Eddy Current Dynamometer Based on Torque Sensor and Ethernet


Dynamometers are mostly used to test the power of engines. They are mainly divided into hydraulic dynamometers, eddy current dynamometers, and electric dynamometers. Because the control performance and response speed of the eddy current dynamometer are obviously better than that of the hydraulic dynamometer, and its price is much lower than that of the electric dynamometer. Therefore, this scheme is designed with an eddy current dynamometer. It uses the motor to measure the mechanical shaft of the engine The output torque is combined with the rotational speed to determine the engine power. At present, most of the dynamometers sold in the domestic market only measure the power of the motor, while the dynamometers that measure the power of automobile engines or aircraft turboprop engines are not very common. Basically, only professional engine manufacturers will have such equipment. At the same time, due to the large size of the existing dynamometer devices, remote monitoring cannot be performed. During the power test, the mechanical part and the control part are placed on the same equipment, and the noise generated during the operation of the equipment is very large, which will directly affect the test results. Moreover, every time the ignition angle is changed, the running equipment must be stopped before the change can be made, which greatly reduces the operability of the equipment. At the same time, the control algorithm of the equipment is complex, the parameter setting method is single, and the flexibility is low. According to the needs of the project, in order to obtain high-precision aircraft engine power parameters, the technical improvement is made on the basis of the original eddy current dynamometer, and the remote monitoring is realized by using Ethernet, which not only maintains many advantages of the existing eddy current dynamometer At the same time, it also overcomes the above-mentioned shortcomings of the dynamometer, and solves the problems of the existing dynamometer with low precision, low operability, poor efficiency and flexibility.

1 How The System Works

The eddy current dynamometer device is mainly composed of two parts: the lower computer and the upper computer.

The lower computer is an embedded device for data acquisition and information processing; the upper computer is a PC client software, which uses the controls to send and receive the relevant instruction information of the lower computer. During the working process, the upper computer of the system uses Ethernet to receive the engine speed, torque and temperature detected by the lower computer, while the user mainly sends commands through the upper computer to control the ignition and shutdown operations of the engine, throttle opening adjustment, excitation voltage control and other functions, set the engine ignition angle, etc.

2 System Hardware Design

The device is mainly composed of power supply module, ignition angle control, Ethernet remote monitoring, engine start control, temperature measurement, throttle control, torque and speed measurement, excitation motor drive and control modules, etc. Among them, the ignition angle control module and the Ethernet remote control module are the characteristic modules of this device. The hardware principle of each module is as follows:

2.1 Ethernet remote monitoring module

The engine is very noisy when it is running, and it is not only unfavorable to the health of the staff in this environment for a long time, but also has a relatively large impact on the test results. In order to prevent the operator from being in a bad environment for a long time, the device adopts Ethernet in the design, and the user can directly control the engine remotely through the Ethernet. The LM3S9B92 main control chip comes with an Ethernet controller, which follows the IEEE 802.3 specification and fully supports the 10BASE-T and 100BASE-TX standards. Therefore, the dynamometer can be remotely monitored by using Ethernet and the host computer.

2.2 Torque/speed measurement module

Engine power is calculated by measuring its torque and speed. This device adopts the existing high-precision torque sensor on the market and is directly connected to the main control board, as shown in Figure 1. The torque sensor transmits the received torque and speed signals through the signal processing circuit to the I/O port of the main control chip, and then the main control chip transmits these data to the host computer through the Ethernet, and the host computer calculates the power through these data.

Torque and rotational speed measurement module

3 System Lower Computer Software Design

The software structure of the lower computer of this device is composed of three parts: data acquisition and transmission, receiving control commands from the upper computer, and automatic control.

Because this design uses Ethernet as the data transmission medium, the realization of TCP/IP protocol stack needs to be considered in the software design. Here, the LWIP protocol stack developed by the Swiss Academy of Computer Science is used as the software basis for Ethernet implementation.

Since the torque and rotational speed information collected from the rotary torque sensor are represented by frequency, the pulse capture pin of LM3S9B92 is used to capture frequency 8 during data collection. The specific method is to record the current time when a rising edge is captured, record the second time when the second rising edge is captured, and calculate the time interval between the two rising edges to obtain the frequency of the signal. Thereby obtaining torque and speed information.

If the frequency and temperature detection subroutines are continuously called in the main program, it will inevitably increase the system burden and affect the execution efficiency of other programs, and this high-speed information refresh is meaningless to the user. Therefore, in order to take into account the system efficiency and the real-time information, it is better to detect the information 5 times per second after actual commissioning. By setting a timer in the system, triggering an interrupt every 200ms, and setting the flag bit in the interrupt service routine. After the main program detects the flag bit, it calls a frequency and temperature detection subroutine. Thus, the trade-off between efficiency and real-time performance can be guaranteed.

The regulation of the excitation voltage adopts the double-loop regulation method of open-loop and closed-loop. By quantifying the influence of the rotation angle of the stepper motor on the voltage, the initial rotation angle of a stepper motor can be obtained according to the voltage value set by the user. Then according to the actual voltage detected by AD, it is filtered by software and compared with the user's voltage, and the rotation of the stepper motor is adjusted accordingly, so as to realize the high-speed and precise adjustment of the excitation voltage.

4 System Host Computer Software Design

The host computer of this device adopts LabView to edit and consists of two parts: virtual instrument panel and user control interface. Among them, the communication with the lower computer adopts the UDP protocol, and transmits the pre-agreed control commands and corresponding data. In the virtual instrument panel part, the lower computer sends the data sampled by each torque sensor to the PC and C terminal. The upper computer reads the data through the preset network port, performs string processing and comparison respectively, and takes out the valid data and convert to a graphical interface for display. The user control interface is designed with the built-in graphic control of LabView. After the input parameters are set by the user, it is converted into a specific string and sent to the network sending port to the lower computer.

5 Analysis of Test Results

In order to test the correctness of the basic functions of the dynamometer, a self-made two-stroke engine was tested by selecting the torque/speed meter as a control. The test results are shown in Table 1. By comparing the test results, it can be seen that the measurement results of this dynamometer and the dynamometers that have been used in the market are basically the same. At the same time, a series of experiments such as high temperature resistance and electromagnetic compatibility were carried out respectively, and the test results and the test results of the torque/speed meter also maintained a high degree of consistency. Thus, the reliability and practicability of the device in practical application are guaranteed.

ItemsOur eddy current dynamometerStandard torque/speed meter
Speed (r/min)55005500
Torque (Nm)51.04951.037
Power (kW)29.4029.39
Table 1. Comparison of test results

6 Conclusion

The device uses the LM3S9B92 chip of the ARM Cortex-M3 core as the controller core of the system, completes the control of the engine and the excitation motor and the collection of related information, and realizes the remote power test control of the engine. Through physical construction and laboratory debugging, 5 prototypes were made for actual comparison test. The test results showed that the new eddy current dynamometer has accurate test results and good control effect, realizes remote data transmission and control, and meets the expected industrial design requirements.

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