Design of Virtual Instrument Test System for Motor Performance Using LabVIEW and TCP/IP Protocol

2.5 Control cabinet

The control cabinet is mainly composed of control switches, switching power supplies, filters and connecting lines. It provides corresponding multi-channel interfaces for each sensor module to connect it to the motor to be tested, and provides auxiliary functions such as safe system power supply, excitation injection, signal isolation, amplitude adjustment and air cooling control, providing strong power support and system emergency measures for the entire motor test system.

3 Software Structure and Algorithm

3.1 Software Structure

The motor performance virtual instrument test system generally adopts a client/server (CS) structure based on TCP/IP protocol. The server architecture is a cFP distributed I/O system, which uses its embedded independent real-time system to realize the signal sampling of the target parameters and complete the real-time monitoring and control of the target parameters; the client adopts a general PC structure, uses the TCP/IP protocol to realize the communication of control parameters and detection data with the server, and provides a GUI graphical user interface to realize human-computer interaction, complete the input of control parameters and the analysis, calculation and chart display of detection data.

The software structure diagram is shown in Figure 3. The system operation process is as follows: after power-on, the server automatically starts the built-in LabVIEW RT real-time program in the memory and listens to the client's "start test" command in real time; the client starts the motor performance virtual instrument test main program, completes user login, hardware configuration, selects test items, sets test parameters, and starts the test program; after the server hears the client's "start test" command, it starts real-time control and data acquisition according to the hardware configuration, test items and test parameters set by the customer, and sends the experimental data to the client through the TCP/IP protocol; the client issues a PID control command and analyzes and processes the experimental data sent by the server. After completing the PID control, it tests according to the test items, analyzes and processes the test data, and displays the experimental results in charts; after the test is completed, the client issues a command to end the test, and the test ends after the server receives and confirms it.

3.2 PID Control Algorithm

This system tests three PID control algorithms: position, incremental and integral separation.

3.2.1 Position control algorithm

The position PID control algorithm is described as:

Among them, k=0, 1, 2... is the sampling number; u(k) is the computer output value at the kth sampling moment; e(k) is the deviation value input at the kth sampling moment; KI=KpT/TI is set as the integral coefficient; KD=KpTD/T is the differential coefficient; Kp is the proportional coefficient; TI is the integral time constant; TD is the differential time constant; T is the sampling period.

The advantages of this algorithm are its simple principle and easy implementation; its disadvantages are that each output is related to the previous state, and e(k) needs to be accumulated, which requires a lot of computational workload. Moreover, the output u(k) corresponds to the actual position of the actuator. If the computer fails, a large change in u(k) will cause a large change in the position of the actuator.

3.2.2 Incremental Control Algorithm

The incremental PID control algorithm is described as:

△u(k)＝Kp△e(k)+KIe(k)+KD△e(k)-△e(k-1)］wherein, △e(k)＝e(k)-e(k-1).

The advantages of this algorithm are: due to the increment of computer output, the impact of malfunction is small; when the computer fails, the output channel or the actuator has a signal latching function, so it can still maintain the original value. The determination of the control increment △u (k) is only related to the sampling value of the most recent k times, and it is easy to obtain a better control effect through weighted processing. Its disadvantages are: large integral truncation effect, static error, and large overflow effect.

3.2.3 Integral separation control algorithm

The integral separation PID control algorithm is described as:

When |e(k)|》ε, that is, when the deviation value |e(k)| is relatively large, PD control can be used to avoid excessive overshoot and make the system respond faster.

When |e(k)|≤ε, that is, the deviation value |e(k)| is relatively small, PID control can ensure the control accuracy of the system.

Figure 4 shows the step response curves of the three PID control algorithms. After experimental comparison, the integral separation PID control algorithm shortens the transition process time from 19.5s for the position type and 16s for the incremental type to 12s; the maximum overshoot is reduced from 36% for the position type and 25% for the incremental type to 18%, with the characteristics of small overshoot, fast response speed, good stability, and strong ability to recover from interference.

4 Performance Evaluation

The motor performance virtual instrument test system realizes the load control of multiple parallel power tools and the real-time monitoring of torque, speed, power and temperature. It uses the TCP/IP protocol to realize the remote control of multiple parallel workstations by the main control computer and the network sharing of test data. The high-precision digital multimeter module DMM-4070 uses a four-wire system to measure the rotor winding inside the motor with a measurement accuracy of up to 6 digits. The power analyzer uses a high-precision power sensor module with a measurement accuracy of up to 0.3%.

The system has the advantages of high measurement accuracy, strong operation stability, and high parallel efficiency. It has been used in industrial sites. It is stable and reliable in actual use and is suitable for durability and comprehensive performance tests of various types of motors. Figure 5 shows the characteristic curve of a motor measured in the experiment, where the horizontal axis is torque. The speed curve, power curve, and current curve are also marked in the figure.