Multi-Axis Synchronization Control Method of CNC Machine Tools

• 1
  Key takeaways
• 2
  What multi-axis synchronization control actually closes
• 3
  Master-slave, cross-coupled and gantry schemes compared
• 4
  Parameters that decide synchronization accuracy
• 5
  Synchronizing rotary axes and RTCP on 5-axis machines
• 6
  Step-by-step tuning procedure
• 7
  Which synchronization scheme fits which machine

Multi-axis synchronization control means two or more servo axes follow one commanded path so their positions stay locked in a fixed relationship. On a gantry mill that relationship is a straight line: the two Y-axis motors must stay square to each other within a few microns. On a 5-axis machine it is a moving geometry, where the rotary table and the spindle head must keep the tool tip on the programmed path while the part rotates.

The controller closes two loops per axis. The inner velocity loop keeps motor speed proportional to the command. The outer position loop compares the encoder count with the interpolated setpoint and corrects the error. Synchronization lives in the outer loop, because that is where the difference between two axes becomes visible. If one loop is tuned softer than the other, the axes drift apart during acceleration and come back together at steady state.

CNC systems add a third layer that ordinary machines do not have: feedback comparison between axes. The control reads both encoders, computes the following error of each, and injects a correction into the lagging axis. This is why a CNC gantry can hold squareness through a reversal that would twist a mechanically coupled machine.

The practical limit is mechanical. A synchronization error you can measure with a dial indicator on the gantry is real, but if the rack has 0.03 mm of backlash or the linear guide preload is gone, no gain change will remove it. Tune the mechanics first, then the loops.

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  Velocity loopInner loop. Sets how fast the motor reaches commanded speed.
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  Position loopOuter loop. Sets how tightly the axis holds commanded position.
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  Cross-axis correctionThe CNC-specific layer that compares two encoders and corrects the lagging axis.

Master-slave is the default on most dual-drive gantries. One drive is master; the second gets the master's velocity command plus a position correction from the encoder difference. It is simple, and it works well when the two sides of the gantry have similar inertia. Its weakness is that the slave only reacts after an error appears, so during a hard reversal the two sides can separate by 10–30 µm before the correction catches up.

Cross-coupled control feeds the difference between the two axes back into both drives. Each axis is corrected toward the average position instead of one axis chasing the other. On a gantry with a heavy cross beam this cuts reversal error roughly in half. The cost is tuning time: two gain sets must be balanced, and an unbalanced pair will oscillate.

Electronic gantry with torque sharing is the scheme we use on machines where the beam is long or the load is off-center. Instead of commanding equal velocity, the control splits the torque command between the two motors based on the position error of each. A heavy cut on the left side loads the left motor more, and the beam stays square without twisting.

For rotary pairing, such as a tilting head driven by two motors, master-slave with a backlash-compensation offset is usually enough. Cross-coupled control is not worth the tuning effort unless the rotary axes carry a large offset load, for example a heavy fixture hanging off a trunnion.

Position gain is the first number to set. It defines how much velocity the drive commands per unit of position error. Too low and the axis lags; too high and the machine rings. On a typical machining center with a 40 mm pitch ball screw and a 1:1 coupling, we start around 30–50 rad/s and raise it in 5 rad/s steps until the axis hums, then back off 20 percent.

Velocity feedforward matters more than most people expect. It predicts the velocity needed for the commanded path and adds it ahead of the position loop, which removes following error during constant feed. With feedforward set correctly, a 1,000 mm/min move should show almost no steady-state following error on either axis. Without it, the two axes can each lag by the same amount and still be synchronized, but any difference in their gains shows up as a contour error.

The synchronization threshold is the window the controller accepts before it flags or corrects an error. Set it too wide and small misalignments pass through into the part. Set it too tight and the machine trips on normal thermal drift. On a gantry with a 4,000 mm travel, we usually allow 0.02–0.05 mm of correction window and alarm at 0.10 mm.

Current or torque limit on the follower axis protects the mechanics. If the slave hits its limit, the master keeps moving and the gantry twists. That twist is far more expensive than a stopped cycle, so we set the follower limit slightly below the master's and let the alarm stop the machine.

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  Position gainStart 30–50 rad/s on a ball-screw axis. Raise in 5 rad/s steps, then back off 20 percent.
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  Velocity feedforwardSet to 100 percent of the axis's velocity loop response, then trim by test cut.
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  Sync window0.02–0.05 mm correction window, 0.10 mm alarm on a 4,000 mm gantry.
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  Follower torque limitSet just below the master so a jam stops the cycle instead of twisting the beam.

On a 5-axis machine, synchronization is not only between two linear axes. The rotary axes must stay coordinated with the linear axes so the tool tip follows the programmed path while the part rotates. RTCP, or tool center point control, does the math: the control recalculates the linear axis positions from the rotary positions so the tip stays on path.

RTCP accuracy depends on the kinematic model stored in the control. The pivot distance, the rotary axis offsets, and the tool length must be measured and entered correctly. A 0.01 mm error in the pivot distance becomes a visible gouge when a Ø10 mm ball nose tool tilts 45 degrees. We measure these values with a dial indicator and a test bar, not from the drawing.

Rotary axes also need backlash and thermal compensation. A trunnion that grows 15 µm as it warms will drift out of sync with the linear axes over a long cycle. Most controls let you apply a compensation table based on running time or encoder temperature. This is worth doing on any cycle longer than 30 minutes.

When the rotary axis is driven by two motors, the same master-slave logic applies, with one addition: the two motors must be phased so their torque ripple does not add. If the rotors are aligned the same way, the combined torque ripple doubles. Rotate one motor's commutation offset by half an electrical cycle during commissioning.

• 1
  RTCP needs real geometryMeasure pivot distance and offsets with a test bar, not from the drawing.
• 2
  Thermal drift is realApply a compensation table for cycles longer than 30 minutes.
• 3
  Phase dual rotary motorsOffset one commutation by half an electrical cycle to avoid doubling torque ripple.