Design of intelligent instruments to support laboratory temperature and humidity measurement

This article describes an intelligent instrument that supports laboratory temperature and humidity measurement, display and remote transmission. This instrument is different from many similar products on the market. It can well meet the requirements of high precision, high resolution and network transmission on site.

1. Hardware design of temperature and humidity display instrument

The hardware circuit part of this instrument mainly includes independent power supply circuits for temperature and humidity channels, temperature and humidity sampling circuits, signal conditioning circuits, analog-to-digital conversion circuits, and photoelectric isolation circuits. The processor uses AT89S52 single-chip microcomputer, as well as LED display circuits and communication Circuit, keyboard circuit, etc., its structure diagram is shown in Figure 1. The following will introduce the circuit of each part accordingly.

1.1 Temperature and humidity transmitter

The non-electricity temperature and humidity signal forms a 4~20 mA electric power signal through the transmitter. This solution uses the EE10-FT6 high-performance temperature and humidity integrated transmitter from Austrian E+E Company.

EE10-FT6 product features:

(1) 24 V DC power supply;

(2) Temperature range: O～50℃; Humidity range: 0～100%;

(3) Temperature accuracy: ±0.1℃; Humidity accuracy: 1.0%;

(4) Input impedance: ≤500 Ω;

(5) Wiring method: three wires (positive terminal of power supply, temperature output line, humidity output line).

1.2 Isolation measurement technology

For conventional temperature and humidity measurements, the temperature and humidity signals are often grounded together and a set of measurement circuits are used. In this solution, the output of the transmitter must first be connected to the temperature and humidity

The degree display (front-end) is connected in series to the back-end of the DDC (Direct Digital Controller) to form the transmitter loop. The transmitter power is provided by the DDC, so the two common ground signals output by the transmitter need to be Make isolated measurements to eliminate effects on back-end measurements. That is, the two ends are connected in series and a pair of common ground signals are measured simultaneously. For analog isolation measurement, the author once considered 3 sets of solutions:

The first option is "virtual" isolation, which uses differential measurement technology and connects one end of the differential to the ground through an appropriate resistor. This option has the lowest cost, but it has very strict requirements for the op amp and is not very stable.

Option 2 uses linear optocoupler isolation, but linear optocoupler is difficult to debug, has high discreteness, requires many peripheral components (operational amplifier), and the front end of the optocoupler still needs to isolate the power supply, and the back end still needs an analog-to-digital converter.

Solution three uses two sets of independent measurement circuits, and then interfaces with the processor through ordinary optocouplers. It requires two sets of isolated power supplies and two sets of measurement parts. The cost is slightly higher, but this solution is the most reliable and easy to debug, so it was eventually adopted, such as As shown in Figure 2.

l.3 4～20 mA current measurement

The sampling resistor uses a 125 Ω/0.1% precision resistor, so that only the O.5~2.5 V voltage signal needs to be measured.

1.4 Amplifier and analog-to-digital converter applications

The analog signal conversion of this temperature and humidity display uses TI's rail-to-rail operational amplifier TLC2252. The biggest feature of this amplifier is that the output can reach the full power supply, and the offset voltage is small, 0.5 mV. The price is low, and it is suitable for this instrument. application.