kind of soil compaction measurement, but that's just an assumption, like all of the members trying to help are doing.

Please @geoffers, tell us what you are trying to do. Something like this goes through a design process in real life. With the first step being to give the statement of what is trying to be achieved. With that many of us can give better ideas about how to do the job needed. Keeping members in the dark because, it's secret or you want to make and sell it, is not something you need to worry about here. Most of us have our own interests and do this as a way to help others, not to 'steal' ideas.

#### KMoffett

Joined Dec 19, 2007
2,770
Sounds like a soil penetrometer.

Ken

#### geoffers

Joined Oct 25, 2010
308
Hi all,
Sorry not trying to keep you in the dark, its for measuring crop height, the tube is attached to a plate which sits on the crop, the rod goes through the crop touches the ground, when you lift it up the pins slides out to full travel. Do many times and you get a good average of the height.

At the moment I'm inclined towards the range finder on the top of the tube but would love to hear any other ideas!

It does need to be gravity driven really and able to slide quite quickly as you walk.
Cheers Geoff

#### cmartinez

Joined Jan 17, 2007
6,594
I thought at first of a electronic laser measure but the crop may give a false reading, so could you attach one of these at all where the tape can be carried down with the rod?
https://www.amazon.ca/eTape16-ET16-75-DB-RP-Digital-Measure-Metric/dp/B015XON8DI/ref=sr_1_1?ie=UTF8&qid=1498663971&sr=8-1&keywords=electronic+tape+measure
Max.
First time I've seen one of those! ... Here's another one with bluetooth output that he could use to interface it to his phone. It would make it the hell of a lot easier to generate a log.

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#### BR-549

Joined Sep 22, 2013
4,938
Tie a helium balloon to the top of each crop. Now you can use a surveyor transit to calculate the average balloon height each evening.

#### BR-549

Joined Sep 22, 2013
4,938
Hang a curtain of sheets along one side of the field. Run a string of lites down low, on the opposite side.

In the evening when dark.....turn lites on and measure silhouette.

#### BR-549

Joined Sep 22, 2013
4,938
If you put a X-Y axis template on the sheets.......you can get a real-time, full-scale plot of the crop everyday.

Details mean everything. Not even enough details.

Is it a row crop.......or a field crop.....like hay or clover. Is it flat.....or on a hill? What is the mature crop height? Do you want an average of the whole field of crop.......or be able to measure each crop?

What is the crop?

#### geoffers

Joined Oct 25, 2010
308
The crop is a grazed sward, could be clover rye grass Lucerne, vetches or a mixture of above, could be drilled in tight rows (1" or so) or broadcast which is more random.

I need to know when it reaches a certain 'cover' (3000kg/ha of dry matter roughly) in order to graze it efficiently, measuring the compressed height or the sward which is compressed by the weight of the plate is the best way to approximate this. Lots of samples are required so I can tell what cover is in each paddock. I was thinking I would add a Bluetooth to the phone feature once I have it working.
I realise I was light on detail but the question is still how can I tell how far my rod has moved!

Max, the tape looks good but I fear the spring would interfere with movement of the rod and wear over time. I could easily be looking at 200 samples a use so the less mechanical the better, but thanks for a helpful suggestion.
Cheers Geoff

#### shortbus

Joined Sep 30, 2009
7,078
What ever did farmers/ranchers do in the past? Without electronics and computer management of grass crops, it's amazing that domestic livestock hasn't gone extinct.

#### KMoffett

Joined Dec 19, 2007
2,770
"the tube is attached to a plate which sits on the crop, the rod goes through the crop touches the ground, when you lift it up the pins slides out to full travel."
What "pins"? Can you post a sketch with this detail.

So, it is a form of a penetrometer.

Ken

#### BR-549

Joined Sep 22, 2013
4,938
Thanks......you want to measure density. And you're doing it with the height of a weight/area plate.

And without rows.....I'm assuming you can't run the tractor down the field?

Were you going to walk to different areas........drop from set distance.....then measure to ground......and try to proportion density/mass?

#### BR-549

Joined Sep 22, 2013
4,938
I am not familiar with your grass over there. How about a hula hoop composed of photo detectors pointing in. The hoop would be mounted on fixed legs........matching the crop. Turn on a LED at the middle of the hoop........and measure the detectors. As the crop grows......the detector output will fall. The hoop can easily be re-located.

Works best at nite and/or tarp. If this is just for you......you can develop a "feel" of an analog device for YOUR crop. And of course you can try different size hoops/LEDs.

If this is for marketing/serious hobby.......digital control processing and more study is needed.

The plate or even a capacitance density measurement might be off due to moisture content. I believe they use that fact to measure moisture in some apps.

The hoop measurement might be off due to storm/wind damage.

If the field looks to be constant density.......you can cut and weigh a small area....and apply math.

#### strantor

Joined Oct 3, 2010
5,160
"the tube is attached to a plate which sits on the crop, the rod goes through the crop touches the ground, when you lift it up the pins slides out to full travel."
What "pins"? Can you post a sketch with this detail.

So, it is a form of a penetrometer.

View attachment 129962

Ken
That's what I was picturing as well. Perfect application for an inductance or LVDT measurement. As I mentioned earlier, the LDC1000 is worth looking into. I have played with the LDC1000-EVM and done exactly this. It is one of my projects on the back burner. I have a 3' piece of PVC with wire tightly spiraled around it, and as i slide a steel rod thru the tube, the inductance changes wildly. With 32 bit resolution you can get a gnat's ass position from it.

#### cmartinez

Joined Jan 17, 2007
6,594
That's what I was picturing as well. Perfect application for an inductance or LVDT measurement. As I mentioned earlier, the LDC1000 is worth looking into. I have played with the LDC1000-EVM and done exactly this. It is one of my projects on the back burner. I have a 3' piece of PVC with wire tightly spiraled around it, and as i slide a steel rod thru the tube, the inductance changes wildly. With 32 bit resolution you can get a gnat's ass position from it.
I would've thought that 24 bits would be fare more than enough, but I'd really like to see 32 bit resolution for this application.

If travel distance is set at 300 mm, then think of this:
Code:
12 bits =         4,095 divisions = 0.07326        mm/div
16 bits =        65,535 divisions = 0.00458        mm/div
24 bits =    16,777,215 divisions = 0.0000178814   mm/div
32 bits = 4,294,967,293 divisions = 0.000000069849 mm/div
Wouldn't you say that 12 bits would be far more than enough for this application?

#### strantor

Joined Oct 3, 2010
5,160
I would've thought that 24 bits would be fare more than enough, but I'd really like to see 32 bit resolution for this application.

If travel distance is set at 300 mm, then think of this:
Code:
12 bits =         4,095 divisions = 0.07326        mm/div
16 bits =        65,535 divisions = 0.00458        mm/div
24 bits =    16,777,215 divisions = 0.0000178814   mm/div
32 bits = 4,294,967,293 divisions = 0.000000069849 mm/div
Wouldn't you say that 12 bits would be far more than enough for this application?
My memory was off. I thought it only came in 32 bit version; actually There is no 32 bit version. There's 12bit and 28 bit. yeah 12bit should be fine.