Measuring the angle of front wheels.

nikpro
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Joined: Tue Jan 23, 2007 12:29 pm

Measuring the angle of front wheels.

Postby nikpro » Tue Mar 31, 2009 11:25 pm

I have fitted a steering angle sensor and I am wondering how to calculate the angle of the steered wheels.

I have got a user defined variable which, using a formula of corner radius and g gives me the required steering angle and I want to compare this against actual steered angle to give me a handling channel.

I plan on putting the front wheels on turnplates to callibrate the sensor but, due to ackerman, the angle of the nearside and offside wheel will be different at any given steering wheel position.

Logic tells me to use the heavilly weighted (Outside) wheel for callibration - is this correct or do I use a mean value from both front wheels?

thanks.
Fraser E White

tristancliffe
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Location: Norwich, UK
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Postby tristancliffe » Wed Apr 01, 2009 9:46 am

I'd start with the outside angle, but you might find the average or inner gives a better result for your comparisons. Calculated steering angles based on corner radii etc are incredibly inaccurate really, as there is no such thing as 'corner radii' - it's changing ALL the time based on line, bumps, camber, wind, tyre condition, driver inputs etc etc. If you can get it to within +-1° you will be doing incredibly well.
Monoposto 2000 - Reynard 883 Toyota

nikpro
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Joined: Tue Jan 23, 2007 12:29 pm

Postby nikpro » Thu Apr 02, 2009 11:27 pm

We have found that the required steering angle is very accurately when you drive the car well within the vehicle limits - this is done on the grid formation lap prior to a race or a period behind the safety car.

We have the sensor for sterring input in volts and when you adjust the scale it overlaps the required steering angle nicley- knowing it in degrees would be more helpful for some theories we have though?
Fraser E White

osborni
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Location: USA, Michigan

Postby osborni » Fri Apr 03, 2009 12:20 am

I have a string pot sensor for steering angle. I was going to use it to look at my driving smoothness and don't care so much about actual degrees. Though I was going to check it for how linear it is. I was just going to put the car on jacks and turn the wheel 90* or so and take a reading, then build a calibration curve from the logged volts. Close enough to see how smooth I'm not.

twister
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Joined: Wed Jul 12, 2006 2:19 am

Postby twister » Wed Apr 22, 2009 3:24 pm

osborni wrote:I have a string pot sensor for steering angle. I was going to use it to look at my driving smoothness and don't care so much about actual degrees. Though I was going to check it for how linear it is. I was just going to put the car on jacks and turn the wheel 90* or so and take a reading, then build a calibration curve from the logged volts. Close enough to see how smooth I'm not.


What sensor did you purchase the RT one or outsourced?

osborni
Posts: 497
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Location: USA, Michigan

Postby osborni » Wed Apr 22, 2009 3:32 pm

celesco SP 25" stroke.

I haven't had time to debug it, but it reads perfectly when not mounted and I pull the string but gives funky readings when mounted up. Don't know what's up with it yet.

twister
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Joined: Wed Jul 12, 2006 2:19 am

Postby twister » Thu Apr 23, 2009 1:16 am

Keep us informed of your progress!
What was the cost of that unit?
Regards Michael

osborni
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Location: USA, Michigan

Postby osborni » Mon May 04, 2009 12:17 pm

I go the thing hooked up. Figured out the hard way that I had wired it wrong. I did a calibration "drive" down my driveway and back up.

I'll post a screen shot of the results in a few days.
BMW 2000 M Coupe

nikpro
Posts: 56
Joined: Tue Jan 23, 2007 12:29 pm

Postby nikpro » Mon May 04, 2009 11:04 pm

String pot fitted to steering column - works perfectly.

Image
Fraser E White

osborni
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Location: USA, Michigan

Postby osborni » Mon May 04, 2009 11:08 pm

Analysis results of a string pot

I did the formula for "Turns". +1 = one full turn to the right, -1 = one full turn to the left, etc.

Image
BMW 2000 M Coupe

chrisp993
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Joined: Fri Apr 20, 2007 2:12 pm
Location: Bloomfield Hills, MI

Postby chrisp993 » Tue May 05, 2009 2:50 pm

Hey, I just did the exact same Celesco SP2 install onto the steering column of my Lotus Exige, with the same goal of comparing steering angle and required steering derived from corner radii.

What i found was that at anything other than very low speeds, the apparent steering angle on track was way more - like more than 40 degrees more than what would be implied by the corner radius ...... but then I remembered slip angle :!:

If you assume a slip angle of 2-3 degrees at the wheel, and a steering ratio of 15.8 (in my case), then the wheel input will be required steering + 2-3 * 15.8, or my observed 40 to 50 degrees off.

So, the data you collect won't be as neatly organized as you think, but it is possible to build a correction into the data to scale it somewhat: in my case, I fooled around with various ways and ended up using:

STEERING ANGLE = (-497.22 * x) + 1678.78 which is my Celesco SP2 input calibrated to output in Degrees

STEERING_NEEDED = 15.8*atan(sign(VAR_0004)*2.301/VAR_0052)*(360/(2*3.142))

LAT_G_ADJUSTED_STEERING = STEERING NEEDED +(VAR_0004/1.4)*60

where VAR_0004 = LAT_G and VAR_0052 = corner radius

but many similar approaches will work - they are all "fudges" to try and fit the data - the aim being to better scale the steering input and steering needed so you can spot oversteer / understeer and corrections etc. and you will certainly see these very easily

You can also work back the other way and define the difference between Steering Needed and Steering Angle Sensor, divide by Steering Ratio = estimated Slip Angle. Plotting this vs. LAT G is interesting - very linear on corner entry / exit but therafter becomes much more noisy toward the limit of grip, above say 3-4 degrees of slip angle, where steering input becomes increasingly divorced from steering response :) For example, this might show you where you are over-using the steering wheel and scrubbing as a result :idea:

pault
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Location: St Charles, IL

Equation Question

Postby pault » Tue May 05, 2009 5:22 pm

I'm looking at the equation for steering needed. I'm assuming that 2.301 is your wheelbase (in meters), is that right?

Paul

chrisp993
Posts: 47
Joined: Fri Apr 20, 2007 2:12 pm
Location: Bloomfield Hills, MI

Postby chrisp993 » Tue May 05, 2009 5:46 pm

Yes:

STEERING_NEEDED = STEERING RATIO * ATAN(WHEELBASE/CORNER RADIUS)

The SIGN(VAR_0004) factor ensures -ve for LH turns and +ve for RH turns

The (360/2 * 3.142) factor converts from Radians to Degrees

nikpro
Posts: 56
Joined: Tue Jan 23, 2007 12:29 pm

Postby nikpro » Tue May 05, 2009 10:03 pm

Thanks for the feedback Chris,

I have calibrated the streering sensor by using turnplates on the wheels to measure the angle the front wheels turn rather than degrees of steering wheel movement.
Fraser E White

chrisp993
Posts: 47
Joined: Fri Apr 20, 2007 2:12 pm
Location: Bloomfield Hills, MI

Postby chrisp993 » Wed May 06, 2009 2:55 am

OK - angle at wheels should equal:

angle at steering wheel / steering ratio

Is this what you found?


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