The essential
A transducer converts a physical quantity into an electrical signal proportional to it. In a load cell, the force strains the body, and strain gauges wired as a Wheatstone bridge change resistance with that strain. The bridge signal is conditioned and amplified before it reaches the data acquisition system. A single-axis load cell measures one direction. A three-axis load cell measures three perpendicular forces; a six-axis load cell adds the three moments.
What is a transducer?
A device that converts one form of energy into a readable signal. The output is electrical and proportional to the quantity measured: force, torque, position, acceleration or temperature.
- Active transducers need no power source. A thermocouple is one.
- Passive transducers need an external power source; their output is a variation of resistance, capacitance or another electrical parameter. A strain gauge load cell or a torque transducer is one.
How does a load cell measure a force?
The force strains the body of the load cell. Strain gauges in a Wheatstone bridge change resistance with that strain. The bridge needs excitation; its output goes through signal conditioning and amplification and comes out as a voltage for the data acquisition system.
Three axes or six?
Three axes if the test needs the forces only; six if it also needs the moments.
- Three axes: Fx, Fy and Fz, in one body. The TR3D series runs from 1.1 kN (250 lb) to 444 kN (100 000 lb).
- Six axes: the three forces and the moment about each axis. Six-axis models: TR6D-C (16K, 20K, 40K) and TR6D-F (5K, 10K).
Where are six-axis load cells used?
Wherever forces and moments act together and have to be separated.
- Biomechanics and orthopedics: joint simulations, prosthetic limbs.
- Robotics: gripping systems and product assembly.
- Automotive: crash, brake and cornering tests.
- Wind tunnels: lift, drag and side force, with yaw, roll and pitch moments.
Can they be used in the field?
Yes. The TR3D and TR6D-C bodies are sealed to IP65, operate from −40 to 149 °C and use weatherproof connectors, so they serve in field data acquisition as well as in the laboratory. When no catalog model fits, the load cell is customized.
To remember
- A strain gauge load cell is a passive transducer: it needs excitation and an amplifier.
- Three axes give the forces; six axes add the moments.
- No catalog model fits: the part itself can become the transducer.