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Fieldview cab margin map
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Posted 10/9/2017 21:19 (#6298290 - in reply to #6287527)
Subject: RE: Fieldview cab margin map

It literally pains me to read these posts again and again. At first I thought you were only here to ask confusing questions as a clever marketing strategy to drive conversation around a particular product line but now I am having doubts.

this is a link to a data sheet on a common airbag that is used on the Deere XP row unit. Lets just work this out so you don't have to keep wondering about it.

Notice how in the middle of it's travel range it has a volume of about 100 in^3 at around the maximum pressure of 100 psi. Lets call this 0.05 ft^3 just as a round number. 

You can discharge this pressure through the 1/8 pipe thread fitting on the top fairly quickly so the control reaction speed of the system is limited by the maximum flow rate into the air bag not the flow rate out of it. Let's assume you want to size this system so that the control response is equal from an increasing and decreasing side.

This airbag will go from maximum pressure to zero in about 5 seconds.

So what do we need to get it from zero to maximum in 5 seconds .05 ft^3 in 1/12 min = ~ 0.6 CFM

So I don't know what the specs are on the compressor but lets look at a typical 12 V compressor like this

Which you can see puts out 1.06 CFM. Awesome if you had a one row planter but you don't so on a 12 row you will only get 0.09 CFM which means on a 12 row planter it would take about 40 seconds to completely fill the whole planter

To achieve a 5 second response time you will need a compressor with capacity of 7.2 CFM

So you can see something like this will get you there but you are already taking 8 GPM of hydraulic flow to do it just to get a 5 second control response, which at 5 mph planting speed is like 35 ft you have traveled.

Again at 100 psi you can flow roughly 20 cfm through a 1/8" orifice which is what you will have through a 1/4" push connect hose

So if you buy a hydraulically driven compressor that gets you to your 7.2 CFM you still aren't having a big problem with the lines.

What you need to do is the same math but for say 1 second or 2 which is what you want. 

It doesn't make any sense.

Here are a couple of axioms about automatic down pressure control.

1. The amount of excess down pressure you need to run decreases as control system accuracy increases

2. The accuracy of the control system is proportional to the rate at which you can increase or decrease the force in the system

The Precision Planting people will tell you the same thing, other than that their engineering team is not here responding to you- unless you actually are them, covertly ;)

You can mess with the gain settings and control parameters all you want but it will never substantially change you system because by the time you increase the air flow rate and orifice sizing to the point where it achieves the desired goal you will be spending so much tractor power doing it that it makes no sense.

This is why the Reflex system has such incredibly high energy density coupled with valves that open and shut in 20 mS

We actually drive this argument even farther and are able to automate the down pressure, the row cleaner and the closing wheel with a 20mS control response with the same hydraulic flow rate (hydraulic horsepower is proportional to pressure * flow- these are all 3000 psi systems) as the the Delta Force uses for just the down pressure.

In reality with a system that can realistically make an adjustment in 10-20 seconds you have to run higher excess gauge wheel load because you can't ever afford to lose ground contact. 99% of the time you are better off running a bit too much than too little.


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