A Detailed Explanation of the Principle of Composition and Control of the CNC Servo-Hydraulic System

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  The principle of composition and control starts with three layers
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  How the control loop closes
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  Where hydraulic servo axes win and where they do not
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  What the principle means for part quality
• 5
  Hydraulic servo axis vs all-electric servo axis

A servo-hydraulic axis is built from three layers that must match each other: mechanical, hydraulic, and electronic. The mechanical layer is the machine frame, the linear guides, the ram or slide, and the ballscrew or rack that carries the load. The hydraulic layer is the pump, the manifold, the servo valve, and the cylinder or rotary actuator. The electronic layer is the NC controller, the drive amplifier, the encoder or linear scale, and the pressure and temperature sensors.

None of these layers works alone. If the guide rails are not straight, no valve tuning will hold a straight edge on the part. If the servo valve is oversized, the axis will hunt at low feed and leave chatter marks. If the encoder resolution is coarse, the loop cannot see the error before it becomes a visible step. Sizing starts from the load, not from the catalog.

The mechanical layer sets the ceiling. Mass, guide friction, and structural stiffness decide how fast the axis can accelerate before it rings. A heavy ram with recirculating roller guides can move at high feed, but the same ram on box ways with poor lubrication will stick and slip. The hydraulic and electronic layers can only work inside that ceiling.

That is the first half of the principle of composition and control: composition is not a parts list. It is a set of matched capacities, and the weakest layer caps the whole axis.

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  MechanicalFrame, guides, ram, and screw. Sets stiffness and moving mass.
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  HydraulicPump, manifold, servo valve, cylinder. Sets force and speed.
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  ElectronicNC controller, drive, encoder, sensors. Sets loop bandwidth and resolution.

The controller compares commanded position with actual position and turns the difference into a valve command. That command is a current, not a pressure. The servo valve converts current into spool displacement, the spool opens a flow path, and the cylinder moves. A linear scale reads the result and the loop repeats, thousands of times per second.

Three loops are stacked. The innermost is the valve spool position loop, usually closed inside the valve itself. Around it sits the cylinder position loop, closed by the NC. The outer loop is the feed drive that interpolates axes together along the tool path. Errors in the inner loops show up as surface marks; errors in the outer loop show up as dimensional error.

Gain is the tuning knob that matters most. Too little gain and the axis lags behind the command, rounding corners and leaving following error. Too much gain and the axis overshoots, then corrects, then overshoots again. On a hydraulic axis the limit usually arrives earlier than on a ballscrew axis because oil is compressible and the valve has its own lag.

Feedforward helps more than raw gain. If the controller knows the acceleration the path needs, it can add the expected valve current before the error appears. That keeps following error small without pushing the loop into oscillation. Many hydraulic controllers expose a separate feedforward gain for exactly this reason.

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  Valve spool loopClosed inside the servo valve, fast, rarely touched by the user.
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  Cylinder position loopClosed by the NC using the linear scale. Main tuning target.
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  Path interpolation loopCoordinates axes. Sets contour error on curves and corners.

Force density is the main reason to choose hydraulics. A cylinder of modest bore can push tens of kN with a small package, which is hard to match with a ballscrew and servo motor. That is why hydraulic axes dominate presses, injection units, heavy ram milling, and some broaching and forming operations. If the axis needs high force at low speed, hydraulics is usually the shorter path.

Stiffness under load is the second reason. Oil under pressure behaves like a very stiff spring, and a well-tuned hydraulic axis holds position against a cutting force without much deflection. That matters on heavy interrupted cuts where a ballscrew axis would be pushed back and then spring forward.

The drawbacks are real. Oil temperature changes viscosity, and viscosity changes response. A machine that is tuned cold will behave differently after four hours of running. Good designs add oil cooling, temperature compensation in the controller, or both. Without that, the axis drifts and the operator retunes it, which is not a stable process.

Cleanliness and maintenance are the other cost. Hydraulic oil leaks, filters load up, and servo valves are sensitive to contamination. A dirty system will show it as erratic motion long before it fails outright. Any shop running hydraulic servo axes needs a filtration and oil-analysis routine, not just a repair crew.

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  Choose hydraulicsHigh force, low speed, high stiffness, heavy interrupted cuts.
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  Choose all-electricHigh speed, low force, clean room, tight thermal control, low maintenance.

On a machined part, a hydraulic servo axis leaves specific fingerprints. Following error during acceleration shows up as a slight taper on a long straight cut. Valve deadband shows up as a flat spot when the axis reverses, often as a small step at a direction change. Thermal drift shows up as a size trend across a shift, not as a random scatter.

That is useful for diagnosis. If parts are consistently oversize at the start of a shift and on size later, look at oil temperature before touching the gain. If a step appears only at reversal, look at valve deadband and backlash compensation. If the surface has a regular ripple at a fixed spacing, look at the loop bandwidth relative to the feed rate.

The same logic applies to choosing a supplier. A shop that runs hydraulic servo axes needs the supporting discipline: oil analysis, filter changes, temperature logs, and a tuning procedure that is written down. Without those, the machine's theoretical capability never reaches the part. With them, hydraulic axes hold very tight tolerances on heavy work.

At GreatLight we run 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, and 16 mill-turn centers, with a maximum processing size of 4,000 mm. We hold ±0.005 mm and finish down to Ra 0.2–0.8 μm on the parts where the drawing calls for it.

If your part needs heavy stock removal followed by a tight finish, the axis type on each operation matters. Roughing on a stiff hydraulic or high-torque axis and finishing on a fast, thermally stable axis is often the better split than forcing one machine to do both.

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  Taper on a long cutCheck following error and acceleration feedforward.
• 2
  Step at reversalCheck valve deadband and backlash compensation.
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  Size trend over a shiftCheck oil temperature and thermal compensation.