Design of driving wheel speed test device based on magnetoelectric sensor

HOU Zhi-wei， CHEN Chang-xin， MA Tie-hua， WANG Yu， JIAO Bin

(1. Science and Technology on Electronic Test & Measurement Laboratory， North University of China， Taiyuan 030051， China；2. School of Electrical and Control Engineering， North University of China, Taiyuan 030051， China)

Abstract： In order to study the transmission efficiency of engine and optimize the structure of driving wheel, the rotational speed storage test device of driving wheel in tracked vehicle based on magnetoelectric sensor was designed. The device consisted of a mounting bracket, a sensor and a tester. The mounting bracket was installed in vehicle body after fixing the tester and sensor to mounting bracket beside the driving wheel. Using the storage test instrument, the wireless trigger technology was applied to synchronously record and stored the rotational speed data of the driving wheel in tracked vehicle. After the experiment was finished, the data was read out through the upper computer. Both valid data and satisfactory results were obtained through both simulated and actual vehicle tests.

Key words： tracked vehicle; storage test; driving wheel test; magnetoelectric sensor

0 Introduction

Compared with wheeled vehicles, tracked vehicles are more and more widely used in modern agriculture, industry and military fields due to their unique tracked structure, strong adhesion to ground contact surface and good environmental adaptability[1]. In a tracked vehicle transmission system, engine speed, torque and effective power are three important indicators for measuring engine performance. The driving wheel is the final link to provide energy for the transmission system and provide power output for the entire transmission system[2]. The effective power can be calculated by rotational speed and torque, so in order to improve the engine performance, study the transfer efficiency of engine, optimize the structure of driving wheel, improve the existing tracked vehicle and develop the next generation of tracked vehicle, the accurate measurement of driving wheel torque and rotational speed is particularly important[3-4]. In addition, the change of the speed can reflect whether the rotating machinery is running normally and smoothly. At the same time, the speed is also an important parameter of the rotating machine. Other parameters of the rotating machine can be obtained by the speed calculation, so it is necessary to measure the rotational speed data in the motion system[5]. In this paper, how to obtain the driving wheel speed data during the driving process of the tracked vehicle is proposed. The storage test instrument is adopted, and the key problems in the test process are studied. Finally, the design of the driving wheel speed test device is completed.

1 Hardware composition of test system

1.1 Overall structure of test system

In the actual test, the tester is always fixed on the tracked vehicle. Due to the complex road conditions of the tracked vehicle, the tester is in bad environment. In order to reduce the influence of electromagnetism, temperature, vibration and noise on the test instrument, improve the working stability of the test instrument and ensure the reliability of the speed data, mature storage test instrument is adopted in the speed test scheme of tracked vehicle driving wheel, so as to realize the local collection and storage of signals[6]. The structural block diagram of the driving wheel speed test system is shown in Fig.1.

Fig.1 Structural block diagram of driving wheel speed test system

1.2 Control module

The main control chip selects TI’s 16-bit ultra-low-power MSP430F149 MCU, which is responsible for receiving external trigger signals and controlling the power management module to achieve low-power design of the system. At the same time, it is responsible for processing the data collected by the sensor, converting the collected data into rotational speed data and storing it in the flash memory. After the experiment is completed, the speed data is read out by the host computer, and analyzed and processed by Matlab software.

1.3 Power management module

The power management module is responsible for powering the entire test system. A lithium battery with a rated voltage of 3.7 V and a capacity of 1 000 mAh is used as an external power supply to power the entire test system. The LP2985 regulator is used to regulate the voltage to 3.3 V, directly supplying power to the main control chip and the wireless module. At the same time, when the main control chip receives the trigger signal, it supplies power to the sensor and the flash memory by controlling the MAX894. When the trigger signal is not received, the MAX894 is off and the system is in low power mode. The schematic diagram of the power management module is shown in Fig.2.

Fig.2 Schematic diagram of power management module

1.4 Wireless trigger module

This device has two trigger modes: manual trigger and wireless trigger. The mode of wireless trigger is proposed considering that the driving wheel speed data can be acquired simultaneously and other test parameters can be acquired synchronously. The purpose is to ensure the speed measurement device of driving wheel keep synchronization with other test devices, such as the torque measurement device. In order to ensure the accuracy of the effective power value, the speed measurement of driving wheel must keep synchronization with the torque measurement[7]. Therefore, the technology combining storage test and ZigBee wireless communication is adopted to realize wireless configuration and synchronous trigger of each test unit of the driving wheel torque and speed by designing communication coding[8]. The system adopts ATZGB-S5 series wireless module with features of small size, simple interface and easy to use, which is embedded in the storage test system. The schematic diagram of the interface between the wireless module and main control chip is shown in Fig.3.

Fig.3 Schematic diagram of interface between wireless module and main control chip

1.5 Storage module

Under the control of main control chip MSP430F149, the storage module mainly records the collected rotational speed data by using the Samsung K9F1G08U0E memory chip.

2 Test method and error analysis

2.1 Test method

At present, there are two kinds of speed measurement methods, namely digital measurement frequency method and digital measurement period method[7].

In both kinds of measurement process, error of speed measurement is inevitable. When the measured speed is higher, that is, when the gear of the measured object rotates once, the number of speed signal is larger, the digital measurement frequency method has higher measurement accuracy and smaller error. Therefore, the digital measurement frequency method is more suitable for measuring the rotational speed of the measured object. Only when the measured speed is low (the time of the two adjacent speed pulse signals is large), the digital measurement period method has higher measurement accuracy, so when the measured speed of the object is low, it is recommended to use the digital measurement period method[7]. Since the driving wheel speed of the measured object is below 800 r/min and the rotational speed is relatively low, the digital measurement period method is used to measure the speed, and the obtained data error is smaller and the rotational speed data is more reliable.

In actual test, the driving wheel contains 13 gear teeth. At a certain moment, the driving wheel speed to be tested isn(r/min). The MSP430 uses the continuous counting mode to count the output pulse of the speed sensor. The test system clock frequencyfis 1 MHz. Assuming that the number of clock pulsesMrecorded by the counter during a speed measurement period is digital, the final rotational speed datancan be obtained by

(1)

2.2 Error analysis

1) When measuring the rotational speed using the dogital measurement periodic method, the error can be obtained by

(2)

It can be seen that the test accuracy is related to the clock pulse frequency. The higher the frequency of the clock pulse, the higher the accuracy. In the actual test, the device uses two 16-bit counters to form a 32-bit counter. Each time the driving wheel rotates one tooth, an instantaneous speed data is obtained and stored in the flash memory.

2) During the actual running of the vehicle, since the size of the 13 teeth of driving wheel and the distance between the teeth are the same, a complete rectangular wave of each output of the sensor corresponds to one of the driving wheels. In the actual test, the sensor scans one tooth and outputs one speed data. Each revolution can get 13 speed data. The more the number of teeth of the driving wheel, the shorter the time of turning each tooth under the same conditions, the more the rotational speed data output per revolution, the higher the accuracy of the instantaneous speed. When the vehicle is running at a constant speed of 20 km/h, the time per revolution of one tooth is 0.024 s, corresponding to outputting one speed data. When the vehicle is driving at a constant speed of 70 km/h at the time, the time per revolution of one tooth is 0.006 9 s, corresponding to outputting one rotational speed data.

3) The effective power can be obtained by multiplying the torque and rotational speed. In order to improve the accuracy of effective power, it is necessary to reduce the measurement error of torque and rotational speed. In the rotational speed measurement system, the test accuracy can be improved by increasing the clock frequency.

3 Design of test equipment

3.1 Speed sensor selection

In the field of rotational speed measurement, the following types of rotational speed sensors are commonly used: variable reluctance sensors, Hall speed sensors, photoelectric speed sensors and capacitive speed sensors[9-13]. The variable reluctance sensor has no output signal at zero speed, the signal amplitude during the rotation of the rotor is not fixed, and the measurement gap must be less than 2 mm. The Hall speed sensor requires that the measurement gap must be less than 2.5 mm, and the Hall speed sensor is very sensitive to external pressure[10]. The photoelectric speed sensor has high requirements for the normal operation of the light source, and the tracked vehicle generates a large amount of dust during driving, which blocks the light source of photoelectric speed sensor.

Each of the above sensors has its own advantages and disadvantages. In actual test, the test environment of continuous vibration during the running of the tracked vehicle, the installation distance of the sensor and driving wheel and the response characteristics of the sensor output signal are comprehensively considered. Through analysis and comparison of various speed sensors, the RP660CY-18B-PC driving magnetoelectric speed sensor with high sensitivity, large output square wave amplitude, difficult transmission interference and wide power supply range is selected. The driving magnetoelectric speed sensor has a response frequency range of 0-20 000 Hz, and the actual vehicle speed range is 20-70 km/h. The corresponding driving wheel speed range is 190-660 r/min, and the speed measurement frequency can be obtained. At 41-143 Hz, since the low-frequency characteristics of the magnetoelectric sensor are not good, the measuring cycle method is used for the low-speed measurement. In the actual test, we only need to detect the rising edge of the sensing output to get speed data that meets the test requirements. At the same time, in this special test environment, the magnetoelectric sensor has good anti-interference performance and less error compared to other types of sensors, which is more suitable for this test condition. The schematic diagram of the sensor installation structure is shown in Fig.4.

Fig.4 Schematic diagram of sensor installation structure

When the number of gear teeth is determined, the corresponding rectangular wave of the sensor output when the driving wheel rotates is shown in Fig.5. The relationship is

F=NZ,

(3)

whereNis the rotational speed, r/s;Zis the teeth number of driving wheel;Fis the output signal frequency of the magnetoelectric speed sensor, Hz.

Fig.5 Output signal waveform of sensor

3.2 Mounting bracket design

Due to the test requirements, the sensor should detect the rotation of the driving wheel teeth. The sensitive surface of the sensor must be opposite to the tooth of the driving wheel, and the measurement gap of the sensor is between 0.5 mm and 8.0 mm. In the actual operation, the sensitive surface of sensor is placed horizontally only 5 mm from the driving wheel teeth. Moreover, the tracked roads are complicated and undulated, so it is impossible to avoid bumps during driving. If the design of mounting bracket is unreasonable, the amplitude of mounting bracket during the running of tracked vehicle may be too large, the sensor collides with the driving wheel and may be damaged, finally the test cannot be completed. Therefore, it is necessary to design a reasonable mounting bracket to fix the sensor.

The tracked vehicle will vibrate during the driving process. The mounting bracket is fixed on the vehicle body by screws. In the bracket design process, in order to reduce the bracket vibration as much as possible and avoid the resonance between the mounting bracket and car body, the following two aspects of shock absorption design are carried out.

1) The mounting bracket vibration is reduced by vibration isolation. In order to achieve the purpose of vibration reduction and noise reduction, a subsystem is usually added between the vibration source and the controlled object to achieve vibration isolation, and the vibration of the controlled object is reduced. By reducing the response of the controlled object to the excitation of vibration source, the vibration energy transmission can be isolated or reduced[14]. Therefore, a rubber pad with long-lasting high elasticity, good vibration isolation and sound insulation performance is added to the joint between the mounting bracket and the body to achieve vibration isolation.

2) Through the scientific design of the mounting bracket structure, the simulation design is simulated continuously to avoid the resonance caused by natural frequency of the mounting bracket being the same or similar to the vibration frequency of the vehicle body.

The vibration phenomenon of the vehicle body during the running of tracked vehicle can be understood as that the driving wheel tooth and track end connector generate a periodic impact force during the meshing process, and the hub and transmission device periodically transmit the impact force to the vehicle body. Under certain working conditions, if the vehicle speed isV(km/h) at a certain time, the number of the driving teeth is fixed toZand the driving wheel meshing radius isR(mm), the frequencyf(Hz) of the impact force can be obtained by[15].

(4)

It can be known from Eq.(4) that when the driving wheel structure is fixed, the vehicle body is periodically vibrated at a certain vehicle speed, and the vibration frequency increases with the vehicle speed. Assume that the tracked vehicle runs at a speed of 90 km/h under a certain road condition, and the vibration frequency of the vehicle body brought into the upper type is 185 Hz. In order to prevent the resonance of the bracket, we use Solidworks software to model, add constraints according to the actual situation, perform vibration modal analysis on the mounting bracket, and continuously improve the structure of the optimized mounting bracket. Finally, the 10th-order natural frequency of the mounting bracket is shown in Table 1, and 6 vibration modes of the mounting bracket are shown in Fig.6.

By optimizing the design, the first-order natural frequency of the mounting bracket was increased from 27 Hz to 667 Hz, bypassing the vibration frequency of the vehicle body. From the modal simulation results, it is known that the natural vibration modes of the bracket at the sensor installation are mainly horizontal vibration and torsional vibration. Finally, after several real vehicle tests, the collision problem between the sensor and driving wheel dose not occur, which proves the rationality of the structural design.

Table 1 10th order natural frequency of mounting bracket

Fig.6 Six modes of mounting bracket

3.3 Tester installation

In the actual test, the tester should be fixed on the mounting bracket, and a test fixture tool with suitable size should be designed according to the test circuit. The test consists of upper and lower end covers and a middle part. The three parts are fixed by screws, and the lower end cover is fixed on the mounting bracket by screws. The upper and lower end covers are 2 mm thick and the middle part is 4 mm thick. They are made of steel. The inner surface of the tool is covered with a layer of rubber paper. The gap is sealed by latex. Finally, the test circuit is potted with paraffin to play the anti-vibration buffer. The physical diagram of the tester is shown in Fig.7.

Finally, the tester and the sensor are screwed to the designed mounting bracket. The test bracket is screwed to the body above the driving wheel. The physical diagram of the final driving wheel speed test device is shown in Fig.8.

Fig.7 Physical diagram of tester

Fig.8 Physical diagram of driving wheel speed test device

4 Experiments and results

After the experiment is finished, the speed data in the flash memory is read out by the host computer software, saved with .dat format, and finally the .dat data is processed and displayed in Matlab software. The measured speed curves of the left and right driving wheels are shown in Figs.9 and 10. It can be seen that the test device records the rotational speed data of the driving wheels on both sides from the acceleration start, the constant speed running to the final deceleration stop within 2 min. The waveform diagram shows that the speed curves of the left and right driving wheels are basically the same. When the vehicle is running, the mounting bracket will vibrate slightly, resulting in a spike in the figure. The constant speed is 220 r/min, which can be calculated according to the diameter of driving wheel. The corresponding speed to the crawler car is 23 km/h, which is the same as the actual driving speed. After comparing with the working condition, the speed curve is basically consistent with the working condition.

Fig.9 Speed curve of left driving wheel

Fig.10 Speed curve of right driving wheel

5 Conclusions

1) This paper presents a driving wheel speed storage test scheme to optimize the structural design of a mounting bracket, which solves the problems that the amplitude of the mounting bracket is too large during the driving process of the tracked vehicle and the sensor collides with the driving.

2) The driving wheel speed storage test device can record the rotational speed of driving wheel at any time in a certain period of time. According to the actual vehicle test, the device can obtain effective driving wheel speed data in various complicated road conditions, such as mountain roads and gravel roads.

3) In addition, the device can be combined with other measurement data of the driving wheel at the same time, such as torque information, to obtain more measurement information, and provide a basis for the optimization of the transmission system and the improvement of the driving wheel structure.