Analysis of Torque Sensor in Automotive EPS

Torque sensor is a high-precision sensor capable of measuring with very little uncertainty. However, in order to be able to obtain highly accurate measurements, it is important to reduce the parasitic loads affecting the sensor. It is often caused by incorrect installation or combination of the drive train. Radial deflection of the shaft end mounted on the measuring flange will generate radial forces and bending moments. Likewise, radial angular deflection will generate axial forces and bending moments on the sensor.

The coupling as a compensation component contains two features, one is have the function of a cardan shaft, and the other is can compensate for angular, radial and parallel errors. The compact form of the compensating part is fully coupled, two elements (semi-coupling) with cardan shaft function are mounted next to each other. Since the gap between each other is very small, the deviation of the compensation is also very small, and a larger compensation requires a larger gap.

Considering the need to install the combination of torque sensor + compensation components on the drive train, a variety of coupling and mounting component variants have been produced.

In practical applications, all offsets and parasitic loads will occur simultaneously. Accurate centering of the drive train is only part of the problem, as inherent offset tolerances cannot be completely eliminated. Therefore, it is necessary to install elastic compensation components with high torsional stiffness, which can almost completely compensate the residual deviation, especially in the case of high rotation speed. The use of compensation components can improve the accuracy of the sensor, and can protect the drive train bearings, with higher wear resistance.

Specific analysis of torque sensor in EPS is as follows:

EPS system is a development trend of future power steering system. EPS is mainly composed of torque sensor, vehicle speed sensor, electric motor, reduction mechanism and electronic control unit (ECU). The sensor detects the magnitude and direction of the torque or angle generated by the steering wheel during the steering operation of driver, and converts the required information into digital signals and inputs them to the control unit. After the command is issued to drive the motor to work, the output torque of the motor is assisted by the action of the transmission device. Therefore, the torque sensor is one of the important components in the EPS system.

Structure and principle of torsion bar torque sensor in EPS

The torsion bar type torque sensor is mainly composed of torsion bar spring, angle-displacement converter and potentiometer. The main function of the torsion bar spring is to detect the torque that the driver acts on the steering wheel and convert it into the corresponding angle of rotation. The rotation angle-displacement converter is a pair of helical mechanisms, which convert the relative rotation angle of the two ends of the torsion bar spring into the axial displacement of the sliding sleeve, which is composed of a rigid ball, a spiral groove and a slider. The slider can move in a helical direction relative to the input shaft, while the slider is mounted on the output shaft through a pin and can move in a vertical direction relative to the output shaft.

Therefore, when the input shaft rotates relative to the output shaft, the slider moves vertically according to the rotation direction of the input shaft and the amount of rotation relative to the output shaft. When the steering wheel is turned, the torque is transmitted to the torsion bar, and the direction of the input shaft relative to the output shaft is deviated. The deviation is the movement of the slider, the movement of these axis directions is converted into the lever rotation angle of the potentiometer, the movement of the sliding contact on the resistance line makes the resistance value of the potentiometer change accordingly, and the change of resistance is converted into voltage through the potentiometer . In this way, the torque signal is converted into a voltage signal.

Design of EPS torsion bar type torque sensor

The torsion bar is an important part of the entire torsion bar torque sensor, so the key to the design of the torsion bar torque sensor is the design of the torsion bar. The torsion bar is connected to the steering wheel shaft through a fine-toothed involute spline, and the other end is connected to the steering output shaft through a radial pin (diameter D). d0=(1.15~1.25)d, length L=(0.5~0.7)d. In order to avoid excessive stress concentration, when using excessive fillet, the radius R=(3~5)d, the effective length of the torsion bar is l, d is the diameter of the effective length of the torsion bar.

The torsional stiffness k of the torsion bar is an important physical quantity of the torsion bar, which can be calculated by referring to the following formula. When it is subjected to torque T, its torsional shear stress τ and deformation angle υ are respectively: its torsional stiffness is: where d-torsion bar diameter, effective length, Ip moment of inertia, and Zi The torsional section coefficient is the test curve of a torsion bar of the torque sensor, the slope of the curve is the torsional stiffness k.

Development trend of EPS torque sensor

With the continuous improvement and development of the EPS system, higher requirements have been placed on the accuracy, reliability and response speed of the torque sensor. EPS torque sensors are showing the following development trends:

  1. From static testing to dynamic online testing.
  2. The test system is developing towards miniaturization, digitization, intelligence, virtualization and networking.
  3. Development from single function to multi-function, including self-compensation, self-correction, self-adaptation, self-diagnosis, remote setting, state combination, information storage and memory.
  4. Development towards miniaturization and integration. The detection part of the sensor can be miniaturized through rational design and optimization of the structure, and the IC part can integrate as many semiconductor components and resistors as possible into a single IC component, reducing the number of external components.

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