Design scheme of ARM and Bluetooth wireless signal acquisition system

Wireless testing technology has broad application prospects in the industrial field. In the case where the connection is complicated and the connection between the equipment under test and the test equipment needs to be disassembled and assembled repeatedly, the use of wireless can reduce the complexity of the work, save a lot of time, and improve the work efficiency of the test link, especially when there is a time limit. The advantage is more obvious. In addition, in the occasions that are not suitable for connection, such as ports, docks, rivers, lakes and dams, field surveys, and measurement of environmental parameters in the depths of oil wells in oil exploration, the use of wireless measurement has incomparable advantages over wired measurement.

1 Introduction

Wireless testing technology has broad application prospects in the industrial field. In the case where the connection is complicated and the connection between the equipment under test and the test equipment needs to be disassembled and assembled repeatedly, the use of wireless can reduce the complexity of the work, save a lot of time, and improve the work efficiency of the test link, especially when there is a time limit. The advantage is more obvious. In addition, in the occasions that are not suitable for connection, such as ports, docks, rivers, lakes and dams, field surveys, and measurement of environmental parameters in the depths of oil wells in oil exploration, the use of wireless measurement has incomparable advantages over wired measurement.

For short-range wireless transmission, Bluetooth uses fast frequency hopping technology to ensure the stability of the link, and at the same time, the possible impact of interference becomes small, and it is suitable for use in industrial test environments where there is a lot of noise interference. It is a digital quantity, so there is no transmission error under normal circumstances, it will not affect the accuracy of the system, and it can be implemented on a single chip, and the volume power consumption can reach a very small level.

2 Chip Select

2.1 Selection of Bluetooth module

The bluetooth chip used in the acquisition module is a commercialized bluetooth module, the core of which is the mainstream CSR BlueCore02-External bluetooth chip, which together with the peripheral devices constitutes the bluetooth module, as shown in Figure 1. The voltage adjustment circuit provides the 3.3 V and 1.8 V required by the Bluetooth module, and the flash memory is used to store the Bluetooth firmware and configuration parameters. The Bluetooth host can communicate with the Bluetooth module through various interfaces (SPI, UART, USB, etc.).

Design scheme of ARM and Bluetooth wireless signal acquisition system

2.2 The choice of microprocessor

The microprocessor is mainly responsible for sampling control, programming and control of the signal conditioning chip, and communication with the Bluetooth module. Due to the adoption of the resident Bluetooth model, the microprocessor communicates with the Bluetooth module through the HCI (Host ControlRF Inteace) interface, including packaging and sending the collected data to the Bluetooth module according to the data format specified by the Bluetooth protocol, and interpreting it from the host computer. The control command of the control system, the collection action of the control system and the working mode of the Bluetooth module.

The microprocessor based on the ARM core has the advantages of fast operation, small size, low power consumption, and abundant peripheral interface resources. It uses the Philips LPC2146 microcontroller based on the ARM7TDMI-S core.

3 Hardware Design

The system hardware is divided into two parts: acquisition module and receiver module:

(1) Acquisition module: It is an 8-channel data acquisition system. After amplification and conditioning, the 8-channel sensor signals become standard analog voltage signals of 0 to 3 V, which are respectively connected to the AD1.0 to AD1.7 pins of the LPC2146. . The ADC in LPC2146 is a successive approximation ADC with a resolution of 10 bits and a conversion rate of 400 ks/s, which supports 8 multiplexed input signals. The LPC2146 samples 8 signals in turn and digitizes them. Since the ADC inside the LPC2146 does not provide a voltage reference during conversion, the LT1461A3, a precision voltage reference of +3 V, is used.

(2) Receiving module: The receiving module also takes the CSR BC02 Bluetooth module as the core, and performs Bluetooth ACL (Access Control List) data communication with the acquisition module. The BC02 chip transmits and receives data with the serial port of the PC through the MAX3232 level conversion chip.

3.1 Signal acquisition module

CSR BlueCore02 Bluetooth module has HCIUART and HCI USB transport layer. This subject uses a relatively simple programming HCI UART interface to connect with the microcontroller LPC2146.

The hardware connection between the Bluetooth module and the ARM is shown in Figure 2. Since both the Bluetooth module and ARM are powered by +3.3 V power supply, there is no level difference between their interfaces, and no level conversion is required. The sending end UART TX of the UART interface of the Bluetooth module is connected to the UART0 receiving end RXD0 of the LPC2146, while the receiving end UART RX of the UART interface of the Bluetooth module is connected to the UART0 sending end TXD0 of the LPC2146.

Design scheme of ARM and Bluetooth wireless signal acquisition system

The reset circuit of the Bluetooth module has three reset functions: power-on reset, manual reset and LPC2146 control reset. When the high level duration on the RST pin is greater than 5 ms, the Bluetooth module will be reset.

In Figure 2, C1 and R3 form a power-on reset circuit; S1 is a long-open button, when S1 is pressed, it realizes manual reset; the P0.16 pin (BTRST) of LPC2146 can also reset the Bluetooth module through the D1 diode.

3.2 Bluetooth receiving module

The receiving module is connected with the monitoring PC to receive the bluetooth ACL data packets sent from the acquisition module and transmit them to the PC. The communication between the Bluetooth module and the PC is based on the HCI RS 232 interface. A level conversion chip MAX3232 is used to convert the TTL level to the RS 232 level. As shown in Figure 3.

Design scheme of ARM and Bluetooth wireless signal acquisition system

The interface between the PC and the Bluetooth module uses hardware flow control, as shown in Figure 4.

Design scheme of ARM and Bluetooth wireless signal acquisition system

It is worth noting that the maximum baud rate that the built-in RS 232 interface of the PC motherboard can achieve is only 115.2 kb/s, which is far lower than the Bluetooth module ACL, and the maximum communication rate of the connection is 721 kb/s. In order to avoid the RS 232 interface becoming the data transmission bottleneck of the data acquisition system, a PCI-RS232 interface card is used, and its baud rate can reach 1 Mb/s.

In addition, the baud rate of the RS 232 interface is also restricted by its transmission distance. After repeated experiments and verification, the final receiver Bluetooth HCI UART transport layer can send and receive data normally at a baud rate of 460.8 kb/s.

3.3 Hardware anti-interference measures

In this system, components such as ARM, signal conditioning chip, and Bluetooth module are susceptible to interference, so anti-interference technology is a key issue that needs to be considered in system design. In the hardware design, the following measures are mainly taken to strengthen the anti-interference ability of the entire test system:

(1) The circuit board is powered by a battery. Powered by a 4.2 V lithium battery, it can provide relatively stable voltage and pure current. Compared with other solutions that use metal slip rings or resolver for power supply, battery power supply eliminates AC noise and power supply fluctuations caused by these two methods.

(2) The power monitoring chip MAX823 and the watchdog circuit are added to the ARM processor. When the fluctuation of the power supply exceeds the safety threshold (3.6- 2.9V), the MAX823 will generate a Reset signal to reset the microprocessor.

4 Conclusion

The wireless testing system based on bluetooth technology in this subject is the application of short-range wireless communication technology in the testing field. The wireless test system has a large number of sampling channels, small size, low power consumption, multiple energy-saving modes, easy installation, and has both signal conditioning and signal remote transmission functions. It can also be used in the working condition monitoring and fault diagnosis systems of other high-speed or low-speed rotating machinery, and has a broad application range and good application prospects.

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