The feasibility of a noncontact sensor is investigated. This type of sensor can potentially be used for torque measurement in a speed-variable power transmission system. Torque can be read by examining the phase difference between two induction signals from respective magnetic sensors that detect the magnetic field intensity of permanent magnets mounted on the surface of a shaft in rotation. A real-time measuring algorithm that includes filtering and calibration is adopted to measure the torque magnitude. It is shown that this new torque sensor can perform well under rotation speeds ranging from 300 rpm to 500 rpm. As an interim report rather than a complete development, this work demonstrates the feasibility of noncontact torque measurement by monitoring a magnetic field. The result shows an error of less than 2% within the full test range, which is a sufficient competitive performance for commercial sensors. The price is very low compared to competitors in the marketplace, and the device does not require special handling of the shaft of the surface.
Torque is an axially twisting moment that is measured on the shafts of rotating machines. For example, in automobiles, electric motors, and helicopters, the technique of torque feedback control is needed to reduce fuel consumption and improve ride comfort. Nonetheless, no torque measurement solution has proven to be reliable and repeatable for successful commercialization in the marketplace. This holds true especially in the automotive industry, which seeks to achieve a low-cost and in situ dynamic torque control without major change of existing designs. The installation method faces great difficulty because it requires reliable and available strain-gauge-type torque sensors in automobiles. For example, common strain-gauge-type sensors must be inserted into the middle of the power transmission shaft using a slip ring, which requires critical maintenance. Another difficulty is the necessity of keeping performance at a high oil temperature of approximately 200°C. The strain-gauge-type sensor is normally reliable only under 80°C, and it is difficult to seal against hot oil.
To resolve these difficulties, studies [
Moving magnet technologies (MMT) and the SAW-type sensor are used successfully in the current market, but the installation of these items requires extensive modification of the target shaft. The best advantage of the new methods is that there are no critical design changes to the current system. For example, MMT can measure the shaft angle between two rotating shafts that are linked by a torsion bar [
This paper proposes a new sensor concept and measurement principle using (
On the other hand, a traditional optical sensor may be used to measure a reflected distance using a specially processed surface of the axis to detect a twist angle. However, this method is limited in some special environments. In this study, we propose a new torque sensor in which multiple permanent magnet pairs make the sensor applicable even in harsh and hot environments such as oil and dust in working machines. Increasing the longitudinal separation
A test bench, which is detailed in Figure
Photo of test bench.
For analysis and visual results, we used the LabVIEW program, as shown in Figure
Front screenshot of torque sensor program.
A Hall sensor is a device that proportionally converts a recognized magnetic field strength to DC voltage, which is typically used for position recognition and detecting rotation speed. Aichi Steel’s AMI302 sensor has high sensitivity and low magnetic field strength (full scale 2 Gauss) and can measure a complex three-axis magnetic field. The AMI302 is not capable of measuring the magnetic field in the three axis directions at the same time. Measuring in the
In order to change the distance between the four pairs of magnets and a pair of AMI302 magnetic sensors, a precision bench was fabricated with a 3D adjustable stage, as shown in Figure
Schematic of torque sensor bench and shaft.
A servomotor (Mitsubishi Electric Corporation MR-J2S-12KA, with a maximum torque of 200 N
The proposed commercial sensor (Haiam HKT-TRC strain-gauge type) is installed and acts as a reference to compare the torque. This sensor has a real-time feedback function and can measure torque up to 200 N
A neodymium magnet (2 mm hexagon size) is designed as a test magnet. The rubber magnet was used in previous experiments [
The neodymium magnet, a widely used type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure [
When the neodymium magnet is set at a 12 mm gap from the sensor, the maximum value of the measured magnetic field reaches a sufficient value of 2 Gauss. The noise and Hall-effect (HE) sensor output-signal contrast ratio are reduced to 10%, which is helpful in improving precision.
Nevertheless, many large-capacity electrical components, such as the servomotor, can generate considerable high-frequency noise. In order to remove this noise, a low-pass filter with a 60 Hz or higher cutoff frequency is applied by software. The major signal frequency is approximately 33 Hz at 500 rpm. To increase the reliability of the data, a higher cutoff frequency is useful at 500 rpm or higher. If a new measurement system is applied to an internal combustion engine (as in the transportation industry) and reduces the distance between the HE sensor and DAQ board, the signal would be much clearer, and the cutoff frequency of the low-pass filter (LPF) could be increased.
A spline fitting [
The concept of this experiment is to develop a detection method of corresponding individual peaks of magnetic field strength waves to estimate a twisted angle. Therefore, detecting peaks from a signal acquisition is the most important factor. Based on a discrete data set, a best-fit spline wave is determined by the
The correlation between torque and twist angle can be expressed in the following equation. Equation (
The constant value
The four magnet pairs are mounted on the shaft at 90° increments to compensate for the dynamic reaction during rotation. The effect of pairing the magnets and the calibration data are analyzed in this chapter. The experiments can be classified into three steps.
The first step is calibration to obtain two critical parameters by measuring an initial angle difference between each pair of magnets and the twist angle per applied unit torque (in rad/N
The second step is the repeated data correction of the first step to check long-term variation and repeatability.
The last step involves comparing the respective data of the commercial sensor and the calculated data from the proposed sensor while changing the rotation speed and applied torque. This step confirms the repeatability and reproducibility of the new suggested measurement method as a torque meter.
A pair of sensors and four pairs of magnets are rarely aligned in an axial position with a no-load condition because it is impossible to install them on an exact axial line. Therefore, one of the important tasks of sensor calibration is to read the initial deviation of each magnet pair at a no-load or specified torque condition. In order to confirm repeatability and reliability as a torque measurement sensor, this parameter has no variation in the short or long term.
As with the experimental results in Figure
Initial angle interval of attached sensors on the shaft.
The data of 300 rpm were compared in Figure
Nonlinear relationship interval at low torque (300 rpm).
In order to consider removing the effect of temperature, a magnet was mounted with AXIA super glue, which can withstand temperatures up to 120°C without changing its physical properties. The variable dependent on temperature in (
Initially, the targeted speed range for the experiment was 100–1000 rpm. However, above 600 rpm, we cannot continuously increase the torque more than 100 N
Twist angle per unit torque in the 100–500 rpm range.
These results can be explained in two ways. The first cause is the misalignment of the shafts. The coupling jointed with the torque motor, the proposed torque sensor, the commercial sensors, and the friction clutch make a comparable vibration. The other cause is a position mismatch of the new sensor and the strain-gauge-type sensor: between the two sensors are ball bearings that cause a loss in torque because the bearings exhibit different friction coefficients for a variable speed range.
For the present sensor system, the general quality of torque measurements is improved except in the low-rpm range. For better dynamic torque measurement in the present system, we isolated the torque-measuring module and shaft from outside vibration. The sensor body would move by a relative motion to maintain the distance between the magnets and the sensors.
However, we failed to obtain an ideal uniform value during testing. We observe that the cause of this failure is probably the ball-bearing gap on both sides of the modules.
Figure
Measured average twist angle at 200–500 rpm.
In a previous approach [
Figure
Measured twist angle of four pairs of sensors at 400 rpm.
After we calibrated the new torque meter, we compared the above theoretical data in (
In this study, a noncontact-type torque sensor was evaluated to examine in variable torque condition its performance through the test bench with a rotating shaft. The shaft torque value was obtained by using four pairs of permanent neodymium magnets attached to the surface of the shaft and a pair of magnetic Hall sensors. The measured magnetic field shows the intervals between peaks. In addition, the frequency detected by the sensor helps to calculate rotational speed, which is the most important variable when converting from the time gap to a twisted angle.
The repeatability of the measured initial-angle difference at each rotational speed between magnets is less than 2%. This does not appear to satisfy the needs of the automotive industry. However, we expect that the result will improve and become more accurate when we employ a higher-grade hardware system.
To guarantee the performance of the torque sensor, a calibration step or phase correction is necessary. Sufficient data regarding constraint, material properties, and geometry are required, in order to apply them to a successful torque sensor by assuming a linear deformation in the rotational direction.
Specifications for the commercial vehicle industry are needed, and future tests should include these important key points. To eliminate vibration effects, the sensor should complement any effect by the fixed part of the axis of rotation. Measurements should be possible even at high rotational speeds. To increase the effectiveness of the hardware configuration and the algorithm for high-speed processing of data, high-speed data acquisition must be possible. Measurements over a low-torque region need to be improved with regard to linearity, in order to obtain accurate data after modifications to the test bench and realignment shaft and after upgrading the motor or bearings. A molding process is needed to protect the Hall sensor from oil and dust in actual conditions and to reduce the size of the entire module. To ensure that operation is free from operating temperature, we should employ the calibration and repeatability test process because the elastic modulus is temperature dependent.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by Inha University.