Robot servo interpolation converts a destination command into a time-ordered trajectory that the position loop can follow. The function remains necessary when only one axis moves: a bare position step requests an instantaneous change in position, which implies unbounded velocity and acceleration. The motor and mechanism cannot produce that motion, so the resulting position error can drive the controller into torque or current saturation, excite structural modes, and leave the axis settling around its destination.
Command and interpolation definitions
The term interpolation here means calculating intermediate commanded positions between motion waypoints. A waypoint states where a segment begins or ends; it does not normally provide one setpoint for every servo update. The interpolator fills that gap by producing a sequence of references over time.
This is distinct from interpretation, which would mean decoding a command or program statement. The controller may perform both functions, but position generation between waypoints is interpolation.
A pulse-count destination specifies displacement. It does not, by itself, define how velocity and acceleration must evolve while reaching that destination. Sending the entire displacement directly to a position loop creates a step command. A step contains broad frequency content, including high-frequency energy that can excite mechanical resonances. Small steps are therefore useful for diagnostic response testing, but a large step is poor production motion practice.
Check 1: Inspect the controller documentation or command trace. Expect a destination waypoint accompanied by generated intermediate position commands, not one unchanged target applied as a large position step.
Waypoint-to-update conversion
The motion planner and servo loop operate at different conceptual levels. The planner defines motion segments and their constraints. The interpolator calculates the reference for each update within a segment. The feedback loop then compares the current interpolated reference with measured position and acts on the resulting error.
For a move from position A to position B, the controller does not need an externally supplied pulse target for every internal update. It generates the intermediate references required to move from A to B according to the selected profile. Coordinated motion adds a geometric requirement: each axis must receive references that keep the tool on the commanded path. Single-axis motion removes that geometric coordination requirement, but it does not remove the need for a realizable time history.
The distinction also explains why interpolation is not a substitute for feedback. Interpolation shapes the command. Feedback rejects disturbances and corrects tracking error. A well-shaped command reduces the error demanded from the feedback controller; it does not eliminate the loop.
Check 2: Trend commanded position and feedback position through one move. Expect commanded position to progress through intermediate values while feedback follows with a finite tracking error rather than responding to one full-distance discontinuity.
Single-axis trajectory constraints
A usable single-axis trajectory places limits on the derivatives of position. Velocity is the first derivative, acceleration is the second, and jerk is the third. The controlled acceleration profiles described for the beginning and end of each motion segment address the transition into and out of motion.
Velocity limiting prevents the planner from demanding travel faster than the selected motion permits. Acceleration limiting controls how quickly velocity changes and therefore moderates the torque demand. Jerk control shapes changes in acceleration. Abrupt acceleration changes transmit shocks into couplings, gear trains, bearings, frames, and payloads; better jerk control reduces this repeated mechanical loading.
A profile that limits velocity and acceleration is materially different from reducing loop gain until a step no longer rings. Lowering gain changes the feedback system’s response to both commands and disturbances. Trajectory shaping removes an unrealistic demand before it enters the loop. Servo tuning and trajectory limits solve different problems and must be commissioned separately.
| Observed behavior | Likely command-side cause | Commissioning action |
|---|---|---|
| Hard launch at segment start | Acceleration begins too abruptly or the axis receives a step-like reference | Select a controlled acceleration profile and inspect the generated command |
| Mechanical impact near the endpoint | Deceleration starts too late or changes too sharply | Review deceleration and jerk shaping for the segment |
| Overshoot followed by settling | Command demands exceed actuator or loop capability | Reduce trajectory demand before retuning feedback |
| Ringing at a repeatable frequency | Command energy excites a mechanical mode | Use smoother trajectory transitions, then evaluate resonance treatment |
Check 3: Trend velocity and acceleration commands. Expect bounded velocity, finite acceleration ramps, and deliberate transitions at both ends of the segment; if jerk is controlled, expect acceleration to change progressively rather than discontinuously.
Position-loop interaction
The position loop acts on the difference between commanded and measured position. Applying the final destination as one large step immediately creates the full move distance as error. The controller responds by increasing its output, but the motor, drive, and mechanics can accelerate only at a finite rate. Once the output reaches a current or torque limit, additional position error cannot produce proportional acceleration.
Interpolation keeps the commanded position close enough to the attainable trajectory for the loop to remain in its useful control range. The axis still develops tracking error under load, during acceleration, and when disturbed, but the error now represents manageable dynamic demand rather than the entire remaining travel distance.
This mechanism matters at the destination as well as at launch. A step command leaves the feedback loop responsible for creating the motion profile and stopping the mechanism through error response alone. A planned trajectory instead reduces velocity and acceleration before the endpoint, so the loop arrives with less stored kinetic energy to dissipate.
Check 4: Compare position error during a shaped move with the error produced by a diagnostic small-step test. Expect the shaped move to show a bounded tracking-error trace that rises during dynamic demand and returns toward its stationary level after settling.
Saturation and integrator management
Saturation occurs when the requested controller output exceeds the available actuator command. During saturation, the relationship assumed by linear loop tuning no longer applies. A large position step can hold the output at its limit while the axis accelerates toward the target.
If the controller contains integral action, error accumulated during saturation can produce integrator windup. The stored integral term remains after the axis approaches the destination, driving overshoot and extending settling time. Anti-windup handling can limit this effect, but it does not make a large production step into a suitable trajectory.
Do not treat reduced proportional gain or increased rate action in a PID loop as the primary correction for an impossible command. Excessively low gain can reduce disturbance rejection and tracking stiffness while masking the original trajectory problem. First keep the command within the drive and mechanism capabilities; then tune the loop using the controller’s approved commissioning method.
| Trace combination | Interpretation | Next decision |
|---|---|---|
| Large position error with output pinned at a limit | Trajectory demand exceeds available actuation | Reduce acceleration demand or reshape the segment |
| Output remains biased after error changes sign | Integral accumulation may be prolonging recovery | Review integral and anti-windup configuration |
| No output limiting, but repeatable oscillation remains | Loop tuning or mechanical resonance requires attention | Evaluate feedback tuning and mechanical frequency response |
| Smooth command with irregular feedback disturbances | Look beyond interpolation | Inspect feedback quality, load variation, compliance, and mechanical play |
Check 5: Trend position error, controller output, and the drive’s current or torque indication. Expect the output to remain below its limit during the commissioned move and to return without a prolonged residual bias at the endpoint.
Motion-profile commissioning
Commission the profile before using feedback gains to compensate for command discontinuities. Use a low-risk move distance and operating condition appropriate to the machine, then increase motion demand only after each trace passes its check.
- Identify the waypoints. Confirm the commanded start and destination and verify the move is truly single-axis. Expect all nonmoving axes to retain their commanded positions.
- Select the controller’s interpolated motion mode. Use the mode that generates intermediate position references and offers controlled acceleration at the beginning and end of the segment. Expect the command trace to contain a ramped trajectory.
- Set trajectory constraints from machine data. Read permitted speed, acceleration, load, and drive limits from the controller, motor, drive, and mechanical documentation. Enter only values justified for the installed mechanism.
- Run at reduced demand. Record commanded position, feedback position, position error, velocity, acceleration when available, and current or torque indication. Expect smooth command derivatives and no sustained output saturation.
- Increase toward the required operating move. Repeat the trace after each change. If saturation, impact, or ringing appears, return to the last passing profile and correct trajectory demand before changing loop tuning.
- Evaluate endpoint behavior. Confirm that deceleration occurs before the destination and that the axis settles without repeated reversals, prolonged integral recovery, or mechanical impact.
- Test disturbances separately. After the trajectory passes, assess feedback tuning under the machine’s permitted load conditions. Do not use a large position step as a routine production test.
Check 6: Repeat the same move several times. Expect repeatable command traces, bounded tracking error, no current or torque limiting, and comparable settling behavior on every cycle.
End-to-end verification
Final acceptance must distinguish command generation, loop response, and mechanical behavior. A position plot alone can hide output saturation, while a current trace alone cannot prove endpoint accuracy.
- Check 7 — command generation: Expect intermediate position references between both waypoints, with controlled acceleration and deceleration at the segment boundaries.
- Check 8 — derivative limits: Expect velocity and acceleration to remain within the configured motion constraints. Where jerk control is configured, expect progressive acceleration transitions.
- Check 9 — tracking: Expect feedback position to follow the generated reference with bounded error and to reach the destination without repeated crossings caused by ringing.
- Check 10 — actuator margin: Expect the current or torque indication and controller output to stay out of sustained saturation through acceleration, travel, and deceleration.
- Check 11 — settled state: Expect position error and controller output to return to their normal stationary levels after the move, with no prolonged bias indicating integrator recovery.
Frequently Asked Questions
How do I tell whether my robot controller is interpolating a single-axis move?
Trend commanded position at the servo-update level if the controller exposes it. An interpolated move shows intermediate references between the start and destination rather than one full-distance position step.
How do I reduce overshoot without lowering the servo gain?
First apply controlled acceleration and deceleration and keep the motion demand below current or torque saturation. Tune the feedback loop only after the command trajectory is smooth and attainable.
How do I distinguish poor interpolation from poor servo tuning?
Inspect commanded position, velocity, acceleration, position error, and current or torque together. Discontinuous commands or sustained output limiting point to trajectory demand; oscillation with a smooth attainable command points toward tuning or mechanical resonance.
How do I perform the final verification for single-axis interpolation?
Run the required move and expect intermediate position commands, bounded velocity and acceleration, bounded tracking error, no sustained current or torque saturation, and a settled endpoint without prolonged controller-output bias.