What is Torque?
Torque is a function of force and length (known as the moment or moment arm), this is shown below in our simple 'seesaw' diagram. The seesaw is balanced even though the forces applied to each side are different, this is because the torque at the middle point is the same. The calculations are as follows:
Left Side = 100N force x 1 metre = 100Nm of torque
The most common metric engineering units for torque is the Newton metre (Nm), whilst the most common imperial engineering units is foot pounds (ft.lb).
Getting an Electrical Signal
When torsion is applied to the shaft causing it to twist, shear stresses are induced. These are measured by bonding the strain gauges at 45° to the horizontal torque axis. As the shear stress induced in the shaft is the same throughout its length, the strain gauges can be bonded at any point along its length. However, it is normal practice to place them in the centre, as far as possible from spurious stress that can be induced at the mechanical interfaces.
Typically four strain gauges are bonded and connected into a Wheatstone bridge configuration with temperature compensation components included within the bridge circuitry. With an excitation voltage applied to the bridge and torque induced into the shaft, an electrical output linearly proportionate to that torque will result.
For the strain gauged shaft to become a useful meauring instrument (torque sensor) it is necessary to calibrate it with a known reference standard. This can be either weights applied to the end of a cantilever arm of known length, or in line with a reference standard torque transducer.
The completed Wheatstone Bridge requires a stable DC supply to excite the circuit. This is usually 5Vdc or 10Vdc, but can be any value from 1Vdc up to 18Vdc.
These low-level millivolt signals are compatible with a vast range of bespoke strain gauge sensors such as load cells and pressure transducers and their associated instrumentation. These instruments include digital displays, analogue and digital amplifiers. Typical analogue amplifiers will generate a higher level voltage (0-5Vdc, 0-10Vdc) or current (0-20mA, 4-20mA) for additional processing.
Typically digital amplifiers also provide RS232 or RS485 output using either industry standard or protocols specific to an industry such as CANbus, MODbus etc. In many cases the signal conditioning is built in to the device, giving a direct analogue or digital output for further processing.
One of the latest developments to be incorporated into torque sensors in radio telemetry operating in the 2.4GHz frequency band. Operating with very low current consumption enables the use of small batteries on a power source. This latest technology has led to the design of contact-less rotary torque transducers.
A further benefit of radio telemetry torque sensors is their low cost (up to 50%) compared with equivalent slip ring or brushless types.
Selection of an appropriate device will be based on the following amongst other factors, all of which will impact on the accuracy and cost of the measurement, these include:
Static / Reaction Torque (less than 1 revolution) vs.
When selecting a transducer to measure dynamic torque, it is important to ensure that Is has a sufficiently wide frequency response bandwidth so that the electrical output will react fast enough to capture the changes (peaks or troughs) as they occur, rather than smoothing or filtering them out.
Power & Signal Transmission in Rotary Torque Sensors
The conductive rings rotate with the sensor and use a series of sprung brushes to contact the rings and transmit the electrical signal. Slip rings are relatively straight forward with only minor drawbacks in that the brushes and to a lesser extent, the rings, do wear and as such have a finite life. Because of this they don't lend themselves to long-term tests, very high speed or to applications where access to the sensor for servicing is limited.
At low speed the electrical connection between the rings and brushes are relatively noise free, however at higher speeds electrical noise will eventually degrade their performance. The maximum speed (rpm) for a slip ring is determined by the surface speed at the interface between the brush and ring. As a result the maximum operating speed will be lower for larger, or higher torque capacity sensors because the larger slip rings will therefore have a higher surface speed at a given rpm. Typical maximum rpm will be in the range of 5,000rpm for a medium capacity torque sensor. For very low capacity measurements, the brush ring interface can be a source of 'drag torque' that can be a problem for the driving torque to overcome.
Rotary Transformer (Inductive Loop)
The rotary transformer system is versatile enough for use in special torque transducers and those where space is limited. However as this design incorporates bearings, the maximum rpm is more than the slip-ring design but limited by the maximum specified performance of the bearing. The system can also be susceptible to noise and errors, which are induced by the alignment of the transformer primary-to-secondary coils. Because of the special requirements imposed by the rotary transformers, specialized signal conditioning is also required in order to produce a signal acceptable for most data acquisition systems.
Wireless Radio Telemetry (Battery Powered, 2.4GHz)
The avoidance of rotor and stator coils means that installing wireless torque sensors is a simplified process over other types as there is no cabling provision or coil alignment to consider, it also means that the measurement signal can be transferred over considerable distances (up to 120m) easily.
The data stream provided by the wireless telemetry system is broadcasted and so can be read by multiple receiver units with different functions such as digital displays, analogue outputs and PC-based USB acquisition making multi-featured systems quick and easy to set up.
The torque transducer will accommodate only a small amount of misallingment, but idearly no misalignment should exist between input and output shafts. Severe misaligment will lead to premature bearing failure due to the repeated stresses developed in the contact areas between the bearings and the raceway, even if the coupling is properly mounted and maintained. In correctly aligned shafts, the bearings will be subjected to loads less than the established rated loads for the bearings in which case will result in extended bearing life.
Popular types used are disk (or plate) / bellows / diaphragm / elastomer / grid / gear and many other proprietary names given by different suppliers. The selection of a suitable coupling type is based on speed, torque rating, environment, access, degree of misalignment and the service life required.
Torque Measurement Summary
To measure torque effectively it is important to understand the factors involved in the creation of the torque as well as those that may alter the torque measurement, as these will impact on the selection of an appropriate transducer. Criteria such as nominal torque rating, speed, available space to mount the sensor, duration of measurement and the environment within which the measurement is taking place are amongst the most significant criteria that will dictate torque sensor selection.
When measuring dynamic torque, the location of the measurement is vital to ensure that the true torque is measured and false measurements are avoided, caused by either adjoining components in the drive train or those that dampen the torque measurement system, including the measurement system itself.
Usually the physical and environmental factors present will pinpoint the selection of appropriate equipment. The selection of the correct transducer rating, transmission system and mechanical connection will ensure the measurement solution is no more expensive than it need be and maximum long-term accuracy and reliability will be achieved.
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