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Data Analysis Fundamentals: Sensor Calibration

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Sensor Calibration


00:00 - The sensors that we use to send data to our loggers will always have an electrical output signal.
00:06 When it comes to analysing the data, we're not interested in an electrical signal, we're interested in a physical quantity.
00:12 In order to convert that electrical signal to a physical quantity, we need to do a calibration inside our logger that helps the logger translate an electrical signal into a physical quantity that we're actually interested in.
00:26 A typical example of that is something like this pressure sensor.
00:29 So this is a sensor that would typically be used for something like measuring coolant pressure or inlet manifold pressure, it's got a physical connection to sense the pressure at one end an electrical connector at the other end that may be run either to an ECU or directly to our logger.
00:44 I just want to show you the spec sheet and we'll go through some of the details in that sheet which will help us with the calibration in the logger.
00:50 This is a typical example of what you'd see in a spec sheet that you'd be supplied when you buy a sensor.
00:56 So typically when you buy a sensor from a supplier they would give you some sort of information pack or spec sheet like this.
01:02 And this is really required in order for you to get the calibration right.
01:06 Starting at the top of the sheet, we've got a specification section here.
01:10 So it gives us some information about the physical connection that we have in the sensor so in this case it's an 1/8 NPT physical connection.
01:18 The electrical connector on it is listed here as well and that's really important when it comes time to spec our loom when it comes to deciding the type of plug that we're using and the connectors that we're going to have to fit.
01:31 This is a typical 5 volt sensor so it just tells us that we need to be supplying 5 volts +/- 0.25 volts there.
01:39 The type of output is going to be a ratiometric output.
01:42 We've got some information about the pinout that the sensor uses which is going to be required when it comes time to connect this sensor to our loom.
01:50 So we need our 5 volt signal, we need our 0 volt reference and our signal output, which this signal output here is going to be what we run to the logger, that's where the signal from the sensor is sent to the logger.
02:03 One of the most important parts that we're going to make use of is the calibration.
02:06 So this particular sensor we're using is a 50 psi and this is, there's some information given for different types of sensor that are packaged in the same body from the supplier.
02:17 So this line here with the 50 psi is the one that we'll be using for our calibration.
02:22 So we can see it gives us some values for voltage with respect to the pressure reading.
02:30 You'll see there's a couple of different inputs here for 0 psi, that'll become clearer why there's multiple inputs there for 0 psi when we do the calibration.
02:38 But essentially this is giving us a mapping between the output voltage and what that physical quantity means for a given output voltage.
02:46 In this case, this is giving us a linear range from 0.5 volts to 4.5 volts.
02:51 The fact that it's linear just means that if you interpolate between these two voltages, it's going to be a linear interpolation between these two physical pressures.
03:00 Opening up the configuration tool for the sensor that we're going to be calibrating today, this is just going to be for a MoTeC logger.
03:08 If I go to the connections tab here we've got a list of some of the inputs that this logger has currently got connected to it.
03:17 And I've got a spare line here for an analogue voltage input which is in this case, this type of sensor, this is an analogue voltage output so the input type that we're going to be using is an analogue voltage so I'm going to start with this spare one here.
03:33 If I just double click on that, in this case I'm going to set up a manifold pressure as an example.
03:41 So I can select that and that's in terms of a pressure in kPa.
03:46 So now if I go and define the relationship between the voltage and the pressure I'm just going to call this MSEL manifold pressure sensor calibration.
03:59 And the method we're going to use is ratiometric which is the typical one we're going to be using for these 0-5 volt sensors.
04:07 Now here I've got this table of calibrations and this is just giving me a way to map the voltage in this column to pressure outputs.
04:14 And we've got this information already supplied to us here in the spec sheet.
04:18 So if I just go ahead and add some of those in, so at 0 volts, I can read off the spec sheet here, that means 0 kPa.
04:25 At 0.5 volts I also have 0 kPa and if I go to the last column here at 4.5 volts, I'm going to have a reading of 344.74.
04:43 So here we've got the graph for these tabular inputs.
04:47 And you can see this area down here with the two entries at 0 volts, what that's doing is really just forcing an output of 0 to stop you getting any erroneous readings.
04:58 So if we didn't have that, anything below 0 volts if there was some small electrical noise in the system, you would end up getting a negative pressure reading which is a little bit meaningless for us so we just force anything below zero volts to read 0 kPa as well.
05:16 We can see here we've got a linear interpolation just from the 0.5 volts at 0 kPa right up 4.5 volts at 344.7 kPa.
05:27 So this is how the logger's going to interpret this signal based on the voltage input.
05:31 It's going to come along here for a given voltage input, it's going to look up the physical quantity and that quantity is what's going to be logged inside the logger so we won't be dealing with voltages, we'll be dealing with physical quantities inside the logger.
05:44 So right now that's all set up, the next thing we would need to do is just set up that channel to actually be logged in the logger but that's as far as we need to go with the calibration for a sensor like that.
05:54 That general method applies to most sensors you're likely to buy.
05:58 In almost all cases you're going to be supplied a spec sheet with that sensor that's going to allow you to do this calibration without doing any measurements yourself.