Design of Micro Inertial Measurement Device Based on ARM and MEMS Devices

In the study of bionic propulsion mechanism, accurate measurement of fish tail fin flapping parameters is of great significance for the study of fish bionic propulsion mechanism and engineering application; however, most researchers currently use the observation method of analyzing images taken by high-speed cameras to obtain parameters. This method is limited by the environment and equipment, and the results are less accurate. This design is a micro-inertial measurement device for biological motion based on MEMS devices. The device is used to achieve accurate measurement of the SPC-III robot fish tail fin flapping parameters, laying the foundation for the first domestic live fish tail fin flapping parameter measurement experiment using MEMS devices, and providing support for the design theory of robot fish bionic propulsion.

1 Design requirements and system structure

According to the biological characteristics of living fish and the characteristics of the experiment itself, the micro-inertial measurement unit should meet the following design requirements: small size, light weight, low power consumption, high acquisition frequency and acquisition accuracy, and good waterproof sealing performance. In order to achieve these requirements, the hardware of the micro-inertial measurement unit consists of two parts: ① microprocessor unit; ② micro-inertial sensor unit. The microprocessor unit mainly includes a microprocessor, an A/D conversion chip and Flash. As the core unit, the microprocessor connects to the A/D conversion chip through the SPI1 port to complete data acquisition, connects to the Flash through the SPIO port to complete data storage, and communicates with the host computer through the serial port. The micro-inertial sensor unit is composed of a MEMS accelerometer and a MEMS gyroscope to complete the task of collecting the original information of acceleration and angular velocity. After the collected original information is processed by A/D conversion, it is written into the Flash chip for storage, or sent directly to the host computer through the serial port for processing. The schematic diagram of the system principle is shown in Figure 1.

2 Microprocessor Unit

2.1 LPC2129 Processor

This device requires the microprocessor to have certain processing capabilities and low power consumption and small size, so Philips' LPC2129 is selected. LPC2129 is based on a 16/32-bit ARM7TDMI-SCPU that supports real-time simulation and tracing, and has 16 KB on-chip SRAM and 256KB embedded high-speed on-chip Flash memory. LPC2129 has a smaller LQFP64 package, extremely low power consumption, multiple 32-bit timers, 4-channel 10-bit ADC, 9 external interrupts, and up to 46 GPIOs.

In the software design of LPC2129, the commonly used uC/OS-II or uClinux operating system on ARM is not used, but a foreground and background timing interrupt structure is used. This foreground and background timing interrupt structure is more suitable for control systems with high real-time requirements, which can ensure that the control loop delay is within a designed range, and the priority relationship between each module is very clear, which is more convenient to use.

2.2 A/D conversion acquisition chip ADS1256

The A/D conversion chip uses the 24-bit serial analog/digital converter ADS1256 from TI of the United States. It can provide up to 23 bits of noise-free accuracy and a data rate of up to 30 ksps. ADS1256 uses a four-wire SPI communication method and is connected to the SPI1 interface of LPC2129, which allows flexible and convenient communication.

ADS1256 adopts a multi-channel cyclic acquisition mode. After the data preparation signal DRDY prompts that data can be extracted, the current acquisition channel is first changed to the next acquisition channel, a new acquisition conversion is started, and then the data in the A/D conversion register is immediately extracted (the data at this time is actually the data converted in the previous round). This method can realize the acquisition and conversion of new data while extracting data, which is a highly efficient working method.

2.3 Flash chip AT45DB041B

Flash1 uses Atmel's programmable serial access chip AT45DB041B. The main storage unit is divided into 2048 pages, each page is 264 bytes, and has two 264-byte static random access memories as data buffers.

AT45DB04lB is connected to the SPI0 interface of LPC2129. The collected data is first written into the buffer 2 of Flash, and then the data in the buffer is written into the main memory page of Flash for preservation, pending offline data analysis and processing.

3 Micro Inertial Sensor Unit

The MEMS sensor unit of the micro inertial measurement device is composed of a micromechanical gyroscope and a micro accelerometer, which can accurately measure one axial angular velocity information and one axial acceleration information of the carrier.

3.1 ADXRS300 Single Angular Rate Gyroscope

ADXRS300 is an angular velocity sensor based on MEMS technology produced by Analog Devices, Inc. of the United States. ADXRS300 is powered by a +5V power supply, and the range of measuring yaw angular velocity is ±300rad/s, the sensitivity is 5mV/(rad·s-1), and the zero output voltage is 2.5V. The range, bandwidth and zero output voltage of the angular velocity can be set by external resistors and capacitors. It adopts BGA-32 packaging technology, the outer size is only 7mm×7mm×3mm, and the weight is only 0.5g. [page]

Assume that the measured angular velocity is αv, in units of (°)/s; the output voltage is Uo, in units of mV; the sensitivity K is 5mV/(°)·s-1, and the zero position output voltage is 2.5V, then there is the relationship:

3.2 ADXLl50 single-axis accelerometer

ADXL150 is a single-axis micro-accelerometer based on MEMS technology produced by Analog Devices, Inc. The main performance characteristics of ADXL150 are: zero-position output bias voltage is Us/2, measurement range is ±50g, sensitivity coefficient is 38mV/g, nonlinearity is 0.2%, zero acceleration drift is 0.2g; it can work with 4~6V power supply; power consumption is very low, and the static current is only 1.8~3.5mA.

Assume that the measured acceleration is av, in g; the output voltage is Uo, in mV; the sensitivity is K, in mV/g; the power supply voltage is Vs, in mV, then there is the relationship:

4 Measurement principles of speed, angle and displacement

After the angular velocity and acceleration information are integrated and calculated, the angle, velocity, displacement and other information can be obtained. Therefore, the device can measure various motion state information of the carrier based on the angular velocity and acceleration information of the carrier.

Assume that the sampling period is T, v(k) is the velocity at time kT, x(k) is the displacement at time kT, a(k) is the acceleration at time kT, and a(k) is the continuous true value of the acceleration. Then, the following integral formula for solving the velocity is obtained:

The area enclosed in the interval [kT, (k+1)T]. Knowing that the sampling values ​​of acceleration at kT and (k+1)T are a(k) and a(k+1), the shape enclosed by acceleration a(t) in the time interval [kT, (k+1)T] can be approximated as a rectangle or trapezoid. The trapezoid is more accurate, as shown in Figure 2.