How do the different types of torque sensors (a.k.a. "torque cell") used for torque measurement and what are the differences between them? We outline the most important facts in this comprehensive introduction to torque transducers for measuring torque.
Torque Sensors manufactured in US by FUTEK Advanced Sensor Technology (FUTEK), a leading manufacturer producing a huge selection of Torque Transducers, utilizing one of the most advanced technologies in the Sensor Industry: Metal foil strain gauge technology. It is is defined as a transducer used for torque measurement (torque sensing) that converts an input mechanical torque into an electrical output signal. Torsions Sensors are also commonly known as Torque Transducer or Torque Cell. There are two main types of Torque Sensors: Reaction Torque Sensors or Rotational Torque Sensors.
By definition, torque sensor is a type of transducer, specifically a torque transducer that converts a torque measurement (reaction, dynamic or rotary) into another physical variable, in this case, into an electrical signal that can be measured, converted and standardized. As the torque applied to the sensor increases, the electrical output signal changes proportionally (torque detector).
There are two main categories of Torsions Sensors: rotary torque sensor and reaction torque sensors. In summary, a reaction sensor measures stationary torque (static or non-rotational), and rotary measures rotational torque (dynamic torque sensor).
Understanding the application and defining the requirements is a crucial part of selecting the right torque sensor.
Rotary Torque Sensors (dynamic or rotational torque)
Rotational sensors (or dynamic torque sensor) are utilized in applications where the torque measurement must be taken at a spinning shaft, engine or a stationary motor. In this case, the transducer must rotate in-line attached to the shaft. A rotary torque transducer is fitted with a slip ring or wireless electronics to transmit the torque signal while rotating (non-contact sensor).
Rotary Torque Transducers are frequently used as testing/auditing tools for motors, power tools, turbines, and generators to measure rotational torque. A shaft-to-shaft Rotating Torque Sensor can be also utilized for feedback control, monitoring torque, and analyzing the efficiency of test stands.
The Rotating Torque measurement is coupled between the motor and the load. As the shaft spins, the Torsions Sensor measures the torque produced by the motor in response to the load applied to the rotating shaft. Some Rotary Sensors are equipped with built-in encoders. These encoders measure the angle/speed produced during the test. Torsion measurements can successfully be monitored on a local digital display, such as a Panel Mount Display, a Hand-Held Display, connected to a PLC, or streamed to a PC using a digital USB instrument.
Rotary Torque Transducer is also a crucial part of dynamometers (or dyno for short), as it provides torque measurement and angular speed (RPM) to easily calculate output power, allowing accurate calculation of motor or engine performance in Kw or HP as well as its electromechanical efficiency.
Reaction Torque Sensors (static)
In some applications, the torque measurement taken with an in-line rotary sensor may be measured at the point where the torque is transferred to the ground using a reaction torque transducer (static torque measurement).
A reaction torsion sensor has two mounting flanges (flange-to-flange sensor). One face is fixed to the ground or a rigid structural member and the other to the rotating shaft or rotary element. Rotation generates shear forces between the flanges, which is captured by the foil strain gauges bonded to the sensor beams and transduced into electrical current by the Wheatstone bridge.
For a given application, a reaction sensor is often less complex and, therefore less expensive than a rotary sensor.
Firstly, we need to understand the underlying physics and material science behind the torque sensor working principle, which is the strain gauge (sometimes referred to as Strain gage). Metal foil strain gauge is a sensor whose electrical resistance varies with applied force. In other words, it converts (or transduces) force, pressure, tension, compression, torque, weight, etc… into a change in electrical resistance, which can then be measured.
Structurally, a reaction torsion sensor is made of a metal body (also called flexure) to which foil strain gauges are bonded. The sensor body is usually made of aluminum or stainless steel, which gives the transducer two important characteristics: (1) provides the sturdiness to withstand high torques; and, (2) has the elasticity to minimally deform and return to its original shape when the torque is removed.
When torque is applied (clockwise or counterclockwise), the metal body acts as a “spring” and is slightly deformed, and unless it is overloaded, it returns to its original shape. As the flexure deforms, the strain gage also changes its shape and consequently its electrical resistance, which creates a differential voltage variation through a Wheatstone Bridge circuit. Thus, the change in voltage is proportional to the torque applied to the transducer, which can be calculated via the torque sensor circuit voltage output.
In a Rotary Torque Sensor, the strain gauge is bonded to the rotating shaft that slightly deforms when torque is applied. The shaft deflection induces stress in the strain gauge that changes its resistance. A combination of strain gauges (usually 4) is arranged in an electrical circuit, the Wheatstone bridge, that converts the resistance variations into a voltage output, which can be calibrated and measured.
Rotary Torque Sensors are designed to measure torque of a rotating shaft. Thus, it is necessary to transfer power to the strain gauge bridge, as well as a means to receive the signal from the rotating shaft. This can be accomplished using slip rings, wireless telemetry, or rotary transformers. Optionally, sensors can also embed encoder for angle or speed measurement.
The sensors must be carefully designed seeking to eliminate off-axis loading (also called side loads or extraneous moments), and must be sensitive only to counterclockwise and counterclockwise torque loading. The sensor output is a function of force and distance (T=F x d), and is usually expressed in inch-pounds (in-lb)., foot-pounds (ft-lb) or Newton-meters (N.m).
For further information, watch our Webinar on how Torque Sensors works.
A question that we frequently hear is: “What is the right sensor for my application?” The reason why it gets asked so often is that it can be tricky to navigate the various sensors offerings on the market.
To help you select your load cell, FUTEK developed an easy to follow, 4-Steps guide. Here is a glimpse to help you narrow down your choices. Check out our “How to choose a Torque Sensor” complete guide for further information.
For more details on our 4-Steps Guide, please visit our How to choose a Torque Sensor” complete guideline.