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LabVIEW continuous sampling and limited sampling mode

2022-06-28 00:27:00 【LabVIEW development】

LabVIEW Continuous sampling and finite sampling mode

Continuous sampling 、N sampling 、 Or other sampling modes , What do they mean ？

In each case, a suitable sampling timing mode is required , And each sampling timing makes a specific DAQ The programming process makes more sense . Before we start , It is necessary to describe the two steps for converting measurements into numbers in your computer .

The first step is by DAQ Hardware determined . This step is to actually convert the input voltage value into RAM The process of bits in a buffer . This is by the analog-to-digital converter in the hardware (ADC) Realized .

The second step is to read out the bits of the buffer . In most cases, these bits are read in the format of floating-point numbers , And this is done by the computer CPU and CPU Program implementation in .

The frequency of these two steps is the main difference between continuous sampling and finite point sampling .

Continuous sampling means DAQ The hardware continuously follows the rate set by the sampling clock , Converts the input voltage into bits of the buffer . If you are worried that the delay of the computer or program will lead to missing sampling points , Then continuous sampling would be a good choice .DAQ The speed of hardware sampling is always determined by the sampling clock setting , The acquisition of this part is completely realized by hardware . Hardware sampling is very fast and stable , At the same time, the software reads asynchronously , And at an indefinite rate . To solve this conflict ,DAQ The board adopts the structure of buffer .

For continuous sampling tasks , When “ Start task ”vi When it's running , The collection begins . Because it requires certain expenses and CPU Interaction , This vi Only allow to prevent in While The front of the loop . after , In a cycle ,DAQmx Start actually reading the buffer . So it explains why you read a lot of points at once , Because if you read too little at a time , It will cause buffer overflow .DAQ The drive only allows two or more samples to be read in one continuous sample . If you find that you only need one sample at a time , Then you should consider redesigning the program or sampling mode . If the set number of read points ( Number of samples per channel ) Exceeded the data prepared by the buffer , that DAQmx Read vi It will wait until there are enough sampling points . This will show that there will be a certain delay in the graphic refresh process , However, it does not mean that some sampling points have been omitted . The number of sampling points is set to -1, As many sampling points as possible will be read each time .

N sampling ( Also called finite point sampling ) It is used for the acquisition task when the total number of sampling points required is known , Especially for sampling tasks with triggers . for example , It may be necessary to acquire after the start of a trigger signal 10 Seconds of data , You can use finite point sampling . The finite point pattern only buffers calls “ Start task ” This VI When .DAQmx Function will read the data from the buffer , Therefore, you need to set the number of samples N. This task is related to While Loops or state machines are matched .

But unlike continuous sampling , Must be called in each loop “ Start task ”, Then proceed “ Read ”, Next “ Stop task ”. otherwise , otherwise DAQ The driver will continue to try to read the data , Although the sampling requirements are over , This will lead to errors . It should be noted that , The time interval between each sampling is timed by the software . If you need to accurately control the timing problem , Continuous sampling is recommended .

Single sampling is performed through hardware timing or software timing, and the previously mentioned N The sampling is consistent . If you need to process or react to the data every time , And not very concerned about accurate timing , So this sampling method is worth recommending . such as , A battery is slowly discharging , The whole process takes a few minutes or hours . In the process , Each time the voltage is lower than a set value , Just send an email to the controller . This is the case with single sampling . Usually , Sampling in “ Start task ” Start when called . If it is hardware timed , Consider hardware synchronization . The sampling point will “ Start task ”VI The next sampling clock after being called is collected .

Select the appropriate sampling method , Program correctly , Will write the most effective program , It will also avoid the occurrence of error messages .