Design of temperature controller based on FPGA

The design of a temperature control system based on FPGA is introduced. The appearance of programmable device FPGA and hardware description language VHDL has shortened the design cycle and difficulty of digital circuits. The system uses FPGA as the core control device for programming. The design adopts a modular idea to realize temperature detection, key input, temperature display and control module, and then integrate it to realize the entire system, achieving the purpose of temperature control.

With the continuous improvement of people's living standards, temperature control systems are widely used. For the temperature controller, it can be realized through relays, PLCs, single-chip microcomputers and special industrial control machines. The relays have zero-voltage conduction, zero-current shutdown, low noise, corrosion resistance, anti-interference, long life, small size, and small Control signals directly drive large current loads and other advantages. However, there are also disadvantages such as on-state voltage drop, heat dissipation measures, output leakage current, AC and DC cannot be used universally, few contact groups, and high cost. PLC is mainly used in the field of power industry control, or the control of the entire pipeline; the power consumption of the microcontroller is large, but the performance is relatively stable; the microcontroller is suitable for real-time industrial control, the price is relatively low, and the programmability and scalability are relatively strong; The dedicated industrial computer is equipped with a small hard disk capacity, low data security and small storage selectivity.

Due to the limitations of volume, cost, operation and other factors, the functional modification and debugging of these control methods require the support of hardware circuits, which increases the difficulty of functional modification and system debugging to a certain extent.

The development of electronic technology and the emergence of FPGA (Field Programmable GateArray) programmable technology have brought a new "technical revolution" to the field of modern industrial control measurement and control. It has millions of logic gates and can be used to implement complex functions.

As the main control part of the temperature controller, FPGA can reduce the number of components of the system, and has the characteristics of flexible design, field programmable, simple debugging and small size, and has a higher cost performance.

1 Principle of temperature collector

There are many ways to achieve temperature acquisition, for example, the traditional two-bit analog control method is used to select an analog circuit, set a fixed value with a potentiometer, and compare the feedback temperature value with the given temperature value after sampling and amplifying the signal. The comparison result controls the current temperature display to determine whether to heat or not. Its characteristic is that the circuit is simple and easy to realize. However, the accuracy of the results obtained by the system is not high, the adjustment actions are frequent, and the system is unstable. Under the control of the loop system and the display of temperature, the man-machine exchange performance is poor. Moreover, an analog circuit is used to compare the feedback temperature value with the given temperature value to determine whether to heat. There are many factors that produce various errors from input sampling to output control, and the accuracy is difficult to meet the requirements. Another method is to use EDA to intelligently control the system, use the FPGA chip as the core controller, use the temperature sensor ADS90 to collect temperature change signals, and process the temperature through the FPGA chip to control the temperature to stabilize it. This method has the advantages of flexible programming and simple control, so that the system can easily achieve temperature control and display. In this paper, FPGA is used to achieve temperature acquisition. Its structure is shown in Figure 1. FPGA has a fast running speed, an internal phase-locked loop and can double the external crystal oscillator to a higher frequency. It has many pins and is easy to implement a large-scale system. FPGA internal programs can run in parallel and have the ability to handle more complex functions.

The temperature sensor module uses the temperature sensor ADS90. ADS90 has the advantages of small size, light weight, good linearity and stable performance. The measurement range is -50 ~ +150 ℃, the full-scale range error is ± 0.01 ℃, its performance can meet the design requirements of this system. In addition, ADS90 is a temperature/current sensor, which is of great help to improve the anti-interference ability of the system.

The heating control part adopts the relay as the control, and the use of the relay is easy to realize the control of the circuit, and the work is reliable under normal conditions. The relay can be electrically isolated by itself without the need for an external coupling. This type of circuit cannot accurately realize the heating wire power control. The heating wire can only work at maximum power or zero power, which will affect the control accuracy. However, it is possible to control the power of multiple heating wires, and the FPGA chip handles the temperature difference to achieve hierarchical power control to improve the dynamic performance of the system.

Two-digit LED seven-segment digital tubes are used to display ten and one digits respectively. The digital tube has the advantages of low energy consumption, low loss, low voltage, long life, aging resistance, and low requirements for the external environment. In addition, the digital tube uses BED coding to display numbers. Programming is easy, and resource consumption is low.

2 System hardware design

The functional modules included in the hardware system are FPGA chip, temperature sensor, power supply, ADC0809, seven-segment digital tube, relay and heater.

2.1 Temperature sensor

The temperature sensor is divided into thermal resistance temperature sensor, thermocouple temperature sensor, radiation thermometer and optical fiber temperature sensor. The thermocouple temperature sensor ADS90 is used in the design. Its single function, small temperature measurement error, low price, fast response speed, long transmission distance, small size, and micro power consumption are suitable for long-distance temperature measurement and temperature control. Linear calibration.

The sensing capability of the AD590 temperature sensor is to increase the current by 1 μA every time the temperature rises by 1 K. When this current flows into a 10 kΩ resistor, a voltage of 0.01 K (0.01 V) will be generated. At 0°C (273 K), when the voltage on the scale is 273 μA, after I\V conversion, a voltage of 2.73 V will be generated. If the measured voltage is Xvs, the temperature to be measured can be obtained from (X-273)\0.01. After the output of the temperature sensor ADS90 passes through the amplifier, the voltage is introduced into the pin of Vin(+) of ADC0809.

2.2 A/D converter

ADC0809 is a CMOS 8-bit analog-to-digital converter. It adopts the principle of successive approximation for A/D conversion. There are two parts of analog multiplex switch and A/D conversion in the chip, which can simulate 8 0~5 V input. The voltage signal is converted in time-sharing. The specific control status is as follows:

When CS and WR are at the same time high level, ADC0809 starts conversion, when the conversion is completed, output high level at INT pin, waiting to read data;

When CS and RD are high level at the same time, read data from ADC0809 through the data bus D[7.0].

When CS=1, WR=1, RD=0, the signal sent by the controller requires ADC0809 to start the analog/digital signal conversion.

When CS=0, WR=0, RD=0, ADC0809 performs the conversion action, and INT raises the low potential to the high potential after the conversion is completed.

When CS=1, WR=0, RD=1, the controller sends out a signal to read the conversion data of ADC0809.

When CS=0, WR=0, RD=0, the controller reads the digital conversion data on the data bus.

2.3 Relay

A relay is an electronic control device that has a control system and a controlled system. It is usually used in automatic control circuits, that is, an "automatic switch" that uses a smaller current to control a larger current. It plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

In this design, an electromagnetic relay is used. This relay is generally composed of an iron core, a coil, an armature, and a contact spring. When an appropriate voltage is applied to the coil, a current flows through the coil to generate an electromagnetic effect. The armature is under the action of electromagnetic force Overcome the tension of the spring, so that the moving and static contacts of the armature are combined. When the coil is de-energized and the pulling force disappears, the moving contact and the stationary contact are separated.

Conclusion

Based on the application of the hardware description language VHDL in the programmable logic device CPLD/FPGA, the temperature controller is designed. Choose to design a simple water temperature control system, use FPGA to control the water temperature automatic control circuit, so that the system can easily achieve temperature control and display, and EDA software and hardware resources, friendly man-machine interface and AFPGA excellent real-time control The functions and flexible programming ability are organically combined, and various control algorithms are realized through software programming, so that the system has the characteristics of high control accuracy and the automatic control of water temperature is of great significance.

I have encountered many problems in the simulation practice. Although I can get a grasp on the program flow, I still cannot be careful and meticulous in handling the details. Especially for languages with strict VHDL syntax, a little care may bring orders. Errors that are difficult to debug, so the debugging process takes a long time.