Pressure measurement and control system of MPXM2010 and its precision improvement method

Introduction

The MPXM2010 device produced by Freescale is a silicon piezoresistive pressure sensor. The MPXM2010 has high accuracy and the output voltage has a good linear relationship with the input pressure. This sensor is a monolithic integrated circuit that integrates a pressure strain gauge and a film resistor network, and has a laser trimming module for temperature compensation and offset correction. The microcontroller 68HC908QT4 is a low-end 8-bit microcontroller with 4 8-bit A/D conversion channels and a 16-bit PWM module, which can be used for A/D and D/A conversion.

Combining the two chips together can form a practical low-cost pressure measurement and control system. The only drawback is that its accuracy is a bit low. If the A/D bit is increased, the cost will increase greatly. This shortcoming can be compensated by hardware matching and software programming, that is, it does not increase hardware costs and can improve product performance.

1. Pressure sensor module design

The MPXM2010 device produced by Freescale is a silicon piezoresistive pressure sensor, and its internal principle is shown in Figure 1. The MPXM2010 has high accuracy, and the output voltage has a good linear relationship with the input pressure. This sensor is a monolithic integrated circuit that integrates a pressure strain gauge and a film resistor network, and has a laser trimming module for temperature compensation and offset correction.

MPXM2010 features are as follows:

• The pressure measurement range is 0～10 kPa, and the accuracy can reach ±0.01 kPa;
• It has temperature compensation function between 0～85℃;
• The linear relationship between the output signal and pressure is good;
• The sensor contact surface can be equipped with or without an outlet.

There is an easy-to-use packaging form of Tape & Reel, the specific sample is shown in Figure 2.

The output signal of MPXM2010 is relatively weak, so an additional MOC2A60 is needed to amplify the small signal and convert DC into AC. In this way, the motor can be directly controlled to cut off or connect the power supply. When debugging the module, separate the various parts for easy debugging. The op amp uses MC33179, and then connects some resistors to output the signal of the pressure sensor, and the size of the output signal can be adjusted by adjusting the resistance value. Figures 3 and 4 are the schematic diagram and PCB board diagram of the pressure sensor module design.

2. Design of pressure measurement and control system and its accuracy improvement

2.1 Direct-connected pressure measurement and control system

Normally, the design can be completed by using the A/D module of 68HC908QT4, just connect the output of the pressure sensor module to the input of the A/D module of 68HC908QT4. Figure 5 shows the block diagram of the pressure measurement and control system.

The features of the microcontroller 68HC908QT4 are as follows:

◇4 KB Flash memory, 128 B RAM memory;

◇4-channel 8-bit A/D converter, 16-bit PWM module;

◇ Low price, the price of each piece can be reduced to less than US$1 for batches of 1,000 or more.

The measurement range of MPXM2010 is 0-10 kPa, and its output voltage signal is limited to 0-5 V, so its accuracy is:

S=5 V／10 kPa="500" mV／kPa

The A/D of 68HC908QT4 is 8 bits and the voltage limit is 5 V, so its accuracy is:

R=5 V／(20-1)bit≈19.61 mV／bit

The pressure accuracy of the entire system is:

R/S=19.61/500 kPa/bit=0.039 22 kPa/bit

If you want to improve the accuracy, increase the A/D to 10 bits, the accuracy is:

R/S=O. 03 922X(28-1)／(210-1)kPa／bit=0.009 776 kPa／bit

After A/D is upgraded to 12 bits, the accuracy is:

R／S=0.039 22×(28-1)／(212-1)kPa／bit=0．002 442 kPa／bit

This can indeed improve the accuracy, but it will increase the hardware cost.Using the PWM module of 68HC908QT4 as D/A converter can improve the accuracy of A/D conversion ingeniously.

2.2 Improved pressure measurement and control system

The reason for the error is that the decimal point is discarded at the A/D, for example, 176.51 bits will be treated as 176 bits. The solution to the problem should also start from here to reduce the error.

The error can be solved by D/A. The data read by A/D is processed by D/A and sent out, and then subtracted from the original data. The error cannot be sent back to A/D directly, but it can be amplified and sent back. Then another A/D in 68HC908QT4 is used to convert the amplified error to A/D. After the MCU obtains the result, it is reduced by the same multiple and added to the original A/D conversion result to obtain a more accurate result. In Figure 6, the entire system can be divided into the pressure sensor module, the analog part, the microcontroller part and the output circuit part. The key to improving the accuracy lies in the design of the analog part. As shown in Figure 7. Assume that the amplification factor of amplifier G is 10. The performance of A/D itself has not been improved, and the accuracy is still R=19.61 mV/bit, which is the limit value. After amplification by 10 times, the original maximum error of 19.61 mV/bit is enlarged to 196.1 mV/bit. A/D processes the amplified data, and its ability is amplified by 10 times. When processing the data, it will be divided by 10 to restore it. From the overall point of view, it is as if the accuracy R is divided by 10, becoming 1.961 mV/bit.

For example, the initial A/D conversion error is 10 mV, which becomes 100 mV after amplification. At this time, after A/D conversion again, the second remaining error is 100 mV-19.61 mV/bit×5 bit="1".95 mV, which becomes 0.195 mV after dividing by 10. The error is greatly reduced, and its limit value is one tenth of the original accuracy.

The magnification of G can be adjusted by yourself, but it must be consistent with the performance of the selected microprocessor and the accuracy of the circuit itself. It is meaningless to choose too high a magnification.

In the circuit shown in Figure 7, Vm, D, and Vc are the same as those shown in Figure 6. The calculated value of D is:

D=(Vm—Vc)×(R14／R13)[l+(R17／R16)]

The magnification of G is (R14/R13)[1+(R17/R16)].

Conclusion

Cost is a very important factor in the product design and development process. By cleverly using the modules in the microcontroller and assisting with the corresponding simple analog circuits, the chip utilization efficiency can be greatly improved and the system performance can be enhanced. Using what you have at hand to improve and recreate can often achieve twice the result with half the effort.