Measure distance of 1.5-2m, accurate to within +/- 2mm

Hi guys, I am currently building a mobile wheel alignment stand so that I can make adjustments to my race car at the track.

In order to measure Toe, two distances are measured between the front wheels, one on the front side of the Wheel, one on the rear side of the wheel. With these two measurements one can calculate the toe angle in degrees/minutes.

To take the measurements my plan was to use TOF sensors on one side of the car, and some reflective surfaces on the other.

I have tested VL530x and VL531x sensors, but they are not very accurate. Here is a list of things I have tried, in order to improve accuracy:

-SPAD Region of interest: 4x4, 8x8, 16x16

-reflective surface: Gloss white, reflective tape, bicycle reflectors, aluminum foil.

-reflector size: 90x40mm, 120x120mm

-ambient lighting: high and low

-software changes:
distance mode: high, medium, low.
Timing budget: various settings between 50ms and 500ms.
Result filtering: exponential filter, running average, and both together.
Measurement modes: continuous, single shot.

Doesn’t matter what I do, I can’t get any better than +/- 5-10mm

Does anyone have any tips on how to improve the accuracy of these sensors, or know of some other sensors that are more accurate?


Is this device intended for measuring in the depot and not live out on the trace track?
If so, I suggest a well calibrated rod and a caliper/micrometer arrangement.

Hi, yes you are correct not while driving, but rather something to use in the pits.
Currently two tape measures do the job, but I am trying to make something a bit more professional/cool.

The plan is to make some stands that replace the wheels, which have a digital readout showing camber and toe.

Here is the general concept:

I think rods with a caliper arrangement might be a bit tricky (I’m thinking portability)

Another idea I had was cables on a roll (like a tape measure). But at the moment I am open to all ideas.

I have a Leica Disto D1 laser-distance measuring device

It has a bluetooth-interface and a smartphone app.
There is a continously measuring-mode that does a measuring once every second

not cheap as you would need two of them and I guess two android-devices to use the App but accurate

same price-range is this one

best regards Stefan

I'm used to railroad tracks. We use like a scale with a moveable caliper like indicator telling, in millimeters, how far, plus or minus, we are from nominal.

A well calibrated rod equipped with a linear potentiometer and a sensing tip, all fed into an Arduino looks doable to me. Calibration is needed (once now and then) for every option. Such a linear pot likely have a range of max some 50 mm, 2". Prepared, known, checked, steady positions for the pot to cover the total span, 1.5 to 2 meter.... Sounds doable and reliable to me.

I would imagine you will need some way to measure the steering is straight ahead with reference to the rear.
Otherwise measuring just these two as you say would be useless due to the geometry of the front steering arrangement.
Perhaps reference to the pitman arm if conventional.

The reference to the rear is really only necessary for Centering the steering wheel, the value that interests me the most is the total toe.

Measuring the rear is where a reference to the opposing axle is needed, as the toe on the rear gives the geometric longitudinal axis, which is responsible for the direction the car wants to drive (if it’s out the car will not drive straight).

I have some laser diodes that will be fitted, which will give an optical reference to the opposing axle.

@steffanL38 how accurate is your rangefinder? Do the measurements value fluctuate, and are the measurements repeatable?

I believe is the manufacturer for the type of sensor modules found in these products, and they can be found relatively cheap.

@Railroader i understand your idea, just the idea of having to transport a couple of ~1.5m rods isn’t very appealing. I was thinking instead of rods that push against a linear transducer, perhaps some calibrated cables that pull against a linear transducer?

You can't be the first in the world to want this, so the coolest thing you can do is find out what the others are doing. If you really need to do this, you may well find your best answer in reply #2

As mentioned previously, as you move the wheels from their centre position, each moves a different amount due to the steering geometry.
Therefore it would be essential to find this central position and lock it in place, otherwise the toe in reference is no longer viable.
Previous answers #2 and #8 might well be a good starting point.

@bluejets dont take this the wrong way, but steering geometry is not the thing I need help with (I know a thing or two about that :wink:), sensor accuracy is the problem at hand.

As for doing research, I have not found anything regarding toe measurements using tof sensors, it was an interesting application for the technology, and I thought it would be worth investigating. The current „professional“ approach is a string box, but it’s a lot of work for a couple of quick changes when working on tire temperature coverage at the track. For this you really just need total toe and left/right camber (unless your working on the rear, as previously stated).

The other recommendation was rods with a linear transducer, which as I previously mentioned sounded like a great idea, only I don’t want to transport the rods around (it would be nice to have something that all fits together in a small box).

There are more accurate TOF sensors but, since they're not hobby grade the price goes up.

Wild idea.

After you've searched on the internet with words such as "arduino machine learning edge detection" and, from your reading decided its worth the learning curve, I am sure you'll get a good working product.

I was just able to get a amg8833, a very small thermal camera with only 8X8 pixels, using a ESP32 to distinguish squirrel, duck, person and upon person detection generate an action signal using machine learning (ML).

I got some data to look at this morning, from the project, then I am going to test out the code I wrote to combine 2 images from 2 amg833's.

Oh, almost forgot. Google has one of the largest open source stores of tensors (trained machine learned models) around. The trick to getting access to it is that you must contribute to the project of tensor collecting. I mention that because there is most likely a trained model for a properly aligned car tire in their store.

When you do this doesn’t the car body work get in the way of any light beam going across the car ( as you need to be half way up the wheel - I can’t imagine any car where you can see straight across unless you make some holes ! ) ?

Also the accuracy you want is not good enough , with toe in typically being a couple mm , knowing that to +- 2mm is useless .
Buying a tracking gauge and be done with it

One way to get around the accuracy/precision issue with the TOF detectors (or any detector) is based on the statistics of confidence limits and confidence intervals. For instance, assuming a standard normal distribution curve for the data, you can calculate that if you have a standard deviation of 7 mm, and desire to know that a value is 10 +/-1mm, i.e., in the range 9 to 11 mm, 99% of the time, you would have to take at least 325 measurements to ensure the accuracy is within the desired range. At 0.1 mm the number of observations goes up to over 32K observations.