Performance of the CNC machine servomotor system during use

• 1
  What a CNC machine servomotor system actually controls
• 2
  Loop gain, damping, and step response in use
• 3
  Resolution and feedback accuracy in the detection chain
• 4
  Speed against stability: where servo tuning goes wrong
• 5
  Servo behavior and what each symptom points to

A CNC machine servomotor system is a closed loop. The control sends a position command, the drive converts it into torque, the motor turns a ball screw or a pinion, and a feedback device reports where the axis really went. Every error you see on a part traces back to one link in that chain.

The loop does not care about the drawing. It only compares command and feedback. If the feedback scale sits on the motor shaft, screw pitch error, thermal growth, and backlash never enter the loop. The table can drift and the drive will still report near-zero error.

That is why machine builders put linear scales on large and medium machines. On a 4,000 mm travel, a 0.01 mm pitch error over the screw length is larger than the tolerance band itself. Direct measurement of the slide removes that term from the error budget.

Open-loop gain K sets how hard the loop pushes back. Typical CNC servo mechanisms run K between 20 and 30 1/s. Below 20, the axis feels soft on direction changes and contour error grows on arcs. Push much above 30 and the axis starts to ring.

• 1
  Feedback location decides what is controllableMotor-mounted encoder cannot see screw or thermal error.
• 2
  Axis inertia sets the ceilingHeavy tables need more gain, not more motor.
• 3
  Resolution is not accuracyA fine scale can still read a loose axis.

In a second-order model, the damping coefficient falls as gain rises. A step command that overshoots or oscillates means the loop has more gain than the mechanics can absorb. The usual fix is not to lower K but to stiffen the drive train.

A servo system with K below 20 is generated by a soft coupling, a worn ball screw, or an oversized load on a small motor. The drive cannot correct a mechanical spring. Gain tuning only exposes it.

Watch the following error during a slow feed. On a healthy axis it stays constant and small. If it climbs as the axis heats up, the problem is thermal, not electrical. If it jumps at one spot on the travel, look for a tight spot or a damaged screw.

The step response test still tells you the most in the least time. Command a small move, capture the encoder trace, and read overshoot, rise time, and settling. No proprietary tool needed. A scope and a drive trace are enough.

• 1
  Constant following errorNormal. It scales with feed rate.
• 2
  Rising following errorThermal growth or a dragging slide.
• 3
  Spike at one positionMechanical defect on the screw or guide.

Resolution is the smallest displacement the system can report. It is a property of the detection element plus the measuring line, not of the motor. A drive rated at 1 μm resolution cannot resolve better than the scale feeding it.

Linear induction synchronizers used on larger CNC machines reach ±0.0001 mm accuracy, 0.05 μm sensitivity, and 0.2 μm repeatability. Those numbers describe the sensor. The machine around it rarely holds that well once load and heat are applied.

In a closed loop, the error from the detection element and the true axis deviation are hard to separate. If the scale reads high by 2 μm, the loop will drive the table 2 μm off and call it correct. Calibration of the feedback device matters as much as its resolution.

For most parts machined to ±0.005 mm, a standard glass scale is enough. Chasing 0.05 μm sensors on a machine with 0.5 μm of thermal drift per hour buys nothing measurable on the part.

• 1
  Match sensor to machineA 0.2 μm scale on a 5 μm frame is wasted.
• 2
  Repeatability beats resolutionRepeatable error can be compensated.

High gain gives fast response and tight contouring. It also amplifies every resonance in the structure. On a machine with a long, thin ball screw, the screw's own axial resonance sits low enough that aggressive tuning will excite it.

The practical approach is to set gain as high as the mechanics allow, then back off slightly. A small amount of following error at high feed is cheaper than a surface that rings on every corner.

Feed-forward terms change the picture. They reduce following error without raising the loop gain, so the axis stays stable while tracking improves. On contoured parts this shows up as better corner accuracy at the same cutting speed.

Mechanical fixes still beat electrical ones. Preloaded ball nuts, a stiffer coupling, and a properly aligned guide remove the error at the source. The servo only has to hold what the mechanics deliver.

• 1
  Raise gain until it ringsThen back off 20–30% for margin.
• 2
  Use feed-forward firstTracking improves without instability.
• 3
  Fix mechanics before tuningGain hides backlash, it does not remove it.