A torque sensor is a transducer that measures the twisting moment acting about an axis, such as the torque transmitted through a shaft, a gearbox output or a robot joint, and converts it into an electrical signal in newton-meters. It works by detecting the tiny elastic deformation that torque produces in a carefully shaped metal element.
In robotics sensors, the torque sensor gives a machine a sense of effort. Joint torque sensors let collaborative robots detect collisions, regulate contact forces and be guided by hand, while torque sensors on tools and test benches verify tightening, drive performance and actuator models. Together with force sensors and tactile sensors, they form the basis of force control.
How a torque sensor measures twist
Most robot torque sensors use strain gauges bonded to a flexure, a metal body designed to twist slightly and predictably under load. Each gauge is a thin foil resistor whose resistance changes in proportion to strain. Four or more gauges are wired as a Wheatstone bridge, which converts the small resistance changes into a millivolt signal and cancels many temperature effects.
The flexure geometry is the core of the design. Spoke-shaped or hollow-shaft elements concentrate shear strain from torque at the gauge locations while staying stiff against bending and axial forces. A good design achieves high torsional stiffness so that the sensor does not make the joint noticeably springy, and low sensitivity to loads on other axes.
The bridge output is amplified, filtered and digitized, often directly in the sensor housing. Modern sensors deliver data over EtherCAT, CAN or analog voltage, with update rates from about 1 kHz to several kHz, fast enough for joint-level force control loops.
Types of torque sensors
Torque sensors differ in sensing principle and mechanical form. The right type depends on whether the shaft rotates continuously, the space available and the required accuracy.
| Type | Principle | Strength | Typical use |
|---|---|---|---|
| Reaction torque sensor | Strain gauges on a static flexure | Simple, accurate, no rotating parts | Robot joints, test fixtures |
| Rotary torque sensor | Strain gauges with contactless signal transfer | Measures spinning shafts | Motor and drivetrain test benches |
| Magnetoelastic sensor | Magnetic property change in shaft | No bonded gauges, robust | Vehicle drivetrains, e-bikes |
| Optical or capacitive sensor | Displacement of two discs | Compact, low drift options | Compact robot actuators |
| Spring deflection sensing | Encoders across an elastic element | Very robust to overload | Series elastic actuators |
A series elastic actuator is a special case: it deliberately places a compliant spring in the drive and estimates torque from the measured spring deflection. This trades bandwidth and stiffness for excellent shock tolerance and low cost.
Specifications that decide performance
Datasheets describe torque sensors with several parameters. Comparing sensors only by measurement range is a common mistake, because crosstalk and overload capacity often matter more in a robot.
| Specification | Typical range | Meaning |
|---|---|---|
| Measurement range | About 0.1 Nm to several kNm | Torque that the sensor resolves accurately |
| Accuracy class | About 0.05-1 percent of full scale | Combined linearity and hysteresis error |
| Overload limit | Often 150-500 percent of range | Torque survived without damage |
| Crosstalk | About 0.1-2 percent | Output caused by bending or axial loads |
| Torsional stiffness | Given in Nm per radian | Added compliance in the joint |
| Bandwidth or sample rate | 1-10 kHz | Speed of the control loop |
Temperature drift of the zero point is especially relevant inside robot joints, where motors heat the structure. Many systems therefore re-zero the sensor when the arm is in a known pose with no external load.
Torque sensor compared with related sensing methods
Robots can perceive force and torque in several ways. A dedicated torque sensor is more accurate and faster than indirect estimates, but it adds cost, cabling and mechanical length to each joint.
| Method | What it measures | Limitation |
|---|---|---|
| Joint torque sensor | Torque at the joint output | Cost per axis, added length |
| Motor current estimation | Motor torque before the gearbox | Gear friction hides small forces |
| Six-axis force and torque sensor | Forces and moments at the wrist | Sees only loads beyond the wrist |
| Load cell | Force along one axis | Not designed for twisting loads |
| Tactile sensor | Pressure distribution on a surface | Local contact only, low force accuracy |
Current-based estimation works reasonably with low-ratio, backdrivable gearboxes, but with harmonic drives or high ratios the friction can exceed the external force a robot needs to feel. That is the main reason why sensitive collaborative arms place torque sensors after the gearbox in every joint.
Integration pitfalls
A torque sensor is only as good as its mechanical integration and calibration. Most field problems stem from loads and signals that the designer did not expect.
- Parasitic bending loads: misaligned bearings push bending moments into the sensor and corrupt readings through crosstalk.
- Electrical noise: millivolt bridge signals pick up motor PWM noise unless amplification happens close to the gauges.
- Missing overload protection: collisions can exceed the safe limit and permanently shift the zero point.
- Uncompensated gravity and dynamics: the controller must subtract torque caused by the arm’s own weight and acceleration before detecting contact.
- Skipping periodic calibration: drift accumulates over months and slowly degrades force control accuracy.
The simple explanation
A torque sensor tells a machine how hard something is twisting. When you open a tight jar lid, your hand feels the effort. A torque sensor gives a robot joint that same feeling.
Inside is a piece of metal that twists by a tiny amount under load. Small sensors glued to it notice the twist and turn it into an electrical signal. With this information, a robot can stop when it bumps into a person or press with just the right strength.
How it is used in practice
An electronics assembler uses a seven-axis collaborative arm to insert flexible connectors into circuit boards. The connectors must be pressed firmly enough to lock but never with more than a few newtons, or the board is damaged.
Each joint of the arm contains a strain-gauge torque sensor behind its gearbox. The controller combines all joint torques with the arm model to calculate the force at the gripper, subtracts the arm’s own weight and moves in force-controlled mode during insertion. When the measured force rises sharply, the connector has locked and the arm retracts.
The same sensors provide collision detection near operators working at the next station. After commissioning, the assembler reports that damaged boards from insertion become rare, and operators can teach new positions by guiding the arm by hand.