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Professional Motorsport Data Analysis: Shock Travel Potentiometer

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Shock Travel Potentiometer

08.52

00:00 - Damper position can be an extremely useful measurement that we'll refer to often in data analysis.
00:06 As it can tell us a lot about how the car is behaving or sometimes misbehaving on track.
00:11 To measure the position of the dampers, either linear or rotary potentiometers are generally used.
00:18 There are many considerations we need to factor in when choosing the potentiometer that will best suit our application.
00:25 But price, packaging, installation, damage tolerance, weather and temperature rating and the voltage range all need to be considered.
00:35 Getting and studying the specification sheets of different damper potentiometers is a non negotiable when we're selecting suitable sensors for our application.
00:44 Putting some effort into researching your options early on this will save you time.
00:48 Each application and suspension layout will determine which packaging constraints are going to apply.
00:55 In some cases, the damper position can be measured directly when it's possible to mount the sensor in parallel with the damper.
01:01 However, in many cases, it's more practical to indirectly measure the damper position by attaching the sensor between some other part of the suspension like an upper or lower control arm and the chassis.
01:14 The installation of a linear potentiometer is generally quite straightforward with each end directly connecting to either the sprung mass or the suspension component via a spherical bearing at each end.
01:26 When using a rotary sensor, the sensor itself needs to be rigidly attached to the chassis with a push rod connecting the moving sensor arm to the suspension component.
01:35 Regardless of whether a linear or rotary sensors is used, care needs to be taken that for all possible suspension positions, the sensor is within is safe travel limits and it's not fouling on any other components.
01:49 This means we need to check at full compression and rebound positions as well as full steering lock in each direction for all combinations.
01:58 We also need to be careful that the spherical bearings actuating the sensor have not run out of angular travel which would otherwise cause damage to the sensor.
02:07 The sensor should be selected and mounted in a way that makes as much use as possible of its available travel.
02:13 So what do we mean by that? Well it would be a poor choice to use a linear potentiometer with 200 mm of travel where the measured maximum travel was only 40 mm.
02:24 As we discussed in the analog to digital conversion section, by doing that, we'd be unnecessarily reducing our measurement precision.
02:32 The same applies to a rotary sensor, where we want to make sure that we're using the maximum practical angular travel of the sensor.
02:39 Maximising that travel is influenced both by the choice of the sensor and how it's mounted.
02:45 Because the positon of the suspension we choose to mount our sensor will determine the amount of travel required.
02:51 Although it's never fun to think about crash damage in motorsport, we should also put consideration into the effect that a big shunt might have on damaging the sensor.
03:00 In some cases, using a rotary sensor can be a good option to reduce the likelihood of damage to the sensor in the case of an accident.
03:09 Because it takes up less space and is generally a bit safer mounted further away from all the moving suspension components.
03:16 We also need to factor in water and dirt resistance as damper position sensors are often operated in relatively harsh environments.
03:24 Each sensor manufacturer will provide an IP or ingress protection rating for each sensor model.
03:31 And it's worth getting the advice of the sensor dealer on this topic.
03:34 They should have a good idea about what level of IP is appropriate for your use case.
03:39 It's important to understand that there are different sensing technologies available, some of which are more resiliant to water and dirt ingress than others.
03:48 This for example is one area where rotary sensors can have an advantage as they tend to have their sensitive components naturally shielded relative to a linear sensor.
03:58 A number of OEM applications use rotary sensors for suspension position which is generally a good indication of the expected longevity.
04:07 Once we've selected and mounted our chosen sensor, the next step is to calibrate it.
04:12 This is where we map the voltage output of the sensor to the travel of the damper.
04:16 Meaning that once the data is logged, we'll be looking at damper position rather than sensor voltage.
04:23 To do this, we need a suitably accurate way of measuring the damper position and a way of consistently moving the suspension through its stroke.
04:30 Using a set of digital vernier callipers and a hydraulic jack is sufficient.
04:35 Make sure to remove the spring, bump stops and anti roll bar connection so that each corner of the car can move freely and independently so that we can calibrate each sensor between the maximum extremes of suspension travel from full droop to full compression.
04:52 Starting at full droop, find a convenient and repeatable place to measure the damper position then zero the vernier callipers.
04:59 This will be our reference, from there, compress the suspension at approximately 10 even steps.
05:05 At each step, record the voltage output of the sensor and the length value of the damper.
05:11 In many logging systems, there's a sensor calibration area where you can click a button at each step in the calibration, enter the damper position value you're measuring with the callipers and the voltage will be automatically recorded at each step.
05:25 In others, you may have to manually enter the values into a damper position vs sensor voltage table.
05:32 It's important to always read this voltage through the logging system rather than measuring directly from the sensor.
05:38 Because this way it means we're always referencing the same voltage that the logger is seeing.
05:43 The goal when plotting a voltage vs position graph is that it should show a smooth relationship with no unusual steps along the way.
05:51 As this tells us we've done a good job in accurately calibrating the sensor which is what we see here in this example.
05:58 Now that we've calibrated our sensors, we can now log differences in damper position over time.
06:04 While we'll make use of damper movement in our analysis, we're often also interested in wheel movement.
06:09 To convert between wheel and damper position, we need to measure the ratio between wheel movement and damper movement.
06:16 Which is often referred to as the motion ratio.
06:19 The particular geometry of a suspension determines this ratio and how much this ratio changes as the suspension moves.
06:27 While it's ideal to define the motion ratio as a table where the ratio is recorded in steps to understand how it changes with suspension position, this introduces some complexities that are a little beyond the scope of this course.
06:40 For the analysis we'll be doing in this course, and in fact, for many situations, it's acceptable to simplify the motion ratio to be a constant value.
06:50 This generally means you have a single motion ratio defined for the front and rear suspension separately.
06:57 To measure the motion ratio, we need to have a known amount of wheel and damper movement that we'll record in steps.
07:03 There are plenty of ways this can be done but here's one simple method.
07:08 In the same way we calibrated the damper position, first remove the spring, anti roll bar and bump stop for each corner of the car.
07:17 We'll be using the live logger output of damper position throughout this process to record the damper position.
07:24 We're interested in measuring the vertical movement of the wheel, relative to the chassis so the first thing we need to decide on is a suitable place to measure wheel position.
07:33 One way is to use a tape to measure the distance from a convenient place on the wheel to a fixed point on the fender or any other convenient place on the chassis.
07:43 You can then use a jack on a suitable part of the suspension to change the wheel position.
07:48 With the suspension in full droop, record the wheel position relative to the fender.
07:54 Then jack the suspension up and record the damper position at each step until the suspension is almost fully compressed.
08:02 Taking care not to damage anything as we go.
08:05 We want to do this in around 10 equal steps from full droop to full compression.
08:11 Now that we have a wheel and damper position table fully populated, we can now calculate the difference in wheel and damper position at each step.
08:19 With that, we calculate the motion ratio at each step by dividing the wheel movement by the damper movement.
08:26 With all the motion ratio values recorded at each step, we can now take the average of all the motion ratios over the full travel of the suspension.
08:35 This average is what we'll use in later calculations.