How to Verify a Pump-Off Controller Position Signal
The position signal is the horizontal axis of every dynamometer card, so if it is wrong the card is twisted no matter how good the load cell is. A pump-off controller uses position to know where in the stroke each load reading belongs and to count strokes, and a position sensor that is reversed, non-linear, or miscounting produces cards that read as pump faults and stroke counts that drift. This procedure verifies the position channel after the load channel is confirmed. It covers the endpoints, the direction, the linearity, and the one-count-per-stroke requirement.
Verify POC Position Signal in one line: To verify a pump-off controller position signal, confirm it reads a repeatable zero at the bottom of the stroke and full scale at the top, that the value increases smoothly as the polished rod rises with no dead spots, and that the sensor produces exactly one clean count per pump stroke. Cross-check that peak load lands on the upstroke half of the card. A reversed, non-linear, or double-counting position signal must be corrected before the controller builds a card or counts strokes.
Confirm the Endpoints and Direction
Cycle the unit slowly, by hand where safe or at low speed, and watch the raw position value. It must reach a consistent low value at the bottom of the stroke and a consistent high value at the top, and those endpoints must repeat every stroke. If the bottom reading wanders or the top clips before the true top of travel, the sensor range or mounting is off and the card will be truncated or shifted. Set or confirm the position span so the full mechanical stroke maps to the full signal range.
Direction is the make-or-break check. The position value must increase as the polished rod rises. If it runs backward, the controller mirrors the card left to right, and a normal full-pump card looks like a pump fault. This single reversal is one of the most common commissioning errors and it invalidates every diagnosis downstream, so confirm it explicitly and fix it in the mounting or configuration rather than trying to compensate elsewhere.
Cross-check position against the already-verified load. The peak load should sit on the upstroke and the minimum on the downstroke. If peak load lands on the downstroke side of the card, position and load disagree on stroke direction, and you must reconcile them. Getting the two signals consistent is what allows the controller to compute a correct card and a meaningful pump-off decision.
Check Linearity and Count Integrity
Between the endpoints the position signal should move smoothly and monotonically. Sweep the stroke slowly and watch for dead spots, jumps, or reversals in the middle of travel, which indicate a worn potentiometer, a marginal inclinometer, or a magnet-and-sensor gap that is too large. A non-linear position axis distorts the dynamometer card in the middle of the stroke, exactly where fluid-pound and gas signatures appear, so linearity errors directly corrupt diagnosis even when the endpoints look fine.
For any position sensor that also serves as the stroke counter, verify it produces exactly one count per pump stroke. A crank magnet placed where the sensor catches it twice per revolution doubles the reported strokes-per-minute; a gap that lets the sensor miss occasionally undercounts. Watch several strokes and confirm the count increments once per full cycle. A wrong stroke count throws off runtime accumulation and any speed-based logic, and it is the same signal behind the well's strokes-per-minute reading.
If the controller derives position from crank angle rather than a linear travel sensor, confirm the angle-to-position mapping matches the unit geometry. A geometry mismatch places the top and bottom of stroke at the wrong angles and skews the whole card. Where the controller offers a geometry or unit-type selection, verify it is set for the actual pumping unit on site, because the shape of the card depends on that mapping being right.
Confirm the Running Card and Stroke Count
With endpoints, direction, and linearity verified, run the well and capture several cards. A correct position channel yields cards that use the full width of the position axis, place the load pickup at the bottom and release at the top, and repeat stroke to stroke. If the card is narrow, shifted, or wandering horizontally, the position signal still needs attention even if the load channel is perfect. The width and placement of the card are the visible proof that position is right.
Confirm the reported strokes per minute matches a manual count. Time the unit against a watch for a minute, or count crank revolutions, and compare against what the controller reports. A mismatch means the position sensor is miscounting and must be corrected before any runtime or speed logic is trusted. This manual cross-check takes a minute and catches double-counting that a card alone will not reveal.
Only when position reads correct endpoints, correct direction, smooth linearity, and an accurate stroke count is the card fully trustworthy. At that point the controller has both a verified load axis and a verified position axis, and the computed card is safe to diagnose from and to base pump-off shutdowns on. Position verification is the second half of what makes a dynamometer-based controller reliable.
Common Mistakes
The classic mistake is a reversed position signal that produces a mirror-image card read as a pump fault. Always confirm the position value increases as the polished rod rises before interpreting any card. The reversal is invisible if you only look at shape and obvious if you check direction.
The second is ignoring mid-stroke linearity because the endpoints look fine. A dead spot in the middle of travel distorts exactly the part of the card where fluid pound and gas show up. The third is not verifying the stroke count against a manual timing, which lets a double-counting sensor inflate the reported speed and corrupt runtime accumulation without ever distorting the card shape.
Frequently Asked Questions
How do I know if the position signal is reversed?
Cycle the unit slowly and watch the raw position value: it must increase as the polished rod rises on the upstroke. If it decreases as the rod rises, the signal is reversed, and the controller mirrors the card so a healthy full-pump card reads as a fault. Confirm direction explicitly, then fix it in the sensor mounting or the configuration rather than trying to compensate downstream.
Why does the position sensor need one count per stroke?
Many controllers use the same position pickup to count strokes and accumulate runtime. If a crank magnet is caught twice per revolution the reported strokes per minute doubles; if the sensor misses occasionally it undercounts. Watch several strokes and confirm the count increments exactly once per full cycle, then cross-check against a manual timing. A wrong count corrupts speed logic and runtime totals even when the card shape looks correct.
What causes a distorted card even when the load cell is good?
A bad position channel. If the endpoints clip before true top or bottom, the direction is reversed, or the signal is non-linear in the middle of travel, the card is truncated, mirrored, or twisted regardless of load-cell quality. Sweep the stroke slowly, confirm smooth monotonic travel between repeatable endpoints, and verify the unit geometry selection matches the actual pumping unit.
Sources and verification
This page references the vendor products and their official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
- Rockwell Automation Literature Library (Allen-Bradley, Studio 5000) - Rockwell Automation
- Siemens SIMATIC and TIA Portal documentation - Siemens
Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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