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Professional Motorsport Data Analysis: Analog to Digital Conversion

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Analog to Digital Conversion

05.42

00:00 - Many of the sensors we use in data acquisition are analog and because dataloggers record digitally, any analog signal needs to be converted to digital before it can be recorded.
00:12 Your typical analog sensors that you're going to see on most cars are things like throttle position, coolant temperature and brake pressure.
00:21 The output of an analog sensor is what we call continuous which we can think of as having a smooth output as we monitor it over time.
00:30 There are no breaks or steps in the signal.
00:33 In order for the smooth signal to be recorded by a modern digital logging system, the sensors measurement range must be broken up into discrete sections.
00:43 Let's look at an example to get a better understanding of that.
00:47 Here we have a generic analog signal which is changing over time.
00:51 We can see the signal is continuous, meaning that the changes in the output are smooth.
00:57 The red data helps us visualise what a conversion to a digital signal might look like.
01:02 We can see that the digital signal is not smooth and is broken up into a number of even steps.
01:09 The size of these steps determines how much the true analog signal must change before the digital signal will also change.
01:17 The conversion from analog to digital is generally handled by the logger itself.
01:22 Or sometimes, by an external expansion module that increases the number of analog inputs the logger can accept.
01:29 What we need to be careful of is that the steps in the digital signal we log are small enough that the signal is useful to us.
01:37 If the conversion is too coarse, we'll end up with a signal that doesn't accurately represent the parameter we're trying to measure.
01:45 Here we can see the same analog signal we looked at before, represented by a few different analog to digital conversions of differing step size, each with a different colour.
01:56 We can see that the number of steps is critical to giving us a useful representation of the signal.
02:02 To get the result we want, we need to match the sensor and the logger for the application of interest.
02:09 To do this, we need to go to the spec sheet of both the logger and the sensor.
02:14 If we're only looking from the perspective of analog to digital conversion, the logging parameter we're most interested in is the resolution for that particular input.
02:23 This will typically be expressed in bits and will have a voltage range associated with it.
02:29 The number of bits describes the resolution which is analogous to the number of steps that we saw in the previous example.
02:36 For example, let's take the case of a logger with an analog input that uses an 8 bit resolution for a 0-5 volt range.
02:46 To convert the number of bits to the number of steps available in this voltage range, it will be 2 to the power of the number of the number of bits.
02:54 So over a 0-5 volt range, with an 8 bit analog to digital converter, you have a maximum resolution of 256 steps.
03:03 This means that there are only 256 discrete values or steps your logger can record.
03:10 There are a number of important parameters to consider on the sensor side but from the perspective of analog to digital conversion, the primary one is the voltage range it operates over.
03:21 Let's look at an example to illustrate how we might match a suitable logger to our application.
03:26 Let's say we're interested in measuring the position of a steering rack with a linear potentiometer and we're in the market for a new logger.
03:33 There are 2 different potential loggers that we're looking at which each have different specifications for analog inputs.
03:41 Logger A has a resolution of 8 bits over a 0-5 volt range and logger B has a resolution of 12 bits over 0-5 volt range.
03:51 The steering rack has a maximum travel from stop to stop of 90 mm that we need to measure over.
03:57 So our sensor will need at least that much travel.
04:00 We found a linear potentiometer with 150 mm stroke that will package well on the rack and it also fits our budget.
04:08 It's a 0-5 volt range sensor and we're only only going to be using 60% of the available travel of the sensor because we're only measuring 90 of the available 150 mm of travel of the sensor which means we're only using 60% of the available voltage range.
04:25 Approximately 3 volts.
04:27 In turns, this means we're only using 60% of the resolution available to us in the analog to digital converter inside the logger.
04:34 Let's look at our example, working through the maths, we can calculate the number of steps available, the number of effective steps we can make use of with this sensor in this arrangement and finally the maximum resolution we'll get.
04:48 You can see this means the smallest difference we can measure in the steering rack position using these components is plus or minus 0.59 mm.
04:58 Working through the same maths, but for logger B, this on the other hand means that the smallest difference we can measure in the steering rack position is plus or minus 0.037 mm.
05:09 Logger B is giving us over 10 times the resolution for this measurement.
05:14 Each situation will have its own requirements for the amount of resolution you need for each measurement and it's up to you to decide how much you need and how you'll use it.
05:24 Hopefully this has been a useful example of why matching both the logger and the sensor to your application's important.