Electronic distance measurement - how ?
Turbo Mini Forums

I want to measure the distance something is moving vertically in real time over a distance of about 50-60mm and it has to be accurate ±0.25mm. I can't attach anything too heavy to the object, and it does vary it's height quite quickly in an irregular manner.
I don't think I can do it with a laser or optically with an off the shelf product, so I'm open to suggestions.

Metric is for people who can't do fractions.

I'm thinking Linear potentiometer?

Yep, me too linear pot is made for this, if you need to you can just measure / log the raw voltage and post process it into a distance after.

Can a linear pot be small and light enough to sit on an SU piston without upsetting the spring balance ?

Metric is for people who can't do fractions.

http://www.ebay.co.uk/itm/RANGE-ROVER-P38-...=item259892d754

I have mainly used Penny and Giles displacement sensors, here's a useful link;

http://www.pennyandgiles.com/Products/Line...on-Sensors.aspx

It shows everything they do

To be light enough to measure a dashpot without upsetting it, I don't know?, linear pots can be a bit inconsistent on how 'stiff' they are throughout the range of travel. I've used LVDTs and string pots when I wanted something lightweight but string pots are good in tension but you have to be a bit careful in contraction as they sometimes can't keep up if the movement is too fast.

LVDT's are none contact so less force / drag involved, but expensive and needs special post processing as AC output

Penny and Giles have PDF spec sheets for all the sensors they do which will quote the force required to move the sensor

I haven't used the specific one Joe linked to above but a lot of the OEM ride height sensors are pretty poor resolution and you need to be sure they have an analogue output and not CAN unless you can deal with / decode the output.

My guess is cost will be the problem as most decent linear pots are around £200 to start

I've been doing some looking on the interweb based around the linear pot idea, and am now wondering if a Hall Effect position sensor would work.

Small magnetic rod in place of, or alongside, the damper with the sensor on the top of the carb.

Metric is for people who can't do fractions.

An eddy current position sensor will measure the distance you require with the accuracy you require, and is non contact, but will be more expensive than a linear pot.

LVDTs aren't fully non contact. They need connecting to what you are measuring in exactly the same way as a linear pot would, although there will be less drag.

Would a visual do ? ie place a camera watching the piston ?

Shane T offers these wastegate actuators, which are doing more or less what you describe. Not sure what the actual sensor is, or how it connects internally. A Wastegate wouldnt move quite as much as an SU piston though.

But you could weld up or even epoxy the top of an SU dashpot and make something with the right sensor

http://www.fastmotorsports.com/products/ef.../wastegates.htm

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Turbosmart are now offering the same, and list a part number for a sensor

http://www.turbosmart.com.au/news/knowledg...gate-sensor-cap

Sensor

http://www.turbosmartdirect.com/Product-Ca...ion-Sensor.html

http://www.turbosmartdirect.com/Product-Ca...ensor-Plug.html

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

those sensors (or ones almost identical) are standard fit to wastegates on some of the newer HDi engines

http://www.ebay.co.uk/sch/i.html?_cqr=true...position+sensor

pretty sure they are even cheaper from peugeot. not quite got the stroke you need tho.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Are you trying to do something similar to brett... (photo of linear pot on 3rd page)

http://www.turbominis.co.uk/forums/index.p...id=159683&fr=50

Is this really simpler than EFI?

http://www.jbperf.com/

Nothing's new is it.

The sliding pot was my first idea (already tried it seems) and a hall effect sensor poking through the dashpot was my second plan...(also tried).

Third plan is to use a plastic strip with an optical pickup as suggested elsewhere.
http://www.usdigital.com/products/encoders...ntal/linear/LIN
http://www.usdigital.com/products/encoders...tal/modules/em1

Metric is for people who can't do fractions.

Sound like it, i have 2 versions one that screws ontop of the damper is good for moving from car to car

And one that is internal and can be kept on the car

I data logged the needle position through megasquirt,

Page 19 of my build thread has some pictures

http://www.turbominis.co.uk/forums/index.p...d=265648&fr=450

Edited by Brett on 7th Dec, 2014.

Yes i moved to the darkside

Instagram @jdm_brett

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Edited by PhilR on 8th Dec, 2014.

Schrödinger's cat - so which one am I ???

another option is to use the linear encoder from a cheap vernier calliper. most have a serial output, and again can be interfaced to an arduino or similar.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

A typical hobbyist IR setup

http://imall.iteadstudio.com/electronic-co...m130619002.html

Drill two holes in the top of the suction chamber offset to clear the dashpot and mount this on two 50-60mm black plastic tubes (to meet its minimum distance requirements) glued/sealed at both ends and paint the top of the piston white.

Analogue output and very linear over your range - see datasheet page 5

ftp://imall.iteadstudio.com/Sensor/IM1306...ring_Sensor.pdf

There are probably lots more suitable, this just happens to be sold by the company that makes my printed circuit boards as they do a lot of arduino stuff as well as PCBs (but you wouldn't even need an arduino as this one is analogue).

Schrödinger's cat - so which one am I ???

That's pretty cool. You could make it permanent and keep the damper in place and not interfere with the piston movement.

It might be trickier to set up, but I'm tempted to try this myself - even though I've got no projects with carbs

EDIT: just thought, my bike has carbs!

Edited by PhilR on 9th Dec, 2014.

Another idea: Could you use a MAP sensor to measure the pressure inside the dashpot, then infer the piston height from that?

Schrödinger's cat - so which one am I ???