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October 4, 2018 17:26

What techniques could be used for sensing coolant level?

The news is reporting issues of engine damage from coolant loss in a modern car engine. It would appear that a dashboard warning is either missed by the driver or is too late to prevent damage. Hence the 'drive' behind this idea, which for a few pounds could save the £8k typical engine replacement costs when a piece of tubing fails...

I am thinking of a simple add on that could detect coolant loss sooner than the current method which seems to be sensing the engine block temperature which will rise rapidly BUT after the coolent is lost. Hence the radiator header tank seems a better sensing point..

My aims are:

1. External to the header tank. I simply do not wish to drill holes and introduce other problems!
2. It has to cope with the engine environment temperature extremes.
3. Work on a typical 'plastic' semi transparent header tank.

An example tank is shown below. Unfortunately Min and Max straddle the welded joint, but sensing in the area of the green line would be an example sensing surface and range.

Just for starters my thoughts are:
1. A multi-capacitive type sensor which will detect the change from coolant being present or not. So think of something like a capacitive touch switch sensor strip.
2. Using opto reflective sensors  127-3057 . Could these sense the coolant (which has colour) through the housing?
3. This could be my favourite to try out! A LED shining through the top of the housing and multiple optical sensors700-0776 down the side. This seems to have the advantage that the coolant surface refractive/reflective properties as well as optical density may provide a nice transition to detect. It may be that the sensor colour could be matched to the coolant colour for improved detection?

This is simply just the 'first idea' step and I'm just thinking of the level sensing at the moment, so issues of fluid splashes and slopping around will be thought about in more detail later.
Any thoughts on the problem, suggested solutions or other techniques?

I will not be selling this but if I actually make anything I will publish on DesignSpark as a project.


October 4, 2018 08:23

Thanks Brad, good point on synchronous detection, I shall add that in.

Using the "stud finder" was a clever idea for a quick test, I shall certainly consider that as a option if I test the capacitive route.

I also consider acoustics and there may be mileage in that route. My initial thoughts were a sound source on one of the hoses, but thought the flexible hoses would attenuate the source. Tapping the tank is a much better idea. I also ruled out acoustic methods for initial tests due to ambient noise, but it's now back on the list.

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October 4, 2018 08:26

I think the optical approach would be the first I'd try. Pulse an LED, and check average detected brightness during pulses vs avg detected between pulses. That synchronous detection technique helps eliminate interference from ambient light levels. Multiple photodetectors (maybe two strips separated horizontally by a goodly span) would help reduce sensitivity to vehicle angle. Play around with location of the LED source and weighting factors on the photosensors in the strips to get best response / ability to differentiate intermediate levels.

As far as capacitive sensing, a quick test with a stud finder (which I'm pretty sure is capacitive) and a two-liter bottle half full of pop also showed some promising results. Surprisingly, I actually had better luck with the stud finder held perpendicular to the bottle rather than flat against it. I'm not sure how well it would handle the electrical noise in the engine compartment, though.

Another approach might be acoustic resonance. Use for stimulus either a small solenoid to tap the surface, a (bone) transducer like Adafruit # 1674, or a cell phone / pager vibration motor, and for sensing a piezo disk (like RS Stock No. 724-3162 ), contact microphone, or maybe something like RS Stock No. 124-5983. The pitch and magnitude of the recovered excitation should both change with level of the fluid in the tank. An advantage of this approach is that the sensed signal will reflect the total fluid level, regardless of whether the vehicle is on a slant. On the other hand, the signal will also be affected by temperature, so will require more testing and adaptation than the other methods.

I think the optical approach will be least affected by noise (electrical or mechanical) in the engine compartment. Components for all three approaches will be required to operate over a broad temperature range, so pay attention to that on data sheets.

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