You need an external 6-axis force-torque (F/T) sensor when your application requires force measurement below 2 Newtons, or needs to detect torque changes independent of the robot's movement. For simpler tasks like detecting a collision or performing basic push-until-contact routines, the built-in force sensing in a Universal Robots e-Series is sufficient. The decision rests entirely on the required precision and the type of force data your process needs.
Built-in vs external F/T sensing
The standard force control on a UR cobot works by estimating force from the electrical current drawn by the motors in each joint. This is excellent for compliant movements and provides the fundamental data for safe operation under ISO/TS 15066. It tells the system the arm has met resistance, but not with the precision needed for delicate tasks.
A wrist-mounted F/T sensor is different. It sits between the cobot's wrist and the end-of-arm tooling, using internal strain gauges to directly measure forces (Fx, Fy, Fz) and torques (Tx, Ty, Tz) at the point of contact. This provides ground-truth data, unaffected by the arm's posture, speed, or joint friction, giving a true picture of what the tool is experiencing.
The table below outlines the core differences our engineers consider when specifying a system. These factors directly impact application success and budget.
| Attribute | Built-in sensing (UR e-Series) | External 6-axis F/T sensor |
|---|---|---|
| Measurement location | At each of the 6 robot joints | At the tool flange, near the workpiece |
| Typical resolution | ~5 to 10 Newtons | Under 0.5 Newtons |
| Data type | Estimated from joint motor current | Direct measurement via strain gauges |
| Primary use case | Collision detection, protective stops | Precision assembly, surface finishing |
| Cost | Included with the cobot arm | £4,000 to £8,000+ per unit |
Source: Universal Robots e-Series documentation and average industry pricing for UR+ certified sensors.
This distinction is critical. Built-in sensing is a safety and general-guidance tool, while an external sensor is a precision measurement instrument.
Applications requiring external F/T sensing
Precision assembly. For inserting a delicate connector or pressing a bearing with under 0.5 mm of clearance, an F/T sensor is non-negotiable. It feels the subtle forces of misalignment and commands micro-adjustments, spiralling or wiggling to find the correct path without jamming. Built-in sensing lacks the resolution to distinguish these small insertion forces from the arm's own friction.
Surface finishing and deburring. Maintaining a constant tool pressure, for example 5 N plus or minus 0.5 N, is impossible with built-in sensing alone. As a cobot moves across a contoured part, its posture and leverage change, altering the force at the tool. An external sensor provides a constant feedback loop so the robot adjusts its path in real time to maintain that exact pressure for a perfect finish.
High-resolution weight and presence checks. Confirming the pickup of a small component weighing under 50 grams is a common challenge in electronics. An external sensor reliably detects these tiny weight changes. We implement this to verify a critical component is correctly gripped before the robot commits to its next move, eliminating a common failure point.
Dynamic part tracing. When applying a sealant bead along an inconsistent edge, an F/T sensor lets the robot feel its way along the contour. This is superior to vision when the feature is defined by a change in plane rather than a visible colour contrast, providing rich data for real-time path generation that accommodates part-to-part variation in mouldings or castings.
Beyond simple contact detection
The standard Force Mode on a Universal Robot is highly effective for tasks with generous tolerances, like placing a part into a fixture where clearance is greater than 0.5 mm. It lets the cobot push gently until it feels contact, then stop or proceed, which is adequate for many machine tending and pick-and-place routines. The challenge arises when just feeling contact provides insufficient data, and your process needs to know how hard the tool is pushing, the direction of that force, and whether any torque is applied. That is the crucial difference between simple contact detection and true force control.
How does application logic dictate the sensor choice?
Most cobot applications are position-limited, programmed with a simple instruction to go to exact X, Y, Z coordinates. Here the internal sensing acts primarily as a safety monitor and programming is straightforward. A force-limited task operates on different logic: move to this surface and maintain 10 N of pressure while sliding 100 mm to the right. Here force is the primary control input, not position, and this is impossible to achieve repeatably without an external F/T sensor. Polishing, grinding, and scientific testing are classic examples where we always specify an external sensor to meet quality standards.
Why is thermal drift a critical factor?
External F/T sensors are sensitive analogue devices, and as the cell warms during a shift their zero point can drift. For a simple presence check on a 1 kg part, this drift is insignificant. For a delicate assembly process relying on detecting force changes below 0.2 N, drift becomes a primary cause of failure. High-end sensors include temperature compensation, but this does not eliminate the issue. Our integration process for precision applications always includes an automated re-zeroing routine in the URCap, where the robot touches a neutral point every 50 to 100 cycles to reset its baseline. Neglecting this step often leads to failed assemblies an hour into a run.
Related pages
Frequently asked questions
Can I use the built-in force sensing for collaborative safety?
Yes. Built-in sensing is a core component enabling UR cobots to comply with ISO/TS 15066 for collaborative operation, allowing the robot to detect a collision with an operator and stop safely. It does not replace the legal requirement for a full risk assessment of the entire application, including the part being handled and the end-effector.
Do I need a special URCap for an F/T sensor?
Yes. Every leading F/T sensor manufacturer provides a dedicated URCap that adds simple blocks for complex tasks like Find Surface, Constant Force Push, and Measure Weight. Using these URCaps turns days of complex scripting into a few hours of configuration.
How much payload do I lose by adding an F/T sensor?
An external sensor and its adapter plate typically weigh 0.5 kg to 1.5 kg and add 30 to 50 mm of height to the tool stack. This mass and offset must be configured in software and subtracted from the cobot's nameplate payload. For a UR10e with a 12.5 kg payload, adding a 1 kg sensor and a 2 kg gripper leaves 9.5 kg of effective payload for the part.
Next steps
The distinction between built-in sensing and an external F/T sensor often defines the success or failure of a precision automation project. If your application involves assembly, finishing, or delicate inspection, an external sensor is a core requirement, not an optional extra.
Our engineers model the forces in your process to validate the correct sensing strategy before any hardware is ordered, guaranteeing the system performs to specification from the first part to the last. Book a no-obligation technical consultation with our automation team today.
