Why controls engineers need to master hydraulic health

Many electrical engineers will ignore mechanical properties of a machine because it’s not their specialty. Control systems engineers cannot do that. Controls require an understanding of the electrical and mechanical interface. Hydraulic fluids come into play when the system has mechanical requirements that use hydraulics. Pneumatics use air. Thus, systems may use electricity to drive hydraulic or pneumatic systems. If the hydraulic fluid is dirty, then the control system will be degraded. If you have a good control system, alarms or faults should be able to indicate a hydraulic problem.

What should a controls systems engineer look for? Systems will show the following symptoms when hydraulics degrade:

• erosion of precision components
• blockage and sticking of valve spools and orifices
• increased friction and stiction
• sensor drift and signal errors
• heat buildup and breakdown
• failure of electric-hydraulic servo valves. 

Component erosion: Control instruments such as servo valves, proportional valves, pressure transducers and positioners rely on extremely tight tolerances, often in the micron range. Dirty hydraulics means that the tolerances are compromised. Contaminants like metal particles, dirt or silica act like abrasives and cause leaks due to worn spool lands and valve seats.

As the system ages, there is a reduced ability to hold pressure or position. The reason yearly calibrations are needed on systems is due to wear. Gradual drift in instrument calibration from original installation is due to mechanical wear and degradation of hydraulic fluids. As this happens, the component life is shortened.

Blockage and sticking of valve spools and orifices: Many control instruments use micro-orifices or narrow passageways. Fine particulate blocks these passages. Spool valves become sluggish or stick mid-stroke. Actuators respond slowly or inconsistently. Control loops oscillate or become unstable. This is especially problematic in servo and proportional valves, which are extremely sensitive to contamination. Friction increases as particulates build up, as well.

Increased friction and stiction: As particulates increase, the fluid loses its lubricity, and deposits build up on moving parts. This results in sticky valve spools, jerky actuator movement and instruments that fail to return to zero or that overshoot setpoints. Technicians can reset the circuit and yet still get higher hysteresis in control loops. Over time, a blockage or a groove could be caused, and a valve may not make it past a specific point or jump. Stiction is often one of the earliest symptoms operators notice.

Sensor drift and signal errors: Pressure, flow and position sensors often rely on diaphragms, strain gauges or delicate transducers and these are the inputs to the system that would monitor increased contamination.

Where do operators see problems? Sluggish pressure transmission due to blocked impulse lines, chemical degradation of sensor diaphragms, fluid-induced swelling of isolation membranes and temperature-related drift from varnish or sludge changing fluid viscosity can lead to misleading readings and poor control quality.

If a machine maintainer keeps the OEM specifications and compares it to the degraded machine conditions, then these changes would be tracked. Also, this is why control system alarms should not be changed without discussion and documentation.

Seal damage and internal leakage: Debris can chemically attack elastomeric seals or physically wear them down. This would lead to mechanical failures. Systems may have to bypass actuators, or pumps may not work. The control authority could be reduced, and there could be a loss in holding pressure. Air ingestion in pump or valve could further destabilize control performance because the hydraulics will oxidize. Once seals degrade, the contamination accelerates rapidly. At that point, the control system should see increased heat.

Heat buildup and fluid breakdown: Contamination increases friction within pumps and actuators. This heat accelerates oxidation and varnish formation. Hydraulics become a stickier fluid that resists precise movement. Carbonaceous deposits may occur on valves. This creates a failure loop for instruments: contamination causes heat; heat creates varnish; varnish causes more contamination; instruments fail sooner.

Failure of electrohydraulic servo valves: Electrohydraulic servo valves are among the most contamination-sensitive components in any hydraulic system. What causes the electrohydraulics to fail? Restricted pilot orifice, malfunction of torque motor flappers/nozzles, spool misalignment or seizure and complete loss of fine control are the symptoms a control engineer should look for.

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Even a few microns of debris can disable a servo valve. Control instruments are typically built with tight tolerances and are dependent on clean, stable fluid properties. Control instruments are sensitive to tiny pressure or flow variations. Large hydraulic actuators are self-cleaning or debris-tolerant, but hydraulic servos are not. Contamination can completely disable an instrument.

What can the OEM do?

OEMs can add high-efficiency filtration: β₁₀ ≥ 200 or better. OEMs should require regular oil analysis that provides particle counts, water content, total acid number/total base number (TAN/TBN), varnish potential. Why? Hydraulic servos and valves have a hydraulic fluid particle count tolerance related to the mechanical build. If the OEM spells out these parameters as part of the system requirements, then the controls engineer will know the hydraulic fluid to choose and the tolerances to use for alarming. The maintenance people will know when to change out filters and oil-change cycles.

What can control system engineers and maintenance techs do?

The control system should maintain specifications. This means running the machine at the correct oil temperature. Systems should monitor temperatures, pressures and tank levels. Control systems also monitor for leaks, blockage, stiction and heat. Error and drift can be captured by monitoring proportional integral loop error values.

Maintenance steps should prevent moisture ingress. Maintenance procedures should include flushing before installing new control instruments. Maintenance personnel should be testing hydraulics weekly and utilizing off-line kidney loop filtration systems to maintain hydraulic fluid cleanliness.

Hydraulic systems require all hands on deck. The OEM should set the boundaries for fluid maintenance based on machine operations. Control system engineers can monitor the system and alarm for preventive activities. Maintenance personnel have responsibilities to manage the status quo.

All these hydraulic parameters are related to three simple ideas. Application speed, application temperature and application load or pressure are the three functions of a system that maintenance, controls and the OEM must agree on. Because a machine is dynamic, the hydraulics need to be tested regularly to make sure that the machine is still operating in the specification that the oil matches.

If speed, temperature and load are maintained, then the control system engineer may not have to change out hydraulic servos before the instrument’s end of life. If hydraulic fluids are not maintained, then hydraulic instruments will be changed out cyclically before the end of life.

The result is that machine costs will increase. One thing to keep in mind is that testing hydraulics and changing out filters and setting up machine alarms for temperature, speed and load is cheaper than changing out expensive instruments.