Torque Sensor Guide: How do they work

Introduction

A Torque Sensor, also known as a torque transducer or torque meter, is a critical device in the field of precision measurements, designed to convert mechanical torque into a quantifiable electrical signal. These sensors find extensive applications across various industries, from automotive to medical, where accurate torque measurement is paramount.

Before delving into the details of how torque sensors operate and their diverse applications, it is beneficial to acquaint ourselves with several pertinent terms related to these devices. Understanding these terms is crucial for gaining insight into the product and its attributes, laying the groundwork for the detailed exploration that follows.

 

Terms and Definitions

Passive Sensor: A passive sensor is a sensor without amplification. The output signal, supplied by the sensor, is in the range of approx. 3 mV/V (depending on the applied strain gauge, the geometry, and the material of the measuring body).

Active Sensor: An active sensor is a device with an amplifier. Usually, the output signal of the sensor is ±10V, ±5V, 0...20 mA, 4...20mA, 10 ±10 mA or 12 ±8 mA. 

Measuring Range: The measuring range is the load range in which the guaranteed error limits may not be exceeded.

Repeatability: The repeatability is the maximum admissible output signal change in relation to the output signal (indication in %).

Nominal Torque: Nominal torque is the upper limit of the measuring range in which the specified error limits may not be exceeded.

Accuracy Class: The maximum single error of the sensor output signal (indicated in %) is smaller than the value related to the accuracy class. Hereby, the tolerance of the sensitivity is not considered.

Service Torque: Service torque is the torque with which the sensor may be stressed with the above nominal torque without changing its specified characteristics. The service torque range should be used in exceptional cases, only.

Limit Torque: Limit torque is the maximum permissible load of the sensor in which destruction of the measuring system is not expected. At limit torque, the specific error limits no longer apply.

Signal: Signal is the digital output signal of the sensor at angle and speed measurement. Generally, the signal is 5V TTL (either 5V or 0V).

Band Width / Max. Dynamic Load: Bandwidth – related to nominal torque – is the sinusoidal varying dynamic load in the direction of the measurement axis of the sensor, which is tolerated by the sensor at a stress of 107 vibration cycles without causing significant changes of the characteristics up to nominal torque.

Bridge Resistance: Bridge resistance is the ohmic resistance of the complete measuring bridge.

Temperature Coefficient of the Sensitivity: The temperature coefficient of the sensitivity is the change – related to the nominal sensitivity - of the actual sensitivity resulting from a temperature change of 10 K.

Temperature Coefficient of the Zero Signal: The temperature coefficient of the zero point is the change – related to the nominal sensitivity - of the output signal of an unloaded sensor resulting from a temperature change of 10 K.

Nominal Temperature Range: The nominal temperature range is the range of the ambient temperature in which the sensor complies with the technical data and error limits.

Service Temperature Range: The service temperature range is the range of the ambient temperature in which the sensor can be operated without the occurrence of permanent changes in the measurement characteristics. Within the service temperature range, specified error limits no longer apply.

Excitation Voltage: The excitation voltage is the required operating voltage to allow error-free operation of a passive sensor.

Sensitivity: Sensitivity is the output signal of a passive sensor at nominal torque less the preload signal.

Nominal Sensitivity: The nominal sensitivity is the theoretically predetermined rated value (sensitivity) of a passive sensor.

Supply Voltage: Supply voltage is the required operating voltage to allow error-free operation of an active sensor.

Level of Protection: The level of protection of housing is determined by the letters IP and a two-digit code number. The number indicates the contact protection, foreign object protection and water protection for electrical equipment.

Control Signal: By a control resistance, a signal, corresponding to the sensitivity of the sensor, is generated inside the sensor.

Advantages: Recalibrations are reduced. Zero point and sensitivity can be controlled before each measurement.

Function: By parallel switching of the resistor RK to the measuring bridge R1, the measuring bridge is electrically de-tuned, so that a measuring signal of 50 or 100 % of the nominal value is available at the output.

Twist Angle: The twist angle occurs at the initiation of the nominal torque between the test side and the drive side of the specimen. By a shift of the torsion angle, the natural resonance of the measuring body changes as well. To avoid interference caused by natural resonance changes, the torsion angle should be minimized.

Ultimate Torque: Ultimate torque is the stress that causes permanent changes or destruction.

Control Signal Excitation: For a sensor with an analog output the control signal can be switched on /off via a voltage signal.

Two switching positions can be defined:

L < 2,0 V (Low-signal) and H > 3,5 V (High-signal). The control signal of a sensor with a digital output signal can be controlled by software.

Pulses / Revolutions: Pulses/revolutions are recorded at angle/speed measurement. By a second 90° offset encoder track and flank evaluation, the pulses/revolutions can be quadrupled.

Reading Rate / Sample Rate: The reading rate or sample rate indicates the number of measurements per second.

Moment of Inertia: The moment of inertia is the moment in which a body opposed to the acceleration torque. The mass moment of inertia should be as small as possible to keep the stress of the measuring section as low as possible during the acceleration process.

Limit Thrust Load: The limit thrust load is the maximum force in the axial direction (axial force) with which the sensor may be loaded.

Limit Shear Force: The limit shear force is the maximum shear force (radial force) with which the sensor may be loaded.

Output Signal: Output signal is a conditioned signal, necessary for the error-free function of an active sensor.

Limit Speed: The limit speed (in rotating devices, usually expressed in r.p.m.) is the upper limit of the speed range that the sensor can permanently withstand.

Explanation of Abbreviations

F.S. – Full Scale

Logos

  Label for compliance with the safety requirements according to the CE guidelines

 

Core Components of a Torque Sensor:

Strain Gauges: The heart of the torque sensor is the strain gauge, a small electronic device that measures the amount of strain (deformation) in the sensor material when torque is applied. These are typically metal foil patterns affixed to a flexible backing, which is then bonded to the sensor's shaft or body. When the material deforms, the strain gauge's electrical resistance changes proportionally, allowing for the measurement of applied torque.

Sensor Body or Shaft: This is the part of the sensor that experiences mechanical strain when torque is applied. In reaction torque sensors, this body is static and doesn't rotate, designed to measure torque in stationary applications. In rotary torque sensors, the shaft is the rotating component that transmits torque from the input to the output, with strain gauges attached to measure the deformation caused by the applied torque.

Electrical Circuitry (Wheatstone Bridge): The strain gauges are connected in a Wheatstone bridge configuration, a type of electrical circuit used to convert the small changes in resistance from the strain gauges into a measurable electrical output. This configuration typically involves four strain gauges in a rotary sensor, ensuring balanced and sensitive measurement.

Slip Rings or Rotary Transformers (Rotary Sensors): These components are used to transfer the electrical signal from the rotating shaft of a rotary torque sensor to the stationary measurement system. Slip rings are mechanical rings that maintain an electrical connection between the stationary and rotating parts. Rotary transformers, on the other hand, provide a non-contact method to transfer the signal, avoiding the wear and maintenance issues associated with slip rings.

Signal Conditioning Electronics: This includes amplifiers and filters that process the raw signal from the Wheatstone bridge to produce a clean and usable electrical output that correlates with the applied torque. This signal is then suitable for digital display, recording, or further analysis.

Torque Sensor: Principle of Operation and Detailed Analysis

How do Torque Sensors Work?

The principle of how torque sensors work hinges on the deformation of a material, typically metal, under the applied torque, which slightly twists or bends the material, causing a change in its physical dimensions where strain gauges are applied. This deformation leads to a corresponding change in the electrical resistance of the strain gauges.

Strain gauges are a fundamental component of torque sensors, consisting of a thin foil and an electrical conductor that are firmly connected. When the material to which the strain gauge is attached deforms under torque, the shape of the foil and conductor changes, altering their electrical resistance. This change in resistance is measured using a Wheatstone bridge circuit, which converts it into an electrical signal. This signal is then amplified and processed to produce a torque reading. The output can be analog or digital, depending on the sensor design.

Unlike load cells or force transducers, which measure forces along one or two axes (tension or compression), torque transducers are designed to measure torsional forces or torque. This distinction is crucial because torque involves a rotational force, similar to a lever action, rather than a straightforward push or pull. The design of torque sensors typically falls into two main categories: measurement shafts and measurement flanges.

Measurement shafts typically consist of a shaft with strain gauges strategically placed, sometimes on a tapered mid-section, to detect torsion. These shafts are usually encased in a housing that allows the shaft to rotate freely, with the housing (stator) remaining stationary.

The entire assembly can be integrated into machines or test setups using hub-type clamps at both ends of the shaft.

Measurement flanges, while similar in function to measurement shafts, feature a distinct design. They consist of a short, often hollow shaft with flanges at both ends. These flanges facilitate the sensor's integration into machinery or test benches through threaded connections. Unlike measurement shafts, flanges do not require a full enclosure or bearing assembly because the rotor (the part of the sensor that rotates) is directly incorporated into the measurement section, minimizing the need for additional support structures.

Beyond accuracy, torque sensors boast high sensitivity, excellent linearity, and minimal hysteresis, making them well-suited for challenging environments. Technological advancements have rendered these sensors more compact, versatile, and economically viable, broadening their application spectrum.

Wheatstone Bridge Circuit

The Wheatstone Bridge Circuit is integral to torque sensors for accurately measuring torque-induced resistance changes. This circuit, usually configured with four strain gauges, detects the minute resistance variations when torque is applied, converting them into measurable electrical signals. The arrangement ensures high sensitivity and accuracy in torque measurement. The "excitation" wires power the sensor, while the "signal" wires convey the output to the measuring instruments. Some models include "sense" wires to correct voltage drops across long distances, ensuring consistent accuracy. The Wheatstone Bridge is pivotal for precise torque readings, making it indispensable in applications requiring meticulous torque monitoring and control.

Types of Torque Sensors: An Overview

Torque sensors are categorized into two main categories: rotary, which measures dynamic torque in rotating elements, and reaction, which measures static torque in stationary setups. For applications requiring the measurement of a wide range of torques, from very low to high, within a single unit, dual-range torque sensors are employed. Detailed insights into each category follow below.

Rotary Torque Sensors

Rotary torque sensors are designed to measure the torque on rotating elements, such as shafts, motors, and crankshafts. They are characterized by their ability to provide real-time torque data from a component in motion. These sensors typically employ strain gauges mounted on a rotating shaft, with data transmission facilitated by slip rings or wireless technologies. Rotary torque sensors are invaluable in applications requiring continuous monitoring of torque in rotating systems, such as in engine testing, drivetrain analysis, and performance evaluations of rotating machinery.

                                                     

Rotary Torque Sensors - Slip Ring vs Non-Contract

Rotary Torque Sensors are divided into two principal categories based on their operational mechanics and data transmission methods: Slip Ring and Non-Contact torque sensors. Each offers unique advantages tailored to specific requirements, ensuring accurate and reliable torque measurements.

Slip Ring Torque Sensors: Mechanism and Applications

Slip Ring Torque Sensors operate based on a physical connection between the sensor's rotating and stationary components. This connection is facilitated by a slip ring assembly, comprising a rotor attached to the dynamic shaft and a stator, which houses the sensor's electronics. Brushes within this assembly ensure continuous electrical contact during rotation, allowing for the direct transmission of torque data from the rotating shaft to the stationary analysis system. Slip Ring Torque Sensors are widely utilized in industrial applications where continuous torque monitoring is essential, such as in automotive drivetrain testing, heavy machinery performance evaluation, and in the monitoring of industrial mixers and conveyors.

Non-Contact Torque Sensors: Advanced Technology and Use Cases

Non-Contact Torque Sensors represent a technological advancement in torque measurement, eliminating the need for physical contact between the sensor's components. This category prominently features Surface Acoustic Wave (SAW) technology, where SAW devices affixed to the shaft undergo distortion proportional to the applied torque. This distortion alters the resonant frequency of the devices, a change that is detected by an adjacent RF pickup without direct contact with the rotating shaft. These torque sensors can generally spin at a higher rate because they are non-contact and are not limited by the slip rings and brushes. Non-contact Torque Sensors find applications in high-end and sensitive environments, including the development of aerospace components, motorsport engineering (notably in Formula 1 for gearbox and engine testing), renewable energy systems like wind turbines, and in pharmaceutical manufacturing for precise control of mixing and dosing processes.

                           

Making the Choice!

The selection between slip-ring and Non-Contact sensors depends on specific requirements such as the need for maintenance, environmental conditions, and the level of precision required. Slip Ring sensors provide a reliable, cost-effective solution for a wide array of applications, while Non-Contact sensors are preferred in scenarios demanding high precision and minimal maintenance.

Couplings for Rotary Torque Sensors

Couplings serve as the critical link between the rotating shafts and torque sensors, facilitating efficient torque transmission. They are meticulously designed to accommodate various operational demands, including misalignments and angular offsets, which are inherent in dynamic systems. The choice between rigid and flexible couplings hinges on the specific application needs—rigid couplings offer a solid connection for precise torque transmission but lack flexibility, whereas flexible couplings provide necessary compensation for misalignments, dampen vibrations, and mitigate stress on connected parts.The absence of a suitable coupling would render the torque transducer incapable of mechanically connecting to the rotating shaft, thus failing to measure the transmitted torque and compromising the system's ability to monitor and analyze torque effectively.

Reaction Torque Sensors

Reaction torque sensors, in contrast, are designed to measure torque without requiring the sensor itself to rotate, similar to a lever arm and a load cell, without actually needing either. They are ideally suited for applications where the torque sensor can be mounted statically, and the torque is transmitted through a stationary or non-rotating element. This type is often used in bench tests, calibration setups, and instances where the torque-producing element remains fixed, such as in the testing of fastening systems, electric screwdrivers, and torque wrenches.

                                                     

Dual-Range Rotary Torque Sensors

The dual-range functionality allows for high-resolution measurements across a broad spectrum of torques, from very low to high, within the same sensor unit. This feature is particularly advantageous in applications requiring precision measurements across varying torque levels without the need for multiple sensors. The smaller measuring range, which can be activated via USB or the operating voltage, provides enhanced accuracy and sensitivity for low-torque applications, while the larger range accommodates higher torque measurements, making dual-range sensors extremely versatile and cost-effective for a wide range of testing and monitoring tasks.

Each type of torque sensor offers unique advantages tailored to specific measurement scenarios, from high-precision laboratory settings to demanding industrial environments. Understanding the distinct characteristics of Rotary, Reaction, and Dual-Range torque sensors is crucial for selecting the most appropriate sensor for a given application, ensuring accurate and reliable torque measurement.

                                                

Applications of Torque Sensors in Various Industries

Torque sensors, with their ability to accurately measure rotational forces, have become indispensable across a broad spectrum of industries. Their integration into various systems enhances precision, efficiency, and safety, making them a cornerstone of modern engineering and technology. From the meticulous requirements of aerospace engineering to the dynamic needs of automotive design, and the critical demands of medical devices, torque sensors play a pivotal role in pushing the boundaries of innovation and quality.

Now, let's delve into the specific applications across different sectors:

Automotive and Industry Applications of Torque Sensors

In the automotive and industrial sectors, torque sensors are pivotal for ensuring precision and reliability. They are integral in systems such as electric power steering, providing real-time feedback to adjust steering assistance. They are extensively used for precise torque/angle measurements in vehicle dashboard controls, ensuring optimal user interaction. They also monitor friction torque in greased ball bearings, using variable-speed AC motors to adjust for grease levels, crucial for machinery maintenance. Furthermore, torque sensors are employed to analyze the start-up and continuous operation characteristics of geared motors, ensuring efficiency and reliability. In applications involving mechanical movements, such as hinges, torque sensors provide valuable data on torque levels, contributing to the design and durability of automotive and industrial components. This technical versatility underscores the importance of torque sensors in advancing automotive engineering and industrial automation.

Torque Sensor in Dashboard Controls                                                                    Torque Sensor in Greased Ball Bearings

                               

Torque Sensor in Geared Motors                                                                                          Torque Sensor in Hinges

                                     

Electronics and Electromobility: In the fast-evolving field of electromobility, torque sensors are key to the development of electric and hybrid vehicles. They measure the efficiency of electric motors, contributing to the refinement of power management systems. This ensures optimal energy usage and extends the range of electric vehicles. Additionally, in electronic devices like e-bikes, torque sensors adjust the motor assistance based on the rider's input, enhancing the user experience and promoting energy conservation.

                 

Medical Technology: In medical technology, precision is paramount. Torque sensors are employed in surgical tools and robotic systems, providing critical feedback for delicate procedures, and ensuring accuracy and patient safety. They also play a role in prosthetics, enhancing the functionality and comfort of these life-changing devices.

                                       

Quality Assurance and Research & Development: In R&D labs, torque sensors facilitate the development of new technologies and materials by providing accurate measurements of mechanical properties. This data is vital for innovating and improving product designs across industries. In quality assurance, they ensure that manufacturing processes meet the highest standards, maintaining the integrity of products from automotive parts to electronic devices.

 

Selecting the Right Torque Sensor

Selecting the ideal torque sensor for your application is not just a matter of convenience; it's a necessity for ensuring both safety and precision in torque measurement. This selection process is critical due to two main factors: the prevention of damage and the assurance of accuracy.

Preventing Damage: The torque sensor, while being a cornerstone for accurate measurements, can also serve as a mechanical fuse within the driveline. Its structural design necessitates directing strain to a specific point for measurement, typically the sensor's most vulnerable spot. This precise requirement makes choosing the right torque sensor crucial to avoid mechanical failures and ensure the longevity and integrity of both the sensor and the system it's integrated into.

Ensuring Accuracy: The reliability of torque data hinges on the sensor's ability to meet all electrical specifications for the application. A well-chosen torque sensor minimizes data uncertainty, providing high-quality, dependable readings that are crucial for the application's success.

When selecting a torque sensor, it's essential to consider several key aspects of your application:

Type of Torque: Determine whether you need to measure dynamic torque with a rotary sensor or static torque with a reaction sensor.

Environmental Conditions: Evaluate the operating environment, including temperature extremes, humidity, and potential exposure to corrosive substances, to ensure the sensor's materials and construction are suitable.

Mounting Configurations: The physical integration of the sensor into your system—how it attaches to the driveline or machinery—must be compatible and straightforward.

Capacity Range: The sensor should be capable of measuring the expected torque range without sacrificing accuracy or risking overload.

Output Signal: Consider the sensor's output compatibility with your data acquisition or monitoring systems to ensure seamless integration and data transfer.

Incorporating the right torque sensor into your application is more than a technical decision; it's a strategic one that impacts the quality, safety, and success of your operations. By carefully considering the factors mentioned above, you can select a torque sensor that not only fits your immediate needs but also accommodates future requirements, driving innovation and ensuring excellence across various industries.

A-Tech engineers can offer the perfect match sensor for any application, emphasizing the importance of accurate torque measurement in quality control, product development, and maintenance across various industries. With A-Tech, you are assured of finding a torque sensor that meets your needs, ensuring precision, durability, and performance.