Common fixes that miss the limiting quantity
Fast positioning pushes actuator effort, current, thermal load, and settling time toward their limits. The number that matters is not speed alone; it is whether the axis can accelerate, decelerate, enter closed-loop control, and dissipate the resulting energy without oscillation or saturation.
Several familiar tuning changes address only part of that problem:
| Attempted fix | Why it fails | Where to check |
|---|---|---|
| Increase closed-loop gains for the entire move | More aggressive correction can excite mechanical or hydraulic dynamics during high-speed travel. Saturated output also prevents the loop from applying the correction implied by its calculated command. | Control output, position error, actuator limit, and feedback trend |
| Reduce gains until travel looks stable | Lower gains may calm the intermediate motion but weaken final capture, disturbance rejection, and settling performance. | Final position error and settling trace |
| Run the whole move open loop | Open-loop travel avoids continuous error correction, but load, friction, pressure, voltage, and temperature changes can shift the stopping point. | Repeated final positions under changing load and temperature |
| Manually switch control modes near the target | An uncoordinated transfer can create a command step, late braking, or a discontinuity in controller state. | Command and actual-position traces at the transfer |
This is heat, not logic, when an electric drive reaches its current or thermal limit. On a hydraulic axis, the analogous limits are available pressure, flow, valve command, and fluid heating. Identify the active physical limit before changing motion or control settings.
Quick Move control mechanism
The RMC function intended for this requirement is Quick Move. It starts the move in open loop and ends it in closed loop. That separates high-speed travel from precision capture: the initial portion applies a planned actuator command without continuously correcting every intermediate position error, while the closing portion uses feedback to reach and hold the final position.
The benefit is not that open loop is inherently faster. It prevents an aggressively tuned position loop from reacting throughout the part of the move where intermediate accuracy is unimportant. Closed-loop control returns where final accuracy and stability matter.
The transition is the critical event. If closed-loop capture starts with excessive position error, velocity, or stored mechanical energy, the controller must remove that energy while correcting position. The result can be overshoot, output saturation, repeated crossings of the target, or a long settling tail. A successful move gives the capture phase enough distance and authority to brake without destabilizing the axis.
Required quantities and limits
Record the motion traces before configuring the transfer. Use actual measurements from the RMC and the actuator system rather than selecting a transition point from distance alone.
| Quantity | Decision it controls | Where to read it |
|---|---|---|
| Actual position and target position | Final error, overshoot, and repeatability | RMC plot or trend data |
| Actual velocity | Energy remaining when closed-loop capture begins | RMC motion trace |
| Position error | Correction demanded after the transfer | Closed-loop diagnostic trace |
| Control output | Whether the controller reaches its command limit during braking or capture | RMC output trace |
| Actuator current, pressure, or equivalent effort | Available acceleration and deceleration authority | Drive, valve, or actuator diagnostics |
| Temperature or thermal utilization | Whether repeated fast moves are sustainable | Drive or actuator diagnostic display |
| Settling time | Whether final stability meets the machine requirement | Time between first target entry and sustained in-tolerance operation |
For an electric axis, acceleration torque generally follows current until a drive or motor limit intervenes. Braking capability can differ from accelerating capability because the drive, supply, load, and regeneration path participate. For a hydraulic axis, available force and speed depend on pressure and flow. Measure both directions and both load extremes.
Quick Move setup procedure
Define the acceptance criteria before tuning: final position tolerance, permitted overshoot, settling requirement, load range, travel direction, and repetition rate. Read allowable actuator effort and thermal limits from the installed equipment documentation.
Plot a representative existing move. Capture commanded and actual position, actual velocity, position error, control output, and the available actuator-effort measurement.
Confirm that the closed-loop axis can hold the target stably at rest. Correct feedback polarity, excessive backlash, mechanical looseness, stiction, output bias, or a noisy position signal before attempting a faster approach.
Select
Quick Movein the RMC help and motion configuration. Use the documented Quick Move fields for the installed RMC software rather than substituting settings from another motion command.Configure the open-loop travel portion conservatively. Check that its command remains within the actuator's usable range and leaves enough distance for deceleration and closed-loop capture.
Configure the transition into closed loop so it occurs before the axis reaches a state that the feedback loop cannot capture cleanly. The measured velocity, position error, and output headroom at transfer decide the setting.
Run at reduced operating demand, plot the complete transfer, and adjust one quantity at a time. Move the transfer earlier or reduce open-loop effort if the closing phase saturates or overshoots.
Increase demand in controlled increments while monitoring thermal utilization and repeated-move behavior. Stop increasing speed when actuator effort saturates, settling degrades, or temperature trends toward the installed equipment limit.
Transition and tuning decisions
Tune the final closed-loop region for capture and holding, not for forcing the entire high-speed trajectory to match a precision profile. A smooth capture has bounded error, useful output headroom, and a decaying response after transfer.
If error grows immediately after the transition, first check feedback direction and whether the axis crossed the target with too much velocity. If output remains pinned at a limit, the problem is available braking authority or transition timing rather than a request for more gain. If output alternates while position rings around the target, reduce excitation in the capture phase and inspect mechanical compliance, backlash, feedback noise, and loop tuning.
Load variation deserves separate tests. An open-loop command produces motion through plant gain, and plant gain changes with payload, friction, temperature, supply conditions, and direction. Place the transition early enough that the closed-loop portion can correct the worst credible open-loop variation.
Verification under operating limits
A single successful move does not establish production stability. Validate the configuration with repeated cycles at the operating extremes.
Test both travel directions and the minimum and maximum expected loads.
Repeat from different starting positions so the open-loop duration and approach conditions vary.
Compare cold-start behavior with thermally stabilized operation.
Record maximum position error, velocity at transition, peak control output, overshoot, final error, and settling time.
Confirm that the axis stays within its final tolerance for the required hold interval and does not drift after capture.
Run the required repetition rate and watch actuator current or hydraulic effort together with temperature. This separates a motion that works once from one that is thermally sustainable.
Keep the trace from the worst passing case as the commissioning baseline. A later increase in capture velocity, output saturation, or settling time then becomes a measurable maintenance symptom rather than a subjective change in motion quality.
FAQ
Why does an RMC axis become unstable during a fast closed-loop move?
The position loop can demand rapid corrections while the mechanics, hydraulics, or electric drive approach an effort limit. Plot position error, velocity, and control output; saturation or ringing identifies whether the limiting factor is actuator authority, transition energy, or loop tuning.
Why does RMC Quick Move finish more accurately than open-loop motion?
Quick Move starts open loop but ends closed loop. The feedback-controlled closing portion corrects the position changes caused by load, friction, temperature, and supply variation before holding the final target.
When should I stop tuning RMC Quick Move and contact official support?
Stop when feedback polarity is verified but the transfer produces uncontrolled motion, repeated output saturation, or behavior that cannot be matched to the documented Quick Move settings. Save the RMC configuration and plots of position, velocity, error, and output, then escalate through the official RMC support channel with the installed hardware and software identification.