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This guide helps you understand the fundamentals of load measurements and how different sensor specifications impact load cell performance in your application. After you decide on your sensors, you can consider the required hardware and software to properly condition, acquire, and visualize load measurements. You can also consider any extra signal conditioning you may need.
What is Torque?
Force is the measure of interaction between two or more bodies: for every action there is an equal and opposite reaction. Force is also described as a push or pull on an object. It is a vector quantity with both magnitude and direction.
Torque is the tendency of a force to rotate an object about an axis. Similar to force being described as a push or pull, torque can be described as a twist to an object. The SI unit for the measure of torque is Newton-meters (Nm). In simple terms, torque is equivalent to force times distance, where a clockwise torque or twist is usually positive and a counterclockwise torque is usually negative. Torque sensors are composed of strain gages that fixed to a torsion bar. As the bar turns, the gages respond to the bar’s sheer stress, which is proportional to the torque.
Figure 1. Rotary slip ring torque sensors can be used to measure startup, running, and stall torque levels.
The two common ways to measure torque are: reaction torque sensors and rotary torque sensors.
Reaction Torque Sensors
Reaction torque is the turning force that is imposed on the stationary portion of a device by the rotating portion as power is either delivered or absorbed. As the load source is rigid while the drive source is trying to rotate, the torque is created. Reaction torque sensors are restrained so they cannot rotate 360 degrees without the cable wrapping up because the housing or cover is fixed to the sensor element. These sensors are commonly used to measure torque of a back-and-forth motion. These types of sensors do not use bearings, slip rings, or any other rotating elements in their installations.
Rotary Torque Sensors
Rotatory torque sensors are complimentary in design and application to reaction torque sensors except that the torque sensor is installed in line with the device under test. As the shaft of a torque sensor is rotating 360 degrees they must have a way to transfer the signals from the rotational element to a stationary surface.
This is accomplished by using one of three mounting methods: slip rings, rotary transformers, or telemetry.
Slip Ring Method:
Figure 2. Slip rings provide an electrical path for excitation and the bridge measurement signal. 
The slip ring method entails that the strain gage bridge is connected to four silver slip rings mounted on the rotating shaft. Precision brushes make contact with these slip rings and provide an electrical path for the incoming excitation and the outgoing signal. You can use either AC or DC to excite the strain gage bridge.
Rotary Transformer Method:
For the transformer method, the rotating transformer differs from conventional transformers by the primary or secondary winding rotating. One transformer is used to transmit the AC excitation voltage to the strain gage bridge and a second transformer is used to transfer the signal output to the nonrotating portion of the sensor. This means two transformers replace multiple rings, and no direct contact is made between the rotating and stationary elements of the transducer.
Digital Telemetry Method:
The digital telemetry method requires no contact points since it consists of a receiver-transmitter module, coupling module, and signal processing module. The transmitter module is integrated into the torque sensor. It amplifies and digitizes the sensor signal into a radio frequency carrier wave that is picked up by the caliper coupling module (receiver). The digital measurement data is then able to be recovered by the signal processing module.
Figure 3. Two transformers are used—one to transmit the excitation signal and the other to transfer the bridge output signal.
Choosing the Right Torque Sensor
Torque sensor selections primarily depend on your capacity and physical requirements.
Capacity—When taking note of application capacity, determine the minimum and maximum torque you expect. Extra torque and moments can increase the combined stress, which increases fatigue and affects overall sensor accuracy. Any load other than an axial, radial, or bending torque, is considered extraneous and should be noted beforehand. If you cannot design or build your setup to minimize the effects of these loads, consult the sensor guide to verify the extraneous loads are within the sensor’s ratings.
Physical and environmental requirements—Evaluate any physical constraints (length, diameter, and so on) and the way the torque sensor can be mounted. Consider environmental factors will be exposed to ensure proper performance across wide temperature ranges, and possible contaminants (oil, dirt, dust).
Revolutions per minute (rpm)—For rotary torque sensors in particular, it is important to understand how long the torque sensor will be rotating and at what speed to calculate the RPM.