Article: Dew Heater Control and the Arduino

[hikerbob (Bob)]

No major updates at the moment then.

There is also some work being done on Ascom control at http://tech.groups.yahoo.com/group/s...ry_automation/

I'm not sure where things are up to with the dew heater stuff but the group was heading in a similar direction (except with Ascom control). I'll revisit that at some point, at the moment the controller run's standalone but it would be nice to have a GUI interface to tweak settings on the fly.

I have some partly made up boards sitting around (still needing some components ). if you wanted to have a play with one I could probably send it over. As they say on the boxes though - some assembly required.

WasYoungOnce also did some work on a dew heater which looked good.

Feedback from mine - the low side switching leaves the outer shroud of the heater connector's at 12Vdc. Some risk there of an accidental short to ground. I've played with other housings for some camping lighting using a 240V moulded box designed for weatherproof mounting of a power point or switch, it had a double wall on the sides which allowed the RCA sockets to be recessed. I've not tried to see how my board would fit in one yet.
In some way's I've wondered if I would have been better off incorporating the control gear into a larger power/connection box which sit's on the ground and running the cables to the scope as a loom. At the moment it's more junk hanging off the telescope with loops of excess cable etc. I may may redo things to do that in the near future.

Bob

[tjsteele (John)]

For me, the case and mounting locations would be the least of my worries. Being in Canada, the dew is pretty intense so having a case on the ground is out of the question. I would want a smaller case and probably mount it on the telescope mount using a secondary bracket. Don't want it on the telescope itself either (by the way, I have a Celestron C6-SGT, nothing fancy but it's my first).

As for the temperature Sensor, i was thinking of using computer temperature sensors such as the one that comes with Asus boards. They are tiny flat ribbon like sensors and could be mounted anywhere on the telescope permanently if it works.

And you would send me a board? I would love that, so just tell me how much and i can do a Paypal thing or whatever you prefer, but as i mentioned, i'm in Canada. I already located most of the parts for the project and was gonna order asap. Spent the better part of the day looking at your diagrams as well. Man my theory is so far gone it's not even funny. But just to let you know, I work in a University so i have access to some pretty knowledgeable people as well if i need assistance...

I did look at the exchanges you had with wasyoungonce as well. I think this is where i got confused with the 2006 thing. If not then i'm totally out of it.

[hikerbob (Bob)]

John PM me your details and I'll find out what the postage will cost.

Bob

[DancesWithWords]

Bob,

Do you have a complete list of parts. I want build your controller. Thanks.

==
Bruce
Member: SPIE, IEEE, IACR
Faintfuzzies.ca

[hikerbob (Bob)]

Bruce the list in the article is pretty close

http://www.iceinspace.com.au/63-597-0-0-1-0.html

I'm working towards a rebuild and going with a larger housing and stick the internals of a USB hub (7 port) inside (with 5 ports facing out) and additional power sockets.

Feel free to ask if you have additional questions.

Bob

[DancesWithWords]

Originally Posted by hikerbob

Bruce the list in the article is pretty close

http://www.iceinspace.com.au/63-597-0-0-1-0.html

I'm working towards a rebuild and going with a larger housing and stick the internals of a USB hub (7 port) inside (with 5 ports facing out) and additional power sockets.

Feel free to ask if you have additional questions.

Bob

Did I read that in your new controller you are using the Arduino Mega? Do the Arduino have the capacity to record temperatures collect from sensors please on mirrors and a remote sensor for ambient temperature. It be really groovy if that info could then be displayed on the LCD/LED screen to see how close the scope is to equilibrium. Bonus points if the Arduino had memory capacity to store this info for later retrieval via USB or bluetooth.

It this project possible for someone with no electronics background? Thanks to my wife, I now have a New Hakko 936 Soldering Iron, and a Extech 300 Multimeter. Birthday presents.

Oh, I've got a Orion XXT12 w/ the Intellicontroller thingie. I'm in Canada and the LCD is the pits in the cold. Anything under 10C and it falls to function. Do you know what the tolerances are on the screen you are using is?

Maybe more questions later.

==
Bruce

[DancesWithWords]

Originally Posted by hikerbob

Bruce the list in the article is pretty close

http://www.iceinspace.com.au/63-597-0-0-1-0.html

I'm working towards a rebuild and going with a larger housing and stick the internals of a USB hub (7 port) inside (with 5 ports facing out) and additional power sockets.

Feel free to ask if you have additional questions.

Bob

What are you going to use the addition USB ports for?

==
Bruce

[hikerbob (Bob)]

Bruce I'm trying to cut down on the number of extra things that hang off the scope. The USB ports are for camera's, focussers, Bluetooth dongle for EQMod and a wireless receiver for a gamepad.

I intend to alter the software for the dew controller at some point to add ASCOM compatibility, not urgent it works fine stand alone but something I'm interested in trying.

Bob

[DancesWithWords]

Originally Posted by hikerbob

Bruce I'm trying to cut down on the number of extra things that hang off the scope. The USB ports are for camera's, focussers, Bluetooth dongle for EQMod and a wireless receiver for a gamepad.

I intend to alter the software for the dew controller at some point to add ASCOM compatibility, not urgent it works fine stand alone but something I'm interested in trying.

Bob

Bob,

How goes the build?

[frolinmod]

I'd like to see something like this where all the smarts are located in software running on a PC connected to the box via USB. The hardware and firmware in the box would only be responsible for sending sense data to the PC, taking control commands from the PC and doing the low level PCM output to the heaters. The software on the PC would have a user interface to change settings and display graphs over time of temperature, relative humidity, dew point, heater output, etc. Oh, I suppose the box could also have a manual mode where everything was controled by the box if/when no PC is connected.

[hikerbob (Bob)]

Danceswithwords, started but on hold at the moment.
To much on my plate lately for late nights in the shed (and it's been cold and windy).

Ernie, I suspect that you would still need most of what's in the controller to do that (or an equivalent such as a PIC).

The next step for that would be to add ASCOM capabilities to the controller. I've thought about it often but still not found a good reason other than interest to do so. I wander out occasionally and look at the temp but don't care to much unless I'm really cold or struggling to heat the corrector plate enough to keep dew off.

Bob

[DancesWithWords]

Bob,

Well I keep and eye out to see if you pick the project back up. In the meantime winter is just around the corner here in Ottawa and I'll be working to more or less replicate your project for use with my XXT12 Orion.

==
Clear Skies
DancesWithWords

[Prolixia]

Hi Bob,

I was in the process of designing an Arduino-controlled dew heater when I came across your article. Thanks for sharing it - helpful doesn't come close!

I have a question about the temperature sensors. From the photos of the finder scope looks like you slip these under the sleeves containing the heater elements, however I'm worried that if I do that the readings will be artificially high - especially when switching the heaters on for the first time on a cold scope (when the heating element will be hot but won't have had time to warm the scope). Have you noticed this at all in practice?

For an eyepiece/diagonal there isn't much scope to move the sensor away from heating element, so I had wondered about switching the heater off for say 30 seconds just before taking each temperature reading to allow the temperatures to equalise and only adjusting the PWM every couple of minutes (perhaps with a longer initial 'burn time' when the controller detects that the scope is well under the dew point).

Does this sound useful, or am I over complicating things?

Thanks again for sharing your excellent materials!

[hikerbob (Bob)]

Prolixia, I've not seen anything to indicate that's an issue. Ideally the temp sensors would be on the optics but I've not thought of a suitable way of doing that. Slipping the sensors under the heaters is the simplest solution I've found but if anyone comes up with a better way of doing that I'd be more than pleased. What I've done is a big step forward from no temp measurements at the heaters but still not ideal.

The only place I've had problems is with a 10" corrector plate, I don't manage to keep the whole thing dry in dewy conditions (but do use the hair dryer a lot less than others). I'd like to add the capability to do a boost on a specific channel, either via ASCOM or using the pot but it's not been a big enough issue to cause me to give it much attention. I suspect the problem is that not enough heat is transfered from the strap to the corrector plate the plate has some tape between it and the housing that supports it so I doubt that tweaking the heater control will fix that. Sometime I'll run the strap without PWM and see if it fixes the dew but that will need a cold winters night.

I'm checking temps every few seconds so turning off the heaters would need a change of approach and reduce the overall duty cycle for the heaters when you wanted them running full bore.

Bob

[Prolixia]

Thanks for clarifying Bob - that's reassuring.

Regarding your corrector plate, I wonder if that might possibly be a symptom of the problem I was suggesting - i.e. because the heat doesn't transfer quickly enough to the corrector plate the temperature sensor warms up well before it and reduces the output on that channel whilst the collector plate is still cold. That could explain why the collector plate never quite gets up to temperature. I guess you could check by looking at the output to the corrector plate heater whilst it's fogging up - if it isn't high then the actual temperature of the collector plate must be significantly lower than the sensor reading.

I was thinking about possible ways to avoid this issue. As you've said, the best approach would be to put the sensor on/in the part being heated. However, I wonder a sacrifice in terms of the frequency with which the temp/PWM update is performed might permit some level of compensation.

What I had in mind was looking at the current temperature and the desired temperature, and taking a guess at how long it would take to bring the current temperature up to the desired temperature running the heater at full-bore. With a bit of experimentation it should be possible to make a rough estimation for a given component (e.g. the corrector plate). Once you have the results for a duty cycle of 1, it would be simple to scale the results for other duty cycles.

When the controller is first turned on, it's reasonable to assume that the temperature at the sensor will be exactly the same as the actual temperature of the scope part (e.g. collector plate). Using the approximation, the controller now takes a guess at the length of time it will need to run the heater to raise the temperature sufficiently. We could reduce the time a little to avoid overshooting.

Having guessed a the required time, the controller then runs the heater for this time, ignoring any sensor readings. At the end of this, the actual temperature of the scope part should be roughly what we're aiming for. The temperature sensor will probably give a higher reading, since it's right next to the (hot) heating element, so we switch the heating element completely off and let it cool down for a short period (some seconds?). Once the heater has cooled down the temperature sensor should be closer to the actual temperature of the scope part, so we can see how close we got.

We now repeat the process using the new temperature sensor reading. Because we should now be closer to the desired temperature, we possibly don't want to perform the heating quite so aggressively, so we can lower the duty cycle (simply adjusting the time we're running the heater to compensate, e.g. duty cycle halved, time doubled).

The idea is that this heater off -> measure -> guess -> heat process would repeat continuously. The fact that we're only turning the heater off when we expect to have reached the target temperature means that we're not slowing down the heating process as per my original suggestion (you're right - that would slow things down considerably).

This is probably all overkill, but it's a change that could be made entirely in software without needing to change the physical circuit. I'm just thinking aloud really. You'd need to make sure that the same scope parts were always heated with the same channel though (and provide a set of approximations for each one) and that would be a disadvantage.

[hikerbob (Bob)]

"You'd need to make sure that the same scope parts were always heated with the same channel though (and provide a set of approximations for each one) and that would be a disadvantage.You'd need to make sure that the same scope parts were always heated with the same channel though (and provide a set of approximations for each one) and that would be a disadvantage."

Might be a workaround for that. In the first iteration I included a current sensor in the circuit. The plan was at startup to run each channel on it's own and measure the current flow and estimate the heater size, eg the bigger the strap the more current they draw.

From what I recall the differences between straps were big enough to know which strap was connected to what.

You could quite possibly get some usable approximations from that for the heating cycles. I found I wasn't using it and dropped it for the second round (extra cost and a little extra complexity). Sparkfun have them. What current measurement would not tell you was the thermal mass of what's being heated or how well heat transfers.

Could well be worth a try, I wanted to keep the unit very generic, all channels behave the same way but that's not an essential.

Might also be worth a look at ASCOM and dew heater control. Not something I've checked any detail on but there has been some work done on that front. Should be possible to tweak individual channels remotely.

Keep thinking out loud, love it as a way of exploring idea's.

Bob

[Prolixia]

Bob, that's a great idea re. identifying the strip on each channel!

I've only just started to order parts for my controller and strips, but I'm really looking forward to having a play around.

Many thanks,

Prolixia

[alistairsam]

Originally Posted by Prolixia

Thanks for clarifying Bob - that's reassuring.

Regarding your corrector plate, I wonder if that might possibly be a symptom of the problem I was suggesting - i.e. because the heat doesn't transfer quickly enough to the corrector plate the temperature sensor warms up well before it and reduces the output on that channel whilst the collector plate is still cold. .

will it help if the temp sensor was not under the actual heater strip but taped say a half inch away with a slightly longer wire?
in this case, the sensor can be mounted onto the corrector plate than under the heater strip.
for eyepieces, it may not be feasible due to limited space, but I think where there is space available, tape the sensor away from the heater strip.

I've been chatting with Bob on developing a similar controller with the picaxes and will let him post his ideas.
it should be easy enough to have a mathematical relation between the pwm frequency and the target temperature, sort of a temperature regulator to ensure it always stays 5 deg above dewpoint and not more, so you don't waste power.
not sure if Bob's project already does this, will have to go back to the article and check.

[hikerbob (Bob)]

The unit's I've built start heating at 5 degree's above due point and ramp up the heating as the sensor's under the heater straps get closer to due point.

One of the idea's that I'd tossed around initially and decided it was looking too difficult with the Arduino was a distributed controller.

Have a master unit to manage the dew point sensing and calculations, display management and comms to a PC if required. Each channel would have a micro controller, mosfet, temp sensor and any other support components required. They could talk to the main controller via 1-wire, I2C or similar. With Arduinos that started to look expensive and bulky.
Using Picaxe 8M2 or similar chips the remote unit's could be very cheap and compact. Eg an 8M2 can be sourced locally for $3.10 http://www.microzed.com.au/index.php...roducts_id=219.

The I2C support in the Picaxe's looks pretty good and that should make comms fairly simple and the whole thing could then be very modular. Not sure of the limit's but it should be viable to have a lot of heaters if required.

Waste heat from switching could go into heating (a slight power saving) if the unit's were small enough to fit under heater straps. It could also use an analogue temp sensor - http://www.phanderson.com/picaxe/lm34.html although I don't know what if any benefit's that would give.

I don't yet know the Picaxe's well enough to be certain how to do all that but from what I've read it looks like it should be simple.

Worth a thought for anyone contemplating building a controller as an alternative approach.

Bob

[alistairsam]

Hi,

I'm working on building a controller with independent pwm channels / distribted controller and as Bob mentioned, the picaxe's should be very cost effective even if individual mcu's are used.
I've used 4 picaxe's on the same I2C bus and they're easy to implement.

I'll carry out some tests and draw up a pcb schematic once I have the prototype working.
I'll try and make it modular so you can add on or remove channels as needed. only bit that will change is the code for the master mcu.

in terms of temp regulation, Bob what would the target temp range need to be at? 3 to 5 deg above dew point till 10 deg above dewpoint or a wider range?
the pwm freq can be linked to the sensor temp so they're not unnecessarily heated but still stay above dew point.