MOOG D661 Spool Feedback Is Correct, but Cylinder Speed Is Wrong

D661 feedback tells the controller where the main spool is. It does not measure cylinder speed. When feedback follows the command but cylinder speed changes, investigate valve pressure drop, available supply flow, load and leakage.

If the command changes while feedback remains stationary or lags substantially, investigate supply voltage, enable status, pilot differential and spool movement. Separating these situations avoids unnecessary valve replacement.

Feedback represents main-spool position

The D661-G…A has several signal codes. Code M uses a ±10 V command and 4–20 mA spool feedback. Code D uses a ±10 V command and 2–10 V feedback. Their centered feedback values are 12 mA and 6 V respectively.

For code M, 16 mA corresponds to (16 − 12) ÷ 8 = 50% of rated positive spool stroke. It does not mean 16 L/min or half of the cylinder stroke. Cylinder position comes from the machine's displacement sensor, a separate measurement.

A PLC that maps 4–20 mA to 0–100% displays 12 mA as 50%. That scale runs from one spool end to the other. For comparison with a bipolar command, use −100% to +100%, with center at 0%. Otherwise, the normal centered position can look like a half-open valve.

Flow changes even when spool position does not

Flow also depends on the pressure drop across the valve. At a fixed opening, reducing pressure drop from 70 to 17.5 bar reduces flow to approximately half, following the square-root relationship.

Consider a linear characteristic rated at 80 L/min with a total rated valve drop of 70 bar. At 50% opening, estimated flow is 40 L/min at 70 bar total drop and 20 L/min at 17.5 bar. This calculation assumes a symmetric circuit with equal flow through the two metering edges and neglects leakage.

For a double-rod cylinder with an effective area of 40 cm², those flows produce approximately 167 and 83 mm/s. Spool feedback can remain at 16 mA in both cases while cylinder speed differs by a factor of two.

Unchanged feedback and reduced speed under increasing load are therefore consistent. Pump pressure, load pressure and return backpressure determine the available valve drop. The cylinder flow-sizing example explains how these conditions enter the calculation.

Three different causes behind the same feedback value

One case is changing load. An unloaded stroke is normal, but motion slows when the tool presses against the workpiece. Spool position follows correctly, while the available pressure drop from P to the working port decreases. Check supply-pressure headroom and pump delivery at that pressure.

A second case is simultaneous operation. One cylinder runs normally until another branch starts. Both slow down without a clear change in spool feedback. Record P pressure and pump delivery during the combined movement to distinguish limited supply flow from rising return backpressure.

A third case is declining effective flow after warm-up. At comparable load and valve pressure drop, feedback stays stable while the cylinder slows. Separate pump, valve and cylinder leakage. Correct position feedback does not rule out internal valve leakage because the position sensor does not measure leakage flow.

In each case, the spool has reached its requested position, but less flow produces useful actuator motion. Opening the valve further may temporarily compensate for some lost flow while leaving a supply or leakage problem unresolved.

Verify tracking during the movement

Do not rely only on the final number on the display. Record command and feedback on the same time axis during starting, acceleration and reversal. This reveals lag, saturation and intermittent signal loss. Correct steady-state feedback at 16 mA does not establish correct acceleration behavior.

Electrical personnel should measure the signal using the matching interface. The 4–20 mA output of codes M and X cannot use the same PLC scaling as code D's 2–10 V output. Circuit loading and wiring must also match. The D661 electrical-code explanation identifies the relevant fields.

For the 4–20 mA output, the catalog identifies 0 mA as a cable-break detection condition; normal center is 12 mA. A field reading of 0 mA also calls for checks of valve power, loop connections and the measurement method. It is not a centered-spool reading.

Record stroke time alongside feedback

Without a flow meter, a permitted low-risk machine movement can provide a useful estimate. Measure a fixed travel and its elapsed time, then multiply average speed by effective cylinder area. For a single-rod cylinder, use cap-end area during extension and annular area during retraction.

“16 mA feedback, slow motion” leaves too much unexplained. Add load, P/A/B/T pressures, oil temperature, travel and time. These distinguish a spool-position problem from reduced effective flow at the same opening. Use existing test points and keep guards and interlocks active.

Correct feedback establishes a position-tracking result, not correct flow throughout the hydraulic system. When command and feedback disagree, investigate valve drive and pilot conditions. When they agree but speed is wrong, investigate pressure drop, supply flow and flow lost to leakage.