智能烘箱温度控制器设计-英文翻译

PID control algorithm

Developed using PID control principle is suitable for the temperature controller is used for low power devices, the controller can achieve good control effect, if carefully chosen the various parameters of the PID temperature control precision can reach 0.05 ℃, can guarantee the normal work of the device. In some control system, first of all measured parameters (temperature) that will need to control by the sensor is converted into a signal and then compared with preset value, the difference compare with the signal after a certain regularity of calculation to get the corresponding control values, send control quantity to control system to carry on the corresponding control, constantly in the work, so as to achieve the goal of automatic adjustment. PID is currently widely used control method, the mathematical model of the control law is: Vo kp e 1de edtTd Vo 1 Tidt

Among them: for proportional coefficient KP; E for the differential signal, e = T - Tset (T: temperature measurement value, the Tset: temperature set point); Ti is constant of integration; Td for differential constant; Where V0, where V0-1 for a moment before then and the amount of control.

Proportional control (P) is one of the most simple control method. The controller output to the input error signal is proportional to the relationship. When only proportional control system output steady-state error.

Integral control (I) in integral control, the controller output is proportional to

the input of the error signal integral relationship. Of an automatic control system, if in the steady state error, there's system, called the control system. In order to eliminate the steady-state error, in the controller must be introduced to "integration". Depends on the time integral, integral study of error with the increase of time, integral will increase. Like this, even if the error is very small, integral item also increases with increasing over time, it pushed further increases the output of the controller makes the steady-state error is reduced, until it is equal to zero. Proportional plus integral (PI) controller, therefore, can make the system in the steady state after no steady-state error.

Differential control (D) in the differential control of controller output is proportional to the input of the error signal differential relations. Automatic control system in the process of overcoming the error adjustment may appear oscillation even instability. The reason is because there are large inertia link, is to inhibit the action of the error and its change is always behind the change of the error. Solution is to inhibit the action of the error change "advanced", namely the error close to zero, to inhibit the action of the error should be zero. That is to say, in the controller only introduced "ratio" is often not enough, the role of proportional error is only amplified amplitude, and the need to increase the "differential", it can predict error change trend, in this way, a proportional plus differential controller can make the inhibitory effect of error control in advance is equal to zero, even negative, avoiding being controlled amount of serious overshoot. So for controlled object with large inertia or lag,

proportional plus derivative (PD) controller can improve system dynamic characteristics in the process of adjustment.

Implementation concrete method of PID control principle varies because of the system. In our system, the incremental calculation method is adopted, and the output of the control volume is adopted the form of the output of the position type. In the numerical control system, the mathematical model of the control law evolved into:

Uo Uo 1 Kp ei ei 1 T ei Td ei 2ei 1 ei 2

Among them: T for acquisition cycle; , ei ei, ei - 1-2 for this moment, a moment again, before the moment of differential signal. Three moments before the benefits of this approach is to simply keep the difference signal, and output control quantity of the initial value is not accurate, can quickly into the stability of control process.

The PID controller parameter setting is the core content of control system design. It is determined according to the character of the charged process of PID controller is the size of the proportion coefficient, integral and differential time. The PID controller parameter setting method are many, in summary there are two broad categories: one is the theoretical calculation of setting method. It is mainly based on the mathematical model of the system, after the theory calculate and determine the controller parameters. Calculation data obtained by this method won't be able to use directly, also must be adjusted and modified through the engineering practice. Second is

engineering setting method, it mainly depends on engineering experience, directly in the test of the control system, and the method is simple, easy to grasp, is widely used in engineering practice. Engineering setting method of PID controller parameters, mainly has the critical ratio method, reaction curve method and decay method. Three methods have their characteristics, their common ground is through test, and then according to the engineering experience formula for setting controller parameters. But no matter which kind of method of controller parameter, the need for final adjustment and improvement in the actual operation. Now commonly used is the critical ratio method. Using the method of PID controller parameter setting procedure is as follows: (1) to first select a short enough sampling period make the system work; (2) only in proportional control link, until the system step response of input appears critical oscillation, jot down at this time the proportion of the amplification coefficient and the critical oscillation period; (3) under a certain degree of control is obtained by the formula to calculate the parameters of PID controller.

MAXIM company recently developed A type K thermocouple signal converter MAX6675 (IC), MAX6675 internal benchmark by precise operational amplifier, power supply, the cold end compensation diode, analog switch, digital controller and ADC circuit composition, complete thermocouple weak signal amplification, the cold end compensation and A/D conversion function, make the front end of the temperature measurement

circuit is very simple. MAX6675 adopted the 12 bit ADC, the temperature measurement range of 0-1024 ℃, within the range of 0-700 ℃ conversion accuracy of plus or minus eight words, power supply, 3.0-5.0 V, a resolution of 0.25 ℃, the conversion time about 0.17 s. MAX6675 encapsulated by 8 feet SO, figure 2-4 for the pin arrangement diagram, T + K type thermocouple's positive electrode (alloy), T - type K thermocouple's negative (nickel or nickel aluminum silicon alloy); CS for high levels of selected signals from this transformation start temperature, low electricity at ordinary times allows data output; SCK for clock input; SO for the data output side, 12 data after temperature conversion output by way of the feet to SPI.

Figure 2-4

Temperature transformation: MAX6675 inside is hot I will signal conversion to and compatible with the voltage signal conditioning amplifier, the ADC input channel and T + T - input end connected to the low noise amplifier A1, to ensure the accuracy of detecting input, at the same time use the thermocouple wires from the interference source. Thermocouple thermoelectric potential output by the low noise amplifier A1 amplifier, after A2 voltage follower buffers, again be sent to the ADC input end. In converting temperature voltage value equal to the value before, it need to

compensate the cold end of thermocouple temperature, cold end temperature is around MAX6675 and 0 ℃ temperature difference between the actual reference value. For K type thermocouple, voltage change rate for uv / 41 ℃, voltage by the formula of linear Vout = (uv / 41 ℃) x (tR - tAMB) to approximate characteristics of the thermocouple. On the type of Vout as the thermocouple output voltage (mV), tR is a temperature measurement point; TAMB is ambient temperature.

The cold end compensation: thermocouple function value is the difference in detection of hot and cold side, thermocouple temperature hot nodes may change in the range of 0-1024 ℃. Cold end namely install MAX6675 circuit board surrounding temperature, the temperature changes within the range - 20 - to + 85 ℃. When the cold end temperature fluctuations, MAX6675 still can accurately detect the hot end temperature changes. MAX6675 is through the cold end compensation of the temperature change around the detection and correction. The device is to convert the temperature of the surrounding temperature through internal testing diode voltage temperature compensation, in order to produce practical thermocouple temperature measurement value. MAX6675 from the output of the thermocouple and test measure the voltage at the output of the diode. The inside the diode voltage circuit and the thermocouple voltage to the ADC conversion, to compute the temperature of the hot junction of the thermocouple. When the temperature of the cold end of thermocouple and chip is equal, MAX6675 can obtain the best accuracy of

measurement. So in actual temperature measurement applications, should avoid as far as possible is placed near the MAX6675 heating device or element, because it will cause the error of the cold end. As shown in figure 2-5 MAX6675 connection.

Figure 2-5 MAX6675 connection

SPI interface: MAX6675 with a standard SPI serial peripheral interface with MCU, and MAX6675 only as from the device. Output MAX6675S0 temperature data format, as shown in the table 2-6 MAX6675SPI interface timing as shown in figure 2-7. MAX6675 from SPI serial interface data output process is as follows: MCU make the CS low and provides the clock signal to SCK, by SO read the measured results. CS is low will stop any conversion process; CS is high will be starting a new conversion process. A complete serial interface reads need 16 clock cycles, read on the falling edge of the clock output of 16 bits, bit 1 and 15 is a false flag, and the total of 0; 14th to third in the MSB to sequential LSB of transformation temperature; 2nd at the bottom, at ordinary times is higher when the thermocouple input open open thermocouple detection circuit implemented by MAX6675 fully, for open thermocouple detector operation, T - must be grounded, and can

make the site as close as possible to GND pin; 1 bit is low to provide MAX6675 device identity code, for the three states of 0.

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