At present, oil, chemical, pharmaceutical, metallurgical and other enterprises have a large amount of cool water tower hampers, special fans and special fans in use, and some fan has no monitoring instrument (especially cool water towers) or some simple local monitoring instruments. For a long time, due to poor operating environment, the monitoring and maintenance means is not perfect, the fan has continuously occurring reducer, the bearing is burned, the transmission shaft bending, the coupling diameter is damaged, and even the blade break. The operation of the fan is very important for this kind of company, so it is necessary to strengthen monitoring, diagnosis, management, and develop a new generation of monitoring diagnostic systems integrating split monitoring, remote monitoring, remote diagnosis and analysis. It is necessary, market demand Big.
Among them, the MAXL320-based fan monitor is a very important part of this monitoring diagnostic system, and its key components are the modes / digital conversion chip. The MaxL320 is a parallel 14-bit 8-channel synchronous A / D conversion chip introduced by Maxim, which is ideal for data acquisitions such as speed, acceleration, displacement, and process amount of fans. Here, a data acquisition interface scheme based on an ARM7 fan monitor is introduced. The A / D chip uses MaxL320, which uses Philips's LPC2290.
1, MAXL320 overview
The MAXL320 is an 8-input channel, a 14-bit high-speed mode / digital converter, a separate sampling / hold (T / H) circuit provides simultaneous sampling for each channel. The device has advanced / first out (FIFO) capabilities to reduce interface overhead and read conversion results between conversion ends or conversions.
1.1 pin and characteristics
The pin and its characteristics are as follows:
CH0 ~ CH7: is an 8-way analog input port.
D0 ~ D13: Output 14-bit parallel digital output, where DO ~ D7 is two-way data lines.
CS, RD, WR: Select signal, read signal, write signal, respectively.
Convst: Convert the start signal. CONVST low-level tracking acquisition analog signals, start conversion when rising.
EOC, EOLC: Conversion End Output (EOC), low level indicates that a channel conversion ends, and can read this channel conversion result. Final conversion knot
Beam output (EOLC), low level indicates the end of the last channel conversion, and then the conversion result of all channels is opened continuously.
CLK, INTCLK / EXTCLK: External Clock Input Pin (CLK), Clock Mode Selection (INTCLK / EXTCLK), which is connected to the internal clock (10 MHz), pick up. AGND selects the external clock input (100 kHz to 12.5 MHz).
Allon: Channel enable input. This pin is highly enabled to open all input channels (CH0 ~ CH7), and then the low level is only the selected channels for A / D conversion.
SHDN: Power down input pin. SHDN = O Select normal mode, SHDN = 1 Select the power down mode.
1.2 MaxL320 work process
The MAXL320 has eight analog input channels that can be used by settings of the register. When Allon is set to low, set the register selection to open the channel, and pull down the CS and WR, then write data to the data line D0 ~ D7, and D0 to D7 correspond to the channel CH0 ~ CH7, by line D0 ~ D7 to the data line D0 ~ D7 Write "1" to select the corresponding channel. If all the ALLON is set to high, all channels are opened without setting up configuration registers. When the analog input channel is determined, send a low level to the CONVST, that is, 1 mode / digital conversion. Suppose the 8-way model channel is used, then the 8 sample / holders inside the chip tracks the input signals before the convst rising edge arrival, and the input analog signal is sampled / held in its rising edge. Once the EOC has a falling edge, it indicates that all the conversion ends, once the falling edge occurs, it indicates that all the model / digital conversion ends. For the result of the conversion, there are two reading results: can read the data after each switch, then the next conversion (see Figure 1 (a)); can also wait until all channels After the conversion is completed, the EOLC signal is low during the entire reading process until the next falling edge of Convst (see Figure 1 (b)).
2. Application of MaxL320 in Data Acquisition of Fan Monitor
The fan monitor is based on the ARM7 processor, and the processor uses Philips's LPC2290. Its data acquisition part of the hardware block diagram is shown in Figure 2.
The actual fan monitor can selectively measure multiple signal inputs according to different requirements, so that the pin Al-Lon is to be grounded, and the register is written according to the requirements of the request, turn on the channel that needs to be opened, the unused channel is closed To reduce power consumption. Because the environment used in this monitor is more harsh, if an external clock is used, the input clock signal is easily interfered, causing the entire data acquisition module unstable, so the pin INTCLK / EXTCLK also picks +3.3 V To select the internal clock (10 MHz), 10 MHz frequencies 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 monitoring the monitoring of the monitoring of industrial fans is the wind turbine shaft vibration acceleration, speed, displacement. This monitor can be connected to the acceleration sensor, the speed sensor, the displacement sensor, and the specific simulation partial circuit block diagram is shown in Figure 3. The access is an ICP acceleration sensor, and is only a set of ICP analog inputs, which are divided into two inputs, AV, A_H, V_V, V_H, S_V, S_H, respectively, respectively, respectively. Output to the vertical direction and horizontal direction of acceleration, speed, displacement. If the speed sensor is used, the first stage output is the speed, the displacement of the second stage output, and the third stage circuit is meaningless. If the sensor is used, the first stage output is displacement, the second, and the third stage circuit is meaningless. The operational amplifiers used by low-pass filter circuits and integration circuits are used with integrated chip MAX4164, which integrates four low power consumption amplifiers. The low-pass filter circuit is a second-order low-pass filter, which increases the RC link to increase the decimation rate for ordinary first-order low-pass filter circuits, and increase the filtering effect better. The integrating circuit is the most typical integrated arithmetic circuit, and a 1μF capacitor is added to the input terminal to filter out the DC component, and the integral capacitor is parallel to prevent the saturation or cutoff caused by the low frequency signal gain and integral drift. It is generally greater than 10 times equal to the input resistance. The programmable amplifier uses LTC6911-2, which is a two-match programmable amplifier integrated chip that can be written to 3-bit programmable interfaces G1, G2, G3 to obtain 0, 1, 2, 4, 8, 16, 32. 64 output magnification.
3.2 Multiple Selection Circuits
The analog signal input of the fan monitor used for industrial scenes is generally more than 8 channels, so it can be there. The 8-channel input of the MAXL320 plus multiple selection circuits, which makes up multiple selection circuits with CD74HC4052. The CD74HC4052 is a dual power input, four sets of channel selection chips, by selecting S0, S1, can make any group in the output four groups, the maximum analog input range is ± 5 V.
4, experimental debugging
4.1 Programming
The underlying driver of MAXL320 is an integrated development environment ADSI. 2 Developed, where the A / D conversion software flow is shown in Figure 4.
In practical applications, in order to control the sampling frequency of the ADC, the design uses the timer to perform the process of performing the entire sample, read data, to change the sampling frequency based on the various requirements of the site. This has changed for the software design of A / D conversion: when the lower sampling frequency (100 Hz ~ 5 kHz) is used, the time is longer, because the software design of this monitor is based on μC. / OS-II embedded system, so the timer interrupt mode is used, which avoids the arrival of the multi-task operating system in sampling this task, reducing the operational efficiency of the multi-task operating system. Put the sample, read the data, read the entire process of the interrupt service, when the time is time, immediately jump to the interrupt service program to perform the sample reading operation, then jump out of the interrupt program, continue to perform the operation behind the main program; When using a relatively high sampling frequency (5 ~ 40 kHz), because the timing time is short, it can be queried by query, and the timer interrupt flag bit is queried. When the logo position bit, the sampling read operation is performed.
4.2 experimental test
The internal clock is used, and the 8 channel is opened, and the channels 0 to 7 are input to 1 kHz sine wave (2 V), and the D0 to D13 of the MAXL 320 is connected to the LPC2290 of the LPC2290, other corresponding The pin is connected according to Figure 3, starting A / D conversion, because data of 8 channels is the same, so only the value after the channel O conversion is read, and the results are shown in Table 1.
The above test results data is the sample point value of 1 kHz sine wave 1 cycle, a total of 38 points, where the positive part sampling point has 19 values, and the negative portion sampling point has 19 values, Table 1 lists only Some of them have a representative value. Target these sampling points on the coordinates, you can restore the input sine waves. It can be seen that the time and theoretical values used by the actual tracking capture signal and the sampling signal are basically the same. However, waiting for the EOLC signal to become low due to the operation of the operation, plus the result of the read conversion is the speed limit of the processor data bus itself, and the entire sampling frequency is lower than the ideal value. You can reduce these two factors on the sampling frequency by some modifications:
(1) The EOLC can be connected to the external interrupt signal pin of the processor, which is interrupted, which is more response speed than the original design;
(2) The number of CPU cycles occupied by the CPU clock cycle or reduces the read and write operations.
5 Conclusion
Generally, the output signal frequency of the general class ICP sensor is between 0.5 Hz ~ 6 kHz, and some high frequency noise is filtered off after low pass filtering. By the above experimental test results and the sampling time (0.3μs or so) and conversion time (3.7μs) measured (3.7 μs), the total time of the sample conversion is about 4 μs, so the throughput of each channel can be calculated. The amount is approximately 250ks / s, so it is fully able to meet the requirements of the field fan monitor data collection.
Editor in charge: GT, read full text
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