Design of dynamic weighing measurement board based on PC104 bus

    When the object being measured is in a non-static state, that is, the object being weighed or measured is in motion, it forms a dynamic weighing measurement state [1]. For example, real-time weighing and measuring of a car on a road or a train on a railway is called dynamic weighing. Overload of freight trucks is a well-known fact in the freight industry. Overload of freight trucks has a huge impact on road transportation safety and road maintenance. The relevant national departments have specially rectified it, and the dynamic weighing measurement system of trucks is one of the powerful technical weapons to control overload of freight trucks.

    There are already some dynamic weighing instruments in use in the field of automobile dynamic weighing measurement, but they are mainly based on single-chip microcomputers, and use low precision, low speed, and multiple components to form the system. Although the manufacturing cost is low, its reliability, working performance and indicators are greatly limited [2, 3, 4]. In view of the actual needs of the project, we designed a high-performance automobile dynamic weighing measurement interface board that cooperates with a portable PC/104 bus computer, overcoming the above shortcomings and achieving the expected design indicators.

    The hardware circuit of the high-performance automobile dynamic weighing measurement interface board matched with a portable PC/104 bus computer consists of two parts: the signal acquisition part and the interface part with the PC/104 bus.

1 Design of signal acquisition part

    The high-resolution A/D converters currently used in measurement and control instruments are mostly independent integrated circuit chips. The high-precision measurement and control systems constructed in this way inevitably have many practical problems that are difficult to solve in terms of anti-interference, conversion accuracy, circuit board volume and price. Using an integrated chip that integrates a high-resolution A/D converter with an MCU (microcontroller or single-chip microcomputer) is a good way to solve the above problems. C8051F060 is a high-speed, mixed digital and analog integrated circuit MCU chip produced by CYGNAL. It has a peak operating speed of 25MIPS, a flexible external memory interface, 59 data I/O interface lines, and two on-chip, independent channel 16-bit successive approximation A/D converters [5]. These two independent channel 16-bit successive approximation A/D converters have the following features:

Ø 16-bit resolution.

Ø ±0.75 LSB INL, guaranteed no missing codes.

Ø Programmable conversion rate, up to 1 Msps.

Ø The chip integrates the tracking and holding and sampling circuits of analog input quantities.

Ø The software can be set as two single-ended input or one differential input A/D converter.

Ø Offset and gain are adjustable within a certain range.

Ø Direct memory access operations can be performed, and data is directly stored in RAM without the need for additional software overhead.

Ø Has a data-dependent window interrupt generator.

Ø Dedicated internal voltage reference or external voltage reference source can be used.

    The 16-bit successive approximation A/D converter with two independent channels is expressed as ADCn (n=0 is the first A/D conversion channel ADC0, n=1 is the second A/D conversion channel ADC1). Among them, ADC0 has 4 conversion start modes; ADC1 has 5 conversion start modes. You can determine when the A/D conversion is completed and process it by querying the ADNINT bit or by interruption.

    Figure 1 is a schematic diagram of the signal acquisition part of the dynamic weighing measurement board. In the figure, the ADC0 IN terminal of channel 1 is connected to the bridge output end of the weighing sensor to collect the dynamic weight signal of the car. The bridge output is a DC voltage signal of millivolt level. Therefore, the instrument amplifier AD620-AMP2 is used as an amplification element to amplify the signal to the analog conversion range (0-2.4V) that the internal A/D converter channel 0 (ADC0) of the MCU (here refers to C8051F060) can accept. C5, R3 and C9 before and after AMP2 are used for anti-interference filtering; D1 and D2 are used for voltage input overload protection of the A/D converter channel ADC0 inside the MCU. The amplification factor of the amplification channel is determined by the resistance value (RG) of the resistor R4. When RG=∞ (no amplification resistor is connected), the amplification factor (G) of the channel is 1.

Figure 1 Schematic diagram of the signal acquisition part of the dynamic weighing measurement board

    The output of AMP2 is filtered and sent to the external input pin AIN0 (pin 18 of MCU) of the 16-bit A/D converter channel (ADC0) in the C80C51F060 chip. The data sampling, holding and A/D conversion are all completed inside the MCU chip through software control.

    In FIG1 , the signal processing process and principle of A/D channel 2 starting from the ADC1 IN terminal are the same as those described above.

    The reference voltage of the 16-bit A/D converter (channel 0 and channel 1) in the C80C51F060 chip is taken from the voltage reference circuit inside the MCU. The voltage reference circuit inside the MCU consists of a 1.2V bandgap voltage reference generator with good temperature stability (typical value is 15ppm/℃) and excellent load regulation (typical value is 0.5ppm/µA) and a two-time gain output buffer amplifier. The internal reference voltage can be connected to the outside of the device through the VREF pin (pin 4 of the MCU). Connect the VREF pin to the VREF0 pin of the reference voltage end of channel 0 (pin 21 of the MCU) and the VREF1 pin of the reference voltage end of channel 1 (pin 6 of the MCU) to provide a reference voltage for the two 16-bit ADCs. The reference voltage value (typical value is 2.43V) determines the analog input range of the ADC.

    The given voltage connected to the input end of the weighing sensor bridge also comes from the reference voltage source inside the MCU. It is composed of the dual operational amplifier AMP3 and R7, R8, R6 and V1 to form the reference voltage amplification, amplitude stabilization and current expansion output circuits to complete the precise setting of the input voltage of the weighing sensor bridge.

    As can be seen from Figure 1, the hardware circuit design of the signal acquisition part of the dynamic weighing measurement board is very simple, including the amplifier circuit and the precise setting of the input voltage of the weighing sensor bridge. Obviously, compared with the traditional A/D converter composed of multiple devices, it has the advantages of reliable operation and strong anti-interference ability, and the price is also very competitive (a C80C51F060MCU costs less than RMB 300).

    When the A/D converters of the two channels are started by the overflow of Timer3 of the MCU and the conversion result value is obtained by interruption, the conversion result value of ADC0 or ADC1 is read in the interrupt subroutine of the corresponding channel and saved or further processed as needed. In the setting, it should be noted that the timing time of Timer3 should be greater than or equal to the sum of the conversion time of ADC and the reading time of the conversion result value (the running time of the interrupt subroutine).

2 Design of the measurement board interface

    PC/104 is the mechanical standard for embedded PCs. It inherits the advantages of the IBM PC open bus structure and is fully compatible with the IBM PC. It meets the special requirements of embedded control: small size, high reliability, long life, and convenient programming and debugging. Therefore, intelligent instruments based on PC104 have been widely used in the test field [6]. Since PC/104 bus devices have the characteristics of small size, high reliability, long life, and convenient programming and debugging, they are suitable for making high-density, small-volume portable test instruments or control devices. The development platform of the PC/104 bus system is exactly the same as that of other existing general-purpose computer systems, so all existing development software can be used. Therefore, intelligent measurement and control systems based on PC/104 have been widely used in the measurement and control field. In order to facilitate the development and use of the system, this dynamic weighing measurement system uses the PC/104 bus system as the system design, development and use platform, and the measurement board cooperates with it to complete signal acquisition, storage and transmission.

    Since it is difficult for the MCU and the PC/104 bus to be consistent in terms of signal operating frequency and interface timing, the I/O data interface between the MCU and the PC/104 bus should adopt an asynchronous parallel buffer interface. That is, devices such as 74HC373 and 74HC374 are used to latch the output data and handshake signals of the data bus on the PC/104 bus side, and the MCU reads or transmits data based on the handshake signals. The output data of the MCU side mostly uses a general parallel port with a data retention function. Therefore, bus driver chips such as 74HC244 and 74HC245 can be used to isolate and drive the data from the MCU to the PC/104 bus, thus realizing the asynchronous data transmission function in both directions [7].

    The external memory interface of C8051F060 is connected to a 128KB RAM (IS62LV1024) chip for data storage. The data interface between MCU and PC/104 bus is completed by the universal parallel interface (P0, P1, P2 and P3, etc.) on the C8051F060 side. The universal parallel port has a data retention function. Therefore, an 8-bit bus driver 74HC245 is used to perform level transfer from MCU to PC/104 bus (C8051F060 is 3V working power supply, PC/104 bus is 5V working power supply) and data driving. The data bus of PC/104 does not have a data retention function. Therefore, an 8D data latch 74HC374 with a three-state output control function is used to perform data transmission and retention from PC/104 bus to MCU (C8051F060 can directly receive 5V signal level). Since the P2 port of C8051F060 needs bidirectional operation function, the data transmission direction between PC/104 bus and MCU is determined and indicated by the latch signal from PC/104 bus. [page]

    Since the measurement board requires more than one decoding address, the 8-bit analog comparator 74HC688 is used as the comparison decoding chip for address decoding, and the 74HC393 (two-to-four decoder) is added for subdivided address decoding [8]. In order to save chips, the address line A0 is abandoned in the design for decoding. Therefore, the address decoding outputs R1/W1 and R2/W2 each occupy two addresses. For example, R1/W1 can be set to 200H-3C0H or 201H-3C1H by changing the jumper of JUM1. In this way, the transmission of commands and data between microcomputers can be realized through the software cooperation between the MCU and the PC/104 bus computer.

Figure 3 MCU program processing flow chart

3. Control software design

    The data transmission between MCU and PC/104 bus adopts master/slave mode, with PC/104 bus system as the host and MCU as the slave. In order to ensure the smooth implementation of data transmission in master/slave mode, MCU responds to the start of data transmission of PC/104 bus system by interrupt mode (the P02 pin of MCU corresponding to D3 of PC/104 bus has been pre-programmed as interrupt response pin INT0 and adopts edge trigger mode) and can perform bidirectional data transmission of any byte value at any time. PC/104 bus system uses C language to operate the interface reading and writing respectively using inportb() function and outportb() instruction, and its workflow flowchart is shown in Figure 2.

    In the programming of C8051F060MCU, the external program interrupt method is used to receive commands from the PC/104 bus microcomputer and exchange data with it; the ADC interrupt method is used for data sampling. Among them, data sampling includes base value sampling (two-way weighing sensor output without wheel pressure condition) and axle weight sampling (two-way weighing sensor measures the weight of one axle of the car at a time). The simplified diagram of the C8051F060MCU program processing flow is shown in Figure 3. In axle weight sampling, the MCU automatically determines the position between the wheel and the weighing sensor according to the given vehicle axle number and sampling threshold conditions and automatically samples the weight value. When the wheel stops on the weighing sensor, the MCU will automatically stop data sampling according to the set over-limit time value.

4 Conclusion

    The types of PC/104 bus interface boards currently on the market are limited and the prices are relatively high. With the widespread adoption of PC/104 bus systems, the design of PC/104 bus interface boards will become the bottleneck of the success or failure of the system. Improving the performance of PC/104 bus interface boards and reducing their prices will greatly promote the application of PC/104 bus systems.

    The design of the dynamic weighing measurement board based on PC104 bus has been proven to have the following advantages after more than one year of actual use:

ü Use fewer components and simplify hardware and software design;

ü Enhanced the system's anti-interference ability and working reliability;

ü Make full use of MCU resources;

ü Have competitive advantage in product price.

 References

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