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jbruce

Geeks only - Raspberry PI controlled kiln

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Today I added the ability to restart a schedule at any point in the schedule.  I did this just in case I have a power outage, or something unforeseen happens. 

I also added an api.  So far, only starting a schedule is supported, but soon, everything will be added.  What this means to me is that I can do things like run my kiln on a schedule. I can pack it the night before, automatically start the schedule [ware drying cycle] at dark thirty and the whole process will finish during the day instead of late at night.

In other news, I twisted my thermocouple leads - this did not stop the noise I'm seeing.  I have capacitors on order to see if they can help in reducing noise

Edited by jbruce

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43 minutes ago, jbruce said:

Today I added the ability to restart a schedule at any point in the schedule.  I did this just in case I have a power outage, or something unforeseen happens. 

I also added an api.  So far, only starting a schedule is supported, but soon, everything will be added.  What this means to me is that I can do things like run my kiln on a schedule. I can pack it the night before, automatically start the schedule [ware drying cycle] at dark thirty and the whole process will finish during the day instead of late at night.

In other news, I twisted my thermocouple leads - this did not stop the noise I'm seeing.  I have capacitors on order to see if they can help in reducing noise

Really nice project and thanks for sharing. If you ever need the PLC code created for the monitor we created ( simple ladder logic for Click PLC stuff) just drop me a message.  You are welcome to it.

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On 12/17/2018 at 9:55 PM, liambesaw said:

11,000 relay cycles, is that ok?  How many cycles are they generally rated for?  Might want to see if you can get away with floating average instead of being super precise.  Is it because it was a slow firing with the elements crAnked to high?  Surprised how cheap those relays are, this is something I'd like to do, where was the cost of 200 dollars to you?  

Sorry for all of the questions, thanks.

Just so you have a point of comparison, I fired my E18T-3 yesterday, a fast bisque firing with a 4 hour preheat, and it averaged about 2,600 relay cycles per kiln section. These are typical mechanical relays. Because SSR's have a longer lifespan, it's okay to have them switch more, plus all that switching can increase element life. When relays switch less, the element has greater variation in temperature. That is, it heats up and cools down every time the relay turns it on and off. The longer the time period switched off, the greater the variation in the temperature of the element. As we know, metal expands and contracts as it heats and cools, and that movement decreases the life span of the element. By switching more often, the element can maintain a more constant temperature, which increases its life. So why don't all the manufacturers use SSR relays? Because they cost more, require an additional safety relay (even more cost), and most people don't care enough to justify the greater cost.

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4 minutes ago, neilestrick said:

Just so you have a point of comparison, I fired my E18T-3 yesterday, a fast bisque firing with a 4 hour preheat, and it averaged about 2,600 relay cycles per kiln section. These are typical mechanical relays. Because SSR's have a longer lifespan, it's okay to have them switch more, plus all that switching can increase element life. When relays switch less, the element has greater variation in temperature. That is, it heats up and cools down every time the relay turns it on and off. The longer the time period switched off, the greater the variation in the temperature of the element. As we know, metal expands and contracts as it heats and cools, and that movement decreases the life span of the element. By switching more often, the element can maintain a more constant temperature, which increases its life. So why don't all the manufacturers use SSR relays? Because they cost more, require an additional safety relay (even more cost), and most people don't care enough to justify the greater cost.

Interesting, thanks

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15 minutes ago, neilestrick said:

Just so you have a point of comparison, I fired my E18T-3 yesterday, a fast bisque firing with a 4 hour preheat, and it averaged about 2,600 relay cycles per kiln section. These are typical mechanical relays. Because SSR's have a longer lifespan, it's okay to have them switch more, plus all that switching can increase element life. When relays switch less, the element has greater variation in temperature. That is, it heats up and cools down every time the relay turns it on and off. The longer the time period switched off, the greater the variation in the temperature of the element. As we know, metal expands and contracts as it heats and cools, and that movement decreases the life span of the element. By switching more often, the element can maintain a more constant temperature, which increases its life. So why don't all the manufacturers use SSR relays? Because they cost more, require an additional safety relay (even more cost), and most people don't care enough to justify the greater cost.

Don’t forget switching at zero crossing, meaning his relays always wait till zero volts and fire. Elements are never subjected to startup at 170 v peak or worst case  340 v. Peak  for  two phase operation. As well as smoother temperature control allows the overall  duty cycle of the element  to match the load more practically. 

Edited by Bill Kielb

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Happy Holidays Potters!

I did another bisque fire today and got very consistent results when compared to the last bisque firing.  I started this firing at 2AM using the new API and it ended just before 3pm.  I also used a new API feature that allows me to start a kiln run anywhere in a schedule.  With this run, I skipped the first two hours because I knew my pots were bone dry.  I added this feature for power outages, but it is handy for this too.

schedule name cone-05-long-bisque
schedule date 12/28/2018
average error in degrees F 0.78
solid state relay cycles 11086
schedule length in hours 15.17
elements on (s) 21955.36
element percent on 47.78
element watts 9640
cost per kwh 0.126
schedule total cost $7.41

The capacitors I added did not improve thermocouple noise or thermocouple errors. I discovered that thermocouple noise is caused by me when I connect a device via wifi.  It is either causing noise from the wifi transmission being picked up by the thermocouple cable... or it could be power supply noise.  I'm not sure, but it does not affect firing at all, so I'm going to ignore it until it becomes a problem. 

bisque2.png.b7337624e860cf4be65e213640a97d40.png

Next is a cone 6 glaze firing.  I'll post results for that.

 

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@liambesaw I think that depends on how fast you want to cool.  If you want to cool at a rate faster than the natural cooling of your kiln, that is not supported today.  If you want to cool slowly , which I am guessing you want for crystal formation, then yes.  Here is my cone6 schedule which cools at 436 F/hour until 1832 and then cools more slowly at 130F/hour to 1400 to maximize crystal formation.

So what happens with the PID controller is that it just tries to maintain the current set point.  It doesn't matter (too much) whether a ramp is going up or down or is static.  If you REALLY want to cool super fast, the original software supported this, I just removed it from the code.  Reflow ovens use a fan to cool faster than the original toaster oven could cool.  This is probably not what you want.

cone6.thumb.png.caf8eb0ca217e7a9a90cc7596be07e48.png

Edited by jbruce

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Downramp is what I was thinking for slow cooling, that's great!  Any tips on doing this project?  Should I get a big kit box and mount the rpi in there and everything?  The schematics look pretty simple as far as the circuitry goes, but it would be cool to see a shot of yours as well.

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ok, I'll open up the bud box I have and send you a pic.  I have to go to dinner first, but sometime soon.  basically it's a 220v male connector on one side, a female 220v connector on the other.  an ssr is mounted to the box with some thermal paste, and the rpi is plopped in there with a plastic case and a breadboard on top.  I have not etched a circuit board for this and might not do so.

The bud box plugs into the wall, the kiln plugs into the bud box. simple.

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1 minute ago, jbruce said:

ok, I'll open up the bud box I have and send you a pic.  I have to go to dinner first, but sometime soon.  basically it's a 220v male connector on one side, a female 220v connector on the other.  an ssr is mounted to the box with some thermal paste, and the rpi is plopped in there with a plastic case and a breadboard on top.  I have not etched a circuit board for this and might not do so.

The bud box plugs into the wall, the kiln plugs into the bud box. simple.

That's perfect, because I will end up needing to bring it indoors when not in use (my kiln is outdoors under cover)

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I'm excited! I'm in the middle of my first cone 6 firing using the rpi controller.  I'll post stats when it's done.  My kiln could not keep up with the aggressive schedule (345 F/hour) from 1880 to 1976, but it was pretty close. After 1976 it slows to 108F/hour and it caught up quickly.

@liambesaw I have not forgotten I promised pics.  I'll get 'em posted for ya.

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Fantastic! Can’t wait to see the firing.

in the next few weeks we are going to design and install a retrofit SSR to one of our existing kilns. We will publish complete with thermal analysis and proper overcurrent protection for the SSR in the hopes that there will be folks who wish to convert and possibly use the raspberry pi as well.

cool stuff!

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The cone 6 glaze firing went pretty well.  You can see in the image that my kiln sitter tripped just before reaching 2232.  It had a cone 7 kiln sitter cone in it.  I think I had a kiln shelf too close to it. No worries, I just by-passed the sitter with slow-flip and used the api to start the cooling phase. You can see I skipped the initial drying phase of this schedule. There is more error this time than previous runs because I included the initial warm up, the part of the schedule where my kiln could not keep up, and the kiln-sitter flip. It was usually within a degree F though.  There were points in the schedule where the elements were on 100% of the time.

schedule name cone-6-long-glaze
schedule date 1/5/2019
average error in degrees F 3.25
solid state relay cycles 11934
schedule length in hours 11:25:12
elements on (s) 6:34:00
element percent on 57.50
element watts 9640
cost per kwh 0.126
schedule total cost $7.98

slow-cone6-glaze.png.33d65be6c00886aaa0598e06310efa41.png

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Yep. It's a small kiln, so I just have one set of witness cones.  In this case, 5,6,7 on the center shelf.  In past firings, I've watched these to determine when to shut down (or move on to the next part of the schedule). This time I did not get the chance because the kiln sitter flipped before I thought it would.  Next time.

 

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I emptied the kiln this morning after the first glaze firing and everything looked good even though the kiln was over-fired by a cone.  The witness cones showed that the kiln reached cone 7... just like the kiln sitter cone did.  The maximum temperature according to the thermocouple was 2221 and cone 7 is 2262 at my firing rate of 108F/hour.  So I guess the thermocouple is just off by 41F.  At the same time, I'm reluctant to believe this because at cone 05, the cone bends right when it should.

 

Maybe someone with more experience with thermocouples can provide advice.  It's a K type thermocouple and I know I have the right polarity on connecting wires and the thermocouple itself.  I'm using NIST linearization for the greatest accuracy. I cannot think of a reason it would be off.

 

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The normal way this is accounted for is to establish 108 degree per hour in approximately the last 250 degrees of firing. I think I mentioned this but if you look at the Bartlett schedules you will see they have perfected firing speeds along with final firing temperatures with a final rate of 120 degrees per hour in the last 250 degrees of firing.

heatwork is difficult to calculate and thermocouple offsets to tabled cone values  are often the solution based on witness cone tests and can be taylored to each cone. Rarely does one offset serve all cones exactly.

just my experience though.

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Thanks Bill.  I changed my schedule from 108F/hour to 120F/hour for that final 250F approach.  We'll see if that changes the heat work a little.  I expect that change will get me part of the way, but probably still a little high somewhere between cone 7 & 6.

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@liambesaw Here is the pic I promised.

wrJNbcp.jpg

Input is a nema male plug with three conductors (2 hot, 1 neutral).  I used an old extension cord for this to make sure it could handle the current.  Turns out it could handle about 4 times the 40A current max, so lots of safety margin there (and free wire). One hot leg is switched by the SSR, everything else runs straight to the output on the right hand side where the female nema plug is mounted.  The RPI is in a plastic case and is covered by a breadboard with all the spaghetti wiring.  You can see the max31855 in there. The thermocouple is the red/yellow wire.  The rpi connects to the control side of the ssr.  There is also power for the rpi (that black cable coming out the square hole in the front).

That big square hole is from a previous incarnation of the controller that used a PID controller purchased from a company.

Note the BIG heatsink with thermal paste and the BIG aluminum box.  If you want your SSR to survive, keep it cool.  There will be times where the SSR is on 100% of the time and that will generate significant heat.

 

Edited by jbruce

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Interesting, and I assume you mean ground instead of neutral?  I'll be direct wiring my kiln so I get to skip all the plug stuff, looks like it will be pretty simple!  Can't wait!  Gonna get started on running the 50 amp circuit to my kiln this weekend, I'm so stoked.  Will probably do a couple firings without the rpi first just to get a feel of what it's capable of, but then I'm making one of these controllers for sure.

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1 hour ago, jbruce said:

@liambesaw Here is the pic I promised.

wrJNbcp.jpg

Input is a nema male plug with three conductors (2 hot, 1 neutral).  I used an old extension cord for this to make sure it could handle the current.  Turns out it could handle about 4 times the 40A current max, so lots of safety margin there (and free wire). One hot leg is switched by the SSR, everything else runs straight to the output on the right hand side where the female nema plug is mounted.  The RPI is in a plastic case and is covered by a breadboard with all the spaghetti wiring.  You can see the max31855 in there. The thermocouple is the red/yellow wire.  The rpi connects to the control side of the ssr.  There is also power for the rpi (that black cable coming out the square hole in the front).

That big square hole is from a previous incarnation of the controller that used a PID controller purchased from a company.

Note the BIG heatsink with thermal paste and the BIG aluminum box.  If you want your SSR to survive, keep it cool.  There will be times where the SSR is on 100% of the time and that will generate significant heat.

 

Are ýour elements always hot? One leg connected?

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4 hours ago, Bill Kielb said:

Are ýour elements always hot? One leg connected?

I ran into a kiln like that once, where the previous repair guy had put a Bartlett controller on a Nabertherm and didn't do it right. I got a nice little shock when I tried to push an element in, even though the controller wasn't actually running. Luckily the teacher at the school had never gotten shocked. This is why two pole relays are used. Switching only one leg, the element won't heat up because the circuit loop isn't closed, but when you touch the element and ground it out you'll get hit.

@jbruce I'm also worried about the extension cord wires. Those wires are only rated for 12-20 amps, depending on the size of the cord. How did you arrive at it being large enough for 4 times the 40 amps? And if the kiln pulls 40 amps, the wires should be rated for 50 amps, which means you should have 6 gauge wire in there.

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This needs to be cleaned up a bunch. Running the cord as parallel feeder will get you a derated 60 amps or 45 amps for 12 and 14 gauge cord respectively. The SSR needs fuse protection , the SSR heat sink is inverted and mounted externally. It is ok as a prototype, but needs a bunch before this could be offered to the general public.

i suggest you get data and perfect the beta project then redesign with an eye  towards electrical safety and device longevity.

 

still a nice project.

 

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