Delays, delays, delays

Can’t you signals just work together?

Usually, in a data acquisition program,  all the signals you measure are “live”, meaning they represent the current conditions at the time they are sampled. However, in some cases you might have some signals which are not live, but delayed. For example, suppose you’re measuring engine operation, and you have gas analyzers sampling the exhaust airstream. These analyzers conduct the gas from the measurement point in the airflow system around the engine to the analyzer mechanism itself. This gas flows through the sampling tubing at a specific rate, and therefore arrives at the sample point at a specific time AFTER it left the main airstream. These delays might be multiple seconds in duration.  There could be other reasons for this delay, such as an echo-measuring device, or the mechanical response time of some piece of hardware.

For analysis purposes, you want to look at the engine holistically and see causes and effects. If the speed changed at a particular point in time, you want to see the CO concentration change as a consequence. But unless you do something to compensate, the change in gas concentration will lag far behind the change in speed that caused it in graphs and tables, making it difficult to judge consequences.

To arrange the signals back into time-synchronous alignment, the solution is to think of ALL signals has having TWO delay lines: one physical, and one logical.  In our example case, the physical part is the gas piping conducting the gas. The logical part is completely within the  DAS software. If EVERY channel has a physical delay time (even if it’s zero), and if EVERY channel also has a logical delay time (even if it’s zero), and if EVERY channel has the sum of the physical delay time and the logical delay time equal to a constant, then the signals coming out of the logical delay lines will be time-aligned.

Example

For example, consider three channels: Channel A records data “live”, i.e. no delay. Channel B has a delay of 3 seconds. Channel C has a delay of 5 seconds.

We want to make the entire chain (physical + logical) the same length for all channels, so we pick the longest delay (5 sec) and make that our total delay time. For channel A, which has no physical delay, we have to have a logical delay of 5 sec. For channel B, which has a physical delay of 3 sec, we add a logical delay of 2 sec for a total of 5. For channel C, which has a physical delay of 5 seconds, we have a logical delay of zero. 5+0 = 3+2 = 0+5, so every channel has the same delay, when we count both physical and logical.

The logical delay is implemented in queues with every channel having its own queue. Every sample received goes into a queue belonging to that channel; and we pull out one sample from the end of the queue to be recorded. Since one goes in and one comes out, the length of the queue never changes. The initial length of the queue corresponds to the delay time we need for a particular channel. If a queue is empty (of zero length), then the sample we put in and the sample we take out are the same sample. If the queue is of length 10, then the sample we get out was taken 10 sample times ago.

At configuration time, the queues are set up, and populated with zero values to set their length according to how much delay is required. After that, the sample process means putting a sample into a queue and taking one out for use.  Every channel operates the same when actually sampling, it’s only the configuration where they differ.

Note that if you are recording a TIMESTAMP value, then the TIMESTAMP signal requires its own queue as well, so that it comes out in alignment with the signals. Of course this means that the data does not become usable until all the zeroes have been flushed out of the queues. That will happen when the longest delay time has expired. We can judge this by putting a RECORD signal into its own queue. We don’t actually record data coming out of the queues until the RECORD signal coming out is true.

You can keep the data flowing into (and out of) these queues all the time.  When you want to start recording, set the RECORD flag TRUE. When you want to stop recording, set the RECORD flag FALSE.  Pay attention to the value out of the RECORD flag’s queue, and act appropriately. (Don’t act on the RECORD flag itself, act on the delayed RECORD flag).

This also means that when the test is over, there is still data remaining in the queues. We have to keep recording until the valuable data has worked it’s way through the queues, meaning 5 seconds after the last sample (in the example case).

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