FPGA in intelligent pressure sensor system design (1)

Introduction

Traditional gas pressure measuring instruments of the sensor part and data acquisition system is the separation of anti-interference ability of the poor, and usually the measured object of pressure change fairly quickly.

So not only system with a faster data throughput rate, but also to be able to meet the complex and ever-changing industrial environment, has better anti-interference capability, self detection and data transmission capabilities.

In this, the use of FPGA has extended flexible, system on chip (SoC), also have a variety of IP cores are available for use, etc., designed to control multiple analog switch, A/D conversion, fast data processing and transmission, error correction, temperature compensation, intelligent sensor systems; at the same time, sensor and data acquisition and processing control system integration, enabling system more compact, improves the system's ability to adapt the industrial field.

1 system properties and components

1.1 intelligent sensor system performance requirements

Sensors pressure measuring range: 0 ~ 5 MPa; system accuracy: ± 0.1% FS; 1-channel analog voltage input (pressure signal) of more than 250 sampies/channel/s; the use of serial RS 232C interface output.

1.2 system main components and performance

According to the system requirements of precision index: select device

Altera FPGA chip selection of C y c l o n e C E P 2 Ⅱ 5, its logical unit has 4 608-LE, 26 M4K RAM blocks, 142 user I/O PIN.

Pressure sensor using PDCR130W, pressure range 0 ~ 7 MPa, operating voltage DC 10 to 30 VDC, output 0-10 V, precision ± 0.05% FS, temperature range-40 to + 125 ° c, temperature ± 0.015% FS/℃.

Temperature sensor with integrated temperature sensor with high precision, sensitivity to LM335 10 mV/K, accuracy to 1 ° c, temperature range-40 to + 100 ℃.

A/D converter selection contains sample and hold for 12-bit A/D converter, convert time AD1674 is 10 μ s, 0-10 V unipolar input or ± 5 V bipolar input, 12-bit parallel output.

Multi-channel analog switch with four selected a multi-channel analog switch AD7502, its PIN is set to EN = 1 enabling signal; A1A0 pin signal for channel selection.

Output level conversion interface system complete with MAX232 chip TTL and RS 232C level conversion.

2 system error correction method

2.1 zero drift and gain error correction method

In intelligent instruments and error model error correction formula is:

Type: b1 and b0-error correction factor.

Error correction circuit model as shown in Figure 1, where x is the measured signal; y is the system output; ε, k, I as an unknown quantity of effects system.

Error correction process is:

When S1 is x = 0, error correction formula-(2), used for zero point calibration system;

When S2 is x = E (standard voltage), formula (3) for the system gain error correction;

With vertical (2), (3) may have error correction factor:

When the actual measurement is closes, use S3 calculated correction factors and error correction formula (1), you can find the correct output signal after y.

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2.2 sensor temperature compensation

The pressure sensor, the environmental temperature on the measurement results have a greater impact, in order to eliminate the error caused by temperature, sensor signal do temperature compensation.

By measuring sensor temperature sensor temperature compensation. Sensor temperature error correction model are:

Type: y is measured value; once the temperature compensation yc measurements; △ φ is the actual temperature sensor and the difference between a standard measurement temperature; a0 to changes in temperature calibration sensor calibration coefficient of variation of temperature, a1 calibration of transducer zero coefficient of variation of drift, the two coefficients reflecting the sensor temperature characteristics.

2.3 random error elimination method

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