Torque Transducers

Torque Transducers

A torque transducer is a sensor for measuring and recording the torque on a rotating system, such as an engine, rotor, or crankshaft, etc.

Torque is the amount of force that causes an object to rotate in a circular motion. Torque is a rotational force, or a force through a distance an object turns. It can be classified into two types: static or dynamic. It is considered static if the object is imparted with no angular acceleration. An example of static torque can the force exerted by a clock spring since there is no rotation involved and therefore there is no angular acceleration.

Torque measurements are very important in a wide variety of engineering products from precision tightening tools to process control. For example, in a wood cutting factory would use a predetermined maximum torque to determine blade changes. This improves the maintainability of the system and reduces downtime while increasing product quality.

Static torque is comparatively easier to measure than dynamic torque. Dynamic torque generally requires the transfer of some parameter that can be electric, hydraulic, or magnetic from the rotating shaft to a static part.

Torque transducers convert torsion which is the surface shearing stress corresponding to a voltage signal and then transfer this signal through the slip ring, brush, or rotary transformer, etc. This allows accurate measurement of the torque from a standstill position to rotation at a high-speed.

A strain gauge is the most common transducer used to measure torque. It converts torque into some change in electrical resistance in a circuit. The strain gauge is fixed to a beam or structural element that deforms when a force is applied. The deflection produced causes stress that changes the resistance. This change of resistance is detected and is converted to the corresponding voltage which can be easily calibrated to give the measure of torque.

There are basically two types of torque sensors:

  • Rotary – A rotary sensor measures the dynamic torque.
  • Reaction – A reaction torque sensor measures the static torque

As the torque transducers use strain gauges applied to a rotating shaft or axle, a method to power the strain gauge bridge is required, as well as to receive the signal from the rotating shaft. To achieve this, various types of slip rings, wireless telemetry, or rotary transformers are used. Some new types of torque transducers can convert the signal to digital data and then send it. The stator electronics can then read the digital data and convert it back to an analog output signal.

A more recent advancement is the use of Rayleigh Wave or Surface Acoustic Wave (SAW). SAW devices are attached to the shaft. The strain on these small devices as the shaft flexes are read remotely and output is received without the need for further electronics on the shaft.

Rotary Torque Sensors are frequently used as testing or auditing tools for motors, power tools, turbines, and generators. They are used for feedback control, torque monitoring, and analyzing the efficiency of test benches.

 

Getting the Measurements

Torque sensor accuracy is quite important in most applications. Although the torque of the tools is essentially measured during the quality assurance process, it is still essential to calibrate them.

Since torque transducers use strain gauges as sensor elements, precise and stable measurement is assured under severe conditions even for long-term operation. Therefore are widely used not only for experiments and research but also for industrial applications.

The design of a reaction torque cell ensures to eliminate side bending and axial loading, and so it is sensitive only to torque loading. The sensor’s output is a function of force and distance and is normally stated in inch-pounds, foot-pounds, or Newton-meters.

Reaction torque sensors can be used in applications where some rotation is acceptable if it will not break the data cable. They are used to calibrate torque wrenches, drills, test the breaking torque of aircraft fasteners, and even measure the opening torque of child-proof medicine bottles.

Common methods for measuring torque are based on mounting transducers on a rotating shaft. Most of these methods also use strain gauges. There are some advantages and limitations of each method, many of those are application dependent. The best solution requires an in-depth understanding of the application.

There are two ways to obtain torque measurements; strain-gauging the shaft and by using in-line torque cells. Both have two technical issues obstacles: getting the power to the gauges over the stationary and rotating parts and getting the return signal. The signals may be obtained by either contact or non-contact methods.

Contact Method

Slip rings are necessary for contact-type torque sensors to supply power to and receive the signal from strain gauges mounted on the rotating shaft. Slip rings are prone to wear and tear. Maintenance of slip rings is a problematic issue in many industrial applications. Slip ring brushes, as well as, the support bearings used in these configurations wear out and require replacement.

Non-Contact Methods

Transmission of data by radio signals offers a good solution for avoiding the slip ring damage issues. A fixed antenna induces a voltage in a loop antenna on the rotating shaft. The power generated from the rotating shaft antenna excites the strain gauges. A radio transmitter mounted on the shaft sends the output signal back to the fixed antenna. In this way, the torque data can be read easily.

Radio telemetry systems though expensive are quite reliable and easy to install. Since there are no mechanical contacting parts to wear out, they have a much longer working life.

Surface Acoustic Wave (SAW)

SAW transducers make use of the Rayleigh Wave phenomenon. By using this method small transducers can be manufactured which are extremely accurate, and also require no physical contact between the revolving shaft and its static part.

The SAW sensor uses the surface waves produced by applying AC voltage on the two interleaved comb-shaped arrays that are bonded on one end of the piezoelectric substrate. At the other end, a similar receiving array converts the wave back into a voltage signal.

These two SAW sensors are bonded to the shaft at an angle of 45 degrees to the axis and it is connected in a “half-bridge” configuration. When force is applied on the shaft to produce a torque, the output signals that are generated can be processed to give a measure of the torque.

The SAW transducers are self-contained and require no physical connection to the shaft housing. The signals are exchanged by capacitive coupling of two disks, one of which is fixed and the other rotates with the shaft.

The SAW technique is two orders of magnitude more sensitive than the resistive strain gauges. The electronic circuitry is based on frequency shift and not voltage levels. This improves the overall signal quality and reduces the noise.

 

Torque Transducer Calibration

Torque calibration is a comparison of measurements between the standard instrument and the test instrument. The standard device is supposed to measure correctly and is perfectly calibrated. The operator then just adjusts the device until they both match. Calibration of the torque transducer is carried out as per standard procedures. Even after you calibrate it, a device or tool might slowly move out of the calibration range over time.

Static torque calibration is a process where discrete torque increments are applied in steps with suitable time intervals for stabilization of the transducer at each applied torque level.

In the continuous torque calibration process, the torque is applied in a continuous linear manner up to the maximum calibration torque delivering the exact amount of torque that is required.

 

Calibration

Calibrating a measuring device involves verifying by how much the device deviates from what it should show. The measurement range must have a higher level of precision than the device being calibrated. The highest measurement standard applied within the organization is called the “reference standard.” Tools requiring calibration are calibrated periodically at an accredited laboratory that maintains national standards and which also ensures traceability.

Torque transducers must be calibrated as per a specified schedule. e2b calibration follows ISO 17025 standards and practices, which establishes guidelines for laboratory testing and calibration and can achieve this level of accuracy on-site as well.
Contact e2b calibration for more information about how we can service your company’s needs.

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