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In the Shop

RSS By: Dan Anderson, Farm Journal

As a farm machinery mechanic and writer, Dan brings a hands-on approach that only a pro can muster. Along with his In the Shop blog, Dan writes a column by the same name as well as the Shop Series for Farm Journal magazine. Always providing practical information, he is a master at tackling technical topics and making them easy for all of our readers to understand. He and his wife, Becky, live near Bouton, Iowa.

In Simple Terms--Potentiometers

Sep 27, 2010

 Potentiometers, sometimes called "pots," are used frequently on modern farm equipment. In simple terms, potentiometers connect to a shaft or linkage that in turn is connected to some part of the equipment that pivots or moves in some way. A soybean platform automatic header height control is a good example. There is a shaft that goes all the way across the skid shoes underneath platforms that rotates as the skid shoes flex. A potentiometer, connected to the end of that sensing shaft, allows the combine to "sense" the position of the shaft/skid shoes and raise or lower the platform as necessary.

Potentiometers merely measure movement and translate the movement into a voltage reading. Of the three wires that go to potentiometers, one is supplied constant voltage, one is ground, and the other is "signal" that varies with the position of the potentiometer's shaft. The amount of signal voltage varies as the potentiometer's input shaft/linkage is rotated.

You often hear mechanics talk about "calibrating" potentiometers. Calibrating is the process of moving the part of the machine the potentiometer is attached to through it's range of motion. This "teaches" the machine the upper and lower limits of voltage the potentiometer will see during the machine's operation. On the bean platform, the potentiometer is calibrated by lowering the platform all the way down, then raising it until the sensor pads no longer contact the ground. The combine "learns" the voltages for upper and lower pad movement, and uses that information to automatically adjust header height.

If something changes about the way the machine operates---the platform hits a rock that bends the sensing shaft, a belt wears into a potentiometer wire, a potentiomenter linkage gets bent--that may change the movement of the potentiometer's shaft and allow it to report higher or lower voltage than normal to the combine. Any variation outside the previously determined voltage ranges can cause the system to throw a warning code and possibly shut down---even as little as 1/1,000 of a volt. 

At that point it's up to the operator or mechanic to determine if there is something wrong that needs repaired (broken wire, broken linkage) or if the system simply needs recalibrated to clear the system and "start over."

My experience is to recalibrate first, maybe several times, tinker with "cheating" the upper and lower setpoints during recalibration, and then replace/repair if needed. Potentiometers are nifty little gadgets, but they can drive you crazy if you let them.

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