Glowplug controller
christer
Posts: 7
Can anybody help! I want some help with measureing the temp of a glowplug on an model engine!! I want to know the temp because I want to correct it when it goes low when idling! The way I can think of is to use the RC-trick. But this is the trouble. The resistance of the glowplug is only ca 0,5 ohm. Somebody who can lead me on the right track?
Thanks.
Christer S
Thanks.
Christer S
Comments
It worked very well but a model plane enthusiast pointed out their solution which is to have a microswitch on the throttle control, and add in a 1.2V nicad when the throttle is low.
However, one big advantage of a constant temp glow is for starting, as you just switch it on, start the engine, and when the engine is running, the constant temp glow switches off.
I used a very low on resistance mosfet (BUK555) with a variable PWM.
You can measure the temperature by measuring the resistance, and you can measure the resistance if you know volts and amps. See http://en.wikipedia.org/wiki/Wheatstone_bridge
You can leave out the resistors R1 and R2. R3 can be a known very low resistance - I used a fixed length (30cm) of automotive cable of known guage so you can work out the milliohm resistance.
Point B is the top of the glowplug so you need 3 wires going to the glowplug = +ve, -ve and sense. The sense wire can be thinner.
Pulse on the power, measure volts between A and B (a difference amp might help). V=IR and you know V and R so you can work out I. And measure volts between B and C and that gives you the volts across the glowplug. Then you can calculate the glowplug resistance which corresponds to temperature.
In practice the analog circuit worked well. I had circuits running on 2 and 3 nicads (2.4 and 3.6V) and the glow temp was very constant. I zapped a few glowplugs on the way, so found it was best to do experiments win 1 nicad.
There was one problem though that I never solved. Point B on that wheatstone bridge needs to be very definitely connected to the glowplug, with no change in resistance. Any sort of plug has a variable resistance, especially in a vibrating environment, and the resistance changes are sufficient to upset the temp readings. The solution was to solder connections to the top of the glowplug, which means out in the field if you needed to change a glowplug you would need a soldering iron.
Of course, with a Propeller you could take this much further, and put the current/volts/temp on a display!
BTW, I have one of the micro switch gizmos on the throttle line. I think it works well. Years ago I even purchase an on-board starter for the glow engine. It was pretty cool to restart an airplane in the air. It caused a lot of puzzled looks at the flying field.
Duane
I don't use the feature an extra servo adds weight.
Is this what you might be trying to do christer?
Ron
RC Car? What glow plug, and fuel do you use. Mixing your own fuel may fix your problem and make more power.
But the glow plug driver is a great project (something I am tempted to reopen), If you use a McCoy #9, custom fuel and the ability to retard the timing at high rpm, or at least control the timing to maintain a certain head temp throughout the rpm range.
You could also use an extra R\C channel for a horsepower increase ability by raising the head temp for a short boost in speed
This is really cool, and would be neat to have in the OBEX. An object that shows how to measure the resistance and convert that to temperature might be helpful in getting folks started on other indirect measurements.
For your vibration issue, could some sort of least squares filtering be used? Or is it be better just to solder up some spares?
EGT you can get them anywhere fro a reasonable price I use them on all my toys even my jet ski