TConnolly
Lifetime Supporting Member
OK, here is a headscratcher for you all. I believe that I have correctly diagnosed this problem, but I want to double check if you all would come to the same conclusion as I did.
Cylinder bore: 17”
Rod Diameter: 8”
Max operating pressure: 4,000 PSI
Max tonnage: 455 tons
Normal operating tonnage: 400 tons.
One year ago the cylinder was retrofitted with a servo valve for position and force control. The piston was replaced by a new piston modified to accept a magnetostrictive position transducer. To keep piping distances between the cylinder and the servo valve at an absolute minimum, a manifold is mounted directly on the cylinder blind end cap. The manifold ports directly into to the blind end cap through a 1” port with an o-ring between the manifold and the cap. The position sensor head mounts in the center of the cylinder cap, so the hydraulic port and manifold are to one side of the cylinder cap, centered along the radius line of the cylinder cap. The manifold also connects to the rod end of the cylinder via hard tubing.. Mounted to the manifold are a bang-bang J spool DCV valve with a P.O. check subplate and a directional control servo valve. Both control motion of the cylinder. The bang/bang valve was included for rapid coarse motion however it is rarely used and not necessary. The piston is always operated by the servo valve. The bang-bang valve is isolated from the servo valve by the P.O. check valve subplate between the bang-bang valve and the manifold. The manifold also has test ports, and pressure transducers for supply pressure, blind end pressure, and rod end pressure. The initial load on the rod is 80 tons. Applied process tonnage is 400 tons.
The hydraulic cylinder operated with the expected precision for nearly a year. Then it began exhibiting anomalous behavior in that the piston would sometimes not move when it was first commanded to move. When the problem occurred the motion controller PID would wind up and fully open the servo valve. Valve spool position feedback confirms that the valve is fully open and doing what it was commanded. Pump pressure was verified at 4,000 PSI. The pressure transducer for the blind end of the cylinder would show 4,000 PSI and the pressure transducer for the rod end would show 0 PSI, however the piston would not move. Pressure transducer readings were corroborated by pressure gages at the test ports. The piston would also not move if the bang-bang valve was activated. However, after about one minute the problem would apparently clear itself and the piston would begin moving and would complete its operation with its usual precision, apply full load to the process, and then retract for the next stroke. The problem would manifest itself randomly a few times a day, but over time the frequency of the problem increased, always with the problem self clearing after about a minute. The cylinder was disassembled and no mechanical damage or seal damage could be found. The seals were replaced and the newly rebuilt cylinder was put back in service. It continued to manifest the problem. An identical cylinder on a second machine began having similar problems. Then the first cylinder quit self clearing the problem; it stalled in the retracted position with full pressure applied to the piston, no pressure in the rod end, and it would remain in that position, unable to move, indefinitely until the cycle was aborted. But if all of the pre load weight was removed from the piston then it would follow its motion profile with the expected precision.
The servo valve, the magnetiostrictive positon sensor, the bang-bang valve, the hydraulic pump, and the pressure transducers are all fully functional. There is nothing wrong with the PLC logic or with the motion control card.
So, what do you all think was going on?
Cylinder bore: 17”
Rod Diameter: 8”
Max operating pressure: 4,000 PSI
Max tonnage: 455 tons
Normal operating tonnage: 400 tons.
One year ago the cylinder was retrofitted with a servo valve for position and force control. The piston was replaced by a new piston modified to accept a magnetostrictive position transducer. To keep piping distances between the cylinder and the servo valve at an absolute minimum, a manifold is mounted directly on the cylinder blind end cap. The manifold ports directly into to the blind end cap through a 1” port with an o-ring between the manifold and the cap. The position sensor head mounts in the center of the cylinder cap, so the hydraulic port and manifold are to one side of the cylinder cap, centered along the radius line of the cylinder cap. The manifold also connects to the rod end of the cylinder via hard tubing.. Mounted to the manifold are a bang-bang J spool DCV valve with a P.O. check subplate and a directional control servo valve. Both control motion of the cylinder. The bang/bang valve was included for rapid coarse motion however it is rarely used and not necessary. The piston is always operated by the servo valve. The bang-bang valve is isolated from the servo valve by the P.O. check valve subplate between the bang-bang valve and the manifold. The manifold also has test ports, and pressure transducers for supply pressure, blind end pressure, and rod end pressure. The initial load on the rod is 80 tons. Applied process tonnage is 400 tons.
The hydraulic cylinder operated with the expected precision for nearly a year. Then it began exhibiting anomalous behavior in that the piston would sometimes not move when it was first commanded to move. When the problem occurred the motion controller PID would wind up and fully open the servo valve. Valve spool position feedback confirms that the valve is fully open and doing what it was commanded. Pump pressure was verified at 4,000 PSI. The pressure transducer for the blind end of the cylinder would show 4,000 PSI and the pressure transducer for the rod end would show 0 PSI, however the piston would not move. Pressure transducer readings were corroborated by pressure gages at the test ports. The piston would also not move if the bang-bang valve was activated. However, after about one minute the problem would apparently clear itself and the piston would begin moving and would complete its operation with its usual precision, apply full load to the process, and then retract for the next stroke. The problem would manifest itself randomly a few times a day, but over time the frequency of the problem increased, always with the problem self clearing after about a minute. The cylinder was disassembled and no mechanical damage or seal damage could be found. The seals were replaced and the newly rebuilt cylinder was put back in service. It continued to manifest the problem. An identical cylinder on a second machine began having similar problems. Then the first cylinder quit self clearing the problem; it stalled in the retracted position with full pressure applied to the piston, no pressure in the rod end, and it would remain in that position, unable to move, indefinitely until the cycle was aborted. But if all of the pre load weight was removed from the piston then it would follow its motion profile with the expected precision.
The servo valve, the magnetiostrictive positon sensor, the bang-bang valve, the hydraulic pump, and the pressure transducers are all fully functional. There is nothing wrong with the PLC logic or with the motion control card.
So, what do you all think was going on?
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I was hoping to have this turn into a good brainteaser.
