Ok, what follows is an excerpt of a position control explanation as given by Johnson City Kubota And Equipment LLC. Believe it or not, =), I will give a brief breakdown of the supplied explanation.
If you have a 3pt hitch control lever that has an up, down AND NEUTRAL position, in order to place the implement repeatedly to the same adjustment height, you must for example lower the implement by pressing the lever "Down". BUT,,, you have to guess when to return the lever to the neutral position when you want it to stop. The problem is, stooping the 3pt hitch is not instantaneous. The lift will have a bit of residual movement after you want it to stop, as well as the fact it takes a certain amount of time to get the lever back to the neutral position, making putting the implement back at an exact position nearly impossible.
Position Control however does not have this "3 Position Lever", allowing you to move the lever to an infinite number of positions along the length of travel. SUCH, that if you had a mechanical stop, something as simple as a bolt and a nut to stop the lever at a certain place along the arc of travel, whala, you have position control.
In this explanation from what should be an "Authority", this notion of there being a "Self Regulating Circuit" that counters "Drift" is not even mentioned, let alone being some kind of prerequisite. For the record, THIS is what the four different dealers I talked to that had any kind of clue stated.
here is a link to the full page:
http://www.johnsoncitykubota.com/Three Point Lift Types.htm
If we accept the Johnson City Kubota and Equipment, LLC explanation, then the Notion that a "Self Regulating Circuit" defines PC is false. NOT that such a circuit does not exist. I would not argue that this technology does not exist. BUT,, and this is hugely important. If Johnson City Kubota is correct, it has NOTHING to do with what PC is.
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Tractor Three Point Lift Types
Let's start by defining what a three point hitch is. Tractor three point hitches are so named because they consist of three major components. They are; the left and right lower lift arms (those long things with the balls in the end that are sticking out the back of the tractor) and the top link; usually a three piece adjustable apparatus with a central body and two threaded ends with balls, one of which attaches to the tractor, and the other which attaches to the implement. The overall length of the top link is adjustable via the threaded ends. The sole purpose of the three point hitch is to place, and hold, the mounted implement in the desired position to accomplish the job at hand. As you may have guessed, there are many different types of systems available to get this done. Some are quite simple, while others are extremely complex. In the simplest form, a hydraulic pump supplies oil to a control valve. The position of the control valve determines where the oil goes next. If the control valve is in neutral, the oil is simply directed back into the reservoir (transmission housing). If "raise" is selected, the oil from the pump is directed, by the control valve, into the lift cylinder, causing the lift arms to rise. If "lower" is selected, the oil from the pump is directed, by the control valve, to the reservoir (just like neutral); however, a passage is also opened which will allow oil to escape from the lift cylinder, allowing gravity to lower the lift arms. Many of the smaller compact tractors are, in fact, just that simple in lift control. They use what might be termed;
"Non position" control. This type of lift is simple to build, easy to troubleshoot/repair, and potentially very aggravating to use, depending on the implement. Let's assume you are using a rear "grooming" mower that is supported, while in mowing position, by four caster wheels (one at each corner of the deck). No sweat. Just shove the lift lever to the "down" position, and the casters take care of it from there. Now, suppose you want to put your box scraper on and put a nice "finish" grade on a yard. In order to do a good job, it is important to have "finite" control of the height of the box scraper. The problem is, you must manually return the control valve to neutral at the very instant the box scraper arrives at the correct height. What usually happens is that the "box" has already gone past where you want it to be before you can "jerk" the control valve into neutral. Need something more user friendly for box scraper work? Enter….
"Position control". This lift type has some extra parts called "feed back" linkage. This linkage can be either internal or external, yet it's purpose remains the same. Feed back linkage serves to return the control valve (automatically) to the neutral state as soon as the lift arms reach the point selected by the relative position of the lift control lever. Whereas the "non position" control type only has three positions (raise, lower or neutral), the position control type allows an infinite number of lift arm positions, from all the way down, to all the way up. If you regularly perform tasks that require precise, repeatable lift arm height control, definitely get the position control type of lift. Next we take a look at….