After the brush is mounted on a compliant stroke and the robot periodically re-references the surface, brush wear no longer translates directly into lost contact force. The number that matters is contact force over time: too little leaves material behind, while too much increases brush deflection, motor current, frictional heating, and tool wear.
Contact load and wear mechanics
A position-controlled robot regulates tool pose, not contact force. As the brush wears, its free length changes. Repeating the same path therefore changes brush compression and the resulting normal force. For an approximately linear compliant element, the relationship is F = kx, where F is contact force, k is effective stiffness, and x is compression. Brush stiffness may vary with wear, speed, temperature, and contact angle, so position correction alone cannot guarantee constant force.
Friction converts mechanical work into heat. Increasing normal force generally raises brush torque and drive current; increasing surface speed raises energy delivered per unit time. This is heat, not logic. A force-control strategy must therefore limit force and travel while the process owner monitors tool current and temperature at the brush or spindle drive.
| Quantity | Why it decides the design | Where to obtain it |
|---|---|---|
| Target normal force | Defines the cleaning load and force-control setpoint | Process trial or tooling specification |
| Maximum permitted force | Protects the workpiece, brush, spindle, and robot | Lowest applicable equipment or process limit |
| Available compliance stroke | Must accommodate expected wear and surface variation | Tool drawing and measured wear range |
| Effective stiffness | Converts displacement error into force error through F = kx
|
Deflection test across the working range |
| Process speed and contact time | Drive frictional heating and cleaning energy | Robot program and conveyor measurement |
| Force feedback quality | Sets achievable accuracy and response | Robot diagnostics or external sensor data |
Control approach comparison
| Approach | Force performance | Wear compensation | Primary limitation |
|---|---|---|---|
| Fixed position with scheduled offsets | Indirect; depends on repeatable brush stiffness | Compensates only for a calibrated wear trend | Misses part variation, brush-to-brush variation, and unexpected wear |
| Move to a surface sensor, then apply a fixed compression | Repeatable when stiffness is stable | Re-establishes the surface reference before cleaning | Still controls displacement rather than measured force |
| Spring-loaded or retractable brush | Passive compliance reduces force change | Mechanism stroke absorbs brush-length change | Force varies with mechanism stiffness, friction, and stroke position |
| Active robot force control | Directly regulates measured or estimated force | Robot advances as the brush wears | Requires suitable hardware, configuration, tuning, and controller entitlement |
A linear motion such as MovL can define the cleaning path, but it does not by itself stop or advance at a force threshold. Likewise, Servofloat should be treated as a licensed force/compliance function rather than a single move instruction. Its availability, supported axes, sensing method, and operating constraints must be read from the installed controller documentation.
Recommended compliant architecture
For a continuously moving surface with gradual brush wear, start with a retractable or spring-supported brush and a sensor-based surface reference. This architecture separates the fast contact mechanics from the robot position loop: the mechanism absorbs small surface motion while the robot periodically restores the mechanism to its intended working deflection.
Select active robot force control when the process specification requires force to remain inside a narrow band despite changing stiffness, uneven surfaces, or rapidly changing contact. Active control needs a valid force signal, a force setpoint, a search direction, a maximum travel limit, and an escape condition. A robot that estimates contact from joint torque may react differently from one using a force sensor at the tool; verify the installed sensing path before tuning.
A wear table or part-count offset is the fallback when neither compliance nor force sensing is available. Derive its interval from measurements of brush length or required correction rather than assuming a fixed amount per batch. Keep a hard travel limit so accumulated offsets cannot drive the tool beyond its mechanical stroke.
Reference and cleaning procedure
- Define the tool direction normal to the surface. Confirm that positive correction moves the brush toward contact and that the cleaning path remains tangential to the surface.
- Record the target force, maximum permitted force, maximum approach distance, mechanism stroke, and allowable spindle or brush current from the applicable process and equipment data.
- With the brush stopped or in the tooling supplier's approved contact state, approach at a controlled search speed. Stop the search when the surface sensor changes state or the validated force threshold is reached.
- Store the detected surface location as the reference for that cycle. Command the calibrated compression needed to place the compliant mechanism in its working range.
- Run the cleaning travel with
MovLor the controller's equivalent linear path while maintaining the intended orientation. If active force control is installed, enable it only for the contact segment and command force along the surface-normal direction. - Supervise force, approach travel, sensor state, and tool current. Retract on excessive force, travel-limit violation, loss of a valid sensor signal, or abnormal tool load.
- At the end of the cleaning segment, disable force mode before retracting unless the controller documentation specifies another sequence. Re-reference often enough that wear cannot consume the usable compliance stroke between searches.
Force-loop verification
Test first at reduced process speed with a representative workpiece. Trend commanded position, measured force or force estimate, surface-sensor state, compliance deflection, and brush or spindle current. The trace should show a controlled rise to contact, stable load during cleaning, and an immediate fall during retraction.
Repeat the test with a new brush and with a brush near its approved wear limit. The robot position should change to accommodate length loss while force remains within the process band. For passive compliance, calculate expected force variation from the measured stiffness and deflection range; if that variation exceeds the process tolerance, active force control or a lower-stiffness, longer-stroke mechanism is required.
Challenge each protective condition separately: surface not found within the allowed travel, sensor already active before approach, force above the maximum, and compliance stroke near an end stop. Each condition must inhibit cleaning and command a controlled retract rather than adding another position offset.
Recurring implementation pitfalls
Mounting compliance in the wrong axis lets the brush follow the path without regulating normal load. Tool-frame errors create the same symptom because a nominally normal correction gains a tangential component. Verify the tool frame and contact direction from robot motion before evaluating force tuning.
Mechanical friction and binding create hysteresis: the force at a given compression differs between approach and retract. Measure both directions. A mechanism that reaches an end stop becomes rigid, so its force can rise sharply even though the program still reports the expected offset.
Aggressive force-loop tuning can produce oscillation when the brush, sensor, robot structure, and moving surface interact. Reduce approach energy, remove mechanical looseness, and tune from a stable low-response condition. Position and force loops must not issue competing commands along the same controlled axis.
FAQ
How do I make a robot move until it reaches a force?
Use a validated force or contact signal as the search stop condition, with limits for approach distance and maximum force. A linear MovL command defines direction but needs separate contact logic or an enabled force-control function.
How do I compensate for brush wear without robot force control?
Mount the brush on a compliant stroke, detect the surface, and command a calibrated compression from that reference. Re-run the reference search before wear consumes the available stroke.
How do I calculate brush force from spring travel?
For a measured linear region, use F = kx. Determine k from a force-versus-deflection test and account for different approach and retract values if the mechanism has friction.
How do I verify that force remains constant while cleaning?
Trend force, robot position, compliance deflection, and tool current with both a new and a worn brush. Position should compensate for length change while force stays inside the process-defined band.
When should I stop testing and contact official support?
Stop if contact force rises uncontrollably, the mechanism reaches an end stop, the force signal is invalid, or the controller cannot retract predictably. Escalate to the robot manufacturer's official support channel when licensing, supported sensing, force-mode sequencing, or controller-specific tuning cannot be verified from the installed system documentation.