Application of the Time Servomotor System in Percussion and Milling Machines

On a percussion or milling machine the servomotor system does not simply spin a spindle. It closes a position loop around the lead screw, the table or the ram, and it does that in real time. The controller issues a new position command every few hundred microseconds. The drive compares that command with encoder feedback, then adjusts current to the motor. The time constant of that loop is what engineers mean by the application time servomotor system.

That loop is not free. Every millisecond of lag shows up as following error during acceleration. A heavy ram on a planer-type mill can carry several hundred kilograms of moving mass. Ask the drive to reverse that mass in 80 ms and the torque peak climbs fast. Size the motor on continuous torque alone and the axis will fault on the first rapid move. Size it on peak torque with no thermal margin and the winding cooks after two hours of roughing.

The practical middle ground is to size for the worst move in the cycle, then check RMS current over the full program. If the RMS value sits below the motor rating with 20–30% headroom, the axis will survive a long lights-out run. If it does not, either slow the rapid or fit a larger frame.

• 1
  Position loop periodTypically 100–250 μs on a modern digital drive.
• 2
  Following errorGrows with feed rate and shrinks with loop gain.
• 3
  RMS currentThe number that decides whether the motor overheats.

Percussion machines cut with discrete blows rather than a continuous chip. The tool leaves the work between strokes, so the axis load is a series of shocks, not a smooth force. A drive with a long time constant cannot react to each shock. The result is a wandering depth of cut and a hole that is not round.

The fix is usually bandwidth, not raw torque. Raising the velocity loop gain lets the drive reject the shock before the tool re-enters the material. Encoder resolution matters here too. A 1 μm linear scale gives the loop something real to correct against, while a cheap rotary encoder on the screw only reports what the screw did, not what the table did.

There are cases where a servo loop is the wrong tool. Deep-hole percussion on a low-rigidity frame will chatter no matter how fast the loop runs. The frame has to be stiff first. Drives cannot fix a machine structure that rings at 40 Hz.

A milling machine spends most of its time moving at constant feed, so the servo demand looks mild. The hard part is the corner. The tool enters a corner, the cutter engagement jumps, and the axis must hold position against a sudden side load. A drive with pure proportional gain will sag, and the corner will show a witness mark.

Feedforward is the usual answer. The controller knows the programmed acceleration, so it can add the required current before the error appears. Add integral action on top and the steady-state error at constant feed drops to near zero. Add too much integral and the axis overshoots on every reversal, which shows up as a ripple along the part edge.

For high-speed finishing at Ra 0.8–1.6 μm the limiting factor is often the mechanical transmission, not the electronics. A belt-driven screw with 0.05 mm of backlash will never hold ±0.005 mm, no matter how good the application time servomotor system is. Direct coupling removes that error source. The trade-off is that the motor sees every load shock without a cushion.

Tune one axis at a time, with the others disabled. Start with the current loop, because nothing above it works if the current loop is soft. Then set velocity gain until the axis starts to hum, and back off by about 30%. Only then close the position loop.

Run a circle-diamond-square test after tuning. It exposes mismatched gains between axes far faster than cutting a real part. If the diagonal shows a step, the two axes are not tracking each other. If the circle shows flat spots at the quadrants, there is backlash or stiction in the screw.

Keep a record of the gain values and the date. Machines drift as bearings wear, and a tuning sheet from six months ago tells you whether the axis has changed or the operator has.