A brief analysis of the design of temperature and humidity monitoring system based on virtual instrument

1 Introduction

Virtual instrument, abbreviated as VI, is a PC instrument with a virtual panel, which consists of hardware devices and interfaces, PC, device driver software and virtual instrument panel. Among them, hardware devices and interfaces can be various built-in function cards based on PC, such as DAQ, general purpose interface bus GPIB interface card, serial port and VXI bus instrument interface, etc., or other programmable external test equipment. Device driver software is a driver that directly controls various controllable hardware interfaces. Virtual instruments communicate directly with real instrument systems through low-level device driver software, and display various controls corresponding to real instrument operation elements on the computer screen in the form of virtual instrument panels. Virtual instrument panel applications are responsible for data analysis, image display, data storage, data printing and other operations on the collected data. The buttons on the panel correspond to various functions and are accompanied by dynamic help. This not only fully realizes single-key operation and OSD, but also makes it more convenient for users to operate virtual instrument surfaces than real instruments.

2 Design Principles

A museum or library of a certain size is generally composed of several collection rooms. This design adopts the principle of allocating time slices to each task in the time-sharing system of the computer to realize the detection and control of the temperature and humidity of multiple collection rooms with one computer and one data card. The analog multi-way switch in the data acquisition card is used to switch between various hardware data channels to achieve the purpose of sampling and controlling each channel in time slices. This method is suitable for the control of parameters that are not very sensitive to time changes, such as temperature and humidity. This can reduce the complexity of the monitoring system to a certain extent.

The structure of the collection room temperature and humidity monitoring system is shown in Figure 1. The controlled parameters such as temperature and humidity are detected by sensors and sent to the computer via the data acquisition card. The computer processes and displays the data according to a certain control algorithm. The processed output control signal is then sent to the actuator air conditioner and dehumidifier via the data acquisition card to achieve the purpose of controlling the room temperature and humidity of the collection.

3 Hardware Design

The hardware of the system mainly consists of PC, data acquisition card, signal conditioning circuit, temperature and humidity sensor and actuator air conditioner and dehumidifier.

3.1 Data acquisition card

The accuracy of data acquisition will directly affect the accuracy of the entire virtual instrument measurement and control system. Therefore, in the selection of data acquisition cards, the two basic indicators of sampling accuracy and sampling speed should be considered first. Taking into account the characteristics of the parameters to be determined in this system, as well as the reliability, accuracy and cost-effectiveness of the data acquisition card, the PCI bus/100Ksps/12-bit 32-channel AD/2-channel DA board/8-way input and output board AD7202 is selected.

3.2 Signal conditioning circuit

Signal conditioning circuits generally include circuits for small signal amplification, filtering, zero point correction, linearization, temperature compensation, error correction, and range switching. In this system, a preamplifier circuit is added to amplify the small signal input by the sensor and a low-pass filter is added to filter out high-frequency noise.

3.3 Sensors

Selection of temperature sensor components: The AD590L temperature sensor is selected, with a temperature measurement range of -55℃～125℃, and a measurement accuracy of ±0.5℃ in the range of -10℃～85℃. This sensor can meet the working requirements of the collection room temperature and humidity measurement and control system. Selection of humidity sensor: The integrated humidity sensor IH3605 produced by HONEYWELL is used, with a measurement humidity range of 0～100RH, an inherent accuracy of ±2 RH, and an operating temperature range of -40℃～85℃.

4 Software Design

The system is developed based on virtual instrument technology, with the graphical programming language LabView 8.2 as the development tool. The computer's own PCI bus is used, and sensors, data acquisition cards, and PCs are used as the basis to form a plug-in virtual system for the instrument hardware platform. The system uses sensors to measure the temperature and humidity of the collection room, and at the same time performs analog-to-digital conversion, result display, control algorithm processing, data query, and control signal output on the collected data. The main functional modules and structural block diagram of this system are shown in Figure 2. [page]

4.1 Parameter setting module

Since the system has a lot of hardware resources, the digital channels on the data acquisition card should be initialized before signal acquisition. There are two acquisition modes for AD cards: single buffer mode and multi-buffer mode. The AD card in this detection system uses a multi-buffer mode. The collection room temperature and humidity measurement and control system needs to set parameters such as the number of channels for collecting signals, sampling frequency, total number of buffers, and number of sampling points for each buffer. By comprehensively setting these parameters, the appropriate signal acquisition time and total number of sampling points can be obtained. The following relationship is satisfied between them:

Total number of points collected = frequency × collection time

Total number of points collected = number of points collected in each buffer × n buffers

4.2 Data Acquisition Module

The data acquisition module mainly includes signal acquisition and analog-to-digital conversion. Since the A/D data acquisition cards used can be directly applied to the unified interface ADCard driver in the .dll format in the LabView software platform, this avoids the use of other languages ​​to develop drivers, and only requires the correct call of the relevant functions under ADCard. The data transmission principle used in data acquisition is: under the control of the driver, the data acquisition card first stores the collected external signals in the buffer on the acquisition card, and the driver then puts the data in the acquisition card buffer into the driver's buffer, and then transmits the data to the data memory of the upper application software through the dynamic link library interface function.

In the background program of LabView, the dynamic link library method provided by LabView is used to implement the input of data acquisition. The entire acquisition process is divided into several steps: calling the DllInitial function to initialize the acquisition function, calling the DllStartIntr function to start the A/D card to start data acquisition, calling the DllADResult function to read the data acquisition results, and calling the DllStopIntr function to stop data acquisition.

4.3 Data Processing Module

The temperature and humidity in the collection room are not only affected by changes in the outdoor environment, but also by the random storage and retrieval of cultural relics, books and documents. This makes it difficult to control the temperature and humidity and is difficult to express using a mathematical model.

This system uses LabView virtual instrument software to implement the fuzzy control method in the intelligent control principle. The basic idea is: when the collected data is within the threshold range of the set value, the system does not adjust. When the collected data deviates from the threshold range of the given value, the newly collected data is compared with the last collected data. If its change tends to the set value, the system still does not adjust. If its change deviates from the set value, the system adjusts. Due to the large time lag of the controlled parameters, this adjustment mechanism can not only avoid system oscillation caused by frequent actuator actions while ensuring control accuracy, but also extend the service life of the actuator.

To prevent unexpected situations such as actuator failure, the system is also designed with upper and lower limit alarm functions for temperature and humidity. Sound and light alarms can be realized through the front panel and block diagram program of the virtual instrument to remind staff to deal with unexpected situations in time.

4.4 Data Display Module

In the data display module, the collected data and alarm signals can be displayed in two ways: curves and values, so as to facilitate human-computer interaction. In addition, a data save function is set to save the collected data for later use. The display of graphics and data can be seen from the virtual instrument panel, as shown in Figure 3. The implementation of data save is reflected in the background block diagram program, and the function - file I/O - write to spreadsheet file node is selected to complete the data storage function.

4.5 Signal output module

After the collected data is processed by the control algorithm, a corresponding control signal will be generated. The system uses two methods for output. If the temperature and humidity of the library collection room are monitored, the corresponding control requirements are relatively low, and switch quantity can be used for control, so the output is a switch quantity signal. If the temperature and humidity of a museum with precious cultural relics are monitored, the corresponding control requirements are relatively high, so analog quantity is used for control. At this time, the output is an analog voltage signal, which achieves the purpose of real-time control by adjusting the speed of the air conditioner or dehumidifier.