Torque is a crucial parameter in motor testing, especially in motor efficiency evaluation, where it is an indispensable measurement. The accuracy of torque measurement directly affects the correctness of motor efficiency assessment. Currently used torque measurement methods can be categorized into the balance force method, the transmission method, and the energy conversion method based on their measurement principles.
I. Balanced Force Method
For a transmission mechanical component operating at a constant speed, a pair of torques T and T′ must exist simultaneously on its main shaft and body, and these two torques are equal in magnitude and opposite in direction. The method of measuring T on the main shaft by measuring T′ on the body is called the balanced force method. Let F be the force acting on the lever arm, and L be the length of the lever arm, then T′ = LF. T′ and T can be obtained by measuring the force F and the lever arm L. The advantage of the balanced force method is that there is no problem in transmitting torque signals, and the force F on the lever arm is easy to measure; the disadvantage is that the measurement range is limited to the constant speed operating state and cannot measure dynamic torque.
II. Transmission Method
The torque transfer method utilizes the change in the physical parameters of an elastic element when torque is transferred. The torque is measured by using the correlation between this change and the torque. Based on different physical parameters, the torque transfer method can be further divided into magnetoelastic, strain gauge, vibrating wire, and photoelectric types, among others. Currently, the torque transfer method is the most widely used in the field of torque measurement.
Figure 1 Classification of Transmission Methods
1. Photoelectric torque measurement method
Two identical circular gratings with the same number of openings are fixed to a rotating shaft. A photoelectric element and a fixed light source are fixed to opposite sides of the gratings. When there is no torque applied to the shaft, the bright and dark fringes of the two gratings are misaligned, completely blocking the light path, and no light reaches the photosensitive element, resulting in no electrical signal output. When torque is applied, the cross-sections of the two circular gratings rotate relative to each other, causing the bright and dark fringes to partially overlap. Some light passes through the gratings and reaches the photosensitive element, resulting in an electrical signal output. The larger the torque value, the larger the torsion angle, the greater the intensity of light reaching the photosensitive element, and the larger the output electrical signal. The magnitude of the applied torque can be measured by measuring the output electrical signal.
Figure 2. Principle of photoelectric torque measurement
The advantages of this method are its fast response speed and ability to monitor torque in real time; its disadvantages are its complex structure, difficulty in static calibration, poor reliability, poor anti-interference ability, and measurement accuracy is greatly affected by temperature changes. This method is not suitable for torque measurement of shafts at startup or low speed.
2. Magnetoelectric Torque Measurement Method
Two identical gears are mounted on an elastic shaft. The magnetic core and coil form a signal acquisition system. A small gap is reserved between the tooth tip and the magnetic core. When the shaft rotates, two alternating electromotive forces are induced in the two coils respectively. Moreover, the alternating electromotive forces are only related to the relative position and intersection position of the magnetic cores of the two gears. The corresponding torque value can be obtained by detecting the magnitude of the electromotive forces.
Figure 3. Principle of magnetoelectric torque measurement
The advantages of this method are high accuracy, low cost, and reliable performance. It is a non-contact measurement method, meaning it requires no power supply or intermediate transmission links. Its disadvantages include complex structure, limited frequency response, difficulty in manufacturing, long response time, and correspondingly large sensor size and mass. The signal is weak at low speeds, while dynamic balancing is difficult at high speeds. The magnetoelectric torque measurement method is suitable for measuring torques that can produce large angular displacements and can measure starting and low-speed torque. However, due to its poor dynamic characteristics, it is not suitable for measuring the torque of high-speed rotating shafts.
3. Vibrating Wire Torque Measurement Method
By utilizing the functional relationship between the natural frequency of the vibrating string and the tension, the force is converted into an electrical charge. First, the electrical charge value is measured and converted into the magnitude of the force, and then the corresponding torque value is calculated.
Figure 4. Schematic diagram of vibrating wire torque measurement principle
The advantages are that the drive shaft can be directly used as the torsion shaft for measurement; it uses frequency signal transmission, resulting in good anti-interference performance; and the sensor part is separate from the force measuring shaft, making it convenient for measurement on ships or vehicles. Its disadvantages are complex structure, low sensitivity, low measurement accuracy, and high requirements for the elastic deformation of the elastic shaft. This method is suitable for torque measurement of large rotating shafts but not for high-speed rotating shafts.
4. Magnetoelastic Torque Measurement Method
Magnetoelastic torque measurement is a method that uses the magnetoelastic effect of ferromagnetic materials and other alloy materials to measure torque. When a torque is applied to an elastic shaft of a ferromagnetic material in a magnetic field, the change in magnetic permeability will reflect the change in the magnetization intensity of the ferromagnetic material. Therefore, the torque signal can be obtained by measuring the change in magnetic permeability.
The advantages of this method are high sensitivity, good stability, non-contact measurement, high output power, fast response speed, good overload capacity, convenient installation and use, strong anti-interference ability, simple structure and circuit, and ability to work in harsh environments. The disadvantages are the existence of "circular arc modulation" error, which limits its application; the inherent deviation in permeability distributed along the torsion axis circumference results in relatively low measurement accuracy, and only the stress value of the magnetostrictive layer material is measured, which still has an error compared to the required torque value. The magnetoelastic torque measurement method is widely used in marine power plants, steel rolling mills, oil drilling rigs, and CNC locomotives.
5. Strain Gauge Torque Measurement Method
Torque causes strain in the drive shaft, and this strain is proportional to the magnitude of the torque. Therefore, the magnitude of the torque can be detected using resistance strain gauges. When the drive shaft is subjected to torque, it undergoes torsional deformation. The maximum shear strain occurs in a direction at a 45° angle to the axis. Attaching a resistance strain gauge in this direction allows the detection of the torque applied to the drive shaft. The working principle is shown in the figure below:
Figure 5. Principle of strain gauge torque measurement
The advantages of strain gauge torque measurement are simple structure, high sensitivity, strong adaptability, low cost, easy operation, mature technology, wide application range, high measurement accuracy, fast response speed, stable and reliable performance, good temperature compensation performance, and adaptability to harsh environments. Its disadvantages are that factors such as humidity, temperature, and adhesives can affect the accuracy of the measurement, and it has poor anti-interference ability. This method is not suitable for torque measurement of high-speed rotating shafts.
III. Energy Conversion Method
The energy conversion method refers to indirectly measuring torque by measuring other parameters such as thermal energy and electrical energy based on the law of conservation of energy. The TN4000 electronic torque meter launched by Galaxy Electric uses this principle to measure motor torque. The TN4000 electronic torque meter utilizes the law of conservation of energy to measure torque through high-precision measurements of electrical parameters, temperature, speed, and other parameters. It is a comprehensive instrument that not only conveniently measures torque but also accurately measures motor voltage, current, power, speed, and other parameters. Furthermore, torque measurement does not require an additional coupling, reducing the difficulty of on-site operation.
