Data acquisition interface solution for ARM7 fan monitor

2 Application of MAXl320 in Data Acquisition of Fan Monitor

The fan monitor is based on the ARM7 processor, which uses the LPC2290 from Philips. The hardware block diagram of its data acquisition part is shown in Figure 2.

The actual fan monitor can selectively measure multiple signal inputs at the same time according to different requirements in different time periods, so the pin AL-LON is grounded here, and the configuration register is written according to the requirements to open the channels that need to be opened, and the unused channels are closed to reduce power consumption. Considering that the environment used by this monitor is relatively harsh, if an external clock is used, the input clock signal is easily interfered, which will cause the entire data acquisition module to be unstable, so the pin INTCLK/EXTCLK is also connected to +3.3 V, and the internal clock (10 MHz) is selected. The 10 MHz frequency can fully meet the sampling requirements of this monitor.

3 Design of MAxl320 peripheral analog circuit

3.1 Analog Input Circuit

The most common measurement signal parameters monitored by industrial fan monitors are fan shaft vibration acceleration, velocity, and displacement. This monitor can be connected to acceleration sensors, velocity sensors, and displacement sensors. The specific analog circuit block diagram is shown in Figure 3. The ICP acceleration sensor connected in Figure 3 is only a set of ICP analog inputs. It is divided into two inputs of ICP_V (vertical direction) and ICP_H (horizontal direction). AV, A_H, V_V, V_H, S_V, and S_H are the vertical and horizontal outputs of acceleration, velocity, and displacement respectively. If a velocity sensor is used, the first stage outputs velocity, the second stage outputs displacement, and the third stage circuit is meaningless. If a displacement sensor is used, the first stage outputs displacement, and the second and third stage circuits are meaningless. The operational amplifiers used in the low-pass filter circuit and the integration circuit are all integrated chips MAX4164, which integrates 4 low-power operational amplifiers. The low-pass filter circuit uses a second-order low-pass filter. For the ordinary first-order low-pass filter circuit, an RC link is added to increase the attenuation slope to make the filtering effect better. The integration circuit is the most typical integration operation circuit. A 1μF capacitor is added to the input end to filter out the DC component. A resistor is connected in parallel to the integration capacitor to prevent saturation or cutoff caused by excessive gain of low-frequency signals and integration drift. The value is generally greater than or equal to 10 times the input resistance. The programmable amplifier uses LTC6911-2, which is a two-matched programmable amplifier integrated chip. By writing values ​​to the 3-bit programmable interface G1, G2, G3, 0, 1, 2, 4, 8, 16, 32, 64 output magnifications can be obtained.

3.2 Multiplexer Circuit

The analog signal input of the fan monitor used in industrial sites is generally more than 8 channels, so a multi-channel selection circuit can be added to the 8-channel input of MAXl320. The monitor uses CD74HC4052 to form a multi-channel selection circuit. CD74HC4052 is a dual power input, four-group channel selection chip. By selecting S0 and S1, any of the four groups can be output, and the maximum analog input range is ±5 V.

4 Experimental debugging

4.1 Programming

The bottom-level driver of MAXl320 is developed in the integrated development environment ADSI.2, and the A/D conversion software flow is shown in Figure 4.

In order to control the sampling frequency of ADC in practical applications, this design uses a timer to perform timing operations on the entire sampling and data reading process of A/D, so that the monitor can change the sampling frequency according to various requirements on site. Among them, the software design of A/D conversion has changed: when a relatively low sampling frequency (100 Hz~5 kHz) is used, the timing time is relatively long. Because the software design of this monitor is based on the μC/Os-Ⅱ embedded system, the timer interrupt method is used, which will avoid waiting for the timing to arrive in the sampling task and reduce the operating efficiency of the multi-task operating system. Put the entire process of sampling and reading data in the interrupt service program. When the timing time is up, it will immediately jump to the interrupt service program to perform the sampling and reading operation, and then jump out of the interrupt program to continue to execute the operations behind the main program; when a relatively high sampling frequency (5~40 kHz) is used, because the timing time is very short, the query method can be used to query the timer interrupt flag bit all the time. When the interrupt flag is set, the sampling and reading operation is performed.

4.2 Experimental Test

Use the internal clock and turn on all 8 channels. Input a 1 kHz sine wave (peak-to-peak value is 2 V) to channels 0 to 7. Connect D0 to D13 of MAXl320 and D0 to D13 of LPC2290. Connect the other corresponding pins according to Figure 3. Start A/D conversion. Because the data of the 8 channels are the same, only read the converted value of channel O. The results are shown in Table 1.

The above test result data is the sampling point value of sampling 1 kHz sine wave for 1 cycle. A total of 38 points are sampled, of which 19 are positive and 19 are negative. Table 1 lists only some representative values. By marking these sampling points on the coordinates, the input sine wave can be restored. Through the oscilloscope, it can be seen that the time used to track and capture the actual signal and the sampling signal is basically the same as the theoretical value. However, waiting for the EOLC signal to become low takes time to execute the program itself, and the reading of the conversion result is limited by the speed of the processor data bus itself, so the entire sampling frequency is lower than the ideal value. Some improvements can be made to reduce the impact of these two factors on the sampling frequency, namely:

(1) EOLC can be connected to the external interrupt signal pin of the processor and the interrupt method can be used, which has a faster response speed than the original query method;

(2) It can increase the CPU clock cycle or reduce the number of CPU cycles occupied by read and write operations.

5 Conclusion

From the above experimental test results and the sampling time (about 0.3μs) and conversion time (3.7μs) measured by the oscilloscope, it can be seen that when the eight channels work simultaneously, the total sampling and conversion time is about 4μs, so it can be calculated that the throughput of each channel is about 250kS/s, which can fully meet the data acquisition requirements of the on-site wind turbine monitor.