Slip Angle Sensor - General Idea

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Caito
13
Joined: Tue Jun 16, 2009 4:30 am
Location: Switzerland

Slip Angle Sensor - General Idea

Post by Caito » Fri May 27, 2016 7:36 pm

Hi Guys!

I was thinking if there was a way to make a fairly "cheap" slip angle sensor. Or put in other ways, a device that could measure longitudinal and lateral speed with respect to ground. Based on other commercial sensor I've seen, I believe ca. 25Hz is enough.

Of course, one alternative is to use optical mouse sensors. Although they have a limited speed capability, you could put another sensor to have a broader speed range. Still, it doesn't sound easy enough, you need optics, sounds unreliable (at least for me).

Nowadays having so much data throughput and processing capability I thought why don't just point a camera to the ground. Then you just find a reference in the image and see where it moved on the next image. It needs quite some processing, but that's offline and is "easily" done. Basically you can move images to find the point of max correlation, or actually search for on piece image on the other, etc. There are many ways to do this.

My question is how blurry would that image be?

As some ball park number. Imagine capturing a spot of 20x20cm and a frame of 500x500 pixel, this means 40mm per pixel. If you want a "still" frame, you would want less than a 40mm movement. Suppose we want this until 50 m/s (180 kph), this would be a 1/1250 shutter speed. With 20cm movement being the maximum allowed between frames, we need a 250Hz frame rate. As you increase spot size you reduce the frame rate, but would increase error I guess.

But I don't know much about photography so that might as well be pure bs.

Do you think something like this could work? Is it worth a try? What problems can you see?
Come back 747, we miss you!!

langwadt
62
Joined: Sun Mar 25, 2012 1:54 pm

Re: Slip Angle Sensor - General Idea

Post by langwadt » Fri May 27, 2016 7:41 pm

Caito wrote:Hi Guys!

I was thinking if there was a way to make a fairly "cheap" slip angle sensor. Or put in other ways, a device that could measure longitudinal and lateral speed with respect to ground. Based on other commercial sensor I've seen, I believe ca. 25Hz is enough.

Of course, one alternative is to use optical mouse sensors. Although they have a limited speed capability, you could put another sensor to have a broader speed range. Still, it doesn't sound easy enough, you need optics, sounds unreliable (at least for me).

Nowadays having so much data throughput and processing capability I thought why don't just point a camera to the ground. Then you just find a reference in the image and see where it moved on the next image. It needs quite some processing, but that's offline and is "easily" done. Basically you can move images to find the point of max correlation, or actually search for on piece image on the other, etc. There are many ways to do this.

My question is how blurry would that image be?

As some ball park number. Imagine capturing a spot of 20x20cm and a frame of 500x500 pixel, this means 40mm per pixel. If you want a "still" frame, you would want less than a 40mm movement. Suppose we want this until 50 m/s (180 kph), this would be a 1/1250 shutter speed. With 20cm movement being the maximum allowed between frames, we need a 250Hz frame rate. As you increase spot size you reduce the frame rate, but would increase error I guess.

But I don't know much about photography so that might as well be pure bs.

Do you think something like this could work? Is it worth a try? What problems can you see?

an optical mouse is a camera

Caito
13
Joined: Tue Jun 16, 2009 4:30 am
Location: Switzerland

Re: Slip Angle Sensor - General Idea

Post by Caito » Fri May 27, 2016 8:31 pm

langwadt wrote:
an optical mouse is a camera
Yes it is, but if you look at the sensors, they're usually tiny and very specific. Quoting one of Agilent's datasheet, max speed is 12ipm (roughly 110kph) and would require specific lenses to make it work at the distances and speeds required. It's not simple enough.
Come back 747, we miss you!!

NL_Fer
63
Joined: Sun Jun 15, 2014 8:48 am

Re: Slip Angle Sensor - General Idea

Post by NL_Fer » Fri May 27, 2016 9:09 pm

The cars have g-sensors and gps right?

mrluke
124
Joined: Fri Nov 22, 2013 7:31 pm

Re: Slip Angle Sensor - General Idea

Post by mrluke » Fri May 27, 2016 9:35 pm

Thinking out loud.

Do you really want a super sharp image?

Using a known shutter speed wouldnt the extent and direction of the blur tell you the movement / slip? Actually if you could measure it very precisely wouldn't it actually give the slip angle?

Tim.Wright
444
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Joined: Fri Feb 13, 2009 5:29 am

Re: Slip Angle Sensor - General Idea

Post by Tim.Wright » Fri May 27, 2016 10:48 pm

Cheap slip sensor? Rig up a caster wheel with a measurement potentiometer. The caster wheel will (more or less) follow the road - therefore the angle between the caster wheel and the vehicle body is your slip angle.

Another method is to calculate it from a lat acc and yawrate sensor. It requires an integration so it will drift over time but its useful for short maneuvers like step steers etc.

Depends what you need the data for in the end?
NL_Fer wrote:The cars have g-sensors and gps right?
That's not enough to calculate slip angle. GPS is horribly noisy and imprecise. You need accelerometer AND gyroscope measurements to correct it and even then its not a particularly robust or precise measurement.
Not the engineer at Force India

Greg Locock
168
Joined: Fri Jun 29, 2012 11:48 pm

Re: Slip Angle Sensor - General Idea

Post by Greg Locock » Sat May 28, 2016 1:32 am

However I think the camera solution has legs. If I were to do it i wouldn't get too caught up with the optical side (a statement I will likely regret), and basically worry far more about the processing side. If you imagine two consecutive frames of a speckled pattern, how quickly can you decide on the 3 possible transformations between the two? (rotation and x and y).

As Tim says, measuring it more than one way is probably necessary. We use differential gps instead of gyros these days, I don't know how much our accuracy has suffered.

Tim.Wright
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Joined: Fri Feb 13, 2009 5:29 am

Re: Slip Angle Sensor - General Idea

Post by Tim.Wright » Sat May 28, 2016 7:42 am

I've experienced, and have heard other stories of problems with even high-end (50k€+) inertial+gps slip measurements. I've had signal drift down straight sections of track and I've heard issues of them exhibiting different signal offsets depending on which direction you drive. Plus numerous reliability problems. So Ive grown pretty skeptical of these systems.

I'm currently of the opinion that if you want to measure the road velocity - you should take a direct measurement ot the road.
Not the engineer at Force India

Shooty81
17
Joined: Fri Sep 25, 2009 1:13 pm

Re: Slip Angle Sensor - General Idea

Post by Shooty81 » Sat May 28, 2016 12:25 pm

Greg Locock wrote:If you imagine two consecutive frames of a speckled pattern, how quickly can you decide on the 3 possible transformations between the two? (rotation and x and y).
What if you use the non-zero exposure time of a single frame and calculate the angle directly out of the streaks of the asphalt pattern of one single picture?

notsofast
7
Joined: Wed Oct 10, 2012 1:56 am

Re: Slip Angle Sensor - General Idea

Post by notsofast » Sat May 28, 2016 6:20 pm

mrluke wrote:Do you really want a super sharp image?

Using a known shutter speed wouldnt the extent and direction of the blur tell you the movement / slip? Actually if you could measure it very precisely wouldn't it actually give the slip angle?
Shooty81 wrote:What if you use the non-zero exposure time of a single frame and calculate the angle directly out of the streaks of the asphalt pattern of one single picture?
This idea (from both of you) seems to be the most straightforward. As long as the asphalt is not too smooth. I guess you would want to aim the camera somewhere between the wheels, away from the tire tracks that are rubbered in.

Greg Locock
168
Joined: Fri Jun 29, 2012 11:48 pm

Re: Slip Angle Sensor - General Idea

Post by Greg Locock » Sat May 28, 2016 11:21 pm

You'd have to write a function that'd identify streakiness..

You are in luck, the following course includes enough image processing to get you started, it is free, not too hard, and I thought it was fun. Plus you get a limited use edition of the latest matlab, which makes me annoyed as I'm stuck with 2009a(more accurately my PC is set up and works, stuffing with matlab versions is a guaranteed way to have to sort out the other programs that use it),

https://iversity.org/en/my/courses/mode ... ouncements

Incidentally I was right to be cautious of my gung-ho attitude to the photgraphic side of things. At 25 Hz I will need a field of view of 100/33.6/25= 1.11 m at 100 kph, bit tricky from underneath a car. My guess is they run a much higher frame rate.

I have a sneaking suspicion that running a camera at each end of the car will give much better results than trying to do everything off one camera, that way you'll see a difference in lateral velocities, rather than a lateral velocity and a slip angle.

Caito
13
Joined: Tue Jun 16, 2009 4:30 am
Location: Switzerland

Re: Slip Angle Sensor - General Idea

Post by Caito » Mon Sep 12, 2016 5:08 pm

I came across this sensor, that might be almost all needed to implement an idea like this one.

https://pixhawk.org/modules/px4flow

Cheers,
Andrés
Come back 747, we miss you!!

Greg Locock
168
Joined: Fri Jun 29, 2012 11:48 pm

Re: Slip Angle Sensor - General Idea

Post by Greg Locock » Mon Sep 12, 2016 8:18 pm

Excellent. (Why can't i mod you up? Android.) well now I have
Last edited by Greg Locock on Tue Sep 13, 2016 9:20 am, edited 1 time in total.

Brian Coat
98
Joined: Sat Jun 16, 2012 5:42 pm

Re: Slip Angle Sensor - General Idea

Post by Brian Coat » Mon Sep 12, 2016 8:37 pm

Raspberry Pi + Picam?
https://www.raspberrypi.org/blog/vector ... stimation/

I must admit "Raspberry Pi!" is my kneejerk response to just about any custom digital look-see project.

But why not? Cheap as chips and what ever you want to do, someone probably already did 80+% of it for you because there is such a high nerd-base using them.

graham.reeds
21
Joined: Thu Jul 30, 2015 8:16 am

Re: Slip Angle Sensor - General Idea

Post by graham.reeds » Tue Sep 27, 2016 10:30 pm

What about gps and a 9dof imu? The gps gives updates 5hz with interpolation from the imu.

The imu will not only give you everything you need so I can't understand why you are thinking of turning the car into a giant wireless mouse...