Rokae XMate Force Control: How Sub-Newton Sensing Changes Assembly

XMate Torque Sensor Architecture

Every joint on XMate carries a dedicated torque sensor — 7 sensors per arm, all measuring simultaneously. The sensor specifications: resolution 0.01 Nm per joint, update rate 2 kHz, measurement range ±150 Nm. The 0.01 Nm joint torque resolution translates to approximately 0.05 N at the end effector for a typical XMate arm geometry — genuine sub-Newton force sensing without an external wrist force/torque sensor.

Why does this matter? A standard force/torque sensor mounted at the robot wrist adds weight, reduces stiffness, and introduces a compliance that makes fast motions less precise. Joint-level torque sensing distributes the measurement across the kinematic chain — the robot uses its own joint torques to infer end-effector forces through a Jacobian transpose calculation. When done at 2 kHz with 0.01 Nm resolution per joint, the result matches what an external wrist sensor provides while adding zero weight and zero stiffness penalty.

Impedance Control Implementation

Rokae’s impedance controller allows engineers to specify independent stiffness, damping, and inertia parameters for each Cartesian axis. A typical connector insertion setup uses: high stiffness (1000 N/m) in the insertion axis to maintain progress once contact is made, low stiffness (50 N/m) in the two lateral axes to allow the connector to self-align with the socket, and a target insertion force of 8 N with a maximum limit of 15 N to abort and report misalignment if exceeded.

This axis-specific compliance is what makes the XMate handle real manufacturing tolerances. A connector socket with ±0.5 mm positional tolerance and ±1 degree angular tolerance is impossible to hit consistently with a position-controlled robot. The same task with impedance control succeeds even when the fixture locating error is at the high end of tolerance, because the lateral compliance absorbs the misalignment and lets the connector geometry guide the insertion.

Applications Where Sub-Newton Sensing Matters

PCB connector insertion: Insertion forces typically 3 to 12 N depending on connector type. The XMate detects the click of full engagement as a force signature drop of 40 to 60% — confirming seat without vision verification. Gear and bearing press-fit: Press-fit interference requires controlled axial force (typically 20 to 200 N) with sub-Newton monitoring to detect the transition from free motion to interference contact, avoiding overshoot that causes bearing damage. Screw driving to torque specification: XMate detects rundown completion by torque signature — the sharp rise when the fastener seats — and stops at the calibrated torque without mechanical clutches. Cable routing and harness insertion: Cable routing requires low-force following of flexible cable geometry — the XMate’s lateral compliance allows the cable to guide itself through clips and conduits.

Programming Force Thresholds

Rokae’s programming environment (both graphical and Python API) exposes force thresholds as task parameters rather than requiring custom controller code. A typical insertion command includes: arm.move_linear(target, speed=50, force_threshold_z=15, compliance_xy=50). The force threshold triggers a fault with a structured error code (CONTACT_FORCE_EXCEEDED) that the PLC can handle with a retry or alarm. This design makes force-controlled assembly accessible to engineers without a control theory background.