Suspension displacement on the cheap

[Menisk]

Hey guys,

So I'm looking for ways to monitor suspension travel on the cheap. What I've thought about so far was using optical distance sensors that you can get for around $20AUD and just mounting one at one end of the damper and putting a plate at the other end for it to measure off. Main thing I'm worried about is whether these are likely to stand up to the abuse of being strapped to the damper of a car, and I'm not sure about how accurate they are. I'm going to end up giving this a shot on one corner at least to see how it goes.

Has anyone tried this before and if so how did it turn out for you? I'm also interested to hear if anyone else has ideas for doing this on the cheap.

[RobbyObby]

I don't know much about distance sensors, but I imagine they'd be pretty sensitive to vibration and would need very secure mounting of the sensor and the plate, in the proper orientation, in order to get useable data. I guess it depends on the accuracy you want.

In the past we've used simple rotary potentiometers hooked up to our rockers. By knowing your motion ratios, you can back track to shock travel. Don't remember the exact components or specs used but I do know we ran 4 pots on each rocker, 1 on the steering column, and an accelerometer with microcontroller for I wanna say less than $100.

Send a PM to PennyMan on these forums, as he's got all the specs and might be able to help you out.

[Pennyman]

No need Robby!

I've had some good success with a linear pot called a "magnetopot" by Spectra Symbol. They're magnetically operated linear potentiometers. There's also a pressure-activated version called a softpot.

spectrasymbol.com/potentiometer/magnetopots

Where they lack in sensitivity (they aren't THAT great, let's get real here), they more than make up for in price. I used one of these for a steering angle sensor in a pinch. They come in various lengths, and they have a plastic backing that you can epoxy to a bracket if you need to.

They are by no means a cure-all, but they work when there isn't an alternative. They're available from DigiKey, Mouser, and probably plenty of other shops.

[Warpspeed]

About the cheapest linear electrical position indicator on the planet is the humble slider potentiometer.
They aren't wonderful, but they work and are readily available.
http://www.discovercircuits.com/DJ-Circuits/SlidePot1.jpg

[Menisk]

My only worry about slider pots is that they'll get trashed super quickly. I'm not sure they're up to the kind of abuse they're going to find being strapped to a shock.

[Warpspeed]

That's certainly true.
Water and dirt will rapidly kill them, and they will wear out or fall to bits fairly quickly.
But at maybe two dollars a go (or for free salvaged from a rubbish dump), they should last long enough to log some quite useful data.

Definitely not recommended for permanent installation, just for some quick and dirty low cost R&D.

[Nath_01]

I remember one of the guys from motec explain to us at one of the fsae workshops that you can get your hands on some pretty cheap rotary pots used on HSV's for body/ ride control.

We used them on our rockers at formula ford to good affect.

Unfortunetly I can't remember exactly how much they were. I do remember being a little surprised however.

Might pay to call the local Holden dealer.

Goodluck

[M. Nader]

Originally posted by Warpspeed:
About the cheapest linear electrical position indicator on the planet is the humble slider potentiometer.
They aren't wonderful, but they work and are readily available.
http://www.discovercircuits.com/DJ-Circuits/SlidePot1.jpg

That's what we will be trying out this year. We are already starting to implement the system

[Warpspeed]

You can get heavy duty plastic resistance track potentiometers intended for robotic applications.
These are designed for constant motion, have proper ball bearings and will last forever in constant motion, but they ain't exactly cheap.

Nath is on the right track, I don't know about HSV stuff, but I owned a 4WD Ford Laser (Mazda 323 clone) that had variable height air suspension. That had two rotary potentiometers mounted exposed under the chassis connected by short links to the front and rear suspension. These were properly sealed units, and were completely trouble free for many years, for as long as I owned the vehicle. Cost of these is unknown.

[Drew Price]

This is a good point.

Almost every modern car with xenon arc discharge headlamps (HID's) that are self-levelling (which is a federal DOT mandate in the U.S. at least) also have rotary pots somewhere in the front and rear. This way the lighting control module knows the pitch angle of the vehicle and can tilt the lamps down if the car pitches up to keep from blinding on-coming drivers.

GM vehicles should all have them where equipped, and even new ones should be cheap.

The ones I saw ran on 5v power, usually switched 5vdc, ground, and you got a 0-5v output signal, and have clip on ball pivot rods.

Any older Cadillac or Buick with self-levelling air suspension as well, along with any of the big Merc's, BMW's, etc.


http://image.made-in-china.com/2f0j00GsETmalKJfkq/Auto-Ride-Height-Sensor-for-BMW-37141093697.jpg

[Menisk]

Alright, guess I'll look around for some decent rotary pots. Thanks guys http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Z]

Menisk,

Or fuel level sender units.

Or if you really want cheap, then make your own! Some resistance wire wrapped around an insulated former (eg. plastic ball-point pen tube), and a springy brass wiper.

Z

[Tim.Wright]

Id avoid the slider pots. They will shake themselves to pieces very quickly.

Best bet is ghetto string pot made with a rotary pot (few dollars), some string and a torsion spring.

Sounds Baja, but string pots are actually a real component.
http://www.firstmarkcontrols.com/s054b.htm

Since a rotary pot can be any price you want (from $0-$500+) you can really have any degree of reliability/cost that you want.

[Warpspeed]

String pots do have a few advantages, the main one being linearity of motion to voltage output. Bell cranks with their changing angles may add a layer of complexity to interpreting the recorded data which we can do without.

The ideal thing to use for "string" is the very fine multi strand stainless steel wire fishermen use between the hook and the fishing line.
This prevents the fish from biting through the line.
This fine stranded stainless wire is extremely strong, very flexible, and absolutely will not stretch.
Perfect for a string pot with a smallish diameter drum.

[Menisk]

I'll have a look at string pots then. My plan with rotary pots on the rockers was to make some i2 maths channels to get back to displacement of the spring/shock.

[Warpspeed]

Rotary pots measure angular position, that may or may not be a problem depending on how the whole thing is applied.

Sliders and string pots more usefully measure linear position.

If you are going to build a string pot, the string will need to seat in a deep helical groove, and may require more than one full turn around the drum to prevent slippage or crossovers.
It can work very reliably, but only if it is very well thought out and built.
A Mickey Mouse sting pot will quickly drive you nuts.

[Menisk]

Yeah, if we go string pots we'll have to build them. I got a quote today on some unimeasure ones. The cheap ones were still 350AUD each. They did say they'd give us a 25% discount, but I'd hardly call that cheap.

We're just going to try a number of things. Our electrical lead is just going to strap a few different things to the rear shock of his bike and log them with an arduino for a while to see how they go.

[SeeGull]

Almost every modern car with xenon arc discharge headlamps (HID's) that are self-levelling (which is a federal DOT mandate in the U.S. at least) also have rotary pots somewhere in the front and rear.

What rule or regulation actually mandates self-leveling of HID Headlamps in the U.S.? I can't find anything specific http://fsae.com/groupee_common/emoticons/icon_frown.gif