Using the pressure sensor design to achieve Level Control System

Abstract: The controller microcontroller, a high level control, alarm, altitude display and other functions, due to increased gas pressure sensor, it has no contact with the surface characteristics, can be used for toxic, corrosive liquid level control, with more of high Research value. The controller can be used in schools is not only teaching and Research, but also for actual production, is the lack of a product comparison.

Keywords: sensor, AD converter, controller, external hardware circuit

0 Introduction
With the rapid development of microelectronic industry, microprocessor controlled intelligent controller is widely used in electronic products, in order to make students intelligent controller MCU deeper understanding. After a comprehensive analysis chosen by the Intelligent MCU control liquid level controller as a Research project, by training sufficient to stimulate students analyze and solve problems and integrated application of the knowledge of the potential. In addition, the level control in high-rise residential tower water level control, sewage treatment equipment and toxic, corrosive liquid solution Level Control is also widely used. By design the model extends well to the specific application case.

1 Comparison of system design description
For the level there are many ways to control, and applications are 2 more, a simple mechanical control device control, a controller is a complex way. Two ways to achieve the following:
(1) simple mechanical control. The common form of float type, electrode type, and so, this form of control has the advantage of simple structure and low cost. Exists the problem is not accurate, not the numerical display, another can easily lead to malfunction, and can only be controlled separately, to communicate with the computer more difficult to achieve.

(2) complex controller control. This control is through pipes installed in the pump outlet pressure sensor, the outlet pressure of standard industrial electrical signals into analog signals, after pre-amplification, multi-channel switching, AD transformed into digital signal sent to the MCU, the MCU operation and comparison of the given parameters, the PID operation, adjust the parameters obtained via the DA conversion to the frequency control voltage input for a given client device to control the output voltage to adjust the motor speed order to control the water level of the goal.

In response to these two kinds of control methods, and design performance requirements to be met, here choose the second control mode, taking into account the cost of the need to remove the PID control. The final formation of the program is to use microcontroller to control the core design of a water supply tank level monitoring system. According to the characteristics of monitored objects, requiring a high degree of real-time detection of liquid level tank, and with the start to compare pre-set value, the MCU control solid state relay breaking the level of adjustment, and ultimately achieving liquid bit of pre-set value. detection value if above the upper limit set value, the requirements of the alarm, disconnect the relay, the control pump to stop Sheung Shui, detection value if less than the lower limit, requiring the alarm, open relays, control pump began to Sheung Shui. live real-time display of measured values, enabling the monitoring of water level.

2 Work
Based Microcontroller AT89C51 chip level controller is based on the core, from the keyboard, digital display, AD conversion, sensors, power and control parts and other components.

Works as follows: the water tank (water tower) level changes, causing connected to the tank (water tower) at the bottom of the air pressure hose changes in air pressure sensor hose is received within the air pressure signal, that is, the variation into voltage signal, the signal is amplified through the signal processing circuit into a range for the 0 ~ 5 V standard signal, into the AD converter, AD converter to the amount of analog signals into digital signals by the microcontroller for real-time data acquisition and processing , according to set requirements control the output, digital display liquid level. through the keyboard setting level high, low and limit values, and force the alarm value. The system controller features an intuitive display the water level, we can control water level.

3 Hardware Design
Level controller hardware includes the microcontroller, sensors (with transmitter), the keyboard circuit, the digital display circuit, AD converter, and output control circuit.

3.1 Microcontroller
Atmel microcontrollers produced by a double row 40 feet AT89C51 chip.

3.2 Sensor
Sensors SY 9411L-D model of a transmitter, which contains an internal pressure sensor and the corresponding amplifier. Pressure sensor is produced by the United States SM 555-2 OEM piezoresistive type pressure sensor, temperature compensation and a full calibration (O ~ 70 ��), the sensor has been specially processed, high-temperature plastic shell with a strong package. reposted elsewhere in the Research Papers Download at the bottom of the tank installation http://www.hi138.com a diameter of 5 mm, hose, one end of the installation in the bottom of the tank, connect the other end of the sensor. tank water levels change, causing changes in air pressure hose, and pressure sensor to convert the voltage signal, transported to the AD converter.

3.3 Keyboard Circuit
P1 port as the keyboard interface, connected to a 4 �� 4 keyboard.

3.4 level display circuit
Level display using dynamic digital display, ranging from 0 to 999 (units can be custom), select the digital connection is a 7-segment common cathode, the model is LDSl8820. Used here to the 74LS373, which is triggered by an 8-bit D device, often used in the SCM system can be extended for the address latch data bus can also be driven as a normal LED devices, the drive alone HEF4511B seven segment display decoder driver to complete the digital display, so 74LS373 here only for the extension of the buffer.

3.5AD conversion circuit and control output
AD conversion circuit play a leading role in the controller, use it to the analog sensor output voltage signals into digital single chip can handle. The controller is made using CMOS technology 8-bit successive approximation AD converter chip ADC0809. In the use of a choice of break, query and delay the preparation of 3 ways to wait for AD conversion process. There are upper and lower control output status display, limit alarm. Also in the design process, the serial port reserved for the further development of the use.

4 Software Design
4.1 Keyboard Program
The keyboard uses a 4 �� 4 structure, so you can use the key 16, the keys are defined as needed, from 0 to 9 for the number keys, 10 to 15, respectively the OK button, modifier keys, Shift key, plus or minus button, cancel button and reset button.

It is noteworthy that, in the controller program written in assembly language, the relative would be more troublesome, if the program will use the C language is much simpler here, do not specify.

5 Conclusion
Level controller model based on single chip design is the key to building the right hardware, and only under the premise of the circuit to conduct accurate software program to be successful.

References:
[1] Huang Zhiwei. Sensor technology .2002,21 (9:31 ~ 33
[2] Dou Zhen. Microcontroller main design principles. Beijing Aerospace University Press .2000
[3] Jia Min level. Testing. Higher Education Press Links http://www.hi138.com Research Papers Download