The resistance furnace temperature detection and control is such a feedback adjustment process. The actual furnace temperature (feedback amount) is deviated from the given furnace temperature, and the control signal is obtained by processing the deviation to adjust the heating voltage of the thyristor regulator. Automatic control of the furnace temperature is achieved.
According to the proportional, integral and differential control of the deviation (referred to as PID control), it is the most widely used control method in process control, and can obtain satisfactory results. The computer-to-PID algorithm is approximated by a difference equation (see the relevant computer)
Control technology reference book), in this calculation process, the computer mainly performs addition, subtraction, multiplication, and division.
System applications are designed with a system-timed interrupt (assumed to be 5s). An interrupt is generated when the Timer/Event Counter overflows. The main program causes the T0 timer/counter to generate a 5s clocked interrupt as the sampling period of the system. Interrupt service routine
Start A/D conversion, read in the sampling data, perform digital filtering, upper and lower limit alarm (such as adding alarm circuit), PID calculation, and then output control signal after D/A conversion.
If a display and printer are connected, the display and print subroutine can also be called in the interrupt service routine to display and print the temperature values for this sample. After the execution of the interrupt service program is completed, it will return to the main program, continue to display the temperature of the sampling furnace, and wait for the next anti-static shoes, anti-static clothing T0 interrupted sampling and processing.
It can be seen that the system application consists of two main parts: the main program and the interrupt service program. Is the system application block diagram. Of course, there are also callable A/D conversion and sampling subroutines, digital filtering subroutines, and digital controller algorithms in the interrupt service routine.
(PID chart algorithm) subroutine, output value processing program, display subroutine, and overrun alarm subroutine. Due to space limitations, this book will not be introduced one by one. Training 2 Application of strain gauge sensors in weighing instruments
The sensors used in the scale generally have anti-static shoes such as resistance strain type and elastic metal structure sensors, and anti-static clothes. Since the use of resistance strain gauges for commercial pricing scales, conventional mechanical scales and grating code scales have been gradually replaced. The weighing error of the resistance strain gauge can be less than 0.02% of full scale. The digital scale with S-shaped double-bend beam strain gauge and single-chip microcomputer has zero tracking and nonlinear correction. Accuracy selection, weighing, tare weight, accumulation, display, printing and many other functions.
Comparing the three working modes of the bridge, it can be seen that when the DC bridge is used as the measuring circuit of the strain gauge, the output voltage of the bridge is linear with the measured strain; under the same conditions (the type of the power supply and the strain gauge are unchanged), Differential operation is larger than single-arm operation output signal, half-bridge differential output is twice the single-arm output, and full-bridge differential output is four times that of single-arm output. Therefore, the output voltage is the largest when the full bridge is differentially operated, and the sensitivity of detection is the highest.
When using the above formula, pay attention to the signs of resistance change and strain value. Anti-static shoes, anti-static clothing, if it is compressive strain, it is substituted with a negative strain value; tensile strain is substituted with a positive strain value.
2. Temperature error of resistance strain gauge and its compensation
(1) Temperature error and its causes
Used as a strain gauge for measuring strain, it is expected that its resistance will only vary with strain and is not affected by other factors. In fact, the change in resistance caused by the temperature change of the strain gauge is almost the same order of magnitude as the change in resistance caused by the strain of the test piece.
If the necessary measures are not taken to overcome the effects of temperature, the measurement accuracy will not be guaranteed. The additional error caused by the change in ambient temperature (deviation from the strain gauge calibration temperature) is called the temperature error of the strain gauge, which is also called the heat output of the strain gauge.
There are two main factors that cause the temperature error of the strain gauge: First, due to the existence of the temperature coefficient of the resistance wire, when the temperature changes, the nominal resistance value of the strain gauge itself changes; the second is the linear expansion coefficient of the resistance wire and the test piece material. At the same time, the temperature change will cause additional deformation of the anti-static shoes and the anti-static clothing, so that the strain gauges generate additional resistance.
When the ambient temperature changes At~C, the temperature coefficient of resistance of the strain gauge sensitive gate material pasted on the surface of the test piece is a. , the change value of the resistance of the sensitive gate resistance wire.
The relative change in the additional resistance due to the change in the ambient temperature is related to the change in the ambient temperature, as well as the performance parameters of the strain gauge itself and the linear expansion coefficient of the test piece. related.
(2) Temperature compensation method of resistance strain gauge
Temperature compensation methods for resistance strain gauges generally include two types: strain gauge self-compensation method and circuit compensation method.
1) Single wire self-compensating strain gauge. The basic basis for manufacturing a monofilament temperature self-compensating strain gauge is the formula (2.30). It is not difficult to see from this formula that the condition for achieving temperature self-compensation is
The advantage of the monofilament self-compensating strain gauge is that the structure is simple, and it is convenient to manufacture and use, but it must be used on the test piece with a certain coefficient of linear expansion coefficient, otherwise the purpose of temperature self-compensation cannot be achieved.
2) Double wire composite self-compensating strain gauge. It is a composite strain gauge made of two kinds of resistance wires with different temperature coefficients in series, if two sections of sensitive grids R, and R. The change in resistance due to temperature change ΔR. . And AR:. Temperature compensation can be achieved with equal size and opposite signs. Resistance R. The ratio of the ratio to R z can be determined by:
The advantage of the compensation method is that the length of the two sections of the sensitive grid can be adjusted during manufacture to achieve better temperature compensation for the test piece of a certain material in a certain temperature range.
3) Circuit compensation method. By using the adjacent arms of the bridge to simultaneously generate the same amount of resistance and the same amount of resistance, the balance of the bridge will not be destroyed to achieve the purpose of compensation.
When measuring strain, two strain gauges are used. One piece is attached to the surface of the test piece, which is called the working strain gauge, and the other piece is attached to the compensation block which is the same material as the material to be tested and is in the same temperature field. Strain gauges. Compensation block during work
Does not withstand strain and only deforms with temperature.
