Position control assumes the workpiece is where the program expects it to be. Grinding, polishing, insertion, deburring, and surface following break that assumption because contact force changes with tolerance, wear, fixture error, and material variation.

Force control closes another feedback loop. The robot measures interaction and adjusts motion or commanded force so the process can tolerate small geometric differences without excessive load.

Wrist sensing measures contact near the tool, but it also sees tool mass, acceleration, and cable forces.

Choose the feedback source deliberately

Robot controllers estimate joint torque from motor current and model data. This built-in sensing avoids extra hardware and can support collision detection or broad compliance. Its accuracy is affected by drivetrain friction, payload calibration, posture, and dynamic motion.

An external six-axis force-torque sensor measures closer to the process and can resolve smaller forces and moments. The tradeoffs are added mass, wiring, overload risk, calibration, and another component in the mechanical stack.

Control mode must match the task

Contact detection stops or changes sequence when a threshold is crossed. Compliance allows displacement under load. Constant-force control regulates a target along selected axes while position control remains active on others. In insertion tasks, search patterns may combine with force limits to find alignment.

Stable contact depends on tool stiffness, robot dynamics, surface variation, and controller tuning.

Separate process control from safety

A force-control threshold used for quality is not automatically a safety function. Personnel protection requires a risk assessment and safety-rated architecture. Tool edges, hot workpieces, trapped energy, and payload momentum remain hazards even when the robot senses torque.

Commissioning should test correct payload and center-of-gravity data, sensor bias, approach speed, maximum force, loss of feedback, collision recovery, and worn-tool behavior. Trends can reveal increasing insertion force, fixture drift, or abrasive wear before quality fails.

Motion hardware can be reviewed in drives and motion control, with robot-specific components available in the ABB Robotics collection.

The integration boundary

The PLC should exchange clear states—ready, contact found, process complete, quality result, and fault—rather than micromanage the robot’s high-rate force loop. That keeps deterministic servo behavior inside the robot controller while preserving machine-level sequence visibility.

Author opinion: force control is not a universal cure for poor fixtures. It delivers the greatest value when mechanical repeatability is already reasonable and adaptive contact is used to absorb the remaining variation while producing diagnostic data.