A torque sensor, torque transducer or torque meter is a device for measuring and recording the torque on a rotating system, such as an engine, crankshaft, gearbox, transmission, rotor, a bicycle crank or cap torque tester. Static torque is relatively easy to measure. Dynamic torque, on the other hand, is not easy to measure, since it generally requires transfer of some effect (electric, hydraulic or magnetic) from the shaft being measured to a static system.
One way to achieve this is to condition the shaft or a member attached to the shaft with a series of permanent magnetic domains. The magnetic characteristics of these domains will vary according to the applied torque, and thus can be measured using non-contact sensors. Such magnetoelastic torque sensors are generally used for in-vehicle applications on racecars, automobiles, aircraft, and hovercraft.
Commonly, torque sensors or torque transducers use strain gauges applied to a rotating shaft or axle. With this method, a means to power the strain gauge bridge is necessary, 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. Newer types of torque transducers add conditioning electronics and an A/D converter to the rotating shaft. Stator electronics then read the digital signals and convert those signals to a high-level analog output signal, such as +/-10VDC.
A more recent development is the use of SAW devices attached to the shaft and remotely interrogated. The strain on these tiny devices as the shaft flexes can be read remotely and output without the need for attached electronics on the shaft. The probable first use in volume will be in the automotive field as, of May 2009, Schott announced it has a SAW sensor package viable for in vehicle uses.
Another way to measure torque is by way of twist angle measurement or phase shift measurement, whereby the angle of twist resulting from applied torque is measured by using two angular position sensors and measuring the phase angle between them. This technique is used in the Allison T56 turboprop engine.
Finally, (as described within the abstract for US Patent 5257535), if the mechanical system involves a right angle gearbox, then the axial reaction force experienced by the inputting shaft/pinion can be related to the torque experienced by the output shaft(s). The axial input stress must first be calibrated against the output torque. The input stress can be easily measured via strain gauge measurement of the input pinion bearing housing. The output torque is easily measured using a static torque meter.
There are essentially two main categories of strain gauge Torque Sensors: rotary and reaction torque sensors.  In summary, a reaction torque sensor measures static torque, and rotary measures dynamic torque. 
Understanding the application and defining the requirements are a crucial part of selecting the appropriate torque sensor. After one clearly defined the application, one should identify what it is to be measured, controlled, or monitored. 
Rotary Torque Sensors
Rotary torque sensors are utilized in applications where the measurement must be taken at a spinning shaft or a stationary motor. In this case, the torque sensor must rotate in-line attached to the shaft. A rotary torque sensor is fitted with a slip ring or wireless electronics to transmit the torque signal while rotating.
Rotary Torque Sensors are frequently used as testing/auditing tools for motors, power tools, turbines, and generators. It can be also utilized for feedback control, monitoring torque, and analyzing the efficiency of test stands.
The Rotary Torque Sensor is coupled between the motor and the load. As the motor rotates, the Torque Sensor measures the torque produced by the motor in response to the load applied to the rotating shaft. Some Rotary Torque 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.
Reaction Torque Sensors
In some applications, the torque measured with an in-line rotary torque sensor may be measured at the point where the torque is transferred to the ground using a reaction torque sensor.
A reaction torque sensor has two mounting flanges. 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 torque sensor is often less complex and, therefore less expensive than a rotary torque sensor.
Micro Reaction Torque Sensor
Micro Reaction Torque Sensor provides a precision torque measurement solution for miniature, DC motors, servo motors. Traditional reaction torque sensors are bulkier. For reference, a micro reaction torque sensor dimensions are 22 mm in diameter and 10 mm in height, which is 5x times smaller than a typical reaction torque sensor. The significant reduction in size allows the micro reaction torque sensors to fit where most reaction torque sensors cannot.
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