A practical overload test system design

Overview

With the rapid development of the economy, modern transportation tools such as cars are increasingly entering many families, and the use of manned tools such as elevators is becoming more and more widespread. The resulting safety hazards cannot but attract the attention of the society. The size of the overload is an important indicator for measuring the comprehensive performance of the motion system and is increasingly being put on the agenda, because the size of the overload is directly related to personal safety and cannot be ignored. This article introduces a portable overload test system that uses lithium batteries as a power source. When an overload occurs, it can collect and store the overload in three vertical directions of space. After the data is collected, it can be sent to a microcomputer for processing, or the results can be directly printed.

System Hardware

The test system is mainly composed of an overload data acquisition system and communication software. The data acquisition system collects data through acceleration sensors in three directions and then stores it in an external data storage device. The communication software reads and processes data through the serial port. The data acquisition system uses an 8751 microcontroller as the main control chip to achieve data acquisition and storage. The composition of the entire system is shown in Figure 1 .

Signal amplifier circuit

The signal of this system comes from the voltage signal output by three acceleration sensors distributed in the three-dimensional direction of space. The sensor uses a 3031 type acceleration sensor with a standard range of ± 50g and a specification of 0.994mv/g . Since the sensor contains a temperature compensation circuit, there is no need to consider the influence of temperature on the measurement signal. Since the acceleration sensor responds to the overload impact, the output of the overload value is small, so a suitable amplifier circuit must be used to amplify the signal. The amplifier circuit must ensure a certain degree of symmetry, reduce the drift, and have high input impedance and high common film rejection ratio to be sensitive to tiny differential mode voltage signals. In addition, since the output characteristics of the sensor are nonlinear, a segmented amplification method must be used to improve the test accuracy. In order to meet the above requirements, we used the signal amplifier circuit shown in Figure 2 .

Using regression analysis, three broken line segments are used to approximate the output curve of the acceleration sensor. In Figure 2, the basic broken line is determined by the linear amplifier A3 , and the other two broken lines are determined by A2 and A1 respectively. When the voltage input is ei , the linear amplification outputs of amplifiers A1 , A2 and A3 are eo1 , eo2 and eo3 respectively , and the total synthetic characteristic curve is: eo=eo1+eo2+eo3 . The use of piecewise linear amplification can greatly reduce the error caused by the nonlinear signal output of the sensor.

Power supply circuit design

Since this test device needs to be easy to carry, the power supply must be able to provide power for a certain period of time and must be small in size, a lithium battery is selected as the power supply, and a matching boost circuit is designed. The specifications of the lithium battery are: 1000mAh 3.6V , and the voltage required by the system is +5V . The battery boost circuit used in this system is shown in Figure 3 .

Since lithium batteries have a high storage capacity, after charging, the boost circuit can provide the entire system with a +5V DC voltage output for up to 20 hours , meeting the requirements of long-term uninterrupted measurement.

Data storage circuit

Due to the large amount of data collected, it is necessary to consider expanding the external data storage. The test system adopts the method of expanding dynamic RAM . Dynamic RAM has the advantages of low cost and low power consumption, and is particularly suitable for occasions that require a large amount of data storage space. Two 62128 chips were selected in the circuit design, and the block diagram of the entire single-chip control system is shown in Figure 4 .

System software design

Lower computer software

In order to facilitate the collection of data to the PC for processing, the system uses RS-232 serial communication to communicate with the computer's RS-232 standard serial port. The RS-232 serial communication interface has the characteristics of high efficiency, reliability, simplicity and low cost, and is very suitable for use in various test devices. When using the serial I/O port as a communication means, the TTL level must first be converted to the RS-232 level. The communication cable only needs to provide three signals: RXD , TXD , and GND .

During the test, the overload data in the three directions of X , Y and Z are collected 4 times respectively, and the data is sent to the PC through the serial I/O after arithmetic average filtering . The system software flow is shown in Figure 5 .

The data filtering process is realized by calling the average value filtering method subroutine. The average value filtering uses the method of discarding the maximum and minimum values ​​to perform average filtering on the measured values.

Host computer software

The communication program of the PC was designed and debugged in Visual Basic 6.0 .

There are two ways to implement serial communication using VB6.0 : (1) using the serial communication control (Mscomm32.ocx) ; (2) using the Windows application programming interface (API) . The MSCOMM.VBX user communication control used in VB enables the user program to send and receive data through the serial port. It not only includes the functions completed by the 16 functions of serial communication in the Windows API , but also develops more object properties, sets standard event functions and procedures for the serial communication control, and provides serial communication parameter settings through the property method. The API provides a complete application interface function and an interrupt-mode communication device driver (COMM.DRV) . Both methods can access the standard interface, but using the Mscomm32.ocx control is simpler, and this method is adopted in the upper computer software design.

When the host computer software initializes the serial communication port, it must ensure that the baud rate is consistent with the test system. When the baud rate of the test system is set to 9600 bit/s , the initialization subroutine of the host computer software is designed as follows:

Private Sub Command1_Click()

With MSComm1

COM1

Information parameters

MSComm1 receive buffer

The area is 40 bytes

MSComm1 send buffer

The area is 2 bytes

.InputMode = comInputModeBinary The data mode is binary

Buffer

.OutBufferCount = 0 Increase the number of packets and send them.

Buffer

End With

End Sub

The data acquisition processing program adopts the MSComm event-driven method. Since only the RXD , TXD and GND of the serial port are connected , it can only be triggered when data is input. The data receiving subroutine is as follows:

Private Sub MSComm1_ OnComm() receives and processes data as soon as it is received .

Dim currentstatus as string

Define the receiving byte array

Receivedata=MSComm1.Input

District Head Address

End Sub

Conclusion

Compared with general test systems, this test system requires a higher sampling frequency and a larger amount of data because it is an overload instantaneous test, so the system must be stable and reliable. In order to meet the requirements of portability, lithium batteries are used as power supply, making the entire device small in size and light in weight. The convenient RS-232 serial communication method facilitates the design of data transmission software. After actual measurement and verification, the overload test device has achieved the expected indicators and is widely praised by users.