Data Analysis Fundamentals: Calibrating Steering Angle
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Calibrating Steering Angle
06.20
| 00:00 | - When we're looking at logged steering angle data, sometimes we're most interested in the steering wheel angle and other times we're interested in the angle the tyres are pointing with respect to the chassis. |
| 00:10 | The most important thing is that we have the same way of checking it from one log file to another so we can make comparisons in the driving style. |
| 00:18 | The amount that the actual tyres will rotate with respect to the chassis for a given amount of steering angle input will depend on the steering rack ratio and the kinematics of the suspension. |
| 00:28 | Most often the part of the steering system we're actually measuring is the angle of the steering column. |
| 00:33 | So that means that's giving us the amount that the steering wheel has turned. |
| 00:37 | If we want to know how far the actual tyres are rotating in a corner, we need to know the relationship between the steering angle input and the tyres. |
| 00:44 | In this example, I'm going to use our RaceCraft GT86 shop car to give you a demonstration how you can make a mapping between the amount of steering input you have and the amount the tyres actually rotate on the car. |
| 00:56 | Depending on the steering geometry you have in your car, the inside and outside wheels will actually turn by different amounts as you apply steering input. |
| 01:03 | The wheel we're most interested in is the outside front tyre in the corner. |
| 01:07 | That's because the outside front tyre has the most effect on grip because it's the most heavily loaded tyre. |
| 01:14 | This means for left hand corners, the only wheel we're interested in is the front right because that's going to be the loaded tyre in a left hand corner. |
| 01:22 | All I'm going to do here is measure the steering input that we get on the steering wheel and I'll also measure the amount the front right wheel has physically rotated for a given amount of steering input. |
| 01:32 | Then I'll do that in a number of steps and we'll get a relationship between them that we can put in the logger so we can log that value directly. |
| 01:39 | I've got my laptop set up here on the car and it's connected to the logger with the ignition on. |
| 01:44 | That's going to allow me to read off live what the logged value is for steering wheel angle. |
| 01:50 | I've got a MoTeC logger fitted to this car and here I've got the MoTeC configuration software open. |
| 01:54 | I'm already connected to the car and the ignition's on so if I go online and monitor channels, if I just come down to steering. |
| 02:06 | We can see here that the steering wheel angle is shown and if I just rotate the steering wheel back and forward, we can see that's changing live. |
| 02:13 | So what I'm going to do for a start is just get that so that is sitting very close to zero to straight ahead and that's plenty close enough there. |
| 02:25 | Now that I've got the steering facing straight ahead, I'm going to go ahead and use this angle gauge and measure the angle of the turn plates that are sitting underneath the front wheels. |
| 02:33 | I'm then going to increment that steering input and take an angle measurement at each point. |
| 02:37 | I've got this really simple spreadsheet set up here. |
| 02:39 | All I'm going to do is measure my steering wheel angle and I'll input them here in this column and I'll also measure the angle of the tyre on the turn plate and I'm just going to do a small correction here for any offset that we have at the start. |
| 02:51 | This is going to give me a plot of the relationship between the input which is the steering wheel angle sensor and output which is the angle of the actual wheel on the front right of the car. |
| 03:01 | So if I head down and just give myself a reference measurement to start with here on the turn plate. |
| 03:10 | I've got an angle of 1.7°. |
| 03:14 | So here I've got a measured steering wheel angle of 0.2° and I've input that here in the spreadsheet as well as measured tyre angle on the turn plate which is our reference starting point which is 1.7°. |
| 03:27 | So next I'm just going to go in about 20° increments here and that should give us enough resolution for mapping that front wheel rotation to the steering wheel angle. |
| 03:37 | Because we're doing the front right here as our calibration, this is for left hand corners so that means we need to turn the steering wheel to the left. |
| 03:44 | So I'm just going to rotate that until we get to about 20°. |
| 03:53 | So there we've got 20.6 so I'll type that in. |
| 03:59 | And in the measured, just take a new measurement here. |
| 04:07 | And we're straight on 0° on that one. |
| 04:12 | So then if I do another 20°. |
| 04:41 | So now that I've measured that relationship between the input steering angle and the output of how far the tyres are rotating, I can go and take that data and input it into a table in the logger so we can log that output channel of the tyre angle directly inside the logger. |
| 04:55 | One of the things to remember here is that because we did that calibration just for the front right, we're going to take a mirror image and invert the readings for the left front. |
| 05:03 | So that means no matter whether we're turning left or right, we're going to get the right polarity for our signal. |
| 05:08 | So here I've got my steering ratio output from the measured steering wheel angle across the top here to the output of the steered angle of the front right wheel. |
| 05:17 | We're going to take this, I'm going to transfer it into a table into my logger configuration. |
| 05:37 | On the left side of the screen you've got the values that I took when we were doing the measurements and then I've, on the right side of the screen you can see where I've put those values into the MoTeC table and I've got them plotted there. |
| 05:48 | One of the things to realise is that plot is mirrored about the vertical axis there. |
| 05:54 | So you can see that covers for both left and right hand corners. |
| 05:58 | So that process for calibrating a sensor like that is pretty generic, you can apply it to almost any situation where you need to define an input and output relationship with something like a table. |
| 06:08 | Obviously if we make any mechanical changes to our steering system, maybe that would be different length steering arms or a different ratio steering rack, we would need to do a new calibration to match this. |
