What is a torque sensor, and how does it work?

Welcome, colleagues.

A torque sensor measures the turning moment transmitted through a shaft or a stationary connection and converts it into an electrical signal. It is also called a torque transducer; a measuring system with a readout may be called a torque meter. The standard unit is the newton-metre (N·m). The sensor tells you how much turning effort is transmitted, rather than simply whether a shaft is rotating.

What happens when the load changes?

The diagram holds speed constant while torque varies between 20 and 80 N·m. The two lines in the lower centre represent the measuring element's end sections, with the relative twist exaggerated for visibility. Actual deformation is very small. The output bar illustrates a 0–100 N·m → 4–20 mA scale: 20 N·m corresponds to 7.2 mA, and 80 N·m to 16.8 mA. Start restarts the cycle; Stop holds the current illustration.

How does torque become an electrical signal?

In a common design, an elastic measuring element twists slightly under torque. Bonded strain gauges convert that deformation into a change in electrical resistance. A Wheatstone bridge produces a small voltage reflecting the change. Load cells use the same underlying principle, but a torque sensor's mechanical structure is designed to respond to a torsional moment.

A signal conditioner amplifies the bridge output and turns it into a usable measurement signal. Depending on the design, outputs include mV/V, ±10 V, frequency or digital data. Interfaces such as 0–10 V and 4–20 mA require suitable electronics. A torque sensor does not automatically provide a standard analogue output. To read an analogue signal and scale it to N·m in a PLC, the supply, input type and scaling must match the measurement chain.

Reaction torque sensors versus rotary sensors

A reaction torque sensor measures the opposing moment carried by a motor housing or another stationary connection; its measuring element does not rotate continuously. A rotary torque sensor is usually installed in the shaft line between the motor and the load. Signals can leave the rotating element through slip rings, inductive transmission or telemetry. Non-contact transmission does not mean that the sensor carries no mechanical load.

A reaction sensor is not limited to measuring constant torque. With suitable frequency response, it can also capture changing moments while remaining stationary. Mounting arrangement and dynamic measurement capability are separate considerations. Before choosing a torque measurement method, identify the connection at which you need the measurement.

Does measuring torque also measure power?

No. Torque and power are different quantities. Calculating mechanical power on a rotating shaft also requires rotational speed in RPM. Speed can come from a separate encoder or an integrated speed measurement system. Using P (kW) = T (N·m) × n (rpm) / 9550, 20 N·m at 1500 rpm gives approximately 3.14 kW. This is mechanical power at the shaft, rather than the motor's electrical input power.

What matters when selecting a sensor?

Choose the measuring range to cover more than normal running torque: consider starting, braking and sudden torque peaks. For rotary sensors, check the maximum permitted speed, shaft or flange interface and mechanical load limits. Measuring a small torque with an unnecessarily large sensor can increase relative measurement error; the effect depends on how the data sheet specifies error.

Linearity, hysteresis, repeatability and temperature effects all contribute to measurement quality. To capture fast changes, consider sensor bandwidth, electronic filtering and acquisition sample rate together. Select an appropriate IP protection rating for the environment. Check the unloaded zero after installation, and remember that zeroing and calibration are different procedures.

Applications and installation

Typical applications include recording a motor's torque curve, validating servo motor performance, monitoring fastening operations and tracking mixer load. Suitable systems can use the measurement as feedback for torque control. Shaft misalignment, axial force and bending moments can affect the reading or overload the sensor. Select couplings and bearing arrangements according to the manufacturer's installation instructions.

Technical references: FUTEK — sensing principle · FUTEK — sensor selection · HBK — rotary sensor selection · HBK — reaction sensor example